modern diagnostics are now providing a much more scientific view of such systems. With contrast agents, the portable receivers Crues mentioned, and a version of MRI being refined that uses fluoroscopy, isotropic three-dimensional imaging can be done on a moving joint, functioning in real time under stresses designed by the diagnostician.

Twenty years ago, cartilage tears were often inferred from observing a characteristic presence of edema and interference with the range of motion. Mistakes were often made, however, given the complexity of the structure and the possible alternative kinds of damage that could be masked by the swelling. Although by the early 1980s the growing use of the arthroscope improved diagnosis significantly, a minor surgical procedure was still required just to confirm the diagnosis. But as MRI has been refined, it now reveals both anatomy and pathology, by "noninvasive diagnosis of joint and soft tissue injuries that was inconceivable 10 years ago. This has virtually eliminated diagnostic surgery in the musculoskeletal system," Crues continued, and now allows surgeons to devise a more definitive and sophisticated approach to specific pathologies. A compact and accessible area like the human knee allows physicians to use a variation on the large, body-size RF coils, which often produces very sharp pictures. Called a surface coil, this compact transmitter-receiver unit fits right over the knee itself, and by this close proximity to the target tissue improves the signal-to-noise ratio (SNR) and therefore image quality. Procedures have been developed so that diagnosis of meniscal and ligament tears is now performed easily in most cases.

Beyond enabling the fairly definitive evaluations of anatomy and pathology Bradley referred to, MRI gives Crues and other musculoskeletal MR specialists a tool with which to conduct proactive studies of the integrity of tissue in the knee. Torn or severed cartilage is a common injury, especially among athletes. Knee cartilage or menisci are small, curved tissue sacs that cushion the major ball-and-socket joint of the leg on either side of the knee joint. One of the underlying causes of meniscal tears is believed to be differences in the coefficient of friction between the top and bottom meniscal surfaces and the bones they slide against as the knee moves, said Crues. This pressure differential can cause shear stresses in the interior and tear collagen fibers, he continued, which the body responds to as it would to trauma or inflammation, leading to a "vicious cycle of . . . 'degenerative changes' and eventual mechanical breakdown." A physician who suspects the presence of such precursors to a rupture will design an MRI sequence to look for the longer T2 that would be expected as the



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