Gradient and Spin-Echo (GRASE) Imaging

Abstract

Magnetic resonance (MR) imaging pulse sequences are developed through an evolutionary process rather than by entirely de nova invention. Useful new imaging sequences evolve by combining new and existing methodology. For example, spin warp phase encoding was incorporated into selective line scan imaging to produce contemporary 2D FT gradient echo imaging (4). The incorporation of a 180° RF refocusing pulse into 2D FT gradient echo imaging yielded spin echo imaging (1). Spin echo images and gradient echo images differ considerably in their static field inhomogeneity and susceptibility artifacts, tissue contrast and the effects of blood flow. Elimination of field inhomogeneity artifacts allowed for much later echo times, increasing T2 contrast in spin echo images, which is extremely useful for tissue characterization in medical diagnosis. Development of pulse sequences may in retrospect appear straightforward. In reality, there are often major difficulties in removing unanticipated image artifacts in new imaging sequences for which solutions often require more creativity than do the original sequence. Imaging with multiple spin echoes was hypothetically suggested as a simple variant of gradient refocused echo-planar imaging (EPI) but without consideration of artifacts from stimulated echo magnetization. It took years and contributions from several scientists [19, 31] to overcome stimulated echo artifacts for successful implementation of spin-echo train imaging.