The AAPM/RSNA physics tutorial for residents. Contrast mechanisms in spin-echo MR imaging.

The majority of sequences used in routine clinical magnetic resonance imaging rely on the concepts involving the spin echo. Spin-echo sequences require long acquisition times (1-10 minutes), but compared with faster gradient-recalled echo methods, spin-echo methods are relatively immune to signal loss and distortions from field inhomogeneity and tissue-induced susceptibility variations. Through modifications of intersequence repetition time (TR), echo formation interval (echo time [TE]), and various gradient moments, image contrast can be altered to emphasize tissue relaxation times T1, T2, or proton density. The TR and TE values control the amount of T1 weighting and T2 weighting, respectively. At long TR intervals (approximately 10 x tissue T1 values) and minimum TE values, the difference in signal intensity arising from relaxation vanishes, and contrast arises solely from the differences in proton density between the two tissues. Images formed with short TR intervals and long TE values exhibit very low signal-to-noise ratio and negligible contrast and should be avoided. Recently, fast spin-echo sequences have partially overcome the limitation of long acquisition times, with up to 16-fold reduction, by acquiring multiple lines in k space with multiecho sequences.