Slides to IAEA Diagnostic Radiology Physics: A Handbook for ...

Slides to IAEA Diagnostic Radiology Physics: A Handbook for ...

Chapter 7: Image Receptors Slide set of 220 slides based on the chapter authored by John A. Rowlands and Ulrich Neitzel of the IAEA publication (ISBN 978-92-0-131010-1): Diagnostic Radiology Physics: A Handbook for Teachers and Students Objective: To familiarize the student with image receptors used in X-ray imaging systems Slide set prepared by K.P. Maher following initial work by S. Edyvean IAEA

International Atomic Energy Agency CHAPTER 7 TABLE OF CONTENTS 7.1 Introduction 7.2 General Properties of Receptors 7.3 Film and Screen-Film Systems 7.4 Digital Receptors Bibliography IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students CHAPTER 7

7.1 7.2 TABLE OF CONTENTS Introduction General Properties of Receptors 7.2.1 7.2.2 7.2.3 7.2.4 7.2.5 Receptor Sensitivity Receptor Noise

Grayscale Response & Dynamic Range Receptor Blur Fixed Pattern Noise IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students CHAPTER 7 7.3 Film & Screen-Film Systems 7.3.1 7.3.2 7.3.3 7.3.4 7.3.5

7.3.6 7.4 TABLE OF CONTENTS Systems The Screen Photographic Film and the Photographic Process Grayscale Characteristics of Film Images Reciprocity Screen-Film Imaging Characteristics Digital Receptors 7.4.1 7.4.2

7.4.3 7.4.4 7.4.5 7.4.6 IAEA Digital Imaging Systems Computed Radiography (CR) Digital Radiography - DR Other Systems Artefacts of Digital Images Comparison of Digital & Analogue Systems Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.1

INTRODUCTION X ray images are formed as Shadows of the interior of the body Since it is not yet practical to focus X rays, an X ray receptor has to be Larger than the body part to be imaged Thus the First challenge in making an X ray receptor is the need to image a large area IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.1 INTRODUCTION

A Second challenge is to make a system which has image quality as good as allowed by the physics i.e. permits the detection of objects whose size and contrast is limited only by the Quantum Statistics This means absorbing most of the X ray quanta and using these in an Efficient, i.e. a quantum noise limited, manner while simultaneously providing adequate spatial resolution IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.1 INTRODUCTION The Capture of an X ray image may conceptually be divided

into three stages (with a possible fourth stage): The Interaction of the X ray with a suitable detection medium to generate a measurable response The temporary Storage of this response with a recording device The Measurement of this stored response Setting the system ready to start again IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.1 INTRODUCTION Example 1

The stages for a Screen-Film system are: 1.The Interaction of an X ray in a phosphor material followed by generation of visible light photons 2.The creation of a Latent Image in the photographic film by these photons 3.The Development of a fixed photographic image 4.For re-usable systems (i.e. those not requiring consumables such as film) is the Erasure of all previous images within the detection system in order to prepare for a fresh image IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.1 INTRODUCTION

Example 2 For a Digital Direct conversion flat panel imaging system: 1. 2. 3. 4. The Absorption of an X ray followed by the release of multiple secondary electrons in a photoconductor The drifting of the electrons and holes to individual electrodes where they are Stored The Readout phase - the charges are transferred to amplifiers where they are digitized line by line This is achieved in step 3 - the readout simultaneously and without further effort performs the essential Erasure IAEA

Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.1 INTRODUCTION Breaking up the stages in this manner is helpful to the understanding of the physics of image acquisition which itself is key to the: Optimization of the receptor design and the Understanding of fundamental limitations on image quality It is also key to developing an understanding of the complementary strengths of the various approaches used in the past, the present and in the future IAEA

Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.2 GENERAL PROPERTIES OF RECEPTORS Before describing in more detail the properties of the different types of image receptor used for projection radiography it is necessary to consider the various: Physical Properties and Quantities which are used to specify their performance IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.2 GENERAL PROPERTIES OF RECEPTORS 7.2.1 Receptor Sensitivity

The initial image acquisition operation is identical in all X ray receptors To produce a signal, the X ray quanta must interact with the receptor material The probability of interaction, or Quantum Detection Efficiency for an X ray of energy E is given by: AQ = 1 - exp [- (E, Z) T ]E, Z) T ]) T ] where is the linear attenuation coefficient of the receptor material, Z is the materials atomic number, and T its thickness IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.2 GENERAL PROPERTIES OF RECEPTORS 7.2.1 Receptor Sensitivity

Because virtually all X ray sources for radiography emit X rays over a spectrum of energies, the Quantum Detection Efficiency must either be specified as a function of energy or as an effective value over the spectrum of X rays incident on the receptor AQ will in general be highest at low E decreasing with increasing E IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.2 GENERAL PROPERTIES OF RECEPTORS 7.2.1 Receptor Sensitivity AQ for

representative examples of: an X ray photoconduct or a-Se a screen phosphor Gd2O2S the scintillator CsI

IAEA The curves are for the primary interaction using the photoelectric coefficient only The thicknesses are for assumed 100% packing fraction of the material which: is realistic for a-Se should be increased by ~2x for a powder screen such as Gd2O2S, and by ~1.1-1.2 x for an evaporated structured CsI

layer Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.2 GENERAL PROPERTIES OF RECEPTORS 7.2.1 Receptor Sensitivity At diagnostic X ray energies, the main interaction process is the Photoelectric Effect because of the relatively high Z) T ] of most receptor materials If the material has a K atomic absorption edge EK in the energy region of interest, then AQ increases dramatically at EK causing a local minimum in AQ for E < EK IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students

7.2 GENERAL PROPERTIES OF RECEPTORS 7.2.1 Receptor Sensitivity The photoelectric interaction of an X ray quantum with the receptor generates a high-speed Photoelectron During the subsequent loss of kinetic energy of the electron in the receptor, Excitation and Ionization occur, producing the secondary signal (optical quanta or electronic charge) The sensitivity of any imaging system therefore depends both on AQ and the primary Conversion Efficiency IAEA the efficiency of converting the energy of the interacting X ray to a more easily measurable form such as optical quanta or electrical charge Diagnostic Radiology Physics: A Handbook for Teachers and Students

7.2 GENERAL PROPERTIES OF RECEPTORS 7.2.1 Receptor Sensitivity Conversion Efficiency can be re-expressed as the Conversion Factor i.e. in terms of the number of secondary particles (light photons in a phosphor or electronhole pairs, EHPs, in a photoconductor) released per X ray For a surprising number of materials and systems this is ~1000 quanta or EHPs per 50 keV X ray The Conversion Factor is closely related to the intrinsic Band Structure of the solid from which the receptor is made IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.2 GENERAL PROPERTIES OF RECEPTORS

7.2.1 Receptor Sensitivity Band Structure IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.2 GENERAL PROPERTIES OF RECEPTORS 7.2.1 Receptor Sensitivity In all receptor materials the Valence Band is almost fully populated with electrons and the Conduction Band is practically empty The Forbidden Energy Gap, Eg, governs the energy scale necessary to release a mobile EHP i.e., to promote an electron from the valence band to the conduction band Although Eg is the minimum permitted by the principle of

conservation of energy, this can be accomplished only for photons of energy Eg IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.2 GENERAL PROPERTIES OF RECEPTORS 7.2.1 Receptor Sensitivity For Charged Particles releasing energy e.g. through the slowing down of high-energy electrons created by an initial X ray interaction conservation of both Energy and Crystal Momentum and the presence of competing energy loss processes necessitate ~3Eg to release an EHP For a photoconductor the maximum number of EHP is 50,000/(2 eV x 3)

~8,000 EHP IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.2 GENERAL PROPERTIES OF RECEPTORS 7.2.1 Receptor Sensitivity This is possible for good Photoconductors but for the only practical photoconductor used in commercial systems at this time (a-Se) there are other losses primarily to Geminate Recombination i.e. the created EHPs recombine before separation by the applied electric field which limit it to ~1,000-3,000 EHP depending on the applied

electric field IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.2 GENERAL PROPERTIES OF RECEPTORS 7.2.1 Receptor Sensitivity In Phosphors, the band gap is usually much higher (~8 eV) so the intrinsic conversion factor is typically lower only ~2,000 EHPs are released 50,000/8 eV x 3 which however in an Activated Phosphor results in emission of only slightly less (1,800) light photons IAEA

Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.2 GENERAL PROPERTIES OF RECEPTORS 7.2.1 Receptor Sensitivity Additional optical losses due to: Light Absorption in the phosphor layer (sometimes with an intentionally included dye) and/or nonreflective backing Dead space between the photoreceptors (Fill Factor) Non-ideal Quantum Efficiency of the photoreceptors further reduces the light per X ray This typically results in ~1,000 EHPs collected in the photoreceptor for our prototypical 50 keV X ray IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students

7.2 GENERAL PROPERTIES OF RECEPTORS 7.2.2 Receptor Noise All images generated by quanta are Statistical in nature i.e., although the image pattern can be predicted from the attenuation properties of the patient, it will fluctuate randomly about the mean predicted value The fluctuation of the X ray intensity follows Poisson statistics so that the variance, 2, about the mean number of X ray quanta, N0, falling on a receptor element of a given area, is equal to N0 Interaction with the receptor can be represented as a Binomial process with probability of success, AQ, and the distribution of interacting quanta is still Poisson with variance: IAEA

2 = N0 AQ Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.2 GENERAL PROPERTIES OF RECEPTORS 7.2.2 Receptor Noise If the detection stage is followed by a process that provides a mean gain g then the distribution will not be Poisson even if g is Poisson distributed It is also possible that other independent sources of noise will contribute at different stages of the imaging system Their effect on the variance will be Additive A complete linear analysis of signal and noise propagation in a receptor system must also take into account the Spatial Frequency Dependence of both signal and noise

IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.2 GENERAL PROPERTIES OF RECEPTORS 7.2.2 Receptor Noise It is important that the number of secondary quanta or electrons at each stage of the image production be considerably greater than N0, to avoid having the receptor noise dominated by a Secondary Quantum Sink Consideration of the propagation of noise is greatly facilitated by the consideration of a Quantum Accounting Diagram The vertical axis represents the average number of quanta or individual particles (electrons or film grains) representing the initial absorbed X ray (assumed to be of 50 keV)

at each stage in the imaging system IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.2 GENERAL PROPERTIES OF RECEPTORS 7.2.2 Receptor Noise Quantum Accounting Diagrams for screen-film, CR and flat panel DR The critical point for each modality is where the minimum number of quanta or EHPs represents a single X ray For flat panel systems this is

~1,000 while for screen-film it is 20 and only 5 for CR This is the Weakest Link IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.2 GENERAL PROPERTIES OF RECEPTORS 7.2.2 Receptor Noise The concept is that the Noise from each stage of the imaging system is related to the number of secondary quanta or electrons at each stage So ideally there should for all stages be many more (if possible exceeding 1,000) such secondary quanta or particles representing each primary quantum (i.e. X ray) The point at which this number is lowest is the

Secondary Quantum Sink IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.2 GENERAL PROPERTIES OF RECEPTORS 7.2.2 Receptor Noise With the examples given (all of which are used commercially) have at least 5 secondaries per primary but it is very easy to find systems in which this is not the case, such as: Non-intensified fluoroscopy or Some optically (lens) coupled radiographic X ray systems IAEA

Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.2 GENERAL PROPERTIES OF RECEPTORS 7.2.2 Receptor Noise The noise in X ray images is related to the Number of X Rays per Pixel in the image and hence to the X ray exposure to the receptor However, the relative noise can be increased by lack of absorption of the X rays, as well as by fluctuations in the response of the receptor to those X rays which are absorbed There are also unavoidable fluctuations in the signal produced in the detection medium even when X rays of identical energy interact and produce a response

IAEA These are caused by the statistical nature of the competing mechanisms that occur as the X ray deposits energy in the medium Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.2 GENERAL PROPERTIES OF RECEPTORS 7.2.2 Receptor Noise Together they give rise to a category of noise known as gainfluctuation or Swank Noise The gain-fluctuation noise can be determined experimentally using the pulse height spectrum, or PHS From this the Swank Factor, AS, is obtained as a combination of the zeroth, first and second moments (MN) of the PHS using the formula:

As = M12 / (E, Z) T ]M0M2) IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.2 GENERAL PROPERTIES OF RECEPTORS 7.2.2 Receptor Noise The ideal PHS, obtained when all absorbed X rays give rise to equal amounts of signal results in a Delta Function and a Swank factor of unity However in practice there are a number of effects which may broaden this spectrum, resulting in a Swank factor of less than unity IAEA

Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.2 GENERAL PROPERTIES OF RECEPTORS 7.2.2 Receptor Noise Swank Factor AS for representative examples of: an X ray photoconduct

or a-Se a screen phosphor Gd2O2S the scintillator CsI These are calculated values and are for the Photoelectric Effect only which effectively means that only K-escape is accounted for IAEA

Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.2 GENERAL PROPERTIES OF RECEPTORS 7.2.2 Receptor Noise K-Escape is the emission of a K-fluorescent X ray following a photoelectric interaction, which then escapes from the receptor without depositing further energy As the energy of the K-fluorescent X ray is below the K-edge, it has a smaller interaction probability than the original incident X ray photon The range of values of As is from 0.7-1 Further losses due to optical effects will be seen in screens, and other losses in photoconductors due to trapping of charge IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students

7.2 GENERAL PROPERTIES OF RECEPTORS 7.2.2 Receptor Noise Swank demonstrated that for many situations the combination of factors can be performed simply by multiplying the component factors For example the K-escape Swank factor can be multiplied by the optical Swank factor to obtain the overall Swank factor Theoretically, for an exponential PHS which can occur for screens with very high optical absorption the optical value can be as poor as 0.5, resulting in a range 0.5-1 for the optical effects Overall therefore the possible range of receptor

Swank factors for screens is 0.35-1 IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.2 GENERAL PROPERTIES OF RECEPTORS 7.2.2 Receptor Noise The noise due to both Quantum Absorption and Gain Fluctuations can be combined to create the zero spatial frequency Detective Quantum Efficiency that is given by: DQE(0) = AQAS Due to its importance it is worth reiterating that, the DQE(0) is the effective quantum efficiency obtained when we compare the noise in the measured image to what it would be if it was an ideal (perfect) absorber and there was no gain fluctuation noise

IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.2 GENERAL PROPERTIES OF RECEPTORS 7.2.3 Grayscale Response & Dynamic Range The Grayscale Response used for an imaging system has to do with the physics of X Ray Detection the Imaging Task to be performed and the response of the human Eye-Brain System to optical images (the most difficult part) In practice many of the decisions made by system designers are empirical rather than fully from theoretical analysis IAEA

Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.2 GENERAL PROPERTIES OF RECEPTORS 7.2.3 Grayscale Response & Dynamic Range However, some Rules of Thumb can be helpful Regarding human vision, there is a spatial frequency range in which the human eye is most acute This is an intermediate frequency range, neither too low nor too high Regarding intensity, it is, as for all human senses, essentially logarithmic in its response, i.e., it is fairly good at seeing fractional differences, provided these are directly juxtaposed Otherwise the human eye is quite poor at quantitative evaluations of intensity IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students

7.2 GENERAL PROPERTIES OF RECEPTORS 7.2.3 Grayscale Response & Dynamic Range In order to separate the Subjective eye-brain response from the more quantitative issues it is usual to: Leave Out (in the case of inherently non-linear systems like film) or to Correct for the optical display part of the system which has a non-linear response (e.g. CRT monitors or LCD flat panel displays) Only then can most systems be, for practical purposes, modelled as being Linear IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students

7.2 GENERAL PROPERTIES OF RECEPTORS 7.2.3 Grayscale Response & Dynamic Range The grayscale response is usually expressed as the Characteristic Curve a plot of the response of the system to a stimulus For example in Fluoroscopy this would be the optical intensity at the video monitor plotted as a function of the irradiation of the sensor at the corresponding point in the image IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.2 GENERAL PROPERTIES OF RECEPTORS 7.2.3 Grayscale Response & Dynamic Range

The range of intensities that can be represented by an imaging system is called the Dynamic Range and it depends on the pixel size that is used in a manner that depends on the MTF However, for any pixel size the dynamic range for an X ray imaging task can be broken into two components: Describes the ratio between the X ray attenuation of the most radiolucent and the most radio-opaque paths through the patient appearing on the same image The required Precision of the X ray signal measured in the part of the image representing the most radioopaque anatomy IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.2 GENERAL PROPERTIES OF RECEPTORS 7.2.3 Grayscale Response & Dynamic Range

If, for example there is a factor of 10 in attenuation across the image field and it is desired to have 10% precision in measuring the signal in the most attenuating region then the Dynamic Range requirement for the receptor would be 100 IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.2 GENERAL PROPERTIES OF RECEPTORS 7.2.3 Grayscale Response & Dynamic Range The Dynamic Range that can be achieved by a practical

linear imaging system can be defined in terms of the response at the output referred back to the input in terms of the X ray exposure: Dynamic Range = Xmax / Xnoise where Xmax is the X ray exposure providing the maximum signal that the receptor can respond to before saturation i.e. that point where the receptor output ceases to respond sensibly to further input Xnoise is the rms receptor noise in the dark IAEA i.e. no X ray exposure Diagnostic Radiology Physics: A Handbook for Teachers and Students

7.2 GENERAL PROPERTIES OF RECEPTORS 7.2.3 Grayscale Response & Dynamic Range X rays are attenuated exponentially: thus an extra tenth-value layer thickness of tissue will attenuate the beam by 10 while a lack of the same tenth-value thickness will increase the X ray exposure by 10 Thus when a mean exposure value Xmean for the system is established by irradiating a uniform phantom, we are interested in multiplicative factors above and below this mean value i.e. Xmean is a Geometric rather than Arithmetic mean IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.2 GENERAL PROPERTIES OF RECEPTORS

7.2.3 Grayscale Response & Dynamic Range For a DR of 100: the correct range is that given by the Geometric Mean of 10Xmean and 0.1Xmean not that given by the IAEA Arithmetic Mean which would be ~2Xmean and 0.02Xmean Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.2 GENERAL PROPERTIES OF RECEPTORS 7.2.4 Receptor Blur Spatial Resolution in radiography is determined both by the

receptor characteristics and by factors unrelated to the receptor The latter includes Unsharpness (blurring) arising from geometrical factors such as: (1) Penumbra (partial X ray shadow) due to the effective size of the X ray source and the magnification between the anatomical structure of interest and the plane of the image receptor and (2) Motion Blurring due to relative motion of the patient with respect to the receptor and X ray focal spot IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.2 GENERAL PROPERTIES OF RECEPTORS 7.2.4 Receptor Blur

In the overall design of an imaging system, it is important that these other physical sources of unsharpness be considered when the Aperture Size and Sampling Interval are chosen If, for example, the MTF is limited by unsharpness due to the focal spot, it would be of little value to attempt to improve the system by designing the receptor with much smaller receptor elements IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.2 GENERAL PROPERTIES OF RECEPTORS 7.2.4 Receptor Blur The major issues related to receptor blur include the

fundamental issues arising within any material which are: (a) Geometrical Blurring (b) the Range of the primary electron (c) the Re-Absorption of K fluorescence IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.2 GENERAL PROPERTIES OF RECEPTORS 7.2.4 Receptor Blur Geometrical Blurring due to oblique incidence of X rays which is especially marked far from the Central Ray

i.e. the X ray which strikes the receptor normally IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.2 GENERAL PROPERTIES OF RECEPTORS 7.2.4 Receptor Blur Range of the Primary Electron the primary electron usually gives up its energy in small amounts, ~100-200 eV at a time but this is sufficient to scatter the electron at any angle, thus the path of the primary is usually a random walk and it does not go with a separation of ~1 m for 10

keV electrons and ~50-100 m for so far from its initial point of 100 keV depending on the medium interaction IAEA in which it is absorbed Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.2 GENERAL PROPERTIES OF RECEPTORS 7.2.4 Receptor Blur Re-Absorption of K Fluorescence X Rays some distance from the primary

photoelectric interaction, something which is likely because of the general rule that a material is relatively transparent to its own K-fluorescence due to the minimum in attenuation below the K-edge IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.2 GENERAL PROPERTIES OF RECEPTORS 7.2.4 Receptor Blur In addition there are material dependent effects specific to direct conversion and indirect conversion receptors shown in the following slides

