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 muscle?

Answer 3

T1 - 1100ms

T2 - 50ms

Question 4

What causes the changes in T1 and T2 between tissues?

Answer 4

Different chemical compositions and physical properties.

Question 5

How is the echo time (TE) measured?

Answer 5

Time elapsed between pulse and centre of acquisition

Question 6

How is the repetition time measured?

Answer 6

Time between the same RF pulse of two adjacent sequences.

Question 7

What type of contrast comes from changing the echo time?

Answer 7

T2

Question 8

What type of contrast comes from changing the repetition time?

Answer 8

T1

Question 9

Describe briefly how a gradient echo sequence produces a signal.

Answer 9

RF pulse knocks M into the x-y plane.
Negative readout gradient causes the signal to dephase due to inhomogeneties 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 10

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

Answer 10

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

Question 11

What causes the dephasing that leads to T2 relaxation?

Answer 11

Spatial variations in magnetic field - caused by differences in the magnetic properties of the tissues.
Temporal variation in the magnetic field - caused by molecular motion and influenced by tissue composition

Question 12

Describe how a spin echo sequence produces a signal.

Answer 12

RF pulse knocks M into the x-y plane (full 90 degrees)
Phase encoding gradient applies, 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 180 degrees.
Signals recombine after the same amount of time (TE/2), causing an increase in the signal strength which is recorded.

Question 13

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

Answer 13

More signal

Some dephasing reversed

Question 14

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

Answer 14

T2

Question 15

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

Answer 15

T2*

Question 16

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

Answer 16

T2* is shorted then T2 so signal decays quicker.

Question 17

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

Answer 17

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

Question 18

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

Answer 18

Short TR, Short TE

TR < 750ms TE < 40ms

Question 19

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

Answer 19

Long TR, Short TE

TR > 1500ms, TE > 75ms

Question 20

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

Answer 20

Long TR, short TE

TR > 1500ms, TE < 40ms

Question 21

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

Answer 21

Large flip angle, short TE

a > 50, TE < 15ms

Question 22

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

Answer 22

Small flip angle, long TE

a > 40, TE > 30ms

Question 23

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

Answer 23

Small flip angle, short TE

a > 40, TE < 15ms

Question 24

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

Answer 24

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 25

How does echo-planar imaging reduce scan time?

Answer 25

Maps all of k-space in on acquisition instead of line by line

Question 26

What gives rise to in and out of phase imaging?

Answer 26

The protons in fat precess at a different frequency to those in water owing to electron shielding effects. In a gradient echo sequence, the fat and water signals are in phase immediately after excitation, but waiting an appropriate echo time (2.2ms at 1.5T, 1.1ms at 3.0T) will cause them to have opposite phase and water and fat signals will cancel. With an echo time of twice the value, the signals will be in phase and add

Question 27

What is a clinical application of in and out of phase imaging in the liver?

Answer 27

It enables us to look for either diffuse fatty liver disease – a general loss of signal in the out of phase image, or to examine liver lesions to see whether they consist of fat or not.

Question 28

What of the following features would have prominent contrast in a T2 weighted brain MRI image:
CSF
Infarct
Oedema
White matter/grey matter contrast
White matter tracts

Answer 28

CSF
Infarct
Oedema

Question 29

On a T1-weighted coronal image of the leg, will the bone marrow appear bright or dark?

What about on a T2-weighted image?

Answer 29

Bone marrow consists largely of lipid. Lipid has a shorter T1 than skeletal muscle and a longer T2 than skeletal muscle and so will be bright on both T1-weighted and T2-weighted images (unless of course they are fat-suppressed!).

Question 30

If MR image signals are collected using a spin echo:

(i) Are the temporal field variations refocused?
(ii) Are the spatial field variations refocused?
(iii) What is the relaxation mechanism of the transverse magnetisation for such a signal?

Answer 30

(i) No.
(ii) Yes.
(iii) T2 relaxation.

Question 31

If the T2* of a liver is 20ms, what percentage of the initial transverse magnetisation will there be at an echo time of 50ms?

Answer 31

exp(-TE/T2*) = exp(-50/20) = 0.082

= 8.2%

Question 32

Suggest three modifications that might be made to a pulse sequence by using prepulses.

Answer 32

Fat saturation, inversion recovery and perfusion imaging.