One of the most frustrating things to diagnose is a single, ill-defined pulmonary nodule. It is usually an animal with cancer, and the clinician wants to check it for metastatic disease. For a clear positive answer, we would like to see multiple, well-defined soft tissue nodules in the lungs. Is it really there? And if it is, could it be benign? There are a few things you can do to try and make this decision.
The first thing to address is whether it is a true finding or an artifact. Many nodules are seen better on one lateral vs. the other, or on a d/v vs. a v/d. This has to do with how inflated the lung is around the nodule. The more air in the lung, the better the contrast between lung and nodule. So a nodule in the right lung will be seen best on the left lateral projection, because the uppermost lung is more inflated. Similarly, a nodule in a dorsal lung lobe will be seen better on a d/v projection.
Sometimes there is superimposition of another structure that makes the nodule unclear. It may be a rib, or the heart, or a vessel. A good technique in this case is to take another radiograph (the same projection as the nodule is seen in) with the center of the beam in a more cranial or caudal projection. Because the x-ray beam is diverging, the rays will strike the nodule and superimposed object in a different way, and you may be able to see them separately.
Of course, some nodules will not be well-defined no matter what you do. And a single nodule doesn’t necessarily mean metastatic disease. So it’s a good idea to take a follow-up radiograph in 4-6 weeks, and see if it has grown. If so, there’s a good chance it’s a metastasis.