Plain radiographs are the initial imaging procedure performed in most patients who present with suspected acute disorders of the gastrointestinal tract. Radiographs of the abdomen and chest can provide essential diagnostic information.

A supine view of the abdomen and an upright view of the chest are the basic views considered essential in most patients. The upright chest radiograph is an important part of the examination: pain from pleural or lung disorders may present initially with abdominal pain and this is the most reliable view for showing free intraperitoneal air. Decubitus and upright views of the abdomen are also occasionally helpful. It may be necessary to proceed to contrast studies, ultrasound, or computed tomography (CT) if plain radiographs are unhelpful or inconclusive.

Spontaneous pneumoperitoneum normally indicates that a duodenal or gastric ulcer has perforated, or that the colon has perforated due to diverticulitis, acute colitis, carcinoma, or trauma. Perforation of the small intestine is uncommon. Free intraperitoneal air is demonstrated in 60 to 90 per cent of plain radiographs performed in patients with pneumoperitoneum, depending on how carefully the examination is performed. The upright posteroanterior chest and left lateral decubitus (right side up) abdominal radiographs are the best views for demonstrating the presence of pneumoperitoneum.

With good radiographic technique as little as 1 ml of intraperitoneal air can be detected. A small amount of free intraperitoneal air is demonstrated as a sickle-shaped collection of air between the liver and the diaphragm on the chest view (Fig. 1) 139 and between the liver and the abdominal wall on the left lateral decubitus view. Larger collections of air may outline the liver. When a relatively large amount of free intraperitoneal air is present characteristic signs may be seen on the supine radiograph. These include gas outlining the outer wall of the intestine (Rigler's signs), a triangular collection of gas between intestinal loops, and gas outlining the gallbladder, the lower border of the liver, lesser sac, and the falciform ligament (Fig. 2) 140. The characteristic ‘football sign’ is seen most frequently in infants, when a large amount of gas outlines the lateral limits of the peritoneal cavity.

Small intestinal obstruction can be diagnosed on plain abdominal radiographs in 60 to 70 per cent of patients, and the supine abdominal view is the most reliable for making the diagnosis. Typical features are gas-distended loops of jejunum and ileum arranged in transverse loops across the central portion of the abdomen (Fig. 3) 141. Little or no gas is seen in the colon in most patients with obstruction of the small intestine, but a moderate or normal amount of colonic gas may be present if the lumen of the small intestine is not completely occluded. If the obstructed loops are fluid-filled they are more difficult to identify, but an upright view in such patients shows the classical ‘string of beads’ sign due to multiple small collections of gas above the fluid (Fig. 4) 142. This is a diagnostic sign of small intestinal obstruction even in the absence of gas-distended loops of intestine.

Plain abdominal radiographs may have a normal appearance in patients with small intestinal obstruction, due to vomiting in cases of high obstruction or because of the intermittent nature of the obstruction.

Closed-loop obstruction occurs when a single loop of intestine is obstructed at two points, one proximal and one distal. Gas and/or fluid may be seen in a round or oval-shaped loop that remains constant in position on different views.

The characteristic appearances of gallstone ileus, caused by impaction of a gallstone in the small intestine, include evidence of intestinal obstruction, visualization of the obstruction calculus, and, in about one-third of cases, air in the biliary tree.

The plain radiographic appearance of obstruction of the large intestine will depend on whether or not the ileocaecal valve is competent. When the valve is competent there is usually considerable dilatation of the colon as far as the obstruction, including marked caecal dilatation, usually with no dilatation of the small intestine. The ileocaecal valve is incompetent in most patients and dilatation of the colon and the small intestine is seen, with the caecum only showing slight dilatation. Fluid-filled distension of the proximal colon is seen when the obstructive lesion is proximal to the splenic flexure.