Also to be considered in Digital Systems are the effects of: Del Aperture and Sampling Interval IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.2 GENERAL PROPERTIES OF RECEPTORS 7.2.4 Receptor Blur Mechanisms of resolution loss in Direct conversion layers (Photoconductors) IAEA

Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.2 GENERAL PROPERTIES OF RECEPTORS 7.2.4 Receptor Blur Mechanisms of resolution loss in Indirect conversion types Powder Phosphor Screens IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.2 GENERAL PROPERTIES OF RECEPTORS 7.2.4 Receptor Blur Mechanisms of resolution loss in indirect conversion types Powder Phosphor Screens and Structured Phosphors (usually called Scintillators) IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students

7.2 GENERAL PROPERTIES OF RECEPTORS 7.2.5 Fixed pattern noise It is important that the radiographic imaging system provide uniformity, i.e. the sensitivity is constant over the entire image If this is not the case, patterns that might disrupt the interpretation of the image will result This is Fixed Pattern Noise In an analogue imaging system, great pains must be taken in the design and manufacture of receptors to ensure that they provide a uniform response In Digital systems post processing can be often be used

to alleviate manufacturing limitations in uniformity IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.2 GENERAL PROPERTIES OF RECEPTORS 7.2.5 Fixed pattern noise In a linear imaging system the fixed pattern noise, which can be expressed as a pixel-to-pixel variation in Gain and Offset (often due to a dark current in the related sensor), can in principle be corrected and their effect completely eliminated The procedure is to correct patient images using: Dark Field (un-irradiated) and Bright Field (uniformly irradiated) images

IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.3 FILM AND SCREEN-FILM SYSTEMS In using a Screen-Film receptor to take an X ray image, a Radiographer must: Load a film into a cassette Carry the cassette to the examination room Insert the cassette into the X ray table Position the patient Make the X ray exposure Carry the cassette back to the processor to develop the film Wait for the film to be developed, and finally Check the processed film for any obvious problems to ensure that the film is suitable for making a medical

diagnosis before returning to the X ray room IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.3 FILM AND SCREEN-FILM SYSTEMS 7.3.1 Systems The Screen-Film Combination & Cassette The screen-film Combination consists of phosphor screen(s) and film designed to work together enclosed within a Cassette The cassette can be opened (in a dark environment) to allow the film to be inserted When the cassette is closed, the film is kept in Close Contact with the screen or more commonly a pair of screens, facing toward the film

IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.3 FILM AND SCREEN-FILM SYSTEMS 7.3.1 Systems The Screen-Film Combination & Cassette Screen-Film receptor: (a) Opened cassette showing placement of film and position of screens, and (b) Cross-sectional view through a dual screen system used in general purpose radiography with the film sandwiched IAEA between two screens Diagnostic Radiology Physics: A Handbook for Teachers and Students

7.3 FILM AND SCREEN-FILM SYSTEMS 7.3.1 Systems The Screen-Film Combination & Cassette Incident X rays first pass through the front of the cassette before reaching the screens When they interact in the screen some of the energy deposited is converted to Light which can travel from the interior to the screen surface where it enters the optically sensitive part of the film called the Emulsion and transfers its information into a Latent Image in the film IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students

7.3 FILM AND SCREEN-FILM SYSTEMS 7.3.1 Systems The Screen-Film Combination & Cassette The film is then Removed from the cassette and Developed so that the latent image is converted to a Permanent Image in the form of: Silver deposited in the Emulsion layer of the film IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students

7.3 FILM AND SCREEN-FILM SYSTEMS 7.3.1 Systems The Screen-Film Combination & Cassette In most cases two screens face the film which has two emulsions: one on either side of the film base with an Anti-Halation layer placed between the two emulsions During X ray exposure, the anti-halation layer is opaque and prevents light crossing over from one emulsion to the other, thus reducing Cross-Talk and hence Blurring IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students

7.3 FILM AND SCREEN-FILM SYSTEMS 7.3.1 Systems The Screen-Film Combination & Cassette The opaque anti-halation layer is removed during Film Development rendering the film Transparent for subsequent viewing For the highest resolution (e.g. Mammography) a single screen in the back of the cassette may be used in contact with a single emulsion film IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.3 FILM AND SCREEN-FILM SYSTEMS

7.3.1 Systems The Screen-Film Combination & Cassette Film emulsions can be used as Direct receptors for X ray images The earliest X ray images were taken with film alone In fact film was used in this way for Mammography up to the 1960s The sole remaining clinical application for film without screens is in dental radiography using Intraoral films However, the X ray absorption efficiency for such films is relatively poor (~1-5%) Thus currently all diagnostic X ray film images are obtained using screen(s) in conjunction with the film IAEA

Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.3 FILM AND SCREEN-FILM SYSTEMS 7.3.2 The Screen Screen Structure Phosphor grains are combined with a Polymer Binder and deposited on a substrate or backing The ratio of binder volume to phosphor volume in the mixture controls the fractional volume of the phosphor layer finally occupied by air pockets or Voids Typically the binder is nitrocellulose, polyester, acrylic or polyurethane and the plastic support or Backing Material is also a polymer e.g. polyethylene terephthalate 200-400 m thick IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students

7.3 FILM AND SCREEN-FILM SYSTEMS 7.3.2 The Screen Screen Structure The use of a black or white backing permits adjustments of the Reflectivity and Absorptivity at the phosphor interface In most screens the typical phosphor Grain Size is 3-8 m Usually a very thin transparent Protective Layer is subsequently applied to complete the screen structure IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.3 FILM AND SCREEN-FILM SYSTEMS 7.3.2 The Screen

Phosphor Screen-film systems employ a phosphor in the initial stage to absorb X rays and produce light Phosphors work by exciting electrons from the valence band to the conduction band so creating Electron Hole Pairs (EHPs) which are free to move within the phosphor Some of these will recombine without giving off any radiant energy IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.3 FILM AND SCREEN-FILM SYSTEMS 7.3.2 The Screen Phosphor

However in an Activated phosphor, most (>90%) EHPs will recombine at an activation centre (created by atomic impurities called Activators) and in the process emit light Since light photons each carry only a small amount of energy (~2-3eV), many light photons can be created from the absorption of a single X ray The specific Colour of the light emitted is related to the optical transitions in the activator By changing the activator, the light colour can be changed IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.3 FILM AND SCREEN-FILM SYSTEMS 7.3.2 The Screen

Phosphor This Quantum Amplification is the Conversion Gain of the phosphor The original screens used until the 1970s were Calcium Tungstate (CaWO4) which is naturally activated and hence not particularly efficient and emits light in the deep blue and UV radiation More recently Rare Earth phosphors with explicit centres for the emission of light at the activator site have resulted in the most commonly used material Gadolinium Oxysulphide Gd2O2S:Tb with Tb in dilute amounts 0.1-1% as an activator IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.3 FILM AND SCREEN-FILM SYSTEMS 7.3.2 The Screen

Phosphor For Gd2O2S, an X ray photon energy of 50 keV is equivalent to that of ~20,000 Green light quanta (E = 2.4 eV) although due to losses typically only 1,800 are produced in practice The Green emission from the rare earth phosphors has also required a change from conventional film in two regards: 1. Ordinary film is sensitive only in the blue, it requires additional sensitization to be green sensitive and then is called Orthochromatic 2. Green light is far more penetrating than blue light and so requires an anti-halation layer to prevent Crossover of images between emulsions IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students

7.3 FILM AND SCREEN-FILM SYSTEMS 7.3.2 The Screen Thickness The choice of the Thickness of a radiographic screen has to balance: the increase in AQ with thickness - which favours a thick screen, and the efficient escape of light and usually more importantly blurring due to spreading of light - which favours a thin screen IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students

7.3 FILM AND SCREEN-FILM SYSTEMS 7.3.2 The Screen Thickness In order to create a sharp X ray image, a transparent phosphor screen would be ineffective since light could move large distances within the phosphor and cause excessive blurring IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students

7.3 FILM AND SCREEN-FILM SYSTEMS 7.3.2 The Screen Thickness Instead X ray screens are made highly scattering or Turbid This is accomplished by using high refractive index phosphor grains embedded in a low reflective index binder Once a light photon exits a grain it tends to reflect off the neighbouring grain surfaces rather than passing through them Thus the lateral spread of the light is confined by Diffusion (multiple scattering) which helps to maintain the spatial resolution of the phosphor layer IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.3 FILM AND SCREEN-FILM SYSTEMS

7.3.2 The Screen Thickness Effect of phosphor thickness on spatial resolution of a Turbid phosphor screen: IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.3 FILM AND SCREEN-FILM SYSTEMS 7.3.2 The Screen Thickness Because of the presence of the binder material the amount of phosphor present in a screen is usually quoted in terms of the Screen Loading or areal density, i.e.

the mass of phosphor per unit area of the screen Typical values for Gd2O2S phosphor screens or screen pairs are from 20-200 mg/cm2 depending on the application IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.3 FILM AND SCREEN-FILM SYSTEMS 7.3.2 The Screen Optical Design Resolution is related to design Screen-Film combinations are not very efficient at absorbing X rays because there is such a severe trade-off between resolution and quantum efficiency

Only if it is acceptable to have a relatively blurred image is it possible to use a screen thick enough to be efficient in absorbing x rays IAEA High resolution screen-film combinations absorb no more than ~10-20% of the X rays whereas general purpose screens may absorb ~30-40% Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.3 FILM AND SCREEN-FILM SYSTEMS 7.3.2 The Screen Optical Design

