Rare pulse sequence

(short for rapid acquisition with relaxation enhancement) an MR imaging pulse sequence of the multiecho spin echo pulse sequence type. Other names for or variations of the RARE sequence are turbo spin-echo (TSE), fast spin-echo (FSE) and HASTE. RARE images provide similar image contrasts to spin-echo images, but there exist distinct differences. Fatty tissues appear brighter in RARE images than in conventional spin-echo images and haemorrhagic and calcified lesions are less well seen in RARE images.

Imaging technique: while in a standard multiecho SE sequence the lines acquired at the echoes are used for different images, in the RARE sequence the lines are used for one or two images only. Typically 8 to 128 lines are acquired after each echo, hence, image acquisition is shortened by this factor. Note that during acquisition, the normal T2 decay (Fig.1) is occuring. Hence, the lines acquired early are enhanced relative to the lines acquired later. Since the lines are acquired in the Fourier transform space of the image (k space), those lines which yield the image contrast are the central lines (see k-space (I), Fig. 2). These central lines should be acquired around the desired echo time; i. e. the first echoes for a T1 weighted image or proton density weighted image and the later echoes for a T2 weighted image. If images with different contrast are desired, the acquired lines can also be divided up among these images as in standard multiecho spin-echo imaging, resulting in e.g. a proton and a T2 weighted RARE image. Note, that the echo time in a RARE sequence is no longer precisely defined as distinct from standard spin-echo images (Fig. 1), as image lines are acquired at different echo times. Strong and fast gradient systems help to shorten RARE pulse sequences, but it should be noted that RARE sequences have a high SAR, since a very high number of strong 180 radiofrequency pulses is applied to the patient during a short imaging sequence.

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