International Congress of the Italian Association of Companion Animal Veterinarians

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1 Close this window to return to IVIS International Congress of the Italian Association of Companion Animal Veterinarians May, 2009 Rimini, Italy Next Congress : 65th SCIVAC International Congress May 28-30, Rimini, Italy Reprinted in IVIS with the permission of the Congress Organizers

2 Thoracic imaging Erik R. Wisner DVM, Dipl ACVR, California, USA Conventional radiography is the mainstay of diagnostic imaging in clinical practice however limitations of the technique must be recognized in thoracic disease. Although inherent density differences of aerated and consolidated lung, soft tissues, and bone provide excellent image contrast on a thoracic radiograph, anatomical superimposition intrinsic to the formation of a radiographic image can often limit its diagnostic value. Although conventional radiography can be used for serial assessment of thoracic disorders without the need for sedation or anesthesia, the improved anatomic detail provided by computed tomography has made this the imaging modality of choice for evaluation of complex or perplexing thoracic disease. The classic airway, interstitial and alveolar pulmonary patterns described for conventional thoracic radiographs can also be appreciated on thoracic CT and the severity and the anatomical extent of disease is often more accurately represented with CT. Mass lesions can often be unequivocally localized to a specific lung lobe, airways can be traced through many branch generations, and primary pulmonary lesions can be more easily discriminated from mediastinal or pleural lesions using CT. As with conventional radiography, computed tomography relies on tissue density differences as the basis for image formation. Each CT image can be conceptualized as a radiograph of a 1 to 10 mm thick slice of the patient. Because each image represents such a thin slab of anatomy, superimposition effects that are inevitable and often the source of confusion with conventional radiography, are minimized. This is particularly important advantage in thoracic CT imaging because thoracic anatomy is complex and pertinent lesions are easily obscured. The contrast resolution of CT is excellent compared with conventional radiographs as there is better discrimination of tissues of similar density. Spatial resolution is also very good with lesion detection on the order of mm. Quantitative measurements of tissue density can also be obtained which can be useful clinically in discriminating between normal and abnormal tissues of similar but different densities. In the past few years the development of helical (sometimes referred to as spiral) CT has revolutionized the value and versatility of thoracic CT due to the speed with which complete studies can be performed. Our standard breath-hold protocol for thoracic CT studies involves hyperventilating the patient for seconds immediately before initiating the scan then applying a forced breathhold at a pressure of 15 cmh20 during the acquisition. All studies are completed in 60 seconds or less. Image slice thickness depends on the size of the patient and typically ranges from 3-7 mm. When an area of concern or interest is identified on the initial study and more information is required, a second scan may be performed over a specific area using thinner slices for better anatomical detail. For these high-resolution (HRCT) images, we use collimation of 1.0 mm or less and acquire images in incremental rather than helical mode. THORACIC WALL AND DIAPHRAGM Thoracic wall masses can be differentiated from peripheral pulmonary masses using a variety of imaging approaches. If the mass involves a rib, bone destruction or periosteal reaction can confirm the origin as being within body wall and is often better appreciated when a high contrast radiographic technique is used. Oblique radiographic views to project a tangential view of the mass may also assist localization. Ultrasound can also be used to differentiate body wall and pulmonary masses as well as to direct fine-needle biopsy. CT provides yet another means with which to localize mass lesions and is particularly useful for determination of resectability and for surgical planning. The most common disorders of the diaphragm include traumatic diaphragmatic tears and true hernias. In addition to plain-film radiography, contrast radiography and ultrasound can be used to confirm these entities. One word of caution; small tears in the diaphragm with minimal abdominal visceral displacement may be difficult to confirm using ultrasound. PLEURA AND PLEURAL SPACE Pneumothorax: Signs of pneumothorax include elevation of the ventral cardiac margin away from the sternum on lateral recumbent views and retraction of lug margins from the thoracic wall and diaphragm as evidenced by a loss of vascular and bronchial markings peripherally. This latter finding is usually best seen in the caudodorsal thorax on lateral views and at the costophrenic angles on a dorsoventral or ventrodorsal view, particularly when the volume of free pleural air is small. In patients with concurrent or previous pleural inflammatory disease, the visceral pleural membrane may also be thickened and appears more distinct when it is retracted from the thoracic wall. Closed pneumothorax may arise from disease or injury to the lungs or major airways or may occasionally occur as an extension of pneumomedi- 542

