Introduction

The main challenge when imaging reptiles is their vast species differences and thus thorough knowledge of the anatomy and physiology of the species is required in order to make diagnostic quality radiographs and to interpret them adequately. Radiography is a useful tool to get an overview of the anatomy and highlight gross pathology; however, limited visualisation of the gastrointestinal and urinary tract as well as the respiratory system is provided. In these cases, additional advance radiography techniques or other imaging modalities such as ultrasound may be indicated. The unique anatomy of certain species, such as chelonians, precludes the use of standard views and warrants modification of standard radiographic technique.

Radiographic Principles

As with any other species being radiographed, basic radiographic principles apply, including obtaining at least two orthogonal views and practising good radiographic technique and radiation safety.

The radiographic unit that is optimal for imaging exotic species is similar to the equipment that is used when imaging small animal patients; however, due to the smaller size of the patients, high-power x-ray machines are particularly useful.¹ Motion blur, often noted when radiographing avian species, due to their high respiratory rate, is seldom a problem in reptiles; however, despite this, machines with a performance of at least 200 mA and a voltage range between 40 kV and 100 kV are recommended.¹ If your caseload includes a high number of reptilian species, then a movable x-ray tube is beneficial, so that the beam can be directed in a horizontal direction. This imaging technique is indispensable to, in particular, chelonians.¹ A film and film-screen combination used in mammalian radiography is suitable for reptilian patients with high detail mammography films beneficial to evaluate patients less than 1000 g.^1,2 Grids are not necessary for patients with a body weight of less than 10 kg (or less than 10 cm in diameter).^1,2 Both digital and film-screen systems produce images of diagnostic quality for exotics work. The use of general anaesthesia for radiography should be assessed on a case-by-case scenario. Often, necessary positioning can be achieved using simple positional aids, such as Perspex tubes for snakes, beakers, Perspex boxes and masking tape.¹ A nifty trick for imaging the limbs of chelonians is to suspend the patient on an upturned bucket or similar structure. This not only calms the animal, but often encourages most chelonians to hang their legs out of their shell.¹

Contrast radiographic studies prove beneficial to assess mainly the gastrointestinal system.^1-4 However, compared to avian species, these studies are often less diagnostic, largely due to wide variation in transit time between and within species.¹ This variation is due to their poikilothermic nature and as a result the transit time can vary depending on the nutritional status and species of the patient as well as environmental factors such as temperature and season.^1,2 Although barium sulphate or Gastrografin can both safely be used to perform the contrast studies in reptiles, all patients should be stabilised and kept under optimal environmental conditions throughout the procedure.^1,2 Studies have shown that the transit of barium is longer than Gastrografin and there is a further risk of the barium solidifying in the gut and delaying transit even further.^1,2,4 For this reason, Gastrografin is preferred.^1,2 Gastrografin provides good visualisation of the gastric mucosa and is less irritating; however, it is hyperosmolar and should not be administered to dehydrated patients.^1,2

Table 1. Suggested views for the imaging of reptilian species

CrCd: craniocaudal; L: lateral; DV: dorsoventral¹

Basic Radiographic Assessment

Despite one's best efforts, radiographic contrast in exotics is often weak due to a number of factors. Furthermore, excessive scatter, intracoelomic fat bodies and superimposition of organs and structures can hamper organ identification and differentiation.¹

It is pertinent to be familiar with the basic anatomy of the species being imaged in order to aid interpretation as much as possible and anatomical and radiographic atlases for exotics besides one's viewing station becomes indispensible.^1,2 If an unfamiliar species is presented for imaging, if possible, a healthy patient of the same species can be radiographed to act as a reference. Similarly, when imaging appendages, comparison with contralateral limbs serves the same purpose.¹ To confound interpretation further, the position and size of the internal organs in reptile patients is often dependant on the size of the surrounding organs and nutritional status of the patient. This is particularly noted with gravidity and reproductive status. Some species make our lives even more difficult by inflating themselves with air as a defense mechanism, distorting the normal respiratory radiographic appearance.

