In order to perform a head CT, the patient is placed on the CT table in a supine position and the tube rotates around the patient in the gantry. In order to prevent unnecessary irradiation of the orbits and especially the lenses, Head CTs are performed at an angle parallel to the base of the skull. Slice thickness may vary, but in general, it is between 5 and 10 mm for a routine Head CT. Intravenous contrast is not routinely used, but may be useful for evaluation of tumors, cerebral infections, and in some cases for the evaluation of stroke patients. Cranial cross-sectional anatomy is very important to know prior to analyzing a head CT. Once the normal structures are identified, abnormalities can be detected and a diagnosis may be possible. Symmetry is an important concept in anatomy and is almost always present in a normal head CT unless the patient is incorrectly positioned with the head cocked at an angle. We will present several representative CT sections with a corresponding sagittal MRI showing the level of the CTsection for you to examine. Epidemiology In the United States, approximately one million people are treated and released from hospital emergency rooms for traumatic brain injury every year. 80,000 Americans experience the onset of long-term disability and 50,000 Americans die as a result of traumatic brain injury each year. Approximately 45% of injuries result from transportation accidents, 26% from falls, and 17% from assaults. Other causes, such as sports injuries, comprise the remainder of cases. Two-thirds of the patients are less than 30 years of age, and men are twice as likely as are women to be injured. The cost of traumatic brain injury is $48.3 billion dollars annually. The estimated cost for a 36-hour stay at the hospital for a minor head injury that resulted in a loss of consciousness is around $13,000. A patient who experiences severe head injury in his mid twenties is estimated to have a lifetime cost of four million dollars including lost work hours, medical and daily care. Thus, head injuries are a substantial concern in the United States. Skull fractures are categorized as linear or depressed, depending on whether the fracture fragments are depressed below the surface of the skull. Linear fractures are more common. The bone windows must be examined carefully. A skull fracture is most clinically significant if the paranasal sinus or skull base is involved. Fractures must be distinguished from sutures that occur in anatomical locations (sagittal, coronal, lambdoidal) and venous channels. Sutures have undulating margins both sutures and venous channels have sclerotic margins. Venous channels have undulating sides. Depressed fractures are characterized by inward displacement of fracture fragments. Linear skull fracture of right parietal bone (arrows) A subarachnoid hemorrhage occurs with injury of small arteries or veins on the surface of the brain. The ruptured vessel bleeds into the space between the pia and arachnoid matter. The most common cause of subarachnoid hemorrhage is trauma. In the absence of significant trauma, the most common cause of subarachnoid hemorrhage is the rupture of a cerebral aneurysm. When traumatic, subarachnoid hemorrhage occurs most commonly over the cerebral convexities or adjacent to otherwise injured brain (i.e. adjacent to a cerebral contusion). If there is a large amount of subarachnoid hemorrhage, particularly in the basilar cisterns, the physician should consider whether a ruptured aneurysm led to the subsequent trauma. Cerebral angiography may be needed for further evaluation. On CT, subarachnoid hemorrhage appears as focal high density in sulci and fissures or linear hyperdensity in the cerebral sulci. Again, the most common location of posttraumatic subarachnoid hemorrhage is over the cerebral convexity. This may be the only indication of cerebral injury. High density blood (arrowheads) fills the sulci over the right cerebral convexity in this subarachnoid hemorrhage. Deceleration and acceleration or rotational forces that tear bridging veins can cause an acute subdural hematoma. The blood collects in the space between the arachnoid matter and the dura matter. The hematoma on CT has the following characteristics: - Crescent shaped - Hyperdense, may contain hypodense foci due to serum, CSF or active bleeding - Does not cross dural reflections High density, crescent shaped hematoma (arrowheads) overlying the right cerebral hemisphere. Note the shift of the normally midline septum pellucidum due to the mass effect (arrow) The hypodense region (arrow) within the high density hematoma (arrowheads) may indicate active bleeding. Subacute SDH may be difficult to visualize by CT because as the hemorrhage is reabsorbed it becomes isodense to normal gray matter. A subacute SDH should be suspected when you identify shift of midline structures without an obvious mass. Giving contrast may help in difficult cases because the interface between the hematoma and the adjacent brain usually becomes more obvious due to enhancement of the dura and adjacent vascular structures. Some of the notable characteristics of subacute SDH are: - Compressed lateral ventricle - Effaced sulci - White matter "buckling" - Thick cortical "mantle" Subacute subdural hematoma (arrowheads). Note the compression of gray and white matter in the left hemisphere due to the mass effect. Chronic SDH becomes low density as the hemorrhage is further reabsorbed. It is usually uniformly low density