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(Gradient- and Spin-Echo) MR Imaging: a New Fast Clinical Imaging Technique’

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(Gradient- and Spin-Echo) MR Imaging: a New Fast Clinical Imaging Technique’ Magnetic Resonance Imaging David A. Feinberg, PhD, MD #{149}Koichi Oshio, MD, PhD GRASE (Gradient- and Spin-Echo) MR Imaging: A New Fast Clinical Imaging Technique’ A novel technique of magnetic reso- W HILE magnetic resonance (MR) ben of image sections and in S/N, the nance (MR) imaging, which corn- imaging has improved rapidly hybrid RARE technique currently has bines gradient-echo and spin-echo oven the last decade, it is remarkable a four- to 16-fold reduction in imag- (GRASE) technique, accomplishes that multisection two-dimensional ing time from that of spin-echo imag- T2-weighted multisection imaging in Fourier-transform spin-echo imaging ing with one excitation and has tissue drastically reduced imaging time, (1) with T2 weighting has remained a contrast similar to that of the spin- currently 24 times faster than spin- standard of reference for routine clini- echo technique. In hybrid RARE im- echo imaging. The GRASE technique cat MR imaging studies of the body aging, the rapid application of a large maintains contrast mechanisms, high and head. Since the earliest studies in number of 180#{176}radio-frequency (RF) spatial resolution, and image quality 1982, there has been a progressive pulses produces considerably higher of spin-echo imaging and is compati- reduction in image acquisition time. deposition of RE energy in the human ble with clinical whole-body MR sys- Improvements in MR-imager hard- body at the standard absorption rate tems without modification of gradi- ware and pulse-sequence design have (SAR). Although SAR increases with ent hardware. Image acquisition time permitted faster imaging times by ne- shorter time intervals between RE is 18 seconds for 11 rnultisection body ducing the number of signals aver- pulses, the speed of RARE imaging is images (2,000/80 [repetition time aged (NSA) used to raise the signal- ultimately limited by the time require- msec/echo time msec]) and 36 sec- to-noise ratio (S/N) in images. ments of section-selective 180#{176}RF onds for 22 brain images (4,000/104). Conjugate synthesis of data (half Fou- pulses during which signal cannot be With a combination of multiple Hahn acquired. rier or NSA = Y2) yields nearly an- spin echoes and short gradient-echo other factor-of-two reduction in imag- In echo-planar imaging (5), signals trains, the GRASE technique over- ing time by taking advantage of a are produced by rapid switching of comes several potential problems of natural symmetry in the k-space data gradient polarity in place of the echo-planar imaging, including large slower selective 180#{176}RF pulses. In this to halve the number of phase-en- chemical shift, image distortions, and way, echo-planar imaging and its coded signals (2,3). For clinical imag- signal loss from field inhomogeneity. modern variants, modulus-blipped ing, it is significant that the conjugate Advantages of GRASE over the echo-planar single-pulse technique synthesis method produces tissue RARE (rapid acquisition with relax- (MBEST) (6) and Instascan (Advanced contrast, chemical shift, and spatial ation enhancement) technique in- NMR, Woburn, Mass) (7), both involv- resolution identical to that of spin- dude faster acquisition times and ing use of a single 180#{176}RF pulse, can echo imaging with one excitation. lower deposition of radio-frequency produce an image in less than 100 Whereas the conjugate synthesis power in the body. Breath holding msec. At high magnetic field method produces an expected 35%- during 18-second GRASE imaging of strengths, methods of echo-planar 41 % reduction in image S/N, this S/N the upper abdomen eliminates respi- imaging produce a large amount of is acceptable for many T2-weighted ratory-motion artifacts in T2- chemical shift on the phase axis of examinations. weighted images. A major improve- images, typically 10-15-pixel misregis- ment in T2-weighted abdominal An alternative approach for fast tration between water and fat. This imaging is suggested. T2-weighted imaging is the rapid ac- problem can be circumvented by us- quisition with relaxation enhance- ing fat-suppression methods. These ment (RARE) technique (4). The hy- Index terms: Abdomen, MR studies, 70.1214 single-shot images, however, have brid RARE technique, also called fast Brain, MR studies, 13.1214 #{149}Magnetic reso- lower spatial resolution and S/N than nance (MR), comparative studies . Magnetic spin-echo imaging, reduces imaging spin-echo images. This can be im- resonance (MR), echo planar . Magnetic reso- time by performing phase encoding proved by using multiple excitation nance (MR), rapid imaging . Magnetic reso- both in trains of multiple spin echoes nance (MR), technology and during multiple cycles (repetition times [TRs]) of signal excitation. Radiology 1991; 181:597-602 While there