Since the X rays also pass through the film: Some darkening will be developed due to Direct interactions of X rays with the film emulsion Usually this is so small that it can be ignored IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.3 FILM AND SCREEN-FILM SYSTEMS 7.3.2 The Screen Optical Design The Optical Design of a phosphor screen critically affects its imaging performance Factors such as: Phosphor grain size

Size distribution Bulk absorption Surface reflectivity Intentionally entrained tiny bubbles to increase scattering can have significant effects on the image quality IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.3 FILM AND SCREEN-FILM SYSTEMS 7.3.2 The Screen Optical Design An Absorptive backing helps reduce blurring by preferentially absorbing the

long path length photons, but at the cost of reduced overall Conversion Efficiency With a Reflective backing, most of the light escapes the front of the screen and is available to be recorded IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.3 FILM AND SCREEN-FILM SYSTEMS 7.3.2 The Screen Optical Design Light absorbing dye can be

added to the screen to enhance the resolution - this is similar to but not identical in effect to an absorptive backing IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.3 FILM & SCREEN-FILM SYSTEMS 7.3.3 Photographic Film & The Photographic Process Photographic Film is a unique material that is sensitive to a very few quanta of light At normal ambient temperature, it can: Record a Latent optical image from a fraction of a second exposure

Maintain this latent image for months, and Eventually be developed without significant loss of information It is also used as a Display and Archiving medium IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.3 FILM & SCREEN-FILM SYSTEMS 7.3.3 Photographic Film & The Photographic Process Film Structure The photographic process uses a thin layer (called the Emulsion) of silver halide crystals called Grains suspended in Gelatine and supported on a transparent film base The grains are primarily Silver Bromide (~95%) with the balance being silver iodide and sometimes trace amounts of

silver chloride The grains are of variable size (~m) and shape (cubic or tabular i.e. flat) depending on the application IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.3 FILM & SCREEN-FILM SYSTEMS 7.3.3 Photographic Film & The Photographic Process Film Structure The Film Base thickness is standardized to ~180 m to allow smooth transport through Automatic Film Processors The emulsion is typically 3-5 m thick and can be on one (Single Sided) or both (Double Sided) sides of the base During the manufacturing process the grains are sensitized by introducing Sensitivity Specks onto the grains by reaction

with sulphur compounds IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.3 FILM & SCREEN-FILM SYSTEMS 7.3.3 Photographic Film & The Photographic Process The Photographic Process The key feature which gives unexposed film its long shelf life is that more than one light photon must impinge on an individual silver halide crystal grain in order to create a stable latent image A single light photon creates an electron that is trapped for a short time (about a second) in a unique point on the grain called the Sensitivity Speck If no other photons are absorbed by this grain, then the

electron will escape from the grain IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.3 FILM & SCREEN-FILM SYSTEMS 7.3.3 Photographic Film & The Photographic Process The Photographic Process However, if a few more electrons are released in the same grain within this time: the electrons stabilize each other at the sensitivity speck and a Latent Image is established The multi-electron process is key to understanding not only

the long shelf life and the non-linear response of film to light but other behaviours to be described such as Reciprocity Failure IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.3 FILM & SCREEN-FILM SYSTEMS 7.3.3 Photographic Film & The Photographic Process Development of the Latent Image After exposure of the film to light, the Latent Image is formed at the sensitivity specks on the individual film grains Film Processing converts the latent image to a viewable permanent image Film processing can be split into three phases: Development

Fixing These processes are facilitated by the suspension of the grains in a Wash IAEA thin layer of water-permeable gelatine supported on a flexible substrate Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.3 FILM & SCREEN-FILM SYSTEMS 7.3.3 Photographic Film & The Photographic Process Development of the Latent Image

Chemicals are transported to the crystals without disturbing their positions when the film is dipped into chemical solutions The Development Process turns a sensitized transparent crystal of silver halide grain into a Speck of metallic silver that absorbs light and therefore is opaque Since these are very small (<1 m), light absorption dominates over reflection due to multiple scattering and they appear black in the same way as any finely powdered metal IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.3 FILM & SCREEN-FILM SYSTEMS 7.3.3 Photographic Film & The Photographic Process Development of the Latent Image The Gain of the system in terms of

the number of silver halide molecules converted into metallic silver per absorbed light photon is a staggeringly large number > 108 which is the basis of its uniquely high Sensitivity IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.3 FILM & SCREEN-FILM SYSTEMS 7.3.3 Photographic Film & The Photographic Process Fixing of the Image After the Latent Image has been Developed, the unexposed and therefore undeveloped transparent silver halide crystals

remain within the gelatine layer Thus the emulsion is still sensitive to light and if further exposed, would sensitize the grains which could self-develop and change the image During the Fixing stage, these undeveloped silver halide crystals are dissolved and removed chemically thereby Fixing the image IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.3 FILM & SCREEN-FILM SYSTEMS 7.3.3 Photographic Film & The Photographic Process Wash - Making it Archival Next after fixing is the Water Wash where the processing

chemicals and any remaining dissolved silver halide are removed leaving only the insoluble silver grains embedded in pure gelatine Drying removes the excess water solvent from the gelatine and results in a completely permanent archival material known as Photographic Film IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.3 FILM & SCREEN-FILM SYSTEMS 7.3.3 Photographic Film & The Photographic Process

Automated Processor Design While the Development Process can be performed manually with trays filled with chemical solutions for a few images per day, or deep tanks for more, these are very labour intensive processes which are difficult to control Automated Processors are therefore used which are: Much faster in operation (typically 90 s from introducing the exposed film to receiving the dried image) Suitable for maintaining consistent image quality by not only keeping the Speed Point consistent but the complete Characteristic Curve IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students

7.3 FILM & SCREEN-FILM SYSTEMS 7.3.3 Photographic Film & The Photographic Process Automated Processor Design The basic concept is to use deep processing tanks kept at a constant temperature and rollers immersed in the tanks to transport the film and ensure that it is subjected to consistent processing conditions A key additional process is the Drying step which means that the film can be used as soon as it emerges from the processor In practice, the simplest practical arrangement is for the processor to be built into the wall of the Dark Room with film removed from the cassette by a Darkroom Technician with a final image deposited in a tray outside the darkroom IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students

7.3 FILM & SCREEN-FILM SYSTEMS 7.3.3 Photographic Film & The Photographic Process Automated Processor Design The cassette is brought into the darkroom by an interlocked pass through or Light-Lock which: Permits the Darkroom Technician to keep his eyes dark adapted and Reduces risk of accidental exposure In more advanced departments the darkroom can be completely eliminated and the film automatically removed from the cassette and reloaded by an automated system IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students

7.3 FILM & SCREEN-FILM SYSTEMS 7.3.3 Photographic Film & The Photographic Process Automated Processor Design Artefacts related to the automatic film processors include: picking up of unwanted debris resulting in Dust Specks on the developed film, and periodic variations in intensity of the film (Roller Marks), which arise primarily from the variation of speed in the tanks due to the difficulty of manufacturing and maintaining the rollers to be exactly round and smooth In some dry environments Electrostatic Discharges can cause characteristic artefacts IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students

7.3 FILM & SCREEN-FILM SYSTEMS 7.3.4 Grayscale Characteristics of Film Images Optical Density A film radiograph is an image of the incident pattern of X radiation in which the distribution of the number of developed grains is related to the distribution of the number of X rays incident on the receptor When viewed on a Viewbox (source of back illumination), the developed grains reduce the light transmission through

the film so that the viewed image is a Negative IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.3 FILM & SCREEN-FILM SYSTEMS 7.3.4 Grayscale Characteristics of Film Images Optical Density The Transmission of light through a film or more precisely its inverse, the attenuation, is expressed as the Optical Density or OD which is defined by: OD = log10 (E, Z) T ] II / IT ) where IT is the intensity of light transmitted through the film due to the intensity of light II incident on it Thus for II / IT =10 the corresponding optical density is OD=1 IAEA

Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.3 FILM & SCREEN-FILM SYSTEMS 7.3.4 Grayscale Characteristics of Film Images Optical Density The transmissions of two absorbers placed in sequence in an optical path are Multiplicative so that the logarithms are Additive Thus two emulsions each of OD=1 would have a total OD=2 IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students

7.3 FILM & SCREEN-FILM SYSTEMS 7.3.4 Grayscale Characteristics of Film Images Optical Density Although the reason for a Negative image is purely historical and determined by the nature of the photographic process the display of images on Digital systems where there is freedom to change is usually very similar to that found with film This is no accident as the look of radiographic film has been tailored over the years to be optimum for its purpose of displaying the image most efficiently to the human observer IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.3 FILM & SCREEN-FILM SYSTEMS 7.3.4 Grayscale Characteristics of Film Images

Densitometry, Sensitometry In a radiology department dependent on Screen-Film, the instability of film processing systems is the single most problematic aspect Maintaining a match in characteristics between films processed from one day to the next, from morning to afternoon and from one film processor to another is an essential chore called Film Quality Control IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.3 FILM & SCREEN-FILM SYSTEMS 7.3.4 Grayscale Characteristics of Film Images

Densitometry, Sensitometry Film Quality Control requires the daily monitoring of the processor with the exposure of a film from a standard batch using a standardized step wedge with a Sensitometer, and the measurement of the developed film with a Densitometer Essential to this approach is consistent testing at the same time each day, measurements of the temperature of the solutions and keeping them chemically fresh Also essential is the need to have predetermined quantitative Action Points established, and associated actions to correct any deficiencies IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.3 FILM & SCREEN-FILM SYSTEMS 7.3.4 Grayscale Characteristics of Film Images

H & D Curve (E, Z) T ]Hurter & Driffield Curve) The curve which relates the Optical Density of the film to the logarithm of the exposure is known as the Characteristic Curve or H&D curve after Hurter and Driffield who first introduced its use IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.3 FILM & SCREEN-FILM SYSTEMS 7.3.4 Grayscale Characteristics of Film Images

H & D Curve (E, Z) T ]Hurter & Driffield Curve) The form of the curve can be explained as follows: For photographic films there is some film density, known as Base plus Fog, even in the absence of any exposure The Base component is the transmission of the film substrate or base The Fog is the absorption in the emulsions due primarily to unwanted self-development of grains unexposed to light IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.3 FILM & SCREEN-FILM SYSTEMS 7.3.4 Grayscale Characteristics of Film Images H & D Curve (E, Z) T ]Hurter & Driffield Curve)

