Gastrointestinal imaging (Proceedings)

This session will cover radiographic and ultrasound imaging of the gastrointestinal tract. Many basic principles and "older" techniques will be discussed. Newer techniques including MR and CT imaging will be briefly discussed.

Radiography

Having evaluated the radiographs carefully and systematically, it is vital important to consider the radiographic findings in light of the clinical signs. There may be clinical findings, which are of no clinical importance or of no significance at the current time. There may be abnormalities which were not expected in light of the present signs but which warrant further investigation. Finally there may be an abnormal finding, which can be linked wholly, or in part to the findings presenting problems and either allow a definitive diagnosis to be made or inform further investigations.

Surgical Abdomen

There are clinical situations where it is important to make a rapid, yet informed, decision that surgery is required immediately or as soon as the patient can be stabilized, rather than further diagnostic tests. Equally it is not in the best interests of the patient for exploratory laparotomy to be considered a routine means of investigating unexplained signs. Radiographic features of the gastrointestinal tract which indicate the need for urgent surgery include:

• Distension and volvulus of the stomach

• Abnormal small intestinal distension in the presence of normal electrolytes levels

• Free air in the peritoneum, which cannot be explained by recent laparotomy, abdominocentesis or a defect in the abdominal wall

• Mottled or streaky gas lucencies in the bowel loops indicative of necrosis

• Overall loss of abdominal detail associated with a septic abdomen, this should be confirmed by cytological testing of the free fluid.

Contrast Radiography (discussed in a following session)

Contrast techniques should be considered when the plain radiographic findings are insufficient to be confident about a diagnosis, but are of concern to further investigate the case. Contrast procedures allow further evaluation of the gastrointestinal tract. When planning a contrast it is very important to:

• Be thoroughly familiar with the procedure

• Have properly prepared the patient for the procedure.

• Have the appropriate contrast medium

• Understand the possible complications and be prepared for these complications

Contrast radiography will be discussed in a following session.

Ultrasound

Diagnostic ultrasound is widely available and offers great potential in the evaluation of the gastrointestinal tract. Not only can ultrasound allow internal architecture of structures to be seen, but also very small structures, such as lymph nodes, can be identified and assessed. Ultrasound should not supplant radiography for the overall evaluation of the abdomen and should be used specifically to answer particular clinical questions or as a supplement to radiographic investigations. Radiography and ultrasound are complimentary procedures and it is often very useful to use both modalities. Abdominal ultrasound is far superior in the investigation of fluid accumulation, but not very useful with large volume free gas in the abdomen.

All portions of the gastrointestinal tract are composed of 5 acoustic layers

Stomach

The most important factor affecting the variation of the radiographic appearance of the stomach is the patient positioning. Indeed the view directly influences the way in which gas and fluid contents are positioned relative to each other in the gastric lumen. Fluid falls to the dependent portion of the stomach whereas gas rises. In the VD view the gas is in the body, crossing the midline of the dogs body. In the DV projection the gas is more likely to be purely in the gastric fundus in the left abdomen. In left lateral recumbency the gas will be right sided in the pylorus and duodenum. This positioning dependency can used to clinical advantage to highly certain lesions or surround lesion with gas for better evaluation. Lesions in the gastric outflow tract or duodenum are seen better in left lateral projections. The stomach wall thickness can only be evaluated with contrast radiography and the attempt to determine wall thickness on plain images is fraught with error.

Ultrasound

Ultrasound has supplanted radiography for the diagnosis of most infiltrative diseases of the gastrointestinal tract. Similar to the liver and spleen, ultrasound is useful to see inside the stomach unlimited by inability to differentiate between fluid and mass. Unfortunately most dogs have gas in their stomachs and gas does limit the ultrasound evaluation of the stomach. Even with this limitation, ultrasound has become the most common pre-endoscopic imaging study.

Small intestines

Radiography has been the mainstay for the evaluation of foreign body and mechanical ileus. The study is inexpensive and quite sensitive for these diagnoses. We use 1.5 times the height of the midbody of L2 and 12 millimeters as our upper limit for normal in dogs and cats, respectively. There are always exceptions for these rules and the most important clinical exception is linear foreign bodies. Many (most?) linear foreign bodies in dogs will have both plication and a mechanical ileus pattern. Most cats only have a plication appearance, without small intestinal dilation.

Ultrasound

Ultrasound is the most important modality to verify small intestinal ileus, determine the cause of the ileus, stage the obstructive disease, verify suspected linear foreign body, evaluate the wall thickness and look for masses in the small intestines of cats and dogs. While it is difficult to completely "run" the small intestines, a thorough examination of the abdomen usually includes all small intestines. This statement works better in cats than 80 Kg Rottweiler's. Similar to the stomach, the small intestines of the dog commonly have large quantities of gas, preventing complete evaluation of the abdomen.

Clinical cases

This session will be case-based, utilizing a case as the beginning point of learning points discussed above. In most cases there will be a comparison of radiography with ultrasound to accentuate differences and relative advantages of each modality.

References

O'Brien, RT and Barr, F. 2009. BSAVA Manual of Canine and Feline Abdominal Imaging. 252pp.