Figure 407 Line drawing of photograph representing radiographic positioning for Figure 406.
Figure 408 Drawing to focus on the cardiovascular system. Right lateral recumbent projection of thorax.
Pericardium and heart
Cranial border
1 Right auricle
2 Right ventricle
In this radiograph the aortic arch is not visible but it can often be seen as a separate structure.Very rarely the cranial border of the pulmonary artery is seen.
Note that the aortic arch can be seen in the respiratory system radiographs and drawings, Figures 418 and 420.
A small indentation can sometimes be seen in the craniodorsal border of the cardiac shadow representing the junction of the cranial vena cava and the cranial border. This is known as the cranial waistline. In this radiograph the pulmonary vessels are obscuring the waistline level.
Caudal border
3 Left atrium. Dorsally obscured by pulmonary vessels.
4 Left ventricle
The junction between the atrium and ventricle is usually marked by a definite sulcus in the caudal border as it curves cranially. This is known as the caudal waistline and is found at the level of the ventral border of the caudal vena cava. Unlike the cranial waistline it is often a consistent finding in the normal cardiac shadow. In this radiograph the caudal waistline is not obvious.
5 Dorsal base
6 Apex
The pericardium, the fibroserous sac surrounding the heart, is not seen as a separate structure. In very obese dogs fat can accumulate within the sac and the middle mediastinum. Even so, the fat opacity may not be readily interpreted as a different tissue and without careful evaluation of the cardiac shadow a misdiagnosis of ‘cardiac enlargement’ can be made.
Note that the right lateral recumbent projection of thorax is preferred for the cardiac shadow as the caudoventral fold of the mediastinum (anatomically the phrenicopericardial ligament) acts to anchor the apex. Although small there are noticeable differences in the cardiac, caudal vena cava and aortic silhouettes in right and left lateral recumbencies (see respiratory system radiographs, Figures 418 and 423).
Vascular
7 Aortic arch. Occasionally an ill-defined shadow representing an outflow vessel from this arch is seen in the right lateral recumbent projection of thorax (not present in this radiograph).
8 Thoracic aorta
9 Level of cranial vena cava
The cranial vena cava cannot be seen as a separate structure but will be found at the ventral level of the cranial mediastinal shadow. It is formed at a level cranial to the thoracic inlet.
10 Caudal vena cava. (In this projection it is seen entering the central tendon of the diaphragm just right of the midline.) In most right lateral recumbent projections of thorax the entry is clearly defined as right sided and can be used to confirm recumbency.
11 Right cranial lobe artery
12 Right cranial lobe vein
13 Left cranial lobe vein
The left cranial lobe artery can not be clearly seen in this radiograph but it will be present just ventral to the right cranial lobe vein.
Radiolucent shadows between the paired cranial vessels are the corresponding lobe bronchial lumen. These should not be mistaken for the abnormal air bronchograms. Often the bronchial walls are also visible and in this radiograph the most dorsal portions are just seen as fine radiopaque lines (14).
15 Artery and vein of caudal segment of left cranial lobe cranially
16 Artery and vein of right middle lobe caudally
17 Right pulmonary artery and veins. Right pulmonary artery when seen as a separate structure, passes ventral to the tracheal bifurcation and is a round/oval soft tissue opacity in its end-on projection.
18 Left pulmonary artery and veins. Left pulmonary artery crosses the trachea, passing cranial to the tracheal bifurcation.
The left vascular trees are located just dorsal to the right but identification is very difficult. Also differentiating between artery and vein is hard. However arteries usually are more opaque, often slightly curved and are more well defined. Veins are usually shorter and stubbier. In addition arteries are located following the bronchial tree whereas veins travel to the left atrium via the shortest route.
Non-cardiovascular structures
19 Tracheal lumen
20 Tracheal walls
21 Level of tracheal bifurcation; carina
21(a) Left cranial bronchus at bifurcation into bronchi for cranial and caudal segments. This shadow is often incorrectly named ‘carina’.
22 Cranial mediastinum occupied by large veins and arteries cranial to the heart, especially the cranial vena cava and brachiocephalic trunk.