Together Base plus Fog in fresh films are ~0.3 OD or less A small amount of light will have difficulty in creating any kind of developable image due to the meta-stability of single electron excitations Thus, small amounts of light cause little darkening of the film When enough light is incident, the film starts to develop (the Toe), then it responds more rapidly and the approximately Straight Line part of the curve emerges IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.3 FILM & SCREEN-FILM SYSTEMS 7.3.4 Grayscale Characteristics of Film Images H & D Curve (E, Z) T ]Hurter & Driffield Curve) The Gradient or Gamma of the curve (i.e. its slope)

actually varies continuously with optical density Once there is sufficient light to develop most of the grains, then saturation begins, giving rise to the Shoulder of the curve, i.e. a flattening at high exposure IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.3 FILM & SCREEN-FILM SYSTEMS 7.3.4 Grayscale Characteristics of Film Images H & D Curve (E, Z) T ]Hurter & Driffield Curve) The Characteristic Curve can be modified in many ways

The film designer can adjust the Slope and/or Sensitivity of the curve allowing adaptation to different applications by changing: the Grains Size distribution grain Loading of the film Development process IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.3 FILM & SCREEN-FILM SYSTEMS 7.3.4 Grayscale Characteristics of Film Images H & D Curve (E, Z) T ]Hurter & Driffield Curve) The Latitude of a film is the range of exposures for which a sensible response of OD occurs

In practice it is measured between the end of the Toe and the beginning of the Shoulder It is desirable that the Gamma be as large as possible to show low contrast objects while simultaneously maximizing the latitude Satisfying these conflicting requirements is the Art of producing a satisfactory film design IAEA Typical values of Gamma for radiography are in the range of 2-3 Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.3 FILM & SCREEN-FILM SYSTEMS 7.3.4 Grayscale Characteristics of Film Images

H & D Curve (E, Z) T ]Hurter & Driffield Curve) When X rays are used to irradiate film directly, an X ray interacting with a silver grain in the emulsion can deposit sufficient energy to create a primary electron Which will in turn deposit its energy in the immediate neighbourhood This will potentially create enough electrons within each of the 10-100 grains near to the primary interaction to sensitise each to the point that it will be developable IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.3 FILM & SCREEN-FILM SYSTEMS 7.3.4 Grayscale Characteristics of Film Images

H & D Curve (E, Z) T ]Hurter & Driffield Curve) Thus, the optical H&D response is bypassed and the initial response to X ray absorption is Linear in exposure In addition there is little risk of Image Fading or Reciprocity Failure which accounts for its usefulness as an imaging radiation dosimeter IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.3 FILM & SCREEN-FILM SYSTEMS 7.3.5 Reciprocity An image receptor which produces the same response for a given exposure independent of the exposure time is said to exhibit Reciprocity.

Film has remarkably good reciprocity in the range of exposures normally used in photographic cameras, i.e. from ~0.001-0.1 s This also covers most exposure times encountered in medical radiography However for the very long exposure times of 2-5 s used in conventional film Tomography and Mammography, reciprocity failure can be important IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.3 FILM & SCREEN-FILM SYSTEMS 7.3.5 Reciprocity The reason for reciprocity failure lies in the photographic process - the photographic grain is designed not to be

sensitised unless sufficient light falls on it in a short time Although this saves it from fogging in the dark, the by-product is reciprocity failure at long exposure times Specifically this means that a long exposure will need a longer time than extrapolation by reciprocity from shorter exposure would indicate This can be of the order of 30-40% increase in time for 2-5 s exposures IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.3 FILM & SCREEN-FILM SYSTEMS 7.3.6 Screen-Film Imaging Characteristics There is a dependence of Speed (i.e. relationship between

darkening of film to radiation) and Beam Quality for screenfilm systems which is usually compensated for by the Phototimer having knowledge of the generator settings Thus it is not usually evident on a well calibrated system For example for a Gd2O2S screen with a K-edge ~50 keV and absorption of ~40% at its Sweet Spot of 80 kV and usual filtration, the speed is maximized IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.3 FILM & SCREEN-FILM SYSTEMS 7.3.6 Screen-Film Imaging Characteristics Speed drops somewhat with increased kV but more significantly at lower kV Similar results are found for most screen-film combinations,

with the Sweet Spot depending on the K-edge of the primary X ray absorber(s) in the screen IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.3 FILM & SCREEN-FILM SYSTEMS 7.3.6 Screen-Film Imaging Characteristics Key factors in the design of a screen-film cassette are: Excellent contact between the screen and the film to prevent blurring or loss of light The front surface of the cassette must be easily penetrated by X rays and not cause scatter Often at the back of the cassette is a Lead Layer to control X ray Backscatter from external objects whose highly blurred image could otherwise be superimposed

Film requires 3-10 photons per grain before a developable grain can be created corresponding to an effective DQE for light of a few percent IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.3 FILM & SCREEN-FILM SYSTEMS 7.3.6 Screen-Film Imaging Characteristics Fortunately, the gain of the screen in terms of the light photons released per complete absorption of an X ray photon is of the order of 800 (for Mammography ~20 keV) and 2,000 (for diagnostic energies ~50 keV) Thus the Effective quantum gain of X rays to developable grains in the film is 800-2000 x 1-2% or

8-16 grains/X ray for Mammography and 20-40 grains/X ray for General Radiography IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.3 FILM & SCREEN-FILM SYSTEMS 7.3.6 Screen-Film Imaging Characteristics This is the information on which the Quantum Accounting Diagram for Screen-Film was established IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students

7.3 FILM & SCREEN-FILM SYSTEMS 7.3.6 Screen-Film Imaging Characteristics Each X ray interacting with the screen creates many light photons but because of the small size of the film grains (<1 m) compared to the screen blurring (~100-500 m) usually only one or at most a few light photons from the same X ray will interact with each individual grain IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.3 FILM & SCREEN-FILM SYSTEMS 7.3.6 Screen-Film Imaging Characteristics

Thus there will be essentially no correlation between individual X ray events and individual grains resulting in the same shape of the response curve for the Screen-Film system when irradiated by X rays as for the Film when irradiated by light i.e. the optical H&D curve as measured with a Sensitometer colour matched to the emission of the phosphor This is a key point in validating Film Sensitometry for Radiography IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.3 FILM & SCREEN-FILM SYSTEMS 7.3.6 Screen-Film Imaging Characteristics The variation of the probability of X ray interaction with

depth in the phosphor screen is exponential so that the number of interacting quanta and the amount of light created will be proportionally greater near the X ray entrance surface The highest-resolution screen-film systems are therefore generally configured from a single screen placed such that the X rays pass through the film before impinging on the phosphor IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.3 FILM & SCREEN-FILM SYSTEMS 7.3.6 Screen-Film Imaging Characteristics This Back Screen configuration improves the spatial resolution of the final image compared to the alternative Front Screen configuration

It can be noted that due to the thickness (~0.7 mm) of the standard glass substrate currently used for active matrix fabrication and its consequent significant absorption of X rays Flat-Panel systems are all configured in the Front Screen orientation IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.3 FILM & SCREEN-FILM SYSTEMS 7.3.6 Screen-Film Imaging Characteristics However, for all but the highest resolution requirements dualscreens are used as they can make a better trade-off between high Quantum Efficiency and high Resolution Screen Nomenclature always stresses the positive:

A high resolution and low AQ screen is referred to as a High Resolution screen a low resolution, as a High AQ IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.3 FILM & SCREEN-FILM SYSTEMS 7.3.6 Screen-Film Imaging Characteristics MTF, NPS & DQE of Screen-Film Systems The MTF of Screen-Film primarily depends on the need of the application It can be excellent especially as measured by the single criterion of Limiting Resolution usually defined at the frequency f for which the MTF(E, Z) T ]f) drops to 4%

The variation of MTF with screen thickness/loading is very significant IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.3 FILM & SCREEN-FILM SYSTEMS 7.3.6 Screen-Film Imaging Characteristics MTF, NPS & DQE of Screen-Film Systems The variation of MTF with spatial frequency (f) for two double screen systems for different applications: Lanex Fine for high resolution - bone - and Lanex Regular for general

radiography and hence different screen thicknesses and optical design IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.3 FILM & SCREEN-FILM SYSTEMS 7.3.6 Screen-Film Imaging Characteristics MTF, NPS & DQE of Screen-Film Systems Only recently have Digital systems been able to approach their capabilities It was controversial for many years as to whether or not high frequency information available only in Screen-Films was necessary in Mammography

IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.3 FILM & SCREEN-FILM SYSTEMS 7.3.6 Screen-Film Imaging Characteristics MTF, NPS & DQE of Screen-Film Systems The answer to this question is obtained by looking at the NPS for Screen-Film which demonstrates that the noise at high spatial frequency reaches an asymptote where the noise is white and quantum noise is negligible due to the negligible MTF of the

screens at these frequencies IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.3 FILM & SCREEN-FILM SYSTEMS 7.3.6 Screen-Film Imaging Characteristics MTF, NPS & DQE of Screen-Film Systems This is seen to be about a factor 20 times lower than the noise power extrapolated to zero spatial frequency This result could have been predicted from the Quantum Accounting Diagram which shows that the Secondary Quantum Sink is 20 silver grains per absorbed X ray IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students

7.3 FILM & SCREEN-FILM SYSTEMS 7.3.6 Screen-Film Imaging Characteristics MTF, NPS & DQE of Screen-Film Systems The dire consequence is that the DQE shows an extremely rapid drop off with f which results in the merging of the standard and high resolution screens above 2.5 mm-1 to an essentially negligible value despite the high resolution screen showing a noticeable MTF even at 10 mm-1 IAEA

Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.1 Digital Imaging Systems In a Digital imaging system, at some stage, the incident X ray image must be sampled both in the Spatial and Intensity dimensions In the Spatial Dimension, samples are obtained as averages of the intensity over receptor elements or dels These are usually square, and spaced at equal intervals throughout the plane of the receptor The Pixel is the corresponding elemental region of the image IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students

