MRI basics lecture 9 26

Physics of MRI, An Overview– Discrete Fourier Transform – Fourier properties – k-space representation in MRI... Frequency encoding - 1D imaging... EPI echo planar imaging... G tReadout R

Magnetic Resonance Imaging:

Physical Principles

,

Lewis Center for NeuroImaging

Physics of MRI, An Overview

– Discrete Fourier Transform

– Fourier properties

– k-space representation in MRI

Many spins in a voxel: vector summation

Rotating

frame

Lamor

precession

Refocus spin phase – echo formation

time

• Invert perturbing field: δ B - δ B

Phase 0 δ Bt φ - δ B(t-TE/2) 0

Phase 0 δ Bt - φ + δ B(t-TE/2) 0

Echo Time (TE)

(gradient echo, k-space sampling)

(spin echo)

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Spin Echo

Frequency encoding - 1D imaging

Gradient Echo FT imaging

G t

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Pulse sequence design

y Gradient

-35000 0 35000

EPI (echo planar imaging)

G t

Readout

Repeat with different phase-encoding amplitudes to fill k-space

Spin Relaxation

dephasing – T2 decay

This can be reversible, unlike T2 decay

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Bloch Equation

magnetic field B and its local environment (relaxation effects)

M T

B

M dt

1 ˆ

1

γ

/ 2 2

*

2

) 0 ( )

(

1 1

1

T t

e M

t M

T T

Free Induction Decay – Gradient echo (GRE)

– Rapid signal decay

– Acquisition must be

disabled during RF

– Don’t get central

e -t/T2*

MR signal

MR Parameters: TE and TR

echo times allow less T2 signal decay

allow the spins to recover their longitudinal magnetization, so the net magnetization available is reduced, depending on the value of T1

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Contrast, Imaging Parameters

) GRE (

e ) e

1 (

or

) SE (

e ) e

1 ( )

TE ,

TR

(

S

* 2 1

2 1

T / TE T

/ TR

T / TE T

/ TR

− ρ

Properties of Body Tissues

MRI of the Brain - Axial

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Brain - Sagittal Multislice T1

Brain - Axial Multislice T1

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Brain Tumor

Post-Gd T1

3D Imaging

Scan time = NyTR

3D Sequence (Gradient Echo)

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3D Imaging - example

•Contrast-enhanced MRA of the

carotid arteries Acquisition time

frequencies (chemicals) can be separated using Fourier transforms

problem

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Spectroscopy - Example

Intensity

Frequency

Future lectures

magnitude, pelc)

navigator…)

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Before frequency encoding and after slice selection, apply y-gradient pulse that makes spin phase

varying linearly in y.

Repeat RF excitation and detection with different gradient area.