THE AAPM RSNA PHYSICS TUTORIAL FOR RESIDENTS - CONTRAST MECHANISMS INSPIN-ECHO MR-IMAGING

The majority of sequences used in routine clinical magnetic resonance imaging rely on the concepts involving the spin echo. Spin-echo sequen ces require long acquisition times (1-10 minutes), but compared with f aster gradient-recalled echo methods, spin-echo methods are relatively immune to signal loss and distortions from field inhomogeneity and ti ssue-induced susceptibility variations. Through modifications of inter sequence repetition time (TR), echo formation interval (echo time [TE] ), and various gradient moments, image contrast can be altered to emph asize tissue relaxation times T1, T2, or proton density. The TR and TE values control the amount of T1 weighting and T2 weighting, respectiv ely. At long TR intervals (approximately 10 x tissue T1 values) and mi nimum TE values, the difference in signal intensity arising from relax ation vanishes, and contrast arises solely from the differences in pro ton density between the two tissues. Images formed with short TR inter vals and long TE values exhibit very low signal-to-noise ratio and neg ligible contrast and should be avoided. Recently, fast spin-echo seque nces 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.