The Small Intestine, Mesentery and Peritoneal Cavity

Postbulbar Ulceration

Postbulbar ulcers are most commonly seen on the concave border of the second part of the duodenum or in the immediate postbulbar area. The ulcer is shown as a typical crater, frequently with spasm of the opposite wall. There may be narrowing of the lumen and thickening of the mucosal folds. In some cases, scar formation may obscure the ulcer crater. Postbulbar ulcers usually fail to heal with medical treatment.

Complications of Peptic Ulceration

Acute haemorrhage is the commonest complication of peptic ulcer disease (PUD), occurring in about 15%, followed by perforation. Contrast-enhanced CT is the technique of choice in the acute setting to detect perforation, bleeding and penetration of adjacent organs. Free perforation may be detected as pneumoperitoneum on the erect chest radiograph. The perforation is sometimes localised or ‘walled-off’ with a marked deformity of the duodenum due to the adjacent inflammatory reaction ( Fig. 21.2 ). Bleeding caused by duodenal ulceration is diagnosed by endoscopy and/or conventional or CT angiography. CT angiography is performed without positive oral contrast and scanning protocol should include unenhanced, arterial and delayed acquisition.

Benign Neoplasms

Benign neoplasms of the duodenum are uncommon, accounting for 10%–20% of duodenal masses, and are often asymptomatic. Adenomatous polyps feature as solitary, mostly sessile, polypoid, intraluminal filling defects on barium studies, or as soft-tissue masses or intraluminal filling defects on CT ( Fig. 21.4 ). Villous adenomas exhibit a characteristic ‘cauliflower’ or ‘soap bubble’ appearance on barium studies, caused by the trapping of barium in the crevices between the multiple frond-like projections of the tumour. Other benign neoplasms of the duodenum include periampullary adenomas, lipomas having low (fat tissue) density on CT ( Fig. 21.5 ) and neurogenic tumours or hamartomas exhibiting the same features as in the SI.

Malignant Neoplasms

Primary Carcinoma

Carcinoma of the major duodenal papilla (also known as the ampulla of Vater) is the type most frequently encountered, usually presenting with jaundice. Non-papillary carcinomas of the duodenum are adenocarcinomas and usually present clinically with duodenal obstruction. On CT, primary carcinomas are seen mostly as focal masses with asymmetric mural thickening, with varying degrees of luminal narrowing ( Fig. 21.6 ). With ampullary/periampullary lesions, biliary or pancreatic duct dilatation is often seen on MRI and CT. When simultaneously present, this is known as the ‘double duct’ sign. Coincident adenopathy or hepatic metastases may also be present. Other malignant primary neoplasms occasionally encountered in the duodenum include gastrointestinal stromal tumour (GIST) and lymphomas.

Pancreatitis

Reactive inflammation of the duodenum may be seen on imaging of acute pancreatitis. Ileus may be seen on plain radiographs and cross-sectional imaging. Mucosal oedema, enlargement of the duodenal loop and thickening of the ampulla of Vater are characteristic findings. Cystic wall changes in the second part of the duodenum and varying degrees of stenosis have been reported on CT and MRI with a specific type of pancreatitis referred to as ‘groove’ pancreatitis. This is a chronic form of inflammation usually confined to the paraduodenal pancreatic parenchyma and the region between the duodenum and head of pancreas ( Fig. 21.7 ).

Crohn Disease

The duodenum is affected in about 4% of patients with Crohn disease. The radiological appearances are similar to those observed in the more distal parts of the SI. The valvulae conniventes are frequently thickened. At a more advanced stage of the disease, there may be strictures with eccentric or concentric narrowing. The disease may cause narrowing of the antrum and proximal duodenum in continuity ( Fig. 21.8 ), resulting in the ‘pseudo post-Billroth I’ appearance.

Progressive Systemic Sclerosis

In patients with connective tissue or neuromuscular disorders, particularly progressive systemic sclerosis and visceral myopathy, the duodenum is frequently involved. There may be dilatation, which is often more pronounced in the second, third and fourth parts. The dilated duodenum may be slow to empty and the grossly dilated, atonic organ may produce a sump effect.

Intramural Haematoma

The most common cause of intramural haematoma is blunt abdominal trauma, classically involving the third part overlying the vertebral column. Infiltration of the duodenum wall with blood and oedema may result in signs ranging from subtle thickening of the valvulae conniventes to a large mixed-attenuation mass ( Fig. 21.9 ). In severe trauma, the duodenum may rupture. This most frequently occurs at the junction of the second and third parts, resulting in retroperitoneal free gas, haemorrhage and fluid.

