Introduction
Diagnostic imaging includes a wonderful collection of technologies which allow the veterinary practitioner the ability to peer inside of a patient, evaluate the health of different organs and help determine the health status of the patient. When ill, reptiles and amphibians, often pose a significant diagnostic challenge and imaging can be particularly useful with these animals. Many of the traditional diagnostic tools in domestic animals (i.e.: hematology and clinical history) are less useful in herptiles (reptiles and amphibians) and thus imaging can be one of the only helpful diagnostic aids. Radiology and ultrasonography are commonly used in conjunction with each providing a complimentary strength. In herptiles, radiology is most useful for skeletal anatomy, pulmonary health and determining if gravid in certain species. On the other hand, ultrasonography plays little role in evaluating bones or lungs, but is extremely useful for visualizing the remaining soft tissue structures (heart, liver, gonads, and in some species the kidneys).
As with domestic animals, it is critical for the veterinary practitioner to have a solid understanding of normal anatomical structures before attempting to evaluate diagnostic images. This is probably the most challenging part of utilizing imaging when diagnosing disease in herptiles. It is strongly encouraged that the veterinarian and technical support staff take advantage of all herptiles which are going to have a surgical exploratory or postmortem examination. Take the time to radiograph and perform and ultrasound exam on these animals prior to the exploratory or post-mortem. Attempt to identify organs radiographically and ultrasound, and then correlate their position and relative health when handling the real tissues. This extra time will be well worth the time investment and allow you to make significant strides in your abilities to interpret and diagnose reptile and amphibian diseases.
Equipment Considerations
Standard radiograph and ultrasound units can readily be used when working with herptiles. Digital radiography does have several advantages including image manipulation and ease of sharing images for consultation. For small patients, less than 3kg, use of dental or mammography radiographic cassettes/units have proven useful. Ultrasonography systems which allow different size and shape of probes to be switched out are the most versatile for the many shapes and sizes of herptiles. A typical 7.5 MHz transducer used with dogs and cats can be utilized with many reptiles and amphibians. If the practitioner sees larger reptiles (i.e.: sea turtles) a 3.5 MHz or 5.0 MHz probe will be required. Additionally, if very small animals are evaluated (i.e.: frogs or small snakes) a 10.0 MHz probe would be very useful. The veterinarian should ensure that appropriate images are being captured and saved during the ultrasound examination. These images are an important part of the animal's record and also a useful learning tool to come back to and reference.
Patient Positioning & Restraint
The decision to utilize manual versus chemical restraint will greatly depend on the animal species, size and degree of safety required for both staff and patient. Most small herptiles can be imaged using manual restraint and some species specific techniques. Radiographically, animals such as lizards and amphibians can be imaged by placing them within a zip-lock plastic bag and then keeping them in a normal resting ventral position on the cassette. The bag should be just large enough for the animal to rest, but not so large that it can turn around. Several small holes can be cut into the bag for normal respiration. The bag prevents them from inadvertently moving off the table while the picture is being taken. For larger lizards (i.e.: monitors and iguanas) a towel placed over the animals head often allows the patient to relax for imaging. Turtles and tortoises (chelonians) are best imaged by placing them on a small platform. This platform should rest under their ventral shell (plastron) and allow their legs to extend, but not touch the cassette. Typically a role of tape, a bucket or a foam block is used as a platform. The above restraint techniques often allow a DV image and a lateral image to be taken without sedation. For lateral views, the radiograph unit is often tilted into a horizontal beam and the animal is left resting on its ventrum. In order to provide high quality images of snakes, it is often necessary to manually restrain them by holding them or via sedation. The image of a coiled up snake is difficult to interpret and provides limited information. Sequential imaging from head to cloacae of snakes including DV and lateral views will provide the most useful images for diagnostic evaluation. These images should be numerically labeled and some overlap from the previous image should be incorporated to help with orientation. Venomous snakes or aggressive animals should be sedated or placed in clear acrylic tubes for imaging.
For ultrasonography, many non-aggressive animals can be imaged without sedation. Most herptiles prefer to remain on their ventrum and are less likely to struggle if elevated off the table rather than rolling them onto their back. As with domestic animals, the ventrum is used for transducer placement for large animals a notched small animal ultrasound table can be used and for smaller animals, the patient can be held in an elevated position, just above the transducer.