The site of transition between dilated gas- or fluid-filled colon and collapsed empty colon normally identifies the site of the obstructing lesion. If there is any doubt about the diagnosis an instant single-contrast barium or water-soluble contrast enema, performed with the contrast medium passing as far as the dilated colonic segments, confirms the presence or absence of obstruction. When obstruction is confirmed the cause is frequently identified. Caecal volvulus should be suspected when a haustrated and disproportionately enlarged air-filled viscus is seen anywhere in the abdomen; the caecum is usually absent from the right iliac fossa and distended small intestine is seen to the right of the dilated caecum. Sigmoid volvulus can frequently also be diagnosed on plain abdominal radiographs: the characteristic appearance is that of a grossly enlarged, gas-filled sigmoid colon arising from the pelvis and deviating to the left or right flank. The apex of the loop is positioned high in the abdomen and may lie under and elevate the diaphragm. Three dense curved lines, representing the walls of the enlarged loop, converge towards the stenosis over the left part of the sacrum (Fig. 5) 143.

The supine view of the abdomen frequently yields important diagnostic information in acute ulcerative colitis. When air is present in the colon the mucosal edge and haustral pattern give an indication of the severity of the inflammation. In the segments where there is faecal residue active mucosal disease is unlikely. Patients with toxic megacolon, a potentially lethal complication of ulcerative colitis, show dilatation of the transverse colon (exceeding 5.5 cm in width). Other signs of toxic megacolon include loss of the normal haustral pattern, an irregular contour to the colonic wall, and numerous, broad-based rounded inflammatory polyps (pseudopolyps) projecting into the lumen of the dilated segment (Fig. 6) 144. Perforation is a serious complication of toxic megacolon.

Acute mesenteric ischaemia and infarction may occur when emboli arising in the heart following atrial fibrillation, myocardial infarction, a left atrial myxoma, or deep venous thrombosis via a patent foramen ovale lodge in the superior mesenteric artery. Other causes of mesenteric infarction include cardiogenic shock and penetrating or blunt trauma to the abdomen. Plain abdominal radiographs show distended loops of small intestine shortly after the onset of symptoms; the number and size of the distended loops increase later. Specific radiological signs may develop, including thickening and oedema of the valvulae conniventes, thickening of the intestinal wall, air in the intestinal wall (Fig. 7) 145, and air in the intrahepatic portal veins.

Paralytic (adynamic) ileus is one of the more common forms of intestinal obstruction and usually occurs throughout the gastrointestinal tract, although occasionally involving only one segment. It is a risk that is present for 3 to 4 days after an abdominal operation. Other causes of paralytic ileus include intestinal ischaemia, sepsis, intraperitoneal inflammation such as acute appendicitis, cholecystitis, pancreatitis, retroperitoneal haematoma, fracture of the spine, ureteric colic, thoracic lesions such as basal pneumonia, rib fractures, or myocardial infarction.

Dilated loops of small and large intestine with air–fluid levels are frequently seen on plain abdominal radiographs. It may be impossible to distinguish adynamic ileus from obstruction and a contrast study may be required to establish the correct diagnosis. When there is localized inflammation, such as in appendicitis, cholecystitis, or pancreatitis, the ileus may develop in one or two adjacent loops of small intestine called ‘sentinel loops’.

Barium-enhanced examination of the upper gastrointestinal tract is used to evaluate the oesophagus, stomach, and duodenum. The double-contrast barium examination, which is now used routinely in the great majority of centres, is quick and easy to perform and takes about 10 to 15 min.

High-density barium is used to coat the mucosal surfaces and a gas-producing agent is used to distend the stomach and duodenum. An intravenous injection of hyoscine butylbromide (Buscopan) or glucagon is given to relax smooth muscle. Double-contrast views of the oesophagus are obtained when the barium is swallowed quickly, so that the swallowed air distends the oesophagus enabling mucosal views to be obtained.

The oesophagus extends from the cricopharyngeus to the gastro-oesophageal junctions. Dysphagia (difficulty in swallowing) is a distressing symptom caused by narrowing of the lumen of the oesophagus. Causative factors include carcinoma of the oesophagus, carcinoma of the fundus of the stomach invading the lower oesophagus, benign strictures, extrinsic neoplasms compressing the oesophagus, and oesophageal webs. A food bolus may impact in the oesophagus during severe oesophageal spasm.

