# Introduction to MR Robert Frost Thanks for images/animations

Introduction to MR Robert Frost Thanks for images/animations provided by Dylan MRI signal from protons Nuclei with odd number of protons/neutrons

have spin or angular momentum spin + charge magnetic moment Bar magnets have magnetic moment + H1 Lars Hanson,

B0, z direction Precession Net magnetisation M

Lars Hanson, Precession Property of something spinning (angular momentum) experiencing a force

https://people.highline.edu/iglozman/classes/astronotes/ Proton precession Frequency, 3T: ~128 MHz 7T: ~298 MHz Radio frequencies (RF)

Magnetic Resonance key points Polarisation net magnetisation M Excitation move away from z axis Precession M ar0und B0 Measure

Relaxation Excitation Apply RF pulse to rotate magnetisation away from z RF pulse has same frequency as the precession Can change duration and magnitude of RF pulse to determine flip angle, e.g. 90 degrees

z x y Precession

A changing magnetic field in x-y plane measure William Overall Measure A changing magnetic field Induces a voltage in coil of

conducting wire Measure A changing magnetic field in x-y plane Signal Relaxation

Two independent components: T2: decay of x-y component Measurable signal decays <100 ms ~ T1: regrowth of z component

Net Mz reforms ~ 1-5 sec Rotating frame If we watch the precession of M while also spinning around the z axis at same frequency then M appears still

T2 relaxation In rotating frame Spins experience inhomogeneous field (created by neighbours) and dephase

T2 relaxation Net magnetisation (x-y component only) T1 & T2 relaxation T1

T2 Contrast T2 decay of Mxy T2 values: WM = 75

ms T1 regrowth of Mz T1 values: WM = 850 ms

Inversion Recovery Excite Inversion recovery T1 contrast

Magnetic Resonance key points Polarisation net magnetisation M Excitation supply energy with RF pulse Precession M ar0und B0 Measure changing magnetic field Relaxation decay of M in x-y, regrowth of M

in z Spatial encoding Field gradients = Spatial encoding

Field gradients ()= () Magnetic Resonance Imaging Polarisation

Excitation Precession Spatial encoding Measure Relaxation Repeat, varying gradients

2D Inverse Fourier Transform

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