The small animal clinician has a number of imaging options available for the evaluation of dogs and cats with gastrointestinal
tract (GI) disease. The purpose of this presentation is to give an overview of these different imaging strategies, some of
the advantages and disadvantages of the different imaging approaches specifically related to the evaluation of GI disease,
and some of the clinical considerations that could suggest the use of one imaging modality over another in planning a diagnostic
strategy for the patient with GI disease. This presentation will not include instructions for performing the imaging procedures,
nor complete details of interpretation of the images generated, but rather some of the clinical considerations that could
lead to choosing one imaging modality over another in the approach to the patient with signs of GI disease. Several cases
will illustrate the application of the varied imaging tools to the evaluation of patients with GI tract disease.
Options for imaging the GI tract
The most clinically useful options for imaging the gastrointestinal tract are plain and contrast radiography (including fluoroscopy),
abdominal ultrasonography, and advanced cross-sectional imaging modalities of computed tomography and magnetic resonance imaging.
Each has advantages and disadvantages (see Table at end of notes) when it comes to imaging the GI tract.
Clinical considerations that influence selection of imaging
The clinical elements that define which imaging choice one initially uses in the diagnostic approach to the patient with GI
disease should reflect the likely differential diagnoses for each patient. The differential list is generated based on patient
history, physical examination findings, and in some cases, results of laboratory testing.
The history is critical for helping with localization of the problem to segments within the GI tract. Careful questioning
should, in many cases, help define a patient that has presented for vomiting as one that is truly vomiting, or a patient that
is regurgitating. For patients that are regurgitating, the imaging strategies focus first on exclusion of esophageal disease,
and so plain thoracic radiographs, possibly followed by contrast esophagrams, become the imaging strategies of choice. For
those patients that have a history strongly suggestive of regurgitation, and for which there is no evidence of esophageal
disease, this author prefers to next perform a gastrogram, preferably with fluoroscopy, to assess gastric emptying; in the
author's experience, dogs with pyloric outflow obstructions may have clinical features more suggestive of regurgitation than
vomiting and may have lesions that are difficult to appreciate with abdominal ultrasound. The gastrogram is easily tacked
on to an esophogram. Animals with good appetites in the face of their GI disease suggest partial obstructions, motility disorders,
and sometimes infiltrative mucosal disease, making them candidates for contrast imaging of the GI tract. Although rare, patients
with lesions suggestive of esophageal masses become candidates for thoracic CT.
For the patient with vomiting that has been localized to the GI tract (that is, non-GI causes of vomiting have been excluded
from the differential list based on results of history evaluation, physical examination and laboratory testing), an initial
fork that can be encountered in the decision making process is whether to pursue plain radiographs, or abdominal ultrasound.
It is important to recall that each modality gives different information, and some would argue that in a patient with GI disease,
a more complete assessment of the GI tract is afforded by both plain radiography (if no lesions are apparent) and ultrasonography.
Patients that have large amounts of intestinal gas evident radiographically can be poor candidates for abdominal ultrasonography
because of the limited information that is gained when imaging gas-filled structures. That said, the author tends to prefer
abdominal ultrasound as the initial imaging step in those patients for which infiltrative or partially obstructive diseases
are particularly high on the differential list, but admittedly this decision is influenced by ready access to abdominal ultrasonography.
Some features that might sway the author toward an abdominal ultrasound include chronicity to the history of GI disease, weight
loss (which suggests chronicity), concurrent diarrhea (so bowel thickness can be evaluated), the palpation of a mass associated
with the intestine or other abdominal viscera in middle-aged to older dogs (so that regional lymph nodes and the liver can
be assessed for changes compatible with metastatic disease, and so that needle aspirates can be obtained from accessible lesions
if desired), or the palpation of thickened loops of bowel (so that regional lymph nodes can be assessed and needle aspirates
obtained). It is appropriate, as implied above, to perform plain abdominal radiographs as a first step in such patients.