Clifford R. Berry1 and Elizabeth Huyhn2 1 Department of Molecular Biomedical Sciences, College of Veterinary Medicine, North Carolina State University, Raleigh, NC, USA 2 VCA West Coast Specialty and Emergency Animal Hospital, Fountain Valley, CA, USA The thorax is ideally suited for radiographic imaging as it provides excellent subject contrast (gas compared with soft tissue), thereby providing excellent radiographic contrast. Thoracic radiographs are inexpensive, rapid to do, and still considered a first line test to survey the intra‐ and extrathoracic structures. Currently, three‐view thoracic radiographs are considered the standard of care in veterinary medicine for the evaluation of the thorax [1–4]. These include the right lateral, left lateral, and either the dorsoventral or ventrodorsal radiographs of the thorax. When describing the right and left lateral radiographs, these images refer to the recumbent side of the dog or cat when placed down against the x‐ray table. The correct point of entrance to point of exit of the x‐ray beam naming of the radiograph would be left to right lateral image (right lateral) and right to left lateral image (left lateral). The ventrodorsal and dorsoventral images appropriately describe the point of entrance to the point of exit of the x‐ray beam as with standard x‐ray labeling nomenclature. For example, with the dog in dorsal recumbency, a ventrodorsal image is made such that the ventral aspect of the patient is the entrance point of the x‐ray beam and the dorsal aspect of the patient is the exit point of the x‐ray beam. The chapters involving the thorax will introduce radiographic concepts for interpretation. It is important to understand that all of the nuances associated with the interpretation of thoracic radiographs in small animals will not be covered, as one cannot see the infinite examples of thoracic diseases, much less normal variations. The purpose of this chapter is to review normal radiographic anatomy of the thorax, thoracic variants, and “fake‐outs,” and present an interpretation paradigm for the routine evaluation of the thorax. High‐quality thoracic radiographs are the end goal for all thoracic imaging studies. High‐quality radiographs require appropriate technique, positioning, and anatomy. For the technique, a high kVp and low mAs are used [5], which will provide the best latitude or overall image gray scale. However, for most digital radiography (computed radiography [CR] or digital radiography [DR]) scenarios, the dynamic range is wide enough, and the processing algorithms mature enough to result in high‐quality images without significant technique manipulation between patients. The most common mistake is photon starvation (underexposure) of the DR or CR plate, resulting in quantum mottle associated with the image. At the other extreme, one can oversaturate (expose) the digital plate which results in absence of information (no vessels, airways, etc.) in areas of the lung, specifically the right cranial lung lobe cranial to the cardiac silhouette and the accessory lung lobe caudal to the cardiac silhouette on the lateral images. Once a mAs (typically 1–3 mAs for the thorax) has been decided, the clinician should use the highest mA station so that the fastest time can be used to obtain the desired low mAs. This will decrease the likelihood of respiratory motion and artifact. The thoracic radiograph should be taken on peak inspiration [6]. This is important for the overall cardiothoracic ratio and detection of pulmonary changes not associated with expiration (Figure 13.1). On expiratory radiographs, there is enlargement of the cardiothoracic ratio (cardiac silhouette appears larger relative to the thoracic volume) and an unstructured interstitial pulmonary pattern is often present [7]. These changes could result in the false‐positive diagnosis of cardiomegaly and pulmonary edema (Figure 13.1A–D). On inspiration, the diaphragmatic crura will cross the thoracic spine at T11–T12 (or further caudal) and there will be an increase in size to the accessory lung lobe, resulting in a larger radiolucent triangle formed between the caudal border of the cardiac silhouette, the ventral border of the caudal vena cava and the cranial border of the central diaphragm when compared with expiratory radiographs, (Figure 13.1A–D). FIGURE 13.1 Right lateral inspiratory (A) and expiratory (B), left lateral inspiratory (C) and expiratory (D), ventrodorsal inspiratory (E) and expiratory (F), and dorsoventral inspiratory (G) and expiratory (H) thoracic radiographs