CT, also called computed axial tomography (CAT), affords the possibility of inspecting cross sections of the skull, brain, ventricles, cisterns, large vessels, falx, and tentorium. It has become a primary tool for demonstrating the presence of abnormal calcifications, brain edema, hydrocephalus, many types of tumors and cysts, hemorrhages, large aneurysms, vascular malformations, and other disorders.
CT scanning is noninvasive and fast. Although it has high sensitivity, its specificity is relatively limited. Correlation with the clinical history and physical examination is an absolute requirement. In the case of a subarachnoid hemorrhage, for example, although a CT scan may quickly localize the areas containing blood (Fig 12–21), additional CT images (Fig 12–22), magnetic resonance imaging, or angiography is often required to determine whether the cause was an aneurysm or an arteriovenous malformation.
The CT scanning apparatus rotates a narrow x-ray beam around the head. The quantity of x-ray absorbed in small volumes (voxels [volume elements, or units]) of brain, measuring approximately 0.5 mm2 × 1.5 mm or more in length, is computed. The amount of x-ray absorbed in any slice of the head can be thus determined and depicted in various ways as pixels (picture elements) in a matrix. In most cases, absorption is proportional to the density of the tissue. A converter translates the numeric value of each pixel to a gray scale. Black-and-white pictures of head slices are then displayed, with black representing low-density structures and white representing high-density structures. The thickness of the slices can vary, from 1.5 mm to 1 cm. The gray scale can also vary; although a setting at which brain tissue is distinguished best is commonly used, in some cases bone, fat, or air need to be defined in great detail.
A series of 10 to 20 scans, each reconstructing a slice of brain, is usually required for a complete study. The plane of these sections is the orbitomeatal plane, which is parallel to both Reid's base plane and the intercommissural line used in stereotactic neurosurgery (Fig 22–13). Usually, a "scout view" similar to a lateral skull roentgenogram is taken with a CT scanner to align the planes of sections (Fig 22–14). With the modern technology now available, each scan takes only a few seconds. Examples of normal and abnormal CT scans are shown in Figures 22–15 and 22–16.
Schematic image of the zero horizontal and coronal planes. The line between anterior and posterior commissures parallels Reid's base plane.
Lateral "scout view" used in CT procedure. Superimposed lines represent the levels of the images (sections). Line 1 is at the level of the foramen magnum; line 4 is at the level of the infraorbitomeatal plane.
CT image, with contrast enhancement, of a horizontal section at the level of the thalamus. Normal image. Compare with Figure 13–5.
Representative examples of CT images. (Courtesy of GP Ballweg.)
CT scanning of the posterior fossa may provide only limited information because of the many artifacts caused by dense bone. Images reformed by a computer from a series of thin sections allow visualization in any desired plane, for example, midsagittal (see Fig 6–15) or coronal. Coronal sections are often extremely useful for structures lying at the base of the brain, in the high convexity area, or close to the incisura. Detailed examination of orbital contents requires planes at right angles to the orbital axis.
Tissue density can change pathologically. Areas of hyperemia or freshly clotted hemorrhage appear more dense (Fig 12–19); edematous tissue appears less dense (Fig 12–14).