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 muscle?
T1 - 1100ms
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 of 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.
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.
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 tissues.
Temporal variation in the magnetic field - caused by molecular motion and influenced by tissue composition
Describe how a spin echo sequence produces a signal.
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.
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 shorted then 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, Short 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 uses 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 time?
Maps all of k-space in on acquisition instead of line by line
What gives rise to in and out of phase imaging?
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
What is a clinical application of in and out of phase imaging in the liver?
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.
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
CSF
Infarct
Oedema
On a T1-weighted coronal image of the leg, will the bone marrow appear bright or dark?
What about on a T2-weighted image?
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!).
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?
(i) No.
(ii) Yes.
(iii) T2 relaxation.
If the T2* of a liver is 20ms, what percentage of the initial transverse magnetisation will there be at an echo time of 50ms?
exp(-TE/T2*) = exp(-50/20) = 0.082
= 8.2%
Suggest three modifications that might be made to a pulse sequence by using prepulses.
Fat saturation, inversion recovery and perfusion imaging.
