Imaging coughing dogs: Thoracic series tips in plain view
At a recent CVC session, Rachel Pollard, DVM, PhD, DACVR, an associate professor and researcher at the University of California, Davis School of Veterinary Medicine, gave a tip-packed talk on obtaining and interpreting thoracic series in coughing dogs. Here are just a few of the things I plan to implement in my own practice:
Broaden your views
For Dr. Pollard, three views—right and left lateral and dorsoventral (DV) projections—are standard for all patients undergoing thoracic studies. Why is three the magic number? Including both lateral projections is the most comprehensive way to check for lesions, and the left lateral projection, in particular, aids in the the detection of pulmonary nodules and dependent pneumonia in the patient’s right side. The DV projection, according to Dr. Pollard, allows the heart to be in its normal anatomic position and is easier for the animal, minimizing restraint needs and personnel exposure.
If you want to confirm whether or not you’ve seen a pulmonary lesion on the DV view or if pleural effusion is making it difficult to see the heart, you can add a fourth view: ventrodorsal (VD).
Keep your eye on the horizon(tal)
Need to confirm whether or not there’s fluid within either the pleural space or a pulmonary mass? Take a horizontal beam radiograph. For this view, the dog is placed between the x-ray tube (positioned so that the beam travels across the table) and the detector in either sternal or lateral recumbency.
Dr. Pollard said that most new machines are able to do this, but because the detector has to be out of the table, additional people may be required to keep it secure.
What’s your interpretation?
Accurately interpreting patterns in pulmonary parenchyma findings is difficult due to the amount of variation in what’s normal. Dr. Pollard described the radiographic lung as having two compartments—one contains air and includes the lumen and alveoli and the other is composed of soft tissue structures, such as airway walls and vessels (also called the interstitium).
According to Dr. Pollard, the presence of air in the alveolar compartment is the most crucial aspect in determining a lung’s particular radiographic appearance, and pulmonary vessels and the walls of large-diameter airways are responsible for the “pulmonary markings” used to establish the lung field’s extent. Any deviance in either structure can cause radiographic lung patterns characteristic of pulmonary parenchymal disease, but to accurately interpret these patterns in thoracic radiographs, Dr. Pollard said you must consider:
Whether or not your study is technically adequate. Consider the presence of positioning artifacts (rotation, limb position, neck position), overexposure, underexposure, processing artifacts and respiration phase.
Whether or not the radiographs are abnormal. This is difficult because, as mentioned above, what’s “normal” widely varies. And the fact that the amount of interstitial density is inversely related to the degree of lung inflation and level of exposure adds further complication. According to Dr. Pollard, overestimating the significance of prominent interstitial patterns in an underexposed or underinflated chest is the most common error.