Computed tomography (CT) permits the imaging of thin slices of tissue in a wide variety of planes. Most CT is done in the axial plane, and many CT scans also provide coronal views; sagittal slices are less commonly used. During CT scanning, the x-ray source and detectors move around the desired region of the body while the patient lies on a table. Modern generations of CT scanners use a spiral motion of the gantry to produce the x-ray data that are then reconstructed by computer. The operator selects the region of the anatomy and the thickness of the slices of tissue to be scanned, along with the kilovolt and milliampere settings. Slice thickness is usually 10 mm through the body and brain and 5 mm through the head and neck, unless threedimensional reconstruction is anticipated. In such cases, the slice thickness is 1.0 to 1.5 mm in order to provide adequate data.
CT scans are usually evaluated on computer monitors although they may also be printed on radiographic film. The contrast and brightness of the image may be adjusted as necessary although the images are usually viewed in two modes: bone windowing and soft-tissue windowing. In bone windowing, the contrast is set so that osseous structures are visible in maximal detail. With soft-tissue windowing, the bone looks uniformly white, but various types of soft tissues can be distinguished (Figure 3-8). Viewing the images in these different formats does not require rescanning the patient.
There are many advantages to CT of the head and neck region compared to imaging this area by plain films or conventional tomography. Thin slices of tissue can be viewed in multiple planes without superimposition by adjacent structures or the blurring out of other layers. Fine detail of osseous and other calcified structures can be obtained. Various soft tissues can be differentiated by their attenuation of the x-ray beam. Fascial planes between muscle groups can be identified, as can lymph nodes and blood vessels. Three-dimensional images that may make it easier to visualize certain abnormalities can be produced, and some software programs will color certain structures (such as tumors) to simplify visualization of the lesion. Three-dimensional models can also be milled out of plastic, based on data from CT scans. Cross-sectional images can also be reconstructed from axial CT scans, producing, for example, views of the mandible for use in preoperative assessments for implant placement.
The major disadvantages of CT relate to the relatively high cost and high radiation dose of this examination compared to those of plain-film radiography. In addition, resolution of fine structures of the head and neck may be less than optimal although the newly developed super-high-resolution orthoCT described above is attempting to address these problems. Careful attention must also be paid to the imaging plane through the jaws if the patient has metallic restorations; these restorations produce streak artifacts that may obscure portions of the anatomy (Figure 3-9).
CT is typically used in dentistry to evaluate (1) the extent of lesions suspected or detected with other radiographic techniques, (2) the degree of maxillofacial involvement in cases of trauma, (3) the integrity and condition of the paranasal sinuses, and (4) the quality and quantity of bone in proposed dental
implant sites, particularly when there are multiple sites or when there has been bone grafting. CT is rarely indicated for evaluation of the TMJ since the osseous structures can be visualized adequately with less expensive techniques such as conventional tomography or panoramic radiography,and disk displacement and other joint soft-tissue information can be better obtained with magnetic resonance imaging. CT may be of value in complex TMJ situations, such as in cases of suspected anky
osis or severe joint destruction or when there is a history of polytetrafluoroethylene or silicon-sheeting TMJ implants.