Barium Studies of the Small Bowel

Normal Small Intestine

The small intestine is extremely tortuous, beginning at the pylorus and extending about 11 feet in the living human from the pylorus to the ileocecal valve. Intestinal length is extremely variable, depending on neuromuscular tone and vascular flow. For example, the denervated, bloodless intestine stretched at autopsy varies from 10 to 30 feet in length. A patient with a small bowel obstruction will have a tortuous, long, and wide small intestine in which the jejunum may fall deep into the pelvis.

The mesenteric portion of the small intestine is suspended from the retroperitoneum by the relatively short root of the small bowel mesentery, extending about 15 cm from the duodenojejunal junction to the right iliac fossa. The jejunum arbitrarily comprises the proximal 40% of the mesenteric small intestine and the ileum comprises the distal 60% ( Fig. 24.1 ). The jejunum typically occupies the left upper quadrant, and the ileum occupies the right lower quadrant and pelvis. The location of the jejunum and ileum is often variable, however, given the mobility of the intestine on the root of the mesentery. Not infrequently, the jejunum flops into the right upper quadrant or changes its position at fluoroscopy.

The small intestine has a smooth curvilinear contour, as seen on abdominal radiographs or cross-sectional imaging studies in patients with free intraperitoneal air. The inner contour of the intestine is characterized by folds that encircle the lumen, known as the valvulae conniventes, plicae circulares, or folds of Kerckring ( Fig. 24.2 ). These folds are composed of mucosa and submucosa and increase the surface area of the small intestine by 300%. The small bowel folds lie perpendicular to the longitudinal axis of the bowel and are thicker, taller, and more numerous in the jejunum than in the ileum. ^, Villi are leaf- or finger-shaped protrusions of epithelium and lamina propria that stud the surface of the folds. Each villus has a core of lamina propria containing a cellular stroma, capillaries, nerves, and a lacteal. Villi are tall and thin in the jejunum and shorter and broader in the ileum. Duodenal villi are more variable and can be short and broad, leaf-shaped, or branched. The villi are about 1 mm in cross-sectional diameter and are just at the limits of fluoroscopic resolution. In contrast, the microvillous brush border of the small intestine is invisible on all radiographic examinations.

The small intestine is one of the largest immunologic organs in the body and a major site of interaction with foreign food antigens, pathogens, and toxins. The host defenses begin at the epithelial surface with a mucus layer containing immunoglobulins (especially secretory IgA) and enzymes. This mucus prevents microbes from adhering to the epithelium and acts as a buffer and lubricant. A variety of other defenses also protect the host from these food antigens, including intraluminal gastric acid, bile salts, and pancreatic enzymes as well as small bowel peristalsis.

The small bowel epithelium is composed of a single layer of cells bound by tight junctions impermeable to large molecules and pathogens. Intramucosal phagocytes include granulocytes, macrophages, and Paneth cells. Three distinct lymphocyte populations are present in the intestine within the epithelium, lamina propria, and Peyer’s patches. Intraepithelial lymphocytes are located in the basal portion of the epithelium and comprise up to 30% of the cell population in the mucosa. Immunocytes in the lamina propria are composed mainly of IgA-secreting plasma cells and lymphocytes. T lymphocytes of helper-inducer and suppressor-cytotoxic types are also present. Lymphoid aggregates span the muscularis mucosae, with portions of these aggregates in the lamina propria and submucosa. These lymphoid aggregates increase in size and number in the distal ileum ( Fig. 24.3 ), forming confluent Peyer’s patches. The follicular areas are composed of B cells, and the parafollicular areas are composed of T cells.

Principles for Performing and Interpreting Small Bowel Studies

The small intestine is a difficult structure to image. The intraluminal environment is hostile to barium preparations. A large amount of fluid (≈9 L) enters the small intestine each day, with only 1.5 to 1.9 L entering the colon. Bile acids, gastric acids, pancreatic secretions, and the epithelial mucus layer interact with barium in the small intestine. Fortunately, most modern barium suspensions no longer suffer from flocculation and clumping, as often occurred in the past.

Because of the inherent length and motility of the small intestine, intestinal loops overlap and change in size, shape, and position at fluoroscopy. Imaging of this structure can therefore take a long time, with normal small bowel transit ranging from 30 to 120 minutes. The transit time can be lengthened dramatically in patients with small bowel obstruction or adynamic ileus from a variety of causes.

The radiologist evaluates the overall location, course, and size of various portions of the small intestine and determines the position of the duodenojejunal junction and first loops of jejunum. The radiologist also evaluates the luminal contour and searches for abnormalities that extend beyond the small intestine, such as diverticula ( Fig. 24.4 ), sacculations, ulcers, or exoenteric masses, and lesions that protrude into the lumen, such as polyps ( Fig. 24.5 ) or abnormal folds. The small bowel folds are best evaluated when the lumen is fully distended and the folds lie perpendicular to the longitudinal axis of the bowel. Fold width also depends on the degree of luminal distention; the greater the distention, the thinner the folds appear ( Fig. 24.6 ). The folds are best shown after mucus has been washed off the luminal surface by the barium column. If folds are evaluated long after the barium column has passed, however, intestinal secretions can lift barium away from the mucosal surface so that the folds erroneously appear thickened. En face mucosal detail is seen by compression of the barium column or by double-contrast technique. Visualization of mucosal detail is necessary for detecting mucosal granularity or nodularity or small ulcers such as the aphthoid ulcers in Crohn’s disease ( Fig. 24.7 ).