3 astinum. Open pneumothorax, as the name implies, arises from an open wound of the chest wall. Tension pneumothroax, which rarely occurs in dogs and cats, can be defined as a pneumothorax in which the pleural space pressure exceeds airway pressure. If unilateral, this results in complete collapse of the lung on the affected side and a midline shift of the heart away from the pneumothorax. Radiographic overexposure or skinfold superimposition can both lead to misdiagnosis of pneumothorax. In severely hypovolemic dogs, the loss of peripheral lung markings cardiac silhouette elevation can also mimic pneumothorax. Pleural Effusion: The list of differential diagnoses for patients with pleural effusion is long and varied but the effusion, in simple terms, is generally composed primarily of blood, pus (exudates), water (transudates, modified transudates), chyle or tumor effusate. In those patients with a large effusion volume, a ventrodorsal view often redistributes the fluid to the dorsal paravertebral gutters providing a better assessment of lung, cardiac silhouette and mediastinum. Ultrasound can also be used to characterize fluid and to detect pleural, mediastinal, pericardial or cardiac masses that may be the source of effusion. Ultrasound should be performed before thoracocentesis since the effusion serves as an excellent window through which to image. Moderate to severe effusions can mimic mediastinal masses due to increased opacity of the cranial thorax and elevation of the trachea. Inflammatory effusion and chronic effusions, such as those associated with chylothorax, will often cause lung lobe volume reduction and rounding of lobar margins due to reactive thickening of the visceral pleura. This can lead to incomplete re-expansion of the lungs and self-limiting, neutral pressure pneumothorax following thoracocentesis. MEDIASTINUM Mediastinal Masses: Cranioventral masses must be differentiated from mediastinal widening due to other causes such as frank hemorrhage (anticoagulant poisoning) or mediastinitis. Cranioventral mediastinal masses generally result in a loss of lucency immediately cranial to the heart, caudal cardiac displacement and loss of definition of the cranial cardiac border dorsal, rightward deviation of the trachea and compression/ atalectasis of the cranial lung lobes. Differential diagnoses include lymphoma, thymoma, chemodectoma, ectopic thyroid and parathyroid tumors, other miscellaneous tumors, Reactive lymphadenopathy from inflammatory diseases such as coccidioidomycosis. Parasternal masses most often represent enlarged sternal lymph nodes due to neoplasia (lymphoma) or response to inflammatory disease. The sternal lymph nodes receive lymphatic drainage from the abdominal side of the diaphragm as well as regionally within the ventral thorax. Hilar masses typically silhouette with the caudodorsal cardiac margin and result in a depression of the distal trachea on the lateral radiograph and a widening of the mainstem bronchi on the VD or DV radiograph. These masses are almost always due neoplasia (lymphoma, regional metastasis of pulmonary neoplasia) or nodal reactivity to granulomatous inflammatory disease cocci). Caudal mediastinal masses are uncommon and are usually associated with the esophagus. Because the caudal mediastinum is thin and surrounded by lung, masses are generally easy to identify radiographically though differentiation from pulmonary or diaphragmatic masses may be more difficult. Although radiography can often provide adequate information to diagnose mediastinal disorders, additional imaging studies can usually provide more specific information regarding lesion origin and etiology. Ultrasound is particularly useful for characterizing suspected mediastinal masses and for guiding fine needle aspiration biopsy. It is also useful for ruling out the presence of a mediastinal mass in fat dogs in which the cranial mediastinal region is obscured radiographically. More recently, CT has been used to diagnose and characterize cranial mediastinal masses for both for determination of resectability and for surgical planning. Esophography is used document and characterize space-occupying lesions in the cranial mediastinum and to verify esophageal involvement in patients with caudodorsal mediastinal disease. Mediastinal Shift: Mediastinal shift, as reflected by displacement of the cardiac and cranial mediastinal silhouettes, can be due to pulmonary atalectasis, pleural adhesions, over or under inflation of lung due to obstructive airway disease, pneumothorax, asymmetrical pleural effusion, large thoracic masses and diaphragmatic hernia. Pneumomediastinum: Pneumomediastinum most commonly results from trauma involving the neck and thoracic inlet, which results in dissection of air into the potential space within the mediastinum, or from direct tama to the trachea or mainstem bronchi. Radiographically this is appears as increased definition of the external tracheal wall margin, the esophagus and great vessels. TRACHEA AND MAINSTEM BRONCHI Common large airway disorders include developmental abnormalities such as tracheal hypoplasia and bronchial dysplasia, large airway trauma, inflammatory airway disease, airway collapse, tracheal or bronchial wall neoplasia, strictures and intraluminal foreign bodies. Because of the inherent contrast between the airway walls and intraluminal air, many of these disorders are readily identified on survey radiographic studies. Inspiratory and expiratory radiographs may be helpful in confirming dynamic abnormalities such as tracheal collapse. GENERALIZED PULMONARY PATTERNS 543 Vascular Pattern It is intuitive that alterations in the pulmonary vasculature usually reflect cardiovascular disease rather than primary pulmonary disease. However, we include a discussion of vascular patterns here because they contribute to the overall complexity of the pulmonary radiographic anatomy and must be considered in the context of other pulmonary imaging findings.

