Shear stresses are most marked at junctions between tissues of differing densities. Lesions in the basal ganglionic regions are usually found along the borders between the ganglia and the internal or external capsules, in other words, the deep graywhite matter junctions of the cerebral hemispheres. On the other hand, shear injuries of the corpus callosum and centrum semiovale are more often nonhemor-rhagic. Attempts to correlate CT findings with acute and chronic sequelae of closed head trauma have been discouraging, largely related to the insensitivity of CT to many cerebral injuries. Chiefly among these, poorly seen by CT and well seen by MR, are the diffuse axonal injuries or white matter shear injuries.
Shear injuries are most often multiple, ovoid and parallel to white matter fiber bundles.
They are hyperintense on T2 and hypointense of T1-weighted scans, unless hemorrhagic components are present, in which case more complex patterns are observed. During transition phases of hematoma evolution, combinations of methemoglobin, hemosiderin rings and peripheral edema can result in layers of differing signal intensity and a target-like appearance. The axial plane is the primary plane of imaging for both cortical contusions and shear injuries, but supplemental coronal views are helpful to assess injuries to the body of the corpus callosum and the inferior frontal and temporal lobes. Fast scan techniques or gradient-echo images have lower resolution but are useful in uncooperative patients. Contrast enhancement has little role in the evaluation of brain contusions.