of a normal 5‐year‐old medium‐sized mixed‐breed dog. In some medium‐ and large‐breed dogs, there will be a radiolucent triangle noted with separation between the cardiac silhouette and the cupula of the ventral diaphragm. In some of the smaller dog breeds, the cardiac silhouette will be in contact with the cranial diaphragm on the lateral and ventrodorsal radiographs on inspiration as well as expiration. These images will still be inspiratory as a dog will breathe (tidal volume changes) by moving its diaphragmatic crura and not the entire diaphragm. If the lateral image was made on expiration, the diaphragmatic crura will be seen at the T8 vertebral body. Similarly, on the ventrodorsal radiograph, the cupula of the diaphragm will be located at the T6–T7 vertebral bodies and there will be cranial displacement of the costophrenic pulmonary angle to the sixth to eighth intercostal spaces (Figure 13.2). Differences in inspiration and expiration in dogs are found in Table 13.1. FIGURE 13.2 Ventrodorsal inspiratory (A) and expiratory (B) thoracic radiographs of a 5‐year‐old medium‐sized mixed‐breed dog. Notice the differences between the position of the cupula and the diaphragmatic crura and relative size of the thoracic volume compared to the size of the cardiac silhouette. In expiration, the crura and costophrenic angle (white arrows) are displaced cranially. TABLE 13.1 Differences in peak inspiration and expiration based on lateral and ventrodorsal radiographs. In cats, all films are typically made on inspiration with the diaphragmatic crura extending caudal to the level of T12–T13 and the cupula (dome) of the diaphragm being caudally displaced to the level of T10. There is usually a separation between the caudal cardiac silhouette and the cranial diaphragm seen on both the lateral and ventrodorsal images (Figure 13.3). This is because cats move all aspects of the diaphragm (crura and cupula) in a caudal direction during inspiration. FIGURE 13.3 Right lateral (A), left lateral (B), and ventrodorsal (C) thoracic radiographs of a normal 5‐year‐old domestic shorthair cat. Notice the difference in position between the diaphragmatic crura on the lateral radiographs (white arrows). Technique and positioning of the thoracic radiographs are accomplished through experience and trial and error. Unfortunately, some veterinarians do not take the time to quality control this aspect of thoracic radiography and thereby do not obtain high‐quality images on a consistent basis. The right and left lateral recumbent radiographs are made such that the sternum and vertebral column align with each other in the same plane parallel to the x‐ray table top (Figure 13.4). The right and left rib heads at the vertebrae should overlap. The thoracic limbs should be taped cranially so that they do not superimpose over the cranial thorax. This is most easily accomplished if the pelvic limbs are taped and pulled caudally as well. A triangular radiolucent sponge can also be placed under the sternum to elevate the sternum away from the x‐ray table top so that it is in the same sagittal plane as the vertebral column and parallel to the x‐ray table top. This is needed for medium‐ and large‐breed dogs. In dogs with a barrel‐shaped thorax, one may need to rotate the sternum toward the table in order to get the sternum horizontal with the thoracic spine and parallel to the table top of the x‐ray unit. FIGURE 13.4 Right lateral recumbent positioning of a dog for a right lateral radiograph. Notice that the collimator light extends from the thoracic inlet caudally to the cranial abdomen (caudal border of the 13th rib) and extends dorsal from the spinous processes of the thoracic spine to the sternum ventrally. The collimation of the x‐ray machine will dictate the anatomy that is projected onto the imaging device. The image should be centered along the caudal border of the scapula (middle of the cardiac silhouette) and include the thoracic inlet cranially, the 13th rib caudally, the spinous processes of the thoracic vertebrae dorsally, and the sternum ventrally. In smaller dogs and cats, collimation helps reduce x‐ray scatter and thereby eliminates degradation of the x‐ray image due to film fog from the scatter radiation as well as decreasing overall scatter radiation to any personnel who are in the x‐ray room at the time of the exposure. However, the goal would be to position the dog or cat such that everyone is out of the x‐ray room at the time of the exposure [8]. In giant‐breed dogs, one might have to split the thorax into cranial and caudal