Carcinoma of the oesophagus appears on barium studies as an irregular stricture with mucosal destruction and shouldering of the margins, as an infiltrating constricting lesion, or as an irregular polypoid mass (Fig. 8) 146.

When carcinoma of the fundus invades the lower oesophagus the primary tumour may be obvious as a large mass in the fundus. In other cases a carcinoma is seen at the oesophagogastric junction with little indication of whether the neoplasm originates in the lower oesophagus or fundus of the stomach.

The oesophagus may be compressed by enlarged neoplastic mediastinal lymph glands or primary carcinoma of the bronchus. Carcinoma of the bronchus occasionally invades the oesophagus, resulting in an oesophagobronchial fistula that can be identified on a barium swallow.

Most benign strictures result from gastro-oesophageal reflux and are usually located in the lower oesophagus just above the oesophagogastric junction. There may be an associated hiatal hernia. Such strictures usually appear as smooth segments of narrowing, although there may be some irregularity of the mucosa without mucosal destruction (Fig. 9) 147. It may be impossible to distinguish a benign oesophageal stricture from primary carcinoma.

Prolonged nasogastric intubation may result in the development of oesophageal strictures, and the accidental ingestion of corrosive acids or alkalis can also result in severe damage to the oesophagus with subsequent stricture formation. Certain medication in tablet form, including tetracycline, quinidine, and potassium chloride, may lodge in the oesophagus at the level of the aortic arch, causing oesophagitis and occasionally strictures.

Hiatus hernias, classified as sliding and rolling, are seen as herniations of stomach through the diaphragmatic hiatus into the thorax. Sliding hiatal hernias are by far the more common type and are present when both the oesophagogastric junction and stomach herniate into the thorax. While only a small amount of stomach may herniate in some patients, in others the whole stomach is affected. Hiatal hernias are reducible when they move in and out of the thorax and non-reducible when part of the stomach remains fixed in the thorax. The rolling type of hiatal hernia, also known as para-oesophageal hernia is seen when the oesophagogastric junction remains in its normal position below the diaphragm, the stomach herniating into the thorax beside the normally positioned lower oesophagus.

Oesophageal webs are seen as shelf-like defects in the cervical oesophagus extending from the anterior wall in a posterior direction. Some webs are circumferential and allow a jet of barium to pass through the centre.

Diverticula may be seen in the oesophagus at the pharyngo-oesophageal junction (Zenker's diverticulum; Fig. 10 148), mid-oesophagus, or at the distal end of the oesophagus (epiphrenic diverticulum). Most mid-oesophageal and epiphrenic diverticula are asymptomatic. Zenker's diverticula may be small and difficult to recognize or can be large, compressing the adjacent oesophagus and soft tissues. The formation of Zenker's diverticulum is often related to gastroesophageal reflux and as a result there is an association between benign oesophageal strictures and Zenker's diverticula. For this reason, a barium swallow should always be performed in patients who present with dysphagia before proceeding to endoscopy, which can result in perforation of the diverticulum, with serious consequences.

Oesophageal varices represent dilated venous collaterals, and usually result from portal venous hypertension. They are seen on barium examination as serpiginous or oval filling defects, mostly in the lower oesophagus and extending upwards to involve the middle third of the oesophagus (Fig. 11) 149. Obstruction to the superior vena cava may result in upper oesophageal varices.

The most frequently encountered disorders of the stomach are ulceration and carcinoma. Gastric ulcers are seen as small or large, round or oval collections of barium with a surrounding zone of radiolucency due to oedema. Folds frequently radiate from the edge of the ulcer crater. The most frequent sites of gastric ulceration are the lesser curve and the posterior wall of the stomach (Fig. 12) 150. So-called ‘sump ulcers’ may develop on the greater curve aspect of the gastric antrum and lower body of the stomach in patients taking analgesic medications (Fig. 13) 151, commonly in the elderly. They develop because of the combined effect of gravity and the corrosive action of the drugs. Occasionally these sump ulcers penetrate through the gastric wall and result in the formation of a gastrocolic fistula.

Erosions are mostly present in the antrum and appear as small collections of barium with surrounding oedema, often located on gastric mucosal folds.

