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9

Normal Radiographic Anatomy
Martha M. Larson

Department of Small Animal Clinical Sciences, Virginia-Maryland College of Veterinary Medicine, Virginia Tech, Blacksburg, VA, USA

9.1 Lungs generally unattached (floating), and the corresponding
costal cartilage is separate from the other ribs. Fragmented
As in the dog, the feline lung is divided into four right‐sided mineralized costal cartilages are a normal variation in
lobes (right cranial, right middle, right caudal, and acces- some older cats.
sory) and two left‐sided lobes (left cranial, divided into cra-
nial and caudal segments and left caudal lobe). The feline
lung extends from the thoracic inlet at the level of the first 9.3 Diaphragm
thoracic vertebra (T1), first pair of ribs, and manubrium,
caudally to the diaphragm. On lateral thoracic films taken The diaphragm is composed of left and right dorsal
on inspiration, the caudal lung lobes typically extend to halves, which attach caudally by means of the crura to
T13 or L1. The longus coli muscle inserts on T6, and inter- the ventral aspect of the third and fourth lumbar verte -
poses between the thoracic vertebrae and lungs, potentially brae, and the central cupula. In the dog, significant dif-
mimicking dorsal pleural effusion on full inspiratory lat- ferences in the radiographic appearance of the diaphragm
eral thoracic radiographs (Figure 9.1). If thoracic films are may be present on left and right lateral, and VD and DV
taken on expiration, the lungs appear more opaque and projections. However, in the cat, these differences are
smaller in volume, extending only to T13 or cranial minimized, likely because of the smaller body size
(Figure 9.2). (Figure 9.4). There may be slightly better visualization of
the caudal lobar vessels on the DV projection, and the
caudal mediastinum and accessory lung lobe may be
9.2 Thoracic Wall better evaluated on the VD view [2].

The thoracic wall is composed of thoracic vertebrae, ribs
and costal cartilages, muscle, skin, fat, and parietal pleura. 9.4 Mediastinum
It is not a common site of disease, but can create significant
radiographic artifacts that mimic pathologic processes. The mediastinum is a true space, located between the left
Large amounts of body wall fat will result in poor inspira- and right pleural sacs. It lies on the midline, although it
tory effort, with subsequent generalized increase in pulmo- deviates slightly to accommodate extension of the right
nary opacity. Scatter radiation can also add to this effect. cranial and accessory lung lobes toward the left side.
Increased prominence of the extracostal musculature sur- Deviation of the accessory lung lobe results in displace-
rounded by more radiolucent fat creates alternating stria- ment of the caudal mediastinum to the left, forming a
tions in the body wall, seen parallel to ribs on ventrodorsal mediastinal reflection visible in both cats and dogs
(VD)/dorsoventral (DV) views (Figure 9.3) [1]. There are (Figure 9.5). A second cranioventral mediastinal reflection
normally 13 pairs of ribs. The costal cartilages of the first is formed by the extension of the right cranial lobe to the
nine ribs articulate with the sternum. Costal cartilages of left. This reflection is commonly seen in dogs, but not
ribs 10–12 form the costal arch caudally. The 13th rib is typically in feline thoracic images.

Feline Diagnostic Imaging, First Edition. Edited by Merrilee Holland and Judith Hudson.
© 2020 John Wiley & Sons, Inc. Published 2020 by John Wiley & Sons, Inc.