Principles of radiological interpretation

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Radiological interpretation is the process by which a radiographic study is critically evaluated and the imaging findings correlated with the clinical presentation, with the aim of advancing the management of the patient. Radiological interpretation is not easy and, although the basic principles can be taught, development of expertise requires examination of a large number of radiographs. Critical comparison of the radiological diagnosis with surgical findings, or the interpretation of the same images by an experienced radiologist, helps develop the skills required for sound radiological interpretation. Errors in radiology are common and often multi-factorial, and may involve problems with radiographic technique, evaluation of the radiograph and interpretation of any findings. Clinical audit and following up the outcome of cases, along with correlating radiological findings with other information such as surgical findings, are important in recognizing radiological errors. This chapter considers Viewing the radiograph; Routine radiographic assessment; Basic approach to radiological interpretation; Radiological report; and Errors in interpretation.

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3.2 Orientation of a DV radiograph of the thorax of a dog using positional markers. The indistinct positional marker (L) is correct (circle). This can be confirmed by the position of gas within the stomach in the left cranial abdomen (*). If the radiograph is oriented correctly, with the left of the animal to the observer’s right, then the findings of enlarged pulmonary artery (arrowed), left auricular appendage (arrowhead) and left ventricle are clearly visible. However, if the film is flipped horizontally, the radiograph appears more ‘normal’ with the cardiac apex superimposed on the ‘left’ hemithorax, but the enlarged pulmonary artery and atrium might then be missed. Diagnosis: patent ductus arteriosus.
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3.4 Lateral abdominal radiograph of a dog presented with reluctance to move. The intra-abdominal structures are unremarkable. Closer inspection of the periphery of the film reveals a large soft tissue mass (arrowed) and associated destructive changes in the distal right femur (*).
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3.5 Lateral thoracic radiographs of a dyspnoeic cat. The lungs are hyperinflated: the diaphragm is flattened (arrowhead), the distance between the cardiac silhouette and diaphragm is increased (double arrow) and the lungs have rounded margins and extend past the last rib. A cause for the air-trapping is not evident at first, but closer inspection of the relatively underexposed cranial thorax and inlet reveals that the air column in the trachea is not of even diameter throughout. With the forelegs pulled forwards and the exposure increased, a plaque-like soft tissue mass within the dorsal trachea (arrowed) is visible. Diagnosis: tracheal neoplasia (carcinoma).
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3.6 Lateral radiographs of the thorax and the abdomen of a cat following trauma. (a) There is pulmonary consolidation (*), presumed to be haemorrhage, diaphragmatic hernia (outlined black arrow) with cranial displacement of the stomach (S), rib fractures and a small volume pneumothorax (white arrows). (b) Overexposure of the ventral abdomen at the periphery of the film obscures the herniation of intestinal loops through an abdominal wall defect (white arrows), avulsion of the prepubic tendon (black arrow) and the ‘empty’ caudal abdomen (*) with herniation of the urinary bladder (B) into the inguinal region. The left kidney (K) is small. These images emphasize the importance, especially in trauma cases, of not suspending the search until all films have been assessed and the periphery and overexposed areas of the radiograph have been evaluated. (Courtesy of Frean and Smyth Veterinary Surgeons)
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3.8 Decubitus VD thoracic radiograph showing the effect of collapse of dependent lung lobes. The cardiac silhouette has descended toward the dependent thoracic wall as a result of the reduction in lung volume (arrowheads). The dependent hemidiaphragm has moved cranially (arrowed). It should be noted that the pathology, whether diffuse or metastatic nodules, within the dependent lung is difficult to distinguish from the collapsed lung tissue.
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3.9 Right and left lateral thoracic radiographs of a dog with mammary adenocarcinoma. No abnormalities are visible on the right lateral view but there is a clear soft tissue nodular opacity superimposed on the cardiac silhouette on the left lateral view. The nodule is consistent with pulmonary metastasis within the right lung. Significant pathology within the poorly inflated dependent lung lobe will be overlooked on lateral recumbent views unless two, preferably three, views are obtained.
