Introduction

Ultrasonography and radiology both help with soft tissue visualization. When used in conjunction, both modalities can offer valuable information regarding a particular disease entity and should therefore be used as complementary diagnostic tools. Despite this, it is important to determine which diagnostic modality will provide the most information for each particular case.

Ultrasound and radiology are common diagnostic tools in small animal medicine. They should be considered as complimentary modalities, providing valuable information on particular pathologies. It is important to consider the advantages and disadvantages of each modality in order to define which test will provide the most information for a particular case.

Radiology has several advantages and provides information regarding the well-known Roentgen signs: size, shape, position, contour and opacity. With radiology, specific techniques like contrast studies (GI, cystourethrogram, vaginogram) can help surpass its main disadvantage, which is differentiating structures with different tissue opacities and function of individual organs.¹ One clear-cut advantage that radiology has over ultrasound is its sensitivity to detect bone diseases such as fractures, subluxations, metastatic or primary bone disease as well as its sensitivity to detect gas in normal and abnormal tissues (pneumoperitoneum, emphysematous cystitis, fetal death, etc.).

Ultrasound, on the other hand, is valuable in evaluating soft tissue composition as well as blood flow within the abdominal cavity.¹ Also, when considering acute abdomen, where peritoneal fluid is suspected, ultrasonography permits an adequate visualization of abdominal organs since liquid acts as an acoustic window. This enables, that despite the presence of liquid, abdominal masses, liver and spleen contour, venous congestion, vascular abnormalities can be visualized.¹

General Considerations in the Acute Abdomen

Gastrointestinal Obstruction

Radiology is still a staple diagnostic test when ruling out gastrointestinal obstruction. There are several rules that should be met in order to correctly diagnose an obstructive pattern that are:

A.  The small intestinal diameter should not exceed the height of a lumbar vertebral body in dogs, and two vertebral bodies in cats.

B.  Two populations of small intestine referring to two distinct diameter sizes of small bowel.

C.  Altered distribution of small bowel within the abdominal cavity referred to as "stacking."

D.  Changes in the gas to fluid relationship that are persistent.

In general, complete obstructions prevent the formation of feces therefore; the empty colon should raise the index of suspicion for a possible obstruction. With this, care must be taken since an empty colon can also be present with colitis and is considered a nonspecific radiographic sign.^2,3

Acute Pancreatitis

Acute pancreatitis is an inflammatory condition that ranges in its severity and by far the most concerning is the development of necrotizing pancreatitis with multi-organ failure.⁴ Abdominal radiology in the case of acute pancreatitis does not help to confirm it, however, it will permit to rule out other pathologies that could be causing the acute abdomen. Some of the radiographic signs that could be appreciated are: loss of detail in cranial abdomen, displacement of the descending duodenum to the right as well as caudal displacement of the colon, functional ileus. In cats these radiographic signs are less sensitive that in dogs, where border effacement is the most frequently reported radiographic sign.

Ultrasonography is more helpful to determine and evaluate the pancreatic parenchyma as well as comorbidities. In acute pancreatitis it is common to visualize the parenchyma as hypoechoic (edema or necrosis) with hyperechoic peritoneum. Abdominal ultrasound will also be able to provide guiding for fine needle aspirates when a particular abnormality is observed such as pancreatic cysts, neoplasia or abscesses.³

Abdominal Neoplasia

As mentioned previously, the ability to visualize abdominal tumors, their extent and characteristics is an advantage of abdominal ultrasound. Furthermore, the ability to aid in sample collection is of added value. The presence of abdominal free fluid or extent of tissue infiltration can be adequately determined with abdominal ultrasound. For these reasons radiology is not the imaging modality of choice in these cases, since the differentiation of soft tissue opacities is more complex and cannot readily diagnose infiltrative pathologies.

CONCLUSION

Imaging modalities for the canine and feline abdomen provide a lot of information for the clinician. Radiology is the first line diagnostic tool for most veterinarians since it is cost efficient and non-invasive. On the other hand, ultrasound provides a more in depth "look" at the soft tissue structures within the abdomen. It is important to consider the advantages and limitations of each diagnostic modality in order to obtain the most information for a particular case.

References

1.  Rendano VT. Abdominal viscera: disease processes defined with radiology and ultrasound (Proceedings). http://veterinarycalendar (VIN editor: Link not available 6/2/2017). http://veterinarycalendar.dvm360.com/abdominal-viscera-disease-processes-defined-with-radiology-and-ultrasound-proceedings.

2.  Fischetti AJ. Abdominal radiology: making the call on the acute abdomen (Proceedings). http://veterinarycalendar.dvm360.com/abdominal-radiology-making-call-acute-abdomen-proceedings.

3.  Strom PR. When to cut when to wait: abdominal radiology in the acutely vomiting patient. http://c.ymcdn.com/sites/www.michvma.org/resource/resmgr/mvc_proceedings_2014/strom.pdf.

4.  Silverstain D, Hopper K. Small Animal Critical Care Medicine. St Louis, MO: Sanders Elsevier; 2009.