halves with overlap in the middle of the image to ensure complete assessment of the thorax (Figure 13.5). FIGURE 13.5 Right lateral (A,B,C) and ventrodorsal (D,E) thoracic radiographs of a 7‐year‐old greyhound. The added soft tissue opacity noted in the right caudal thorax is due to superimposed skinfolds and not pulmonary pathology. This was confirmed on the left lateral images. Additionally, on the right lateral images there is mineralization noted to the aortic root and potentially the coronary arteries. Other radiographic projections of the thorax include oblique, horizontal beam, and “humanoid” projections. Oblique projections are typically obtained with the patient in ventral or dorsal recumbency and obliqued to the right or left so that rib or pleural space abnormalities can be highlighted (Figure 13.6). Use of horizontal beam projections can only be accomplished if the tube head can be rotated from a vertical to a horizontal position with the patient placed in lateral or ventral/dorsal recumbency (Figure 13.7). This view is obtained to confirm pneumothorax or to shift pleural fluid away from a given area to determine if an intrathoracic mass is present [9]. The “humanoid” view is a special radiographic projection, taken from the dorsal recumbent position, where rather than extending the thoracic limbs cranially, they are pulled caudally alongside the thorax. This will shift the right and left scapulae in a caudal and lateral position relative to the cranial thorax and thereby produce better visualization of the left and right cranial lung lobes (Figure 13.8). FIGURE 13.6 Ventrodorsal (A), ventrodorsal left oblique (B), and ventrodorsal right oblique (C) thoracic radiographs of an 8‐year‐old medium‐sized mixed‐breed dog. When the sternum is obliqued to the left side of the dog, the main pulmonary artery appears prominent; when the sternum is obliqued to the right side of the dog, the right atrium and right side of the cardiac silhouette appear enlarged. This emphasizes the need for straight VD positioning as shown in (A) (sternum and vertebral column superimposed). FIGURE 13.7 Right lateral (A) and horizontal beam (B) thoracic radiographs of a 15‐year‐old domestic shorthair cat with a large cavitated mass of the cranial subsegment of the left cranial lung lobe. On the right lateral radiograph, the intralesional fluid portion cannot be appreciated. On the horizontal beam radiograph, the intralesional fluid portion is further characterized by a fluid line oriented parallel to the sternum and x‐ray table. The metallic hemoclips are from a prior lobectomy and lung lobe resection for a pulmonary carcinoma. FIGURE 13.8 Ventrodorsal (A) and “humanoid” ventrodorsal (B) thoracic radiographs of an 8‐year‐old medium‐sized mixed‐breed dog. In the “humanoid” image, the scapulae are no longer superimposed over the right and left cranial lung lobes. Knowledge of thoracic anatomy is power when it comes to interpretation of thoracic radiographs. Conventional orthogonal radiographs provide a two‐dimensional representation of a three‐dimensional object with images taken at 90° relative to each other so that one can triangulate the anatomy. However, due to border effacement and summation, interpretation is hampered by geometric changes of the thorax and superimposition of three‐dimensional structures, making the radiographic images a “summary” image of the original anatomy (Figure 13.9). FIGURE 13.9 Three‐dimensional reconstruction of the thorax in sagittal plane including the osseous structures of the thorax and cardiovascular structures (A), in dorsal (B) and sagittal (C) planes delineating the air‐filled structures of the thorax, in sagittal plane including the cardiovascular structures of the cervical region and thorax (D). Border effacement is where two objects of the same radiographic opacity are in contact with each other, resulting in a loss of border distinction for each of the objects (Figure 13.10). Summation is the added radiographic opacity of two superimposed structures that are not in contact with each other (Figure 13.11). FIGURE 13.10 Ventrodorsal thoracic radiograph of a 3‐month‐old Rottweiler. There is a diffuse alveolar pulmonary pattern of the right lung, border effacing and limiting the complete evaluation of the cardiac silhouette and pulmonary vasculature. Several pleural fissure lines are noted along the left side of the inner thoracic wall. The widening of the costochondral junctions is more prominent on the left than the right due to the leftward rotation of the sternum relative to