Computed tomography (CT), originally known as computed axial tomography (CAT) is a medical imaging method employing tomography where digital geometry processing is used to generate a three-dimensional image of the internals of an object from a large series of two-dimensional X-ray images taken around a single axis of rotation. The word "tomography" is derived from the Greek tomos (slice) and graphia (describing). CT produces a series of axial images, which can be manipulated, through a process known as windowing, in order to recreate the image in a different plane.

X-ray slice data is generated using an X-ray source that rotates around the object; X-ray sensors are positioned on the opposite side of the circle from the X-ray source. Many data scans are progressively taken as the object is gradually passed through the gantry. They are combined together by the mathematical procedure known as tomographic reconstruction.

Since its introduction in the 1970s, CT has become an important tool in medical imaging to supplement X-rays and medical ultrasonography. Although it is still quite expensive, it is the gold standard in the diagnosis of a large number of different disease entities.

Diagnosis of cerebrovascular accidents and intracranial hemorrhage is the most frequent reason for a "head CT" or "CT brain". Scanning is done without intravenous contrast agents (contrast may resemble a bleed). CT generally does not exclude infarct in the acute stage, but is useful to exclude a bleed (so anticoagulant medication can be commenced safely).

For detection of tumors, CT scanning with IV contrast is occasionally used but is less sensitive than magnetic resonance imaging (MRI). CT can also be used to detect increases in intracranial pressure, e.g. before lumbar puncture or to evaluate the functioning of a ventriculoperitoneal shunt. CT is also useful in the setting of trauma for evaluating facial and skull fractures.

In the head/neck/mouth area, CT scanning is used for surgical planning for craniofacial and dentofacial deformities, evaluation of cysts and some tumors of the jaws/sinuses/nasal cavity/orbits, and for planning of dental implant reconstruction.

Chest CT is excellent for detecting both acute and chronic changes in the lung parenchyma. For detection of airspace disease (such as pneumonia) or cancer, ordinary non-contrast scans are adequate. For evaluation of chronic interstitial processes (emphysema, fibrosis, and so forth), thin sections with high spatial frequency reconstructions are used. For evaluation of the mediastinum and hilar regions for lymphadenopathy, IV contrast is administered.

CT angiography of the chest (CTPA) is also becoming the primary method for detecting pulmonary embolism (PE) and aortic dissection, and requires accurately timed rapid injections of contrast and high-speed helical scanners. CT is the standard method of evaluating abnormalities seen on chest X-ray and of following findings of uncertain acute significance.

With the advent of subsecond rotation combined with multi-slice CT (up to 64 slices), high resolution and high speed can be obtained at the same time, allowing excellent imaging of the coronary arteries. Images with high temporal resolution are formed by updating a proportion of the data set used for image reconstruction as it is scanned. In this way individual frames in a cardiac CT investigation are significantly shorter than the shortest tube rotation time. It is uncertain whether this modality will replace the invasive coronary catheterization.

Dual Source CT scanners, introduced in 2005, allow higher temporal resolution when acquiring images of the heart, allowing a greater number of patients to be scanned.

Many abdominal disease processes require CT for proper diagnosis. The most common uses include diagnosis of renal/urinary stones, appendicitis, pancreatitis, diverticulitis, abdominal aortic aneurysm, and bowel obstruction. CT is also the first line for detecting solid organ injury after trauma. Oral and/or rectal contrast is usually administered (more often iodinated contrast than barium due to the tendency of barium to cause imaging artifacts that limit evaluation of abdominal structures).

CT has limited application in the evaluation of the pelvis. For the female pelvis in particular, ultrasound is the imaging modality of choice. Nevertheless, it may be part of abdominal scanning (e.g. for tumors), and has uses in assessing fractures.

CT is also used in osteoporosis studies and research along side DXA scanning. Both CT and DXA can be used to asses bone mineral density (BMD) which is used to indicate bone strength, however CT results do not correlate exactly with DXA (the gold standard of BMD measurement), is far more expensive, and subjects patients to much higher levels of ionizing radiation, so it is used infrequently.

CT is often used to image complex fractures, especially ones around joints, because of the ability to reconstruct the area of interest in multiple planes.