Introduction

As with most exotic species, imaging avian species can be challenging due to their small size, rapid respiration (initiating subtle motion) and interspecies anatomical variation. This communication will focus on radiographic principles to optimise imaging and basic interpretive principles, once diagnostic images are obtained.

Radiographic Principles

The key to obtaining diagnostic quality images is highly detailed radiographic studies.¹ Some people falsely believe that due to the small nature of the patients, lower output machines will suffice for such studies, but this is actually the opposite. High-power x-ray machines with an output of at least 300mA and a kilovoltage range of 40–90 kVp which can be adjusted in 2 kV increments is ideal.^1,2 Although the range of kV mentioned above is ideal, the anode voltage is often kept between 45–55 kV to allow for short scale radiographic technique which provides a high degree of contrast and wide grey scale range.^1,2 Additionally, due to the fast respiration of bird, extremely short exposure times are warranted (0.015–0.05 s). To further optimise radiographic detail, fine film screen combinations should be used. Mammography film can also be used for this purpose. A grid is not used even in birds with a width of more than 10 cm as the air sacs minimally attenuate x-rays to generate scatter radiation.^1,2

Digital radiographic systems are becoming popular amongst private practice, slowly replacing film-screen radiography in veterinary practice. Although digital systems operate at a higher kV and mAs, special algorithms are available from manufacturers that are applicable to avian species. Studies have shown that digital systems are equal, if not superior, to film-screen systems for avian studies.³

Patient Preparation

If the patient is not compromised, then starving for 2 hours (small birds) and 3–5 hours (large birds) is recommended. This is advocated as the stress of manual restraint as well as anaesthesia can increase the risk of regurgitation and aspiration pneumonia. If the digestive tract is full on radiographs, this can also hamper interpretation and lead to misdiagnosis of certain conditions.^1,2

Despite the stress associated with anaesthetising the birds for the studies, the risk and stress levels are less than experienced during manual restraint. GA also allows for improved positioning which ultimately leads to less retakes. To further facilitate good-quality studies, the use of easily obtainable positional aids is encouraged.^1,2 If manual restraint is needed, strict adherence to safety guidelines is warranted and should be enforced amongst all staff.

Radiographic Projections

Precluding a compromised patient condition, it is always advocated to take two radiographic projections at orthogonal angles. Most often the standard view for imaging the coelomic cavity is a laterolateral and a ventrodorsal view.^1,2 Although either lateral projection is sufficient, many clinicians prefer a right lateral view.¹ For pectoral extremity studies, mediolateral and craniocaudal views are suggested; however, very often the coelomic cavity and wing views can be conducted in one study.^1,2 Laterolateral and ventrodorsal views are standard for the head/skull with additional oblique views depending on the pathology present.¹ The use of foam wedges, sponges and tape can facilitate symmetrical positioning of the patient without the use of manual restraint.¹

In severely compromised patients or in certain clinical scenarios, such as when searching for metal ingesta, an "exploratory" radiograph can be made. For this technique, a horizontal beam radiograph with the patient perched on a stand may provide adequate information to guide further diagnostics.

Basic Radiographic Assessment

Below, the basic radiographic anatomy of psittacine birds will be discussed, with further case studies and clinical cases being discussed during the contact session.

Skeletal System

There is some variation in the anatomy amongst species and thus it encourages to invest in a good radiographic anatomy atlas to have close at hand when viewing images of unfamiliar species.

In order to aid in flight, flighted birds have a high degree of pneumatised bones in the proximal skeleton. These include the humerus, coracoid, parts of the rib and sternum, pelvic girdle and femur. The appearance of pneumatised bone radiographically is thin, fine cortices and a network of intramedullary bony trabeculae. The ulna is the prominent antebrachial bone in birds, and there is extension fusion and reduction in the bones of the wing and foot.¹

During reproductive activity, female bones have an increased bone density of the femur and other long bones due to high circulating level of oestrogen.^1,2

Cardiovascular System

The heart is situated on the VD projection between the 2nd and the 5th–6th ribs and together with the liver silhouette, the cardiohepatic silhouette has an hourglass appearance.¹ The width of the heart should not exceed 51–61% of the maximal width of the thorax. The heart apex and cranial liver silhouette are superimposed on the lateral view in most granivores except cockatoos, where the apex can be differentiated from the liver due to the position of the air sacs. On both the lateral and VD views the major blood vessels that can be seen are soft tissue tubular structures, and in older patients, calcification of the blood vessel walls can lead to increased visualisation.¹

Respiratory System

The trachea is clearly visualised on both lateral and VD projections with the syrinx located at the terminal portion of the trachea at the bifurcation into the bronchi. The lungs of bird are nonelastic structures which can be seen as honeycomb-like shadows on the lateral views. Due to the fact that birds do not have a diaphragm and lung lobe volume is constant, taking views at maximum inspiration is not possible.¹

Birds have an extensive air sac system which is usually not radiographically visible unless pathological. There are nine air sacs.

Liver

As discussed previously, the liver and cardiac silhouette on the VD view present an hourglass shape. The size and position of the liver can vary depending on the nutritional status of the bird and whether ingesta is present in the proventriculus or not. Generally, on a VD view, the lateral margin of the liver should not extend beyond an imaginary line drawn between the coracoid and the acetabulum.¹

Spleen

The spleen is easily visualised as a bean- or egg-shaped, soft tissue opacity dorsal to the proventriculus on the lateral projections. Superimposition on the VD views precludes its visualisation unless pathological.¹

Gastrointestinal Tract

There is some species differentiation with regard to the visibility of the GIT; however, in the psittacine, the crop (ingluvies), proventriculus, ventriculus and small intestines can all be visualised on lateral views. In some instances, grit (normal ingesta) in the ventriculus facilitates its localisation.^1,2,4

Further imaging of the gastrointestinal tract is facilitated by utilising contrast studies. Due to the rapid transit time of ingesta, these studies can often be successfully completed. It is advisable, if clinically possible, to withhold food prior to contrast administration and that the crop and proventriculus are empty. Food in the digestive tract will degrade the detail visualised at the interface of the contrast and the GIT mucosa and food in the crop and proventriculus prior to the study will limit the volume of contrast that can be administered, a factor that can affect the efficacy of the study.^1,2,4

Due to the risk of metabolic and fluid imbalances in birds, the use of Gastrografin is not recommended, and a dose of 20 to 50 ml/kg barium sulphate 30% weight to volume is advised by most texts. However, barium is contraindicated in cases of suspected perforation, and in these cases iodine-based contrast agents should be used with caution.^1,2,4

References

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

2.  Silverman S, Tell LA. Radiology of Birds: An Atlas of Normal Anatomy and Positioning. 1st ed. Missouri: Saunders Elsevier; 2012.

3.  Bochmann M, Ludewig E, Krautwald-Junghanns ME, Pees M. Comparison of the image quality of a high resolution screen-film system and a digital flat panel detector system in avian radiography. Vet Radiol Ultrasound. 2011;52(3):256–261.

4.  Vink-Nooteboom M, Lumeij JT, Wolvekamp WTC. Radiography and image intensified fluoroscopy of barium passage through the gastrointestinal tract in six healthy amazon parrots (Amazona aestival). Vet Radiol Ultrasound. 2003;44(1):43–48.