CT stands for computer tomography and like conventional x-rays uses x-ray beams. The CT scan is a test where the interior of the human body is imaged in three dimensions. The x-ray beam passes through the human body in a thin axial slice, which is repeated in various directions (fig. 4).

General technique of a CT scanner.

Figure 4. General technique of a CT scanner.

The detectors on the opposite side measure the radiation transmission through the patient. This enables the computer to determine the degree of absorption in very small volume elements, the so-called voxels. The size of the voxel depends, among other things, on the matrix size(number of pixels) and slice thickness. The information on the voxels is then converted into ‘CT numbers’, better known as Hounsfield unit (HU). More about this later.

When interpreting a CT scan, you should picture yourself standing at the patient’s feet looking at his or her head; top is the side of the abdomen, bottom the side of the back (examination table) and left & right have been inverted.

Nowadays 3rd-generation CT scanners are mostly used (1st, 2nd and 4th-generation CT scanners will not be discussed in this module). In 3rd-generation CT scanners, the x-ray tube and detectors synchronously rotate around the patient. The detector row covers the full width of the fan-shaped x-ray beam (fig. 5).

Third-generation CT scanner.

Figure 5. Third-generation CT scanner.