Introduction
Access to advanced diagnostic imaging modalities, such as ultrasound and computed tomography is becoming increasingly available to private practitioners in recent years; however, routine radiography is by far the most frequently utilized for both economic and diagnostic reasons. Due to the relatively small size of many exotic animal patients, detail is often a primary concern. Detail can be maximized in any radiographic setup by using a small focal spot, the shortest possible exposure time, adequate focus-film distance, a collimated beam, single emulsion film and a rare earth, high-detail screen. Over the past several years, human mammography machines and film have been used by many institutions to produce detailed images of small patients, and today digital radiography with its speed, excellent image quality, and ease of image storage and manipulation is becoming more and more popular in the private sector.
Avian Radiology
Proper radiographic positioning is absolutely essential for the accurate interpretation of radiographic lesions. Some birds may be adequately restrained using commercial devices (for example, the Plexiglas "bird board") or items such as sand bags, foam blocks, or tape. Provided the patient is tolerant of manual restraint and not highly stressed, dyspneic, or injured, these methods can provide diagnostic images. However, it is often in the best interest of the patient to mask a bird down with isoflurane or sevoflurane anesthesia. This provides appropriate relaxation to ensure proper positioning and optimize the diagnostic value of the films. It is also far less stressful for the patient and minimizes the time needed to get diagnostic images. Most frequently, both ventrodorsal (VD) and right lateral, whole body views are obtained for diagnostic purposes. In a well positioned VD view, the spine and sternum are superimposed, and the scapulae, acetabula, and femurs are parallel. A good lateral projection shows the ribs, coracoids, acetabula, and kidneys superimposed on one another.
Musculoskeletal System
The avian skeleton is unique in that it has been specially modified for flight. It is lightweight and many bones of the axial skeleton are fused to form a rigid framework for stability. Air sacs extend into the medullary cavity of major bones including the humerus, coracoid, pelvis, sternum, and vertebrae (and additionally the femur, scapula, and furcula in some birds). The spine is separated into highly mobile cervical and largely fused thoracic, synsacrum, and caudal vertebrae segments. The pectoral girdle consists of the clavicle, coracoid, and scapula, and is frequently a point of concern that can benefit from radiographic evaluation in both wild and companion birds. The pelvic girdle is made up of the fusion of the ischium, pubis, and ileum and is rigidly fused to the synsacrum. 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. Developmental abnormalities, associated with poor husbandry and nutrition, are seen in young animals, and can be due to deficiencies or imbalances in calcium, phosphorus, or vitamin D. Generalized osteopenia, valgus deformity of the tibiotarsi, kyphosis, lordosis, and sternal compression can all be evident radiographically in these cases. In some animals, if the spinal or sternal abnormalities are severe enough to compromise the thorax, respiratory distress may occur.
Traumatic injury may result in fractures, soft tissue injuries, or luxations. In evaluating a fracture, it is important to consider the location, proximity to joints, bone density, presence or absence of a periosteal response, extent of soft tissue involvement, number of fragments, and whether the fracture is open or closed. In general, acute fractures will possess sharp well-defined margins, will not have a significant periosteal response, and will have concurrent soft tissue swelling. In contrast, older chronic fractures have more indistinct, rounder fracture ends, tend to have a greater periosteal response, and there will be minimal soft tissue involvement, perhaps even atrophy. Lack of visualization of the fracture lines and the presence of a smooth, well-defined callus bridging all cortices indicate complete healing.
Bone density is one of the most important qualities to evaluate on avian radiographs. Avian long bones are characterized by thin cortices, but normal pre-ovulatory hens will 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 presumably producing estrogens. Osteolysis is the predominant radiographic change with infectious or neoplastic processes, and differentiation between these etiologies may require biopsy or cytology. Acute infection may show bone destruction with minimal periosteal reaction, but chronic infections usually present with significant periosteal response. Osteomyelitis or septic arthritis can result from an open fracture, penetrating wound, hematogenous spread, iatrogenic contamination, or an extension from air sac disease or pododermatitis. Fungal osteomyelitis may cause pronounced periosteal reaction or increased medullary opacity due to granuloma formation. Mycobacterial infections may also cause medullary granulomas as well as septic arthritis and bone lysis. Acute septic arthritis may initially appear as joint effusion only with very little radiographic change. As the infection progresses, however, the articular cartilage degenerates resulting in a narrowed joint space and osteolysis, and periosteal changes may occur on the epiphyses and metaphyses of the involved bones. The presence of osteophytes or sclerosis of subchondral bone indicates a more chronic form of degenerative joint disease. Primary bone neoplasia (i.e., osteosarcoma) is uncommon in avian patients, and most tumors affecting bones occur secondary to soft tissue neoplasia.1
Cardiovascular System
In general, the heart extends from the second rib to the fifth or sixth rib, with the exact location varying by species.1,4 Echocardiography is probably the most accurate means of assessing cardiac size and function, but traditional radiographs can provide a good estimate of the size and shape of the cardiac silhouette. In the VD view of a normal Amazon parrot, the distance across the heart base at the level at the atria should be about 50% of the width of the coelomic cavity at the fifth thoracic vertebra.1 Additionally, on the VD view, the lateral margins of the heart and liver should create an hourglass shape in psittacines. Pericardial effusion can be recognized radiographically as a symmetrical globoid enlargement of the cardiac silhouette. Cardiomegaly is usually asymmetrical and 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.
