chap 7 Flashcards
increasing the field strength increases SNR because
the NMV increases in size because the low energy population of spins increases
what maximizes resolution ?
decrease slice thickness
the frequency difference between fat & water is called
chemical shift
decreasing the rBW can increase artifacts like
chemical shift & susceptibility
increasing the rBW (wide) has what affect
decreases SNR, chemical shift, & scan time
pixel area =
FOV / matrix size
T/F: it is possible to null the signal of other tissues than fat and water
true
isotropic
symmetrical voxels & equal resolution in every plane
anisotropic
voxels are not symmetrical
increase resolution, decrease SNR
small voxel (slice thickness)
decrease resolution, increase SNR
large voxel (slice thickness)
long TR, short TE
increase SNR
short TR, long TE, lower flip angle
reduces SNR
which matrix has low resolution, high SNR, & short scan times
coarse matrix
which matrix has high resolution & low SNR
fine matrix
what parameters can affect scan time?
TR, phase matrix, NSA
doubling the NEX _______ scan time & _______ SNR ______
doubles, increases, by the sq root of 2
halving the square FOV reduces the SNR by
3/4
what occurs when the receive band width is reduced?
- SNR increases
- TE increases
- number of slices available decreases
what happens when you decrease the TR from 2500 ms to 500 ms
scan times decreases
trade off for increasing NEX
maximizes SNR, but increases scan time
decreasing the phase matrix will
makes the scan shorter , would result in a coarser matrix
trade off for increasing slice thickness
maximizes SNR, but decreases resolution
trade off for decreasing TE
maximizes SNR , but decreases T2 weighting
trade off for increasing TR
maximizes SNR, but decreases T1 weighting
trade off for decreasing TR
minimizes scan time, but increases T1 weighting, lowers SNR, & less # slices
trade off for increasing TE
increases T2 weighting, and reduces SNR
do we achieve greater SNR with a long or short TR?
long
T/F: a wider receive band with would be beneficial to SNR
false
what sequence is NOT used with gadolinium
STIR
which sequence nulls signal from fat with contrast enhanced sequences
SPIR
T/F: a large coil will include more area of received signal but this is more susceptible to artifacts
true
