Daniel A. Feeney, DVM, MS, DACVR
Professor of Radiology, University of Minnesota College of Veterinary Medicine, St. Paul, MN, USA
A. Positioning for Thoracic Radiography
1. Is Any Thoracic Preparation Necessary?
In general, no. However, care should be taken to assure that artifactual atelectasis that could be confused with unilateral lung infiltrates is avoided, if possible. Therefore, sedation should be avoided unless absolutely necessary due to poor patient control. If radiographs must be made under general anesthesia, the animal should be "bagged" several times to limit the effects of anesthesia-induced dependent atelectasis (will see mediastinal shift toward the side of lung collapse).
2. What About Positioning for Thoracic Radiography?
Routine positioning for small animal thoracic radiography should be in either right or left lateral and ventrodorsal (VD) or dorsoventral (DV) recumbency and exposures made using a vertically (downward) directed X-ray beam and a grid to control scatter radiation reaching the screens. I recommend the VD position because the animal is often easier to get "straight" and there is improved visualization of the cranial lung lobes. Although often suggested, there is no evidence to confirm the claims that the DV view is more physiologic and therefore better to view the heart. To the contrary, in the DV position, the diaphragmatic cupola (dome) is actually displaced forward and shifts the axis of the heart which is equally artifactual as is dorsal recumbency, but at least with dorsal recumbency, you can see more lung.
For the lateral views, the front legs should be pulled cranially to allow more clear visualization of the thoracic inlet and parasternal region. For the VD view, the front legs should be pulled somewhat cranially to limit superimposition on the craniolateral lung fields. Reading through the inescapable scapulas is bad enough. The goal should be to include the region from the thoracic inlet to the caudodorsal aspect of the diaphragmatic crura on all routine views (use multiple films on giant breeds as necessary). Expose the film during inspiration to limit confusion of lung stroma with interstitial infiltrates.
If there is pleural fluid, the VD view is more sensitive for picking it up than is the DV. In addition, unless there is a massive pleural fluid accumulation, you can still see the heart on a VD when it would be masked by the same amount of fluid on a DV view! Inpatients were there are equivocal or confusing evidence of pulmonary disease, particularly for nodules, it is advisable to make views with both sides down.
B. Basics of Thoracic Interpretation
Use the Geometric Roentgen Signs (size, shape, margination/contour, number, & location), the Density Roentgen Signs (air, fat, soft-tissue/water, bone/mineral, & metal) and the Silhouette Sign concept (two structures of equal radiographic density cannot be distinguished at the point of contact). These will allow you to determine whether each of the organs and spaces listed below are morphologically normal (within limits of radiographic sensitivity) and to infer something about the Functional Roentgen Signs (continuity, intact borders or walls, propulsion, & excretion). Special radiographic procedures, if necessary, can be used to confirm or refute functional or morphologic abnormalities suspected on the survey radiographs.
C. Overview of the Thoracic Radiograph
1. Identify the following:
a. Diaphragm - all parts
b. Ribs and sternum
c. Thoracic inlet and cranial mediastinum
d. Peripheral great vessels
e. Caudal mediastinum
f. Pleural space (normally not seen)
g. Lungs all lobes (interlobar fissure lines)
h. Pulmonary vessels
i. Paracardiac mediastinum
j. Heart and central great vessels:
i. Ascending aorta and pulmonary trunk
ii. Right & left atria
iii. Right and left ventricles
2. Are there any masses or abnormal densities found that cannot be localized to specific organs or regions?
3. Formulate rule-outs & determine if more complex imaging procedures are indicated/necessary?
Radiographic Evaluation of the Routine and Acute Thorax
D. Evaluation of Thoracic Radiograph
1. Is there a massive (clinically significant) accumulation of air or fluid in the pleural space, mediastinum or pericardial sac? Pericardial fluid causes smooth, rounded enlargement of the cardiac shadow and with clinical evidence of poor perfusion may suggest tamponade. Entertain and rule out diagnosis of flail chest and tension pneumothorax in presence of pneumothorax.
2. Is there evidence for primary pulmonary disease or pulmonary disease due to other causes such as congestive left heart failure? Determine type of pulmonary disease (i.e., alveolar, interstitial, bronchial, the lobar distribution and extent within each lobe. Look for cranial lobar pulmonary venous distension in comparison to the paired artery in combination with left atrial enlargement for congestive left heart failure.
3. Is there evidence of obstruction or discontinuity of the major airways? Look for mediastinal or intrapulmonary air dissection or accumulation.
4. Is there evidence for an intrathoracic space occupying mass(es) large enough to cause the observed clinical signs. (i.e., mediastinal lymphoma, diaphragmatic hernia?) If so, assess site of origin.
5. Is there evidence for organ displacement or lack of clarity to intrathoracic organs (except those in mediastinum) to suggest hernias, incarcerations, lung lobe torsions, tension pneumothorax etc. with or without some fluid?
6. Are there abnormal densities in the thoracic cavity (i.e., bullets, rocks, bones, ingesta etc.) and if so to what intrathoracic organs are they related?
7. Is there evidence for severe damage or dysfunction to the thoracic boundary structures? Specifically consider flail chest (multiple segmental rib fractures) tension pneumothorax and partial or complete diaphragmatic paralysis if fixation of normally variable structures or paradoxical movement of motile structures. Multiple films at different phases of respiration may be necessary.
8. Is there evidence for low cardiac input from relative or absolute hypovolemia or electrolyte imbalance as evidenced by pulmonary hypoperfusion or microcardia? Observe cranial lobar pulmonary vessels (magnified artery and vein pair, lateral view) and they should normally be 0.5–1.0 x diameter of the fourth rib.
9. Is there evidence of perithoracic disease? Specifically look for evidence of trauma, masses, subcutaneous emphysema of the thoracic wall, spine, proximal forelimbs, etc. Also, in cases of extreme inspiratory dyspnea, palpate and auscultate the larynx for paralysis, avulsion, or obstruction, Radiograph the larynx if necessary. Prominent caudal displacement (suction) may be the result of laryngeal paralysis.
10. Compare and further analyze the patient clinical demeanor, history, physical exam combined with 1–9 to determine patient status:
a. Patient is critical ⇒ immediate intervention (e.g., chest drain, surgery)
b. Patient is stable ⇒ can tolerate detailed evaluation
E. Causes of acute thorax in small animals:
1) Obstruction/overdistention, 2) Inflammation/sepsis, 3) Organ displacement/malposition, 4) Space occupation, 5) Perforation/rupture, 6) Heart failure or embolism, or 6) Other (e.g., gunshot, drug reaction, neurogenic, allergy...)