Multiple spin-echo MR imaging of the body: image contrast and motion-induced artifact.

For a given TR and TE, image quality changes when the number of spin echoes obtained is varied. To investigate the importance of this in clinical imaging, a total of 4 patients and 9 volunteers had MRI examinations of the abdomen (n = 7) and/or pelvis (n = 8) which included at least 2 sequences with identical TR (2000 or 2500 ms), TE (80 ms) and other parameters, but with a different series of refocusing pulses. Sequences included single-echo (S), asymmetric and symmetric double-echo (AD and SD) and quadruple-echo (Q) techniques. Image contrast and severity of motion-induced artifact was measured via blind examination by 3 independent MRI radiologists and calculation of signal-difference, signal-difference-to-noise ratios and intensity of motion-induced "ghost artifact." The order of decreasing signal differences was S, SD, AD and Q, and all of three liver lesions were better seen with S than with SD techniques. These observations are consistent with signal loss from cumulative inaccuracies from multiple 180 degrees RF pulses. The order of increasing intensity of ghost artifact was Q, SD, AD and S, consistent with the beneficial motion artifact-reducing effects of even-echo rephasing. Knowledge of these effects of multi-echo imaging allows one to make informed decisions about imaging protocols rather than to simply obtain multiple echoes "because they are free."