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Radiographic imaging of small bowel disease.

The small bowel performs several vital physiological functions. First, it serves an essential role in the absorption of nutrients. In performing this task, the small bowel acts like sort of a human Cuisinart, mixing its contents together and breaking them down into smaller pieces. Proteins, fats and carbohydrates are broken down by a variety of enzymes present within the intestine so that these nutrients can be effectively absorbed as they pass through the lumen of the small bowel toward the cecum. The tremendous surface area of the small bowel facilitates the transportation of these important nutrients from the bowel into the lymph or blood.

In addition, the small bowel reabsorbs some 75% to 80% of the fluid that passes through it each day, helping the body to maintain its electrolyte and mineral balance. The small bowel also serves as a passageway for excess solids and liquids that are not absorbed.(1)

Disorders of the small bowel are numerous. Although some diseases of the small bowel (such as Crohn's disease) may present as a combination of different signs and symptoms, most disorders tend to materialize as one of a limited number of clinical scenarios, including gastrointestinal hemorrhage, obstruction, perforation or ischemia; or the malabsorption of fluid, electrolytes or nutrients. Congenital abnormalities also may be present, resulting in small-bowel obstruction that can lead to death if left untreated.(1)

Although the anatomy and physiology of the small bowel is generally well understood, the accurate diagnosis of small bowel disorders remains a challenge for clinicians despite recent advances in diagnostic imaging techniques. This article reviews the anatomy of the small bowel and discusses imaging techniques used to diagnose small bowel disorders.

Anatomy of the Small Bowel

The small bowel or small intestine begins at the pylorus and consists of the duodenum, the jejunum and the ileum. (See Fig. 1.) In vitro, the small bowel measures approximately 6 meters to 7 meters in length; however, it is significantly shorter in vivo, measuring between 230 cm and 370 cm. The shorter length in vivo is attributed to its high muscle tone and contractility. In adults, its length seems to be related to height and weight, with the small bowel being somewhat shorter in women than in men.(1,2)

The duodenum is the first and widest section of the small bowel, measuring approximately 20 cm to 25 cm long and 4 cm to 5 cm in diameter. It consists of four segments: the first superior) portion, the second (descending) portion, the third portion (horizontal) and the fourth (ascending) portion, which together form a C-shaped curve that encircles the pancreas. The jejunum begins distal to the ascending portion of the duodenum, and the ileum extends from the junction with the jejunum to the ileocecal opening. Although there are distinct morphological differences between the jejunum and ileum, the transition is gradual and there is no distinct boundary between the two.(2)

The small bowel consists of several distinct intestinal layers. Beginning from the innermost portion of the bowel, bound at the bowel lumen, these layers are the mucosa, the submucosa, the muscularis and the serosa.(2) (See Fig. 2.)


Most digestion and absorption in the small bowel occurs in the mucosal layer. From the lumen to the submucosa, it consists of three sections: the epithelium, the lamina propria and the muscularis mucosa. The mucosa is thicker in the jejunum than the ileum and forms circular folds covered with villi. Villi are tiny, thread-like projections located on the mucosal surface that help increase the area of absorption.(3) (See Fig. 3.)


The submucosa consists of moderately dense connective tissue and sometimes clusters of adipose cells. It also houses numerous blood and lymphatic vessels.(4)


The muscularis of the small bowel is located between the submucosa and the serosa and consists of an outer longitudinal and inner circular layer of smooth muscle. The muscularis is responsible for peristalsis, the wave-like action that travels along the length of the intestine to propel the luminal contents toward the colon. Peristaltic waves begin at intervals of a few minutes and propagate for a short distance before fading out. A single wave rarely travels the entire length of the small bowel, and several may occur simultaneously. Segmental movements (alternating contraction and relaxation) do not move the intestinal contents toward the large intestine, but serve rather to mix the contents of the lumen.(5)

In the terminal portion of the ileum, the thickened muscularis forms an ileocecal sphincter that remains partially contracted to delay emptying of the contents from the ileum into the cecum. The presence of food in the ileum prompts ileal peristalsis and initiates movement of its contents into the large intestine, causing relaxation of the ileocecal sphincter. A bicuspid valve at this juncture, composed of mucosal folds, relaxes as a meal passes through it and closes once the cecum is filled to prevent reflux from the cecum to the ileum.(1)


The serosa is the outermost layer of the small intestine. It is a continuous sheet of squamous epithelium and mesothelium. It is separated from the underlying muscularis by a thin layer of loose connective tissue.(1)

Mucosal Folds

The mucosal folds of the small bowel function to slowly advance the luminal contents of the bowel toward the colon, and thereby serve to increase absorption of nutrients and fluids within the small bowel. These folds are circular in nature and run almost the entire length of the small bowel.(1) (See Fig. 4.)