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3.10 Lateral and DV thoracic radiographs of a dog presenting with a cough. No abnormalities are evident on the left lateral view, but complete consolidation of the left cranial lung lobe (arrowheads) is apparent on the DV view. Extensive consolidation or large mass lesions in the lung may not be distinguished without a sufficient number of radiographic views.
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3.11 Lateral radiograph of a dog with a thoracic mass. On this view, the large mass (arrowheads) could be located at any point in the direction of the X-ray beam and involve the chest wall, pulmonary parenchyma or the mediastinum. DV thoracic radiograph of a dog with a thoracic mass. This view localizes the mass (arrowheads) to the midline. The list of differential diagnoses is narrowed to a caudodorsal mediastinal mass lesion (e.g. oesophageal mass, mediastinal abscess or primary mediastinal neoplasia).
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3.12 Lateral and DV thoracic radiographs of a cat. Relying on a single view, it is not possible to determine whether both air-gun pellets lie within the thorax. Assessment of orthogonal views, together with the location of the entry wounds, can indicate which important structures may have been damaged along the path of the pellet.
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3.13 Right and left lateral radiographs of a dog presenting with vomiting. On the right lateral view, the abnormal mixed gas and soft tissue opacity caudal to the stomach may be the transverse colon. However, on the left lateral view, the lumen of the duodenum is highlighted with gas (black arrows). It has a tortuous or zigzag path across the abdomen. Other dilated small intestinal loops are of soft tissue opacity. Diagnosis: small intestinal linear foreign body.
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3.14 Right and left lateral abdominal radiographs of a dog presenting with vomiting. On the right lateral view, the pylorus is of soft tissue opacity (*) and distinguishing between ‘normal’ fluid within the dependent pylorus and disease (mass or intraluminal foreign body of a soft tissue opacity) is not possible. On the left lateral view, a sharply demarcated soft tissue structure in the pylorus is surrounded and highlighted by gas (arrowheads). This appearance is consistent with an intraluminal gastric foreign body. Two bouquet garni muslin bags were removed at surgery. (Courtesy of Palmerston Veterinary Group)
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3.15 Right lateral radiograph of the abdomen of a large-breed dog. There is a large bilobed gas-filled viscus in the region normally occupied by the stomach. Both the body of the stomach and the pylorus are filled with gas (and quite a large amount of fluid). The fundus (arrowheads) is located ventrally and the gas-filled pylorus (arrowed) is located dorsally, indicating malpositioning of the stomach. The descending duodenum (*) is not usually visible on the right lateral view, but can be seen here due to rotation of the stomach to the left. Diagnosis: gastric dilatation–volvulus.
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3.17 Röntgen sign: size. Lateral thoracic radiograph of a dog showing marked cardiomegaly. Moderate to marked cardiomegaly can usually be distinguished from a normal cardiac silhouette; however, differentiating mild cardiac enlargement is difficult given the variability of chest conformation in the dog. Not all ‘classic’ signs of left-side enlargement, such as elevation of the trachea (arrowed), are recognized in all dogs (e.g. in deep-chested breeds, such as the Dobermann, the trachea may remain divergent from the spine even in advanced disease).
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3.18 Röntgen sign: size. Lateral and VD abdominal radiographs of a cat with unequally sized kidneys. The right kidney (arrowheads) is small and irregular, representing hypoplasia or end-stage disease. In comparison, the left kidney (LK) is moderately enlarged due to secondary renal hypertrophy.
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3.19 Röntgen sign: size. Mediolateral radiograph of the right antebrachium of a dog with shortening of the radius (arrowed). Comparison with the normal left limb is helpful to confirm the reduced length and compare the degree of subluxation of the radial head.
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3.20 Röntgen sign: contour and shape. Dorsopalmar radiograph of the carpus of a dog with irregular spiculating new bone on all aspects of the distal radius. The appearance of the periosteal reaction is used to categorize the changes as that of aggressive disease. Diagnosis: primary malignant bone tumour. Lateral radiograph of a cat with a lemon-shaped soft tissue opacity (arrowed) in the mid-ventral abdomen, representing expansile change from a structure such as the intestine or, in this case, the tail of the spleen. Diagnosis: splenic mass. Lateral radiograph of the caudal thorax and cranial abdomen of a dog with an intestinal perforation. The intestinal loop crossing the abdomen has a corrugated margin (white arrows). This change, which is consistent with bowel irritation and spasm, is indicative of severe inflammation. Taking into consideration the free abdominal fluid, reduced serosal detail, and free gas (black arrows), the most likely cause is septic peritonitis.
