INTRODUCTION TO THORACIC RADIOLOGY
The thoracic radiographic study is probably the most common study performed in
veterinary medicine.
Indications for performing thoracic radiographs:
• Coughing
• Dyspnea/ Tachypnea
• Heart Murmur, Collapse
• Primary or Secondary Neoplasia
– Check for metastasis
• Thoracic Trauma
• Chest Wall Mass
• Exercise Intolerance, Weight Loss
Technical Factors
• Potential for Movement
– Decrease mAs – with a shorter time, this will minimize the possibility of
motion
• High inherent contrast area
– High kVp – we are not worried about the loss of contrast by using a high
kVp technique since the thorax has so many different radio opacities
present.
• Collimation – the beam should be collimated to include from the thoracic inlet
to the diaphragm. The tighter the beam is collimated the less scatter will be
present.
• Centering – caudal aspect of the scapula – this is usually easy to palpate
– Thoracic inlet to diaphragm
– Pull forelimbs forward – it is imperative that the forelimbs are pulled
cranially so the triceps musculature does not overlie the cranial mediastinal
area.
Determining the Phase of Respiration
• Always expose at peak inspiration
– Maximizes lung contrast – allow better visualization of pulmonary
parenchyma. The diaphragm will be displaced caudally so the lung
parenchyma is not compressed.
– Inspiratory lateral view
• Caudodorsal aspect of lung caudal to T12
• Increased aeration of accessory lung lobe
• Separation of heart silhouette and diaphragm
– Inspiratory VD/DV view
• Diaphragmatic cupola caudal to mid T8
• Lung tips caudal to T10
DV vs. VD
• DV
– Less stressful for patients who are dyspenic, very large or difficult to
handle. Many believe the DV is better for evaluation of the heart.
– Diaphragm rounded – like one large dome
– Caudal pulmonary vessels better visualized – this is especially important
when evaluating for heartworm disease.
– Better to see small amount of pleural air – the air will rise to the tips of the
thorax caudally
• VD
– Better for lungs – most people routinely do the VD view
– Hear appears elongated
– Flat diaphragm – Mickey Mouse ears – 3 lumps
– Better to see small amount of pleural fluid
• Right Lateral – most people routinely do the Right lateral
– Better cardiac detail
– R crus forward
– See Cava go into the right crus
• Left Lateral – done if pathology is suspected in the right lung lobes or if a
metastasis check is being performed
– Heart appears round
– L crus forward
– See Cava go past the first crus (left) and enters the caudal one (right)
• Anesthesia
Rads made while under general anesthesia or while heavily sedated
- This should be avoided if possible.
- Will cause the pulmonary parenchyma particularly on the dependent side
to become congested or show evidence of partially atelectasis
o Due to decreased expansion of the down lung
o Due to change in blood flow to the lung
- If the radiographs must be made while the patient is under anesthesia, the
position of the patient should be changed to allow for expansion of the
lungs with a re-breathing bag.
A breath hold technique can be performed to inflate the lungs before the
exposure
• Breed Differences
- The appearance of the thoracic cavity including the heart and pulmonary
parenchyma can appear slightly different depending on the breed of dog
Deep chested Dobe, giant breeds ect.
 Cardiac silhouette usually 2 ½ intercostals spaces on the
lateral
 Cardiac shape is more upright with less contact of the
diaphragm
 The cardiac shape is round on the VD view
Barrel chested  round body short leg dogs  bull dog, Bassett
hound, dachshund and many toy breeds
 Cardiac silhouette usually 3 ½ intercostals spaces on the
lateral
 Cardiac shape is more rounded on the lateral and there is
increased contact with the diaphragm
Average  in between
The Effects of Lateral Recumbency
Which “side” of the hemithorax is best seen on laterals?
- The dependent lung will become compressed because the lung can not
fully expand with the weight of the patient on that lung thus causing an
increase in radiopacity
- The non dependent lung will be better aerated thus highlighting a soft
tissue pulmonary change in that lung
- In practice this is used in many ways:
o Pneumonia – commonly occurs in the right middle lung lobe  take
a left lateral
o Solitary pulmonary mass – if is located in left cranial lung  the
right lateral would allow better visualization of the mass
Critically thinking about which lateral will show the pulmonary change you are
expecting is imperative.
• Lung lesions (mass, nodule, infiltrate) may only be seen on 1 view!!!
• Only the non-dependent (up) lung can be critically evaluated
– Dependent lung loses aeration (atelectasis)
• Increases in opacity
• Silhouettes with lesions
Special Views
• Horizontal beam
– Upright VD view
• Pleural fluid will fall caudally so CMM can be seen
– Recumbent lateral VD
– Position patient to move pleural fluid away from area of interest
• Cranial mediastinal mass
• Lung mass
– Check for free air – side up
Interpretation of Thoracic Radiographs
• Develop your own routine
• Systematically evaluate everything on every view
• Evaluate a specific structure simultaneously on both views (i.e. assess lungs
on VD and lat before moving on to mediastinum)
Interpretation of Thoracic Radiographs
• Heart
• Lungs
• Mediastinum
• Pleural space
• Chest wall
• Bones, Abdomen, Neck
Normal Cardiac Silhouette
• Subjective on the lateral view
– Dog = 2 ½ - 3 ½ intercostal spaces
– Cat = 2 – 2 ½ intercostal spaces – as cats age the heart may lie more
parallel to the sternum
• 65% or less on VD/DV view
• Objective
– Buchanan method
Clock Face
• 11-1 Aortic Arch
• 1-2 Main Pulmonary Trunk
• 2-3 Left Auricle
• 2-5 Left Ventricle
• 5-9 Right Ventricle
• 9-11 Right Atrium
• Centrally – Left Atrium
Lateral View
• Make a Plus sign
• Bermuda triangle = right atrium, aorta and main pulmonary trunk
• Left atrium
• Left Ventricle
• Right Ventricle
Thoracic and Pulmonary Vessels
• Aorta
• Caudal Vena Cava
• Cranial pulmonary vessels – best seen on lateral view
– The size should be smaller than the proximal portion of the third rib
• Caudal pulmonary vessels – best seen on DV view
– The vessel should form a square with the 9th rib where it crosses it (as
opposed to a rectangle.
