Abdominal radiography can occasionally be sufficient to determine whether surgery is necessary or not in a patient with gastrointestinal (GI) disease. For example, in most cases of gastric dilatation volvulus or when a GI foreign body (FB) is radiographically visible no further imaging is required prior to surgery.

In many cases, however, GI foreign bodies are not identified on plain radiographs despite being the cause of GI symptoms. Ultrasound is routinely used to differentiate between mechanical and functional ileus. When available, it has eliminated the need for gastrointestinal contrast radiography. Luminal dilatation resulting in jejunal diameter of more than 1.5 cm has been suggested as a useful indicator for presence of small intestinal obstruction in dogs.¹

Once intestinal dilatation is observed, a thorough ultrasonographic search for a possible cause of obstruction should be undertaken. Ultrasound enables us to identify and distinguish between intraluminal obstructive structures and mural masses. Foreign bodies on ultrasound are characterised by variable degrees of distal acoustic shadow and surface reflection. The ultrasonographic appearance of foreign bodies is totally dependent on the nature of the material. Many common foreign bodies (e.g., bones) have a hyper-reflective interface and cause a strong acoustic shadow, however, some foreign bodies have ultrasound propagation characteristics similar to soft tissue; therefore, they do not cause a strong acoustic shadow.

Partial intestinal obstruction, recent obstruction, and duodenal obstruction may present without obvious radiographic signs of intestinal dilatation. In cases of duodenal obstruction, the duodenum is dilated but the dilatation is not clearly identified on the radiographs because the duodenum contains soft tissue/fluid dense material and no gas. The absence of gas in the dilated loop of intestine decreases the chance to confidently diagnose an obstruction on the basis of radiography alone, except when the FB is radiographically obvious. In these cases, further investigation with ultrasound or abdominal CT is necessary.

Linear foreign bodies and intestinal intussusceptions are readily identified with ultrasound. In the case of linear foreign bodies, the intestine presents with a typical plicated appearance and the intraluminal linear foreign body is very commonly seen. Intestinal plication must be differentiated from intestinal corrugation on ultrasound because the two are very different entities; bowel wall corrugation has been associated with pancreatitis, enteritis, peritonitis, neoplasia, or bowel wall ischaemia.² Intestinal intussusception has a pathognomonic appearance of multiple concentric mural layers when viewed in transverse section. In cats, intestinal intussusception is more frequently associated with an underlying intestinal neoplasia compared to dogs that seem more likely to develop it due to underlying inflammatory disease.

In private practice ultrasound is commonly used to differentiate between mechanical obstruction, gastroenteritis, and pancreatitis. The ultrasonographic finding of an enlarged, hypoechoic pancreas surrounded by hyperechoic mesenteric fat and possibly by some peritoneal fluid is compatible with pancreatitis.

In a recent study comparing ultrasound and computed tomography for the diagnosis of canine GI mechanical obstructions, with findings of exploratory surgery as the reference standard, the sensitivity and specificity of abdominal ultrasonography were 100% and 67%, respectively. The positive predictive value (PPV) for ultrasonography was 93%, and the negative predictive value (NPV) was 100%. For CT, sensitivity and specificity for the diagnosis of mechanical gastrointestinal obstruction were both 100%, and the PPV and NPV were also both 100%.³

In another study comparing abdominal radiology and CT for detecting mechanical intestinal obstruction in dogs, CT was more sensitive (95.8% vs. 79.2%) and specific (80.6% vs. 69.4%) compared to radiographs, but the difference was not statistically significant.⁴

In some cases of gastrointestinal disease, the diagnostic utility of abdominal ultrasound over other diagnostic tests (such as endoscopy) is debated. Investigation of the stomach with ultrasound can be challenging, particularly in deep chested dogs. It is reported that in dogs presenting with chronic vomiting the contribution of abdominal ultrasound was high only in 27% of the cases.⁵ This study suggests that ultrasound is more indicated in older patients, in patients presenting with a greater number of vomiting episodes and weight loss, and when a neoplastic disease is suspected.

Ultrasound and CT are used to look for non-perforated gastrointestinal ulcerations whereas radiology can only be used to detect pneumoperitoneum in case of perforated GI ulcers or abdominal fluid in case of peritonitis. In a recent study sensitivities of radiography, ultrasonography, and CT were 30, 65, and 67% in dogs with non-perforated ulcers and 79, 86, and 93% in dogs with perforated ulcers, respectively.⁶ In this article the reviewed cases span over 10 years and have been seen in ultrasound by various radiologists; no information is provided about the possibility of operator dependent accuracy of ultrasound for detection of GI ulcerations. However, in practice, this variable is likely to affect the results. Other factors that may affect the ultrasonographic ability to identify ulcerations are location (gastric versus intestinal), number, and size of the ulcers.

The use of ultrasound to detect focal accumulation of peritoneal fluid and guide sampling of the fluid facilitates the early detection of septic peritonitis secondary to GI ulcerations.

In conclusion the clinician should decide whether radiology, ultrasound, CT, or a combination of these imaging techniques is most appropriate in each particular case depending on the nature of the underlying GI disease.

References

1.  Sharma A, Thompson MS, Scrivani PV, et al. Comparison of radiography and ultrasonography for diagnosing small-intestinal mechanical obstruction in vomiting dogs. Vet Radiol Ultrasound. 2011;52(3):248–255.

2.  Moon ML, Biller DS, Armburst LJ. Ultrasonographic appearance and etiology of corrugated small intestine. Vet Radiol Ultrasound. 2003;44(2):199–203.

3.  Winter MD, Barry KS, Johnson MD, et al. Ultrasonographic and computed tomographic characterization and localization of suspected mechanical gastrointestinal obstruction in dogs. J Am Vet Med Assoc. 2017;251(3):315–321.

4.  Drost WT, Green EM, Zekas LJ, et al. Comparison of computed tomography and abdominal radiography for detection of canine mechanical intestinal obstruction. Vet Radiol Ultrasound. 2016;57(4):366–375.

5.  Leib MS, Larson MM, Panciera DL, et al. Diagnostic utility of abdominal ultrasonography in dogs with chronic vomiting. J Vet Intern Med. 2010;24:803–808.

6.  Fitzgerald E, Barfield D, Lee K, et al. Clinical findings and results of diagnostic imaging in 82 dogs with gastrointestinal ulceration. J Small Anim Pract. 2017;58(4):211–218.