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Imaging techniques for avian lower respiratory diseases
Imaging Techniques for Avian Lower Respiratory Diseases Susan M. Newell, DVM, MS, Dip.ACVR, GregoryD. Roberts, DVM, MS, Dip.ACVR, and R. Avery Bennett, DVM, MS, Dip.ACVS
Radiography is the most frequently used imaging technique to assess avian lower respiratory disease. The unique architecture of the avian respiratory system results in variations of radiographic appearances that are markedly different from the traditional pulmonary patterns frequently seen in mammals such as alveolar, bronchial, or interstitial. Alveolar lung disease, the hallmark of pneumonia in mammalian imaging, is not identified in avian lungs because the terminal airways consist of interconnecting bronchi and parabronchi without well-developed smaller compartments. Pathological changes of avian lungs are most commonly recognized radiographically as increased soft-tissue opacity with loss of the normal reticulated pattern. Avian lungs are relatively noncompliant and firmly adhered to the thoracic wall; thus, total lung volume is relatively constant throughout respiration, and conditions such as atelectasis do not occur. This review will address the common radiographic findings associated with avian lower respiratory disease. In addition, the potential applications of computerized axial tomography (CT) and magnetic resonance imaging (MRI) in avian respiratory disease will be discussed. Copyright 9 1997 by W. B. Saunders Company. Key words: Avian, respiratory, imaging, diseases, radiology.
WO i m p o r t a n t factors in accurate radioraphic interpretation are a knowledge of n o r m a l anatomy and creation of a diagnostic film: T h e n o r m a l anatomy of m a n y species of b o t h c o m p a n i o n a n d wild birds has b e e n described 1-6and a reference atlas 7 is often useful in the clinical setting. N u m e r o u s protocols for obtaining diagnostic radiographs in avian medicine have b e e n described. 8-1~Either general anesthesia or a positioning b o a r d and masking tape can be used to restrain the patient. Radiographs From the Department of Small Animal Clinical Sciences, College of VeterinaryMedicine, the University ofFlorida, Gainesville, FL. Address reprint requests to Susan M. NeweU, DVM, MS, Dip.ACVR, Department of SmaU Animal Clinical Sciences, College of Veterinary Medicine, the University of Florida, PO Box 100102, Gainesville, FL 32610. Copyright9 1997 by W. B. Saunders Company. 1055-937X/97/0604-000255.00/0
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should be m a d e with a fast exposure time (1A0 second or faster), small focal spot, closely collim a t e d beam, and high-detail film; screen combinations such as Q u a n t a Detail screens with Cronex 10 film (Sterling Co, [formerly DuPont] Wilmington, DE) or Kodak Min-R single-emulsion m a m m o g r a p h y film and cassettes (Eastman Kodak Co, Rochester, NY) are r e c o m m e n d e d . T h e cassette is placed on the table top because a grid is unnecessary for avian radiography, except in large birds such as ratites. A technique chart based on exact bird m e a s u r e m e n t s or general bird size (ie, small budgerigar, m e d i u m Amazon Parrot, large cockatoo) can be derived using an extremities chart for small animals as a guideline. An example of a simple avian technique chart that is routinely used with m a m m o g r a p h y cassettes and film is presented in Table 1. A lateral and a ventrodorsal view o f the patient are routinely obtained, These radiographs should be centered on the mid-coelom, with the limbs e x t e n d e d to avoid superimposition with the coelomic cavity. Accurate positioning is vital to avoid artifactual changes that can either mimic or obscure i m p o r t a n t radiographic signs. Radiographs should be exposed during p e a k inspiration to optimize visualization of the air sacs when possible.
