Tomography is an X-ray examination technique in which only structures in a particular plane produce clearly focused images. Whole-body computed tomography was introduced in 1977 and has already made a major impact in the investigation and management of medical and surgical disease. The technique is particularly valuable where a mass distorts the contour of an organ (e.g. a pancreatic tumour – see PANCREAS, DISORDERS OF) or where a lesion has a density different from that of surrounding tissue (e.g. a metastasis in the LIVER).

Computed tomography can distinguish soft tissues from cysts or fat, but in general soft-tissue masses have similar appearances, so that distinguishing an inflammatory mass from a malignant process may be impossible. The technique is particularly useful in patients with suspected malignancy; it can also define the extent of the cancer by detecting enlarged lymph nodes, indicating lymphatic spread. The main indications for computed tomography of the body are: mediastinal masses, suspected pulmonary metastases, adrenal disease, pancreatic masses, retroperitoneal lymph nodes, intra-abdominal abscesses, orbital tumours and the staging of cancer as a guide to effective treatment. Where MRI offers an equally good or better image, it is preferred since CT scanning inevitably involves the patient receiving a dose of irradiation.