Vascular Conditions

Abdominal aortic aneurysms may compress the third part of the duodenum and occasionally cause obstruction. The duodenum, when faintly opacified with oral contrast medium and stretched around an aneurysm, may be misinterpreted as a contained leak or as a patch of perianeurysmal inflammation ( Fig. 21.10 ). Aortoenteric fistulae most often involve the duodenum, particularly the third part of the duodenum. An aortoenteric fistula should always be suspected in patients who have undergone aortic graft surgery and present with gastrointestinal haemorrhage. The superior mesenteric artery compression syndrome is a rare form of high intestinal obstruction, which is believed to be caused by the narrowing of the normal angle between the aorta and superior mesenteric artery. This entity may be diagnosed on multidetector computed tomography (MDCT) when a transition point between dilated and collapsed duodenum is observed in the third portion of the duodenum.

Plain Radiographs

Patients who present acutely with suspected perforation or obstruction of the SI are usually investigated initially with plain abdominal radiographs. The erect chest x-ray, which may reveal free subphrenic air, is usually the more important.

Barium Studies

CT and MR enterography have largely replaced the use of barium studies for non-acute intestinal imaging. Yet, barium studies are still widely available and provide a cost-effective means of imaging the SB where availability or access to modern MRI or CT is limited. Optimally performed barium studies, as a result of their superior spatial resolution, offer superior mucosal detail when compared with MR and CT imaging. Two principal methods are used in this context: the SI follow-through study, during which a large volume of barium or water-soluble contrast agent is ingested orally, and enteroclysis, which entails naso-intestinal intubation and high-volume infusion of contrast challenging and exerting control over intestinal distension. This is then followed by fluoroscopic interrogation of the SI with various patient body habitus manipulation and compression techniques. The control over bowel distension in enterocolysis through intestinal intubation and infusion of contrast achieves optimal bowel distension that offers excellent detail of mucosal changes, especially when double-contrast techniques are employed involving the instillation of intraluminal air after the contrast agent to outline the mucosa and walls. Radiation exposure poses a limitation for barium studies. Even though doses may not be as high as for CT or CTE studies, current opinion is that the diagnostic yield may not justify such exposure values ( Fig. 21.11 ).

Computed Tomography

CT has a major role in evaluating mural and extramural lesions and in assessing mesenteric involvement and ancillary intra-abdominal findings associated with inflammatory or neoplastic small intestinal diseases. Careful choice of CT imaging and reconstruction technique is essential to identify and characterise subtle intestinal abnormalities. Intravenous contrast administration is essential for a comprehensive CT examination of the SI. Peak enhancement of the small intestinal mucosa, known as the enteric phase, occurs approximately 50 seconds(s) after intravenous injection or 14 seconds after peak aortic enhancement.

For routine CT imaging, opacification of the intestinal lumen is achieved using orally administered positive contrast material such as iodinated or dilute barium solutions, starting approximately 1 hour before the examination. CT enterography ( Fig. 21.12 ) involves the use of larger volumes of neutral Hounsfield unit (0–30 HU) oral contrast agents, ingested orally at a faster rate to distend the SI. Many neutral Hounsfield unit oral contrast agents have been described in the literature, ranging from commercial products such as PEG-based bowel preparation, agents such as VoLumen (Bracco Diagnostics, Princeton, NJ), which is a low-density (0.1%) barium sulphate solution, to simpler solutions such as water or milk. These negative oral contrast agents may be used with or without the addition of bulking agents such as methylcellulose, locust bean gum and Mucofalk (Dr F. Pharma, Freiburg, Germany), a plant husk extract . Neutral oral contrast agents are gaining widespread acceptance and are more frequently used when a detailed CT study of the SB wall or mesenteric vessels is required . Recent studies comparing the efficacy and tolerability of enteric distension agents suggest that best results are achieved by PEG preparation as they are relatively less absorbable and remain inside the lumen for longer; a disadvantage of this, however, is that patients experience more abdominal cramps and discomfort. Although the site and severity of mucosal disease and mural enhancement patterns of the SI are much better assessed with neutral or negative oral contrast agents, positive enteric contrast agents remain valuable in patients with suspected perforation, abscess or extramural complications of Crohn disease. Some authors also maintain that positive enteric contrast agents may increase the conspicuity of cystic and subtle soft-tissue disease of the mesentery and peritoneum.

