Jennifer N. Langan, DVM, DACZM
University of Illinois College of Veterinary Medicine & Chicago Zoological Society's Brookfield Zoo, Brookfield, IL, USA
Introduction
Radiography can be a very useful diagnostic tool in exotic animal medicine and should be considered part of a minimum database with many presenting concerns. Exotic animals are often exceptionally stoic and do not show outward clinical signs associated with disease. Radiography can provide clinicians much needed information about the general health of these exotic species that can otherwise not be determined by physical examination alone. Developing efficient, consistent radiograph techniques for these species will improve patient safety and minimize complications that can occur. Sedation greatly reduces the patient's stress and allows for appropriate positioning to improve diagnostic image quality. The use of digital radiography provides the clinician superior image quality, greater versatility, better image storage/manipulation and greater efficiency compared to traditional radiographic systems when evaluating exotic animal species. Due to the relatively small size of many exotic animals it is important to make all possible adjustments to maximize detail by using a small focal spot, the shortest possible exposure time, adequate focus-film distance, a collimated beam. Recommendations for radiographic positioning and interpretation of commonly seen abnormalities will be discussed.
Avian Radiology
Some birds may be adequately restrained using commercial devices (e.g., Plexiglas "bird board") or items such as sand bags, foam blocks, or tape. Provided the patient is tolerant of manual restraint these methods can provide diagnostic images. However, it is often in the best interest of the patient to anesthetize these patients. Most frequently, both ventrodorsal (VD) and right lateral, whole body views are obtained for diagnostic purposes. Wings are extended laterally and dorsally for VD and lateral views respectively. Positioning the right wing and leg cranial to the left ensures they can be differentiated consistently during interpretation.
Musculoskeletal System
Bony changes associated with metabolic bone disease and pathologic fractures as well as those associated with traumatic injury or infection are commonly identified on radiographs. Avian fractures heal faster than in mammals. Lack of healing or lysis are signs associated with osteomyelitis. Mycobacterial infections are not uncommon and can cause medullary granulomas and bone lysis. Normal pre-ovulatory hens have an increased medullary bone density referred to as polyostotic hyperostosis. This condition has also been reported in hens with oviductal tumors and in cocks with Sertoli cell tumors.
Cardiopulmonary System
The heart is best evaluated on the VD view. The lateral margins of the heart and liver create an hourglass shape. Cardiomegaly can be seen as an elongation of the cardiac shadow, loss of the hourglass indentation between the heart and the liver lobes, and loss of the caudal and cranial waists.
Birds have unique respiratory anatomy including: diverse tracheal anatomy, air sacs, and parabronchi. Tracheal disease is relatively common and best appreciated on lateral views. Air sac inflammation causes the membranes to thicken and nodular infiltration develops with progressive disease. Normal lung parenchyma is honeycombed in appearance, with the air densities mostly consisting of end-on views of the gas-filled parabronchi.2 In diseased lungs, prominent ring shadows may be present and are usually seen near the hilum and mid-portions of the lungs with pneumonia.1,4 If the parabronchi fill with fluid or exudate, or become replaced with neoplastic or granulomatous infiltrate a blotchy, mottled appearance can be seen.1,4 Pulmonary edema and hemorrhage tend to appear more diffuse whereas tumors, or granulomas are more discrete.
