EISAGOGIKO EUROPEAN ASSOCIATION OF VETERINARY DIAGNOSTIC IMAGING EUROPEAN COLLEGE OF VETERINARY DIAGNOSTIC IMAGING ARISTOTLE UNIVERSITY OF THESSALONIKI SCHOOL OF VETERINARY MEDICINE SECTION OF RADIOLOGY ANNUAL MEETING 2007 CONFERENCE GUIDE & ABSTRACTS BOOK PORTO CARRAS, CHALKIDIKI, GREECE 2 9. 0 8-0 1. 0 9. 2 0 0 7
Thoracic Imaging: Ultrasound with Radiographic Correlation John S. Mattoon, DVM, Dipl. ACVR Associate Professor of Radiology Washington State University Ultrasound examination has become a valuable diagnostic procedure for assessment of thoracic disease in small animals. Thoracic sonography is limited by the barriers to ultrasonic transmission caused by aerated lung, air within the pleural cavity, and the bony thorax. Its usefulness is potentiated when used in conjunction with thoracic radiography and in the presence of pleural fluid, which serves as an acoustic window to allow imaging of otherwise sonographically obscured structures. the presence of pleural fluid radiographically often masks concurrent lesions such as mediastinal, pleural, or pulmonary masses. Ultrasound may be used to detect and characterize pleural fluid, pleural, pulmonary and mediastinal disease, and assess the integrity of the diaphragm. Ultrasound is invaluable in differentiating cardiac from non-cardiac disease and is routinely used to aid needle placement during thoracocentesis and to safely procure biopsy samples. Body WALL and Lung Surface Scanning via an intercostal approach produces an image of subcutaneous tissue, intercostal musculature, pleural lining of the thorax (parietal pleura) and the lung surface (visceral pleura). Subcutaneous fat and connective tissue are quite echogenic, and just deep to this the thin intercostal muscles are seen as relatively hypoechoic tissue with coarse parenchymal texture. Unless high frequency transducers are utilized, or transducers capable of high near field resolution (linear array, convex or microconvex electronic transducers), these tissues will not be resolved. the parietal pleura is recognized as a highly echogenic, thin, smooth linear structure. the lung surface is also seen as a thin, smooth echogenic line directly adjacent to the parietal pleura. It glides to and fro along the parietal surface during respiration and is thus readily identified by respiratory motion, termed the gliding sign. A cobblestone or roughened appearance of pleural surfaces is seen in cases of pleuritis, carcinomatosis, chronic pleural effusions and some types of pulmonary disease. Lung has a very characteristic appearance due to reverberation artifact, created as the ultrasound strongly reflects off the surface of the aerated lung. A series of reflections ensue between the lung surface and the transducer creating a sequence of highly echogenic linear echoes known as reverberation artifact. Accordingly, if peripheral lung tissue is aerated, pathology deeper within the pulmonary parenchyma cannot be imaged. PULMONARy Disease Since only the visceral pleural surface of normal lung can be imaged, ultrasound assessment of pulmonary disease is dependent on mass lesions, nodules, consolidation or atelectasis within the periphery of lung tissue. the presence of aerated lung between the transducer and the lesion will effectively block the transmission of the ultrasound and the lesion cannot be imaged. As fluid or cellular infiltrate invade the interstitial and/or air spaces, an acoustic window develops (provided this occurs in the periphery of the lung). the lung parenchyma begins to appear as solid tissue, owing to the acoustic properties of the fluid or cellular infiltrate. the sonographic appearance of pulmonary lesions varies. the degree of remaining air-filled alveoli and bronchi and conversely, the extent to which the air spaces are fluid-filled have a great effect on the sonographic appearance of diseased lung. At the lung surface, pulmonary lesions are seen as sonographic interruption of the linear, highly echogenic visceral pleural surface. Echogenic pathologic tissue extends into the parenchyma, displacing aerated lung. the pathologic region will be bordered by highly characteristic reverberation artifact. the deep margin of the lesion will be distinct and echogenic, as the ultrasound reflects at the interface of the diseased and normal lung. Adhesions of lung to the parietal pleura are referred to as lesion-pleural symphysis, diagnosed when the gliding sign is absent. This finding may represent extension of pulmonary neoplasia to the chest wall or be present with inflammatory lesions. It should be noted that the gliding sign may be barely perceptible or absent in patients with rapid, shallow respiration. Absence of the gliding sign may also be due to loss of diaphragmatic excursion, indicating the possibility of phrenic nerve paralysis. 70
