HISTORY

Computerized axial tomography was developed in the late 1960s and early 1970s, independently, by Hounsfield and Cormack. CT was first clinically applied in trauma situations in the mid-1970s. Initially, it was only possible to image the head and brain. After a period in which CAT scanners were not common, by the mid-1980s these devices were available in hospitals throughout the United States. This availability, as well as advances in CT technology, spurred the development of new applications and techniques.

Computed tomography is performed by passing rotating fan beams of x-rays through the patient in an axial plane. In the early scanners, the x-ray tube rotated around the patient to obtain a single image or “slice.” The table then moved for the next slice. This process was quite time consuming. In modern scanners, the tube rotates continuously while the table is in motion, yielding a spiral or helical scan. The more recent scanners also have multiple rows of detectors that can obtain as many as 64 slices simultaneously. This makes it possible to scan the entire body from head to pelvis in only a few minutes.

The continuous data from the spiral scanning are stored in computer memory, which allows the data to be manipulated in various ways. If, for example, the chest and abdomen have been scanned, detailed reconstructions of images of the thoracic and lumbar spine can be obtained without exposing the patient to additional radiation. The computer can also generate three-dimensional (3D) rotating images, which can be useful in the interpretation of CT angiography and complex fractures.

Similar to plain radiographs, there are four basic densities on a CT image. Air is black; fat is dark gray; soft tissue is light gray; and bone/calcium and contrast agents are white. The x-ray absorption of a specific tissue can be measured in Hounsfield units (HU). The density of water is zero. Blood measures 40–70 HU. Urine, ascites, and bowel content measure close to water, 0–30 HU. Various “windows” for bone, brain, lung, abdomen, and so on, are used to best display tissues of various densities.

COMPUTED TOMOGRAPHY OF HEAD/BRAIN (CRANIUM)

Computed tomography of the head is indicated in patients with clinical evidence of traumatic brain injury, including loss of consciousness, amnesia, depressed level of consciousness, a mental state that is difficult to evaluate due to recreational or therapeutic drug administration, hemotympanum or cerebrospinal fluid leak, suspected skull fracture, severe headache, persistent nausea and vomiting, or post-traumatic seizures.

Scanning protocols may vary slightly from institution to institution. In adults the scan is usually performed with 5-mm cuts from the skull base to the vertex. In infants, the scan is commonly performed with finer cuts starting at the second cervical vertebra.

The study is performed without contrast. Acute hemorrhage appears white on the scan (Figure 1) except in very anemic patients, where it may appear isodense with brain. Active intracranial bleeding may also appear as gray swirls within the white clot, indicating hyperacute hemorrhage.

Figure 1 Computed tomography scan demonstrating a classic lenticular-shaped, acute epidural hematoma.

Computed tomography is very sensitive for intracranial hemorrhage, edema, and mass effect. As CT technology has advanced, detection of smaller lesions has become possible, leading to controversy regarding the significance of these lesions, and the indications for CT in mild head injury.¹ CT of the brain may be normal and not correlate well with the clinical picture in patients with diffuse axonal injury.

Computed tomography of the head may also detect fractures of the upper facial bones and orbits; however, dedicated CT of the face is required to provide adequate resolution to plan operative intervention. High-resolution CT of the base of the skull may be required to evaluate basilar skull fractures.

COMPUTED TOMOGRAPHY OF FACE AND ORBITS

Computed tomography is the best method to study facial fractures. Patients with clinical suspicion of fractures due to facial swelling, deformities, visual changes, or bony tenderness or instability on examination should undergo CT of the facial bones, concomitant with head scan (if necessary). If facial fractures or fluid in the sinuses are seen on CT of head, a formal CT of the face should be performed, as the resolution of head CT is insufficient.