• Veins are ventral and central
Trachea, Bronchial Tree
• Carina – then splits to the main stem bronchi then lobar bronchi
• Tracheal rings can mineralize – this is particularly seen in brachycephalic
breeds
• Decreased tracheal diameter
– Tracheal narrowing (stenosis, extramural compression), Tracheal
hypoplasia, Tracheal collapse
Lungs
• Normal anatomy
– Left
• Cranial (cranial subsegment)
• Cranial (caudal subsegment)
• Caudal
– Right
• Cranial
• Middle
• Caudal
• Accessory
• Normal lung boundaries
– 4th to 5th ICS on VD
• Fissure b/w L cranial lung sub-segments
• Fissure b/w R cranial and middle lung lobes
– 6th to 7th ICS on VD
• Fissure b/w L cranial and caudal lobes
• Fissure b/w R middle and caudal lobes
• Regions of a specific lung lobe
– Perihilar (hilar)
– Midzone
– Periphery
• Distribution of disease may lead to etiology
– Edema
– Pneumonia
The Mediastinum
• Cranial, middle, caudal compartments
• Routinely visible structures:
– Heart, trachea, cvc, aorta, +/- thymus, +/- esophagus
– Cranioventral mediastinal reflection
– Caudoventral mediastinal reflection – seen on left side of thorax
Mediastinal Abnormalities
• Shift
• Masses
• Fluid
• Pneumomediastinum
Mediastinal Shift
• Assess on VD or DV
– Position of heart, trachea, aorta, cvc
• ***MUST BE STRAIGHT or may be artifactual!!!
• Ipsilateral shift
– Unilateral decrease in lung volume (atelectasis)
• Contralateral shift
– Increase in lung volume
– Intrathoracic mass
Cranial Mediastinal Masses
• Lie on or adjacent to midline
• Lateral or dorsal displacement of mediastinal structures
– Elevation of trachea
• ***Diff dx= large volume pleural effusion
• Widening of mediastinum on VD
– Should be < 2x width of vertebrae on VD
– Fat may artifactually widened, esp. Bulldogs
• Increased opacity in mediastinum
Mediastinal Fluid
• Increased soft tissue opacity in mediastinum
• May appear as a soft tissue mass
• Common causes
– Feline infectious peritonitis
– Hemorrhage
• Trauma
• Coagulopathy
– Esophageal perforation
Pneumomediastinum
• Free air in mediastinum
– Enhanced visualization of mediastinal structures
– Not dyspneic
• Can progress to pneumothorax
– Pneumothorax does NOT progress to pneumomediastinum
• ***Mediastinum communicates with neck and retroperitoneal space
– Subcutaneous emphysema
– Pneumoretroperitoneum
Causes of Pneumomediastinum
• Air escaping into lung interstitium from ruptured alveoli
– Trauma, hyperinflation during anesthesia
• Extension of gas from neck fascia
• Tracheal perforation
– Trauma, venipuncture, TTW, cuff over inflation
• Esophageal perforation
• Extension of retroperitoneal gas
• Gas producing organism in mediastinum
The Pleural Space
• Two layers
– Parietal
• Lines thoracic wall and diaphragm
– Visceral
• Lines outer lung surface
• Normal pleura not usually visible
• May be visible with pleural thickening or if beam strikes normal pleura
tangentially
– Between right middle and right caudal lobes on Left Lateral
• Visible with pleural effusion or pneumothorax
Pleural Effusion
• Radiographic signs
– Interlobar fissure lines
• VD more sensitive with small volumes
– Retraction of lungs
– Increased soft tissue in thorax outlining lungs
• Esp. dorsal to sternum on lateral
– Silhouetting of heart/ diaphragm
Pneumothorax
• Air in pleural space
– External, lung, or mediastinum
• Radiographic signs
– Retraction of lungs
– Lucent space between lung and chest wall
• ***Vessels do not extend to chest wall
• Use a hotlight
– Dorsal “displacement” of heart on lateral
•
Actually sliding into dependent thorax
Causes of Pneumothorax
• Trauma
• Lung rupture
• Chest wall rent
• Extension of pneumomediastinum
• Rupture of cavitary lung mass
Tension Pneumothorax
• Pleural space pressure exceeds atmospheric pressure during both phases of
respiration
• Severe lung collapse
– Lungs lose normal shape
• If unilateral, may cause contralateral mediastinal shift
• Caudal displacement of the diaphragm
Extrathoracic Structures
• Sternum
• Vertebrae
• Ribs
• Adjacent soft tissues
• Diaphragm
• Extrathoracic changes may indicate cause of intrathoracic findings
– Examples
• Pneumothorax
– Rib fractures may suggest secondary to trauma
• Pleural effusion secondary to rib or chest wall mass
The Diaphragm
• Cupula
– Cranioventral convex portion
• Right and left crura
– Attach to cranioventral border of L3 and body of L4
– May cause irregularity on these surfaces
• Appearance depends on centering of X-ray beam