4 Decreased vascularity: Hypovolemia causes a reduction of pulmonary arterial and venous diameter that may result in pulmonary hyperlucency. Other imaging evidence confirming hopovolemia includes microcardia and reduced caudal vena cava diameter. Right-to-left shunting lesions also result in pulmonary underperfusion and are often accompanied by right-sided cardiomegaly. Pulmonary hypertension may result in abrupt tapering of pulmonary arteries peripherally and comparable reduction in venous diameter. Pulmonary arteries may be enlarged proximally in these patients. Increased Pulmonary Vein Diameter: Venous distension is most often due to left ventricular insufficiency. With early failure, pulmonary venous congestion may occur alone or with signs of interstitial edema. Increased Pulmonary Artery Diameter: A generalized increase in pulmonary arterial diameter occurs with left-toright shunting lesions resulting in pulmonary vascular overcirculation. This is often best appreciated when evaluating the tertiary and more distal pulmonary arterial braches toward the periphery of the lung. Pulmonary arterial enlargement also occurs with acquired disorders such as heartworm disease that induce peripheral arterial obstruction, pulmonary hypertension and arteritis. Airway Oriented Patterns Bronchial patterns are usually associated with infectious or non-infectious inflammatory disease (allergic or immunemediated airway disorders). The airway pattern can sometimes be difficult to identify because it can be obscured by an overlying interstitial pattern. The radiographic hallmark of an airway oriented pattern is apparent bronchial wall thickening that is often accompanied by luminal narrowing resulting in the classic description of tram tracks (bronchi viewed in long axis) and donuts (bronchi viewed end-on). Increased bronchial wall prominence may be due to bronchial mucosal edema, hyperplasia, exudate accumulation or peribronchial cellular and fluid infiltrates that radiographically mimic wall thickening. On thoracic CT images, individual bronchi can be more easily seen and more accurately characterized. Additionally, bronchial wall thickness can be directly measured on a computer work-station. Moreover, the peribronchial and interstitial tissues can be more accurately assessed for pathologic change. Bronchial plugging: Inspissated exudates can sometimes accumulate in the distal airways of patients with longstanding airway disease. Radiographically, this may appear as a nodular interstitial pattern at first glance but the presence of a branching pattern reveals the intraluminal distribution. On thin section CT images of the lung, this branching pattern is more easily seen and has been referred to as the tree-inbud sign. Bronchiectasis: Bronchial wall malacia resulting from chronic inflammatory insult leads to bronchial dilatation and sacculation. Bronchial walls appear thickened and additional radiographic findings may include pulmonary consolidation from concurrent bronchopneumonia. On thoracic CT images, individual bronchi can be more easily seen and more accurately characterized. Additionally, bronchial wall thickness can be directly measured on a computer work-station. Moreover, the peribronchial and interstitial tissues can be 544 more accurately assessed for pathologic change. In people, the CT diagnosis of bronchiectasis is made using bronchial to pulmonary arterial diameter ratios. In our experience in dogs with airway disease, a bronchial/arterial ratio of 2 or more is highly suggestive of bronchiectasis. In patients with atelectasis or pulmonary fibrosis, traction bronchiectasis can also occur due to increased radial forces on the affected bronchial wall. This is sometimes seen on survey radiographs but is much more evident on CT images of affected lungs. Radiographic features of feline airway disease are similar to those in dogs and include prominent bronchial markings, increased peribronchial opacity, diffuse unstructured interstitial pulmonary opacity, soft tissue accumulation in airways indicative of mucous plugging, lung hyperinflation, lung lobe atalectasis, and occasional bronchiectasis. Many of the radiographic features of feline airway oriented disease are better recognized on CT images due to the lack of anatomic superimposition. Interstitial Patterns Structured or nodular patterns: Generalized nodular patterns are most often due to widespread pulmonary metastasis or granulomatous inflammatory disease. From CT imaging literature in people, there is evidence to suggest that metastatic nodules arising from pulmonary vascular seeding may result in better-defined nodules than those neoplastic or inflammatory nodules