Diagnostic Imaging: Skin mass or pulmonary nodule
Sharpness of the margin
We can see pulmonary nodules or skin masses as separate from lung or skin because they are surrounded by air, creating a soft-tissue/air interface that has very high contrast. A mass on the thoracic wall is surrounded by air only. Nodules in the lung are surrounded by air, small vessels and pulmonary tissue, and the thoracic wall. This means that skin masses have much sharper margination than lung nodules. Compare the sharpness of the body wall mass on the caudoventral right thorax to the pulmonary nodules in the images. This dog has multiple pulmonary metastases from a large liver mass as well as a thoracic wall mass. Digital radiographs such as these will make the margination of external masses even sharper than on film because of the increased contrast of the images.Completeness of the margin
Use a marker
If you still can't tell if the soft-tissue structure you are looking at is a skin mass or a pulmonary nodule, then place a marker on the skin mass and repeat the radiograph. A dab of liquid barium or metallic adhesive markers both work well. This technique helps you to document which masses are accounted for externally.
Since radiographs are a two-dimensional representation of a three-dimensional body, we need two views to pinpoint the location of a nodule. For example, the pulmonary nodule between ribs 4 and 5 on the left lateral projection is visible in the same rib space on the dorsoventral projection.
Next time you look at radiographs, compare some of these features using structures outside and within the body. Notice the sharpness and incomplete margin of nipples compared to end-on vessels near the heart base, as well as their position on two projections. With practice, these small but useful details become part of your radiographic interpretation skill set.
Dr. Zwingenberger is a veterinary radiologist at the University of California-Davis.