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3.21 Röntgen sign: number. Dorsopalmar radiograph of the manus of a cat with six digits. The change is of no clinical significance and consistent with congenital polydactyly.
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3.22 Röntgen sign: number. Lateral thoracic radiograph of a dog showing complete consolidation of the right middle lung lobe (arrowed). The loss of normal air-filled lung is easily overlooked as it is completely superimposed on the cardiac silhouette.
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3.23 Röntgen sign: position. Lateral thoracic radiograph of a cat following oral administration of barium suspension. The gastric body is displaced cranially into the thorax. Diagnosis: diaphragmatic rupture. B = fundus; D = duodenum; P = pylorus.
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3.25 Röntgen sign: gas opacity. Lateral and right decubitus VD horizontal beam abdominal radiographs of a dog with sudden collapse. (a) The abdomen is distended and serosal detail is poor, indicating a large abdominal effusion. The large gas bubble superimposed on the abdominal contents is easily overlooked as the margin is indistinct (arrowheads). The serosal surfaces of the stomach and colon are highlighted by gas (arrowed). These are additional signs that there is free gas in the abdomen. (b) The volume of free gas (*) collects under the costal arch and surrounds the fundus of the stomach (S). A fluid–gas interface is present due to the large volume of free abdominal fluid.
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3.26 Röntgen sign: soft tissue opacity. Lateral thoracic radiograph of a dog with multiple soft tissue opacities throughout the lung fields. Summation of multiple nodules results in a marked increase in opacity. Some of the summated nodules are equal to or greater in opacity than the thin bones such as the ribs. As neither the physical density nor the effective atomic number of the soft tissue nodules change, the increase in opacity is entirely due to the increase in the thickness of the tissues through which the X-rays must pass. The combined effect of greater physical density of bone and larger effective atomic number means that an equal thickness of bone attenuates X-rays six times more than soft tissue. Therefore, the increase in thickness of the soft tissue needs to be considerable for the opacity to be similar to that of bone. Diagnosis: pulmonary metastases.
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3.27 Röntgen sign: soft tissue opacity. VD abdominal radiograph of a cat. The body (B) of the spleen is a relatively radiopaque soft tissue triangle, visible craniolateral to the left kidney. The head (H) and tail (arrowheads) of the spleen are less well defined and less opaque oblong radiopacities extending medially from the body of the spleen. The increase in thickness of the body of the spleen as it curves round results in the increased opacity.
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3.28 Röntgen sign: bone opacity. Lateral abdominal radiograph of a dog with generalized osteopenia and a folding fracture (arrowed) of one tibia. The flat bones of the pelvis are almost unrecognizable, and the soft tissue structures are more conspicuous and opaque compared with the bones. Diagnosis: nutritional secondary hyperparathyroidism.
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3.29 Röntgen sign: dystrophic mineralization. Lateral abdominal radiograph of a cat. The discrete rounded structures (arrowed) with fine mineralization of the periphery are due to fat necrosis (a form of dystrophic mineralization). They are usually single and may be attached to the mesentery or free floating within the abdomen. DV thoracic radiograph of a cat with dystrophic mineralization of the soft tissue lateral to the spine. The dystrophic mineralization has a fine granular appearance (arrowed). The cause is unknown but tissue damage from a bite wound or trauma is suspected. Lateral abdominal radiograph of a cat with extensive mineralization of markedly enlarged visceral lymph nodes. Diagnosis: metastatic carcinoma. VD abdominal radiograph of a cat with extensive mineralization of markedly enlarged visceral lymph nodes (arrowed). Diagnosis: metastatic carcinoma.
