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