the thoracic vertebral column. FIGURE 13.11 Right lateral abdominal radiograph of a cat. There is summation of the left and right kidneys. Note the soft tissue opaque oval made by the caudal pole of the right kidney (cranially located) and the cranial pole of the left kidney (caudally located). The relative radiolucency noted in the center of the kidneys is secondary to fat deposition in the renal sinus at the renal hilum. Computed tomography provides a three‐dimensional approach to radiographic anatomy of the thorax. The initial volumetric data set acquired of the thorax represents raw data that can be reformatted into one of the standard anatomic planes (transverse, sagittal, dorsal). Transverse images can be used to review normal thoracic anatomy as thin section slices of the thorax presented in a lung (WW/WL: 2000/−700), soft tissue (400/100), and bone window (2000/1000) with contrast medium to highlight the intravascular space (Figure 13.12) [10]. One should review these images in the sagittal and dorsal plane images to better understand the relationships of the structures of the thorax as well as the complexities of individual lung lobes and bronchial anatomy (Figure 13.13). FIGURE 13.12 Transverse computed tomographic images of the thorax of a dog at the level of the heart in the lung window (A), soft tissue window (B), bone window (C), and soft tissue window (D) after intravenous contrast administration. FIGURE 13.13 Computed tomographic study of the thorax of a dog in lung window in the transverse plane (A), dorsal plane (B), and sagittal plane (C) images. Patient positioning dictates the appearance of the radiographic anatomy of the thorax. The radiographic anatomy changes between the laterals and between ventrodorsal and dorsoventral images, particularly for medium‐ and large‐sized dogs. In right lateral radiographs, the right thoracic limb will appear smaller than the left and will be in a more caudal position. The cardiac silhouette is in the ventral two‐thirds of the middle thorax and has an oval or egg‐shaped appearance. The cardiac silhouette is located dorsal to the sternum without separation from the sternum (except for deep‐chested breeds and sight hounds). The right and left diaphragmatic crura parallel each other with the right crus being cranial to the left (Table 13.2). This is because the dependent (right) lung is less aerated, and the gravity‐dependent abdominal contents displace the right side of the diaphragm cranially. The dorsal border of the caudal vena cava will become confluent with the right diaphragmatic crus (cranial crus) at the level of the caval foramen. In addition, the caudal vena cava will terminate cranially at the level of the caudal border of the cardiac silhouette where the caudal vena cava becomes confluent with the right atrium (Figure 13.14). The fundic and body portions of the stomach will contain gas and are located caudal to the left crus (caudal crus). TABLE 13.2 Differences between right and left lateral thoracic radiographs in medium‐ and large‐breed dogs. These changes are not seen routinely in the small breeds or cats. FIGURE 13.14 Left lateral (A) and right lateral (B) thoracic radiographs from a large mixed‐breed dog. The cardiac silhouette is oval on the right lateral radiograph and round on the left lateral radiograph. On the left lateral, the cardiac silhouette apex is noted to rotate away from the sternum. This gap between the ventral border of the cardiac silhouette and the sternum should not be mistaken for a pneumothorax. The right diaphragmatic crus is located cranial to the left diaphragmatic crus on the right lateral radiograph. The left diaphragmatic crus is located cranial to the right diaphragmatic crus on the left lateral radiograph. The intrathoracic caudal vena cava is confluent with the right diaphragmatic crus on both lateral radiographs. With respect to the cranial lobar vasculature and bronchi, the right cranial lung lobe bronchus is the first radiolucent circle located superimposed over the carina (terminal portion of the trachea) on the right lateral image (Figure 13.15). One can then trace the right cranial lobe bronchus in a cranioventral direction with the pulmonary artery being dorsal and the pulmonary vein being ventral to the bronchus. The pulmonary artery and vein are equal in diameter to each other and should not be larger than the thinnest point of the proximal portion of the right fourth rib (Figure 13.16). In right lateral recumbency, the right cranial lung lobe bronchus is initially dorsal to the left