Imaging after head trauma: why, when and which.

CT scanning is the current first imaging technique to be used after head injury, in those settings where a CT scan is available. The first scan is usually done without contrast enhancement. The value of CT is the demonstration of scalp, bone, extra-axial hematomas and parenchymal injury. It is rapid and easily done in the presence of the multiple monitors that many trauma patients have in place. It can be used to demonstrate the bony anatomy of the spine and is good for evaluation of abdominal and chest trauma also. MRI is more sensitive for all posttraumatic lesions other than skull fracture and subarachnoid hemorrhage, and can demonstrate parenchymal spinal cord injury. The cons are a longer scanning time, interference of the imaging by certain ICP monitors and problems with the positions of the monitoring equipment and ventilators outside the MRI magnetic field. MRI will be used increasingly to study early head injury because of its ability to measure cerebral blood flow, cerebral blood volume and the location and extent of cerebral edema. If the CT does not demonstrate pathology adequate to account for the clinical state, MRI is warranted. Follow up is best done with MRI as it is more sensitive to parenchymal change than is CT.