The small intestine, often referred to as the small bowel, is a long, coiled tube connecting the stomach to the large intestine. It is divided into three segments: the duodenum, the jejunum, and the ileum, where the majority of nutrient and water absorption occurs. Evaluating this organ presents a unique challenge for medical professionals due to its considerable length, up to 25 feet, and its difficult-to-access position. Since traditional endoscopy cannot reliably reach the entire small intestine, specialized diagnostic methods have been developed to inspect its inner lining, assess its structure, and check its function.
Direct Visualization Techniques
Direct visualization involves methods that allow a doctor to obtain a clear, magnified view of the small intestine’s inner surface, or mucosa. One such widely used non-invasive technique is Video Capsule Endoscopy (VCE), sometimes called a PillCam. The patient swallows a vitamin-sized capsule that contains a miniature camera, a light source, and a radio transmitter. The capsule is propelled naturally through the entire gastrointestinal tract by peristalsis, capturing thousands of high-resolution images as it moves along its path.
These images are transmitted wirelessly to a recorder worn on a belt by the patient, typically over an eight-hour period. VCE is particularly useful for detecting mucosal abnormalities such as obscure bleeding sources, small ulcers, polyps, or early signs of inflammatory conditions like Crohn’s disease. The high magnification of the capsule camera allows for the detection of subtle lesions that may not be visible with other imaging methods.
However, the passive movement of the capsule means the physician cannot control its speed or direction. A significant limitation is the inability to perform therapeutic interventions, such as stopping a bleed, removing a polyp, or taking a tissue sample for biopsy. Patients with known or suspected narrowing of the small bowel (stricture) risk the capsule becoming lodged and causing an obstruction, which is an absolute contraindication for the procedure.
When visual inspection and intervention are required, doctors use enteroscopy, which involves passing a long, flexible endoscope deep into the small bowel. Conventional endoscopes only reach the duodenum, necessitating specialized devices to explore the jejunum and ileum. Earlier methods, such as push enteroscopy, allowed limited access beyond the duodenum by simply pushing the scope forward.
Modern advanced methods primarily rely on balloon-assisted enteroscopy, which includes both single-balloon and double-balloon systems. These devices use inflatable balloons attached to the scope and an overtube to systematically “pleat” or gather the small intestine onto the scope, similar to pushing a curtain onto a rod. This telescoping action allows the physician to achieve significantly deeper insertion, potentially reaching the entire length of the small bowel.
The advantage of balloon-assisted enteroscopy is that it allows for real-time visual inspection and therapeutic procedures. The doctor can pass instruments through a working channel in the scope to take biopsies from suspicious tissue, remove polyps, or treat sources of bleeding. The procedure can be performed either orally (antegrade) to examine the upper small bowel or rectally (retrograde) to access the lower portion.
Structural and Cross-Sectional Imaging
Beyond seeing the inner lining, doctors often need to evaluate the full structure of the small bowel wall and the tissues surrounding it. Cross-sectional imaging techniques achieve this by providing detailed, three-dimensional views of the abdominal contents. Computed Tomography Enterography (CTE) and Magnetic Resonance Enterography (MRE) are the primary tools used for this type of assessment.
Both procedures require the patient to drink a large volume of an oral contrast solution to fully distend the small intestine loops. This distension is crucial for separating the coiled segments and clearly outlining the bowel wall for examination. For both CT and MR enterography, an intravenous contrast dye is typically administered to highlight active inflammation, which appears as increased enhancement or brightening of the bowel wall.
CTE uses X-rays to create its images and is generally faster than MRE, which makes it particularly useful in acute situations, such as when a bowel obstruction, perforation, or abscess is suspected. CTE provides excellent spatial resolution, allowing doctors to quickly identify complications and the extent of pathological processes.
MRE, conversely, uses strong magnetic fields and radio waves instead of ionizing radiation, making it the preferred modality for younger patients or those requiring frequent follow-up monitoring for chronic conditions like Crohn’s disease. While MRE acquisition takes longer than a CT scan, it offers superior soft-tissue contrast resolution. This is highly effective for detecting subtle findings like edema, fistulas, and perianal disease.
Older fluoroscopy techniques, specifically Barium Studies, are still used in select cases, though they have largely been replaced by CTE and MRE. The Small Bowel Follow-Through (SBFT) involves the patient drinking a liquid barium contrast medium, and then a series of X-ray images are taken over several hours as the contrast progresses through the small bowel. This method checks for blockages, abnormal motility, or significant structural changes.
A more detailed variation is Barium Enteroclysis, where the contrast is delivered directly into the jejunum through a nasogastric tube. This intubation allows for controlled and better distension of the small bowel loops, which can improve the visualization of subtle lesions compared to the standard follow-through. However, these barium methods are limited because they only provide a view of the lumen and not the full thickness of the bowel wall or surrounding structures.
Assessing Function Through Lab and Breath Tests
When structural problems are ruled out, doctors may investigate how well the small intestine is performing its digestive and absorptive tasks using non-invasive functional tests. Breath tests are a common method used to detect imbalances in the gut bacteria or the inability to process specific carbohydrates. These tests rely on the fact that only bacteria in the gut, not human cells, produce certain gases like hydrogen and methane when they ferment sugars.
For example, a test for Small Intestinal Bacterial Overgrowth (SIBO) involves consuming a specific sugar, such as lactulose or glucose, followed by collecting breath samples at regular intervals. A rapid rise in hydrogen or methane gas levels suggests that excessive bacteria are fermenting the sugar in the small intestine. Similarly, a breath test for lactose or fructose intolerance measures gas production after ingesting the specific sugar, indicating malabsorption.
Indirect assessments of small bowel function can also be gathered through routine blood and stool analyses. Blood tests can measure levels of essential vitamins, such as B12 or iron, whose malabsorption can point to dysfunction in the ileum or duodenum. Stool tests provide insight into the efficiency of fat digestion and absorption, with elevated fecal fat levels suggesting a malabsorption syndrome.
Specific blood markers can also provide indirect evidence of a problem, such as antibody tests for Celiac disease or inflammatory markers suggesting generalized inflammation within the digestive tract. These lab-based chemical and microbial analyses complement the direct visualization and cross-sectional imaging techniques.