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.
These same principles of tomographic imaging can also be applied to radionuclide scanning, in which the sensors for emitted radiation encircle the patient and computer techniques convert the sensor data into tomographic images; examples include single-photon emission CT (SPECT) and positron-emission tomography (PET).
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.
CT is the most accurate study for detecting and localizing urinary calculi.
CT may be done with or without IV contrast.
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
- Identifying inflammatory bowel disease
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
- 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.
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). 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.
Some CT scans use IV contrast, which has certain risks (see Radiographic Contrast Agents and Contrast Reactions). However, oral and rectal contrast also has risks, such as the following:
- If barium, given orally or rectally, extravasates outside the gastrointestinal tract lumen, it can induce severe inflammation in the peritoneal cavity. Iodinated oral contrast agents are used if there is a risk of intestinal perforation.
- Aspiration of iodinated contrast agents can induce severe chemical pneumonitis.
- Barium retained in the intestinal tract can become hard and inspissated, potentially causing intestinal obstruction.