Gastrointestinal imaging strategies in the evaluation of gastrointestinal disease (Proceedings)

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.

Localization of diarrhea to the small or large bowel can also help influence imaging approaches. Patients with small bowel diarrhea are often better imaged with abdominal ultrasound unless obstructive disease is high on the differential list, in which case, plain and contrast radiographs could be quite helpful. Again, it may be reasonable to start with a plain abdominal film in such patients to exclude an obvious obstructive lesion, or gas-filled bowel, before proceeding with an abdominal ultrasound. GI imaging is not routinely employed in the evaluation of patients with strictly large bowel diarrhea. Animals with features of both small and large bowel disease could have diffuse GI disease, or could have focal lesions (usually partially obstructive) localized to the distal ileum-ileocecocolic region. Plain/contrast radiographic approaches, as well as abdominal ultrasound, are good imaging approaches to those patients.

From a laboratory standpoint, findings suggestive of protein-losing enteropathy (PLE), which can develop as a consequence of diffuse or focal disease, will often provoke an abdominal ultrasound over plain or contrast radiographic approaches. The author often uses the results of abdominal ultrasonography to dictate whether a PLE patient is a better candidate for obtaining intestinal biopsies endoscopically, or surgically: endoscopic approaches are appropriate in the face of evidence of diffuse disease, whereas surgical interventions may be more appropriate for patients with focal, or segmental-appearing, disease as surgery could be not only diagnostic, but potentially therapeutic, a distinct advantage over endoscopic examinations.

Gastrointestinal foreign bodies and obstructive disease are relatively common causes of vomiting in dogs and cats, particularly those of young age, and patients for which these diseases are high on the differential list (usually based on key historical or physical examination features) are often easily assessed by plain abdominal radiographs as a first imaging strategy. One needs to be careful not to misinterpret small, circular densities in the region of the antrum and pylorus, particularly if seen on radiographs obtained in the right lateral projection. Fluid in the stomach can pool in the antrum, which can appear very circular (like a round foreign body) when the patient is in right lateral recumbency, and the author has seen many animals over the years that have had exploratory laparotomies on the basis of this finding only for the surgeon to find no foreign body or mass. If there is any doubt about the significance of such a density, one should evaluate an orthogonal view, or the left lateral projection, to see if the density persists or not.

While abdominal ultrasound is increasingly available to small animal clinicians, there are regions where ultrasound accessibility is still limited. In those situations, or when an owner refuses referral, the author still believes in the utility of contrast studies of the GI tract. Contrast radiography can suggest the presence of mucosal disease, and can be quite helpful in defining partial GI obstructions; contrast radiography of the intestinal tract is also considered superior to plain radiography for assessment of intestinal wall thickness. As with abdominal ultrasonography, results of contrast radiography could influence the approach to biopsy (endoscopic vs surgical) in those patients with diffuse or focal disease. If interpretations are difficult, and the patient's clinical status would allow, having the study reviewed by a radiologist could be prudent before proceeding with additional diagnostic tests. It should be emphasized that obstructive diseases or foreign bodies can not be absolutely excluded as causes of clinical signs even if not readily apparent on radiographic studies.

It goes without saying that availability, and cost, are also considerations when developing a GI imaging strategy. Plain and static contrast radiographic imaging is within reach of most practitioners, while abdominal ultrasound may have more limited availability. Thus, it is rarely inappropriate to perform plain abdominal radiography as a starting point in the imaging approach to a patient with GI disease. The equipment needed for fluoroscopic examinations, CT and MRI will typically limit availability to referral centers. These latter imaging studies are also usually more costly than plain and contrast radiography, and abdominal ultrasonography.

Fluoroscopic vs static images

Some imaging centers offer the potential for fluoroscopic evaluation of the patient. From the standpoint of GI disease, fluoroscopy is usually best used for patients that have abnormalities of swallowing, or that have regurgitation or vomiting suggestive of proximal GI obstruction. Fluoroscopic examination is often critical for the diagnosis of cricopharyngeal disease (uncommon, but difficult to diagnose without fluoroscopy), and diseases such as hiatal hernia which may not always be readily apparent on static images of the abdomen. The author has also found fluoroscopic examinations very helpful in the assessment of patients that proved to have function pyloric obstructions given the potential for real time evaluation of gastric contractions timed with pyloric relaxations.

CT and MRI

The specialized nature of these imaging modalities limits their availability and utility in GI disease patients as compared to plain and contrast radiography, and abdominal ultrasound. These modalities are, however, extremely useful in the evaluation of some GI diseases, in particular neoplastic diseases. The advantages of CT and MRI lie in their ability to provide more anatomic detail than is possible with other imaging modalities, anatomic detail that could be crucial in the assessment of patients with GI masses. MRI and CT are commonly used at WSU to better define the local extent of disease in animals with large anal sac masses or lesions involving the rectum, as this area can be difficult to image with abdominal ultrasound. Regional lymph nodes can be evaluated for size as part of the staging process afforded by these cross sectional imaging strategies. CT and MRI scan are more sensitive for the detection of intrahepatic lesions (the liver is a common site of metastasis of primary GI tumors) than abdominal ultrasound, particularly when contrast agents are administered during the course of these radiographic examinations. CT would be an excellent imaging strategy to apply to patients that have masses associated with the esophagus, and is an excellent imaging choice for the assessment of metastatic lung disease: the enhanced sensitivity of thoracic CT makes it a valid staging option before aggressive medical or surgical interventions in patients with neoplastic GI disease.

Summary

Imaging techniques are useful tools at the disposal of small animal clinicians, and results of imaging studies can play key roles in dictating the direction that further diagnostic, or treatment, approaches will take with a given patient. It can not be overemphasized, however, that results of GI imaging have to be interpreted in light of all other pieces of information known about the patient lest diagnostic or treatment errors are made. Each patient should be viewed, and approached, as individual and unique so that the appropriate imaging approach can be employed for each.

Imaging modalities useful in the evaluation of patients with GI disease.