Department of Veterinary Clinical Sciences, Purdue University, West Lafayette, IN, USA
The Thorax Examined By the Radiologist: How to Avoid Mistakes
Two most commonly mistakes that can be avoid easily in reading thoracic radiographs occurs during the radiographic process, which are taking too few views and inappropriate positioning.
Too Few Views
A minimum of 2 orthogonal views are needed for a thoracic study. A minimum of 3 views are needed for a metastatic check for metastatic neoplasia to the lungs. As aspiration pneumonia commonly occurs in the right cranial and right middle lung lobes, a left lateral view is needed to diagnose this condition. A recent trend of performing 3 views (2 laterals and a VD) is recommended for all disease condition in thorax. A DV view is needed for the evaluation of the caudal lobar vessels in the investigation of cardiac disease. At our institute, we performed a DV, a VD and a lateral views for study of cardiology.
Shape and size of the cardiac silhouette is very sensitive to positioning. Any obliquity of the DV or VD views will distort the cardiac silhouette and give us a wrong impression of cardiomegaly. Misdiagnosis of right cardiomegaly is commonly make with shifting of the cardiac silhouette to the right secondary to oblique positioning. The spine and the ribs are not optimize for reading. Wrong diagnosis of spinal fracture in cases of motor vehicle accident is common.
A designated person for quality control for all images taken is essential in any hospital. At our institute, the radiology residents and radiologists approve all studies. Occasionally, patients need sedation or general anesthesia to achieve optimal positioning. Patient not suitable for sedation or general anesthesia may not have a good positioning radiograph and making reading difficult.
The other commonly make mistakes after radiography are mistaken normal structures as abnormal, fail to recognize breed variation, fail to recognize normal anatomical variation and fail to recognize physiology effect on radiographs.
Mistaken Normal Structures as Abnormal
The caudal esophagus is commonly seen as a soft tissue tubular structure with a distinct ventral border, on the left lateral recumbency view. This may be mistaken as esophageal foreign body. Sternal lymph node is normally seen on a right lateral recumbency view but not the left lateral recumbency view. Thus is it crucial to be able to detect a sternal lymph node in both lateral recumbency views to make a definitely of mild enlargement of the sternal lymph node in cases of patients with peritoneal disease. Radiographic images are 2 D image of a 3 D structure. Superficial structures such as skin tag, nipples and wet hair may be seen as the lung parenchyma. One of the challenges of reading thoracic radiograph is to differentiate the superficial structures such as nipple from a real pulmonary soft tissue nodule.
Fail to Recognize Breed Variation
The appearance of the cardiac silhouette is breed and body conformation dependent. The most common error in reading thoracic images is overestimation of the size of the cardiac silhouette. This is especially true for the barrel-chested dogs when there is a heart murmur detected during physical examination. The cardiac silhouettes of the deep-chested dogs are taller and the barrel-chested dogs are slightly shorter and rounder. Oblique positioning of the VD sometimes will make the cardiac silhouette appear larger on the right and smaller on the left. There is a positive spinal trachea angle, mainly in deep chested dogs. The reduction of spinal trachea angle is normal due to left ventricle enlargement and presence of a heart base mass. In small breed and barrel chested dogs, the trachea normally runs parallel with the spine, thus it may have a zero spinal trachea angle. One of the common mistake is to look at this small spinal trachea angel in small breed dog and make a misdiagnosis of left-sided cardiomegaly secondary to mitral valve insufficiency.
Pleural indentation is commonly seen in Basset Hounds on both VD and DV views. This mimics retraction of lung lobes, thus mistaken as presence of pleural effusion.
Fail to Recognize Normal Anatomical Variation
Presence of pericardiac fat will occasionally lead to enlarged cardiac silhouette. This is especially true in the VD view of cats. Sometimes, the cardiac silhouette could not be see with the abundant of pericardial fat. In older cats, the cardiac silhouette will have a more horizontal position and a bulge may be present at the aortic arch. This should not be mistaken as an aortic aneurysm. Widening of the cranial mediastinum is one of the radiographic signs of cranial mediastinal lymphadenomegaly. In some cases of obese or brachycephalic dogs, the mediastinum could be very wide due to presence of abundant mediastinal fat. This is sometimes misinterpreted as a mediastinal mass.
The cranial sub segment of the left cranial lung lobe is located slightly more cranial to the right cranial lung lobe, crossing right ward over the mediastinum and part of it appears to be in the “cranial aspect of the right hemithorax”. On the lateral view, this could be mistaken as a large pulmonary bulla.
Fail to Recognize Physiology Effect on Radiographs
It is common to detect a small amount of gas in the mid-thoracic esophagus in healthy animals. When patients are under sedation or general anesthesia, there may be a slightly increased in the gas in the esophagus.
Thus the status of sedation and general anesthesia of the patients should be considered when reading thoracic radiographs. This is easily confused with megaesophagus. Obese animal normally hypoventilate, thus will produce expiratory thoracic radiograph. This will lead to increased soft tissue opacity of the caudodorsal lung field, mimicking pulmonary edema. In some cases, the less expended lungs appear to be “retracted” from the thoracic wall. This could be mistaken as presence of pleural effusion. In deep-chested dogs, they tend to be able to inhale easily even though on lateral recumbency. Thus inspiratory radiograph with mild degree of over expansion of lungs may occur. This could be seen as a radiolucent area at the apex of the cardiac silhouette. The differential diagnosis for this is pneumothorax.
As a conclusion, number of views and positioning should be taken into consideration even before interpretation of a thoracic study. The knowledge of normal anatomy variation appearance on a radiograph, especially with breed variation, is essential. Last but not least, the physiological state of the patient will influence the appearance of the lungs and esophagus, and should be taken into consideration when interpreting thoracic study.