Contrast in MRI

Perturbation of Mo

• When you perturb Mo, it will recover back to equilibrium with time constants T1 and T2

T2 decay

TE

Signal (measure of Mx, My)

S= So e -TE/T2TI

Signal (measure of Mz)

S = So (1-e-TI/T1)

Dipole-dipole interaction

The dipole field around the proton at A alters the field experienced by the proton at B

Macromolecule

Proton exchange

Spinexchange

MolecularexchangeWater

Molecular relaxation mechanisms

Protons

Dipole coupling

Measuring T1 by inversion recovery

TI

Signal from amplitude of FID

RFpulse

Mz

Mxy

FID (T2* envelope)

S = So (1-2e-TI/T1)

Repeat for different TIs

Time

180o 90o

T1 measurement

TI

Signal from amplitude of FID

S = So (1-2e-TI/T1)

TI = 100, 250, 500, 1000, 1500, 2000, 3000, 5000

T2 (top) and T1 (bottom) maps

genu

splenium

CC_mid

IC

CSTISS

ESS

genu

splenium

CC_mid

IC

CSTISS

ESS

T1 of normal appearing white matter in MS

Vector coherence

Schering

Dephasing

Mxy at t=0

x’

Fast spins(+ve)

Slow spins( -ve) Complete

dephasing

Mxy=0

Time

T2* artefacts

Phantom with coin near it

Good(ish) shim

Bad shim

BOLD effect due to T2* effects around blood vessels

Spin echo

FID

90o

180o

T2 envelope

T2* envelope

fast

slow s

f

T= 14 mins T = 90 mins

Fate of multiphase meals

liquid

viscous

36 min 48 min 72 min

Spleen Spinal cord

L R

LiverMeal

Anatomical reference

Slow mixing of a porridge meal

Monitoring dilution

T2 for measuring brain iron and myelin

genu

splenium

CC_mid

IC

CSTISS

ESS

genu

splenium

CC_mid

IC

CSTISS

ESS

Weighted images

T1 weighting T2 weighting

TI

TE

Signal Signal

High contrast High

contrast

Low contrast Low

contrast

SMA blood flow

Elastography- internal palpation

Pre and post Gd

Contrast enhancement

Gd-DTPA molecule

Structure of Ln(III)-DTPA complexes. Oxygen atoms are red, nitrogen atoms blue, carbon atoms grey and lanthanides green.