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3.30 Röntgen sign: architecture/integrity. Lateral thoracic radiographs of a dog. The normal tapering air-filled bronchi are lost and have been replaced by a sacculated dilated bronchial pattern (arrowheads). Although the function of the bronchi cannot be directly assessed, the bronchial pattern represents an irreversible change in architecture (bronchiectasis) and this indirectly suggests abnormal clearance by the bronchi. Some of the bronchi are filled with exudate and mucus and appear as soft tissue nodules (*), which should not be mistaken for metastases. Lateral abdominal radiograph of a cat showing loss of the normal contour of the caudoventral body wall and loops of gas-filled small intestine located cranial to the stifles. Diagnosis: body wall rupture.
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3.31 Lateral abdominal radiograph of a dog presenting with vomiting following surgery for bile duct obstruction (cholelith). Ultrasonography demonstrated that the stomach was distended with fluid and ruled out post-surgical pancreatitis. Radiography provided a better assessment of the degree of gastric distension. The pylorus (black arrows) and body (white arrows) are markedly dilated, extending almost to the umbilicus, and have displaced the contents of the small intestines caudally. The small intestinal loops are not dilated. This pattern is consistent with a gastric outflow obstruction in this dog (due to adhesions) and is similar in appearance to that seen with congenital or acquired pyloric stenosis. There is no need for a barium study in this dog. Restraint of the dog in lateral recumbency was not possible, hence the radiograph was obtained using a horizontal beam. F = fundic gas cap.
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3.32 DV thoracic radiograph of a dog. The pulmonary vessels (arrowed) can be clearly seen, even over the cardiac silhouette, as they are separated from the heart by air-filled lung. The coronary vessels cannot be seen as they are in contact with the heart.
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3.33 DV thoracic radiograph of a cat with border effacement of the cardiac margins due to pleural fluid. The cardiac margins cannot be clearly seen as the opacity of the fluid and the heart is the same (arrowed).
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3.34 VD radiograph of the pelvis of a male dog with marked unilateral hip dysplasia and secondary osteoarthritis. The prepuce (arrowed) can be clearly seen to the left of the caudal lumbar spine, due to the sharp interface between air and the soft tissue and because the margins are parallel to the beam.
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3.35 Lateral abdominal radiographs, illustrating change in relative opacity. Survey radiograph of a male dog with radiopaque cystic (white arrow) and urethral (black arrow) calculi. Retrograde vaginourethrogram series showing the change in appearance of cystic calculi as the nature and opacity of the surrounding substance alters. The calculi are markedly opaque compared with gas, lucent when surrounded by a small volume of iodine, but become more opaque as the surrounding volume of iodine increases, partially obscuring them. This emphasizes the importance when performing a double-contrast cystogram (see Chapter 4) that neither the concentration nor the volume of the contrast medium should be so great as to obscure small lesions.
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3.36 Lateral abdominal radiograph of a cat. The composite shadow in the craniodorsal abdomen ventral to L2–L3 is caused by the overlapping of both kidneys (*). Note that one kidney is smaller than the other and a large right adrenal gland mass (arrowheads) is present.
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3.37 Lateral thoracic radiographs of a dog. Superimposition of the ribs and pulmonary vessels creates the impression of nodules. These ‘nodules’ (arrowheads) are particularly visible in the caudodorsal thorax at the sixth, seventh and eighth ribs.
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3.38 Dorsoplantar radiograph of the tarsus of a dog. Fine radiolucent lines (arrowed) are apparent where the abaxial and axial cortices of the third and fourth metatarsal bones respectively, overlap. These are Mach lines, which may be misinterpreted as fissure fractures. They are optical illusions that appear at the sharp boundaries between structures with large differences in opacity. A similar effect is produced when using computed or digital radiography systems (see Chapter 2) only the appearance is the result of image processing.
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3.39 Effect of background on perceived opacity. The squares in the centre are of the same opacity, but the one on the right appears less opaque due to the darker background.
