2.1 Computed tomography (CT) scans

A conventional X-ray of a bone fracture is a two-dimensional (2D) image, taken from the front of the patient by a single camera, and is sometimes known as a planar X-ray.

However, a computed tomography (CT) scan produces many 2D images of sections throughout the body using detectors arranged in a circular field, which can then be computer processed to give a three-dimensional (3D) reconstruction of the body.

With carefully controlled conditions, even changes in soft tissues indicating tumours can be picked up and located by this method. The resolution can be as good as 1 mm or less.

The following video shows a CT scan being done in a hospital for a patient with a suspected injury to his spine.

So, how is a CT image produced?

The X-ray source is rotated around the patient and the intensity recorded on the opposite side of the patient. Using data from a large number of angles, a computer generates a two-dimensional map of the tissues in a slice of the body.

Note that there are three directions in which slices through the brain (or the body in general) are typically reported in imaging, as illustrated in Figure 3: axial, sagittal and coronal. You’ll meet these terms again when looking at MRI.