is a reduction in the num- Abbreviations: CPMG = Carr-Purcell-Mei- boom-Gill, GRASE = gradient echo and spin echo, MBEST = modulus-blipped echo-planar single-pulse technique, NSA = number of sig- I From the Department of Radiology, Harvard Medical School, Brigham and Women’s Hospital, nals averaged, RARE = rapid acquisition with Boston. Received April 18, 1991; revision requested June 1; revision received June 25; accepted July relaxation enhancement, RF = radio frequency, 1. Address reprint requests to D.A.F., Department of Radiology, New York University Medical Cen- SAR = standard absorption rate, S/N = signal- ter, 560 First Ave. New York, NY 10016. to-noise ratio, TE = echo time, TR = repetition © RSNA, 1991 time. 597 cycles (multiple TRs) that lengthen imaging times. Other stringent re- quirements of echo-planar imaging include static magnetic fields with good homogeneity (better than 0.3 ppm for head imaging with a 1.5-T -i system) to avoid signal loss and image Phase error ottleld (nhomogenetty T..... distortions resulting from inhomoge- and themlcal shift. neity-induced phase errors. All echo- it1 2 Si s2s3 s4s5s6 s7s8s9 planar imaging variants require strong magnetic gradients with fast rise times, which to date has limited RF&slgnal ir/2 JVJ’tAJ& JVJ’\lJ(J& their application to a handful of re- search centers. It is significant that the subsecond imaging time of echo-pla- nar techniques eliminates artifacts (Read out) f\_/\ I\_[\ caused by both respiratory and car- ‘k_I diac motion. In this article, a novel technique of MR imaging, which combines gradi- Gy tt fi ent echo and spin echo (GRASE) tech- (Phase) vu vu vu niques, enables T2-weighted abdomi- nat imaging in tess than 18 seconds. This is sufficiently fast to permit Gz n n breath holding during image acquisi- (slice select) tions to eliminate respiratory-motion Figure 1. Pulse-sequence diagram of the GRASE technique. An echo train is formed with the artifacts, currently not possible in din- Carr-Purcell-Meiboom-Gill (CPMG) sequence with selective 180#{176}RE pulses (rr, ‘rr, . .) and ical T2-weighted spin-echo studies of multiple signal readout gradient (Gx) reversals. With the multiple 180#{176}pulses, phase errors the abdomen performed with acquisi- due to field inhomogeneity and chemical shift are greatly reduced in magnitude and are peri- tion times of 4-8 minutes. odically reproduced throughout the echo train. In echo-planar imaging, with the absence of 180#{176}pulses there would be a large accumulation of phase errors and chemical shift (dashed line). The 24 echoes are phase-encoded differently by the gradient pulses in the phase-en- coded direction (Gy). In each excitation cycle (TR), the strength of certain Gy pulses are MATERIALS AND METHODS changed (down-directed arrows) to offset the phase position in k space, while the pairs of negative Gy pulses maintain constant large jumps in the k-space trajectories. (The k-space lo- GRASE Technique cation of signals [sl-s9] is shown in Fig 2.) By using refocusing pulses (up-directed arrows), the net accumulated Gy phase shift is returned to zero before each 180#{176}RF pulse. Here, three The technique of combining gradient readout gradient reversals (NCR = 3) are shown, and only three of the eight RE refocused spin- echoes with spin echoes is shown in the echo periods (N5) are shown. pulse-sequence diagram (Fig 1). A train, or number of RF refocused spin echoes (NSE), is produced by using the CPMG spin-echo sequence. Centered about each spin echo, read (frequency) axis as a result of fre- small phase increments are imposed dun- a number of gradient-recalled echoes (NGR) quency sensitivity through the echo train. ing each 180#{176}-180#{176}interval along the total are produced by switching the polarity of In GRASE imaging, the chemical shift is echo train. Subsequent computer reorder- the readout gradient. The speed advan- proportional to the time interval of the ing or interleaving of the signals in k space tage over standard spin-echo imaging is short echo train with NCR echoes, whereas (Fig 2) effectively removes the peniodicity proportional to the total number of echoes in echo-planar imaging, the chemical shift of field inhomogeneity errors on the phase pen 90#{176}excitation and equals NGR X N5. is proportional to the longer time of the axis. The effective echo time of the image is the gradient-echo train composed of 64-128 In this way, GRASE phase encoding or- time at which the origin of k space (zero echoes. den is significantly different from that of order phase encode) is sampled, near the The GRASE pulse sequence is more RARE and echo-planar imaging, which middle of the echo train. complicated than a simple combination of continuously increase their phase-encod- In GRASE imaging, the use of 180#{176}RE spin echo with gradient echo, since the ing steps as a function of time in the echo pulses to nutate on reverse the position of relatively small field-inhomogeneity phase train (Fig 2).
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