Normal RadiographicAppearance T h e trachea can b e identified on the ventrodorsal view on the right side of midline in the cervical region, centrally at the thoracic inlet, a n d terminates at the syrinx which, in most birds, is located at one of the first few intercostal spaces. T h e syrinx is difficult to visualize in most birds except male waterfowl, where it is often large and mineralized. T h e lungs can be visualized as finely reticulated, " h o n e y c o m b " structures located on either side of the cardiac silhouette in the cranial half of the coelomic cavity on
Seminars in Avian and Exotic Pet Medicine, Vol 6, No 4 (October), 1997: pp 180-186
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Figure 1. Lateral (A) and ventrodorsal (B) views of a clinically normal adult Amazon Parrot (Amazona sp). Note the symmetrical appearance of the air sacs and fine reticulated lung pattern. appear more opaque because of the concave shape of the coelom and superimposition of adjacent musculature. Most authors agree that the lungs do not significantly change size or shape with phases of respiration. 11'12 Portions of the cavities of several air sacs are visible radiographically in birds, but the thin walls cannot usually be identified in n o r m a l patients. The normal opacity of the caudal thoracic and abdominal air sacs is generally less than that of the lungs and approximately equal to that of the surrounding air. O n the ventrodorsal view, the caudal thoracic and abdominal air sacs are located lateral to the viscera, with the abdominal air sacs extending caudally to the level of the cloaca2 O n the lateral view, the air sacs are not as visible, but form the lucent triangle between the lungs cranially, the kidneys dorsocaudally, and the proventriculus-ventriculus ventrally. An example of the n o r m a l radiographic appearance of the avian lower respiratory system is shown in Fig 1. Table 1. Avian Technique Chart
the ventrodorsal projection. They are located between the heart and spine on the lateral projection. The h o n e y c o m b appearance is created by the closely opposed soft-tissue dense bronchi and parabronchi filled with air. The most lateral and dorsal aspects of the lungs may
Patient
k Vp
mAs
Parakeets, Love Birds Cockatiels, Pigeons African Grey Parrots Amazon Parrots Cockatoos Macaws
42-44 46-48 48-52 50-52 52-54 52-56
4-6 6 8 8 8 8
NOTE. This technique chart is for use with mammography film and cassettes, table top (no grid) technique. Adjust technique as needed for patient body condition.
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NeweU, Roberts, and Bennett
Radiographic Abnormalities Abnormalities of the trachea and syrinx are u n c o m m o n 1~ and most are not radiographically evident, except focal mass lesions caused by abscesses, granulomas, parasites, neoplasms, or foreign bodies. Tracheal lesions cranial to the thoracic inlet are relatively easy to identify as soft-tissue masses either within or externally compressing the otherwise air-filled tracheal lumen. Masses associated with the syrinx may be difficult to identify radiographically because of superimposition of other soft-tissue structures. Radiographic abnormalities of the lungs include blurring and thickening of the normal honeycomb appearance, increased p r o m i n e n c e of the parabronchi (ring shadows), focal consolidation of the lungs, and discrete soft-tissue dense mass lesions. Acute p n e u m o n i a may appear as an accentuated parabronchial pattern in the hilar and mid-portion of the lungs, s As p n e u m o n i a progresses, an increased opacity with an uneven, blotchy appearance may be seen in the middle and caudal aspects of the lungs (Fig 2). Ring shadows, f o r m e d by markedly thickened parabronchi with lucent centers, are often seen in the hilar region associated with aspergillosis. 1~ Diffuse blurring of parabronchi may be secondary to p u l m o n a r y h e m o r r h a g e or e d e m a whereas focal areas of consolidation are usually secondary to exudative processes (Fig 3). Abscesses, fungal granulomas, or, less likely, neoplasms 14,15 may completely obliterate the lung parenchyma with discrete soft-tissue dense masses, which can also displace adjacent viscera (Fig 4). Knowledge o f normal vascular anatomy is i m p o r t a n t to differentiate small p u l m o n a r y masses from large vessels. Lesions in the cranial air sacs may appear to be in the lung because of the m a n n e r in which the air sacs overlap the lungs in both radiographic projections. 9 In addition, with many respiratory diseases, both the lungs and air sacs are affected and differentiation between abnormalities of one or both organs can be difficult. An example of this phenome n o n occurs with chronic obstructive p u l m o n a r y disease (pulmonary hypersensitivity syndrome), which has been described in Blue and Gold Macaws (Ara ararauna) and is believed to occur secondary to chronic airborne irritants and p o o r
ventilation. Radiographically, these birds have hyperinflation of the air sacs with increased p r o m i n e n c e of the parabronchial markings and right-sided cardiomegaly. 16 Radiographic abnormalities of the air sacs include focal or diffuse regions of increased soft-tissue opacity, overdistension with rounding of the air sac margins, and compression or collapse, particularly in the caudal air sacs. Generalized air sacculitis may cause an evenly distributed increase in density with occasional softtissue lines located at sites where air sacs overlap (Fig 5). The distal tips of the air sacs often appear r o u n d e d or blunted and may be asymmetrical (Fig 3). These birds may physically and radiographically appear to have an exaggerated barrel-shape secondary to p o o r compliance because of chronic air sac overinflation, aa The possible causes for air sacculitis are multiple and include viral, bacterial (including Chlamydia and Mycoplasma), and fungal agents as well as nutritional deficiencies52 In general, mycotic diseases are more often associated with multifocal- or focal-increased opacities caused by granuloma formation within the air sacs (Fig 5). 15 Severe overdistension of the air sacs is believed to be specific for syringeal stenosis] ~ whereas milder overdistension is associated with many diseases. Rupture of the air sacs is u n c o m m o n , yet it does occur and can lead to a pneumocoelom with dorsal displacement of the heart from the sternum and increased visualization of other coelomic organs. Small volumes of free coelomic air could persist for several days following laparoscopy. The air sacs can be externally compressed by enlargement o f other coelomic organs (eg, gastrointestinal, urogenital), or diffuse coelomic abnormalities such as obesity, ascites, or peritonitis. In these cases, the presenting complaint is usually respiratory distress. These patients often appear radiographically similar with a loss of visualization of abdominal or caudal thoracic air sacs and decreased intracoelomic detail.