Respiratory System
The avian respiratory system is unique in that the lungs are small, undergo little change in volume when breathing, and have air sacs which act as bellows but do not participate in gas exchange. This bellows system allows for continual one-way air flow through the system and results in increased oxygen absorption efficiency.1,2 Each primary bronchus runs through the entire length of the lungs and terminates in the caudal thoracic air sacs, but gives rise to secondary bronchi terminating in parabronchi along the way. The parabronchi are where blood gas exchange takes place and they make up the bulk of the lung tissue. Most birds have eight air sacs including the cervical, clavicular, and paired cranial thoracic, caudal thoracic and abdominal air sacs. Some birds also possess paired cervical sacs bringing their total to nine. The avian trachea consists of complete, rigid rings and varies between species. In some species like the Trumpeter swan and cranes, the trachea forms loops within the keel before coursing through the thoracic inlet to the lungs. In toucans and mynah birds, the trachea normally deviates ventrally at the level of the thoracic inlet. In waterfowl, males have a distinct syringeal bulla which allows sex determination in species that do not have dimorphic plumage.4
Normal lung parenchyma will appear honeycombed in appearance, with the air densities mostly consisting of end-on views of the gas-filled parabronchi.2 Because of their unique anatomy, the air bronchograms and atelectasis commonly seen in mammals do not occur, but with pulmonary disease the normal honeycombed parenchyma may be enhanced by parabronchial infiltration causing prominent ring shadows.1,4 These parabronchial patterns consistent with pneumonia are usually seen near the hilum and mid-portions of the lungs. As disease progresses, the parabronchial lumen may fill with fluid or exudate, or becomes replaced with neoplastic or granulomatous infiltrate causing a blotchy, mottled appearance.1,4 This change is most easily seen at the caudal aspects of the lungs on a VD view. Pulmonary edema and hemorrhage tend to appear more diffuse where as discrete well defined masses in the lungs are more common caused by tumors, abscesses, or granulomas.
Air sac inflammation causes these structures to thicken and become apparent radiographically (in normal birds, the borders of the air sacs cannot be distinguished). Air sac disease may also present as nodular infiltration or consolidation, and decreased compliance may cause a barrel-shaped appearance to the thorax. Subcutaneous emphysema may result from trauma to an air sac and its rupture.
Gastrointestinal System
The crop is present in the right lateral thoracic inlet area on a VD view, while the proventriculus is most easily seen lying dorsal to the liver on a lateral view. The ventriculus can generally be viewed on both the VD and lateral projections caudal and ventral to the proventriculus. The intestinal tract normally occupies the caudodorsal abdominal cavity and no gas should be present in a normal bird. Specific areas of the gastrointestinal tract are best visualized using barium as a contrast agent, but the presence of gas, abnormal distension, or change in position in normal survey radiographs suggests a disease process. The spleen appears as a rounded structure to the right of midline between the proventriculus and ventriculus on a VD view, and slightly dorsocaudal to the proventriculus on a lateral view if visible. Splenomegaly may be caused by infectious (Chlamydia, mycobacteria, viral, bacterial), neoplastic, and metabolic (lipidosis, hemochromatosis) diseases. Splenic, testicular, ovarian, and renal masses tend to compress the gastrointestinal tract ventrally and either cranially or caudally. The liver does not normally extend beyond the sternum on a lateral view, and in psittacines, should not extend laterally across a line drawn from the coracoid to the acetabulum on a VD view.1,4 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 Symmetrical liver enlargement can be caused by infectious, neoplastic, parasitic (toxoplasmosis), and metabolic (lipidosis, hemochromatosis) causes, while asymmetrical enlargement is more likely caused by neoplasia or granulomatous disease. Abdominal effusion is recognized radiographically by a homogenous appearance and obscured visualization of specific organs. It can be associated with liver disease, neoplasia, metabolic disorders (hemochromatosis), sepsis, inflammation (yolk peritonitis), and cardiac failure.1,4
Urogenital System
Avian kidneys are best visualized on a lateral view, and oblique views may be necessary to visualize each kidney. The kidneys are not well visualized on a VD projection, but may be if they are grossly enlarged or have an increased opacity. The kidneys are normally surrounded by air, and loss of the air shadow 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. An excretory urogram may be the only way to confirm renal disease when severe kidney enlargement causes the abdominal viscera to become indistinguishable from one another on radiographs. Masses involving the spleen, oviduct, testicles, ovary, and intestines may occupy space in the caudodorsal abdomen and mimic renal lesions.1,4 Testicular abnormalities are uncommon but may include orchitis or Sertoli cell tumors. In a hen, an active ovary's follicles may resemble a bunch of grapes just cranial to the kidneys radiographically, and while mineralized eggs are easily visualized, soft-shelled eggs may be difficult to distinguish from other soft tissue masses without ultrasound.