Mucosal folds are larger and more numerous in the duodenum and jejunum, becoming increasingly smaller and fewer as they reach the distal portion of the ileum, where there are no folds. Accordingly, the jejunal wall is thicker than the ileal wall due to the presence of larger and more numerous folds in the jejunum.(1)

Physiology of the Small Bowel

The primary function of the small bowel is the digestion and absorption of nutrients such as protein, carbohydrates, fat and vitamins from food. It also reabsorbs fluid, electrolytes and bile salts from gastric, biliary, pancreatic and intestinal secretions. The tube-like structure of the small bowel provides an extensive absorptive surface area facilitating digestion. In addition, alternating contraction and relaxation of the small bowel helps mix its contents, while peristaltic movement of the muscularis helps propel the luminal contents toward the large bowel.(5)

Digestion and Absorption

Various nutrients are absorbed at different sites within the small bowel. For example, calcium and iron are primarily absorbed in the duodenum; carbohydrates, fats and amino acids are absorbed in the jejunum; and vitamin B-12 and bile salts are absorbed in the ileum.(6)

Although most nutrients and water are absorbed in the duodenum and jejunum, the ileum may function as an area of enhanced absorption following resection of the jejunum. The secretion of bicarbonate from the pancreas and biliary tract helps neutralize the gastric contents and creates the ideal environment for digestion in the small bowel. Absorption can be characterized as active or passive transport. In passive transport, absorption depends on concentration, electrical or osmotic gradients, whereas active transport requires the addition of energy.

Digestion of carbohydrates begins in the mouth with enzymes present in the saliva. However, most carbohydrates are hydrolyzed into trisaccharides and disaccharides in the duodenum and jejunum, where they undergo further hydrolysis into monosaccharides. Monosaccharides such as glucose, fructose and galactose are actively absorbed by the villi. In patients with celiac sprue, a chronic syndrome marked by the loss of villous structure, malabsorption of carbohydrates may occur.

Digestion of fats or triglycerides starts in the stomach and continues in the small bowel, where they are hydrolyzed into free fatty acids and monoglycerides, which combine with bile acids to form micelles, watersoluble particles that are absorbed by passive diffusion into the enterocytes.(7) In the enterocytes, fatty acids and monoglycerides resynthesize into triglycerides, which are absorbed with phospholipids and cholesterol into the lymphatic system and finally enter the systemic circulation. Even minor damage to the tips of the villi can result in disruption of fatty acid absorption.(8) Most bile acids can be reabsorbed from the small bowel. Bile acid deficiency due to impaired hepatic synthesis or disorders of the ileum can cause fat malabsorption.

Protein digestion also begins in the stomach, where it is converted by pepsin into peptides and amino acids. Peptides are converted in the small bowel into oligopeptides, dipeptides and amino acids. These are further digested into single amino acids that are absorbed by the enterocytes. Pancreatic disease, surgical resection or celiac disease resulting in loss of intestinal mucosa can affect the absorption of protein.

0Each day, approximately 7 liters to 9 liters of water pass through the small bowel. Approximately 1000 mL to 1500 mL of this water comes from diet, with the remainder derived from biliary, gastric, pancreatic and salivary secretions. Of this, the small bowel can absorb as much as 5 liters to 7 liters. The rest enters the colon.8 Water is absorbed passively and is therefore contingent upon an osmotic gradient. Active absorption of water occurs in the villous epithelial cells. In healthy people, water is absorbed with electrolytes or may remain in the lumen with other nonabsorbable substances to help maintain the osmotic gradient.

The small bowel also absorbs electrolytes like sodium, chloride and potassium and excretes bicarbonate.(9) Methods of electrolyte transport include passive diffusion, convention and carrier-mediated active transport.