Gastrointestinal System
The intestinal tract normally occupies the caudodorsal abdominal cavity. The presence of gas, abnormal distension, or change in position suggests a disease process. Splenomegaly may be caused by infectious, neoplastic, and metabolic diseases. Splenic, testicular, ovarian, and renal masses tend to compress the gastrointestinal tract ventrally and either cranially or caudally. Hepatomegaly is a common radiographic finding, and can be recognized by a loss of the hourglass waist on a VD view, rounded liver lobe margins, compression of abdominal air sacs, extension of the liver lobes beyond the scapula/coracoid line, cranial displacement of the heart, dorsal elevation of the proventriculus, and caudodorsal displacement of the ventriculus.1,4
Urogenital System
Kidneys are best visualized on a lateral view. Loss of the air shadow around the kidneys indicates renal enlargement, dorsal displacement of abdominal organs, or the presence of abdominal fat or fluid.1,4 Bilateral symmetrical nephromegaly can occur with infection, metabolic disease, dehydration, post-renal obstruction, and neoplasia. Masses involving the spleen, oviduct, testicles, ovary, and intestines may occupy space in the caudodorsal abdomen and mimic renal lesions.4 Testicular abnormalities include orchitis or Sertoli cell tumors. In hens, follicles resemble a bunch of grapes just cranial to the kidneys on the lateral view. Mineralized eggs are easily visualized whereas soft-shelled eggs may be difficult to distinguish from other masses without ultrasound. Loss of detail in the coelom is a common finding with effusions.
Reptile Radiology
Radiographs of reptiles can be difficult to read because they are often characterized by poor image contrast. This is caused by the close anatomic proximity of internal organs, a lack of internal fat, the lack of a clearly demarcated thorax and abdomen, and the image distortion that can result from the superimposed shell or scales of these patients.3 Horizontal beam studies are utilized for lizards and chelonians. Turtles and tortoises are relatively easy to restrain and position for radiographic studies, and it is best if the head and limbs are not retracted inside the shell. A craniocaudal horizontal view is usually preferred for evaluating the lungs. The digestive system, urinary bladder, and skeleton are best examined on traditional dorsoventral (DV) views. In snakes, the lateral view provides the most diagnostic information. In positioning lizards both DV and lateral (horizontal beam) projections are most commonly taken. The lateral view is most important for evaluating the spine and respiratory tract.
Musculoskeletal System
Metabolic bone disease is very common in reptile patients. Bone radiopacity is based on the opacity of the pectoral girdle in chelonians, the ribs in snakes, and the contrast of the long bones and skull in lizards.3 Soft tissue calcification (especially of vascular structures) is a common abnormal finding in lizards that have been chronically over supplemented with vitamin D.3 Appendicular skeletal fractures are the most common type of skeletal injury seen in lizards while rib fractures are common in snakes. Complete bony fracture healing may not be evident for more than six months. Proliferative spinal osteopathy is characterized by a proliferative segmented spondylosis and snakes seem to be especially susceptible. Appendicular osteomyelitis and infectious arthritis generally appear as slowly progressive lytic processes in reptiles.
Cardiopulmonary System
The heart in chelonians may be visualized as a soft tissue opacity in the ventral portion of the body adjacent to the tracheal termination on radiographs of chelonians.3 The lungs are best appreciated on horizontal radiograph beam views, especially craniocaudal and lateral projections. In lizards and snakes, the heart and lungs are best appreciated on the lateral views. The location of the heart varies between species of lizards, ranging from the pectoral girdle in iguanas to mid-coelom in monitors and has indistinct borders.
The lungs of turtles and tortoises are composed of irregularly shaped areas of respiratory tissue, interspersed with muscular bands which are closely adhered to the carapace.2,3 In snakes and most lizards, the lungs are more sac-like, and most snakes have only a single functioning lung (the right), with the left side either absent or vestigial.2,3 Pneumonia presents as increased pulmonary opacity on radiographs, which is usually diffuse in snakes but can be focal areas in chelonians or lizards.
Gastrointestinal System
The DV view is generally best for visualizing the gastrointestinal tract in lizards, turtles, and tortoises, while the lateral view is superior in snakes. The gastrointestinal tract, liver and pancreas are difficult to visualize due to the close apposition of internal organs and the small amount of internal fat.3 The stomach can sometimes be visualized in the left mid portion of the coelom in chelonians, but often is not seen at all in lizards or snakes unless it is gas filled or radiopaque food was recently ingested. Digestive tract gas is more commonly seen in lizards than in snakes or chelonians, but is usually not prominent unless aerophagia is present.3 Rocks, gravel, or sand are commonly seen in reptile digestive tracts, but in large volumes may cause an obstruction. Gastrointestinal radiographic contrast studies are often necessary for confirming obstruction and foreign body ingestion, and either barium sulfate or a nonionic iodinated contrast medium may be used.