23 Pleural cupola. Area of lung extending cranial to 1st rib.
24 Sternum
24(a) Manubrium of sternum
24(b) Xiphoid process
25 1st thoracic vertebra
26 11th thoracic vertebra
27 Diaphragmatic shadow
27(a) Left ‘crus’
27(b) Right ‘crus’
28 Mineralised costal cartilages
29 Gas-filled gastric fundus
Fluid-filled oesophagus can often be seen in the caudodorsal thoracic cavity in the right lateral recumbent projection of thorax. (Not present in this radiograph but can be seen in the mediastinal structures section, Figures 432 and 433.)
Figure 409 Drawing of right lateral recumbent projection of thorax to provide guidance on normal vascular sizes.
A Artery of cranial lobe (dorsal to vein)
B Vein of cranial lobe (ventral to artery)
C End-on blood vessel of cranial lobe (appears as distinct circular soft tissue opacity and is associated with adjacent longitudinal blood vessel)
R Width of proximal 3rd of 4th rib
Normal artery size crossing 4th rib
Width of A approximately 75% of R
Width of A should not exceed R
A = B or A may be slightly larger than B
Normal blood vessels
Generally blood vessels decrease in their size towards the periphery of the lung lobes, but the end-on vessel widths are always similar to adjacent longitudinal blood vessel widths. The latter allows differention of end-on blood vessels from nodules.
Figure 410 Schematic drawing of right lateral recumbent projection of thorax to illustrate cardiac chambers and major vessels. (Corresponds to drawing Figure 408 but with the exclusion of some thoracic cavity details seen in radiograph.)
Left side with associated vessels
a Left atrium with pulmonary veins (1)
b Left auricle
c Left ventricle (drawing does not indicate wall thickness)
d Aorta with left subclavian artery (2), and brachiocephalic trunk (3)
M Left atrioventricular valve; mitral
A Aortic valve
Right side with associated vessels
e Right atrium with cranial vena cava (4), and caudal vena cava (5), plus azygos vein (6)
f Right auricle
g Right ventricle (drawing does not indicate wall thickness)
h Pulmonary trunk. Main pulmonary artery or pulmonary artery segment.
L Ligamentum arteriosum. Remnant of foetal ductus arteriosus.
P Valve of pulmonary trunk
T Right atrioventricular valve; tricuspid
Figure 411 Drawing of right lateral recumbent projection of thorax to provide guidance on normal cardiac size.
L Maximum length of heart. Measurement from ventral aspect of ‘carina’ and heart apex.
W Maximum width of heart. Measurement of maximum width perpendicular to L.
T Cranial endplate of 4th thoracic vertebral body.
Cardiac size using vertebral heart size (VHS)
Normally L+W does not exceed 10.6 vertebral body lengths.
Heart size over 10.6 may usually be considered as enlargement.
Caution
Some breeds of dog regularly exceed 10.6, e.g. Golden Retriever, Boxer, Pharaoh Hound and Cavalier King Charles Spaniel.
Always remember to look for signs of cardiac failure when cardiac size is being assessed. Even if cardiac enlargement is apparent, clinical signs will be due to failure not enlargement.
Serial cardiac size using VHS
Although the VHS is only a guidance for cardiac enlargement, it can act as a useful size monitor for serial thoracic radiographs of the same dog. The latter will aid in assessing response to cardiac therapy.
Figure 412 Drawing of right lateral recumbent projection of thorax to provide guidance on normal cardiac and major blood vessel sizes.
H Height of cardiac shadow
W Maximum width of cardiac shadow
Normal cardiac size
At 5th rib H should not be greater than 2⁄3rds of the thoracic cavity height.
W is a range of 2.5 to 3.5 intercostal spaces wide depending on chest conformation.
A Aortic width
B Caudal vena cava width
Normal major blood vessel size
Caudal vena cava
At the greatest width, at the same intercostal space, B:A is less than 1 or equal. Greater than 1.5 is abnormal.
Figure 413 Projection to focus on the cardiovascular system. Dorsoventral projection of thorax. Radiograph taken during general anaesthesia with full inflation of lung lobes. Beagle dog 2.5 years old, entire male (same dog as in right lateral recumbent projection of thorax, Figure 406). (Approximately 75% of original size.)
Figure 414 Line drawing of photograph representing radiographic positioning for Figure 413.
