Diagnostic CT Colonography
Incomplete Optical Colonoscopy
Incomplete or failed conventional colonoscopy is one of the most frequent indications for CT colonography. Conventional colonoscopy is considered incomplete if the cecum could not be reached or if the terminal ileum could not be intubated. An incomplete colonoscopy may be due to inadequate bowel preparation or to anatomic features such as an elongated colon (Fig. 1.1a, b), diverticular disease, or postoperative adhesions. It is also not uncommon for the procedure to be interrupted by the patient due to discomfort. Obstruction of the bowel lumen due to neoplastic or nonneoplastic stenosis is another reason for an incomplete examination. CT colonography almost always enables a complete examination of the colon, as the gas used to distend the colon in CT colonography can pass such obstacles much more easily than an endoscope. The radiologist therefore is able to evaluate the entire colon on the basis of the acquired CT images, and can usually readily identify what prevented completion of the optical colonoscopy. It is also known that, in patients in whom colonoscopy has failed once, there is a greater risk that a follow-up examination will also be incomplete. Thus, rather than attempting a second colonoscopic examination, it is preferable to refer these patients to CT colonography.
Examination schedule. A follow-up examination with CT colonography may be performed immediately after incomplete colonoscopy as long as the patient has not undergone polypectomy or biopsy. The advantage of doing this is that the colon is already prepared and no further bowel cleansing procedures are needed. If fecal tagging is to be performed after an incomplete colonoscopy, 50–60 mL of an iodinated oral contrast agent is given to the patient after optical colonoscopy. CT colonography should not be performed for at least 2 hours after oral administration of the contrast medium. If optical colonoscopy was performed under sedation or anesthesia, specific care must be taken that the patient is able to drink the contrast agent safely and to follow the instructions for the examination. Depending on the reason for the incomplete colonoscopy (e.g., mass lesion causing stenosis or elongated colon), intravenous administration of contrast agent may be advisable.
If a polypectomy or deep biopsy has been performed during the previous examination, colonic distension should be avoided because there is an increased risk of perforation. In such patients, CT colonography should be performed no earlier than 10 days after conventional colonoscopy. Some authors recommend an even longer interval since granulation tissue remains in the bowel wall up to 1 week after the intervention, increasing the risk of perforation. As a rule, the referring colonoscopist should be consulted to find out why the colonoscopy was incomplete, and whether polypectomy or biopsy was performed. If there is any uncertainty, an unenhanced low-dose CT scan of the abdomen may be performed prior to colonic insufflation for CT colonography to rule out a colonoscopy-related perforation.
If suboptimal bowel preparation is the reason for incomplete colonoscopy, the examination should be scheduled for another day with a second preparation.
Preoperative Evaluation of Patients with Stenotic Colorectal Carcinoma
CT colonography is a powerful tool for the detection and preoperative evaluation of colorectal carcinoma. It allows one to determine the precise colonic localization of the primary tumor, to perform TNM staging, and to detect any synchronous colonic lesions. Three-dimensional (3D) colon maps (3D global views) provide an exact anatomic depiction of the colon that allows precise localization of the tumor. This information is particularly useful for surgical planning. CT colonography also enables an evaluation of local tumor infiltration into the surrounding area. Two-dimensional (2D) multiplanar images of the colon may be used for T-staging, that is, to detect extraluminal tumor spread or infiltration into adjacent organs (stages T3 and T4). If intravenous contrast agent has been administered, N-staging and M-staging may also be performed. In patients with stenosing tumors, synchronous lesions in the prestenotic colon that cannot be reached by conventional colonoscopy may also be detected. In patients with colorectal cancer, the prevalence of synchronous carcinomas and polyps is 5% and 27%–55%, respectively.
There is increasing evidence of the usefulness of CT colonography in the detection as well as the preoperative evaluation of colorectal cancer. A recent meta-analysis by Pickhardt et al. (2011) showed that CT colonography is highly sensitive for the detection of colorectal cancer, with an overall sensitivity of 96.1%. A comparative study by Neri et al. (2010) has shown that CT colonography is superior to conventional colonoscopy in the identification of colonic masses, the completeness of colonic evaluation, and the precise definition of the segmental location of the carcinoma. Recent studies by McArthur and colleagues (2010) and by Park and colleagues (2011) have shown that CT colonography is accurate in assessing significant synchronous lesions in patients with colorectal cancer.
The potential of CT colonography in patients with colorectal cancer is appealing since it combines local and distant staging of disease with detection of synchronous colonic lesions. It may replace optical colonoscopy, particularly where histological proof of malignancy has already been obtained by previous incomplete endoscopic examinations. Thus, preoperative evaluation of a colorectal carcinoma is considered an indication for CT colonography (Fig. 1.2).