Hybrid RARE: Implementations for abdominal MR imaging

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Abstract
Hybrid RARE (rapid acquisition with relaxation enhancement) is a family of magnetic resonance (MR) imaging techniques whereby a set of images is phase encoded with more than one spin echo per excitation pulse. This increases the efficiency of obtaining T2‐weighted images, allowing greater flexibility regarding acquisition time, resolution, signal‐to‐noise ratio, and tissue contrast. Hybrid RARE techniques involve several important new user‐selectable parameters such as effective TE, echo train length, and echo spacing. Choices of other parameters, such as TR, sampling bandwidth, and acquisition matrix, may be different from those of comparable conventional T2‐weighted spin‐echo images. Different hybrid RARE implementations can be used for abdominal screening, with T2‐weighted or T2‐weighted and inversion‐recovery contrast, or for characterizing liver lesions or imaging the biliary system with an extremely long TE. High‐resolution images may be obtained by averaging multiple signals during quiet breathing, or images may be acquired more rapidly during suspended respiration. In this review, the authors discuss the basic principles of hybrid RARE techniques and how various imaging parameters can be manipulated to increase the quality and flexibility of abdominal T2‐weighted MR imaging.