The AAPM/RSNA physics tutorial for residents. Contrast mechanisms in gradient-echo imaging and an introduction to fast imaging.

The strengths of spin-echo sequences in clinical magnetic resonance imaging are its relative insensitivity to magnetic field inhomogeneities and its versatility; however, they require relatively long imaging times. Use of gradient-echo sequences largely circumvents the problem. A gradient echo is created by means of a gradient reversal following a single radio-frequency (RF) pulse, typically with a flip angle that is less than 90 degrees, compared with a spin-echo sequence, which requires at least two RF pulses, typically a 90 degree excitation pulse followed by a 180 degree refocusing pulse. The shortened repetition times (TRs) in gradient-echo sequences together with small flip angles enable maintaining adequate image signal-to-noise ratios while still achieving a short imaging time. Numerous gradient-echo sequences have been developed, with the most common being (a) fast low angle shot (FLASH), which uses small flip angles and short TRs; (b) gradient-recalled acquisition in the steady state and fast imaging with steady-state precession (GRASS/FISP); (c) magnetization-prepared rapid gradient echo (MP-RAGE); and (d) echo planar. The shorter imaging times made possible through gradient-echo sequences are used clinically to minimize patient motion artifacts and to allow isotropic three-dimensional imaging with acceptable imaging times.