This technique allows direct imaging of living tissue by exposure to x-rays, but otherwise in a completely non-invasive way. After the first application to human in 1973, G.N. Hounsfield and A.M. Cormack were honoured with the Nobel prize in 1979. The absorption of x-rays by the body of the patient is measured by detectors and calculated in a slice-wise manner resulting in black-and white images of the different x-ray densities of the tissues. During the examination, the patient is lying on a special examination table and is slowly pushed through the wide-open gantry of the scanner. Numerous transversal thin “cuts” of the region of interest are acquired in this way which can be reformatted later in other directions. Thus, a computed tomogram usually consists of a number of slices but 3D images can be calculated as well. In contrast to the conventional x-ray, bones and soft tissue organs can be presented without any superposition. The usually small differences in radiodensity between different tissues can be enhanced by injection of contrast medium.
Because CT can be performed fast and easy, it is usually the first imaging modality in the acute patient, e. g. cerebral bleedings, traumatic injuries or infarctions. Blood supply to the brain tissue (CBF) can be imaged as well as occlusion of cerebral arteries and veins. In addition, a biopsy can be carried out by advancing a puncture needle to the abnormal tissue under CT control. Histological examinations of the biopsy may clarify the differential diagnosis between various kinds of tumours and infections along the skull, neck and spine, and major surgery is no longer necessary. Advancement of puncture needles under CT control is also used to find the right position for injection of local analgetics and cortison for relief of facet joint pain caused or to stabilize fractured vertebrae by injection of bone cement (vertebroplasty).