special subjects
Computed Tomography (CT)
Variations of CT
Forvirtual (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 enterographyis 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 Overview of Inflammatory Bowel Disease None 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 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. 