Chapter 5 Mark D. Herbst, M.D., Ph.D.

The MR Imaging Process Two major functions –Acquisition of RF signals –Reconstruction of images

k-space Where acquired signals are stored in the computer –Filled one line at a time –Must be filled completely before reconstruction –The number of lines = the resolution in the phase direction –The number of dots per line = the resolution in the frequency direction

To get a 256 x 256 image Need 256 lines in k-space Each line must have 256 dots per line Each line takes time to get, and that time is TR in conventional single echo spin echo imaging, so this would take 256 TRs.

To get a 192 x 256 image Need 192 lines in k-space Each line must have 256 dots per line Each line takes time to get, and that time is TR in conventional single echo spin echo imaging, so this would take 192 TRs.

Acquisition Usually one line of k-space for each repetition of the imaging sequence = each repetition time = each TR Fast methods produce multiple lines for each TR

Definition of TR Repetition time Time between 90 degree pulses in a spin echo technique

Imaging Protocol Various methods—spin echo, gradient echo, inversion recovery Various image types-T1-weighted images (T1WI), T2WI, T2*WI, STIR, FLAIR, MRA Spatial characteristics—slice thickness, number of slices, pixel size, voxel size Detail/noise—matrix, pixel, and voxel size Selective signal suppression Artifact reduction techniques

The Imaging Cycle Two distinct phases –Longitudinal relaxation – T1 weighting –Transverse relaxation – T2 weighting –Amount of T1 or T2 weighting depends on TR and TE –Echo event is produced by an RF pulse followed by either an RF pulse or a gradient pulse

T1 contrast

PD contrast

T2 contrast

T1, PD, and T2 contast all contribute to the final brightness of the pixel on the MR image