Carcinoma of the stomach is seen as an ulcerating, polypoid, or infiltrating lesion. Ulcerating carcinomas, sometimes called malignant ulcers, show thickening or distortion of the folds at the edge of the crater and fusion or amputation of the folds by an area of induration at the ulcer edge. Malignant ulcers are often shallow with a nodular or uneven pattern in the base of the crater and have ill-defined or irregular outlines. If malignancy is suspected an adequate number of endoscopic biopsies should be obtained at the earliest opportunity.

Polypoid carcinomas appear as irregular polypoid filling defects in the stomach (Fig. 14) 152. Infiltrating carcinomas characteristically produce marked narrowing of the lumen of the stomach and when they involve the whole stomach, show the characteristic ‘linitis plastica’ appearance (Fig. 15) 153; however, mucosal biopsies obtained at endoscopy from areas of linitis plastica often fail to show evidence of malignancy. Metastatic carcinoma of the breast may infiltrate the wall of the stomach and also result in a ‘linitus plastica’ appearance.

The clinical and radiological features of primary gastric lymphoma, which accounts for 2.5 per cent of malignant gastric neoplasms, frequently resemble those of gastric lesions, particularly carcinoma. Since the prognosis of primary gastric lymphoma is much better than that of carcinoma, accurate diagnosis is important. Characteristic radiological features include a mass that may be partially effaced by the barium, gross hypertrophy of the mucosal folds that become more effaced as the stomach is distended (Fig. 16) 154, and one or more large gastric ulcers seen in association with mucosal hypertrophy. Narrowing and rigidity of the gastric antrum may be seen, sometimes extending across the pylorus into the duodenum. Duodenal ulceration is occasionally found in association with a gastric mass.

Benign peptic ulceration is common and is the most frequently encountered disorder in the duodenum. The barium examination is an accurate technique for detecting and demonstrating duodenal ulcers, provided a good double-contrast examination technique is used. Ulcer craters in the duodenum appear as single or multiple sharply defined, constant collections of barium, sometimes with a surrounding zone of oedema or with folds radiating from the crater. Most ulcers have a diameter of less than 1 cm (Fig. 17) 155. Some patients may have considerable deformity of the duodenal cap due to previous ulceration, making it difficult to detect an active ulcer crater. The degree of deformity varies considerably and, when marked, can result in duodenal stenosis.

A number of malignant neoplasms may involve the duodenum. Primary neoplasms of the duodenum are uncommon, and can be classified into carcinoma of the papilla of Vater and true carcinoma of the duodenum. Carcinoma of the papilla of Vater appears as an enlarged papilla with irregular borders, sometimes with ulceration. Non-papillary carcinomas of the duodenum are adenocarcinomas and are seen as ulcerative, polypoid or annular lesions, similar to the appearances of carcinomas in other parts of the gastrointestinal tract. Lymphomas and sarcomas of the duodenum are sometimes encountered.

The duodenum may be invaded by malignant neoplasms from adjacent organs or may be the site of metastic deposits. Carcinoma of the head of the pancreas frequently involves the duodenal loop, causing widening, a double contour, irregularity of the inner border, or stricture formation. The reversed ‘3’ sign of Frostberg is a characteristic but infrequent finding. Carcinoma of the body or tail of the pancreas may invade the distal duodenum. The duodenum may also be invaded by malignant neoplasms in adjacent organs such as the colon, right kidney, and gallbladder. The duodenum may be the site of metastatic deposits from malignancies elsewhere, particularly malignant melanoma.

The duodenum is affected by Crohn's disease in about 4 per cent of patients who have the disease elsewhere in the small intestine or colon. The appearances are similar to those in the more distal parts of the small intestine. Crohn's disease may cause tubular narrowing of the gastric antrum and proximal duodenum in continuity, resulting in the ‘pseudo post-Billroth I’ appearance.

Intramural duodenal haematoma can result from blunt abdominal trauma, anticoagulant therapy, or blood dyscrasia. On barium studies an intramural haematoma is seen as a concentric obstructive lesion in the second or third part of the duodenum, sometimes giving a ‘coiled spring’ appearance.

