T2*-weighted Sequences and Clinical Applications
Most GRE sequences are T2 * sensitive in some way because no 180° radiofrequency pulse is used, and therefore, as mentioned previously, the dephasing caused by magnetic field inhomogeneities is not compensated. There are two broad categories of GRE sequences: incoherent (spoiled residual transverse magnetization) and coherent (refocused transverse magnetization). Residual transverse magnetization is spoiled in incoherent GRE sequences after signal acquisition so that it will not interfere in the next cycle. In coherent GRE sequences, the residual transverse magnetization is refocused so that after a few repetition cycles, it leads to a steady state of transverse magnetization ( 6 ). Most of the clinically used T2 * -weighted sequences are related to or modifications of spoiled GRE (eg, fast low-angle shot [FLASH]) and postexcitation refocused GRE (eg, fast imaging with steady-state precession [FISP]) sequences.
Echo-planar imaging is a fast MR imaging technique in which all of the phase-encoding steps to form an image can be acquired either in a single TR (single-shot echo-planar imaging) or in a few TRs (multishot echo-planar imaging). In echo-planar imaging, rapid oscillation of high-amplitude gradients is used to form gradient echoes, thus avoiding radiofrequency refocusing. Lack of radiofrequency refocusing makes echo-planar imaging sequences sensitive to T2 * decay. The effective TE in echo-planar imaging is usually listed as that TE at which the origin of k-space is collected.