Overview

Adequate radiographic technique and patient preparation, and familiarity with normal anatomy, variants and pitfalls in interpretation are crucial when interpreting radiographs. This lecture will cover common mistakes in interpretation of small animal radiographs, anatomic variations and technique faults.

Radiology of the Thorax

Technique, positioning and respiration (“TPR”): An optimal thoracic radiograph should be obtained with a high kVp and a short exposure time. A grid should be used if the patient is thicker than 10 cm. Additionally, in digital radiography, application of a thorax algorithm is necessary for optimized evaluation. Straight positioning is a must as even slight obliquity interferes with evaluation of cardiac morphology and affects the appearance of other thoracic tissues. With the exception of dynamic studies, all thoracic radiographs should be obtained at maximum inspiration. Expiratory radiographs will artificially increase pulmonary parenchymal opacity which may mimic diffuse lung disease. Relative cardiac size will also be exaggerated on expiratory radiographs. While radiographs under sedation are advisable, general anaesthesia may result in extensive pulmonary atelectasis and possibly render the study nondiagnostic. Finally, a radiographic study should in most instances include at least 2 orthogonal views, and opposite lateral views are indicated for full assessment of lung parenchyma.

Anatomic variations and physiologic status: There is marked variability in thoracic structures (thoracic wall, cardiac size and shape) between dogs of different breeds. Lack of familiarity; e.g., with thoracic radiographs in chondrodystrophic or very deep chested dogs may result in misinterpretation. Age related changes such as costal cartilage mineralization, a mild unstructured interstitial and bronchial pulmonary pattern, and physiologic cardiovascular changes in old cats (parallel orientation of heart with the sternum and redundancy of aortic arch and descending aorta) can also be mistaken for pathology.

Thoracic wall: Like in radiography of the musculoskeletal system (see below), an optical illusion can occur along the boundaries of adjacent (linear) structures of different opacities (“Mach line”) which can lead to the erroneous diagnosis of a rib fracture. Subcutaneous fat planes or subcutaneous emphysema superimposed over osseous structures can also result in the misdiagnosis of rib lesions.

Diaphragm: Pitfalls in the evaluation of the diaphragm are rather rare. However, a true (congenital) diaphragmatic hernia can be mistaken for a mass, and intrathoracic masses bordering the diaphragm (e.g., accessory lung lobe tumour) may mimic a diaphragmatic rupture.

Pleural space: Chondrodystrophic chest wall conformation and abundant intrathoracic fat deposits may be confused with pleural effusion. Deep chested patient conformation, skinfolds, microcardia and overexposure of the radiograph may give the impression of pneumothorax. These pleural space “abnormalities” can usually be remedied by thorough evaluation of orthogonal radiographs and correction of exposure factors.

Mediastinum: Appearance of the cardiac silhouette is greatly influenced by positioning. Additionally, pulmonary atelectasis and resultant mediastinal shift can affect the appearance of the heart. Abundant intrathoracic fat can mimic a (cranial) mediastinal mass, especially in barrel chested dogs and on VD views. However, unlike with a mediastinal mass, the cranial margin of the cardiac silhouette remains visible on the lateral view, and the trachea is in a normal position. Finally, tracheal position is to some degree influenced by flexion of the neck which has to be taken into consideration when evaluating the mediastinum.

Lung: The normal lung is air-filled and of low opacity. Underexposure, expiratory status and pulmonary atelectasis will result in increased pulmonary opacity which may mimic unstructured interstitial or alveolar pulmonary infiltrates. Pulmonary nodules can be mimicked by a variety of normal structures and variants. End-on-vessels are circular, are superimposed over vessels of equal or larger size, and are more opaque than to be expected for a true pulmonary nodule of similar size. Pulmonary osteomas are punctate mineralized foci within the lung, especially seen in older dogs. These are small (1–2 mm) and are more opaque than pulmonary soft tissue nodules of this size. Skin nodules, nipples, parasites (ticks) on the skin and surface debris can mimic pulmonary nodules when superimposed over pulmonary parenchyma. Remedies include close examination of orthogonal radiographs to identify the “nodule”, physical evaluation of the patient surface, and marking of any superficial skin tags with positive contrast medium prior to repeating radiographs.

Radiology of the Abdomen

Technique: Unlike for thoracic radiographs, high contrast is desirable for an abdominal radiograph to maximize margin definition between soft tissue opaque organs and abdominal fat. This is achieved by choosing a technique relatively low in kVp. With very few exceptions, each radiographic study should include at least one lateral and a ventrodorsal view, and especially for evaluation of the gastrointestinal tract opposite lateral views are very valuable.

