Introduction

Diagnostic imaging is an art and a science. The science of diagnostic imaging is didactic information that is learned during veterinary training. The art is the experiential learning that takes place over the course of a lifetime as one interprets radiographs and the ability to extract information from an image. As one studies radiographs, one moves from a lower order of learning (pattern recognition) to a higher level of interpretation where different aspects of the interpretation process impact the final conclusions. Then, the interpretation is filtered through the clinical information relevant to the patient at hand.

Interpretation of a radiograph should be directed by a paradigm. An interpretation paradigm is a map that guides you along a path of thorough and complete evaluation of a radiograph. The paradigm is an essential tool to use for evaluation of all radiographic studies, and examples are provided in each section. This chapter presents an approach to and overview of the philosophies shared by the editors of this textbook regarding radiographic interpretation.

Why Radiographs?

Why bother with radiography, or diagnostic imaging in general? Specifically, radiography is relatively fast and readily available as a diagnostic imaging test. The procedures for most standard examinations are well defined, and expectations for the capabilities of the modality are relatively well understood. In the realm of diagnostic imaging tests, it is also inexpensive and noninvasive, a rapid test to perform. Therefore, it is a great tool to monitor and stage disease and evaluate anatomy. We also use radiographs to document the results of patient management, to figure out if a treatment is working or not. And of course, when possible, we use radiographs to actually diagnose disease.

​​Most often, imaging is used to decrease the level of uncertainty about a diagnosis. In most cases, we do not end up with a definitive diagnosis, but we do use the imaging findings as well as any other information to narrow the list of probable diseases. We do this by gathering data. If we think of each individual finding as a test, with each of those tests as having a particular value, then when we add them together, we should hopefully paint a picture or pattern of disease. So, think of each finding as a piece of a larger pattern. As we fit more and more of these diagnostic puzzle pieces together, the pattern becomes more clear. As that pattern emerges, hopefully we can recognize that it is consistent with a specific disease, or perhaps a subset of diseases. The result should be a shorter list of potential or probable pathologic processes (differential diagnoses), and we can direct our next steps accordingly.

Imaging Findings as Tests

As mentioned, tests can be characterized by their value, and that value is best described by sensitivity, specificity, and accuracy as tested against a normal and abnormal population of animals. If we think of each radiographic finding as a test, we realize that each finding can be associated with a certain number of true and false positives as well as true and false negatives when compared to a gold standard [1]. The gold standard would be the test considered to be the best available to diagnose a given disease. That said, the test may be relatively unavailable, too expensive, or perhaps very invasive, and therefore cannot always be done. The sensitivity or specificity for each and every finding for each and every disease are not always known, but we do have data for some of this, and we often can extrapolate. We also have our clinical experience and diagnostic acumen to draw on, which continue to grow over time. As more and more clinical research is done, we get new information on the value of findings as tests through science. Perhaps most importantly, the combination of findings can be most powerful as a diagnostic tool, and can further increase the sensitivity, specificity, and accuracy of radiography as a diagnostic tool for a given disease.

It is important to recognize that some individual findings may be very nonspecific, and that they are not exact for any particular disease and can be features of many different, completely unrelated diseases [1]. This means that, individually, they do not contribute to the reduction of uncertainty that we hope to attain. However, when we combine multiple findings, the added value of each finding narrows our scope in the “cone of certainty” (Figure 1.1).

For example, an unstructured interstitial pulmonary pattern that is moderate in severity and hilar in distribution could result in a large list of potential differential diagnostic considerations from multiple etiologies. If we combine this finding with other radiographic changes, such as left‐sided cardiomegaly, elevation of the carina on the lateral images, widening of the caudal bronchi on the ventrodorsal image and enlargement of the pulmonary veins in a dog, our differential list narrows very quickly to pulmonary edema secondary to left‐sided cardiac disease. Depending on other signalment and physical examination findings (small breed dog with a grade IV/V pansystolic cardiac murmur), our differential list narrows even more to mitral valve degenerative disease (endocardiosis) with secondary left heart failure.

This is why one of the most important and fundamental interpretation skills is learning to describe abnormal radiographic anatomy in an organized and systematic fashion. Being systematic and organized helps us to recognize patterns that might otherwise elude us.

Describing Abnormalities: Roentgen Signs

The fundamental language of radiographic interpretation is the Roentgen signs. These are the six features that we describe for every organ or body system that we evaluate: location, size, shape, number, margin, and opacity. The definitions as well as some terminology for use in description of abnormalities are listed in Table 1.1. Figure 1.2 is a radiograph that contains all radiographic opacities.

Opacity

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