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Computed Tomography

By

Mehmet Kocak

, MD, Rush University Medical Center

Last full review/revision Apr 2021| Content last modified Apr 2021
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In CT, an x-ray source and x-ray detector housed in a doughnut-shaped assembly move circularly around a patient who lies on a motorized table that is moved through the machine. Usually, multidetector scanners with 4 to 64 or more rows of detectors are used because more detectors allow quicker scanning and higher-resolution images, which are particularly important for imaging the heart and abdominal organs.

Data from the detectors essentially represent a series of x-ray images taken from multiple angles all around the patient. The images are not viewed directly but are sent to a computer, which quickly reconstructs them into 2-dimensional images (tomograms) representing a slice of the body in any plane desired. Data can also be used to construct detailed 3-dimensional images.

For some CT scans, the table moves incrementally and stops when each scan (slice) is taken. For other CT scans, the table moves continuously during scanning; because the patient is moving in a straight line and the detectors are moving in a circle, the series of images appear to be taken in a spiral fashion around the patient—hence the term helical (spiral) CT.

Uses of CT

CT provides better differentiation between various soft-tissue densities than do x-rays. Because CT provides so much more information, it is preferred to conventional x-rays for imaging most intracranial, head and neck, spinal, intrathoracic, and intra-abdominal structures. Three-dimensional images of lesions can help surgeons plan surgery.

Noncontrast CT is used

  • To detect acute hemorrhage in the brain, urinary calculi, and lung nodules

  • To characterize bone fractures and other skeletal abnormalities

IV contrast is used

  • To improve imaging of tumors, infection, inflammation, and trauma in soft tissues

  • To assess the vascular system, as when pulmonary embolism, aortic aneurysm, or aortic dissection is suspected

Oral or occasionally rectal contrast is used for abdominal imaging; sometimes gas is used to distend the lower gastrointestinal (GI) tract and make it visible. Contrast in the GI tract helps distinguish the GI tract from surrounding structures. Standard oral contrast is barium-based, but low-osmolar iodinated contrast should be used when intestinal perforation is suspected.

Variations of CT

Virtual colonoscopy and CT enterography

For virtual (CT) colonoscopy (CT colonography), oral contrast is given, and air is introduced into the rectum via a flexible, thin-diameter rubber catheter; then thin-section CT of the entire colon is done. CT colonoscopy produces high-resolution 3-dimensional images of the colon that closely simulate the detail and appearance of optical colonoscopy. This technique can show colon polyps and colon mucosal lesions as small as 5 mm. It is an alternative to conventional colonoscopy. Virtual colonoscopy is more comfortable than conventional colonoscopy and does not require conscious sedation. It provides clearer, more detailed images than a conventional lower gastrointestinal (GI) series and can show extrinsic soft-tissue masses. The entire colon is visualized during virtual colonoscopy; in contrast, in about 1 in 10 patients, conventional colonoscopy does not allow the right colon to be evaluated completely.

The main disadvantages of virtual colonoscopy include

  • The inability to biopsy the polyps at the time of examination

  • Radiation exposure

CT enterography is similar, but it provides images of the stomach and entire small intestine. A large volume of low-density oral contrast agent (eg, 1300 to 2100 mL of 0.1% barium sulfate) is given to distend the entire small intestine; use of neutral or low-density contrast helps show detail of intestinal mucosa that might be obscured by use of contrast that is more radiopaque.

Thus, the unique advantage of CT enterography is in

CT enterography often involves using IV contrast. Thin-slice high-resolution CT images of the entire abdomen and pelvis are obtained. These images are reconstructed in multiple anatomic planes, forming 3-dimensional reconstructions.

CT enterography can also be used to detect and evaluate disorders other than inflammatory bowel disease, including the following:

  • Lesions obstructing the small intestine

  • Tumors

  • Abscesses

  • Fistulas

  • Bleeding sources

CT IV pyelography (CT IVP) or urography

IV contrast is injected to produce detailed images of the kidneys, ureters, and bladder. IV contrast concentrates in the kidneys and is excreted into the renal-collecting structures, ureters, and bladder. Multiple CT images are obtained, producing high-resolution images of the urinary tract during maximal contrast opacification.

CT urography has replaced conventional IV urography in most institutions.

CT angiography

After a rapid bolus injection of IV contrast, thin-slice images are rapidly taken as the contrast opacifies arteries and veins. Advanced computer graphics techniques are used to remove images of surrounding soft tissues and to provide highly detailed images of blood vessels similar to those of conventional angiography.

CT angiography is a safer, less invasive alternative to conventional angiography.

Disadvantages of CT

CT accounts for most diagnostic radiation exposure to patients collectively. If multiple scans are done, the total radiation dose may be relatively high, placing the patient at potential risk (see Risks of Medical Radiation Risks of Medical Radiation Ionizing radiation (see also Radiation Exposure and Contamination) includes High-energy electromagnetic waves (x-rays, gamma rays) Particles (alpha particles, beta particles, neutrons) Ionizing... read more ). Patients who have recurrent urinary tract stones or who have had major trauma are most likely to have multiple CT scans. The risk of radiation exposure vs benefit of the examination must always be considered because the effective radiation dose of one abdomen CT is equal to 500 chest x-rays.

Current practice dictates that CT scanning use the lowest radiation dose possible. Modern CT scanners and revised imaging protocols have dramatically lowered radiation exposure from CT. Also, newer, investigational methods are evaluating the use of even much lower radiation doses for certain CT scans and certain indications; in some cases, these doses would be comparable to the radiation delivered by x-rays.

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