Cross-Sectional Imaging C o m p u t e d axial tomography (CT) and magnetic resonance imaging (MR/) are two crosssectional imaging techniques in veterinary medi-
Avian Respiratory Imaging
183
Figure 2. Lateral (A) and ventrodorsal (B) views of a juvenile Blue and Gold Macaw (Ara ararauna) with severe diffuse bacterial air sacculitis and p n e m n o n i a . Patchy increased soft-tissue opacity areas (white arrows) and ring shadows (small white arrows) are present within the lungs. Thickened thoracic and abdominal air sacs walls (curved white arrows) can be visualized.
Figure 3. Ventrodorsal view of an adult African Grey Parrot (Psittacus erithacus) with focal increased lung and air sac opacity caused by aspergillosis. Note the asymmetry between the right (affected) and left (normal) caudal thoracic and abdominal air sacs.
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Figure 4. Lateral (A) and ventrodorsal (B) views of an adult Red-Lored Amazon Parrot (Amazona autumnalis) with a large soft-tissue dense mass (white arrows) in the left lung or cranial thoracic air sac. The differential diagnosis includes abscess, granuloma, or neoplasia. A large fungal granuloma (aspergillosis) was identified at necropsy.
cine. CT uses ionizing radiation identical to standard radiography; however, because of the cross-sectional imaging and improved detail, precise identification of the lung and air sac margins is possible. This allows for m o r e accurate localization of lesions within either the p u l m o n a r y
parenchyma, adjacent air sacs, or both o r g a n s ) ,15 CT is particularly valuable in p u l m o n a r y disease where small lesions such as air sac plaques, which may be missed or incorrectly localized radiographically, are easily identifiedJ 5,17 CT can also assess mass lesions at the thoracic inlet and, by calculation of CT numbers, can potentially distinguish between fat density lipomas, cavitary thyroid goiters, and solid soft-tissue granulomas. T h e terminal trachea, syrinx, and mainstem bronchi are often a b n o r m a l in birds with acute respiratory distress secondary to aspergillosis. These regions are difficult to assess radiographically because of superimposition of other softtissue dense organs. CT can accurately identify mass lesions, such as fungal granulomas within major airways in humans, and could potentially localize airway obstructive lesions in c o m p a n i o n and wild birds. Magnetic resonance imaging uses magnetic fields and precisely timed radio frequency waves to generate an image with unparalleled softtissue contrast. MRI has b e e n used most extensively in neurologic and musculoskeletal diseases in veterinary medicine; however, their routine use in h u m a n medicine includes respiratory disorders, particularly those of the u p p e r airway and mediastinum. Magnetic resonance imaging may not be as valuable for air sac or p u h n o n a r y diseases because of the n o r m a l low tissue density of these organs. Any respiratory abnormality where there is increased soft-tissue density is easily assessed with M R / i n h u m a n s and could be visualized in birds. T h e n o r m a l MRI a p p e a r a n c e
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Figure 5. Lateral (A) and ventrodorsal (B) views of an adult Amazon Parrot with focal increased softtissue opacity within the left caudal thoracic air sac with probable extension into the left cranial thoracic air sac and possibly the left lung. There is r o u n d i n g and blunting of the left caudal thoracic air sac (compared with the opposite side).
