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Technical Innovation |
Department of Radiology, Technische Universität München, Ismaninger Str. 22, Münich, Germany D-81675.
Received November 3, 2004;
accepted after revision January 31, 2005.
Address correspondence to K. Woertler
(woertler{at}roe.med.tum.de).
Abstract
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CONCLUSION. Our 2D multislice turbo spin-echo sequence combined with a driven equilibrium pulse provides a bright signal of joint fluid with otherwise unchanged signal intensities as compared with a normal T1-weighted turbo spin-echo sequence at high spatial resolution and short scan times. Thus, it might represent a useful adjunct for routine joint examinations.
In recent years, MR arthrography with the use of T1-weighted pulse sequences after application of gadolinium chelates has regained increasing attention. At present, it has developed into the method of choice for indications such as imaging of the shoulder and articular cartilage and in the detection of several specific abnormalities of the wrist, elbow, hip, knee, and ankle [1-3]. MR arthrography provides optimal contrast between intraarticular fluid, fibrocartilaginous and capsular structures, articular cartilage, and subchondral bone [1, 2].
Several investigators have therefore developed native pulse sequences, such as the DESS (dual-echo steady-state) sequence, which show arthrographic contrast without the use of contrast media [4]. However, since gradient-echo techniques are applied to produce these bright-fluid images, the relative signal intensities of fibrocartilage, hyaline cartilage, and bone are different from those in T1-weighted spin-echo or turbo spin-echo arthrograms.
A driven equilibrium spin-echo sequence named DEFT (driven equilibrium Fourier transform) has been proposed by Hargreaves and colleagues [5, 6] and Yoshioka and colleagues [7]. The main drawback of the DEFT sequence as it is published is its long acquisition time by running a 3D acquisition with a TR of 400 msec. Despite an echo-planar imaging readout of five echoes, which requires additional phase-correction steps to minimize ghosting artifacts and an imaging matrix of 256 x 192, the acquisition time still amounts to 8 min 29 sec [7].
We present a novel fast 2D interleaved multislice turbo spin-echo sequence (turbo factor, 3) in combination with a driven equilibrium (DRIVE) pulse [8]. After readout of three spin-echoes for spatial encoding and a final 180° refocusing pulse, a resonant -90° radiofrequency restoration pulse (DRIVE pulse) at the instant of the final spin-echo flips the residual transverse magnetization back up into the longitudinal direction. This primarily affects the signal intensity of free water with its long relaxation times. The long T2 ensures an essentially unchanged amount of residual transverse magnetization and thus results in a significant increase of longitudinal magnetization after the restoration pulse compared with the long T1 longitudinal magnetization in a standard T1-weighted pulse sequence. This technique can be used either for T2-weighted imaging (long TEs) with relatively short TRs or, as in our application, for T1-like imaging (short TEs) with an artificially increased signal intensity of free water. Thus, instead of decreasing the signal intensity of cartilage below that of intraarticular fluid by using long TEs, this technique increases the signal intensity of free water above that of cartilage at short TEs (Figs. 1A and 1B).
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In musculoskeletal imaging, driven equilibrium techniques have so far predominantly been used to increase the contrast of T2-weighted pulse sequences, for example, to enhance myelographic contrast of T2-weighted spin-echo images of the spine [9]. The DEFT sequence described by Hargreaves and colleagues [5, 6] provides mixed T1/T2 contrast in combination with fat-suppression resulting in bright cartilage signal and a high contrast between articular cartilage and surrounding tissue [7]. Thus, this technique has so far mainly been applied in imaging of articular cartilage of the knee and, probably due to its relatively long acquisition times, has not been widely used in joint imaging.
Our application provides bright signal of joint fluid with otherwise unchanged signal intensities compared with a normal T1-weighted turbo spin-echo sequence (Figs. 1A and 1B). In combination with fat-suppressed proton density or T2-weighted sequences, our fast arthrographic sequence can help to improve delineation of articular cartilage surfaces (Figs. 1A, 1B, 2A, and 2B), lesions of fibrocartilaginous or ligamentous joint structures (Figs. 2A and 2B), and tendon tears. Thus, in our experience, it represents a useful adjunct for routine examinations of joints where application of contrast media is not possible or not desirable.
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