Duodenal diverticula are seen fairly frequently on barium examination and these have little clinical significance in the majority of patients.

Barium studies play an important role in the investigation of patients with known or suspected disorders of the small intestine. There is continuing debate as to which of the several available techniques should be used for routine examination of the small intestine. The barium follow-through and small bowel enema (enteroclysis) are used most frequently. The barium follow-through is a well established technique and is the preferred method of many radiologists, but the small bowel enema is being increasingly adopted in many centres. The barium follow-through is normally performed after the barium examination of the oesophagus, stomach, or duodenum by taking films when the barium passes into the small intestine. The small bowel enema is performed by passing a special radio-opaque nasogastric tube so that its tip lies in the distal duodenum or proximal jejunum and infusing dilute barium to outline the small intestine. A double-contrast technique using barium and an aqueous solution of methylcellulose is used in some centres. The small bowel enema is superior to the follow-through as the loops of jejunum and ileum are distended during the examination, making it easier to detect any abnormalities that may be present. The terminal ileum is frequently shown when barium refluxes through the ileocaecal valve during barium enema examinations.

Disorders that are disclosed as morphological changes in the small intestine on barium examination include Crohn's disease, neoplasms, chronic radiation enteritis, Meckel's diverticulum, jejunal diverticulosis, tuberculosis, and ischaemia.

Crohn's disease is characterized by a variety of radiological signs. Ulceration is common and is seen as fissure ulcers, discrete mucosal ulcers, longitudinal ulcers, sinuses, and fistulae (Fig. 18) 156. Single or multiple strictures are a frequent finding: tight strictures cause obstruction and result in dilatation of the more proximal intestine. Other signs of Crohn's disease including thickening of the valvulae conniventes, cobblestoning, asymmetrical lesions, skip lesions, and a mass in the right iliac fossa displacing adjacent loops of intestine.

Primary neoplasms of the small intestine are uncommon. Carcinoid tumours, located mostly in the distal ileum, may give a variety of radiological appearances. Characteristically, a carcinoid tumour appears as an intraluminal filling defect, but it may also be seen as a localized stricture or as a mass in the right iliac fossa causing distortion of a number of adjacent ileal segments. Primary carcinoma of the small intestine is nearly always located in the jejunum and has a radiological appearance similar to that of carcinoma of the colon. Lymphomas, which are multiple in 40 per cent of patients, are mostly seen in the ileum, although they may be located anywhere in the small intestine. Ulceration and cavitation is a frequent finding in lymphomas. Leiomyomas and leiomyosarcomas may be seen as either round intraluminal filling defects, or as cavitating masses related to the small intestine.

Chronic radiation enteritis develops in a small number of patients following radiotherapy to the abdomen and pelvis. The radiological changes include thickening of the valvulae conniventes, single or multiple stenoses, adhesions, sinuses, and fistulae. Meckel's diverticulum is seen as a solitary pouch arising from the antemesenteric border of the ileum.

Barium examination in patients with small intestinal obstruction shows the site of obstruction as an abrupt transition from distended or dilated small intestine to collapsed distal loops. Causes of obstruction that may be identified include neoplasms, Crohn's stricture, adhesions, or internal herniae. Other disorders of the small intestine that may be demonstrated on barium examination include jejunal diverticulosis, tuberculosis, and ischaemia.

The barium enema remains a widely used technique for the detection of carcinomas and adenomas in the colon, the diagnosis and evaluation of diverticular disease and its complications, and for assessing the extent and severity of inflammatory bowel disease. The double-contrast barium technique is the method of choice in most centres, although a single-contrast examination is performed in patients with suspected colonic obstruction. Water-soluble contrast studies are performed mostly to examine the anastomosis in patients who have undergone recent sigmoid resection.

Digital examination of the rectum and sigmoidoscopy are the initial diagnostic procedures in the investigation of colonic disorders, and these should always be performed before a barium enema is requested. If a rectal biopsy is performed, an interval of at least 7 days should be allowed before a barium enema is performed to avoid the risk of perforation.

