Working Principle of CT

In CT, the X-ray tube revolves around the craniocaudal axis of the patient. A beam of X-rays passes through the body and hits a ring of detectors. The incoming radiation is continuously registered, and the signal is digitized and fed into a data matrix, taking into account the varying beam angulations. The data matrix can then be transformed into an output image ( Fig. 1.1 ). The result is usually displayed in “slices” cross-sectionally. Different tissues attenuate radiation to varying degrees, allowing for the differentiation of tissue subtypes ( Table 1.2 ). This absorption is measured in Hounsfield units. When one views an image, two values are displayed with the image: Window and Level. The Window refers to the range of Hounsfield units displayed across the spectrum from black (low) through the grayscale to white (high). Level refers to the Hounsfield unit on which middle gray is centered. By adjusting the window and level, certain features of the image can be better assessed or emphasized.

Recent advances have improved the quality of CT imaging. Multidetector scanners have several rows of photoreceptors, enabling the simultaneous acquisition of several slices. Helical techniques allow the patient table to move continuously through the scanner instead of stopping for each slice. These advances have significantly decreased scan times and radiation exposure while improving spatial resolution. Improved resolution and computing power enable cross-sectional images to be reformatted into any plane (axial, coronal, sagittal), as well as three-dimensional anatomy or subtraction images to be displayed when necessary or helpful (e.g., three-dimensional reconstruction of airways). Newer in-office flat-plate cone-beam scanners can rapidly acquire 1-mm slice thickness images of the sinuses and temporal bone with very low radiation exposure.

Contrast Media

Intravenous contrast media are used in CT to visualize vessels and the vascularization of different organ systems. This allows better differentiation of vessels versus other structures. Some tissues also take up greater amounts of contrast natively, as well as in certain disease states (e.g., infection, neoplasm, edema). Luminal contrast material containing iodine or barium can also be used in some structures (e.g., gastrointestinal tract) to clarify anatomy.

Computer-Assisted Surgical Navigation

CT scanning data can be utilized for computer-assisted surgical navigation. There are several systems in use. The axial CT image data, acquired at 1-mm slice thickness or less, are loaded onto the image guidance system in the operating room. The system utilizes the CT data and compares them to the patient′s facial features or landmarks via an infrared camera or electromagnetic field disturbance to determine point location in three-dimensional space. Various surgical instruments can be registered and detected. The location of an instrument tip is then displayed on the previous CT images in three planes. This is most often used in sinus and skull base surgery.