GASTROINTESTINAL DIAGNOSTIC RADIOLOGY

GASTROINTESTINAL (GI) TRACT
Contrast radiographic investigations, including CT, are generally accepted as the methods of choice for investigating diseases of the GI tract. Although ultrasound is not considered a primary tool in the investigation of bowel lesions, as the gas-filled lumen makes visualization difficult in many cases, ultrasound is remarkably successful in diagnosing GI tract pathology in the hands of an experienced operator. GI tract ultrasound can be time-consuming, but a wealth of information can be obtained with a high-frequency linear probe in a symptomatic patient. Considerable diagnostic benefit has been shown for careful, targeted, percutaneous ultrasound of the large and small GI tract using high-frequency transducers.

It is important to be aware of the variable ultrasound appearances of normal bowel, as it may be responsible for mimicking other pathology. Normal bowel is frequently difficult to examine on ultrasound as the gas-filled lumen reflects the sound, requiring careful compression techniques. Abnormal bowel is particularly accessible to ultrasound, however. A fluid-filled lumen also make easy the demonstration of valvulae conniventes of the small bowel and haustra of the large colon.

Oesophagus and stomach
The oesophagus is not usually accessible to percutaneous ultrasound; however, the lower end can be demonstrated as it passes through the diaphragm in the midline, just anterior to the aorta. Its normal appearances should not be confused with a mass. Occasionally, ultrasound demonstrates the thickened wall associated with an oesophageal carcinoma involving the lower oesophagus.

Endoscopic ultrasound (EUS), with its high frequency and proximity to the relevant structures, is able to demonstrate the layers of the gut wall, and to demonstrate pathology and accurately stage malignant disease in both the oesophagus and stomach, and also to guide invasive procedures. Barium X-ray studies are still the first-line investigation of choice for many potential GI tract conditions; however, endoscopy is regarded as the gold standard for investigating the lining of the stomach and duodenum and can be combined with biopsy when necessary. Although percutaneous ultrasound has had modest success in revealing stomach masses if the stomach is filled with water, it can never replace endoscopy. However, if such lesions are discovered, this helps to direct subsequent radiological management.

Malignant tumours
The most common site for a bowel tumour in the adult is around the caecum. It is useful to target this area in patients with altered bowel habit in whom bowel carcinoma is suspected, although detection with ultrasound is usually incidental. The mass tends to be hypoechoic, or of mixed echogenicity, with a small, eccentric, gas-filled lumen. This cannot be differentiated, however, from an inflammatory mass on ultrasound. Vigorous Doppler flow can usually be visualized in both inflammatory and malignant masses. The finding of a colonic mass would normally prompt a barium enema, to delineate the nature, extent and position of the mass, with subsequent staging by CT if malignancy is confirmed. The advantage of ultrasound over barium enema is that of displaying the tumour itself, rather than just the narrowed lumen.

The role of ultrasound in patients with known bowel carcinoma is to identify and document the presence of distant metastases, particularly in the liver, as metastases from colorectal carcinoma are particularly amenable to curative resection. Bowel tumours should be considered in the list of differential diagnoses when the origin of a mass discovered on ultrasound is unclear. Endosonography may be used to detect and stage rectal cancers, although it is only able to demonstrate perirectal nodes and cannot evaluate distant disease. Endosonography is ideal however, in the follow-up of rectal cancer, and can detect early recurrence of disease.

Obstruction
Ultrasound has been found to be helpful in the investigation of acute obstruction. It can confirm obstruction, by demonstrating dilated, fluidfilled bowel loops with ineffective peristalsis. These fluid-filled loops of bowel are highly amenable to ultrasound scanning, which has the advantage of being able to visualize peristalsis directly, unlike a plain X-ray. It is possible to trace the dilated bowel to the site of obstruction, distal to which are normal loops of collapsed bowel.

The confirmation of obstruction with ultrasound has been proved to be as sensitive as and more specific than plain X-rays and can potentially reduce the need for surgery in such patients, save costs and reduce radiation dose. However, identifying the actual site and cause of obstruction is time-consuming and frequently unsuccessful. Patients with suspected bowel obstruction, therefore, usually proceed straight to CT.

OTHER RETROPERITONEAL ABNORMALITIES
Ultrasound is successful in identifying retroperitoneal masses, but CT and MRI are more effective at establishing the extent and nature of many of these masses, particularly those partly obscured by gas-filled bowel. The majority of malignant retroperitoneal tumours are renal or adrenal in origin. Other primary tumours, apart from lymphomas, are rare, and include liposarcoma and leiomyosarcoma. These tend to be large when they present, and of variable/complex ultrasound appearance. Encasement of major vessels by tumour is a further characteristic of the retroperitoneal origin of the mass, together with anterior displacement of structures such as the pancreas, kidneys, aorta and IVC. Ultrasound is also able to identify peritoneal and omental deposits in patients with late-stage carcinoma. These are particularly amenable to diagnosis when surrounded by ascites and usually arise from gynaecological or urological tumours. Benign retroperitoneal masses identifiable on ultrasound include haematomas, psoas abscesses, lymphadenopathy and pancreatic pseudocysts.