A clean colon, a satisfactory barium suspension, the use of a smooth muscle relaxant, and good examination technique are necessary to obtain consistently good results. The colon is cleansed by using a combination of cathartics, low residue diet, and increased fluid intake on the day before the examination. A cleansing enema may be required on the morning of the examination. The examination is performed by infusing barium into the colon and replacing much of the barium with air to give a double-contrast effect before taking the radiographs. Hyoscine butylbromide or glucagon is given intravenously to produce smooth muscle relaxation.

Most carcinoma of the colon has reached a fairly advanced stage by the time the patient presents with clinical symptoms. Carcinoma is shown as a constricting lesion with mucosal destruction, a narrow irregular lumen and shouldered margins, and a sharp transition between the neoplasm and adjacent normal colon. In some cases the carcinoma is seen as an irregular intraluminal polypoid filling defect (Fig. 19) 157; other carcinomas appear as asymmetrical infiltrating lesions with mucosal destruction. Occasionally the carcinoma is shown as a large ulcerating mass. Multiple primary carcinomas are seen in the colon at the time of presentation in 3 to 5 per cent of patients.

Since strong evidence suggests that adenomas are the precursors of the great majority of colorectal carcinomas, the detection and removal of adenomas in the colon and rectum is important. Adenomas are shown as either sessile (Fig. 20) 158 or pedunculated small filling defects, usually less than 1 cm in diameter. Villus adenomas may be larger and have a frond-like appearance.

Flexible sigmoidoscopy, and colonoscopy are now widely available for examining the colon. The relative roles of barium studies, flexible sigmoidoscopy and colonoscopy have not been established. A small number of centres now perform flexible sigmoidoscopy before barium enemas. The majority of polyps and carcinomas develop in the sigmoid colon and investigation by combined flexible sigmoidoscopy and barium enema improves the detection rate, particularly in patients with diverticular disease, in whom small carcinomas and polyps may be obscured by the diverticula.

Ulcerative colitis, Crohn's colitis, and ischaemic colitis account for the great majority of patients with inflammatory bowel disease who are assessed with barium studies. The diagnosis of ulcerative colitis should be firmly established on the basis of a rectal biopsy taken at sigmoidoscopy. The double-contrast barium enema is an excellent technique for demonstrating the extent and severity of inflammation and the presence or absence of an associated carcinoma (Fig. 21) 159. Mucosal ulceration may be mild, moderate or severe and will extend in a proximal direction from the rectum in continuity to involve part or all the colon with associated loss of the normal haustral pattern. Patients with chronic ulcerative colitis show narrowing of the lumen and shortening of the colon.

The typical features of Crohn's colitis are inflammation, often in the form of aphthous ulcers, strictures, asymmetrical lesions, skip lesions, and predominant involvement of the right side of the colon. The distal sigmoid colon and rectum are nearly always spared, although perianal sinuses and fistulae (Fig. 22) 160 are a recognized feature. Ischaemic colitis characteristically shows oedematous changes in the splenic flexure that either return to normal in about 4 to 6 weeks, or result in stricture formation.

Diverticular disease is a frequent finding in middle-aged and older patients. Diverticula appear as single or multiple small out-pouchings, most frequently in the sigmoid colon, although they may be present in any part of the colon. Acute inflammation (diverticulitis) complicates diverticular disease in a small minority of patients. A paracolic abscess may be shown on barium enema as displacement and narrowing of the intestinal lumen with an altered mucosal pattern. Unlike carcinoma of the colon, the mucosal pattern in the narrowed segment is intact, although it may be distorted, and there is no shouldering of the margins. In some cases it may be impossible to distinguish a paracolic inflammatory mass from carcinoma. A soft tissue mass, gas lucency, air–fluid level, or barium in an extraluminal cavity may be seen in diverticulitis. The characteristic drape sign is occasionally seen, and is caused by bending of adjacent empty diverticula towards the abscess. Paracolic abscess may result in colonic or sometimes small intestinal obstruction. Fistulae may be identified as tracks of contrast medium passing from the colon to adjacent viscera. The more common fistulae are colovesical and coloenteric, fistulae to the skin, genital tract, ureter, stomach, hip, perineum, and soft tissues of the thigh being less common. Longitudinal paracolic fistulae appear as a track of barium running parallel to the colon in the paracolic tissues.