Anatomic variations and physiologic status: Optimally, animals should be fasted prior to a radiographic examination of the abdomen as a large amount of food within the stomach and faeces within the colon can obscure other abdominal organs, result in visceral crowding by displacing organs from their normal position, and create pseudo-lesions by superimposition of gastrointestinal contents over abdominal organs.

Serosal margin detail: Failure to choose adequate expos ure factors may affect serosal margin detail and may lead to a misdiagnosis of peritoneal effusion (underexposure or high kVp-low mAs technique) or pneumoperitoneum (overexposure). Serosal margin detail is greatly affected by presence of abdominal fat. Young animals have physiologically poor serosal margin detail as brown fat appears radiographically similar to soft tissue. Similarly, lack of abdominal fat in very thin animals may give the impression of abdominal effusion.

Liver: Deep chested dogs often have a subjectively small liver, and in some older dogs the liver becomes “saggy” resulting in the caudal ventral liver margin extending beyond the costal arch with maintained sharp margination and normal alignment of the gastric axis. These deviations from “normal” liver size should be evaluated in light of laboratory analysis findings.

Gastrointestinal tract: On a right lateral view fluid within the stomach will accumulate in the pylorus which is frequently mistaken for a cranial ventral abdominal mass. A left lateral view will aid in distinguishing pylorus from mass as the normal pylorus will now fill with air. Luminal contents within the gastrointestinal tract greatly affect its appearance. It is important to understand that measurements of intestinal wall thickness on survey radiographs are unreliable as luminal fluid silhouettes with the wall which gives the impression of wall thickening.

Urinary tract: Skin nodules/nipples and external debris superimpose d over kidneys, retroperitoneal space or urinary bladder may mimic mineralization or calculi. The deep circumflex iliac artery arises ventral to the L5-6 vertebrae. Seen end-on it may mimic a ureteral calculus. Additional end-on vessels arising from the aorta are occasionally seen on digital radiographs and should also not be mistaken for pathologic findings.

Radiology of the Musculoskeletal System

Technique: High contrast radiographs are desirable for the musculoskeletal system to high light differences between fat, soft tissue and bone. Almost all indications require acquisition of at least 2 orthogonal views, and oblique views or special techniques such as stress views may be needed for full patient evaluation. “Mach lines” are an optical phenomenon which can be misinterpreted as fissure lines. An important artefact to be familiar with in digital radiography is the “Ueberschwinger” or rebound artefact which appears as a dark halo around structures with a large density difference to surrounding tissues and which can mimic loosening of metallic implants.

Anatomic variations and physiologic status: There are numerous normal anatomic variations of the musculoskeletal system in small animals, and any potentially abnormal finding has to be interpreted in light of history, signalment and clinical findings. Examples include vertebral anomalies (e.g., bloc k vertebrae, transitional vertebrae or hemivertebrae) and variability in conformation of the antebrachium between large breed and chondrodystrophic dogs. Open physes in young animals frequently lead to confusion when trying to identify the reason for a patient’s lameness. Nutrient foramina can be misinterpreted as fracture or fissure lines. Purchase of a normal radiographic anatomy textbook, establishment of an in-house case library in which normal cases are stored for reference, and obtaining comparison radiographs (either in a normal dog of similar age and breed, or of the contralateral limb in the same animal) can all be helpful to develop a sense of normal. Finally, degenerative changes are common especially in older dogs and may or may not be associated with clinical signs. Examples include spondylosis deformans and degenerative joint disease.

References

1.  Drost WT, Reese DJ, Homof WJ. Digital radiography artifacts. Vet Radiol Ultrasound. 2008;49(1 Suppl 1):S48–56 . Review.

2.  Nuth EK, Armbrust LJ, Roush JK, Biller DS. Identification and effects of common errors and artifacts on the perceived quality of radiographs. J Am Vet Med Assoc. 2014;244:961–967.

3.  Thrall DE. Introduction to radiographic interpretation. In: Thrall DE, ed. Textbook of Veterinary Diagnostic Radiology. 6th ed. St. Louis, MO: Elsevier Saunders; 2013.

4.  Thrall DE, Robertson I. Atlas of Normal Radiographic Anatomy and Anatomic Variants in the Dog and Cat. 2nd ed. St. Louis, MO: Elsevier Saunders; 2015.