MRI: Sequences

Question 1

What are the T1 and T2 of Grey Matter?

Answer 1

T1 - 1000ms

T2 - 100ms

Question 2

What are the T1 and T2 of White Matter?

Answer 2

T1 - 600ms

T2 - 90ms

Question 3

What are the T1 and T2 of CSF?

Answer 3

T1 - 3000ms

T2 - 600ms

Question 4

What are the T1 and T2 of Liver?

Answer 4

T1 - 600ms

T2 - 30ms

Question 5

What are the T1 and T2 of Muscle?

Answer 5

T1 - 1100ms

T2 - 50ms

Question 6

What are the T1 and T2 of Kidney?

Answer 6

T1 - 800ms

T2 -50ms

Question 7

What causes the changes in T1 and T2 between tissues?

Answer 7

Different chemical compositions and physical properties.

Question 8

How is the echo time (TE) measured?

Answer 8

Time elapsed between pulse and centre of Acquisition

Question 9

How is the repetition time measured?

Answer 9

Time between the same RF pulse on two adjacent sequences.

Question 10

What type of contrast comes from changing the echo time?

Answer 10

T2

Question 11

What type of contrast comes from changing the repetition time?

Answer 11

T1

Question 12

Describe briefly how a gradient echo sequence produces a signal.

Answer 12

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.

Question 13

How does altering the TE affect the signal from a gradient echo image?

Answer 13

Longer TE gives less signal due to dephasing of magnetisation in the x-y plane.

Question 14

What causes the dephasing that leads to T2 relaxation?

Answer 14

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.

Question 15

Describe briefly how a spin echo sequence produces a signal.

Answer 15

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.

Question 16

What are the advantages of spin-echo imaging over gradient echo imaging?

Answer 16

More signal

Some dephasing reversed

Question 17

What is the spin-spin relaxation mechanism for a spin-echo imaging sequence?

Answer 17

T2

Question 18

What is the spin-spin relaxation mechanism for a gradient-echo imaging sequence?

Answer 18

T2*

Question 19

Why does a gradient echo sequence have less signal than a spin echo sequence?

Answer 19

T2* is shorter than T2 so signal decays quicker.

Question 20

What is the disadvantage of a spin-echo sequence over a gradient-echo sequence?

Answer 20

Minimum echo time is longer so sequences take longer to complete.

Question 21

What sort of TRs and TEs do we want for a T1-weighted scan?

Answer 21

Short TR, Short TE

TR < 750ms, TE < 40ms

Question 22

What sort of TRs and TEs do we want for a T2-weighted scan?

Answer 22

Long TR, Long TE

TR > 1500ms, TE > 75ms

Question 23

What sort of TRs and TEs do we want for a PD-weighted scan?

Answer 23

Long TR, Short TE

TR > 1500ms, TE < 40ms

Question 24

What sort of flip angle and TE do we want for a T1-weighted scan?

Answer 24

Large flip angle, Short TE

a > 50, TE < 15ms

Question 25

What sort of flip angle and TE do we want for a T2-weighted scan?

Answer 25

Small flip angle, Long TE

a < 40, TE > 30ms

Question 26

What sort of flip angle and TE do we want for a PD-weighted scan?

Answer 26

Small flip angle, Short TE

a < 40, TE < 15ms

Question 27

Name three types of imaging that use pre-pulses to alter contrast.

Answer 27

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.

Question 28

How does echo-planar imaging reduce scan times?

Answer 28

Maps all of k-space in one acquisition, instead of line by line.