Patient Stabilization

Depending on the unit, CBCT examinations are made with the patient sitting, standing, or supine. Supine units are physically larger, have a greater physical footprint, and may not be accessible for patients with some physical disabilities. Standing units may not be able to be adjusted to a height low enough to accommodate wheelchair-bound patients. Although seated units are the most comfortable, they may not allow scanning of physically disabled or wheelchair-bound patients. With all systems, immobilization of the patient’s head is more important than patient positioning because any head movement degrades the final image. Immobilization of the head is accomplished by using some combination of a chin cup, bite fork, or other head-restraint mechanism.

X-Ray Generator

During the scan rotation, each projection image set is made by sequential single-image capture of the remnant x-ray beam by the detector. X-ray generation may be continuous or pulsed to coincide with the detector activation. It is preferable to pulse the x-ray beam to coincide with the detector sampling; this means that actual exposure time is up to 50% less than scanning time. This technique reduces patient radiation dose considerably.

The ALARA (As Low As Reasonably Achievable) principle of dose optimization necessitates that CBCT exposure factors should be adjusted on the basis of patient size. This adjustment can be achieved by appropriate selection of tube current (milliamperes [mA]), tube voltage (kilovolt peak [kVp]), or both. In some cases, time also can be adjusted with faster scans producing images with fewer basis images (see later section on scan factors). Although both kVp and mA may be fixed on some units, they are automatically modulated in near real time on other units by a feedback mechanism detecting the intensity of the transmitted beam, a process known as automatic exposure control. On other units, exposure settings are automatically determined by the initial scout exposure. This feature is highly desirable because it is operator independent. The variation in exposure parameters together with the presence of pulsed x-ray beam and size of the image field are the primary determinants of patient exposure.

Scan Volume

Scan Factors

The number of images constituting the projection data throughout the scan is determined by the detector frame rate (number of images acquired per second), the completeness of the trajectory arc (180 to 360 degrees), and the rotation speed of the source and detector. The number of basis images making up a single scan set may be fixed or variable. Higher frame rates have both desirable and undesirable effects. Higher frame rates increase the signal-to-noise ratio, producing images with less noise and reducing metallic artifacts. However, a higher frame rate is associated with a longer scan time and higher patient dose. In addition, more data are obtained, and primary reconstruction time is increased.

CBCT imaging systems often use a complete circular trajectory or a scan arc to acquire adequate projection data for volumetric software reconstruction. However, increasingly, CBCT units are based on panoramic platforms, which have scan arcs less than 360 degrees. Most CBCT units have fixed scan arcs; however, some provide a choice of manual controls to reduce the scan arc further. A limited scan arc potentially reduces the scan time and patient radiation dose and is mechanically easier to perform. However, image/>