Elodie E. Huguet
DVM, DACVR (DI)
Dr. Huguet grew up in France before moving to South Carolina in 2001. She obtained her veterinary degree at the University of Georgia College of Veterinary Medicine, followed by radiology and small animal rotating internships in private practice and a radiology residency at Veterinary Specialty Hospital of the Carolinas and the University of Florida, respectively. Dr. Huguet is currently working part-time as a clinical assistant professor of diagnostic imaging at the University of Florida and is part of the IDEXX teleradiology team. When not working, she is an active long-distance runner and enjoys spending time with her dog, Arya, traveling, oil painting, and competing her horse, Stan, in the sport of dressage.Read Articles Written by Elodie E. Huguet
Robert C. Cole
DVM, DACVR (DI, EDI)
Dr. Cole is a professor of diagnostic imaging at Auburn University College of Veterinary Medicine. After obtaining his DVM degree from Auburn University, he spent 4 years in general mixed animal practice. He then completed a residency in diagnostic imaging at the University of Tennessee and spent 7 years in Texas in both academia and private practice before returning to Auburn University as a faculty member in the department of clinical sciences.Read Articles Written by Robert C. Cole
Clifford R. Berry
Dr. Berry is an adjunct professor of diagnostic imaging at the University of Florida and a clinical assistant professor of diagnostic imaging at North Carolina State University College of Veterinary Medicine. He received his DVM from University of Florida and completed a radiology residency at University of California–Davis. He has a specific interest in diagnostic imaging of the thorax.
Updated October 2022Read Articles Written by Clifford R. Berry
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
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.
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.
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.
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.
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.
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
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.
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
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).
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
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
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.
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
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.
- Krupinski EA. Current perspectives in medical image perception. Atten Percept Psychophys. 2010;72(5):1205-1217. doi:10.3758/APP.72.5.1205
- Thrall DE. Principles of radiographic interpretation of the thorax. In: Thrall DE, ed. Textbook of Veterinary Diagnostic Radiology. 7th ed. Elsevier; 2017:568-581.
- Schwarz T, Johnson V. Basics of thoracic radiography and radiology. In: BSAVA Manual of Canine and Feline Thoracic Imaging. British Small Animal Veterinary Association; 2008:9-26
- Schwarz T, Johnson V. The heart and major vessels. In: BSAVA Manual of Canine and Feline Thoracic Imaging. British Small Animal Veterinary Association; 2008:86-90.
- Thrall DE, Roberston I. The thorax. In: Atlas of Normal Radiographic Anatomy and Anatomic Variants in the Dog and Cat. 3rd ed. Elsevier; 2022:176.
- Olive J, Javard R, Specchi S, et al. Effect of cardiac and respiratory cycles on vertebral heart score measured on fluoroscopic images of healthy dogs. JAVMA. 2015;246(10):1091-1097. doi:10.2460/javma.246.10.1091
- Martin M, Mahoney P. Improving the diagnostic quality of thoracic radiographs of dogs and cats. In Practice. 2013;35(7):355-372. https://doi.org/10.1136/inp.f4460
- Samei E, Flynn MJ, Eyler WR. Detection of subtle lung nodules: relative influence of quantum and anatomic noise on chest radiographs. Radiology. 1999;213(3):727-734. doi:10.1148/radiology.213.3.r99dc19727