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 Nevertheless, radiographs can still be valuable diagnostic tools to the reptile clinician for evaluating both skeletal and soft tissue structures. Due to the many diverse body shapes of reptile species, the radiographic views that are most helpful in examining the animal's internal anatomy can vary quite a bit depending on the patient. Horizontal beam studies are utilized for lizards and chelonians, so it is important that the practitioner's radiograph tube has the capability to rotate 90 degrees so that the radiograph beam can be directed parallel to the table top. 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. For non-anesthetized patients, this can sometimes be accomplished by elevating the animal on a radiolucent object like a bucket. 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. General anesthesia is usually recommended for radiographic studies of snakes, especially for thorough examination of the spine. In snakes, the lateral view provides the most diagnostic information, and if the radiograph beam is well collimated, several exposures can be made on a single piece of film. 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
Thorough examination of the skeletal system is a common motivating factor for pursuing radiographic studies of reptile patients. Chelonian shells are formed around dermal bone plates and represent a formidable complication to the production of detailed radiographic studies in these animals. The pectoral and pelvic girdles occupy a position inside the ribs acting like vertical pillars to give extra strength to the shell. Snakes have extremely kinetic skulls and have no distinct cervical region, but the first two cervical vertebrae lack ribs. There are often up to 400 vertebrae precloacally, each with its own pair or ribs and large axial skeletal muscles.2 All vertebrae except the cervical ones bear ribs in lizards. Burrowing lizards have lost their limbs, but unlike snakes, they still retain their pectoral and pelvic girdles.2
Metabolic bone disease is a term that encompasses many skeletal diseases which can be caused by nutritional, dietary, or hormonal causes including dietary deficiencies of calcium or vitamin D, insufficient exposure to ultraviolet light, calcium/phosphorus imbalances, or renal or parathyroid disease.3 Evaluating bone radiopacity is based on the opacity of the pectoral girdle in chelonians (outline should appear crisp and cortical radiodensity should be homogenous), the opacity of the ribs in snakes, and the radiopacity and contrast of the long bones and skull in lizards.3 Angular deformities of the long bones and ribs tend to be more common in lizards than in snakes and chelonians and can progress to bulbous distortion of long bones in lizards due to deposition of fibrous connective tissue around the poorly mineralized bones in the body's attempt to stabilize them. 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. In fracture healing, fibrous callous formation is relatively more important in reptiles than it is in mammals and clinical fracture stability often occurs far earlier than radiographic healing.3 Complete bony fracture healing may not be evident for more than six months. Prolonged and sometimes lifelong visualization of the radiolucent fracture line is not uncommon because many of the appendicular fractures heal with a combination of a radiopaque callus and fibrous tissue.3 Displaced rib fractures with no radiopaque callus are common but are usually incidental findings in snakes.
Almost completely opposite of most reptiles' response to appendicular bony trauma is their response to vertebral insult. The response of the vertebrae can be exuberant in snakes and lizards, with radiographic changes often being more dramatic than the clinical signs (spinal deformity, rigidity). Proliferative spinal osteopathy is generally 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, and a persistent lytic defect in the bone is often still seen after resolution of the osteomyelitis.
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 survey radiographs of chelonians but evaluation of size, chamber volume, and contractility are really best performed with ultrasound.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 midcoelom in monitors and has indistinct borders. If a discrete border is seen, it may indicate pericardial fluid or cardiomegaly is present.3 Positive pressure ventilation can be very helpful in improving the radiographic appearance of the lungs, especially in lizards and snakes.