Figure 415 Drawing to focus on the cardiovascular system. Dorsoventral projection of thorax.
Pericardium and heart
Right side
1 Right atrium
2 Right ventricle. Shadow of ventricle just crosses the midline.
Left side
3 Left auricle
4 Left ventricle
5 Apex. Formed by the wall of left ventricle.
Please see right lateral recumbent projection of thorax to highlight cardiovascular system, Figure 408, for more details.
Vascular
6 Aortic arch
7 Aorta
8 Pulmonary trunk. Main pulmonary artery or pulmonary artery segment.
9 Level of cranial vena cava within cranial mediastinum soft tissue shadow.
10 Caudal vena cava
11 Arteries to caudal lung lobes. Originate cranial to tracheal bifurcation level, carina, and travel lateral to principal caudal lobe bronchus (12).
The carina is not clearly seen in this radiograph but will be located at (13).
14 Veins to caudal lung lobes. Located caudal to tracheal bifurcation level, carina, and travel medial to main caudal lobe bronchi. In this radiograph the right vein is obscured by the shadow of the caudal vena cava. Veins are slightly smaller than arteries.
Care must be taken in identifying arteries and veins in this projection as the mineralised costal cartilage shadows ‘mimic’ vessels. To distinguish between the two sets of shadows one must trace the full course of the shadows.
Shadows of the mineralised costal cartilages will be seen to travel caudally then curve cranially at, or near, the costochondral junctions.
To avoid confusion the mineralised costal cartilages and bony rib shadows have been excluded from the drawing but the reader should now identify these shadows. A drawing of the mineralised cartilages and ribs, Figure 395, can be found in the axial skeleton section.
Non-cardiovascular structures
15 Tracheal lumen
16 Tracheal wall
17 Right cranial bronchial lumen
18 Cranial mediastinum. The right border is formed by the cranial vena cava while the left is formed by the left subclavian artery. In addition, both the trachea and oesophagus lie within the cranial mediastinum.
19 Pleural cupola
20 1st rib
21 8th rib
22 Diaphragmatic shadow
23 Gas-filled lumen of gastric fundus
24 Caudoventral fold of the mediastinum (anatomically the phrenicopericardial ligament). The fold (or reflection) is a fibrous thickening of the ventral portion of caudal mediastinum.
Although the fold is usually seen at the cardiac apex a more lateral position can sometimes occur, as in this radiograph. The attachment of the fold is pericardial not heart.
Figure 416 Drawing of dorsoventral projection of thorax to provide guidance on normal vascular sizes.
A Artery of caudal lung lobe (lateral to vein)
B Vein of caudal lung lobe (medial to artery)
R Width of 10th rib
Normal artery size
At the level of 10th rib the width of A does not exceed R.
A = B or A may be slightly larger than B.
Figure 417 Schematic drawing of dorsoventral projection of thorax to illustrate cardiac chambers and major vessels. (Corresponds to drawing Figure 415 but with the exclusion of some thoracic cavity details seen in radiograph.)
Left side with associated vessels
a Left atrium
b Left auricle
c Left ventricle (drawing does not indicate wall thickness)
d Aorta, root plus arch, with brachiocephalic trunk (1)
A Aortic valve
M Left atrioventricular valve; mitral
Right side with associated vessels
e Right atrium with cranial vena cava (2) and caudal vena cava (3)
f Right auricle
g Right ventricle (drawing does not indicate wall thickness)
h Main pulmonary artery
P Valve of pulmonary trunk
T Right atrioventricular valve; tricuspid
Guidance on normal cardiac size
Maximum width of cardiac shadow is 2⁄3rds the width of the thoracic cavity.
Respiratory system: Figures 418–430
Left and right lateral recumbent, ventrodorsal and dorsoventral projections with schematic drawings
Guidance drawing on normal tracheal size: Figure 421
Figure 418 Projection to focus on the respiratory system. Left lateral recumbent projection of thorax. Radiograph taken during general anaesthesia with full inflation of lung lobes. Beagle dog 7 years old, entire male (same dog as in all projections of thorax to focus respiratory system, Figures 423, 426 and 429). (Approximately 70% of original size.)
Figure 419 Line drawing of photograph representing radiographic positioning for Figure 418.
