MRI study of the upper extremity is routinely performed using dedicated phase-array coils, with the patient in a supine position and the arm by the side. The nerves of the upper extremity are in general best visualized on axial images. T1-weighted sequences provide superb anatomical resolution. Watersensitive sequences such as fat-suppressed T2-weighted and Short-tau inversion recovery (STIR) sequences are useful in identifying signs of neuritis/neuropathy and acute denervation muscle injury. Acute axonal nerve lesions are manifested by T2 hyperintensity and increased girth of the nerve at and distal to the site of injury. Proximal increased girth may also be encountered. Increased T2 signal and post-gadolinium enhancement of denervated muscle fibers is most likely related to fluid shift from intracellular to extracellular compartment, and changes in the intramuscular vascular bed leading to capillary engorgement and increased muscular blood volume [5,6]. These changes were noted as early as 24 hours after complete transection of the sciatic nerve in rats . MRI has also been successfully used experimentally as an indicator of nerve degeneration, healing, and reinnervation . New contrast media, such as superparamagnetic iron oxide, may be useful in detecting macrophage invasion into the degenerating nerve distal to an axonal lesion . Other agents such as gadofluorine M-enhanced MRI may aid in the assessment of nerve regrowth and regeneration .
In the following pages, the authors review the normal anatomy, the MRI anatomy, and the MRI manifestations of sports-induced entrapment/compres-sive neuropathies of the upper extremity.
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