Debate remains whether imaging is required for GCS of 15. Clear indications for neuroimaging include GCS less than 15; unexplained transient or persistent loss of consciousness; other forms of altered sensorium; focal neurologic signs; depressed skull fractures; seizure; persistent variations; progressive headache; and penetrating injuries.89 In these instances, NECT plays an essential role for diagnosis and for detecting processes that require emergent intervention, such as epidural and subdural hematomas, increased intracranial pressure, and depressed skull fractures. Although discussed elsewhere, a number of points related to optimal imaging are worthy of emphasis.
The differential density of blood facilitates the detection of intraparenchymal and extraaxial hemorrhage. Density is displayed on a relative scale referred to in Hounsfield Units (HU). For example, water or CSF is assigned a value of zero, bone or metal varies between l00 and 1000 HU (hyperdense), fat is approximately -100 HU (hypodense), and air is -1000 HU. Further, acute blood has a density of 60 to 100 HU, and normal brain parenchyma has a density of approximately 35 (white matter) to 40 HU (gray matter). This differential density enables the increased density of blood or the decreasing density with accumulating edema to be detectable. Images are filmed in a manner that accentuates differences in the parenchymal range of densities. To optimize sensitivity for detecting parenchymal injuries, a gray scale is assigned such that regions of higher density are not easily differentiable. As such, blood in proximity to bone can be difficult to detect using parenchymal windows (Fig.229-6.^. and Flg,.,.,2.2..9.-.6,§).
FIG. 229-6. A. Axial CT demonstrates a somewhat subtle left subdural hematoma (curved arrows). Note secondary signs of mass effect with the contralateral sylvian fissure detectable but compressed on the side of the subdural hematoma (straight arrow). B. Axial MR demonstrating the increased conspicuity of blood due to the lack of obscuration by adjacent bone.
When evaluating for the presence of subdural and epidural collections it is important that images are formatted in a fashion so that the high densities are also distinguishable (i.e., blood windows). At many institutions it may not be routine to print these windows due to associated added film costs, but at a minimum, images should be reviewed on the CT console to exclude small blood collections. This is particularly true in the pediatric population where subdural hematomas can be a marker of intentional trauma. Detection is also facilitated by attention to the subtle secondary signs of extra-axial blood including loss of sulcal demarcation, mass effect on a ventricle, or displacement of the gray white junction from the inner table ( Fig 229-6.). These secondary signs are also important because not all acute blood is hyperdense. In anemic patients (hemoglobin level 8 to 10 g/dL) or patients with a coagulopathy, acute blood may be isodense and thus difficult to visualize.
Occasionally, MRI can be of value in assessing trauma patients. For example, in assessing for small acute subdural hematomas or for subacute/chronic hematomas, CT may be limited due to the hematomas' proximity to bone or because over time the density of blood becomes isodense to normal brain (subacute) or to normal CSF (chronic) (Fjg:iMi229-6B,). However, these blood products will remain abnormal with MRI for months to years. This can be of value in children when new and old coexistent extra-axial hematomas are essentially pathognomonic for child abuse.
MRI is also more sensitive in detecting diffuse axonal injury in which differential movement of parenchyma results in neuronal shearing. Diffuse axonal injury should be suspected when coma occurs immediately after severe injury and the degree of compromise is out of proportion to the CT findings. MR may be beneficial in that it will show multiple punctate areas of blood products below the threshold of CT detection. Specific anatomic structures that tend to be involved include the corpus callosum, adjacent to the superior cerebellar peduncle, the internal capsule, and gray-white matter junction.
Intra- or extracranial arterial dissection may be the result of trivial trauma, blunt trauma, or penetrating injury. MR can detect the presence of an acute dissection with T1 hyperintense blood seen in the vessel wall. In combination with MR angiography, it provides a fairly sensitive screening for a dissection. It enables an adequately sensitive but noninvasive method for vascular assessment and simultaneous assessment of the brain parenchyma and the extra-axial spaces. Other vascular lesions associated with trauma are pseudoaneurysms, which may occur in any dissected vessel, both intracranially and extracranially, and are prone to bleed. Catheter angiography remains the gold standard, but should be restricted to those patients with equivocal or abnormal MRA findings or a high-level of clinical suspicion.
Catheter angiography is also recommended in all patients with penetrating injuries to zone 1 and zone 3 of the neck, although assessment of zone 3 injuries is slightly more equivocal.10 This is imperative for evaluating vessels of the thoracic inlet and above the mandible, respectively, because they are not accessible to visual inspection. Further, surgical exploration when indicated will require a vascular road map. Zone 2 injuries can be observed without angiography, which is recommended for expanding hematomas or where there is the development of respiratory or neurologic compromise.11
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