cranial lobe bronchus, but then the two airways cross and the left cranial lobe bronchus becomes dorsal and cranial to the right cranial lobe bronchus cranial to the cardiac silhouette (Figure 13.17). FIGURE 13.15 Right lateral thoracic radiographs from the same dog as in Figure 13.14. The right cranial lung lobe bronchus (white arrow) is the first radiolucent circle superimposed over the carina. The left cranial lobe bronchus exits the ventral aspect of the carina (red arrow) and immediately bifurcates into cranial and caudal subsegments. FIGURE 13.16 Right lateral (A), left lateral (B), and ventrodorsal (C) thoracic radiographs of a dog. The pulmonary artery (black arrowhead) and pulmonary vein (white arrowhead) are normal and symmetric. The pulmonary arteries and veins do not measure greater than the width of the fourth rib (white arrow) on the lateral radiographs and do not measure greater than the length of the ninth rib (black arrow) on the ventrodorsal radiograph. FIGURE 13.17 Right and left lateral thoracic radiographs from a 7‐year‐old golden retriever. (A) Right lateral radiograph. (B) Right lateral radiograph with the right cranial bronchus highlighted in yellow and the left cranial bronchus highlighted in green. The two airways cross each other at the cranial aspect of the heart base. (C) Left lateral radiograph. (D) Left lateral radiograph with the right cranial bronchus highlighted in yellow and the left cranial bronchus highlighted in green. The two airways parallel each other over the region of the heart base. In addition, if one traces the ventral margin of the trachea caudal to the opening of the right cranial lobe bronchus, a radiolucent gap will intersect the line so that there is a bronchus exiting the carina in a ventral direction (Figure 13.18). This is the common opening into the left cranial lobe bronchus. Within several millimeters of exiting the carina, the left cranial bronchus will divide into the cranial and caudal subsegments. The cranial subsegmental bronchus can be traced in a cranial direction and overlaps the right cranial lung lobe bronchus, prior to crossing into a dorsal position relative to the right cranial lobe bronchus. The caudal subsegment of the left cranial bronchus extends ventrally over the middle of the cardiac silhouette and can be visualized on the right lateral projection. Occasionally, the right middle bronchus might be visualized along the caudal border of the cardiac silhouette on the right lateral image but is more commonly seen on the left lateral image. FIGURE 13.18 Collimated right lateral thoracic radiograph of a dog. At the opening of the right cranial lobe bronchus (white arrow), a radiolucent gap will intersect the line so that there is a bronchus (white arrowhead) exiting the carina in a ventral direction. When the dog or cat is in right lateral recumbency, a lesion in the left lung will be better visualized due to better aeration of the nondependent lung lobes. In addition, there will be atelectasis of the right lung lobe and thereby border effacement with soft tissue opaque lesions (Figure 13.19) [11]. However, radiolucent lesions in the right lung lobe might be better visualized on the right lateral for the same reasoning. Lesion conspicuity in the right cranial lung lobe where it wraps around the cranial aspect of the cardiac silhouette (just caudal to the cranioventral mediastinal reflection) and the right accessory lung lobes caudal to the cardiac silhouette are less influenced by recumbency due to the central position of these lobes. FIGURE 13.19 Right lateral (A), left lateral (B), and ventrodorsal (C) thoracic radiographs of a dog with a right middle lung lobe aspiration pneumonia. This is best visualized on the left lateral image with a lobar sign and a central air bronchogram. This lesion is not seen on the right lateral, but is clearly visualized on the ventrodorsal image with border effacement of the right side of the cardiac silhouette. In the left lateral image, the left diaphragmatic crus is displaced cranial to the right crus due to the fact that the dog’s abdominal contents are cranially displaced along the dependent left side (down side of the patient) secondary to gravity and atelectasis of the left lung lobes. Any gas within the fundus or body of the stomach will be seen caudal to the left crus. From the level of the diaphragmatic cupula, the right crus and left crus diverge away from each other dorsally (Figure 13.20). The cardiac silhouette elevates away from the sternum secondary to aeration of the right middle