Diarrhea (increased fluid in the stool) is a disorder of intestinal fluid and electrolyte movement. For example, if absorption in the small bowel was decreased by 60%, the resulting excess fluid flowing to the colon can cause diarrhea.(9) In patients with celiac disease, atrophic villi decrease absorptive function and crypt cells increase secretions. Other factors such as bacterial endotoxins, hormones and detergents also can cause diarrhea because of excess fluid and electrolyte secretion.

Motor function of the small bowel occurs during the interdigestive state, the time between the intake of a meal and the emptying of the intestine. After eating, peristalsis may begin anywhere along the small bowel by contraction of the mucosal folds. This contraction propels the contents toward the cecum and thereby serves to increase the absorptive area of the small bowel. Segmental contraction can separate the intestinal lumen into several sections, promoting the mixing of food. Contractions occur faster in the duodenum than in the ileum.(10,11)

Normal small bowel transit time is 73 minutes [+ or -] 6.5 minutes, fluctuating in duration from 31 minutes to 139 minutes. On average, after eating, the first appearance of food in the cecum occurs in approximately 74 minutes [+ or -] 5.2 minutes,12 and within about 4 hours after eating a meal, 50% of the food ingested enters the colon.(12) It is interesting to note that transit time is no related to the speed of contraction, but rather to the presence of nutrients in the ileum, which delays transit.

During fasting, intestinal movement can be divided into three phases: complete lack of contractions, random contractions, and one contraction with each slow wave known as the migrating motor complex (MMC). The MMC occurs every 84 minutes to 112 minutes at a speed of 6 cm/min to 8 cm/min.(10) The MMC is mitigated by neural and hormonal factors and moves the luminal contents forward to prevent bacterial growth. In patients with systemic sclerosis and primary visceral myopathies, contractions may resemble a pseudoobstruction. Any decrease in intestinal motility can result in the overgrowth of bacteria and malabsorption.


Disorders of the small bowel generally present as one of a limited number of clinical manifestations. These are generally related to intestinal hemorrhaging obstruction, perforation, ischemia and malabsorption of fluid, electrolytes or nutrients. Some diseases, such as Crohn's disease, may present with two or more symptoms. However, diagnoses of small bowel disorders are rarely made with information derived from physical examination alone. Detailed radiographic evaluation is generally required for a definitive diagnosis.


Hemorrhage of the small bowel is fairly obvious, and the presentation of bleeding often is characteristic of the type of lesion. For example, the presence of melena (dark, clumpy, tar-like stools stained with blood) suggests lesions located above the ligament of Treitz, while lesions below that level generally produce hematochezia (bloody stools).(13) In general, however, physical examination of gastrointestinal hemorrhaging is useful only in determining the amount of blood loss.


Obstruction of the small bowel is characterized by cramping, pain, nausea, vomiting, abdominal distension and constipation. The quality and timing of vomiting can help locate the source of the obstruction, as distal obstruction is characterized by vomitus with sediment and a longer time between eating and vomiting.(14)


In general, perforation of the small bowel is rare. When perforation is present, it is generally accompanied by severe abdominal pain, vomiting, rigidity of the abdominal wall and extreme weakness.(14)


Ischemia of the small bowel usually is due to occlusion of the superior mesenteric artery as a result of thrombosis or embolus. Abdominal pain is disproportionate to the physical findings.(14) If ischemia progresses, signs and symptoms of intestinal infarction with peritonitis supervene. Bleeding also may occur during the progression of small bowel ischemia.


Small bowel diseases due to malabsorption encompass a variety of signs and symptoms, including diarrhea, cramps, abdominal distention, flatulence, weight loss, paresthesias, pathological fractures, easy bruising, edema and night blindness. 14 Many of these symptoms are caused by specific vitamin and mineral deficiencies.

