Effects of Positioning, Respiration, and Technique on Interpretation of Thoracic Radiographs of Dogs and Cats

Interpretation of radiographic images is complicated by the superimposition of body parts because 3-dimensional anatomical structures are projected as 2-dimensional images.¹ Interpretation of 2-dimensional radiographs then involves recreating 3-dimensional mental images of the patient’s anatomy.^1,2 Being familiar with the radiographic appearance of different organs requires a systematic and consistent approach. Having standardized views with a high level of radiographic detail is optimal for recognizing normal anatomy and lesions.

A radiographic study of the thorax should include at least 2 orthogonal projections, including lateral and ventrodorsal/dorsoventral projections. However, for a complete evaluation of the lungs, opposite lateral projections are also needed as inherent collapse and atelectasis within the dependent lung lobes can consequently camouflage lesions or be misinterpreted as disease, such as aspiration pneumonia or bronchopneumonia. For this reason, acquiring radiographs under sedation is preferred over general anesthesia because anesthesia can lead to atelectasis and further limit interpretation (FIGURE 1).^2-4

Figure 1A. Right lateral view of a dog acquired with the patient under anesthesia. With the patient under anesthesia, a mild to moderate multifocal unstructured interstitial pulmonary pattern can be seen in all lung lobes and represents atelectasis. Anesthesia can exacerbate the amount of atelectasis, resulting in border effacement of lesions, and can reduce the conspicuity of normal anatomical structures.

Figure 1B. Right lateral view of a dog acquired the next day with the patient under sedation. The unstructured interstitial pulmonary pattern is resolved.

Using a checklist will help ensure that the study is of optimal quality for interpretation (BOX 1).³ To minimize the need for retakes, the diagnostic quality of radiographs should be assessed at the time the images are acquired. At that time, special views may be elected to further help with interpretation.

The ability to recognize or exclude lesions on radiographs depends on the radiographic accuracy of a study,¹ which requires patients being properly positioned and the radiographic technique being adequately adjusted. The primary focus of this article is the effects of positioning, respiration, and technique on thoracic radiographic interpretation.

Patient Positioning

Radiographic interpretation is based on viewing images in a similar manner every time so that abnormalities are more easily recognized and on evaluating radiographs acquired with a standardized positioning protocol.

Viewing

When radiographs are viewed, image interpretation depends on established standard display protocols. The cranial or right side of the patient should always be on the viewer’s left,⁵ which requires appropriate positioning of the patient across the length of the imaging plate. Although it may be tempting to position larger patients diagonally across the imaging plate to reduce the number of radiographs needed, doing so renders interpretation more difficult (FIGURE 2); therefore, acquiring multiple projections of the cranial and caudal aspect of the thorax is preferred.

Figure 2A. Right lateral radiograph of a cat diagonally positioned in relation to the detector plate, hindering the ability to view the obtained images using standard hanging protocols.

Figure 2B. Ventrodorsal radiograph of a cat diagonally positioned in relation to the detector plate, hindering the ability to view the obtained images using standard hanging protocols.

Positioning

Patient Alignment

Rotating the patient results in a nonstandardized projection of anatomical structures with less familiar superimposition of the anatomy, which can create confusion and result in lesions being missed or overinterpreted. Therefore, the patient must be properly aligned as even small degrees of obliquity can alter the appearance of the cardiac silhouette and other intrathoracic structures. On ventrodorsal views, right or left rotation of the thorax can result in a mediastinal shift and lead to the false impression of cardiac chamber enlargement or dilation of the great vessels (FIGURE 3). Similarly, on lateral views, rotation of the thorax can also lead to misinterpretation as cardiomegaly and may affect vertebral heart score measurements.

Figure 3. Ventrodorsal radiograph with mild leftward rotation of the thorax. There is subsequent artifactual rounding of the right cardiac margin (arrowheads) with a bulge at the 1–2 o’clock position in the region of the main pulmonary artery (asterisk), which may be falsely interpreted as main pulmonary artery enlargement and right-sided cardiomegaly, as seen with pulmonary hypertension.