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3.42 Ultrasonogram and right lateral radiograph of the abdomen of a dog with marked weight loss and chronic vomiting. (a) Foreign material (black arrows), creating distal shadowing, was identified within the stomach on the ultrasonographic study, but the amorphous shape and strong shadowing limit a confident assessment of the extent and significance of the material. The white arrows denote the stomach wall. (b) The radiographic study confirms the ultrasonographic findings and that the foreign material (arrowed) is radiopaque and of such size to be clinically significant. A large mass of plastic material was removed at gastrotomy and the clinical signs resolved. (Courtesy of Mandeville Veterinary Hospital)
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3.43 VD radiograph of the pelvis and mediolateral radiograph of the right stifle of a 16-week-old puppy with pyrexia, lethargy and reluctance to move. Neurological disease was suspected and radiographs of the spine (including the pelvis) were obtained. Close scrutiny of the pelvis revealed irregular areas of lysis within the proximal and distal metaphyses of both the left and right femur. A full survey of the appendicular skeleton revealed similar changes in other metaphyses (arrowed), including the proximal and distal tibia. The changes are consistent with metaphyseal osteomyelitis (an aggressive haematogenous osteomyelitis). Two important radiographic principles are demonstrated in this case: firstly, all areas of the radiograph were assessed, allowing the peripheral lesions to be recognized, and secondly, the metaphyseal lesions must be differentiated from the normal appearance of the physes in a dog of this age.
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3.44 DV thoracic radiograph of a dog with dyspnoea. A cursory examination of the film reveals pleural fluid; however, a thorough search reveals a left thoracic wall lesion (destruction of the tenth rib; arrowed), which is the likely cause of the pleural effusion.
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3.45 Lateral radiograph of the caudal abdomen of a male dog. There is an ill defined mineral opacity (arrowed) superimposed on the soft tissues caudal to the os penis. This may be considered a displaced fabella or an artefact, but was confirmed as an urethral calculus. Calculi and other lesions within the penile urethra may be overlooked if the pelvis is not included when imaging male dogs presenting with signs of lower tract disease. An additional view with the hindlimbs pulled forward should be obtained for a complete assessment of the urethra.
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3.46 Right and left lateral thoracic radiographs and left lateral barium gastrogram of a Cavalier King Charles Spaniel receiving treatment for congestive heart failure. Recent unproductive coughing and retching unresponsive to further diuresis was reported. (a–b) Radiography was performed but not considered to demonstrate pulmonary congestion. On review of the radiographs, dilatation of the caudal oesophagus (arrowed) was noted. The body of the stomach (F) is dilated with air, as is the pylorus (P); the latter is an abnormal finding on the right lateral view. On the left lateral view, the stomach has an abnormal C-shape with caudal rotation of the body and fundus (S). (c) Barium administration confirmed a chronic gastric torsion. This case emphasizes the importance of evaluating all parts of the film, correlating the radiograph with clinical expectations and the value of reviewing the films. D = duodenum. (Courtesy of Frean and Smyth Veterinary Surgeons)
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3.47 Lateral view of the skull of a dog. The basihyoid bone (arrowed) can be seen as a relatively radiopaque structure ventral to the tip of the epiglottis. It can be confused with a foreign body.
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3.48 Lateral abdominal radiograph of a dog showing apparently thickened small intestinal walls (arrowed) highlighted by gas within the lumen. This artefact is commonly misinterpreted as pathology. The appearance is caused by the fluid contents of the small intestine silhouetting with the walls, with a gas cap on top, creating apparent wall thickening. Intestinal wall thickness should only be measured after contrast medium administration or with ultrasonography.
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3.49 Lateral abdominal radiograph of a male dog following positive-contrast cystography. The bladder is poorly filled with contrast medium. There has been inadequate patient preparation as the colon is still filled with a large volume of faeces. The quality of this study is poor and the resulting artefacts might be misinterpreted as a structural abnormality of the bladder.
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3.50 Lateral thoracic radiograph of a dog with multiple small mineral opacity nodules throughout the lung fields. These are consistent with pulmonary heterotopic bone and should not be mistaken for metastatic disease.
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3.51 Lateral thoracic radiographs of a cat: expiratory and inspiratory views. (a) On the expiratory view, pleural fluid and possibly even a cranial mediastinal mass might be suspected. In addition, any pathological changes in the lungs would not be detected. (b) The inspiratory view shows a normal thorax. Poor inflation is usually considered to be a technical error in larger patients, but these images demonstrate that this artefact may also occur in the cat.
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