Compared with EPI, the formation of multiple spin echoes in GRASE allows more k-space lines to be sampled with each excitation RF pulse. Originally developed nearly thirty years ago when high-performance gradient coils were still uncommon in clinical whole-body MR imaging systems and hence EPI was a luxury, GRASE was proposed as a practically feasible fast sequence offering single-shot capability with T2-weighted contrast. GRASE, gradient- and spin-echo RF, refocusing radiofrequency TE, echo time. Note that phase accumulation from off-resonance effects occurs within each spin echo, whereas signal decaying from T2 relaxation causes amplitude modulations across the spin echoes.
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A number of refocusing pulses (three in this example) are applied after the excitation pulse to generate spin echoes at different TEs, whereas within each echo spacing a series of gradient-echoes (five here) are formed to sample the spin echo. Therefore the GRASE sequence can be viewed as a hybrid of FSE and EPI.įigure 1 Schematic drawing of the pulse sequence diagram for the GRASE sequence, including the RF pulse, the slice selection gradient, and the frequency-encoding gradient channels. Each gradient-echo is preceded by appropriate phase encoding gradients (not shown) to be filled in the k-space at corresponding locations to form an image. It consists of multiple refocusing radiofrequency (RF) pulses after the excitation RF pulse just like the fast spin-echo (FSE) technique ( 8), but with series of gradient-echo readouts in a way similar to EPI inserted in the echo spacing interval between successive refocusing pulses. The pulse sequence diagram of the GRASE sequence is schematically shown in Figure 1 ( 6, 7). Principles and properties of the GRASE sequence The gradient- and spin-echo (GRASE) sequence ( 6, 7) in our opinion, although not as obvious as the two aforementioned examples, may well be another imaging technique with strong potential but was long overlooked.
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Another instance is balanced steady-state free precession imaging, whose banding artifacts precluded its inclusion in routine clinical examination protocols before the advancements of gradient coils and shimming technology ( 4), but nowadays has become an essential tool in cardiovascular MR imaging ( 5).
Well known examples in the magnetic resonance (MR) imaging literature include clinical diffusion-weighted imaging ( 1) that relies largely on the commercialization of single-shot echo-planar imaging (EPI) ( 2) which, in turn, requires high-performance gradient coils to reduce artifacts related to off-resonance to diagnostically acceptable level ( 3). There are plenty of technical innovations that were regarded in early days as certainly impractical during initial usage, yet later proved to become the predominant method of choice in certain applications after key improvements of the associated equipment or devices. Professor, Graduate Institute of Biomedical Electronics and Bioinformatics, National Taiwan University, Taipei, Taiwan.
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