DESIGN OF DEDICATED SHIM COILS

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Introduction: Diffusion-weighted imaging of cartilage may provide insight into normal cartilage structure, cartilage defects, and transplants. Specifically, diffusion-weighted images may demonstrate structural orientation of the collagen fibers. We have developed a method for imaging diffusion effects in articular cartilage. Materials and Methods: The diffusion-weighted imaging sequence for cartilage uses conventional Stejskal-Tanner diffusion gradients, a circular echo-planar readout trajectory, and a spiral navigator pulse to reduce motion effects. Since the T2 relaxation time of cartilage is short, echo time is kept to a minimum (51 ms) while still allowing diffusion weighting to be applied. In-plane spatial resolution is 1.3 x 1.4 mm. Nine slices can be acquired in about 5 minutes. B-values were 0, 393, and 486 s/mm^2. All imaging was done on a 1.5T GE Signa with Echospeed gradients. Results: Ten normal volunteers and one patient with a cartilage transplant have been studied. Normal articular cartilage shows an apparent diffusion coefficient (ADC) ranging from 1 to 2x10^-3 mm^2/s, which compares well with literature results obtained on cartilage/bone plug specimens. ADC values increase in normal cartilage going from the cartilage/bone interface to the joint surface. Conclusion: In-vivo diffusion-weighted imaging of articular cartilage can be performed on a conventional whole-body scanner. This type of imaging may yield new information about the orientation and type of collagen fibers present. It may be possible to follow the development and integration of chondrocyte transplants, and to distinguish fibrocartilage from hyaline cartilage at the transplant site.