Figure 6. Dorsal T1 weighted image of the terminal trachea (white arrow), syrinx, and mainstem bronchi (small white arrows) of an adult macaw. Note the increased signal intensity (white) appearance of the marrow cavity of the radii (due to fat) compared to the pneumatic appearance of the humeri.
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of the distal trachea, syrinx, and mainstem bronchi of a macaw is presented in Fig 6. With the increasing availability of advanced imaging techniques such as CT and MRI in veterinary medicine, the potential of these modalities to noninvasively provide diagnostic information previously available only by surgical endoscopy or exploratory celiotomy will certainly be explored.
References I. Orosz SE, ToaI RL: Tomographic anatomy of the Golden Eagle (Aquila chrysaetos).J Zoo Wild Med 23:39-46, 1992 2. Hillyer EV, Orosz S, Dorrestein GM: Respiratory system, in Altman RB, Clubb SL, Dorrestein GM, Quesenberry K (eds): Avian Medicine and Surgery. Philadelphia, PA, Saunders, 1997, pp 387-411 3. Smith BJ, Smith SA, Spaulding KA, et al: The normal xeroradiographic and radiographic anatomy of the cockatiel (Nymphicus hollandis). Vet Radiol Ultrasound 31:226234, 1990 4. Smith BJ, Smith SA, Spaulding KA, et al: The normal xeroradiographic and radiographic anatomy of the Orange-Winged Amazon Parrot ( Amazona amazonica amazonica). Vet Radiol Ultrasound 31:114-124, 1990 5. Smith BJ, Smith SA: The normal xeroradiographic and radiographic anatomy of the Red-Tailed Hawk (Buteo jamaicensis) with reference to other diurnal raptors. Vet Radiol Ultrasound 31:301-312, 1990 6. Smith BJ, Smith SA: The normal xeroradiographic and radiographic anatomy of the Great Horned Owl (Bubo virginianus) with special reference to the Barn Owl ( Tyto alba). Vet Radiol Ultrasound 32:6-16, 1991
7. Rubel, GA, Isenbugel E, Wolvekamp P (eds): Arias of Diagnostic Radiology of Exotic Pets, Philadelphia, PA, Saunders, 1991 8. McMillan MC: Imaging techniques, in Ritchie BW, Harrison GJ, Harrison LR (eds): Avian Medicine: Principles and Application. Lake Worth, FL, Wingers, 1994, pp 246-326 9. Smith BJ, Smith SA: Radiology, in Altman RB, Clubb SL, Dorrestein GM, Quesenberry K (eds): Avian Medicine and Surgery. Philadelphia, PA, Saunders, 1997, pp 170199 10. Krautwald-Junghanns ME: Avian radiology, in Rosskopf WJ, Woerpel RW (eds): Diseases of Cage and Aviary Birds, ed 3. Baltimore, MD, Lea & Febiger, 1996, pp 630-663 11. McMillan MC: Radiology of avian respiratory diseases. Compen Contin Educ Vet 8:551-558, 1986 12. Tully TN, Harrison GJ: Pneumonology, in Ritchie BW, Harrison GJ, Harrison LR (eds): Avian Medicine: Principles and Application. Lake Worth, FL, Wingers, 1994, pp 556-581 13. McMillan MC, Petrak ML: Retrospective study ofaspergillosis in pet birds.J Assoc Avian Vet 3:211-215, 1989 14. Leach MW: A survey of neoplasia in pet birds. Sem Avian Exotic Pet Med 1:52-64, 1992 15. Krautwald-Junghanns ME, Schumacher F, Telhelm B: Evaluation of the lower respiratory tract in psittacines using radiology and computed tomography. Vet Radiol Ultrasound 34:382-390, 1993 16. Fudge AM, Reavill DR, Rosskopf WJ: Diagnosis and management of avian dyspnea: A review, in 1993 Proceedings of the Association of Avian Veterinarians, Nashville, TN, Association of Avian Veterinarians, 1993, pp 187-195 17. Krautwald-Junghanns ME: Radiology of the respiratory tract and the use of computed tomography in psittacines, in 1992 Proceedings of the Association of Avian Veterinarians, New Orleans, LA, Association of Avian Veterinarians 1992, pp 366-373
Radiography is the most frequently used imaging technique to assess avian lower respiratory disease. The unique architecture of the avian respiratory system results in variations of radiographic appearances that are markedly different from the traditional pulmonary patterns frequently seen in mammals such as alveolar, bronchial, or interstitial. Alveolar lung disease, the hallmark of pneumonia in mammalian imaging, is not identified in avian lungs because the terminal airways consist of interconnecting bronchi and parabronchi without well-developed smaller compartments. Pathological changes of avian lungs are most commonly recognized radiographically as increased soft-tissue opacity with loss of the normal reticulated pattern. Avian lungs are relatively noncompliant and firmly adhered to the thoracic wall; thus, total lung volume is relatively constant throughout respiration, and conditions such as atelectasis do not occur. This review will address the common radiographic findings associated with avian lower respiratory disease. In addition, the potential applications of computerized axial tomography (CT) and magnetic resonance imaging (MRI) in avian respiratory disease will be discussed. Copyright 9 1997 by W. B. Saunders Company. Key words: Avian, respiratory, imaging, diseases, radiology.