Urogenital System
Urinary bladder calculi are seen most commonly in turtles and tortoises but do also occur in lizards. In chelonians, the urinary bladder is expansive in volume and bilobed in shape, so calculi can vary in position.3 Cloacal calculi can also be seen in lizards and chelonians, but may be radiolucent on traditional radiographs.
Ovaries are only visible during peaks of follicular activity and can be recognized as bilateral large clusters of round soft tissue opacities. After ovulation, eggs take on an oval appearance and are arranged in a linear overlapping pattern.3 Shells of the eggs are thinner and less radiopaque than in birds.
Amphibian Radiology
Radiographs can usually be obtained without chemical restraint in amphibians by placing animals in containers that restrict excessive movement. Small plastic bags work well, even for aquatic amphibians and can be used for short procedures to take DV and horizontal lateral views.6 Chemical restraint with tricaine methanesulfonate or isoflurane allows improved positioning especially to visualize extremities. There is significant diversity of anatomy among amphibians however in general radiographs typically yield excellent images of skeletal anatomy, lungs (if present) and to a lesser extent the gastrointestinal tract. The vertebrae, urostyle, radio-ulna, tibiofibula and the pelvic girdle are fused portions of the anuran skeleton. Coelomic anatomic structures can rarely be individually identified. Contrast radiography can greatly enhance the ability to differentiate the gastrointestinal tract.
Common abnormal radiographic findings in amphibians include: anasarca/ascites, osteopenia, folding fractures, spinal abnormalities, gastrointestinal foreign bodies (stones, sand), and cystic calculi.
Small Mammal Radiology
Rodents, rabbits, guinea pigs, chinchillas and ferrets are best radiographed under anesthesia to obtain films of diagnostic value. Inhalant anesthesia is convenient and efficient for most small mammals. Standard radiographic views are VD and lateral but DV and oblique views of the skull are often helpful. Because of the relative small patient size, whole-body radiography is usually performed. Gastrointestinal contrast films provide useful diagnostic information, particularly in rabbits and ferrets which commonly present with physiologic and mechanical obstruction. Radiologic examination of the urinary tract can be enhanced by giving intravenous iodinated contrast agents for diagnostic urograms. Cystography can be performed (primarily in ferrets and rabbits) to assist with evaluation of lower urinary tract if patient size allows a urinary or adapted intravenous catheter to be placed for contrast administration.5
Monogastric exotic small mammals (i.e., rodents, ferrets, hedgehogs, sugar gliders) have anatomy more similar to cats and dogs. Of these, ferrets most closely resemble domestic small animals but are more tubular in form. Splenomegaly is a common finding in ferrets but does not necessarily correlate with disease. Gastrointestinal foreign bodies, cardiomegaly and organomegaly associated with lymphoma are common disease conditions identified on radiographs in ferrets.
Rodents have a major disparity in body cavity size. The abdomen is much larger than the thorax making visualization of thoracic structures difficult. Feces and a small amount of gas in the intestines are normal abdominal findings.6 Rats commonly present with pneumonia as well as soft tissue masses associated with primary mammary gland neoplasia.
Rabbits, guinea pigs and chinchillas are hindgut fermenters whose digestive tract more closely resembles that of the horse. Body cavity disparity, as in rodents, is exemplified on radiographs, where the abdominal cavity is much larger (and more voluminous) than the thoracic cavity.6 In general, serosal detail is poor in these species. The cecum, usually visualized in the right hemiabdomen, should be full of ingesta and may contain small amounts of gas.6 Gastric stasis, trichobezoars, calciuria, cystic calculi, and dental disease are common radiographic findings. These species have large, thin-walled auditory bullae which are most pronounced in chinchillas. Animals which present with a head tilt often have radiographic evidence of otitis interna, including fluid accumulation within the bullae and osteolysis of the bulla in chronic cases. Older, intact, female rabbits are predisposed to uterine adenocarcinoma and commonly present with an enlarged uterus secondary to neoplasia and secondary pyometra.
