THE AAPM RSNA PHYSICS TUTORIAL FOR RESIDENTS - CONTRAST MECHANISMS INGRADIENT-ECHO IMAGING AND AN INTRODUCTION TO FAST IMAGING

The strengths of spin-echo sequences in clinical magnetic resonance im aging are its relative insensitivity to magnetic field inhomogeneities and its versatility; however, they require relatively long imaging ti mes. 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 requ ires at least two RF pulses, typically a 90 degrees excitation pulse f ollowed by a 180 degrees refocusing pulse. The shortened repetition ti mes (TRs) in gradient-echo sequences together with small flip angles e nable maintaining adequate image signal-to-noise ratios while still ac hieving a short imaging time. Numerous gradient-echo sequences have be en developed, with the most common being (a) fast low angle shot (FLAS H), which uses small flip angles and short TRs; (b) gradient-recalled acquisition in the steady state and fast imaging with steady-state pre cession (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 patie nt motion artifacts and to allow isotropic three-dimensional imaging w ith acceptable imaging times.