Perfusion - ASL Flashcards
What is the effect of an off-resonance B1 pulse on the magnetisation?
The off-resonance pulse leads to a residual z-component of B0 in the rotating frame, so M rotates around Beff, not B1.
What is the adiabatic condition?
dtheta/dt «_space;gammaBeff
M follows B under this condition
What are adiabatic pulses?
small B1 applied far from resonance
B1 increased and frequency swept
This brings B1 into the transverse plane
What is the benefit of adiabatic pulse excitation?
flip angle indep. of B1, so insensititive to B1 inhomogeneites
cons
What is the magnetisation transfer effect?
bound water: short T2, KHz spectral BW
free water: long T2, Hz spectra BW
Apply off-res pulse to excite bound water
Free water exchanged with bound water
=> signal from free pool reduces
How can magnetisation transfer effect be used clinically?
image with off-res pulse - image without
signal reduced in proportion to amount of bound water
e.g. reveal areas of de-myelination in brain (less bound water)
Define the magnetisation transfer ratio
MTR= S with off-res pulse / S without
What is ASL?
Use of tagged (magnetisation inverted) arterial blood to measure perfusion to a brain voxel (CBF).
What is CASL?
Continuously excite inflowing arterial blood using local coil outside of imaging slice to achieve magnetisation inversion.
Subtract control image from tagged image
How is inversion achieved in CASL?
Flow driven adiabatic inversion.
Adiabatic inversion flow through G replaces frequency sweep.
Three main types of ASL?
CASL, PASL, pCASL
What are the four competing processes in CASL experiment?
1) flow in and blood-tissue water exchange
2) tissue-blood water exchange - flow out
3) inversion recovery
4) magnetisation transfer effect
Disadvantages of CASL?
Requires use of local Tx coil
Long labelling pulse - high SAR
Long labelling pulse - high duty cycle req.
What type of ASL is FAIR and what does it stand for?
PASL
Flow-sensitive Alternating IR
How does PASL differ from CASL?
Use standard slice-selective 180 pulse instead of adiabatic excitation with local coil.
Control uses non-selective 180 pulse.