but may be loculated. Rebleeding often occurs and causes mixed density and fluid levels. Crescent shaped chronic subdural hematoma (arrowheads). Notice the low attenuation due to reabsorbtion of the hemorrhage over time. This chronic subdural hematoma (arrowheads) shows the septations and loculations that often occur over time. An epidural hematoma is usually associated with a skull fracture. It often occurs when an impact fractures the calvarium. The fractured bone lacerates a dural artery or a venous sinus. The blood from the ruptured vessel collects between the skull and dura. On CT, the hematoma forms a hyperdense biconvex mass. It is usually uniformly high density but may contain hypodense foci due to active bleeding. Since an epidural hematoma is extradural it can cross the dural reflections unlike a subdural hematoma. However an epidural hematoma usually does not cross suture lines where the dura tightly adheres to the adjacent skull. Biconvex (lenticellular) epidural hematoma (arrowheads), deep to the parietal skull fracture (arrow). Diffuse axonal injury is often referred to as "shear injury". It is the most common cause of significant morbidity in CNS trauma. Fifty percent of all primary intra-axial injuries are diffuse axonal injuries. Acceleration, deceleration and rotational forces cause portions of the brain with different densities to move relative to each other resulting in the deformation and tearing of axons. Immediate loss of consciousness is typical of these injuries. The CT of a patient with diffuse axonal injury may be normal despite the patient's presentation with a profound neurological deficit. With CT, diffuse axonal injury may appear as ill- defined areas of high density or hemorrhage in characteristic locations. The injury occurs in a sequential pattern of locations based on the severity of the trauma. The following list of diffuse axonal injury locations is ordered with the most likely location listed first followed by successively less likely locations: - Subcortical white matter - Posterior limb internal capsule - Corpus callosum - Dorsolateral midbrain Hemorrhage of the posterior limb of the internal capsule (arrow) and hemorrhage of the thalamus (arrowhead). Hemorrhage in the corpus callosum (arrow). Cerebral contusions are the most common primary intra-axial injury. They often occur when the brain impacts an osseous ridge or a dural fold. The foci of punctate hemorrhage or edema are located along gyral crests. The following are common locations: - Temporal lobe - anterior tip, inferior surface, sylvian region - Frontal lobe - anterior pole, inferior surface - Dorsolateral midbrain - Inferior cerebellum On CT, cerebral contusion appears as an ill-defined hypodense area mixed with foci of hemorrhage. Adjacent subarachnoid hemorrhage is common. After hours, hemorrhagic transformation or coalescence of petechial hemorrhages into a rounded hematoma is common. Multiple foci of high density corresponding to hemorrhage (arrows) in an area of low density (arrowheads) in the left frontal lobe due to cerebral contusion. Traumatic intraventricular hemorrhage is associated with diffuse axonal injury, deep gray matter injury, and brainstem contusion. An isolated intraventricular hemorrhage may be due to rupture of subependymal veins. Intraventricular hemorrhage (arrow) found in a trauma patient. Note the subarachnoid hemorrhage in the sulci in the subarachnoid space (arrowheads). Epidemiology Stroke is a clinical term for sudden, focal neurological deficit. The following are statistics from the American Heart Association and the National Stroke Association. Stroke accounts for one out of every 15 deaths and it is the third leading cause of death in the United States. On average someone suffers from a stroke every 53 seconds and every 3.3 minutes someone dies of a stroke. Each year about 600,000 people suffer from a new or recurrent stroke. About 500,000 of these are new. The incidence and prevalence of stroke are equal among women and men, however women account for 61% of the deaths. In a given year 28% of those who suffer from a stroke are under 65. There are approximately 4.5 million stroke survivors alive today. Stroke is the leading cause of serious, long-term disability in the United States. Fifty to seventy percent of survivors regain functional independence, but fifteen to thirty percent are permanently disabled. Three months after stroke, 20% of survivors require institutional care. The estimated direct and indirect annual cost of stroke in the United States is 43.4 billion dollars. Strokes are classified into two major types - hemorrhagic and ischemic. Hemorrhagic strokes are due to rupture of a cerebral blood vessel that causes bleeding into or around the brain. Hemorrhagic strokes account for 16% of all strokes. An ischemic stroke is caused by blockage of blood flow in a major cerebral blood vessel, usually due to a blood clot. Ischemic strokes account for about 84% of all strokes. Ischemic strokes are further subdivided based on their etiology into several different categories including thrombotic strokes, embolic strokes, lacunar strokes and hypoperfusion infarctions. Hemorrhagic strokes account for 16% of all strokes. There are two major categories of hemorrhagic stroke. Intracerebral hemorrhage is the most common, accounting for 10% of all strokes. Subarachnoid hemorrhage, due to rupture of a cerebral aneurysm, accounts for 6% of strokes overall. Hemorrhage in the cerebellum