Selected Species - Specific Imaging Features

Chelonians

Chelonians have paired multichambered lungs which occupy the dorsal three-fifths of the coelomic cavity.^1,2,5 The dorsal surface is adhered to the peritoneal lining of the coelomic cavity and the ventral surface is separated from the coelomic viscera by a "pseudo-diaphragm"; a non-muscular septum horizontale.⁵ The size of the lung field can vary depending on the relative size and position of other internal organs.¹ Interstitial and alveolar diseases are best visualised on CrCd horizontal beam views.¹

Peritonitis can be suspected on radiographic studies with the subtle, nonspecific finding of a granular texture to the coelomic cavity or free fluid, gravitating to the ventral portions on horizontal beam views.¹

As with most other species, GIT obstructions are strongly suspected based on the presence of gas-dilated intestinal structures.^1,2

Most terrestrial species and some terrapins have hard-shelled eggs; however, there are some chelonians which have soft-shelled eggs, which can make identification and differentiation from other masses difficult.¹

Snakes

The lungs are sac-like structures with a large central blood vessel coursing the length of the lung in a ventral to dorsal direction.^1,2,6 Some species (pythons) have a vestigial lung lobe, normally the left lung lobe.¹ It has been suggested that respiratory pathology in snakes is best detected on a lateral view and even then, only gross opacity changes will be detected.¹ Computed tomography is a far superior modality for imaging this system. Radiographic evidence of respiratory disease in snakes only becomes apparent late in the course of disease and very often, if acute pneumonia is noted, then the clinician should rely heavily on the clinical signs and institute therapy.¹

The musculoskeletal system is often the primary reason for requesting radiographs in these species, and fractures, osteomyelitis, osteitis deformans (Paget's disease), spinal osteopathy and neoplasia are some of the more common conditions affecting this species.⁶

Lizards

As with snakes, the lungs are sac-like structures located dorsal to the liver and against the dorsal wall of the coelomic cavity.^1,6 In some species, this organ system can extend extensively caudally occupying a large portion of the coelomic cavity. The heart is poorly visualised and lies in the thoracic inlet in most species.^1,6

Eggs in most lizard species are soft-shelled and can extend to the region of the heart.^1,6 The exception is geckos.¹ Normal eggs may be difficult to differentiate from follicles which can give rise to preovulatory egg binding which can progress to egg peritonitis, if these structures rupture.^1,6 In such cases, ultrasound proves more sensitive to differentiate.

References

1.  Krautwald-Junghanns ME, Pees M, Reese S, Tully T. Diagnostic Imaging of Exotic Pets. 1st ed. Hannover: Schlutersche; 2011.

2.  Schumacher J, Toal RL. Advanced radiography and ultrasonography in reptiles. Semin Avian Exotic Pet Med. 2001;162–168.

3.  Smith D, Dobson H, Spence E. Gastrointestinal studies in the green iguana: technique and reference values. Vet Radiol Ultrasound. 2001;42(6):515–520.

4.  Grosset C, Daniaux L, Sanchez-Migallon Guzman D, Webber ES, Zwingenberger A, Paul-Murphy J. Radiographic anatomy and barium sulphate contrast transit time of the gastrointestinal tract of bearded dragons (Pogona vitticeps). Vet Radiol Ultrasound. 2014;55(3):241–250.

5.  Lim CKL, Kirberger RM, Lane EP, Elliot DL. Computed tomography imaging of a leopard tortoise (Geochelone pardalis pardalis) with confirmed pulmonary fibrosis: a case report. Acta Veterinaria Scandinavica. 2013;55:35.

6.  Banzato T, Hellebuyck T, Van Caelenberg A, Saunders JH, Zotti A. A review of diagnostic imaging of snakes and lizards. Vet Rec. 2013;173:43–49.