Computed tomography (CT), also known as computed axial tomography or CAT scanning, is a medical imaging method that uses x-rays to generate a view of "slices" through the body. CT produces a volume of data which can be manipulated, through a process known as windowing, in order to demonstrate various structures based on their ability to block the X-ray beam.
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. It has more recently begun to also be used for preventive medicine or screening for disease, for example CT colonography for patients with a high risk of colon cancer. Although a number of institutions offer full-body scans for the general population, this practice remains controversial due to its lack of proven benefit, cost, radiation exposure, and the risk of finding 'incidental' abnormalities that may trigger additional investigations.
Diagnosis of cerebrovascular accidents and intracranial hemorrhage is the most frequent reason for a "head CT" or "CT brain". Scanning is done with or without intravenous contrast agents. CT generally does not exclude infarct in the acute stage of a stroke, but is useful to exclude a bleed (so anticoagulant medication can be commenced safely).
For detection of tumours, 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 tumours of the jaws/paranasal sinuses/nasal cavity/orbits, diagnosis of the causes of chronic sinusitis, and for planning of dental implant reconstruction.
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-slice), high resolution and high speed can be obtained at the same time, allowing excellent imaging of the coronary arteries. Images with a 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.
Cardiac MSCT carries very real risks since it exposes the subject to the equivalent of 500 chest X-rays in terms of radiation. The relationship of radiation exposure to increased risk in breast cancer has yet to be definitively explored.
The positive predictive value is approximately 82% while the negative predictive value is in the range of 93%. Sensitivity is about 81% and the specificity is about 94%. The real benefit in the test is the high negative predictive value. Thus, when the coronary arteries are free of disease by CT, patients can then be worked up for other causes of chest symptoms.
Much of the software is based on data findings from Caucasian study groups and as such the assumptions made may also not be totally true for all other populations.
Dual Source CT scanners, introduced in 2005, allow higher temporal resolution so reduce motion blurring at high heart rates, and potentially require a shorter breath-hold time. This is particularly useful for ill patients who have difficult holding their breath, or who are unable to take heart-rate lowering medication.
The speed advantages of 64-slice MSCT have rapidly established it as the minimum standard for newly installed CT scanners intended for cardiac scanning. Manufacturers are now actively developing 256-slice, true 'volumetric' scanners, primarily for their improved cardiac scanning performance.
Abdominal and Pelvic CT
CT is a sensitive method for diagnosis of abdominal diseases. It is used frequently to determine stage of cancer and to follow progress. It is also a useful test to investigate acute abdominal pain. Renal/urinary stones, appendicitis, pancreatitis, diverticulitis, abdominal aortic aneurysm, and bowel obstruction are conditions that are readily diagnosed and assessed with CT. CT is also the first line for detecting solid organ injury after trauma.
Oral and/or rectal contrast may be used depending on the indications for the scan. A dilute (2% w/v) suspension of barium sulfate is most commonly used. The concentrated barium sulfate preparations used for fluoroscopy e.g. barium enema are too dense and cause severe artifacts on CT. Iodinated contrast agents may be used if barium is contraindicated (e.g. suspicion of bowel injury). Other agents may be required to optimize the imaging of specific organs: e.g. rectally administered gas (air or carbon dioxide) for a colon study, or oral water for a stomach study.
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 tumours), 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 assess 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). CT is far more expensive, and subjects patients to much higher levels of ionizing radiation, so it is used infrequently.
CT of Extremities
CT is often used to image complex fractures, especially ones around joints, because of its ability to reconstruct the area of interest in multiple planes.