References
1. McMillan, M. C. 1994. Imaging Techniques. In: Ritchie, B. W., G. J. Harrison, and L. R. Harrison (eds.). Avian Medicine: Principles and Application. Wingers Publishing, Inc., Lake Worth, Florida. Pp. 246–326.
2. O'Malley, B. 2005. Clinical Anatomy and Physiology of Exotic Species: Structure and Function of Mammals, Birds, Reptiles, and Amphibians. Elsevier Saunders, Philadelphia, Pennsylvania.
3. Silverman, S. 2006. Diagnostic Imaging. In: Mader, D. R. (ed.). Reptile Medicine and Surgery. 2nd ed. Elsevier Inc., St. Louis, Missouri. Pp. 471–489.
4. Smith, B. J., and S. A. Smith. 1997. Radiology. In: Altman, R. B., S. L. Clubb, G. M. Dorrestein, and K. Quesenberry (eds.). Avian Medicine and Surgery. W.B. Saunders Co., Philadelphia, Pennsylvania. Pp. 170–199.
5. Stefanacci, J. D., and H. L. Hoefer. 1997. Small Mammal Radiology. In: Hillyer, E. V., and K. E. Quesenberry (eds.). Ferrets, Rabbits, and Rodents Clinical Medicine and Surgery. W. B. Saunders Co., Philadelphia, Pennsylvania. Pp. 358–377.
6. Stetter, M. D. 2001. Diagnostic Imaging of Amphibians. In: Wright K. M. and B. R. Whitaker (eds.). Amphibian Medicine and Captive Husbandry. Krieger Publishing Co., Malabar, Florida. Pp. 253–272.
Additional Exotic Animal Radiology References
1. Capello, V., M. Gracis, and A. Lennox. 2005. Rabbit and Rodent Dentistry Handbook. Zoological Education Network, Lake Worth, Florida.
2. Capello, V., and A. Lennox. 2008. Clinical Radiology of Exotic Companion Mammals. Blackwell Publishing/John Wiley & Sons, Ames, Iowa.
3. Farrow, C. S. 2009. Veterinary Diagnostic Imaging: Birds, Exotic Pets and Wildlife. Mosby Elsevier, St. Louis, Missouri.
4. Hernandez-Divers, S., and M. Lafortune. 2004. Radiography. In: McArthur, S., R. Wilkinson, and J. Meyer (eds.). Medicine and Surgery of Tortoises and Turtles. Blackwell Publishing, Oxford, United Kingdom. Pp. 195–212.
5. Mader, D. R. 2006. Radiographic Anatomy. In: Mader, D. R. (ed.). Reptile Medicine and Surgery. 2nd ed. Elsevier Inc., St. Louis, Missouri. Pp. 1097–1102.
6. Orosz, S. E., P. K. Ensley, and C. J. Haynes. 1992. Avian Surgical Anatomy Thoracic and Pelvic Limbs. W. B. Saunders Co., Philadelphia, Pennsylvania.
7. Rübel, G. A., E. Isenbügel, and P. Wolverkamp. 1993. Atlas of Diagnostic Radiology of Exotic Pets. W.B. Saunders Co., Philadelphia, Pennsylvania.
8. Samour, J. H., and J. L. Naldo. 2007. Anatomical and Clinical Radiology of Birds of Prey. W. B. Saunders Co., Philadelphia, Pennsylvania.
9. Silverman, S., and L. Tell. 2005. Radiology of Rodents, Rabbits and Ferrets: An Atlas of Normal Anatomy and Positioning. Elsevier Mosby, St. Louis, Missouri.
10. Silverman, S., and L. Tell. 2010. Radiology of Birds: An Atlas of Normal Anatomy and Positioning. Elsevier Mosby, St. Louis, Missouri.