7.4 DIGITAL RECEPTORS 7.4.1 Digital Imaging Systems In the Intensity Dimension, the signal is digitized into one of a finite number of levels which are expressed in binary notation as bits To avoid degradation of image quality, it is essential that the Del Size and the Bit Depth n (when n is given by 2n) are appropriate for the requirements of the imaging task The Matrix Size or the coverage of the array is different depending on the application and the size of the body part to be imaged and the magnification IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS

7.4.1 Digital Imaging Systems The fractional area of the del which is active has an upper limit of unity but can often be smaller than that due to a reduced Fill Factor The linear dimension of the active portion of each del defines an Aperture The aperture determines the Spatial Frequency Response of the receptor IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.1 Digital Imaging Systems If the aperture is square with dimension, d, then the MTF of the receptor will be proportional to

|sinc|(E, Z) T ]fd) times the intrinsic MTF of the equivalent analog receptor where f is the spatial frequency along the x or y directions the MTF will have its first zero at the frequency f = 1/ d expressed in the plane of the receptor IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.1 Digital Imaging Systems For example

A receptor with d = 200 m will have an MTF with its first zero at f = 5 cycles/mm Also of considerable importance is the sampling interval, p, of the receptor, i.e. the Pitch in the receptor plane between corresponding points on adjacent dels The Sampling Theorem states that only frequencies f in the object less than <1/2p (the Nyquist frequency, fN) can be faithfully imaged IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.1 Digital Imaging Systems Thus if the pattern contains higher frequencies, then a

phenomenon known as Aliasing occurs wherein the frequency spectrum of the image beyond the fN is: mirrored or folded about the fN in accordion fashion and added to the spectrum of lower frequencies, increasing the apparent spectral content of the image below fN It is important to realize that both signal and noise can show aliasing effects IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.1 Digital Imaging Systems In a receptor composed of discrete elements, the smallest possible sampling interval in a single image acquisition is

p=d Even in this most favourable case, fN =1/2d while the aperture response only falls to zero at 2fN If the Del Dimension is less than the Sampling Interval, then the zero is at >2fN, further increasing the aliasing IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.1 Digital Imaging Systems Aliasing effect on the image of a bar pattern: Top: original Centre: pixel pitch sufficiently small, all bar patterns are

resolved correctly despite some blurring Bottom: pixel pitch too large to resolve the finest bar pattern, aliasing occurs IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.1 Digital Imaging Systems Aliasing can be avoided by Band Limiting the image before sampling i.e. attenuating the higher frequencies such that there is no appreciable image content beyond fN This may be accomplished by other blurring effects intrinsic to the receptor

These mechanisms can have different effects on noise and signal and may not necessarily reduce noise and signal aliasing in the same manner IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.2 Computed Radiography (CR) In CR the imaging plate or IP (a screen made using a Photostimulable Phosphor) is: Positioned in a light tight cassette Exposed to a patient X ray image and then Produces a digital image in a system which Extracts the exposed plate from the cassette while

protecting it from ambient light Reads it out Erases the image and Returns it to the user in the cassette ready to be reused IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.2 Computed Radiography (CR) CR system based on the use of reusable Photostimulable Phosphor plates housed in cassettes Readout of plate in Laser Scanner with photostimulated

light collected in Light Guide and detected by a PMT IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.2 Computed Radiography (CR) CR belongs to a class of systems which could be called Reusable Plate Technologies and directly replace Screen-Film There are currently no competing technologies for the reusable plate despite the fact that

the Image Quality of CR is poorer than DR it requires a larger exposure to produce acceptable images IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.2 Computed Radiography (CR) Method of Latent Image Formation The Photostimulable Phosphor screen in the IP is very similar to a conventional X ray screen except it uses a phosphor that contains traps for excited electrons Most photostimulable phosphors are in the Barium Fluorohalide family typically BaFBr:Eu

X ray absorption mechanisms are identical to those of conventional phosphors IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.2 Computed Radiography (CR) Method of Latent Image Formation The photostimulable phosphors differ in that the useful optical signal is not derived from the light emitted in prompt response to the incident radiation as in conventional screen-film systems but rather from subsequent Stimulated Light Emission when EHPs are released from traps

The initial X ray interaction with the phosphor crystal causes EHPs to be generated IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.2 Computed Radiography (CR) Method of Latent Image Formation Some of these electrons produce blue/green light in the phosphor in the normal manner but this is not used for imaging Instead the phosphor is intentionally designed to contain metastable EHP traps that store a latent image as a spatial distribution of trapped electrons and holes By stimulating the phosphor by irradiation with red light, these EHPs are

released from the traps and are free to move in the valence band (holes) and conduction band (electrons) IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.2 Computed Radiography (CR) Method of Latent Image Formation These mobile EHPs subsequently trigger the emission of shorter wavelength (blue) light CR screens (also called Imaging Plates) are exposed in a cassette with only one screen This reduces the Absorption Efficiency compared to a dual

screen combination and this is an intrinsic disadvantage of CR compared to Screen-Film imaging IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.2 Computed Radiography (CR) Image Readout The Readout system for photostimulable phosphor plates uses a red laser beam Flying Spot scanning system to stimulate the screen on a point-by-point basis exciting Photostimulated light

(usually blue) from the screen in proportion to the previous X ray irradiation of that point In practice, depending on the laser intensity, readout of a photostimulable phosphor plate yields only a fraction of the stored signal IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.2 Computed Radiography (CR) Image Readout The blue light is collected by a Light Guide that is a critically important component in the avoidance of a Secondary

Quantum Sink i.e. the light is funnelled to a PMT that detects and amplifies the signal Note that in contrast to the situation with screen-film the scattering of the blue light does not degrade the image resolution However the scattering of red light on its way to the photo centres does IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.2 Computed Radiography (CR) Image Readout

The Primary Source of light blurring in CR: the spreading by scattering of the incident exciting light gives rise to the blurring the light that comes out of the phosphor grains is irrelevant and does not contribute to resolution loss Finally, the image plate is then flooded with light to erase any residual image and is then ready for reuse IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.2 Computed Radiography (CR) Image Readout

The advantages of Photostimulable Phosphor systems are that they are digital systems with a very high Dynamic Range IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.2 Computed Radiography (CR) Image Readout The Quantum Accounting Diagram for scanned readout of the flying spot laser readout of CR shows that the incident absorbed X ray although originally

represented by multiple quanta is at a critical stage in the chain represented by only ~5 electrons in the PMT IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.2 Computed Radiography (CR) Image Readout This is not adequate to keep the additional noise from the system negligible when the relatively poor MTF is also factored in which can be seen in the NPS for CR:

IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.2 Computed Radiography (CR) Image Properties The underlying physical phenomenon of the Photoluminescence process is expected to be linear and this is demonstrated over the exposure range relevant for medical imaging The variation of the signal with exposure is essentially linear over the four orders of magnitude relevant for

diagnostic radiography IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.2 Computed Radiography (CR) Image Properties In practice, the Photomultiplier signal in most CR readers is processed logarithmically before being output It appears at first sight that the Characteristic Curve for CR is radically different from all other imaging systems This is due to the fact that images using both film-screen and CR are designed for radiography and it is conventional to use negative images (sometimes called White Bone) IAEA

Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.2 Computed Radiography (CR) Image Properties The conventional mode of displaying the H&D curve hides this Depending on the application and the plate type used the relationship is different but consistent between settings Various Latitude and Speed points can be set to mimic the behaviour of screen-film systems IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students

7.4 DIGITAL RECEPTORS 7.4.2 Computed Radiography (CR) Image Properties The MTF is shown for high resolution and normal resolution IPs: It can be seen that they are comparable to those for corresponding screen-film systems This is not surprising as they are based on similar concepts for screens, and manufacturers can make any resolution simply by changing the thickness IAEA

The real test of equivalence will only come when we compare DQE Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.2 Computed Radiography (CR) Image Properties The overall image quality for CR can be evaluated from the DQE for a normal resolution plate It is however interesting to see how closely the screen-film systems match the results for CR This is related to the high level of secondary quantum noise evident in the NPS of CR, worse in fact

than for screen-film, but not unexpected when the Quantum Accounting Diagrams for the two modalities are compared: that both have Secondary Quantum Sinks in the range 5-20 quanta per X ray IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.2 Computed Radiography (CR) Image Properties All screens are made in similar ways and it is impossible to make them completely uniform in properties despite best

efforts The image from an entirely uniform exposure will therefore contain as well as Quantum Noise, noise due to the Non-Uniformity and Granularity of the underlying structure IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.2 Computed Radiography (CR) Image Properties How does this manifest itself in images? Suppose there was a 1% variation in effective thickness

of the plate then all images would reflect that change giving an effective signal-to-noise ratio of 1% On the other hand, the SNR due to Quantum Mottle decreases as the square root of exposure Thus, the apparently surprising conclusion that structural noise will be most important when the exposure is high IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.2 Computed Radiography (CR) Image Properties The same effect will be present in Screen-Film systems, although here variations in screen uniformity and film

granularity will both be involved For Screen-Film systems it is impossible to correct for structure noise, it could perhaps be accomplished for CR if efforts were made to ensure that the IP was always positioned exactly within the reader However, this is not done and is probably unnecessary as the loss of DQE due to structural noise occurs at relatively high exposure levels, generally outside the clinically important regions of the image IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.3 Digital Radiography - DR The key digital technology permitting an advance in medical X ray applications is the

