SHEAR INJURIES

      Severe head injuries are often associated with rotational forces that produce shear stresses on the brain parenchyma. The brain itself has very little rigidity and is extremely incompressible. Brain volume can be decreased only by exerting great pressure. On the other hand, the brain is soft and malleable. Relatively little effort is required to distort the shape of the brain. The parenchyma is of relatively uniform density, except for differences between the CSF of the ventricles and surrounding brain tissue. Slight differences in density also exist between gray and white matter.

      When the skull is rapidly rotated, it carries along the superficial brain parenchyma but the deeper structures lag behind, causing axial stretching, separation and disruption of nerve fiber tracts. Shear stresses are most marked at junctions between tissues of differing densities. As a result, shear injuries commonly occur at gray/white matter junctions, but they are also found in the deeper white matter of the corpus callosum, centrum semiovale, brain stem (mostly the midbrain and rostral pons) and cerebellum. 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. The thalamic and basal ganglia injuries are hemorrhagic in slightly more than 50% of cases. On the other hand, shear injuries of the corpus callosum and centrum semiovale are more often nonhemorrhagic.       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. These injuries constitute the most frequent findings on MR in head trauma, comprising as high as 40% of all lesions. 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. Endnote  

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