Fluoroscopy is a key component of any small bowel examination. The radiologist examines the head of the barium column to understand the course of the small intestine and to detect contour abnormalities or filling defects in the barium column. The radiologist also assesses small bowel motility, distensibility, and pliability at fluoroscopy. Mobility or fixation of intestinal loops can also be recognized by manual palpation of the bowel.

Small Bowel Follow-Through

There are many ways to perform a small bowel follow-through examination. This chapter discusses the technique employed at my institution. The small bowel follow-through begins as a single-contrast examination of the esophagus, stomach, and small intestine that uses barium most appropriate for the small bowel. For this examination, the patient drinks a large volume (500–1000 mL) of low-density (35%–45% w/v) barium specifically prepared for evaluating the small intestine.

The patient should not eat or drink after 9:00 to 11:00 p.m. the day before the examination. If a peroral pneumocolon is to be performed, the patient should receive a barium enema preparation to cleanse the terminal ileum and right side of the colon.

A single-contrast upper gastrointestinal (GI) series is often performed (using 250–350 mL of low-density barium) as a prelude to examining the small bowel. The purpose of the upper GI series is to show gross upper GI involvement by diseases that affect the small bowel, such as Crohn’s disease and scleroderma. Esophageal or gastric abnormalities may also be detected as incidental findings, given the high frequency of upper GI disorders such as gastroesophageal reflux disease. However, a double-contrast upper GI examination using high-density barium is not performed because this barium is not designed for evaluating the small intestine and, if used, high-density barium may prevent adequate visualization of pelvic small bowel loops. The radiologist therefore sacrifices double-contrast evaluation of the upper GI tract to ensure a more optimal examination of the small intestine. After the esophagus, stomach, and duodenum are evaluated, the patient leaves the fluoroscopic suite and slowly sips an additional 250–350 mL of low-density barium.

In some practices, a technologist obtains overhead radiographs and a radiologist evaluates the overheads views, only palpating the small bowel at fluoroscopy when an abnormality is suspected on the overhead radiographs or when barium has reached the terminal ileum. Such an approach is strongly discouraged. Instead, a properly performed small bowel follow-through relies on fluoroscopic detection and spot image documentation of all abnormalities. Ideally, each loop of small bowel is palpated when it is optimally distended by low-density barium ( Fig. 24.8 ). While diseases affecting long segments of small bowel, such as Crohn’s disease, ischemia, and radiation enteropathy, are readily detected on small bowel follow-through ( Fig. 24.9 ), detection of short lesions such as the skip lesions in Crohn’s disease requires particularly careful palpation of the small intestine. The radiologist should therefore evaluate the patient at least several times—about 15 to 30 minutes after the single-contrast upper GI series is performed and then at 15- to 45-minute intervals, depending on how fast the barium column is progressing through the small intestine. The patient is turned into various positions (including supine, lateral, and prone positions), and manual palpation is routinely employed to splay out individual small bowel loops. At my institution, overhead radiographs are no longer obtained. Instead, digital spot images at the lowest magnification are usually adequate for this purpose.

The length of the examination can be shortened by administering a standard dose of 20 mg of metoclopramide (Reglan) orally 20 to 30 minutes before the study or 10 mg intravenously (IV) at the beginning of the examination. ^, Metoclopramide accelerates gastric emptying and small bowel transit. Unfortunately, metoclopramide also increases resting muscle tone, resulting in incomplete small bowel distention. The result is a faster but less optimal examination.

Some radiologists use a premade mixture of 24% w/v barium suspended in methylcellulose. This barium suspension produces greater luminal distention than a routine small bowel follow-through as well as a transradiant effect that mimics enteroclysis. However, the barium is not as dense as that used for a routine small bowel follow-through, so it can be more difficult to detect filling defects in the barium column.

The small bowel follow-through has two important limitations. Even with the use of metoclopramide, the pylorus delays emptying of barium from the stomach so the small bowel may be incompletely distended. As a result, it can be difficult to evaluate luminal contour or detect filling defects in the barium column. Because normal transit time for the small bowel is 30 to 120 minutes, it also is not feasible for the radiologist to remain in the fluoroscopy suite for the entire examination. As a result, the small bowel can only be evaluated intermittently, and lesions can be missed, depending on the degree of filling and distention of individual small bowel loops at the time of fluoroscopy.

Peroral Pneumocolon

A peroral pneumocolon may be performed in conjunction with a small bowel follow-through. This procedure is used to obtain double-contrast views of the terminal ileum and/or cecum and ascending colon when Crohn’s disease is suspected in this region or when the right side of the colon is inadequately evaluated on a barium enema or colonoscopy.

For a peroral pneumocolon, the patient undergoes a barium enema preparation to clear feces from the terminal ileum and right side of the colon. After a routine small bowel follow-through has been performed, 1 mg of glucagon is administered IV, and air is insufflated into the rectum via a Foley catheter. The colon is slowly distended with air as the patient is turned into various positions to manipulate administered air into the cecum and terminal ileum. Air can be refluxed successfully into the terminal ileum in 85% to 90% of patients. Double-contrast spot images of the pelvic ileum, terminal ileum, and right side of the colon are then obtained ( Fig. 24.10 ; see Fig. 24.7 ).

Enteroclysis

The small intestine has been examined via intubation techniques since the 1920s. All of these techniques entail positioning a tube beyond the pylorus, overdistending the small bowel with various contrast agents, and detecting abnormalities at fluoro-scopy. The multiplicity of techniques for performing enteroclysis reflects the imperfections of each individual technique. Lack of widespread acceptance of this procedure by the radiologic community is related to the relatively high level of expertise and effort needed to perform enteroclysis and also to patient discomfort during the intubation procedure. As a result, this technique has largely been replaced by computed tomography (CT) and magnetic resonance (MR) enterography.