Parameters Flashcards
Contrast parameters that affect SNR
TR, TE, TI, flip angle
As NSA is increased by 2, the SNR increases by…
By the square root of the change. Square root of 2 = 1.41, 40%
Scan time formula for conventional 2d acquisition
Tr x Ny (phase encodings) x NSA
Scan time formula for 3d acquisition
TR X Ny X NSA X # slices (partitions)
Scan time formula for 2d FSE Acquisition
TR X Ny X NSA/ETL
Scan time formula for EPI
TR X NSA X #shots (# shots is matrix / ETL)
Increasing TR has what affect on SNR?
Increasing
What effect does increasing TR have on artifacts?
None apparent
What effect does increasing TE have on SNR?
Reduces
What affect does increasing TE have on artifacts?
Increases susceptibility artifacts and can change chemical shift
What sequences are useful in pediatric brain exams?
Inversion sequences for heavy T1 weighting. Relatively short TR with short TE and TI to null CSF
Reducing the flip angle has the same effect on contrast as…
Increasing the TR, less T1 weighted
Increasing flip angle has what effect on SNR?
Increase up to the Ernst angle
Increasing NSA had what effect on artifacts?
Reduce some forms of motion artifacts
What affect did increasing FOV have on SNR?
Increase SNR by the square
What affect does increasing as FOV Have son artifacts?
Increased susceptibility artifact
Increased chemical shift artifact
Decreases aliasing when FIV is larger than the anatomy
Increases partial volume averaging
What effect does increasing matrix have on SNR?
Overall reduction
Increase by the square root in sampling
Decrease directly by voxel size
What effect does increasing matrix have on artifacts?
Decreases Gibbs or truncation artifact
And partial volume averaging
What effect does increasing slice thickness have on artifacts?
Increased susceptibility artifact
Increase partial volume averaging
What effect does increased ETL have on scan time?
Decreases scan time
What effect does increasing ETL have on resolution?
Decreased resolution thru increased blurring
What effect does increasing the number of slices in a 3d acquisition have on SNR and Resolution?
Increased SNR with # of slices (more sampling),
increased resolution with decrease in thickness
What effect does narrowing receiver bandwidth have on TE?
Increase,
More t2 weighting, less slices
What effect does narrowing receiver bandwidth have on artifact?
Increased chemical shift
When imaging vascular structures using TOF, what order should slices be acquired in to maximize signal?
Against the flow
