Diseases of the Colon and Rectum: CT Colonography

Abstract

The public health need for colorectal cancer screening is compelling. Colorectal cancer is common, accounting for approximately 50,000 deaths yearly in the USA [1]. The benign precursor adenoma can be detected by several differ-ent imaging techniques, and, if removed, can prevent malig-nant transformation. The approximately 10-year polyp dwell time allows ample opportunity for patients to be screened and polyps detected and removed. Potentially, under ideal screening circumstances, an entire class of cancers could be prevented. Unfortunately, barriers exist to ideal screening, including suboptimal performance of many existing colorec-tal screening tests, reluctant compliance by patients to follow recommended screening guidelines, and variable insurance coverage of examination charges. In many ways, CT colo-nography (CTC) approaches an ideal screening test by addressing issues and problems inherent with other tech-niques. This syllabus will highlight many key issues for CT colonography today. 16.1 Technique Proper technique at CT colonography is imperative for high performance. The three main steps include preparation, CT scanning, and interpretation. 16.1.1 Preparation 1. Dietary restriction. Preparation begins with a recommen-dation for patients to consume clear liquids or a low-residue diet for at least 1 day prior to beginning the cathartic bowel preparation. Although this step is opti-mal, it can be skipped if patients are following the subse-quent instructions carefully. 2. Stool tagging. 10–20 mL of barium or iodinated contrast material is administered orally for three meals prior to beginning the cathartic preparation. This contrast agent mixes with stool and ideally is evacuated with the cathar-tic preparation. Residual stool will contain high-attenuation contrast and can be easily discriminated from soft tissue attenuation polyps. Some prefer to perform stool tagging after the cathartic agent, but before fluid tagging. 3. Cathartic. The cathartic preparation can be performed with a number of different formulations including poly-ethylene glycol electrolyte solution and magnesium citrate or bisacodyl tablets. Phospho-soda agents are not recommended because of prior issues with renal insuffi-ciency and sodium retention. Bisacodyl tablets require the patient to be well hydrated for the purgation process to be successful. Bisacodyl tablets are easy to consume and well tolerated by patients. Polyethylene glycol elec-trolyte solution provides the cleanest colon, but some patients find it more difficult to consume in its entirety. 4. Fluid tagging. Following the cathartic preparation, the small bowel will continue to secrete soft tissue attenuation fluid into the colon. This fluid can obscure immersed pol-Learning Objectives