Contrast Application in Exotic Species

Contrast application in exotic species.

Annemie Van Caelenberg, Ugent

As recently exotic species are more commonly seen as a pet, medical imaging techniques as x-rays and CT-scan are also more frequently performed in this kind of animals. With x-rays the basic roentgen and cheap modality that is routinely used in veterinary practice. CT-scan is an axial and more advanced roentgen technique to visualize especially hard tissues as bones and teeth. However, when contrast medium is used, also soft tissue structures can be evaluated in detail. Disadvantages of this modality is the need foranaesthesia, the higher costs and the availability. Moreover, this examination asks for experienced radiologists to get a correct performance and diagnosis.

The specific anatomy of the exotic species is an important factor with a prominent impact on the imaging: eg. the presence of wings and the lack of a diaphragm in birds, a partial or no diaphragm in reptiles, the presence of an exoskeleton (carapax, plastron) in turtles, different shape of the entire body in snakes, ...

Also the different physiologic characteristics have specific consequences: birds and small mammals have a high metabolic rate, while this is low in others eg. reptiles. These are important factors for the respective medical imaging examinations (artifacts due to movement, contrast transit time,...).

As in general exotic species are very sensitive to stress, it's really important to work fast and efficient. In case of x-rays the use of/need for anaesthesia can be discussed. In literature CT -scan on conscious animals is described recently. So can snakes be positioned extended in a tubular device or curled in a plastic box. Rabbits can be positioned in a plastic device and for avian species a plexiglass restraint plate plus head covering can be used.

More specific for small mammals, the main indications for CT-scan are dental and skull pathology, metastatic lung disease and vertebral pathology. Usually the animals are positioned in dorsal or ventral recumbency.

Also birds are positioned dorsally or ventrally during the CT examination. The main indications for this kind of animals are visualization of the skeletal system (especially the cranium and the spine) and the respiratory tract (sinus and lungs). Frequent avian pathology are fractures of the spine and cranium (especially the hyoid bones and beak apparatus), changes in the size of musculoskeletal structures, soft tissue anomalies of the head (upper respiratory infection; control of effect of treatment). As in other species individual cases/indications are possible.

Differences can be made for the CT-scan of several types of reptiles: chelonians and lizard can be scanned completely, while snakes are usually partially examined. Chelonians can be "wrapped" before the examination, so only the head is free; this prevents movement of the limbs while the CT-scan can be performed without any use of sedation or anaesthesia. The main indications for CT-scan in reptiles is multiple. The most important one is the skeletal region: metabolic bone disease, mineralization, fractures of the limbs or exoskeleton, infection of bones or joints, neoplasia, musculoskeletal structures. Also the respiratory tract is a frequent indication: foreign body, obstruction, pneumonia, neoplasia-abscesses-granuloma, haematoma, lungfield perforation. The examination of the GI tract can be interesting to evaluate several pathologies: constipation, foreign body, neoplasia. CT-scan of the liver can be indicated to confirm hepatic pathology:eg. fatty liver, congestion, infection, abscesses, neoplasia. Scanning the urogenital tractcan be done to exclude the presence of follicles and eggs, renomegaly and mineralization, the presence of concrements and stones in the allantois.

The parameters and protocols used for CT-scan of exotic species are dependent on several factors as the kind of species, general condition, age, weight, metabolic rate, anaesthesia... The mean parameters used during the examination are: 120 KvP, 130 mA, a slice thickness of 6mm and an interval of 3mm. The algorithm used is mainly depending on the suspected pathology and the contrast use (bone window, soft tissue window, lung window,...)

In general the contrast use in exotic species is comparable to this in small animals with a native scan before the contrast administration and a post-contrast series reply. The type and use of contrast medium depends especially on the indication eg. inflammation, neoplasia, obstruction.

Iodine contrast is generally injected intravenously with the port species specific, while barium sulfate is choosen for use per os/per cloaca in radiology (CT-scan less likely) to visualize the GI tract.

In small mammals (rabbits, guinea pigs, chinchillas, rats, mice,...) barium sulfate is admitted per os (10-20ml/kg) to visualize the GI tract. The main indication is a gastrointestinal obstruction. Other indications are oesophageal pathology and intestinal displacement (due to masses or diaphragmatic herniation). Iodine contrast medium is especially intravenously (2 ml/kg; v. auricularismedialis, v. cephalicaantebrachii, v. saphenalateralis, v. coccygica) used for inflammation or neoplasia. In cases of GI pathology with suspicion for perforation iodine contrast can be admitted per os.

In avian radiology barium sulfate is applied per os (25-45% suspension; 20 ml/kg) to visualize the GI tract. The x-ray timing is depending on several factors: eg. condition, age and size of the patient. Also the type of bird is important: eg. garnivore, fructivore or carnivore, with the consequences on the length of the gut. Alternatively iodine contrast is used for GI tract examination if perforation is suspected. A dose of 10 ml/kg (250mg I/ml product) is administrated per os. However, usually the iodine contrast (600mg I/kg) is given intravenously (ulnar vein/intra osseus).

In reptiles barium sulfate is especially used per os if GI pathology is suspected. A dose of 20 ml/kg (35% solution) is administrated. However some factors such as the nutritional status, the temperature andthe season has to be evaluated. Also the kind of species is important as the intestinal passage is longer in herbivorous patients (eg. iguana vs. turtle). Iodine contrast is mainly used IV (v. coccygica, v. jugularis); with few and different doses mentioned in literature (1-2 ml of 240-300mg I/ ml; 2 ml/kg). Double contrast is also described for visualization of the wall of the upper and lower GI tract.