Gastrointestinal Tract: Normal Imaging

NORMAL IMAGING

Upper Gastrointestinal Tract

The organs that can be examined in the upper gastrointesti-nal tract include the pharynx, esophagus, stomach, and duo-denum. The pharynx and esophagus may be evaluated separately or as part of more complete examinations of these organs. Various techniques are available to assess the func-tion and structure of the pharynx depending on the indica-tions for the examination. Motion recording (now using mainly digital technology) of pharyngeal function and im-aging of pharyngeal structures are often combined for a more thorough examination. Also, materials of variable vis-cosity can be used in patients to determine aspiration risk and dietary needs; the latter is often called a “modifiedbarium swallow” and is done in conjunction with a swallow-ing therapist. An alternate method of assessing swallowing is fiberoptic endoscopic evaluation of swallowing (FEES), which is also performed by a swallowing therapist with training in this technique.

Pharyngeal function is complex and is best evaluated with motion-recording techniques that allow slow-motion review. Imaging of the pharynx is usually done with the patient in the frontal and lateral positions (Figure 10-1). In the frontal view, the paired valleculae and piriform sinuses are sepa-rated. The lateral view of the pharynx superimposes these structures, but permits better visualization of the base of the tongue, hyoid bone, and epiglottis anteriorly, and the poste-rior pharyngeal wall and cervical spine posteriorly.

The esophagus, stomach, and duodenum are usually ex-amined together as part of the upper gastrointestinal series. A variety of radiographic techniques are used and usually com-bined to optimize the upper gastrointestinal examination; techniques include observation of esophageal motility, film-ing of the organs with varying amounts of barium suspen-sion, gas, or air, and obtaining views of the mucosal surface (Figures 10-2, 10-3). The upper gastrointestinal tract is bestexamined by luminal contrast studies or endoscopy because mucosal abnormalities are often the cause of disease. CT and MR imaging can also evaluate these organs and detect focal masses, wall thickening, and extrinsic processes, such as an invasive pancreatic malignancy. These modalities can also contribute to the staging of malignancies, especially in the esophagus (Figure 10-4).

The esophagus consists mainly of a tubular portion with a bell-shaped termination called the esophageal vestibule. The esophagogastric junction normally lies within or below the esophageal hiatus. When the esopha-gogastric junction lies above the hiatus, hiatal hernia is present, which is the most common structural abnormality of the upper gastrointestinal tract (Figure 10-5 A). The esophageal mucosal surface has a smooth appearance when distended and shows smooth, thin longitudinal folds when collapsed (Figure 10-5 B). Esophageal peristalsis can be ob-served by having the patient swallow single volumes of bar-ium suspension.

The stomach has a complex shape and varies considerably depending on the degree of distention (see Figures 10-2, 10-3). When the stomach is collapsed, the rugal folds are seen prominently and may mimic focal or diffuse gastric disor-ders, whether on luminal contrast or CT studies. With gastric distention, the rugal folds are flattened and the mucosal sur-face of the stomach is seen more effectively. Barium studies of the upper gastrointestinal tract evaluate gastric function poorly; radionuclide gastric emptying studies are more effec-tive for this purpose.

The duodenum is attached to the stomach at the narrow pylorus and consists of the duodenal bulb and the de-scending and ascending portions, although a horizontal segment is often added (Figure 10-6; see Figure 10-4 B). The duodenum terminates at the duodenojejunal junction, which is attached to the ligament of Treitz. The duodenal bulb has a triangular or heart-shaped appearance normally tapering to the apex of the bulb with its junction to the de-scending portion. The bulbar mucosal surface is normally smooth. The duodenum assumes a C-shape configuration within the upper abdomen, and the mucosal folds are sym-metric. The duodenum is surrounded by many structures, particularly the pancreas, and is often involved secondarily by diseases in other adjacent organs; cross-sectional CT or MR imaging is useful in demonstrating this involvement (see Figure 10-4).

Small Intestine

The radiographic examination of the small bowel evaluates the mesenteric portion of the organ, which consists of the jejunum and ileum. The following luminal contrast meth-ods can be used to examine the small intestine: (1) peroral small-bowel series; (2) enteroclysis; and (3) various retro-grade techniques (eg, via an ileostomy). The peroral small-bowel series is often done immediately after an upper gastrointestinal examination and following ingestion of additional barium suspension. Serial images of the ab-domen are obtained, and abdominal compression applied to better visualize the small-bowel loops, including the terminal ileum (Figure 10-7). However, the peroral small-bowel study is probably the least effective method of exam-ining this organ; techniques that better distend the small bowel with higher volume are now preferred depending on the indications. These include enteroclysis and CT or MR imaging with volume instillation by oral ingestion or via a tube, that is, CT or MR enterography or CT or MR entero-clysis (Figure 10-8).

Enteroclysis is an intubated examination of the small in-testine and can be done by a variety of techniques and using a number of different modalities (as discussed earlier). The small intestine is intubated by a nasal or oral route with a small-bore enteric tube placed with fluoroscopic guidance. A variety of luminal contrast methods exist, but filming is done similarly to the peroral examination. The enteroclysis tech-niques permit better control of small-bowel distention and more exact visualization of small-bowel loops (Figure 10-9).

Retrograde examination of the small bowel involves filling of the organ from the opposite direction. Various techniques can be used depending on the patient’s anatomy. Reflux of the small intestine through the ileocecal valve can be done as part of a barium enema. If the patient has an ileostomy, various devices can be introduced into the ostomy site and a barium suspension instilled directly.

The length of the mesenteric small bowel in adults aver-ages about 20 feet, but varies considerably among individ-uals. The jejunum comprises just over one-third of the length and the ileum the remainder, although no discrete transition is seen between the two segments. The normally distended small bowel has a caliber of 2 to 3 cm, being slightly larger more orad in the jejunum. Depending on the degree of distention, the mucosal folds (valvulae con-niventes) may have a feathery appearance or may be trans-versely oriented across the intestinal lumen with more complete distention. The mucosal folds are more numerous in the jejunum and gradually decrease in number and size in the ileum.

Large Intestine

The radiographic examination of the large bowel evaluates the entire organ from the rectum to the cecum. Reflux of bar-ium suspension into the ileum and the appendix, if present, occurs commonly. The colon can be evaluated by several techniques, which include single-contrast and double-contrast barium enemas; different types of rectal tips and barium suspensions are used for each examination (Figure 10-10). The single-contrast method simply involves filling the colon with a dilute barium suspension, whereas the dou-ble-contrast technique requires a denser, more viscous bar-ium suspension and air. In both methods, large and small compression images of all segments of the colon are ob-tained. Colonoscopy and CTC have both had a substantial impact on the numbers of barium enemas now being per-formed (Figure 10-11).

The large intestine consists of the rectum, sigmoid colon, descending colon, splenic flexure, transverse colon, hepatic flexure, ascending colon, and cecum (see Figure 10-10). The length of the colon varies considerably, mainly because of differences in length and redundancy of the sigmoid colon and colic flexures. The colon also varies in caliber depending on location and luminal distention achieved. The mucosal surface has a smooth appearance, and the colonic contour is indented by the haustra, which are less numerous in the de-scending colon. The rectal valves of Houston are often seen, especially on double-contrast imaging. The ileocecal valve has a variety of appearances and may be large if infiltrated by fat (Figure 10-12).