As pulmonary consolidation becomes more severe, it spreads to the deeper parenchyma. It may be homogeneous, but more often the sonographic appearance is that of inhomogeneous tissue. This inhomogeneity is due to some airways filling with fluid or exudate while others remain air-filled. Residual air within alveoli and bronchi create multiple, very echogenic foci. These are termed air bronchograms. They are recognized by intense, short linear echogenicities. Fluid accumulation within air spaces appear as small anechoic foci, termed fluid bronchograms. They are non-pulsatile (to differentiate them from pulmonary vascular structures), sometimes branching tubular anechoic structures. Without Doppler, the differentiation of small pulmonary vessels from fluid bronchograms is not possible, but the presence of one or both is still indicative of pulmonary consolidation since neither is seen in normally aerated lung. Consolidation of lung is termed hepatization, a gross anatomic description, sometimes be used to describe the sonographic appearance.the sonographic appearance of consolidated lung can resemble liver tissue, especially when larger bronchi are fluid-filled, which mimic hepatic vessels. This appearance is seen with lobar pneumonia, lobar neoplasia and lung torsion. Because of its resemblance to hepatic parenchyma, consolidated lung may be incorrectly identified as herniated liver. the classic appearance of a pulmonary abscess is a cavitary lesion. the central cavitary area may contain anechoic or echogenic fluid. the cavitation may be also be compartmentalized or loculated, characterized by echogenic internal septations. Layering can occur, with sedimentation of heavy echogenic cellular debris ventrally with less echogenic fluid dorsally. Echogenic gas can be found within the abscess cavity. the presence of gas is indicative of anaerobic or aerobic microbial infection and/or communication with an airway. the lining of the abscess cavity may be rough and irregular or relatively smooth. Fibrous encapsulation will occur if the abscess is chronic, imaged as a hyperechoic outer margin. Atelectasis, or lung lobe collapse, occurs either in the presence of pleural fluid or with a pneumothorax. Atelectic lung is easily imaged in the presence of pleural fluid but cannot be imaged when a pneumothorax is present. Atelectic lung lobes are identified as small triangular structures floating within the pleural fluid, attached dorsally to the stem bronchi. the lung parenchyma is quite mottled and heterogeneous with highly echogenic foci representing residual air-filled alveoli. CRANIAL Mediastinum the cranial mediastinum is difficult to image in the normal patient and in cases in which only small lesions are present, or pleural fluid is absent. Mediastinal masses which are large enough to contact the thoracic wall are easily imaged via an intercostal approach, however. the thymus, largest at 4-5 months of age, and may be seen as tissue of moderate echogenicity with a granular yet homogeneous texture. in the presence of pleural fluid, the normal mediastinum in a lean animal is a very thin, echogenic and distinct structure. Varying amounts of echogenic fat may be present, increasing the thickness accordingly. Vessels will be readily apparent. Mediastinal widening is usually first detected radiographically. Ultrasound plays a valuable role in helping differentiate incidental widening secondary to fat accumulation from pathologic conditions of the mediastinum or from the presence of pleural fluid. Mediastinal masses are often accompanied by pleural effusion and may thus be masked radiographically. Mediastinal lesions may be small or become large enough to contact the thoracic wall. in the latter instance, imaging is quite easy from an intercostal approach since lung will be displaced away from the chest wall by the mass. Lymphoma is the most common type of mediastinal mass lesion found in dogs and cats. the classic appearance is of hypoechoic nodular masses with a thin distinct echogenic periphery. These may coalesce sonographically to become quite large with lumpy margins. Some cases of lymphoma have a more heterogeneous echotexture. Color-flow Doppler analysis may indicate marked vascularity. Other types of mediastinal masses such as mast cell tumor, undifferentiated neoplasia and even reactive lymphadenopathy may have an identical sonographic appearance to lymphoma. PLEURAL Effusion the presence of pleural fluid, particularly moderate or large amounts, greatly enhances the ability to image intrathoracic structures, contrary to radiography. Mediastinal fat and vessels are readily apparent as are surfaces of the heart, lungs and diaphragm. the caudal vena cava may be seen, traced into the right atrium. Occasionally, the esophagus, trachea and mainstem bronchi are noted. Pleural effusion is recognized as anechoic or echogenic material within the pleural space, between the thoracic wall or diaphragm and lung. Transudate or modified transudate of recent origin are generally anechoic while exudate, hemorrhage or carcinomatosis are more echogenic. Chronic effusions can lead to fibrin formation, seen as linear, irregular echogenic strands floating within the fluid (pleural septations). Moderate or large effusions are recognized immediately from virtually 71
any window into the thorax. Lungs and heart will literally be floating. As the amount of fluid increases, lung volume will diminish, to the point of total atelectasis. Small amounts of fluid may be difficult to detect, noted as separation of the lung surface from the thoracic wall or small anechoic spaces between the lung and the diaphragm. in the cranial thorax, pleural fluid can be seen as two discrete pockets separated by the mediastinum. the mediastinal vessels will be visible as anechoic tubular structures intimately associated with echogenic fat. Asymmetric fluid accumulation may be difficult to detect or missed entirely on an ultrasound examination unless there is direction provided by thoracic radiographs. Examples include focal hemorrhage from trauma, pleural disease such as inflammation secondary to migrating foreign bodies or neoplastic processes involving the pleura or ribs. the contribution of thoracic radiographs to successful