X-Ray examination of the colon depict the distribution of gas in the intestines, calcifications, tumor masses, and the size and position of the liver, spleen, and kidneys. In the presence of acute intra-abdominal disease, erect, lateral, and oblique projections are helpful.
Although plain radiographs of the abdomen are generally nonspecific, they often give clues to the underlying problems. Free air is best seen on upright views. An obstructing colon cancer may demonstrate dilation of the proximal colon with a paucity of gas distal to the mass. Volvulus of the sigmoid colon or cecum may demonstrate their characteristic radiographic findings.
The lumen of the colon can be seen radiographically by instilling a suspension of barium sulfate through the anus (barium enema). Adequate preparation of the bowel is imperative before barium enema examination so that the colon will be as free as possible of fecal material and gas. Although many rectal lesions can be demonstrated by barium enema, x-rays are not as accurate here as with lesions above the rectosigmoid. Proctosigmoidoscopy is the best method for inspecting the rectum. Postevacuation films reveal the mucosal pattern and small lesions.
Barium enemas are performed as single-column or double-column studies. In the double-column (air contrast) barium enema, a higher-density, more viscous barium is used. After the mucosa is first coated with barium, carbon dioxide or air is insufflated to distend the colon and provide a second contrast medium. The double-column barium enema is more sensitive for detection of small lesions, but it is more strenuous and for that reason less well tolerated by frail or elderly patients.
Water-soluble contrast such as Gastrografin or diatrizoate sodium (Hypaque) may be used as alternatives to barium. Fine resolution with these agents is not as good as with barium; however, they can be used when barium is contraindicated, such as when there is a concern for perforation.
CT scan is useful in the diagnosis of masses (neoplasms and abscesses) and is also the most sensitive for detecting intra-abdominal free air and acute inflammatory processes such as appendicitis or diverticulitis. CT colography, or “virtual colonoscopy,” is a new technique that utilizes 3D reconstruction of the air-distended colon. In a series of 1223 average-risk adults who subsequently underwent conventional (optical) colonoscopy, virtual colonoscopy was better at detecting relevant lesions. It may be less accurate in surveillance populations. Studies are ongoing to evaluate the efficacy of virtual colonoscopy in both screening and surveillance. Thus far, the major limitations include the need for full bowel preparation and follow-up colonoscopy for tissue diagnosis of radiographic abnormalities. Because virtual colonoscopy is considerably time- and labor-intensive from the standpoint of the radiologists, active investigations into methods of automating the evaluation process are ongoing.
MRI is proving reliable for staging of cancer. Sonography (external, endorectal, and endovaginal) is useful in the diagnosis of masses as well in the evaluation of anatomy, such as depth of penetration of rectal cancers or presence of pelvic nodal metastases. PET has emerged as an increasingly valuable tool in colon and rectal cancer. PET has been shown to be 95% sensitive, 98% specific, and 96% accurate in the detection of cancer recurrence. The technique utilizes the glucose analogue fluorodeoxyglucose, which accumulates in metabolically active tissues. When used appropriately, it can help to distinguish patients who would benefit from surgery for recurrent cancer from those who have unresectable disease, particularly when the other imaging modalities fail to localize the disease.