Veterinary CPD webinar – Canine Thoracic Radiograph Interpretation

Interpreting thoracic radiographs is a core skill in veterinary practice, but it can also be one of the most challenging. Between patient positioning, variations in anatomy, and overlapping structures, there’s a lot to take in. This overview highlights the foundational principles and walks through real case examples to build confidence in recognising normal versus abnormal thoracic imaging.

The Basics: Radiographic Opacities

When reviewing any radiograph, it helps to start with the basics: how tissues interact with X-rays.

• Air appears black (lucent) because X-rays pass through easily.
• Fat is slightly more opaque than air.
• Soft tissue and fluid share the same opacity, meaning a mass and fluid can look identical.
• Bone is more opaque, appearing lighter.
• Metal blocks nearly all x-rays, creating the whitest areas.

Remember: darker = lucent, lighter = opaque.

Why Orthogonal Views Matter

The first commandment of radiology? Always take orthogonal views. A single projection can easily hide pathology. For the thorax, three views are ideal:

• Right lateral
• Left lateral
• Ventrodorsal (VD) or dorsoventral (DV)

Two laterals prevent missing disease that might only appear on one side, such as pneumonia confined to a single lung lobe.

Summation vs. Silhouetting

Two key principles in radiology interpretation:

• Summation: When structures overlap but maintain visible borders, creating the impression of increased opacity.
• Silhouetting: When structures of the same opacity overlap, their margins disappear. Example: a mass within fluid.

Keeping these concepts in mind prevents misinterpretation of overlapping anatomy.

A Stepwise Approach: “Outside-In”

To avoid missing important findings, use a consistent approach every time:

1. Check inclusion and positioning – Are the cranial and caudal lung margins, spine, sternum, and cranial abdomen visible? Is the patient straight?
2. Skeletal structures – Scapulae, ribs, spine, and sternum may show concurrent disease.
3. Cranial abdomen – Look at the liver and serosal detail.
4. Cardiac silhouette – Assess size and shape; use objective measures (VHS, VLAS) cautiously, mainly for monitoring over time.
5. Pulmonary vasculature – Remember: pulmonary veins are ventral and central compared to arteries.
6. Lungs – Evaluate for interstitial, bronchial, or alveolar patterns.
7. Mediastinum and pleural space – Check for widening, effusion, or masses.

Think of it like a physical exam: repeat the same sequence every time.

Case Highlights

Case 1: Aspiration Pneumonia

• Signalment: 6-year-old Pit Bull with vomiting, cough, and anorexia.
• Findings: Alveolar pattern with air bronchograms in the right middle and left cranial lung lobes. Clear lobar signs separating diseased from normal lung.
• Diagnosis: Aspiration pneumonia, supported by history and distribution.
• Key Lesson: Always use three views to avoid missing focal pneumonia.

Case 2: Cardiogenic Pulmonary Oedema

• Signalment: 10-year-old Cavalier King Charles Spaniel with cough and dyspnea.
• Findings: Enlarged cardiac silhouette with prominent left atrial enlargement compressing the left principal bronchus. Perihilar interstitial-to-alveolar opacity.
• Diagnosis: Cardiogenic pulmonary oedema secondary to mitral valve disease.
• Key Lesson: Evaluate both the lateral and VD/DV views for perihilar changes. Don’t miss airway compression from left atrial enlargement.

Case 3: Hiatal Hernia

• Signalment: 3-year-old French Bulldog with intermittent regurgitation.
• Findings: Gas- and soft-tissue–filled structure caudal to the heart, visible on one lateral projection but not the others.
• Diagnosis: Sliding hiatal hernia.
• Key Lesson: If a lesion disappears on the orthogonal view, consider transient or positional conditions like hiatal hernia.

Case 4: Multiple Concurrent Diseases

• Signalment: 11-year-old Maltese with cough.
• Findings: Cardiomegaly with left atrial enlargement and tracheobronchomalacia. Evidence of perihilar increased opacity.
• Diagnosis: Mitral valve disease with secondary pulmonary oedema, plus airway collapse.
• Key Lesson: Small-breed dogs often have more than one condition contributing to respiratory signs—interpret radiographs with this in mind.

Final Thoughts

Thoracic radiograph interpretation is both art and science. The images are two-dimensional, but the diseases are not. Using a systematic, outside-in approach, orthogonal views, and a solid understanding of radiographic principles will help you avoid missed diagnoses and improve patient outcomes.

Just like a physical exam, consistency is key: interpret radiographs the same way every time.