Computed tomography (CT) is only occasionally used as the initial investigation when disorders of the hollow organs of the digestive system are suspected. CT can, however, provide useful further information about neoplasms and other conditions that involve the gastrointestinal tract.

Initial optimism that CT would be a reliable method for staging oesophageal carcinoma prior to surgery has not been confirmed. It is sensitive in detecting liver metastases and invasion of the tracheobronchial tree but is unreliable for assessing mediastinal soft tissue extension, aortic invasion, and coeliac axis lymph node involvement. Endoscopic ultrasound is proving to be a more accurate technique for staging oesophageal carcinoma, since the depth of infiltration can be accurately assessed and lymph node metastases can be detected. However, endoscopic ultrasound has the disadvantage that an expensive dedicated endoscope is required.

CT has a limited role in evaluating the stomach: it is most useful for the preoperative staging of gastric carcinoma and for helping to confirm the diagnosis of linitis plastica. The characteristic appearances of gastric varices on CT can be helpful when the diagnosis is difficult on barium studies. The role of CT in visualizing the duodenum is mostly limited to showing changes in adjacent organs such as the pancreas that also involve the duodenum.

CT also has a limited role in the investigation of the small intestine, although in certain patients with Crohn's disease CT is useful for investigating abdominal masses, and in particular for demonstrating abscesses and fistulae. An abscess appears as a fluid collection in the mesentery or retroperitoneum bordered by a thick contrast-enhancing wall. Gas within the fluid in the form of scattered bubbles or air–fluid levels is diagnostic. The extraintestinal extent of lymphomas and other intestinal neoplasms can be defined by CT. Dilated fluid-filled loops of obstructed intestine show characteristic appearances on CT and this can be an important finding when obstruction is not suspected.

In the colon CT is important in evaluating patients who have undergone surgical resection for carcinoma. In the immediate postoperative period surgical complications can be diagnosed. It is also a sensitive method for detecting local and distant recurrent neoplasm. CT is being used increasingly in the initial evaluation of suspected acute diverticulitis. Sigmoid diverticulitis is seen on CT as localized thickening of the colonic wall in association with inflammatory changes in the pericolic fat or an adjacent abscess. Fistulae, distant abscesses, intestinal or ureteric obstruction, and peritonitis are complications that can be identified on CT.

The main indication for performing gastrointestinal angiography is in the diagnosis and treatment of gastrointestinal bleeding. If endoscopy fails to identify the origin of an acute bleeding episode, selective catheterization of the coeliac axis, superior mesenteric artery, and inferior mesenteric artery is normally performed. Angiography is successful in locating the bleeding site in up to 90 per cent of patients who continue to bleed during the investigation. Embolization may be undertaken during angiography in patients who are unsuitable for surgery. Single vessels such as the left gastric, gastroduodenal, and gastroepiploic arteries can be embolized because of the rich collateral circulation in the upper gastrointestinal tract.

Diagnostic angiography plays an important role in the location of bleeding from the small intestine, which is beyond the reach of endoscopy. Some patients will present with acute bleeding, while others have a long history of obscure bleeding with negative endoscopy and barium studies. Superselective arteriography may be required to pinpoint the site of bleeding in the small intestine accurately. If a lesion demonstrated by angiography in the small intestine is likely to be difficult or impossible to identify at surgery, intraoperative angiography is indicated. The catheter is left in the superselected branch vessel supplying the lesion during the subsequent operation. The anatomical location of the abnormality is then confirmed by an intraoperative angiogram of the individual loops. A modification of the technique involves injecting a small amount of methylene blue through the superselectively placed catheter.

Angiodysplasia, which is usually located in the caecum and ascending colon, can cause obscure gastrointestinal bleeding. It cannot be identified on barium studies but it may be recognized by an experienced endoscopist. Angiography is an excellent technique for diagnosing this condition.

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