In this procedure, x-radiation is attenuated as it passes successively through the scalp, skull, CSF, cerebral gray and white matter, and blood vessels. The intensity of the exiting radiation relative to the incident radiation is measured, the data are integrated, and two-dimensional images are reconstructed by computer. This major achievement in methodology, attributed to Hounsfield and others, permitted the technologic advance from plain radiographs of the skull to reconstructed images of the cranium and its contents in any plane. The differing densities of bone, CSF, blood, and gray and white matter are distinguishable in the resulting picture with great clarity. One can see and measure the sizes of hemorrhage, infarction, contusion, edematous brain, abscess, tumor, and also determine the shape and position of the ventricles and midline structures. The radiation exposure is only modestly greater than that from plain skull films. The machinery can be manipulated to reduce radiation exposure where this limitation is desirable, for example, in children.
As illustrated in Fig. 2-1, in transverse (axial) section of the brain, one sees the cortex and underlying subcortical white matter, the caudate and lenticular nuclei and the internal capsules and thalami. The position and width of all the major sulci and fissures can be measured, and the optic nerves and medial and lateral rectus muscles stand out clearly in the posterior parts of the orbit. The brainstem, cerebellum, and spinal cord are easily visible in the scan at appropriate levels. The scans are also useful in imaging parts of the body that surround peripheral nerves and plexuses, thereby demonstrating tumors, inflammatory lesions, and hematomas that involve these nerves. Intravenous administration of radiopaque material (contrast) can be used with CT to visualize regions where the blood–brain barrier has been disrupted from tumors, demyelination, and infection.
In imaging of the head, CT has a number of advantages over MRI, the most important being safety when metal may be present in the body, shorter examination time, and the clarity of blood from the moment of bleeding. Other appealing aspects are its broader availability, lower cost, larger aperture of the machine that reduces patient claustrophobia, and equivalent or superior visualization of calcium, fat, and bone, particularly of the skull base and vertebrae (see Fig. 2-1D). If constant monitoring and use of life support equipment is required during the imaging procedure, it is accomplished more readily by CT than by MRI. Advances in CT technology have greatly increased the speed of the scanning procedure and have also made possible the visualization, with great clarity, of the cerebral vasculature (CT angiography; see further on).
CT also demonstrates the bony structures of the vertebral column in greater detail than is available with conventional x-ray. Herniated lumbar and cervical discs, cervical spondylotic bars and bony spurs encroaching on the spinal cord or roots, and spinal cord tumors can be visualized with clarity. MRI provides even sharper visualization of the spinal canal and its contents as well as the vertebrae and intervertebral discs as discussed further on.