Focal parenchymal injuries such as contusions and intraparenchymal hemorrhage usually develop as a result of contact of the brain with the osseous walls of the cranial cavity. The coup-type injuries occur at the point of contact, and the contrecoup injuries occur on the opposite side of the brain. Contusions often occur in areas where the walls of the cranial cavity are irregular, such as the anterior and middle cranial fossae. Therefore, frontal and temporal lobe contusions are common as the brain glides along these irregular surfaces (85) (Fig. 6-90A,B).
FIGURE 6-90. A: Nonenhanced CT scan shows a small left frontal hyperdense hemorrhagic foci (arrow ). Acute extra-axial bleed is also noted (arrowhead ). B: Left temporal post-traumatic hemorrhagic contusions (arrow ). Overlying acute extra-axial bleed is noted (arrowhead).
Cerebral contusions are heterogeneous lesions containing edema, hemorrhage, and necrosis, with any element predominating. When blood makes a major contribution, the contusion appears on CT as a poorly delimited irregular area of hyperdensity. A contusion with mostly edema or necrosis may not be detectable immediately, but after a few days it appears as a hypodense region. Where there is a general admixture of elements, contusions may have a heterogeneous density. Old contusions appear as hypodense areas. By MRI, the edematous and necrotic areas have low signal intensity on T1-weighted images and high signal intensity on T2-weighted images, and thus MRI is more sensitive than CT in identifying these non- hemorrhagic contusions. The areas of hemorrhage in a contusion older than a few days will be hyperintense on both T1- and T2-weighted images.
Intraparenchymal hemorrhage differs from contusions by having better demarcated areas of more homogeneous hemorrhage. The CT and MRI characteristics of acute and evolving intraparenchymal hemorrhage are the same as for hemorrhagic stroke.