Flat-Panel Active Matrix Array originally developed for laptop computer displays The unique technology underlying active matrix flat-panel displays is large area integrated circuits called Active Matrix Arrays because they include an Active switching device - the Thin Film Transistor (TFT) IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.3 Digital Radiography - DR Active matrix arrays are an example of an important class of readout methods which are called Self Scanned A self-scanned readout structure may be defined as one in which the image created in a certain plane is readout in that

same plane The advantage of such structures is that they are thin in the third dimension Active matrix arrays use hydrogenated amorphous silicon as the semiconductors deposited on a thin (~0.7 mm) glass substrate which facilitate large area manufacture IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.3 Digital Radiography - DR The current scale on which the Substrate is made exceeds several metres on a side, thus permitting several sensors to be made on a single substrate which facilitates mass production of monolithic devices Although long promised, it is not yet feasible to make all the

readout components on the glass substrate Gate drivers, readout amplifiers and ADCs for example are still made separately and bonded to the substrate A complete imaging system on glass remains elusive IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.3 Digital Radiography - DR Another large class of self scanned receptors: Charge Coupled Devices (CCDs) and CMOS sensors Although used extensively in optical systems incorporating

demagnification such as those for XIIs they need to be made from single crystal silicon IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.3 Digital Radiography - DR Active matrix technology allows the deposition of semiconductors across large area substrates in a well-controlled fashion such that the physical and electrical properties of the resulting structures can be modified and adapted for many different applications Coupling traditional X ray detection materials such as Phosphors or Photoconductors with a large-area active matrix readout structure forms the basis of flat-panel X ray imagers

IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.3 Digital Radiography - DR Large area active matrix array concept which is applicable for both (a) Direct conversion (using a photoconductor layer) and (b) Indirect conversion systems (using a phosphor layer) depending on whether a pixel electrode or photodiode is used on the active matrix array In both cases thin film transistors (TFTs) are made using the semiconductor hydrogenated amorphous silicon (a-Si:H) IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students

7.4 DIGITAL RECEPTORS 7.4.3 Digital Radiography - DR All the switches along a particular row are connected together with a single control line (Gate Line) This allows the external circuitry to change the state of all the switching elements along the row with a single controlling voltage Each row of pixels requires a separate switching Control Line The signal outputs of the pixels down a particular column are connected to a single Data Line with its own readout amplifier IAEA This configuration allows the imager to be readout one horizontal line at a time

Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.3 Digital Radiography - DR Unlike the Charge Transfer Readout method used in many modern CCDs active matrix arrays do not transfer signal from pixel to neighbouring pixel but from the pixel element directly to the readout amplifier via the Data Line This is similar to CMOS imaging devices IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS

7.4.3 Digital Radiography - DR A distinction is made between flat-panel X ray imaging devices that: incorporate a photoconductor to produce electrical charges on detection of an X ray (Direct Conversion) those that use a phosphor to produce visible wavelength photons on detection of an X ray (Indirect Conversion) IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.3 Digital Radiography - DR

In the Direct Conversion approach, a photoconductor is directly evaporated onto an active matrix array The charge released in the bulk of the photoconductor is collected by a large applied field, which brings the electrons and holes to their respective Electrodes Those reaching the upper continuously biased electrode are neutralized by the power supply providing the bias, effectively replenishing the loss of field which would otherwise be caused IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS

7.4.3 Digital Radiography - DR The Charge reaching the readout pixel electrode is stored temporarily on the Capacitance of the pixel until readout The Magnitude of the signal charge from the different pixels contains the imaging information inherent in the intensity variations of the incident X ray beam IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.3 Digital Radiography - DR In the Indirect Conversion approach, a phosphor layer (e.g. a structured scintillator such as CsI:Tl) is placed in intimate contact with an active matrix array

The intensity of the light emitted from a particular location of the phosphor is a measure of the intensity of the X ray beam incident on the surface of the receptor at that point Each pixel on the active matrix has a photosensitive element that generates an electrical charge whose magnitude is proportional to the light intensity emitted from the phosphor in the region close to the pixel This charge is stored in the pixel until the active matrix array is read out IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS

7.4.3 Digital Radiography - DR There are in principle two advantages of direct conversion compared to indirect: 1) the fewer number of Conversion Stages make it possible to have a significantly higher conversion efficiency of X ray energy to EHPs on the active matrix ~8,000 direct cf 1,000-2,000 indirect 2) Much higher resolution due to the elimination of blurring during the charge (direct) or photon (indirect) Collection Phase IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.3 Digital Radiography - DR

Direct Conversion Proposed photoconductors include Hg2I, CdTe and PbO However, there is only one currently practical photoconductor, a-Se Its Conversion Efficiency is in fact comparable to phosphors, and technical issues limit its thickness so it is used mostly in Mammography where its low Z) T ] is in fact ideally matched to the absorption requirements and high resolution is attained IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.3 Digital Radiography - DR

Thus at the current time the dominant approaches to digital General Radiography use the indirect conversion approach due to the higher specific absorption of available phosphors compared to a-Se In Mammography however, a-Se is superior due to the need for a low energy X ray spectrum increasing the quantum efficiency beyond what is possible with a phosphor and because it simultaneously increases resolution IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.3 Digital Radiography - DR Indirect Conversion

One of the main issues with the design of Powdered Phosphor screens is the balance between spatial resolution and X ray detection efficiency As the phosphor is made thicker to absorb more X rays, the emitted light can spread further from the point of production before exiting the screen This conflict is significantly eased by the use of a Structured Phosphor such as CsI When evaporated under the correct conditions, a layer of CsI will condense in the form of needle-like, closely packed crystallites IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students

7.4 DIGITAL RECEPTORS 7.4.3 Digital Radiography - DR In this form the resolution is better than for a powder phosphor screen However, resolution may be further enhanced by fracturing into thin pillar-like structures by exposure to a thermal shock This has the disadvantage of reducing the effective density of the structured layer to ~80-90% that of a single CsI crystal IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS

7.4.3 Digital Radiography - DR The hope was that these columns would act like fibre optic light guides due to the difference in refractive index n between: CsI (n = 1.78) and the Inert gas or air (n~1) which fills the gaps between the pillars Taking this model literally, light photons produced by the absorption of an incident X ray will be guided towards either end of the pillar if they are emitted within the range of angles that satisfy conditions for Total Internal Reflection IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS

7.4.3 Digital Radiography - DR Theoretical calculations predict that ~83% of the isotropically emitted light will undergo internal reflection within a perfectly uniform pillar The other ~17% will scatter between pillars and cause a reduction in the spatial resolution IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.3 Digital Radiography - DR Actual layers of CsI have a somewhat reduced light collimating capability due to the:

Unavoidable non-uniformity of the surface of the pillars Unavoidable contacts between adjacent pillars at various points within the layer and Defects in the cracking In addition, in practice the layers form with an initial layer which is not columnar and the columns develop beyond a certain thickness (~50 m) IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.3 Digital Radiography - DR However, they maintain significantly higher resolutions for a given thickness of phosphor than powder screens As a Rule of Thumb they seem to have twice the resolution

of a powder phosphor screen of the same physical thickness and optical design e.g. presence or absence of reflective backing This corresponds to 3-4 times the Mass Loading for the structured phosphor layer when packing density and atomic number are accounted for IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.3 Digital Radiography - DR To increase the light collection capabilities of the layer, a Reflective Backing can also be added to the X ray entrance surface of the CsI to: redirect the light photons emitted in this direction back towards

the exit surface This significantly increases the light output of the layer but at the cost of a reduced spatial resolution IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.3 Digital Radiography - DR MTF, NPS & DQE of DR Systems The MTFs of the direct and indirect systems show a distinct qualitative difference The overall MTF of each system is the product of the MTF of the X ray detection medium and the del aperture function which for a uniform response is the sinc function The intrinsic spatial resolution of a photoconductor is extremely

high, which means that the system MTF for a direct conversion receptor will be to all intents and purposes the sinc function as all other components are close to unity IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.3 Digital Radiography - DR MTF, NPS & DQE of DR Systems In a system that utilizes a phosphor as the X ray detection medium, the spatial response of the phosphor is much poorer than the sinc function The MTF of the combination will then be practically equivalent to the phosphor MTF

In other words for the a-Se system it is the Aperture Function which defines the Presampling MTF whereas for phosphor based system (even columnar CsI) the phosphor blurring dominates and defines the overall MTF IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.3 Digital Radiography - DR MTF, NPS & DQE of DR Systems Furthermore, for the parameters usually chosen in practical medical systems the MTF at the fN is: ~60% for the Direct system and closer to 10% for the Indirect system

This implies that noise aliasing will be severe for the direct and almost negligible for the indirect detection approach IAEA in general Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.3 Digital Radiography - DR MTF, NPS & DQE of DR Systems The degree of Aliasing permitted by the system designer can be established from an evaluation of the pre-sampled MTF and a calculation of the area above fN compared to the area below Examples of MTFs for wellmatched systems using 200

m pixels and 200 m sampling pitch for both Direct and Indirect conversion systems at the same incident energy IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.3 Digital Radiography - DR MTF, NPS & DQE of DR Systems The NPS of Direct and Indirect receptors also demonstrate striking differences The NPS of the Direct receptor, due to its minimal

spatial filtration prior to sampling, combined with aliasing of the frequencies above the fN is almost white (i.e. is independent of spatial frequency) IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.3 Digital Radiography - DR MTF, NPS & DQE of DR Systems In contrast the Indirect receptor shows a marked drop in NPS with increasing frequency, due to the greater presampling blurring inherent in the phosphor layer demonstrated by

comparing the MTFs The NPS for higher frequencies show a Linear Exposure Dependence but the NPS does not go to zero at zero exposure - this is the Electronic Noise which at the pixel level is of the order of several thousand electrons rms This noise is to be compared with the signal per X ray which is of the order of 1,000 IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.3 Digital Radiography - DR MTF, NPS & DQE of DR Systems Finally by combining MTF and NPS the DQE(f) can be obtained:

In the particular Direct conversion receptor illustrated the DQE(0) is somewhat smaller due to the relatively poor X ray absorption efficiency of the photoconductor whereas DQE drops very little with spatial frequency due to the very high MTF (which is close to the ideal sinc function) IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.3 Digital Radiography - DR MTF, NPS & DQE of DR Systems

In contrast the DQE(0) of the Indirect conversion system is higher due to better X ray Absorption Efficiency but the DQE drops more rapidly with increasing spatial frequency due to the poorer MTF which is mitigated to some degree by the reduction of NPS with frequency due to the drop in MTF IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.4 Other Systems Optically Coupled Systems Indirect conversion flat panel systems use Optical Coupling of an imaging screen to an active matrix array

