1 Mandibular bodies
2 Temporomandibular joints
3 Tympanic bullae of temporal bones
4 External acoustic meatus of temporal bone
5 Petrous temporal bone
6 Occipital condyle
C 3rd cervical vertebra
D Stylohyoid bones
E Epihyoid bones
F Ceratohyoid bones
G Basihyoid bone
H Thyrohyoid bones
7 Soft palate
9 Cranial limit of laryngeal part of pharynx
10 Caudal limit of laryngeal part of pharynx. Caudal limit is the caudal border of the cricoid cartilage.
11 Caudal limit of larynx
12 Position of the rostral cornu of thyroid cartilage
13 Rostral limit of the arytenoid cartilage. The orniculate process of the arytenoid cartilage is missing in the cat.
14 Vestibular fold
15 Glottic cleft. The cat lacks lateral ventricles and has instead shallow depressions.
16 Infraglottic cavity
21 Intrapharyngeal ostium
22 Laryngeal vestibule
23 Thyropharyngeal muscle
24 Cricopharyngeal muscle
Right lateral recumbent and dorsoventral projections with schematic drawings
Pericardium and heart
1 Right auricle
2 Right ventricle
In this projection the aortic arch is not visible but a large aortic arch is more commonly seen in the cat than the dog. It appears as a distinct bulge at the cranial border, at or below the right auricle level, extending into the cranial mediastinum. Where the aortic arch covers the cranial waist the cardiac shadow appears cranially tilted with an increase in sternal contact.
3 Left atrium
4 Left ventricle
Although the cat has a cranial and caudal waistline, as in the dog, using these as a guideline for cardiac enlargement is not as straightforward, particularly for the caudal waistline. Separation of left atrium and left ventricle is more difficult and enlargements often do not affect the waistline.
Fat accumulation within the pericardial sac is only occasionally seen in the cat but care must be taken to differentiate between soft tissue and fat opacities. In this radiograph fat opacity is visible outside the pericardium on the ventral thoracic cavity wall (7).
The right lateral recumbency is preferred for the cardiac shadow, as discussed in the dog section. In addition the projection should be at full inflation of the lung lobes.
This film is not as fully inflated as the left lateral recumbent projection of thorax of the same cat, Figure 733. As such the vascular shadows are more prominent and the cardiac shadow is slightly cranially tilted. However craniocaudal and dorsoventral cardiac measurements for both projections are the same.
Right lateral recumbent projection, with drawing, and left lateral recumbent projection of thoracic cavity in forced inflation, or over-inflation, of lung lobes, Figures 728–730, have been included to show the effect of ‘loss’ of pulmonary radiographic opacity and ‘upright’ cardiac shadow caused by hyperinflation.
8 Thoracic aorta. (Aortic arch is not clearly seen as a separate structure in this radiograph.)
9 Ventral limit of cranial vena cava
10 Caudal vena cava
11 Cranial lobe artery
12 Cranial lobe vein
The radiolucent shadow between the paired cranial vessels is the lumen of the cranial lobe bronchus. It should not be mistaken for an air bronchogram. An air bronchogram is a characteristic radiographic feature of an alveolar pattern. The latter occurs in diseases which cause an infiltration into the alveoli. Soft tissue opacity replaces the alveolar air lucency. This results in adjacent air-filled lumens of the bronchi becoming visible.
Bronchial walls are not generally visible in the cat unless they are diseased.
13 Pulmonary artery and veins
The arrangement of pulmonary veins is different in the cat compared to the dog. In the dog the veins are symmetrically arranged as left and right sets of lobar veins.
In the cat there are three groups draining: the two parts of the left cranial lobe; the middle and cranial lobes of the right side; and caudal lobes of right and left lungs. Each of these groups is of two or three veins and they are not symmetrically arranged. Hence radiographic differentiation of veins in the cat is very difficult.
14 Tracheal lumen
14(a) Endotracheal tube
15 Tracheal walls
16 Cranial mediastinum
17 Ventral mediastinum. (See juvenile section, Figure 742, for appearance in kittens when the thymus occupies the entire cranioventral mediastinum at a level ventral to cranial vena cava. It also extends 1 to 2 cm cranial to the 1st ribs into the neck.)
18 Diaphragmatic shadow
18(a) Right ‘crus’
18(b) Left ‘crus’
In this radiograph the appearance of the right and left ‘crura’ would suggest left rather than right recumbency. Identification of lateral recumbency is more difficult in the cat than the dog. Often ‘crura’ are superimposed and, as in this radiograph, are misleading.
Entry of the caudal vena cava is usually unhelpful leaving only the gastric gas, caudal to the left ‘crura’, as a guide for recumbency analysis. In this radiograph insufficient gas was present to show the presence of the gastric fundus caudal to the left ‘crura’.
19 Caudal border of scapula
21 Manubrium of sternum
22 Xiphoid process
23 2nd thoracic vertebra
24 11th thoracic vertebra
25 Mineralised costal cartilages
26 Skin and muscle masses of forelimbs
It is widely reported that the cat has a fairly standard shape and size for its cardiac shadow, compared to the breed variation seen in dogs.
With this assumption radiographically normal thoracic projections, from clinically normal cats with no evidence of cardiac abnormalities, should have been relatively easy to obtain (see ‘Normality’ in the Introduction). This was not the case as many cats were found to have cardiac ‘enlargements’ from their radiographic shadows.
Guidance drawing on normal cardiac size: Figure 723
A Apicobasilar length at tracheal bifurcation
B Maximum width between cranial and caudal borders of heart at right angles to A
C Distance between the cranial border of 5th rib and caudal border of 7th rib
D 5th rib
E 7th rib
F Cranial endplate of body of 4th thoracic vertebra.
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). Aortic arch forms the most cranial structure of the ‘heart and major vessels’.
e Left atrioventricular valve; mitral. Length is usually about 25% the craniocaudal width of the heart.
f Aortic valve
Right side with associated vessels
g Right atrium with cranial vena cava (4) and caudal vena cava (5) and azygos vein (6)
h Right auricle. Right auricle is in contact with aortic arch and almost forms the cranial border of the ‘heart’ mass at the cardiac base.
i Right ventricle (drawing does not indicate wall thickness)
j Pulmonary trunk; main pulmonary artery or pulmonary artery segment
k Right atrioventricular valve; tricuspid. Length is usually about 50% of the craniocaudal width of the heart. It overlaps both aorta and pulmonary artery.
l Pulmonary valve
Guidance information on normal cranial mediastinal size: Figure 726
Pericardium and heart
1 Right atrium
2 Right ventricle
3 Left auricle
4 Left ventricle
5 Apex. Formed by left ventricle.
6 Aortic arch. In young cats the aortic arch is often obscured by thymic tissue while in aged cats this structure may become enlarged and distorted (see Figure 732).
8 Pulmonary trunk; main pulmonary artery
9 Level of cranial vena cava within cranial mediastinum soft tissue opacity
10 Level of caudal vena cava. (The vein is is not seen as a separate shadow in this film.)
11 Artery to caudal lung lobe
12 Vein to caudal lung lobe
13 Artery to cranial lung lobe
14 Vein to cranial lung lobe. A linear radiolucent shadow has been created between the artery and vein (14a). As in the right lateral recumbent projection of thorax, Figure 721, the lucent shadow must not be mistaken for an air bronchogram. (Air bronchograms appear in disease as a characteristic radiographic feature of an alveolar pattern.)
15 Tracheal lumen
16 Tracheal wall
Guidance on normal size of cranial mediastinum
Width should be less than the width of the thoracic vertebral body.