Ultrasonography in amphibians is a very useful tool and because most of these animals are comfortable in water, positioning them in a water-bath is ideal. Amphibians can be placed in a rigid plastic container (aka: Tupperware) with enough water to cover their legs. The animal does not need to be sedated and the use of water actually enhances the ultrasound image. The water in the container should be room temperature and free of disinfectants or other chemicals. The author commonly uses bottled water for this purpose. The transducer can be applied directly to the bottom of the plastic container with coupling gel, or can be placed directly into the water (without gel), lateral to the body wall.
Image Interpretation & Clinical Applications
Reptile and amphibian radiographs provide excellent images of skeletal anatomy and the respiratory system. Unlike mammals, it is often difficult or impossible to individually identify soft tissue organs such as the liver, spleen, kidney and gonads. The gastrointestinal tract may be identified if it is full of air or radiographic contrast material. Contrast studies are commonly used to help delineate location and function of the GIT and adjacent organomegaly or masses. In many cases, normal GI transit times for reptiles have not been published, but are much slower then those of mammals. In many species, it may take days or even weeks for the contrast material to reach the terminal colon. Barium and iodinated agents have both been used in herptiles, but caution should be taken when using barium in dehydrated herptiles as the barium can pull needed fluids into the bowels. In general, the author uses approximately 10cc/kg of contrast material via gavage for contrast studies.
Snakes and chelonians commonly have respiratory disease and radiology is very useful in confirming and staging the disease. In snakes, a lateral radiograph focusing on the area just caudal to the heart is most useful. In turtles and tortoises, the X-ray unit should be turned to a horizontal position and an anterioposterior view taken with the beam pointing directly toward the patients head. It is important that legs are not pulled in, but are hanging down, to ensure an unobstructed view of the lungs.
Radiology can also be very helpful when doing reproductive work in reptiles. Calcified eggs in the oviduct of snakes, lizards and chelonians can be seen and in those species which are viviparous, fetal skeletal anatomy can be imaged. Radiographs have also been used to determine the sex of monitor lizards who are not sexually dimorphic. Males have a pair of calcified hemi-baculum just caudal to their vent, while females do not.
When performing an ultrasound examination on herptiles it is important to systematically evaluate each organ system. The author prefers to start at the heart and move caudally towards the cloacae. The heart provides an ideal landmark to begin each examination. In general it is easy to locate and allows the practitioner a consistent organ system to adjust their image quality (contrast, gain, magnification, etc), prior to moving onto other organ systems. In many lizards, the heart can be located just between the front legs. In monitor lizards and crocodilians the heart is more caudally located. In snakes the heart is generally located about 1/4th to 1/3rd the distance between the head and cloacae. In chelonians, the heart is best visualized by placing the transducer in the soft fleshy area between the neck and front legs.
Remember that herptiles do not have separate abdominal and chest cavities (crocodilians are an exception) and thus the heart will be immediately adjacent to the liver and stomach in many species. In snakes, their organs are lined up in a very linear progression with the lungs following behind the heart, which then transitions to the liver. The liver terminates about ½ way down the snake and then the stomach, gall bladder, spleno-pancreas, small bowels, fat bodies, ovary and colon are seen. In the author's experience, testes and kidneys are rarely identified. When imaging chelonians, the shell is too thick for ultrasound penetration. The soft areas adjacent to the legs provide an ideal window into the coelom. Placement of the transducer just cranial to the femur will allow visualization of the ovary, intestines, liver, urinary bladder and gall bladder.
Ultrasonography is a very useful tool for reproductive work in herptiles. This is especially true in immature animals for sex determination or in animals which may not have calcified eggs (i.e.: amphibians), and determining if the animal is gravid. When amphibians are gravid, their coelomic cavity fills up with eggs from their cloacae to their pericardium. Ultrasonography can rapidly determine if the animal is gravid versus organomegaly or ascites.
Advanced Imaging Modalities
Computerized Axial Tomography (CAT scan) and Magnetic Resonance Imaging (MRI) have both been successfully used in reptiles and amphibians. Both provide excellent, high quality imaging of internal anatomy. In most cases, both CAT scans and MRI will require sedation and/or anesthesia for imaging. Since there are few published references of normal images for these advanced imaging modalities in herptiles, when possible, there is great value in bringing a "normal control" which can also be imaged for comparison. CAT scans are usually less expensive, but are not useful for CNS imaging and often do not offer the image detail that a high quality MRI can provide.
Literature Cited
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