WO i m p o r t a n t factors in accurate radioraphic interpretation are a knowledge of n o r m a l anatomy and creation of a diagnostic film: T h e n o r m a l anatomy of m a n y species of b o t h c o m p a n i o n a n d wild birds has b e e n described 1-6and a reference atlas 7 is often useful in the clinical setting. N u m e r o u s protocols for obtaining diagnostic radiographs in avian medicine have b e e n described. 8-1~Either general anesthesia or a positioning b o a r d and masking tape can be used to restrain the patient. Radiographs From the Department of Small Animal Clinical Sciences, College of VeterinaryMedicine, the University ofFlorida, Gainesville, FL. Address reprint requests to Susan M. NeweU, DVM, MS, Dip.ACVR, Department of SmaU Animal Clinical Sciences, College of Veterinary Medicine, the University of Florida, PO Box 100102, Gainesville, FL 32610. Copyright9 1997 by W. B. Saunders Company. 1055-937X/97/0604-000255.00/0
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should be m a d e with a fast exposure time (1A0 second or faster), small focal spot, closely collim a t e d beam, and high-detail film; screen combinations such as Q u a n t a Detail screens with Cronex 10 film (Sterling Co, [formerly DuPont] Wilmington, DE) or Kodak Min-R single-emulsion m a m m o g r a p h y film and cassettes (Eastman Kodak Co, Rochester, NY) are r e c o m m e n d e d . T h e cassette is placed on the table top because a grid is unnecessary for avian radiography, except in large birds such as ratites. A technique chart based on exact bird m e a s u r e m e n t s or general bird size (ie, small budgerigar, m e d i u m Amazon Parrot, large cockatoo) can be derived using an extremities chart for small animals as a guideline. An example of a simple avian technique chart that is routinely used with m a m m o g r a p h y cassettes and film is presented in Table 1. A lateral and a ventrodorsal view o f the patient are routinely obtained, These radiographs should be centered on the mid-coelom, with the limbs e x t e n d e d to avoid superimposition with the coelomic cavity. Accurate positioning is vital to avoid artifactual changes that can either mimic or obscure i m p o r t a n t radiographic signs. Radiographs should be exposed during p e a k inspiration to optimize visualization of the air sacs when possible.
Normal RadiographicAppearance T h e trachea can b e identified on the ventrodorsal view on the right side of midline in the cervical region, centrally at the thoracic inlet, a n d terminates at the syrinx which, in most birds, is located at one of the first few intercostal spaces. T h e syrinx is difficult to visualize in most birds except male waterfowl, where it is often large and mineralized. T h e lungs can be visualized as finely reticulated, " h o n e y c o m b " structures located on either side of the cardiac silhouette in the cranial half of the coelomic cavity on
Seminars in Avian and Exotic Pet Medicine, Vol 6, No 4 (October), 1997: pp 180-186
Avian Respiratory Imaging
181
Figure 1. Lateral (A) and ventrodorsal (B) views of a clinically normal adult Amazon Parrot (Amazona sp). Note the symmetrical appearance of the air sacs and fine reticulated lung pattern. appear more opaque because of the concave shape of the coelom and superimposition of adjacent musculature. Most authors agree that the lungs do not significantly change size or shape with phases of respiration. 11'12 Portions of the cavities of several air sacs are visible radiographically in birds, but the thin walls cannot usually be identified in n o r m a l patients. The normal opacity of the caudal thoracic and abdominal air sacs is generally less than that of the lungs and approximately equal to that of the surrounding air. O n the ventrodorsal view, the caudal thoracic and abdominal air sacs are located lateral to the viscera, with the abdominal air sacs extending caudally to the level of the cloaca2 O n the lateral view, the air sacs are not as visible, but form the lucent triangle between the lungs cranially, the kidneys dorsocaudally, and the proventriculus-ventriculus ventrally. An example of the n o r m a l radiographic appearance of the avian lower respiratory system is shown in Fig 1. Table 1. Avian Technique Chart
the ventrodorsal projection. They are located between the heart and spine on the lateral projection. The h o n e y c o m b appearance is created by the closely opposed soft-tissue dense bronchi and parabronchi filled with air. The most lateral and dorsal aspects of the lungs may
Patient
k Vp
mAs
Parakeets, Love Birds Cockatiels, Pigeons African Grey Parrots Amazon Parrots Cockatoos Macaws
42-44 46-48 48-52 50-52 52-54 52-56
4-6 6 8 8 8 8
NOTE. This technique chart is for use with mammography film and cassettes, table top (no grid) technique. Adjust technique as needed for patient body condition.
