Sequences Flashcards
What is contrast (a general explanation) and how is it generated in MRI? (4 marks)
Contrast is differences in signal intensity in an image.
It is generated by exploiting differences in proton density, T2 and T1 relaxation times in different tissues.
Give T1 and T2 values for CSF, liver, kidnet, grey/white matter and fat.
T1 (s) T2 (ms) ---------------------------------- CSF 3.0 600 GM 1.0 100 WM 0.6 90 K 0.8 50 L 0.6 30 F 0.24 85
What causes T2 relaxation? (5 marks)
Incoherent exchange of energy between neighboring atoms. Also known as spin-spin interactions. Can be classed as spatial variations, such as nearby ferromagnetic materials or inhomos in applied B (could also be lung/air interface, or temporal variations such as blood flow or movement of fluid (e.g. brain).
What mechanisms does a SE or a GRE compensate for and hence why is the signal greater in a SE sequence? (6 marks)
GRE does not compensate for spatial or temporal. Hence why T2* is faster.
SE corrects for spatial due to flipping of the magnetisation, but cannot correct for temporal.
More signal is acquired due to the re-focussing of the signal and more time allowed to acquire it before it decays.
How can a T2WI, T1WI and proton density image be generated? Explain this with a suitable sketch and give estimates of TE and TR values for each of these. (8 marks)
Sketch the M_z and M_xy for a 90 degree pulse.
Explain the below.
T2 = long TR (3 s) and long TE (100 ms) T1 = short TR (0.6) and short TE (10 ms) PD = long TR and short TE
Explain what pre-pulses are? (2 marks)
RF pulses and gradients applied before the main imaging sequence to modify what is seen in the image, e.g. fat sat or inversion recovery.
Sketch and explain how a fat-sat pre-pulse sequence works? (4 marks)
Pre-pulse sequence with a narrow bandwidth used to selective target and dephase any fat signals, so that fat is suppressed.
Prior to the main imaging sequence, a RF pulse is applied with crusher gradients to dephase the magnetisation in the fat. Then the normal imaging sequence is applied, with the fat suppressed.
What is the purpose of T1 inversion recovery and explain how it works? Explain with reference to FLAIR imaging. (8 marks)
Sketch the pulse sequence and the Mz diagram for flair.
Apply 180 degree pulse and wait a time TI till the CSF is zero. Then apply normal sequence with the CSF essentially nulled due to zero Mz component to excite and put in M_xy phase.
Suppresses CSF while enhancing GM/WM contrast. FLAIR is a type of T1 inversion recovery.
What is the difference between T1 saturation recovery and T1 inversion recovery? (2 marks)
Saturation recovery uses a 90 degree pulse and is faster, but the suppression is not as effective.
How can faster imaging be achieved? Sketch the pulse sequence for this. (6 marks)
Sketch it out and explain it.
Acquire the whole of k-space in one pulse sequence by rapidly switch in the phase and read-out gradients, while acquiring.