CT enteroclysis is performed by administering contrast medium directly into the SI through an 8–10-F nasojejunal tube. A total of 1.5 –2 L of enteric agent is instilled via a pump initially at a rate of 100–150 mL/s and then increased to 150–200 mL/s after the first 0.5–1 L to induce an ‘ileus’ state in the bowel wall in an effort to improve bowel distension. Water or methylcellulose solution is typically used as a neutral luminal contrast agent but an iodinated water-soluble contrast medium or a dilute barium solution can be used to provide positive luminal contrast. Patients are usually pre-medicated with an intravenous antiperistaltic agent. Although there are few comparative studies, diagnostic accuracy rates do not seem significantly different between CT enterography and CT enteroclysis, but a majority of studies report greater patient tolerance with CT enterography.

Ultrasound

Unlike CT, angiography and fluoroscopy, ultrasound does not employ ionising radiation and, therefore, is particularly useful when imaging young patients with Crohn disease who often require repeated imaging. Ultrasound of the SI requires high-frequency (5 to 17 MHz) linear array probes, which provide increased spatial resolution of the intestinal wall. Colour or power Doppler imaging and contrast-enhanced US (CEUS) provide more detailed information on mural and extraintestinal vascularity, which may reflect inflammatory disease activity. High-resolution ultrasonography is quick and non-invasive but diagnostic accuracy is dependent on operator experience. In clinical practice, US plays a rather limited role in the diagnosis and management of other small intestinal disorders but small intestinal obstruction may also be recognised and primary intestinal neoplasms may be identified using a dedicated US technique.

Magnetic Resonance Imaging

MRI provides excellent soft-tissue contrast and three-dimensional imaging capabilities, which are crucial when studying the SI. Prerequisites for a state-of-the-art MRI examination of the SI include adequate bowel distension, which may be achieved by enterography or enteroclysis, homogeneous intraluminal positive or negative contrast and dynamic multiphase intravenous contrast enhancement. Imaging sequences should have a high temporal resolution to reduce bowel motion artefacts. Many MR protocols incorporate cine sequences to facilitate the dynamic assessment of intestinal motility. Cine sequences aid in distinguishing peristaltic narrowing from true strictures and for assessing the severity of strictures ( Fig. 21.13 ) The sensitivity for detecting small tumours by gadolinium-enhanced T ₁ MR imaging with fat suppression has been shown to be greater than that of CT ( Fig. 21.14 ).

Angiography

In acute active bleeding, angiography has been shown to demonstrate haemorrhage when the rate of blood loss exceeds 0.5 mL/min. Less severe gastrointestinal bleeding is frequently intermittent rather than continuous, resulting in a high rate of negative angiographic examinations. In the clinical context of acute active bleeding, it is important to avoid barium, iodine or other positive oral contrast agents before angiographic studies.

Nuclear Medicine Studies

Nuclear medicine studies with labelled red cells are useful for detecting lower grade or intermittent intestinal haemorrhage. Leucocyte scintigraphy and fluoro­deoxyglucose-positron-emission tomography (FDG-PET) are helpful in the diagnosis of suspected inflammatory bowel disease and/or the assessment of current disease activity and treatment response. Older techniques, like pertechnetate scintigraphy for the detection of a Meckel diverticulum that contains gastric mucosa, have yet to be surpassed by more modern imaging methods. Carcinoid tumours of the SB may be elegantly demonstrated by scintigraphy ( ¹¹¹ In-octreotide (Octreoscan)). In addition, flurodeoxyglucose (FDG) - positron emission tomography (PET)/Computed tomography (CT) [FDG-PET/CT] techniques are already proving useful in SB oncology.

Radiological Appearances

Crohn disease of the SI has a variety of radiological appearances, some of which are pathognomonic. Characteristic features and the multiplicity of imaging findings vary depending on the severity, stage and extent of the disease. These are listed in Table 21.1 .

Barium studies may demonstrate aphthoid ulcers, which appear as small collections of barium with surrounding radiolucent margins due to oedema. Blunted, flattening and thickening of the valvulae conniventes may occur in approximately 50% of patients with Crohn disease.

As the inflammation proceeds to deeper mural layers, fissure ( Fig. 21.15 ) and longitudinal ulcers ( Fig. 21.16 ) may develop. Cobblestoning is fairly common and is caused by a combination of longitudinal and transverse ulceration, separating intact portions of mucosa ( Fig. 21.17 ).

Narrowing of the intestinal lumen is common, as are strictures, which may be short, long, single or multiple, the latter being virtually diagnostic of Crohn disease. Solitary strictures ( Fig. 21.18 ) may be accompanied by proximal dilation and, in the absence of additional evidence of Crohn disease, have to be differentiated from other causes of stricture ( Table 21.2 ). Discontinuous involvement of the intestinal wall is shown either as skip lesions or asymmetry. Asymmetrical involvement of the intestinal wall produces characteristic ‘pseudodiverticula’, which represent small patches of normal intestine in an otherwise severely involved segment. Inflammatory polyps (pseudopolyps) are seen in Crohn disease as small, discrete round filling defects, but are not a frequent finding. A smooth, featureless outline replacing the normal mucosal pattern, without any significant changes in the calibre of the lumen, may occasionally be seen. Mesenteric abnormalities are commonly seen, with mesenteric inflammation causing hypertrophy of mesenteric adipose tissue known as fibrofatty proliferation being a characteristic feature. This usually occurs along the mesenteric side of bowel causing mass-effect giving the characteristic finding of bowel loop separation. Fibrofatty proliferation remains present even during clinically non-active disease.