Imaging Techniques

The goal of radiographic evaluation of the small bowel is to identify pathologies or verify normalcy. This is possible only with methods that are capable of accurately depicting bowel morphology. Barium contrast studies remain the primary methods of choice for diagnostic evaluation, despite recent advances in cross-sectional imaging with computed tomography, ultrasound and magnetic resonance imaging. A detailed small bowel examination can yield invaluable information provided that the underlying pathology is indicative of small bowel disease.(15)

Ideally, barium studies should identify the mucosal surface, clearly outlining mucosal folds, and test the distention of each bowel loop. To properly evaluate normally overlapping bowel loops, each loop must be separated from adjacent loops using compression techniques. Loops also may be evaluated through another loop using double-contrast methodologies. It is important to note that surface details and patterns of folds can be accurately visualized only in the distended bowel, and caution should be observed when evaluating minor variations in the small bowel, particularly with poorly coated or less than optimally distended portions of the small bowel. (See Fig. 5.) Although movement within the small bowel may be assessed during radiographic examination, overall transit time within the small bowel can vary significantly between individuals and provides little diagnostic information.

Peroral Small Bowel Examination

Despite the increasing popularity of enteroclysis, the conventional peroral small bowel series remains a popular diagnostic tool because of its simplicity. In simple terms, it consists of feeding a barium suspension directly into the small bowel and recording its progression throughout the small intestine. In the past, small volumes of barium (approximately 8 oz) were preferred, since it was believed that small amounts of barium would avoid the problem of overlapping loops. However, this also required that numerous films be taken as the barium traveled through the small bowel. Thus, it is thought that segments of the small bowel may not have been visualized because the small bolus of barium may have passed through these areas between images. Furthermore, the small volume of barium used was not enough to stimulate peristalsis, resulting in a much longer transit time.

The large-volume technique was popularized by Richard Marshak, M.D.,(16) because it stimulated peristalsis.(5) However, it is important to note that the quality of the results depend on the technique used. For example, the variable appearance of the normal small bowel, overlapping of loops and variation in individual transit times can make definitive evaluation of the normal small bowel difficult. (See Fig. 6.)

Small Bowel Follow-through Series

After a single-contrast upper gastrointestinal study, as much as 500 mL of a 42% weight/volume (w/v) suspension of barium may be used to examine the small bowel. In cases in which a double-contrast upper GI examination using high-density barium is performed, an initial bolus of 200 mL of dilute (24% w/v) barium suspension is administered followed by 300 mL of a 42% w/v suspension. In either case, compression radiography of the proximal jejunum is recommended. The patient then is monitored every 15 to 20 minutes until compression fluoroscopy and spot filming of the bowel are completed. When the barium has reached the colon, the series is completed with a prone radiograph of the abdomen.

Small Bowel Meal

The small bowel meal study is used specifically in cases in which there has been no prior upper GI series. Approximately 500 mL to 600 mL of a 42% w/v solution is used. This volume of barium is sufficient to produce a continuous contrast study, leaving the stomach and cecum relatively quickly. Prolonged transit time may be avoided with the administration of a promotility agent such as metoclopramide administered intravenously during the examination. Fluoroscopy and compression radiography are performed at 15- to 20-minute intervals. The small bowel meal also may be enhanced with a peroral pneumocolon study.

Peroral Pneumocolon Examination

Peroral pneumocolon examination provides detailed imaging of the ileocecal area using double contrast. This study is used to supplement routine spot films of the distal ileum. [17] The study is performed once the right colon is filled with barium following a small bowel meal or SBFT. The cecum then is distended with air that is introduced via the rectum. Once the rectum is filled, the air then is refluxed into the distal ileum through prone positioning of the patient. This examination is recommended in cases of suspected inflammatory bowel disease or whenever the distal ileum cannot be properly evaluated because of poor distention, low-lying pelvic cecum or pelvic adhesions.

Small Bowel Enteroclysis

The use of small bowel enemas can be traced back to the works of Pesquera in 1929. [18] Additional contributions were made by Gershon-Cohen and Shay as well as others. [19] Ultimately, the technique was popularized by Johann Sellink, [19] and his version of the procedure remains the preferred technique to this day. [19,20]

Enteroclysis represents a significant improvement in small bowel examination. It has demonstrated high reliability and yielded accurate diagnosis in a variety of small bowel diseases. While Sellink advocated the presence of an empty colon prior to examination, it is not a requirement. [19] In general, patients are instructed to take nothing by mouth for a period of 10 to 12 hours prior to examination.