As in birds, the unique anatomy of reptilian lungs means that the standard radiographic patterns (alveolar, interstitial, bronchial, pleural) used in mammals do not exist. 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 reptilian digestive tract is short compared to the mammalian digestive tract, but the transit time of ingesta is generally much longer. Unfortunately, the gastrointestinal tract and accessory digestive organs like the liver and pancreas are often difficult to visualize on survey radiographs 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. Intestinal obstruction is characterized by enlargement of the diameter of the digestive tract; however, a prominent obstructive gas pattern is not always seen.3 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 and may be located laterally.3 Calculi tend to have an irregularly lamellated appearance and an irregularly rounded shape. Cloacal calculi can also be seen in lizards and chelonians, but may be radiolucent on traditional radiographs. Pregnancy in lizards and snakes is radiographically diagnosed with the identification of oval to round, soft tissue opacity structures in the caudal abdomen that are arranged in a linear overlapping pattern that resembles a clumped string of pearls.3 In turtles and tortoises, the shells of the eggs are typically radiopaque.
Amphibian Radiology
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 examples of fused portions of the anuran skeleton. A prominent nutrient foramen is commonly visualized in the tibiofibula bones. Coelomic anatomic structures can rarely be individually identified although contrast radiography can greatly enhance the diagnostic value of radiographs when gastrointestinal disease is suspected.
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. The use of magnification with or in addition to mammography and dental film is often a helpful tool with these species and can improve resolution with exceptionally small patients.
Common abnormal radiographic findings in amphibians include: anasarca/ascites (commonly associated with sepsis), generalized osteopenia with folding fractures and spinal abnormalities caused by metabolic bone disease, gastrointestinal foreign bodies (stones, sand) that are not always associated with clinical disease and cystic calculi.
Small Mammal Radiology
Rodents, rabbits, guinea pigs, chinchillas and ferrets are best radiographed under anesthesia to obtain films of diagnostic value. Gas anesthesia (isoflurane, sevoflurane) delivered via face mask or in a chamber is convenient and safe for most small mammals. Standard radiographic views are VD and lateral but DV and oblique views can be helpful for specific areas of interest. 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 compared to the avian and herptile counterparts discussed above. 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 large 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.
Conclusion
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. These species 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. Sedation greatly reduces the patient's stress and allows for appropriate positioning to improve diagnostic image quality. Developing efficient radiograph techniques for these species will improve patient safety and minimize complications (i.e., hypothermia) that can occur. 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.
Acknowledgements
Thank you to Brookfield Zoo's veterinary staff for their help in acquiring radiographic studies as well as sharing many of the cases presented. Special thanks to Dr. Julia Whittington for her collaboration in preparing the teaching materials.
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. Saunders Elsevier, St. Louis, Missouri. Pp. 471-489.
4. Smith, B.J., Smith, S.A. (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., Hoefer, H.L. (1997) Small Mammal Radiology. In: Hillyer E. V. and Quesenberry K.E. (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 Whitaker B.R. (eds.). Amphibian Medicine and Captive Husbandry. Krieger Publishing Co., Malabar, Florida. Pp. 253-272.
Additional Exotic Animal Radiology References
Capella V., Gracis, M., Lennox, A. (2005) Rabbit and Rodent Dentistry Handbook. Zoological Education Network, Lake Worth, Florida.
Capello V., Lennox, A. (2008) Clinical Radiology of Exotic Companion Mammals. Blackwell Publishing / John Wiley & Sons, Ames, Iowa.
Farrow C.S. (2009) Veterinary Diagnostic Imaging, Birds, Exotic Pets and Wildlife. Mosby Elsevier, St. Louis, Missouri.
Hernandez-Divers, S., Lafortune, M. (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.
Mader, D.R. (2006) Radiographic Anatomy. In: Mader, D. R. (ed.). Reptile Medicine and Surgery. 2nd ed. Saunders Elsevier, St. Louis, Missouri. Pp. 1097-1102.
Orosz S.E., Ensley, P.K., Haynes, C.J. (1992) Avian Surgical Anatomy Thoracic and Pelvic Limbs. W.B. Saunders Co., Philadelphia, Pennsylvania.
Rübel, G.A., Isenbügel, E., Wolverkamp, P. (1993) Atlas of Diagnostic Radiology of Exotic Pets. W.B. Saunders Co., Philadelphia, Pennsylvania.
Samour J.H., Naldo, J.L.(2007) Anatomical and Clinical Radiology of Birds of Prey. W.B. Saunders Co., Philadelphia, Pennsylvania.
Silverman S., Tell, L. (2005) Radiology of Rodents, Rabbits and Ferrets An Atlas of Normal Anatomy and Positioning. Elsevier Mosby, St. Louis, Missouri.
Silverman S., Tell, L. (2010) Radiology of Birds An Atlas of Normal Anatomy and Positioning. Elsevier Mosby, St. Louis, Missouri.