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MR-Imaging-of-the-Wrist-with-a-Portable-Extremity-Scanner Powered By Docstoc
					MR Imaging of the Wrist with a Portable Extremity Scanner Garry Gold1, Daphne Theodorou1, Thanmaya Blair1 , Gaspar Garcia1, Chris Crowley2 , Freeman Rose2, Debra Trudell1 , and Donald Resnick 1
University of California San Diego Department of Radiology, San Diego CA 921221 MagneVu Corporation, Carlsbad CA 920082 Purpose New methods offer MR imaging systems with substantial cost savings. In order to evaluate these new systems, it is important to compare performance in a clinical application. Tears to the Triangular Fibrocartilage Complex (TFCC) of the wrist are an important cause of wrist pain [1]. The purpose of this study was to demonstrate MR imaging of TFCC tears using a portable extremity scanner. Images obtained from the portable extremity system are compared with images obtained from a conventional 1.5T system as well as histologic data. Materials and Methods The MagneVu OrthoVu 1000 is a 0.2T system weighing about 200 lbs. that is portable and can be operated in an unshielded room [2]. A non-uniform magnetic field is used (T2* ~25 µsec) to form small images, primarily of shallow structures. The magnetic field establishes a permanent slice selection gradient, while phase encoding gradients are applied in the selected image plane. Averaging a CPMG pulse train of rapidly refocused spin echoes offsets the loss of signal to noise ratio (SNR) that results from diffusion and the non-uniform field [2]. Three cadaver wrists were imaged in the MagneVu system with T1, T2 and inversion recovery (IR) weighting. Scan time was about 13 minutes per slice for a resolution of 0.8 to 1.2 mm in-plane. Slice thickness was 3 mm. T1, T2, and IR weighted images were also obtained in a 1.5T GE Signa system at the same and higher resolution using standard imaging parameters. Imaging results were compared with histology. Results Three wrist specimens were imaged, sectioned, and examined for TFCC tears. One specimen showed a clear tear on the radial aspect of the TFCC. The second specimen showed a partial TFCC tear. The third specimen showed no tear. Both the Signa and MagneVu images showed the complete tear. Neither MR system was able to distinguish the partial tear from a complete tear. The IR images on the MagneVu system show the TFCC tear most clearly. Discussion A variety of recent technical developments have provided portable MR imaging systems that can potentially be operated at the point of care with low cost. Although such systems have limitations in SNR and imaging depth, this paper shows that this portable extremity scanner is capable of imaging TFCC tears.

Figure 1. The portable extremity scanner used for this study. The computer and electronics console is on the left, and the integrated magnet, gradient, and RF coil assembly is on the right.

Figure 2. a) T1-weighted image (TE 20 ms, TR 600 ms, in-plane resolution 0.8 mm resolution). b) Inversion recovery image (TE 20 ms, TR 600 ms, TI 40 ms, 1.2 mm resolution) of a wrist specimen obtained from the system shown in figure 1. A tear in the Triangular Fibrocartilage Complex (TFCC) is seen (arrow). Slice thickness is 3 mm and in-plane resolution is 1.2 mm.

1. Schweitzer, M. et al., “Chronic wrist pain: spin-echo and short tau inversion recovery MR imaging and conventional and MR arthrography”, Radiology. 1992, 182(1), 205-11. 2. C.W. Crowley and F.H. Rose, “Remotely Positioned MRI”, U.S. Patent # 5,304,930, 1994.

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Description: MR-Imaging-of-the-Wrist-with-a-Portable-Extremity-Scanner