A variety of tubes are available for enteroclysis, varying in length and stiffness and in the location of the holes at the distal end of the tubes. The tubes are somewhat stiff and are provided with guide wire sufficient to reach the proximal jejunum. The Malinte tube [21] was introduced in the early 1990s. It has a balloon on its distal end that was designed to be inflated in the duodenum to prevent reflux of the barium suspension into the stomach.

Although the tube may be introduced nasally or orally, the easiest route of administration is by mouth. Patients often are able to swallow the tube without help if encouraged properly. Oral administration avoids possible damage to the nasal terminates and is more comfortable for most patients. After the tube has been swallowed and reaches the distal portion of the esophagus, a guide wire is inserted, leaving the distal portion of the tube flexible.

The tube should be inserted with the patient in the upright position, which stretches the stomach downward, encouraging the tube to follow the curvature toward the pylorus. (See Fig. 7.) Gentle forward pressure is maintained until the tube reaches the pylorus and enters the duodenum.

Once the tube has entered the duodenum, the guide wire is retracted, allowing the tube to more easily negotiate the curves of the duodenal loop. The tube should be advanced to the proximal jejunum when possible, since a barium suspension that is injected beyond the ligament of Treitz seldom refluxes into the stomach. The procedure can be performed with the tube in the third or fourth portion of the duodenum. However, reflux of barium into the stomach is more likely to occur, and the patient may vomit during the procedure.

Direct intestinal administration of barium at a sufficient rate results in the simultaneous visualization of the entire small bowel in a state of distension. The rate of flow should be tailored to the individual. The correct rate of flow will permit moderate distension of the bowel while permitting peristalsis to advance the barium distally. If the barium is administered too quickly, the bowel will become overextended, peristalsis will be inhibited, and reflux of the barium into the stomach may occur with subsequent vomiting.

During the administration of barium into the small bowel, spot compression films are taken of each section of loops as they fill with contrast. Filming the intestine as it fills with contrast minimizes the possibility of overlapping loops. As the examination continues, filming of the small bowel proceeds in conjunction with compression. Just as compression is used during the peroral administration of barium, compression is used during enteroclysis to separate the intestinal loops. Compression, forcible exhalation, an air enema or filling the bladder with water helps push the intestinal loops upward, where they can be easily compressed and separated for better visualization.

Compete evaluation of all bowel loops is readily obtained with enteroclysis, and folds are straightened and arranged parallel to the lumen. Prestenotic dilation is visible at sites of minimal stricture, and small sinus tracts and fistulae are easily recognized. The examination usually can be completed in as few as 20 to 30 minutes, minimizing the degradation of the barium solution by the intestinal fluid. Once the barium has reached the distal ileum and all loops have been spot-filmed using compression, the tube is withdrawn and the patient is placed in the prone position. A large prone film of the entire abdomen is taken to document the entire appearance and character of the small bowel. (See Fig. 8.)

Certain disadvantages are associated with this procedure, including the possibility of improper placement of the tube, increased patient exposure to radiation, and the need for direct involvement of the radiologist at all times throughout the procedure. of the tube flexible.

Single-contrast Technique

The concentration of barium solution used during enteroclysis is contingent upon the size of the patient, and ranges between 28% and 42% w/v for adults. In most cases, 700 mL to 1000 mL of barium is sufficient. Fluoroscopy and compression of the entire bowel is performed at various phases of filling. Single-contrast examination may require the use of secondary double contrast. Infusion with water may be used to achieve better distension and to help delineate the ileocecal region. Infusion of water is a transient effect only, because the water readily mixes with barium solution. Infusion of air also helps in the evaluation of ileal loops, particularly in patients who have adhesions.

Biphasic Technique

The biphasic imaging technique combines the single-contrast method of enteroclysis with a double-contrast depiction that is achieved with methyl cellulose. A complete single-contrast and a double-contrast examination both can be accomplished in one examination by combining 450 mL of a 50% w/v suspension of barium with an infusion of 0.5% methyl cellulose. The infusion of methyl cellulose is initiated after barium has filled the pelvic segments of the ileum. Thus, adequate amounts of luminal barium are pushed distally, permitting continuation of single-contrast evaluation of the distal small bowel while proximal segments are beginning to become trans-radiant. The timing of the infusion with methyl cellulose provides a quick second look at each segment of the small bowel. [22-24] (See Fig. 9.)

