Chapter 11 Lecture Mark D. Herbst, M.D., Ph.D.

Acquistion Time and Protocol Optimization Time of scan = (# phase encoding steps xTR x NEX)/ETL

Ways to save time Reduced matrix always done in phase direction (why do it if it doesn’t save time?) No change in frequency matrix Reduces number of phase encoding steps

Ways to save time Rectangular FOV Done in phase enc direction, to match reduced matrix size to make square pixels

Ways to save time Half acquistions Half scan or half Fourier Uses the phenomenon of Hermitian Symmetry to make up the empty part of k-space with known info Equal to ½ NEX Results in decreased S/N Incorrect in book---says increases noise…not really, it decreases S

Signal Averaging Decreases noise, but increases time (NEX)

Protocol Design Usually all are optimized by manufacturer or techs that are working at that center Consider contrast sensitivity For example, uterus is shown well with T2WI, not so well with T1WI Image detail – rule of thumb is to make the slices half the size of the thing you want to see, or less.

Protocol Design Consider rectangular FOV and matrix Don’t overdo detail, since small voxels will give you decreased S/N Consider “3D” MR, which gives more signal Artifact reduction (see ch 14) Speed factor = turbo factor = ETL for Fast Spin Echo (FSE, TurboSE)