Cross-sectional imaging is increasingly important in the initial diagnosis and follow-up of patients with Crohn disease, because of its ability to directly image the intestinal wall and thereby assess any mucosal inflammation; it also facilitates the accurate diagnosis of extramural disease/complications such as sinuses, fistulae, phlegmon and abscess formation ( Fig. 21.19 ). Identification of interloop, pelvic and abdominal wall abscesses is essential for appropriate patient management as their presence often contraindicates the use of advanced immunosuppressive medications such as the anti-tumour necrosis factor (TNF) agent infliximab.

On CT, the thickened bowel wall may enhance homogeneously or may demonstrate a stratified ‘target’ appearance (see Fig. 21.19 ). Mural stratification is often demonstrated in active lesions during IV contrast enhancement. Multisegmental discontinuous SI disease, leading to ‘skip lesions’, is highly suggestive of Crohn disease, particularly when the bowel wall thickening is asymmetric, or ulceration is seen (see Fig. 21.17 ). Perienteric changes that occur in mesentery and peritoneum include mesenteric hypervascularity leading to the ‘comb sign’, fat stranding and fibrofatty proliferation, which is caused by an accumulation of mesenteric fat related to the chronic inflammatory process ( Fig. 21.20 ).

The important role of CT in Crohn disease means that radiation exposure is an ongoing concern due to the repeated imaging that is often necessary due to the relapsing, remitting nature of the disease. The development of low-dose CT protocols or alternatives to CT has gained increasing interest. Recent studies comparing low-dose CT images, reconstructed with noise-reducing iterative reconstruction techniques, with conventional-dose CT examinations demonstrate successful results preserving diagnostic yield whilst considerably reducing radiation exposure.

MR enterography is an excellent alternative technique for the evaluation of the SB in patients with Crohn disease. Characteristic lesions such as bowel wall thickening ( Fig. 21.21 ), linear and fissure ulcers, fistulae and cobblestoning are accurately depicted on T ₂ weighted MR sequences. Fat-suppressed, T ₂ weighted sequences are required to identify submucosal oedema and T ₁ weighted post-contrast sequences are required to identify mucosal enhancement ( Fig. 21.22 ). Magnetic Resonance Enterography (MRE) is comparable with conventional enteroclysis in assessing the number and extent of involved SB segments and in demonstrating luminal narrowing and/or pre-stenotic dilatation. MRE has a clear advantage over conventional enteroclysis in demonstrating extramural manifestations and/or complications of Crohn disease, including abscesses. Sinuses and fistulous tracts manifest as high signal tracts on T ₂ weighted fat-saturated MR images with avid enhancement following gadolinium-based contrast material administration ( Fig. 21.23 ). Several recent studies have suggested that the addition of diffusion-weighted sequences to MRE protocols may improve the detection of acutely inflamed segments, especially in younger patients where bowel loops are overlapping because of a scarcity of intra-abdominal fat. Furthermore, apparent diffusion coefficient (ADC) values have been reported as useful in determining severity of inflammation and distinction of normal versus inflamed bowel.

Ultrasound represents another ‘radiation-free’ alternative but has a limited role. Variable sensitivity and specificity values have been reported for ultrasound for initial diagnosis of CD because of its reliance on operator skill and experience. Segmental thickening of bowel wall greater than 3 mm, non-compressibility and hypervascularity on Doppler evaluation are markers of active inflammation in patients with known CD. US examinations serve as a fast and cost-effective tool for the investigation of some extra-intestinal complications, primarily abscesses and hepatobiliary complications.

Evaluation of all imaging studies in patients with Crohn disease should always include a search for the extra-intestinal manifestations of Crohn disease, which include renal calculi, erosive sacroiliitis and for evidence of sclerosing cholangitis. Standard MRI is commonly deployed for hepatobiliary or perianal complications.

Complications of Crohn disease directly related to the intestinal disease process include both gastrointestinal adenocarcinomas and carcinoid tumours, but the radiologist should also be aware of emerging reports of an increased risk of lymphoma in Crohn patients receiving long-term immunosuppressive medications such as azathioprine or anti-TNF agents.