Earlier systems used smaller receptors arrays in conjunction with a fibre optic or a lens to couple the image to other optical devices, such as a CCD or a video camera Lens Coupling Fibre Optic Coupling IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students

7.4 DIGITAL RECEPTORS 7.4.4 Other Systems Optically Coupled Systems However, there is then a considerable loss of light depending on the Collection Angle of the optical system compared to the broad angle over which light is emitted from screens These methods therefore have significant problems in maintaining good noise properties This is because the Collection Efficiency of the light from the screen by the imaging device is generally rather poor IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS

7.4.4 Other Systems Optically Coupled Systems For example, even in the best optically coupled lens system with a pair of Relay Lenses of the same focal length placed back to back, the coupling efficiency cannot practically exceed 20% and this only with very high aperture lenses (f/0.75) This is the case for 1:1 imaging With demagnification M greater than unity, the coupling efficiency drops by M2 M: ratio of the side of the screen to the corresponding image sensor dimension IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students

7.4 DIGITAL RECEPTORS 7.4.4 Other Systems Optically Coupled Systems Thus for commonly seen demagnifications of 20 the coupling efficiency is 0.05% Under these conditions only ~1 light photon on average represents the interaction of each X ray This is a serious Secondary Quantum Sink IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.4 Other Systems

Optically Coupled Systems This may manifest itself in two general ways: 1. The inability to fully represent the position of the X ray resulting in a decrease in both DQE(E, Z) T ]0) and a much more rapid decrease in DQE with f and 2. Due to the very small gain resulting in amplifier noise becoming dominant at much higher exposure levels than it would otherwise IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.4 Other Systems Optically Coupled Systems

In the presence of these kinds of Secondary Quantum Sinks the transfer of light from the screen to the receptor becomes the limiting stage in the imaging chain and significantly degrades the overall performance of the system The use of a Fibre Optic Taper can alleviate, but not eliminate the loss due to demagnification for the reason that the acceptance angle for light decreases in the same way as for lenses IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.4 Other Systems

Optically Coupled Systems Thus a major technical advantage for the flat panel receptor (in an Indirect detection configuration) is that it can be placed in Direct contact with the emission surface of the screen Its Collection Efficiency for the emitted light is consequently much higher (~50% and approaches 100% for special configurations) than with the demagnifying approaches IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.4 Other Systems

Optically Coupled Systems It is interesting to compare the situation for an XRII: Electron optical demagnification in an XRII IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.4 Other Systems Optically Coupled Systems Here

by converting light photons from the Phosphor to electrons in the Photocathode in direct contact with the phosphor and the ability to bend the path of electrons, which is impossible for light is critical in maintaining much higher Collection Efficiency and so avoiding a Secondary Quantum Sink This is a critical reason why XRIIs were important for so long despite their other problems IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.4 Other Systems Photon Counting

Photon Counting receptors and their close relatives Photon Energy Discriminating receptors interact with incident photons one by one and report back to the system that: a photon has been detected (a Count) or a photon within a specific energy range has been detected (Energy Discriminated Count) The potential advantage in image quality of these systems is significant IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.4 Other Systems

Photon Counting There are two reasons: The First is that they can entirely eliminate the effect of amplifier noise This is because the signal from a single X ray emerges in a very short time and the signal can thus be made to be >5 times the noise This advantage makes it possible to work at very low exposure rates where other systems would be dominated by amplifier noise IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.4 Other Systems

Photon Counting The Second reason is that by knowing the size of the signal, the energy of the incident x ray may be estimated, and thus correction for Swank noise performed And, in the context of the complete system, better weighting of the importance of: High Energy (highly penetrating, low contrast) versus Lower Energy (less penetrating, higher contrast) X rays can be accomplished IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.4 Other Systems

Photon Counting This can perhaps increase the SNR by factors of ~2 Unfortunately these improvements pale in comparison to the increase of complexity of the circuitry necessary at each pixel, which may generally be of the order of 1,000-100,000 fold This practical factor has to this point limited the application to Mammography but improvements in microelectronic circuitry are reaching the point where more general application may be feasible in the near future IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS

7.4.4 Other Systems Scanning Geometries What X ray geometry should be used? is the first fundamental decision facing the designer of an X ray imaging system Conventionally, producing an X ray image involves exposing the entire area of interest simultaneously and detecting it with an area sensor as in screen-film radiography IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.4 Other Systems

Scanning Geometries Other approaches The simplest is to obtain a Pencil Beam of radiation and scanning it over the patient, one point at a time pencil beam accomplished by collimating the broad area flux from the XRT by means of a lead blocker with a small hole in it A single sensor aligned with the pencil beam creates an image of the patient IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS

7.4.4 Other Systems Scanning Geometries Other variations between a pencil and area beams: IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.4 Other Systems Other approaches Scanning Geometries Slit Irradiation is obtained with a fan beam of radiation and an aligned single line receptor which is scanned

perpendicularly to the line across the patient Both pencil beam and slit beam scanning are extremely inefficient in the utilization of X rays Most of the X rays are removed by the collimator and a full scan imposes an enormous Heat Load on the tube IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.4 Other Systems Scanning Geometries Other approaches It is possible to improve the efficiency of such systems by employing a multi-line or

Slot Receptor where the X ray beam extends across the full image field in one dimension and is several lines wide in the other IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.4 Other Systems Other approaches Scanning Geometries There are two types of Slot receptors: 1. A slot is moved discontinuously across the width, a

single exposure made and the multiple lines readout this process is repeated until the entire area is covered 2. In Time Domain Integration, TDI, the slot beam receptor moves continuously and the image is read-out one line at a time IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.4 Other Systems Scanning Geometries Why use these complicated scanning methods to produce images when it appears that irradiating a static area receptor is much simpler? Several concepts must be balanced: Simplicity of Construction

Scatter Tube Loading History has shown that when Area Receptors are feasible they are preferred IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.4 Other Systems Scanning Geometries The first is relative Simplicity of Construction In an X ray scanning system to image patients there must be no significant wasted radiation, which means that very accurate pre-collimation of the X ray beam must be used

This is made more difficult by the requirements of scanning the system In the early development of digital radiographic systems it was only technically feasible to create Linear Receptor Arrays and no practical area arrays existed, thus the mechanical complexities were acceptable since there were no alternatives IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.4 Other Systems Scanning Geometries Image Quality Each method has image quality advantages and disadvantages

but the most important consideration is Scattered Radiation A reduced area receptor can, be much more efficient than area receptors in eliminating scatter IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.4 Other Systems Scanning Geometries Tube Loading The shortest exposure and the least loading of the tube

are huge strengths of the area receptor which make it the preferred option unless control of Scatter is paramount IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.4 Other Systems Scanning Geometries Existing systems which use scanning systems are highly specialized devices where Scatter Control: 1) to ensure quantitative imaging is essential For example Dual-Energy Imaging for Bone Densitometry

where the complete and exact elimination of scatter overwhelms other concerns IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.4 Other Systems Scanning Geometries 2) to permit Photon Counting approaches where the technical demands of the additional counters and discriminators needed at a per pixel basis are currently prohibitive for an area receptor but are feasible for example by using several silicon receptors in an edge-on configuration

IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.5 Artefacts of Digital Images The raw image information acquired from the current generation of flat-panel receptor systems is unsuitable for immediate image display It must be processed to remove a number of Artefacts to obtain a diagnostic quality radiograph IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS

7.4.5 Artefacts of Digital Images Moir A particularly visually disturbing effect, which is to be avoided at all costs, is Moir Fringing arising from spatial interference between the periodic structure of flat panel receptors and a Stationary Grid Moving the grid perpendicularly to the grid lines during the exposure using a Potter-Bucky grid arrangement should eliminate these problems IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.5 Artefacts of Digital Images

Ghosting & Lag Corrections for: Image carry over or Lag - effects seen in dark field exposure after prior exposure or Ghosting - effects producing change in gain related to prior images and so are seen in flood field images may sometimes be necessary IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.5 Artefacts of Digital Images Ghosting & Lag These phenomena may be particularly problematic

After large exposures to the imager or When the imager is used in Mixed Mode (i.e. receptor designed to be capable of both fluoroscopic and radiographic imaging) and the system is moved to Fluoroscopy after a large Radiographic exposure IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.6 Comparisons of Digital & Analogue Systems Advantages of Digital over Analogue Systems

for Radiography Advantages related to Image Quality and Dose: Lower dose needed Higher resolution Greater dynamic range IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.6 Comparisons of Digital & Analogue Systems Advantages related to Convenience in use: Elimination of handling and carrying of cassettes Immediate evaluation of images for image quality and

positioning Transmission of digital images Digital archiving, searching PACS Elimination of unique image (film) Image processing to more optimally present the image information to the reader Elimination of distortion and shading (c.f. XRIIs) Enabling advanced applications (e.g. digital tomosynthesis, cone beam CT, dual energy imaging and IAEA CAD) Diagnostic Radiology Physics: A Handbook for Teachers and Students 7.4 DIGITAL RECEPTORS 7.4.6 Comparisons of Digital & Analogue Systems

The image quality of radiographic detectors has experienced a quantum jump in the last decades as Flat Panel Imagers have become feasible However, there is still no completely satisfactory system, and the cost is very high compared to the systems they have replaced There is still much to be done to provide Quantum Limited performance for all radiographic imaging receptors at reasonable cost IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students Bibliography DAINTY, J.C., SHAW, R., Image Science, principles, analysis and evaluation of photographic-type imaging processes, Academic Press,

London (1974) ROWLANDS, J.A., TOPICAL REVIEW: The physics of computed radiography, Physics in Medicine and Biology 47 (2002) R123R166. ROWLANDS, J.A., YORKSTON, J., "Flat panel detectors for digital radiology", Medical Imaging Volume 1. Physics and Psychophysics, (BEUTEL, J., KUNDEL, H.L.VAN METTER, R.L., Eds), SPIE, Bellingham, (2000) 223-328 IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students

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