lung lobe and should not be mistaken for a pneumothorax. The silhouette will also have more of a rounded or circular shape. The caudal vena cava is seen superimposed over the caudal aspect of the cardiac silhouette and extends caudally over the left crus, becoming confluent with the right crus at the caval foramen. The left and right cranial lung lobe bronchi will parallel each other, with the left cranial lobe bronchus (cranial subsegment) being in a dorsal and cranial position relative to the right cranial lung lobe bronchus (that appears larger due to magnification). The right middle lung lobe bronchus is seen over the mid to caudal aspect of the cardiac silhouette. FIGURE 13.20 Left lateral thoracic radiograph of a dog. The left diaphragmatic crus (white arrow) is located cranial to the right diaphragmatic crus. The mild accumulation of gas in the gastric fundus (black arrow) is seen superimposed over the left diaphragmatic crus. Because the animal is in left lateral recumbency, a lesion in the right lung will be better visualized due to atelectasis of the left lung lobe and thereby border effacement with soft tissue opaque lesions. However, radiolucent lesions in the left lung lobe might be better visualized on the left lateral for the same reasoning. There are several variants and “fake‐outs” (normal radiographic changes seen that are not equated with pathology) that can be seen on left lateral radiographs when compared with the right lateral image. On the left lateral radiograph, it is common to see a focal area of pleural thickening between the right middle and right caudal lung lobes superimposed over the caudal aspect of the cardiac silhouette (Figure 13.21). Although the incidence of this finding has not been reported, in the authors’ experience this is routinely seen in approximately 40% of normal thoracic radiographic studies, particularly in older dogs, and should not be mistaken for a pleural effusion. This finding is not commonly seen in cats. In addition, gas can be seen in the thoracic esophagus at the level of the heart base or fluid seen in the caudal thoracic esophagus just cranial to the esophageal hiatus on the left lateral image (Figure 13.22). FIGURE 13.21 Left lateral (A), right lateral (B), and ventrodorsal (C) thoracic radiographs of a dog. On the left lateral radiograph, there is pleural thickening (white arrowhead) between the right middle and right caudal lung lobes and increased conspicuity of the pericardial and/or middle mediastinal fat (white arrow), which are commonly mistaken for pleural effusion. On the right lateral and ventrodorsal radiographs, there is no evidence of pleural fissures. FIGURE 13.22 Right lateral (A), left lateral (B), and ventrodorsal (C) thoracic radiographs of a brachycephalic dog. There is focal gas and soft tissue opacity in the region of the caudal thoracic esophagus consistent with a sliding hiatal hernia as seen on the left lateral radiograph (white arrows). This is not seen on the right lateral or ventrodorsal radiographs. Differences between the dorsoventral (DV) and ventrodorsal (VD) images primarily involve the positioning and shape of the diaphragm, visualization of the caudal lung lobes, cardiac shape, and superimposition of the scapula over the cranial thorax. In the VD image, there are three surfaces seen on the thoracic side of the diaphragm forming three convex borders. The cupula (dome) of the diaphragm is central and cranial. Each diaphragmatic crus forms a convex surface on its respective side of the thoracic diaphragm. These are located cranial to the respective right or left side and caudal to the cupula (Figure 13.23). In some dogs and cats, there will be better separation between the cardiac silhouette and the cupula of the diaphragm as the accessory lung lobe is more elongated and apparent. This results in better visualization of the caudal vena cava. Changes in the descending thoracic aorta and great vessels may be more apparent on the DV image compared with the VD image (Figure 13.24). FIGURE 13.23 Ventrodorsal thoracic radiograph of a normal adult dog. There are three convex surfaces noted along the diaphragm that include the right and left diaphragmatic crura (labeled and red lines) and the cupula of the diaphragm (red dashed line and labeled; central and cranial). FIGURE 13.24 Dorsoventral radiograph without labels (A) and with labels (B) from a normal dog. The caudal lung lobe vasculature is labeled: RCPA – right caudal pulmonary artery; RCPV – right caudal pulmonary vein; LCPA – left caudal pulmonary artery; LCPV – left caudal pulmonary vein; and CVC – caudal vena cava. There is fat noted between the cranial and caudal subsegmental fissure of the left cranial lung lobe at the left fourth intercostal space. In the VD image, the airways should be traced from their origin in the carina and caudal trachea (superimposed over or to the right of the spinous processes of the thoracic spine; Figure 13.25). The trachea terminates at the carina, with the bifurcation into the right and left principal bronchi. The right cranial lung lobe bronchus is the first branch from the right principal bronchus and extends in a cranioventral direction. The right middle bronchus originates from the ventrolateral aspect of the continuation of the right caudal lobar bronchus. Next, the accessory lung lobe bronchus originates in a ventromedial position along the right caudal lung lobe bronchus. The right caudal lobe bronchus then continues into the right caudal lung lobe in a caudodorsal direction (Figure 13.26). FIGURE 13.25 Ventrodorsal thoracic radiograph of a normal dog. Note the right accessory lung lobe bronchus branches from the craniomedial aspect of the right caudal lung lobe bronchus. T, trachea; RCr, right cranial lung lobe bronchus; RMid, right middle lung lobe bronchus; RCd, right caudal lung lobe bronchus; Acc, accessory lung lobe bronchus; C, carina; LCrCr, cranial subsegment of the left cranial lung lobe bronchus; LCrCd, caudal subsegment of the left cranial lung lobe bronchus; LCd, left caudal lung lobe bronchus. FIGURE 13.26 Ventrodorsal (A) and left lateral (B
CHAPTER 13
Anatomy, Variants, andInterpretationParadigm
Overview
Thoracic Radiographs – Basic Principles
Image
Inspiration
Expiration
Right and left laterals
Separation of ventral cardiac silhouette from sternum; postcardiac triangle larger with flattened diaphragm; relative and absolute cardiac size are decreased (smaller cardiothoracic ratio); minimal or no contact between cardiac silhouette and diaphragm; diaphragmatic crura caudal to the level of T10; relatively flat angle (horizontal) of the caudal vena cava; soft tissue opaque lesion conspicuity is best in the nondependent lung lobes
Lack of separation of the cardiac silhouette and sternum; overlap between the cardiac silhouette and cranial diaphragm; diaphragmatic crura to level of T8 (cranial to T10); relative and absolute cardiac size are increased (larger cardiothoracic ratio); postcardiac triangle smaller with curved diaphragm; dorsal angle of the caudal vena cava; recumbent atelectasis will decrease overall lesion conspicuity
Ventrodorsal/dorsoventral image
Elongation of the cardiac silhouette; separation of the cardiac silhouette from the cupula of the diaphragm; caudal positioning of the costophrenic angles of the lungs along the lateral margins of the thorax at the level of the 9th and 10th intercostal spaces; increased detail associated with the caudal lobar pulmonary vasculature
Smaller length of the cardiac silhouette; overlap between the cardiac silhouette and diaphragm; cranial positioning of the costophrenic angles of the lung at the level of the 6th intercostal space; decreased detail of the caudal lobar pulmonary vasculature
Thoracic Radiographic Anatomy
Computed Tomographic Anatomy of the Thorax
Right Lateral Radiograph
Anatomic differences
Right lateral
Left lateral
Cardiac silhouette
Oval or egg shaped
Circular/round
Crura
Right crus cranial; crura parallel to each other
Left crus cranial; crura diverge away from each other starting at the level of the caudal vena cava
Caudal vena cava
Dorsal border blends with cranial crus; the cranial end of the caudal vena cava ends at the cardiac silhouette
Dorsal border blends with caudal crus (passes over cranial crus); the cranial end of the caudal vena cava is superimposed over the caudal aspect of the cardiac silhouette
Gas in fundus/body of the stomach
Caudal to left crus
Caudal to the left crus but more conspicuous over the caudodorsal thorax
Cardio‐sternal distance
Decreased and normally seen in juxtaposition with each other
Increased and away from each other
Cranial lobar bronchi and vessels
Criss‐cross over each other cranial to the cardiac silhouette; right cranial bronchus, artery, and vein are dorsal to the left cranial subsegmental bronchus
Parallel to each other cranial to the cardiac silhouette; the left cranial subsegmental bronchus, artery, and vein are cranial and dorsal relative to the right cranial bronchus and vessel pair 
Left Lateral Radiograph
Ventrodorsal Radiograph
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