Figure 420 Drawing to focus on the respiratory system. Left lateral recumbent projection of thorax.
1 Cranial limit of the left cranial lung lobe. Extends beyond the 1st pair of ribs into pleural cupola in well-inflated lung lobes. Occasionally this shadow may appear more lucent than the adjacent right cranial lung lobe shadow.
2 Cranial limit of the right cranial lung lobe
3 Lucent shadow created by right middle lung lobe. This lung lobe is larger than the corresponding caudal segment of the left cranial lung lobe. Its presence at full inflation between the cardiac and sternal shadows should not be mistaken for free pleural gas. Pulmonary opacities can be seen in the lucent region so confirming lung tissue.
Note that there is no left middle lung lobe in the dog.
4 Dorsal limit of caudal lung lobes. (The limit is more dorsal than normal, caused by positional rotation of the dog during radiographic exposure, as seen by rib shadows superimposed over vertebral canal.) In a non-rotated lateral projection of the thorax the limit is at the level of the dorsal edge of vertebral body.
5 Ventral border of caudal lung lobe
From plain radiographs of the thorax it is not possible to identify many individual lung lobes due to extensive superimposition. The above labels the peripheral extents of some of the lobes which can be recognised and the schematic drawing attempts to demonstrate the areas covered by the individual lobes.
6 Pleural fissure line between middle and caudal lobes. Incidental finding which is occasionally seen in left lateral recumbency of larger breeds of dog.
7 Cervical tracheal lumen
8 Thoracic tracheal lumen
9 Tracheal walls
10 Level of tracheal bifurcation into right and left principal bronchi; carina
11 Radiolucent circular shadow demonstrating end-on projection of left cranial lobe bronchus at bifurcation into bronchus for cranial and caudal segments. This shadow is often incorrectly labelled ‘carina’. The carina cannot be seen as a distinct shadow on lateral thorax projections.
12 Right cranial bronchial lumen
13 Right cranial bronchial wall
14 Left cranial bronchial lumen
15 Left cranial bronchial wall
16 Linear opacities representing bronchial walls
17 Circular opacities representing bronchial walls seen end-on
Bronchial markings, as labelled above, are a normal feature in older dogs. Also the appearance of the pleura is not uncommon. In addition nodular, linear and circular interstitial opacities will be found, as in this 7-year-old dog. The lung shadows in this Figure should be compared with the ones found in the cardiovascular system right lateral recumbent radiograph, Figure 406. In the latter the dog is 2.5 years old.
Opacity changes will start to appear from approximately 4 years of age but environmental factors have to be considered e.g. town versus country (urban/rural). The opacity changes represent fibrous tissue and, or, mineralisation of bronchial and interstitial tissues. They are more pronounced, and occur at a younger age, in the chondrodystrophic breeds.
Note that this radiograph has been taken at full inflation to avoid radiographic error of ‘increase lung opacity’.
18 Cranial mediastinum occupied by large veins and arteries cranial to the heart especially cranial vena cava and brachiocephalic trunk
19 Cardiac shadow including aortic arch at cranial border and at dorsal and ventral extremities the extensions of the pericardium
20 Aorta
21 Caudal vena cava seen entering the central tendon of the diaphragm at a level just right of the midline
22 1st thoracic vertebra
23 11th thoracic vertebra
24 Manubrium of sternum
25 Xiphoid process
26 Mineralised costal cartilages 27 Diaphragmatic shadow
27(a) Left ‘crus’
27(b) Right ‘crus’
27(c) Cupola
28 Lumbodiaphragmatic recess
29 Ventral skin folds superimposed on thorax shadows
30 Caudal border of scapula
31 Caudal angle of scapula
32 Spine of scapula
Figure 421 Drawing of left lateral recumbent projection of thorax to provide guidance on normal tracheal size.
T Width of trachea
I Width of thoracic inlet
R Width of proximal 3rd of 3rd rib
Normal tracheal size
T is not less than 20% of I
T is approximately 3 times R but always in excess of R
Caution
In the English Bulldog T may be only 14% of I
Figure 422 Schematic drawing of left lateral recumbent projection of thorax to illustrate lung lobes. (Corresponds to drawing Figure 420 but with exclusion of thoracic cavity details seen in radiograph.)
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