182
NeweU, Roberts, and Bennett
Radiographic Abnormalities Abnormalities of the trachea and syrinx are u n c o m m o n 1~ and most are not radiographically evident, except focal mass lesions caused by abscesses, granulomas, parasites, neoplasms, or foreign bodies. Tracheal lesions cranial to the thoracic inlet are relatively easy to identify as soft-tissue masses either within or externally compressing the otherwise air-filled tracheal lumen. Masses associated with the syrinx may be difficult to identify radiographically because of superimposition of other soft-tissue structures. Radiographic abnormalities of the lungs include blurring and thickening of the normal honeycomb appearance, increased p r o m i n e n c e of the parabronchi (ring shadows), focal consolidation of the lungs, and discrete soft-tissue dense mass lesions. Acute p n e u m o n i a may appear as an accentuated parabronchial pattern in the hilar and mid-portion of the lungs, s As p n e u m o n i a progresses, an increased opacity with an uneven, blotchy appearance may be seen in the middle and caudal aspects of the lungs (Fig 2). Ring shadows, f o r m e d by markedly thickened parabronchi with lucent centers, are often seen in the hilar region associated with aspergillosis. 1~ Diffuse blurring of parabronchi may be secondary to p u l m o n a r y h e m o r r h a g e or e d e m a whereas focal areas of consolidation are usually secondary to exudative processes (Fig 3). Abscesses, fungal granulomas, or, less likely, neoplasms 14,15 may completely obliterate the lung parenchyma with discrete soft-tissue dense masses, which can also displace adjacent viscera (Fig 4). Knowledge o f normal vascular anatomy is i m p o r t a n t to differentiate small p u l m o n a r y masses from large vessels. Lesions in the cranial air sacs may appear to be in the lung because of the m a n n e r in which the air sacs overlap the lungs in both radiographic projections. 9 In addition, with many respiratory diseases, both the lungs and air sacs are affected and differentiation between abnormalities of one or both organs can be difficult. An example of this phenome n o n occurs with chronic obstructive p u l m o n a r y disease (pulmonary hypersensitivity syndrome), which has been described in Blue and Gold Macaws (Ara ararauna) and is believed to occur secondary to chronic airborne irritants and p o o r
ventilation. Radiographically, these birds have hyperinflation of the air sacs with increased p r o m i n e n c e of the parabronchial markings and right-sided cardiomegaly. 16 Radiographic abnormalities of the air sacs include focal or diffuse regions of increased soft-tissue opacity, overdistension with rounding of the air sac margins, and compression or collapse, particularly in the caudal air sacs. Generalized air sacculitis may cause an evenly distributed increase in density with occasional softtissue lines located at sites where air sacs overlap (Fig 5). The distal tips of the air sacs often appear r o u n d e d or blunted and may be asymmetrical (Fig 3). These birds may physically and radiographically appear to have an exaggerated barrel-shape secondary to p o o r compliance because of chronic air sac overinflation, aa The possible causes for air sacculitis are multiple and include viral, bacterial (including Chlamydia and Mycoplasma), and fungal agents as well as nutritional deficiencies52 In general, mycotic diseases are more often associated with multifocal- or focal-increased opacities caused by granuloma formation within the air sacs (Fig 5). 15 Severe overdistension of the air sacs is believed to be specific for syringeal stenosis] ~ whereas milder overdistension is associated with many diseases. Rupture of the air sacs is u n c o m m o n , yet it does occur and can lead to a pneumocoelom with dorsal displacement of the heart from the sternum and increased visualization of other coelomic organs. Small volumes of free coelomic air could persist for several days following laparoscopy. The air sacs can be externally compressed by enlargement o f other coelomic organs (eg, gastrointestinal, urogenital), or diffuse coelomic abnormalities such as obesity, ascites, or peritonitis. In these cases, the presenting complaint is usually respiratory distress. These patients often appear radiographically similar with a loss of visualization of abdominal or caudal thoracic air sacs and decreased intracoelomic detail.
