Magnetic resonance (MR) imaging has a less important role in acute trauma. Relative to CT, MR is insensitive and nonspecific in the detection of acute hemorrhage. By contrast, MR imaging is far superior to CT in demonstrating cerebral ischemic lesions caused by trauma. Diffusion and T2-weighted images, which emphasize the water content of cerebral tissue, can demonstrate ischemic brain tissue minutes to a few hours after trauma (see Chapter 2). MR angiography, which images blood flow in the cerebral vasculature, may demonstrate the source of ischemia due to arterial dissection. This noninvasive method of angiography is particularly helpful in pediatric trauma patients, for whom catheter angiography is technically difficult and has a higher complication rate than in adults. Finally, MR imaging is helpful in the rehabilitation phase of severe head injury for prognosticating recovery from diffuse axonal injury (see below).
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