MRI Sequences Flashcards
What does T2 contrast depend on?
TE and tissue T2
What does T1 contrast depend on?
TR and tissue T1
What makes good MRI sequence?
Acceptable to patient (time, noise)
Good contrast
Good anatomical resolution
Information on structure, blood supply, function
Reproducible
Properties of T2 weighted scan
Spin echo (so not T2*)
Long TE (100ms), long TR (3000ms) - so not T1 weighted
Fluid decays slowly, solid decays quickly so fluid is bright. Good for edema.
Properties of T1 weighted scan
Spin echo or gradient echo
Short TE (10ms) - so not T2 weighted, short TR (450ms)
Fluid has long T1 so dark, fluid is dark. Good grey-white matter contrast.
Properties of proton density scan
TR long (3000ms), TE short (10ms)
Grey matter T1 and T2
T1 1.1s
T2 90ms
White matter T1 and T2
T1 0.56s
T2 80ms
CSF T1 and T2
T1 2.1s
T2 600ms
Liver T1 and T2
T1 0.8s
T2 26ms
Muscle T1 and T2
T1 0.9s
T2 33ms
What causes contrast in MRI?
Differences in:
T1
T2
Water content (proton density)
Blood flow
Water diffusion properties
Why are there large differences between T1 and T2 in tissues with similar water contents?
Different chemical compositions and physical properties
What can we change to change contast?
Echo time TE (to centre of ACQ) for T2
Repetition time TR for T1
Spin echo vs gradient echo for T2
Spin echo refocuses spatial field variations, relaxation mechanism is T2 not T2*
Spin echo therefore has more signal but takes longer than gradient echo
Fat saturation
Remove lipid signals from an image
Use fatsat prepulse - narrow RF pulse exciting only lipid protons, then have crusher gradients and then begin imaging
Inversion recovery
Inversion prepulse - 180 degree pulse and delay prior to imaging, inverts magnetisation and wait for recovery.
Short T1 tissues have significant recovery, long T1 has little relaxation
Example of inversion recovery
FLAIR imaging - fluid attenuated inversion recovery
Brain images with good contrast between grey and white matter and suppression of signal from CSF - inversion time chosen for zero signal from CSF.
Faster imaging
All of k-space in one acquisition
Echo planar imaging, move through k-space by increasing slice select gradient a bit each time and varying readout gradient positive and negative.
T1w and T2w in brain
T2w good for pathology - eg oedema, fluid is bright
T1w good for anatomy - good WM/GM contrast, fluid is dark
Prepulses
Additional RF and gradient pulses prior to imaging sequence, aim is to alter contrast by modulating magnetisation
Examples: fat-saturation, inversion recovery and perfusion imaging