Cross-Sectional Imaging C o m p u t e d axial tomography (CT) and magnetic resonance imaging (MR/) are two crosssectional imaging techniques in veterinary medi-
Avian Respiratory Imaging
183
Figure 2. Lateral (A) and ventrodorsal (B) views of a juvenile Blue and Gold Macaw (Ara ararauna) with severe diffuse bacterial air sacculitis and p n e m n o n i a . Patchy increased soft-tissue opacity areas (white arrows) and ring shadows (small white arrows) are present within the lungs. Thickened thoracic and abdominal air sacs walls (curved white arrows) can be visualized.
Figure 3. Ventrodorsal view of an adult African Grey Parrot (Psittacus erithacus) with focal increased lung and air sac opacity caused by aspergillosis. Note the asymmetry between the right (affected) and left (normal) caudal thoracic and abdominal air sacs.
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Figure 4. Lateral (A) and ventrodorsal (B) views of an adult Red-Lored Amazon Parrot (Amazona autumnalis) with a large soft-tissue dense mass (white arrows) in the left lung or cranial thoracic air sac. The differential diagnosis includes abscess, granuloma, or neoplasia. A large fungal granuloma (aspergillosis) was identified at necropsy.
cine. CT uses ionizing radiation identical to standard radiography; however, because of the cross-sectional imaging and improved detail, precise identification of the lung and air sac margins is possible. This allows for m o r e accurate localization of lesions within either the p u l m o n a r y
parenchyma, adjacent air sacs, or both o r g a n s ) ,15 CT is particularly valuable in p u l m o n a r y disease where small lesions such as air sac plaques, which may be missed or incorrectly localized radiographically, are easily identifiedJ 5,17 CT can also assess mass lesions at the thoracic inlet and, by calculation of CT numbers, can potentially distinguish between fat density lipomas, cavitary thyroid goiters, and solid soft-tissue granulomas. T h e terminal trachea, syrinx, and mainstem bronchi are often a b n o r m a l in birds with acute respiratory distress secondary to aspergillosis. These regions are difficult to assess radiographically because of superimposition of other softtissue dense organs. CT can accurately identify mass lesions, such as fungal granulomas within major airways in humans, and could potentially localize airway obstructive lesions in c o m p a n i o n and wild birds. Magnetic resonance imaging uses magnetic fields and precisely timed radio frequency waves to generate an image with unparalleled softtissue contrast. MRI has b e e n used most extensively in neurologic and musculoskeletal diseases in veterinary medicine; however, their routine use in h u m a n medicine includes respiratory disorders, particularly those of the u p p e r airway and mediastinum. Magnetic resonance imaging may not be as valuable for air sac or p u h n o n a r y diseases because of the n o r m a l low tissue density of these organs. Any respiratory abnormality where there is increased soft-tissue density is easily assessed with M R / i n h u m a n s and could be visualized in birds. T h e n o r m a l MRI a p p e a r a n c e
Avian Respiratory Imaging
185
Figure 5. Lateral (A) and ventrodorsal (B) views of an adult Amazon Parrot with focal increased softtissue opacity within the left caudal thoracic air sac with probable extension into the left cranial thoracic air sac and possibly the left lung. There is r o u n d i n g and blunting of the left caudal thoracic air sac (compared with the opposite side).
Figure 6. Dorsal T1 weighted image of the terminal trachea (white arrow), syrinx, and mainstem bronchi (small white arrows) of an adult macaw. Note the increased signal intensity (white) appearance of the marrow cavity of the radii (due to fat) compared to the pneumatic appearance of the humeri.
186
Newell, Roberts, and Bennett
of the distal trachea, syrinx, and mainstem bronchi of a macaw is presented in Fig 6. With the increasing availability of advanced imaging techniques such as CT and MRI in veterinary medicine, the potential of these modalities to noninvasively provide diagnostic information previously available only by surgical endoscopy or exploratory celiotomy will certainly be explored.
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