MRI: Sequences Flashcards
What are the T1 and T2 of Grey Matter?
T1 - 1000ms
T2 - 100ms
What are the T1 and T2 of White Matter?
T1 - 600ms
T2 - 90ms
What are the T1 and T2 of CSF?
T1 - 3000ms
T2 - 600ms
What are the T1 and T2 of Liver?
T1 - 600ms
T2 - 30ms
What are the T1 and T2 of Muscle?
T1 - 1100ms
T2 - 50ms
What are the T1 and T2 of Kidney?
T1 - 800ms
T2 -50ms
What causes the changes in T1 and T2 between tissues?
Different chemical compositions and physical properties.
How is the echo time (TE) measured?
Time elapsed between pulse and centre of Acquisition
How is the repetition time measured?
Time between the same RF pulse on two adjacent sequences.
What type of contrast comes from changing the echo time?
T2
What type of contrast comes from changing the repetition time?
T1
Describe briefly how a gradient echo sequence produces a signal.
RF pulse knocks M into the x-y plane (might not be complete, could be at any angle).
Negative readout gradient causes the signal to dephase due to inhomogeneities in B field.
Gradient reversed, and signals recombine after the same amount of time, causing an increase in the signal strength which is recorded.
How does altering the TE affect the signal from a gradient echo image?
Longer TE gives less signal due to dephasing of magnetisation in the x-y plane.
What causes the dephasing that leads to T2 relaxation?
Spatial variations in magnetic field - caused by differences in the magnetic properties of the tissue.
Temporal variation in the magnetic field - caused by molecular motion and influenced by tissue composition.
Describe briefly how a spin echo sequence produces a signal.
Rf pulse knocks M into the x-y plane (full 90deg)
Phase encoding gradient applied, dephasing of signal begins.
Second RF pulse (twice as long) applied half way between initial rf pulse and acquisition time (TE/2), flips signal by 180deg.
Signals recombine after the same amount of time (TE/2), causing an increase in the signal strength which is recorded.
What are the advantages of spin-echo imaging over gradient echo imaging?
More signal
Some dephasing reversed
What is the spin-spin relaxation mechanism for a spin-echo imaging sequence?
T2
What is the spin-spin relaxation mechanism for a gradient-echo imaging sequence?
T2*
Why does a gradient echo sequence have less signal than a spin echo sequence?
T2* is shorter than T2 so signal decays quicker.
What is the disadvantage of a spin-echo sequence over a gradient-echo sequence?
Minimum echo time is longer so sequences take longer to complete.
What sort of TRs and TEs do we want for a T1-weighted scan?
Short TR, Short TE
TR < 750ms, TE < 40ms
What sort of TRs and TEs do we want for a T2-weighted scan?
Long TR, Long TE
TR > 1500ms, TE > 75ms
What sort of TRs and TEs do we want for a PD-weighted scan?
Long TR, Short TE
TR > 1500ms, TE < 40ms
What sort of flip angle and TE do we want for a T1-weighted scan?
Large flip angle, Short TE
a > 50, TE < 15ms
What sort of flip angle and TE do we want for a T2-weighted scan?
Small flip angle, Long TE
a < 40, TE > 30ms
What sort of flip angle and TE do we want for a PD-weighted scan?
Small flip angle, Short TE
a < 40, TE < 15ms
Name three types of imaging that use pre-pulses to alter contrast.
Fat Saturation - removes fat from the signal with fat-specific rf pulse.
Inversion recovery - measures T1, or removes some component if inversion delay corresponds to the time required for the signal to reach the x-y plane.
Perfusion imaging - sensitive to blood flow.
How does echo-planar imaging reduce scan times?
Maps all of k-space in one acquisition, instead of line by line.