When patients are positioned for thoracic radiographs, cranial extension of the thoracic limbs is necessary to reduce superimposition of the appendicular musculature over the cranial aspect of the thoracic cavity.^3,4 With increased muscle mass over the cranial thorax, particularly on lateral projections, soft tissue opacity will appear increased and can be confused with cranial mediastinal disease or can mask pulmonary disease (FIGURE 4).^3,4 In addition, caudal retraction of the thoracic limbs with or without hunching of the patient will cause the sternum to deviate dorsally and can be falsely interpreted as pectus excavatum (FIGURE 5). Also to be avoided is excessive cranial extension of the thoracic limbs as it can result in distortion of the thoracic anatomy.

Figure 4A. Right lateral radiograph of dog, acquired with the limbs mildly caudally retracted

Figure 4B. Right lateral radiograph of dog, acquired with the limbs cranially extended. Cranial extension of the thoracic limbs reduces superimposition of the appendicular musculature over the cranial thoracic region, increasing the conspicuity of a cranioventral mediastinal mass.

Figure 5. Right lateral radiograph of a cat with caudally retracted thoracic limbs and subsequent dorsal deviation of the sternum cranially (arrowheads), not to be misinterpreted as pectus excavatum.

The patient’s neck should be positioned in a neutral to slightly stretched cranial position as ventroflexion can result in dorsal deviation of the trachea within the cranial thoracic region on lateral views (FIGURE 6), with possibly more pronounced focal rightward deviation of the trachea on a ventrodorsal/dorsoventral view, which can mimic a mass effect, as occasionally seen with cardiac neoplasms.^4,5

Figure 6A. Lateral radiograph of a dog with a ventrally flexed neck and consequent dorsal deviation of the trachea within the cranial thoracic region (arrowhead).

Figure 6B. In another lateral radiograph of the same dog with a straightened neck, the tracheal deviation is resolved.

Magnification

Because x-rays produced by the generator travel in a straight path from a single source, the structures that will first interact with x-rays (farthest from the detector plate) will be magnified in relation to structures that are closer to the detector plate.^2,5 Therefore, lesions can be more accurately measured when they are placed closer to the detectors. Similar positioning is also helpful for projecting lesions in a comparable manner when obtaining recheck radiographs at different times.

Geometric Distortion

Emitted x-rays diverge from the source and interact with tissues outside of their direct path from the generator to the detector plates.² The entire imaging plate within the set collimation is subsequently exposed; however, because the incoming x-rays interact with tissues of the shortest path between the generator and detector plate, some degree of geometric distortion is expected and is more pronounced peripherally. Therefore, anatomical structures at the outer periphery of the field of view should be cautiously interpreted.² This concept also emphasizes the value of centering the x-ray beam over the cardiac silhouette for a more accurate interpretation of cardiovascular structures.

Radiography of the vertebral column is best achieved by acquiring several collimated projections centered on multiple vertebral segments. As a result, the area of the vertebral column centered within the field of view will be more accurately represented, especially with regard to intervertebral disk space widths.^3,5 Because of the divergent nature of the x-rays, the intervertebral disk spaces that are in the periphery of the image will be artifactually narrowed (FIGURE 7).^2,5

Figure 7. Divergence of the x-ray beam from the main source will cause intervertebral disk spaces that are off-centered to be artifactually narrowed. (A) The T11–T12 intervertebral disk space is artifactually narrowed due to tangential projection of the x-rays (arrowhead).

Figure 7B. Centering the x-ray source over the T11–T12 intervertebral disk space will result in a more accurate projection of the intervertebral disk space width and resolution of the previously noted narrowing (arrowhead).