sonography of the thorax cannot be overemphasized. Identification of pleural fluid dictates the sonographer to search for an etiology, both within the thorax as well as the abdomen. Ultrasound may contribute to the diagnosis of congestive heart failure, neoplasia (mediastinal, pulmonary, or pleural), pneumonia, lung torsion, trauma, mediastinal disease and diaphragmatic hernia. Hepatic disease, pancreatitis, glomerulonephritis, pyometra and systemic effects secondary to postparturition may cause pleural effusions, indicating the potential need for abdominal ultrasound. It should be emphasized that although general sonographic appearances of various types of pleural effusions have been described, it is still essential that proper laboratory analysis of the fluid be performed, including cytology and culture and sensitivity (aerobic and anaerobic). PLEURAL Masses Pleural masses may be detected radiographically and thus prompt ultrasound evaluation. Pleural masses may also be hidden radiographically, silhouetting with pleural fluid and subsequently diagnosed via ultrasound examination. Deciding if a peripheral thoracic mass is pleural or pulmonary in origin can be challenging based on radiographic findings alone. Ultrasound can readily distinguish between the two by observing if the mass moves with respiratory motion (pulmonary origin) or is stationary and originating from the parietal pleural surface. ThORACIC wall lesions Ultrasound assessment of externally evident thoracic wall lesions may provide information regarding the appearance, size and extent of the lesion, whether there is penetration into the pleural space or lung, if localized pleural fluid is present and assess body wall integrity. DiaphRAGMATIC Hernias Ultrasound examination of the diaphragm has proven to be a fairly reliable technique for the diagnosis of hernias. Because it is noninvasive, ultrasound examination may be the next logical diagnostic procedure following survey radiography. Accurate assessment is augmented by the presence of pleural and/or abdominal fluid. Positive contrast peritonography is more sensitive than ultrasound, especially if the diaphragmatic defect is small, and may be considered the practical reference standard. Recognition of the continuous echogenic interface between the liver/diaphragm and lung is the principle landmark. Discontinuity of this curvilinear echogenicity indicates the presence of a diaphragmatic hernia. Careful attention to the presence or absence of herniated abdominal contents through the hernia is important. Recognition of the commonly encountered mirror image artifact from herniated liver is essential, the key distinction being the presence or absence of the characteristic echogenic line representing the diaphragm/lung surface. the presence of mirror image artifact is evidence that disruption of the diaphragm at that location is not present. Ultrasound is an excellent noninvasive method by which to confirm the presence of CPDH. Generalized enlargement of the cardiac silhouette is the common radiographic abnormality, prompting differential diagnoses of various cardiac diseases and pericardial effusion. Caudal sternal abnormalities or reduced number of sternebrae are sometimes present. the diagnosis can be made sonographically by visualization of liver (or other organs) surrounding the heart, contained within the pericardial sac. This may be accomplished using the heart as an acoustic window or a transabdominal, subcostal approach via the liver. Scanning the diaphragm will reveal the absence of the echogenic diaphragmatic landmark centrally and continuation of the liver parenchyma into the pericardial sac. If only falciform fat is herniated, the diagnosis may be more difficult, as the falciform fat may be similar to pericardial fat in appearance. in this case, careful attention to the continuity of the echogenic diaphragm is essential. 72
Interventional Procedures Sonographic visualization of pulmonary or mediastinal lesions and pleural effusion allows ultrasound guided biopsies or aspirates to be performed. An 18 gauge biopsy needle is generally used while 20-23 gauge needles are used for tissue aspirates, 18 gauge or larger for effusions. Needle aspirates may be the first choice in mixed pulmonary lesions, as there is increased risk of pneumothorax if aerated lung is still present. This has not been a common sequela of lung biopsies or aspirates in my experience. While solid masses pose little risk of pneumothorax, hemorrhage is still of concern. Avoidance of large anechoic tubular pulmonary vessels is essential. the procedure may be done with or without sedation in a cooperative patient, perhaps with a local anesthetic. Smaller patients or small lesions may require the use of heavy sedation or general anesthesia for safety and comfort. A biopsy guide allows confident and precise needle placement, but limits accessibility in some instances because of rib interference. Therefore, freehand aspirates or biopsies with direct ultrasound guidance are often performed. Immediately following the procedure, the sonographer should assess the patient for potential hemorrhage. Anechoic or echogenic fluid accumulation around the biopsy site not noted prior to the procedure is indicative of hemorrhage. Detection of pneumothorax post-biopsy may be done with ultrasound and thoracic radiography. Emergence of a gassy pleural effusion and disappearance of the lung lesion sonographically indicates a pneumothorax. These cases should be monitored radiographically. 73