that arise from the lymphatics or directly from pulmonary interstitium. Poorly defined nodules can often mimic an unstructured interstitial pattern on survey radiographs. Thin-section CT is being used with increasing frequency to confirm the presence of nodules suspected from plain radiographs. In a recent review of veterinary patients with confirmed pulmonary metastatic disease, less than one in 10 nodules seen on CT were detected radiographically. Further, while nodules as small as 1 mm in diameter could be detected by CT, nodules less than about 8 mm in diameter were inconsistently identified on survey radiographs. In our practice, it is fast becoming a standard of care to include thoracic CT pre-operatively for any patient with surgically managed neoplasia with metastatic potential. Unstructured interstitial patterns are associated with wide range of underlying disorders including pulmonary fibrosis, edema, hemorrhage, interstitial inflammatory disease and neoplasia. In general, overall pulmonary opacity is increased and there is a variable loss of definition of the bronchial and pulmonary vascular margins. In most instances, the interstitial pattern is non-specific and radiographic findings may be useful for reaching a diagnosis only when combined with current history and supporting clinical signs. Thin-section CT imaging is sometimes more specific than survey radiography for reaching a diagnosis because the source for increased pulmonary density can often be determined anatomically (e.g. perivascular infiltrates from left ventricular failure or high permeability edema). Interstitial markings on CT may appear multifocal and coalescing and are often described as having a ground glass appearance. Reticular opacities which are non-tapering and peripheral are also occasionally seen and are thought to represent areas of atlectasis and fibrosis.

5 Alveolar pattern Alveolar infiltrates may also result from a wide range of disorders with varying etiologies but, in simple terms, invariably consist of blood, inflammatory infiltrates, edema fluid or tumor cells. The distribution of the infiltrates will often suggest a specific etiology (dependent lung lobes = bronchopneumonia, caudodorsal = noncardiogenic edema, hilar = cardiogenic edema) as will the rate of progression or resolution (pulmonary hemorrhage/contusion = resolution in 2-3 days). The radiographic characteristics of alveolar disease include air bronchogram formation, soft tissue opacification of lung, effacement of adjacent soft tissue structures including vascular and cardiac margins and delineation of lobar margins. Common disease processes associated with this pattern are bronchopneumonia, neoplasia, pulmonary edema, and hemorrhage. CT can prove valuable in patients with chronic alveolar pulmonary consolidation that are non-responsive to conventional therapies. It may reveal pulmonary masses obscured by overlying alveolar infitrates or small pulmonary nodules not visualized with conventional radiography. Additionally, bronchial foreign bodies or pulmonary architectural abnormalities such as bronchiectasis may be recognized that predispose patients to chronic recurrent pneumonia. Additional reading 1. Scherrer W, Kyles A, Samii V, Hardie E, Kass P, Gregory C. Computed tomographic assessment of vascular invasion and resectability of mediastinal masses in dogs and a cat. N Z Vet J Dec; 56(6): De Rycke LM, Gielen IM, Simoens PJ, van Bree H. Computed tomography and cross-sectional anatomy of the thorax in clinically normal dogs. Am J Vet Res Mar; 66(3): Prather AB, Berry CR, Thrall DE. Use of radiography in combination with computed tomography for the assessment of noncardiac thoracic disease in the dog and cat. Vet Radiol Ultrasound Mar-Apr; 46(2): Morandi F, Mattoon JS, Lakritz J, Turk JR, Jaeger JQ, Wisner ER. Correlation of helical and incremental high-resolution thin-section computed tomographic and histomorphometric quantitative evaluation of an acute inflammatory response of lungs in dogs. Am J Vet Res Aug; 65(8): Morandi F, Mattoon JS, Lakritz J, Turk JR, Wisner ER. Correlation of helical and incremental high-resolution thin-section computed tomographic imaging with histomorphometric quantitative evaluation of lungs in dogs. Am J Vet Res Jul; 64(7): Johnson VS, Ramsey IK, Thompson H, Cave TA, Barr FJ, Rudorf H, Williams A, Sullivan M. Thoracic high-resolution computed tomography in the diagnosis of metastatic carcinoma. J Small Anim Pract Mar; 45(3): Nemanic S, London CA, Wisner ER. Comparison of thoracic radiographs and single breath-hold helical CT for detection of pulmonary nodules in dogs with metastatic neoplasia. J Vet Intern Med May-Jun; 20(3):