Thoracic Imaging in Small Animal Practice

Thoracic Imaging

• The presentation covers thoracic imaging in small animal practice.
• Learning outcomes include describing positioning for interpretable chest radiographs, determining necessary views for diagnostic quality, and identifying confounding or problematic features.

Thoracic Radiography - Indications

• Coughing: Examples include pulmonary disease, left-sided congestive heart failure (CHF), parasitic diseases, neoplasia, and inhaled foreign bodies (FB).
• Dyspnoea: Related to airway obstruction, pulmonary disorders, and pleural disorders, including murmurs, congestive heart failure, and arrhythmias.
• Cardiovascular disease: Murmurs, congestive heart failure, and arrhythmias are noted.
• Thoracic Trauma: Includes pneumothorax, haemothorax, rib fractures, and diaphragmatic rupture.
• Neoplasia: Primary or metastatic disease, foreign bodies.
• Regurgitation: Megaesophagus and congenital disorders.
• Thoracic wall lesions: Neoplasia, thoracic deformity.

General Considerations

• Exposure: High kV and low mAs are recommended to minimize movement blur.
• Inspiratory view: Essential for detecting bullae, air trapping (in feline asthma), and small pneumothoraces. Exceptions may apply.

Patient Preparation - Sedation and Anaesthesia

• Advantages: Better positioning, less risk of movement blur, less stressful for the patient, timing of radiographs for end inspiration, and ability to perform manual inflation (for general anaesthesia).
• Disadvantages: Atelectasis, dependent lung collapse (general anaesthesia), manual inflation potentially obscuring small nodules and resolving pathological atelectasis, and considerations for protective clothing during manual inflation.

Standard Radiographic Views

• Patient positioning: Dorsoventral (DV), Ventrodorsal (VD), Right Lateral, Left Lateral, lesion-orientated oblique, decubitus view (horizontal beam DV/VD), standing horizontal beam.

Patient Positioning - Minimum Views

• Cardiac conditions: Right lateral and DV views.
• Lung pathology: Right lateral and VD views.
• Pulmonary metastases: Right lateral, Left lateral, VD views.

Review of a Thoracic Radiograph

• Review includes surrounding soft tissues, neck, cranial abdomen and diaphragm, bones (including ribs), pleural space, mediastinum, trachea and carina, bronchi, cardiac silhouette, great vessels and pulmonary vasculature (including the aorta), and lungs.

Mediastinum

• The space between pleural cavities, extending from the thoracic inlet to the diaphragm.
• Size varies on DV/VD radiographs.
• Specific structures (azygos vein, main pulmonary artery, vagus nerve) are present but not always visible.

Mediastinal Shift

• Movement of the mediastinum or structures away from the midline (DV or VD projection), indicative of a volume change in one hemithorax.
• Possible causes include unilateral lung collapse, pleural disease, unilateral pleural effusion/pneumothorax, large single or multiple pulmonary masses, and unilateral diaphragmatic rupture.

Review of a Thoracic Radiograph - Lymph Nodes

• Normal lymph nodes are not typically visible.
• Visible lymph nodes include cranial mediastinal, sternal, and tracheo-bronchial.
• Enlargement may indicate rounded soft tissue masses and cause increased size of the mediastinum.
• Causes include reactive lymph nodes, lymphoma, and metastatic disease.

Review of a Thoracic Radiograph - Heart

• Factors affecting heart size and appearance include conformation/breed, age, respiratory phase, and systemic disease.
• Heart size is typically 2-2.5 intercostal spaces (cat) or 2.5-3.5 intercostal spaces (dog). Maximum size is typically no more than two-thirds the width of the thorax on inspiratory views.
• The vertebral heart score (VHS) is a useful tool to measure left and right heart, which should correspond to breed standard values.

Vertebral Left Atrial Size

• A method to assess left atrium size using a line from the carina to the caudal aspect of the left atrium, intersecting the caudal vena cava border (point 1).
• A second line of equal length is drawn from the cranial edge of vertebra T4, extending caudally (point 2).
• Size is measured from vertebral body length and calculated to the nearest 0.1.

Vertebral Left Atrial Size - Diagnostic Value

• This method assists in evaluating left atrial size, which is helpful in certain conditions such as myxomatous mitral valve disease in dogs.

Review of a Thoracic Radiograph - Trachea

• Lateral views are useful for assessing the trachea; the head must be in a neutral position (no artifacts).
• The trachea usually forms an angle with the thoracic spine, is roughly parallel in a lateral view, and superimposed on the spine in a dorsoventral view.
• Size should not change during the respiratory cycle, and narrowing during tracheal collapse is often difficult to diagnose on radiography.

Review of a Thoracic Radiograph - Oesophagus

• Located in the dorsal mediastinum.
• The trachea stripe sign can be seen in mega-oesophagus. The oesophagus appears as outlined in the picture with a stripe sign.

Review of a Thoracic Radiograph - Lungs

• The DV position is often more sensitive for detecting pleural effusions compared to lateral views. Conversely, lateral views can reveal lung lobe distortion in cases of collapse.
• Ideally the examination begins with a dorsoventral view (DV).
• In lung opacities, artificial increases in opacity can be due to obesity, under-exposure, expiration (during breathing exercise), atelectasis (collapse), or pleural disease; and genuine opacity is due to reduced air volume or increase in soft tissues/fluid within the lung.

Review of a Thoracic Radiograph - Lung Patterns

• Alveolar pattern – alveoli fill with fluid (edema, exudate, blood, neoplastic cells)
• Marked increases in lung opacity are seen in focal, multifocal, or diffuse types.
• Border effacement can occur.
• Air bronchograms may be visible.
• Lobar signs can be apparent.
• Interstitial pattern – diffuse swelling or thickening of the tissue between alveoli, often associated with conditions like pulmonary fibrosis
• Bronchial pattern – thickened bronchial walls and peribronchial changes, often seen in cases of inflammation

Review of a Thoracic Radiograph - Vascular Structures

• Arteries are positioned close to the bronchus and are displayed as white arrows on the radiographs.
• Veins are more ventral and central than the arteries, and are displayed as black arrows on the radiographs.
• Pulmonary vessel size should be similar at the corresponding level.
• Arteries and veins in cranial and caudal areas relate to 9th rib width and 1.2x diameter, respectively (for comparison).

Review of a Thoracic Radiograph - Pleural Space

• Pleural effusion (fluid buildup in the pleural space) is typically bilateral.
• Common signs include a widened interlobar fissure, retraction of lung lobes from the thoracic wall, scalloped lung lobe borders, and an obscured cardiac silhouette.
• Causes for pleural effusion include congestive heart failure, pyothorax, hemorrhage, and chylothorax.
• Pneumothorax (air in the pleural space) results in radiolucent, collapsed lung areas and retraction.

Radiography vs Ultrasound vs CT

• Ultrasound: Useful for assessing cardiac function, diagnosing pleural effusions (and performing thoracocentesis), and performing ultrasound-guided biopsies of thoracic masses. It has limitations in assessing lung pathology.
• CT: Superior for assessing the entire thorax and identifying nodules. It also allows for greater contrast resolution and surgical planning, which is beneficial for staging neoplasia.

Summary

• The summary covers patient positioning, views required, interpretation strategies, and common pathologies.