Fig. 33.1
Normal small bowel follow through
The small bowel follow through is performed for a wide variety of clinical problems. Along with CT enterography and MR enterography (see below), the small bowel follow through is an important test to diagnose and manage Crohn’s disease (Figs. 33.2 and 33.3). It is also commonly used for patients with possible bowel blockage from adhesions (scar tissue) or cancer. Patients with diarrhea, malabsorption or protein loss or pain are often studied with a small bowel study. SBFT is occasionally helpful to evaluate gastrointestinal blood loss, and to evaluate anatomy after surgery [3, 4].
Fig. 33.2
Small bowel follow through shows terminal ileal strictures and abdominal separation of small bowel loops in the right lower quadrant secondary to mesenteric fat proliferations
Fig. 33.3
Small bowel follow through demonstrates abnormally dilated small bowel loops consistent with small bowel obstruction secondary to stricture (red arrow, transition point). (I am assuming we are showing a stricture in right lower quadrant with delayed transit time)
The small bowel study is usually initiated early in the day after an overnight fast. No other preparation is necessary. In most practices, the patient will discuss his or her symptoms with the radiologist at the start of the exam. The study begins with a regular X-ray before any barium is given, to see if any abnormalities are present which would later be lost behind the barium. This is followed by the administration of a large cup of thin barium. This is the chalky, white, nearly tasteless material that is dense to X-rays. The progress of the barium through the small bowel is monitored with still images of the abdomen obtained by a radiology technologist at 10–30 min intervals. As the barium moves through the small bowel, the radiologist will chose the appropriate times to do a real-time, detailed evaluation with fluoroscopy. These are “live” X-ray images of the intestine, much like watching a video clip on the internet. This step allows the radiologist to observe how well the small bowel contracts, and may help him see abnormalities that are not apparent on still images. The radiologist can sometimes find abnormalities by asking the patient to point to areas of discomfort or pain during fluoroscopy. The radiologist will likely ask the patient to drink more barium during the exam. This helps keep the small bowel well distended, making abnormal areas more obvious (Fig. 33.3). Some radiologists may have the patient take gas crystals during the exam. Gas in the small bowel can help show abnormalities not well seen in dense areas of barium, like the pelvis. The entire exam is usually done in 45 min to 2 and one-half hours, depending on how fast the small bowel fills and empties. The only discomfort experienced by patients is a mild sense of bloating, particularly if gas crystals are used [3–6].
While there is extensive research on radiation exposure and cancer risk, the best and most comprehensible risk assessment is only available for humans who have received moderate to high doses of radiation [6, 7]. Real and measurable risk is well documented for survivors of the atomic blasts of Hiroshima and Nagasaki, and for the occurrence of second cancers in patients who receive large doses of radiation to treat cancers such as lymphoma, thyroid cancer, cervical cancer, and other neoplasms. The risk from a single radiology exam such as a CT scan or small bowel follow through is so small as to be nearly unmeasurable, and certainly falls into other acceptable risks such as travelling by car, swimming, or being a pedestrian in a city. Further, risk only has meaning when balanced by benefit, and the value of accurate disease detection and monitoring almost always outweighs the small risk of radiation. However, it is prudent to exercise care for patients who may require many exams over the course of a chronic disease process such as Crohn’s disease or young patients. In these cases, MR exams may give fairly similar information with no known biologic risk. Finally, pregnancy status should be clear in women of childbearing age before exams using ionizing radiation.
CT Enterography
Over the last 15 years, a specific set of modifications to routine CT scanning of the abdomen and pelvis have been devised to make CT more sensitive for the detection of disease in the small bowel. CT enterography is accomplished with three modifications to routine CT technique. First, a large volume (up to 2 L) of neutral oral contrast (fluid) is given to distend bowel. The fluid can contain sorbitol, locust bean gum, polyethylene glycol, or small amounts of barium. Unlike water, these agents are chosen because they are not rapidly absorbed in the small bowel. As such, they distend the small bowel, making abnormalities of the wall more obvious—consider the analogy of the difficulty in reading a crumpled newspaper versus a page that has been spread out. The oral contrast is ingested over 60 min with last 200–300 mL given immediately before the scan. Glucagon has been used by some investigators, but unlike MR enterography its efficacy has not be well established with CT enterography. Second, IV contrast is given and timed to show blood flow to the bowel wall, approximately 45 s after IV contrast injection. Disease states such as inflammation or tumor will often alter blood flow, and increased or decreased flow can be detected with CT enterography (Fig. 33.4). Finally, computer techniques are used to display images in three planes, which often makes abnormalities more detectable [8–10].
Fig. 33.4
Coronal image from CT enterography in a 32-year-old patient with Crohn’s disease shows two short segment strictures with mucosal hyperenhancement (circles) suggesting active disease
The patient experience for CT enterography is similar to other CT tests that use IV contrast (X-ray dye). First, the patient’s past experience with IV contrast must be reviewed for allergies. Blood work is usually needed to assure that kidney function is normal, since IV contrast can worsen kidney function in patients with kidneys that do not work well. Large volume of liquid ingested before the exam can make some patients bloated. The CT exam itself is quite fast—the images are obtained in as little as 20 s. Modern scanners are high speed devices that can generate detailed images even if some motion is present. In certain situations, more than one trip through the scanner is necessary. For instance, X-ray contrast from the blood supply may pool in a bowel loop at a site of bleeding, and this collection may be most apparent a minute or two after the contrast was given [10, 11].
MR Enterography
Overview
Over the past several years, MRI has become an increasingly popular modality in evaluation of small bowel owing to lack of ionizing radiation, improved soft tissue contrast, ability to provide dynamic information regarding distention and motility, and relatively safer intravenous contrast agent profile. Continuing improvements in MR software and hardware have enabled small bowel MR to assume a major role in the evaluation of the small bowel. Optimal distention of the small bowel loops is crucial for the correct evaluation of the bowel wall because collapsed bowel loops may hide lesions or mimic disease by mistakenly suggesting that the collapsed segments are actually an abnormality-related thickened bowel wall. Two techniques are currently performed: MR enteroclysis and MR enterography. Although MR enteroclysis produced better distention of small bowel, it is less well tolerated as it requires nasoenteric intubation (often needing sedation) and infusion of 1500–2000 mL of contrast agent. Although there are few studies indicating better performance of MR enteroclysis over MR enterography, from an evidence-based medicine point of view, the overall level of evidence is weak. Because MR enteroclysis is more time intensive and less efficient than enterography, it is not widely performed. In comparison, MR enterography requires that patient ingest large volume of oral contrast material (up to 1.0–1.5 L) but is better tolerated. A limitation of MR enterography is variability in bowel distension, especially jejunal loops. Although the debate of MR enteroclysis vs MR enterography will continue, the decision of using one technique over other must take into account indication, performance of one technique over other, patient acceptance, resources, and institutional expertise. At our institution, MR enterography is routinely used for evaluation of small bowel disease [11–13].
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