Reflux Small-Bowel Enemas

The small bowel also can be filled with a barium suspension injected through an ileostomy, colostomy or via the rectum after a single contrast enema. These examinations can be used to study the entire small bowel, but generally are reserved for cases in which an obstruction in the distal intestine is suspected. (See Fig. 10.)

Examination of the small bowel through an ileostomy requires the use of a Foley catheter. The catheter should be inserted until the balloon is past the abdominal wall. The balloon then is inflated with 5 mL to 10 mL of air and the catheter is retracted until the balloon lodges against the interior of the ileostomy and functions as a ball valve.

Barium is injected until the entire length of the small bowel is filled. Radiography is performed using compression with the patient in the supine position. Again, a supine film of the entire abdomen should be performed upon conclusion of the examination to demonstrate the nature and appearance of the small bowel.

This examination is important in evaluating ileostomy dysfunction, which often is due to adhesions that partially obstruct the distal ileum proximal to the ileostomy. It also is useful in the detection of recurrent Crohn's disease of the ileum in patients who have undergone a colonic resection. Reflux examination of the small bowel via colostomy is performed in much the same manner as examination via an ileostomy.

Reflux examination of the small bowel also can be accomplished with a rectal barium enema, although this procedure is often a very uncomfortable examination for the patient. A standard single-contrast examination is performed using an 18% w/v suspension of barium. The colon is imaged only if indicated by the patient's history.

Once the colon is filled with barium, a water infusion is begun to push the barium into the small bowel. The subsequent administration of water also helps to displace the barium from the colon so that the barium in the small bowel can be filmed without being obscured by barium remaining in the colon. As in the aforementioned imaging techniques, the small bowel then is spot-filmed using compression.

The rectal barium enema procedure seldom is used today, except in cases in which lesions are believed to be located in the distal small bowel. Enteroclysis remains the preferred method of examination.

Cross-sectional Imaging

The advantage of cross-sectional diagnostic imaging techniques is directly related to their ability to visualize the small bowel in relation to other organs. In certain disorders of the small bowel, CT may provide complementary information to barium studies by demonstrating the extent of involvement of other organs or of the mesentery.

In neoplastic disorders, intra-abdominal metastasis can be demonstrated with CT. (See Fig. 11.) Computed tomography also is useful in evaluating both local and extraintestinal complications of Crohn's disease, such as the degree of bowel wall thickening, inflammatory reaction of the mesentery and abscess formation. (See Fig. 12.)

In general, sonography is not used to specifically diagnose small bowel disorders. However, since sonography often is used as a screening procedure in patients complaining of abdominal pain or masses, it may be the first study performed that suggests an abnormality or pathology. Solid lesions and neoplastic masses can produce a characteristic sonographic pattern, known as the target or pseudokidney sign. While gas-filled loops of the small bowel cannot be imaged with sonography, fluid-filled, distended loops can be visualized and may be useful in confirming small bowel obstruction. (See Fig. 13.)


Although the anatomy and physiology of the small bowel are well understood, accurate diagnosis of disorders of the small bowel remains elusive. Despite the availability of a variety of highly sensitive diagnostic imaging procedures, the quality of imaging techniques is contingent ultimately upon the experience of the radiologic technologist who performs the exam.

Barium contrast studies remain the method of choice for evaluating the small bowel, with the introduction of enteroclysis significantly improving the small bowel examination. As imaging techniques continue to be refined, there is no doubt that enhanced evaluation of the small bowel will be achieved in tandem with greater patient satisfaction and less patient discomfort.

Careful and accurate imaging of the small bowel will continue to be paramount in the accurate diagnosis and treatment of small bowel disorders, and radiologic imaging will remain the most important diagnostic tool available in the diagnosis and treatment of small bowel disease.


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Julliana Newman, B.A., ELS, is a certified editor of the life sciences who lives in Albuquerque, N.M. She won the 1997 Award for Excellence in Scholarly/Professional Articles in the International Technical Publications Competition of the Society for Technical Communication.

Reprint requests may be sent to the American Society of Radiologic Technologists, Publications Department, 15000 Central Ave. SE, Albuquerque, NM 87123-3917.

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Publication:Radiologic Technology
Date:Jul 1, 1997
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