Effects of Respiration

Motion will result in blurred images.² The subsequent loss of image detail can hide disease or create artifacts that mimic disease. In some instances, anatomical structures may be completely unrecognizable. With thoracic radiography, motion usually results from patient movement or respiration. Therefore, proper immobilization of patients with sedation and positioning devices will minimize degradation of image quality.⁴

Acquiring thoracic radiographs during peak inspiration is essential for optimal aeration of the lungs and avoiding effacement of lesions by atelectasis,² which can be falsely interpreted as disease. Obtaining radiographs during peak inspiration is also essential for separating intrathoracic structures, removing superimposition, and facilitating interpretation. The reduced superimposition during inspiration may be particularly apparent when assessing the triangular arrangement between the caudal margin of the cardiac silhouette, diaphragm, and caudal vena cava because greater separation and improved conspicuity of these structures will be noted.⁴ In addition, reduced intrathoracic volume on expiration can result in kinking of the trachea within the cranial thoracic region (FIGURE 8).

Figure 8. Ventrodorsal radiograph of a dog, acquired during the patient’s expiration. With reduced pulmonary volume, bunching of the intrathoracic structures may cause kinking of the trachea within the cranial thoracic region and should not be misinterpreted as a pathological mass effect.

The decreased intrathoracic volume during expiration can result in the false impression of cardiomegaly as the cardiac silhouette will occupy a greater portion of the thoracic cavity (FIGURE 9), thus making measurements of cardiac silhouette to thoracic width less reliable. In addition, Olive et al noted variation in vertebral heart score measurements during peak inspiration and expiration; larger measurements were obtained during expiration.⁶ Therefore, consistent acquisition of thoracic radiographs will reduce variability, improve the accuracy of measurements, and optimize pulmonary contrast to aid with disease recognition.^2,6

Figure 9A. Ventrodorsal radiograph of a dog, acquired during the patient’s expiration. With reduced pulmonary volume during expiration, the cardiac silhouette occupies a larger portion of the thoracic cavity and may be falsely interpreted as cardiomegaly.

Figure 9B. Ventrodorsal radiograph of a dog, acquired during the patient’s inspiration.

Radiographic Technique

When thoracic radiographs are acquired, respiration and cardiac motions are inherent. Motion can reduce the sharpness of intrathoracic structures, rendering them less well defined and more difficult to assess. For this reason, when the thorax is radiographed, exposure time should be adjusted.² A short exposure time is preferred. Technique using high kilovoltage peak (kVp) and low milliampere-seconds (mAs) is advised, taking into consideration the patient’s size and body condition. Using a technique chart is advised to decrease the probability of having to repeat radiographs if overexposed or underexposed.⁷

Overexposure

Given the inherent high contrast of the thorax, the radiographic technique should be carefully adjusted to not overpenetrate regions of lesser density, such as the lungs. Radiographs that are overexposed will be diffusely darkened with complete blackening and loss of detail in regions of lower tissue density (FIGURE 10A).^2,4,7 The anatomy in affected regions will be inconspicuous, which in some cases may be falsely interpreted as pneumothorax. Therefore, recognizing this artifact will help avoid misdiagnosis.

Figure 10A. Right lateral radiograph of a dog, showing saturation artifacts within the cranial thoracic region resulting from overexposure.

Figure 10B. Right lateral radiograph of a dog, showing appropriate radiographic technique.

Underexposure

Radiographs that are underpenetrated do not have adequate contrast and will have generalized whitening of the image (FIGURE 10C).^2,4,7 Thoracic structures and lesions may subsequently be inconspicuous, especially in overconditioned or larger patients.^2,4,7 When mAs are set too low, a smaller number of electrons will be created and, therefore, fewer x-rays will be projected through the patient, resulting in a grainy image (i.e., quantum mottling) (FIGURE 11).⁸

Figure 10C. Right lateral radiograph of a dog, showing underexposure with reduced pulmonary contrast.

Figure 11. Right lateral radiograph of a dog, showing a grainy appearance resulting from radiographic underexposure (i.e., quantum mottle).

Summary

Radiography is a valuable diagnostic tool used routinely in veterinary practices for disease identification. The positioning of patients and timing/settings used for image acquisition are crucial for acquiring high-quality radiographs and enhancing recognition of lesions. Using a methodological approach will increase efficacy by reducing the number of retake radiographs and is well worth the extra time to more accurately interpret radiographs and to guide further diagnostic and treatment recommendations.