Radiography is a routine diagnostic procedure in veterinary practice. Most of the radiographic techniques that are used for dogs and cats are applicable to birds. Birds contain considerable amounts of air that makes their radiographs even more revealing. All the x-ray machines in small animal practices will be able to produce adequate exposures for avian work. The majority of radiographs can be produced with 50 or 55 kV, which will give good contrast when using a cassette with intensifying screens. A high mA is useful as it will allow fast exposure times but perfectly adequate radiographs can be produced with 20 mA and exposure times of 0.1 to 0.12 seconds, provided that modern intensifying screens are used.

Non-screen film can be used but increases the exposure time. The screens that are used for radiographs of the extremities in man, cats and dogs are also perfect for birds. Fast screens, which are suitable for larger dogs etc., lose a lot of the detail that is vital for avian radiology. Mammography cassettes, screens and film are also very useful and give excellent detail but processing the film is more critical than usual.

Positioning

Perfect positioning is vital to produce a diagnostic radiograph. Poor positioning will produce radiographs that are often non-diagnostic and sometimes dangerously misleading.

Lateral and ventrodorsal whole body radiographs should be used as part of the routine examination of most ill birds. The lateral view should have the wings extended dorsally and the legs extended caudoventrally. Beware: with old or caged birds that are not used to flying, it is possible to fracture the humerus when attempting to draw the wings into the correct position for a lateral view.

Usually two views are sufficient one view taken 90° from the first. Correct positioning may be difficult with small birds or some views. Special positioning devices, such as Plexiglas screens, can be used to restrain a conscious bird. These are sometimes useful but often the positioning is inadequate and there is some risk to the struggling patient. A general anaesthetic using isoflurane and oxygen is best. Even very ill or dyspnoeic birds can be more safely restrained with an anaesthetic than manually. Sandbags, zinc oxide tape, or suction-attached ties are useful for positioning. Occasionally very ill birds require examination for lead in their gizzard. Wrapping the bird in a towel or putting it in a cloth bag allows the clinician to obtain a radiograph that shows the presence or absence of lead.

Respiratory system

The trachea terminates at the syrinx. The syrinx leads into the primary bronchi, which are not usually seen. The lungs are visible on both views in a normal bird. The lungs overlie each other on a lateral view. The parabronchi can be seen with careful radiography and gives a guide to the diseases status of the lung. On a lateral view the parabronchi look like a sponge; on the ventrodorsal view, a thumb print. The air sacs are visible surrounding the heart and abdominal viscera as dark, air-filled spaces. The clavicular air sac can be seen just below the shoulder joint, again dark against the denser surrounding muscle because of the air contained in it.

Pneumonia and air sacculitis can be seen on both lateral and ventrodorsal views. The detail of the parabronchi is lost and the lung structure looks poorly defined. Abscesses in the lung may show as areas of increased density. In cases of airsacculitis the air sacs become more dense and when there is gross pathology the walls of the air sacs thicken visibly. A bubble like appearance is typical of aspergillosis.

Cardiovascular system

The heart is visible on both views. The main arteries appear as a pair of twin dots cranial and also lateral to the heart on the ventrodorsal view. On the lateral view these blood vessels are much more recognisable. Enlargement of the heart or calcification of the major arteries can be seen when present. Oedema due to congestive heart failure is impossible to differentiate radiographically from the effusion caused by a visceral tumour, purulent air sacculitis, or an enlarged liver. In these cases it is important to determine the position of the liver and alimentary tract. Ultrasonography may be useful. If this is not available then paracentesis is required. The fluid should be removed before anaesthesia if possible or there is a great risk that the fluid will enter the lungs and drown the bird.

Coelomic contents

The liver can be seen best on a ventrodorsal view. In many birds the distance from the sternum to the edge of the liver should be half the distance between the sternum and the body wall. If the bird has eaten recently the food-filled proventriculus and gizzard (or pathologically dilated proventriculus) will spread the liver laterally and make it appear enlarged. This must be differentiated from pathological enlargement of the liver. A clinical history is useful but a barium meal will give a final answer.

The proventriculus is seen most clearly on the lateral view. It can be distended normally with food but will enlarge and distend pathologically with a number of conditions. Barium will often help differentiate these conditions. Most pathological distensions slow or stop the passage of food through the gut. Increased time for the passage of ingesta is only an indication that the bird is ill and is not necessarily diagnostic of any particular disease.

The gizzard is visible on both lateral and ventrodorsal views. The muscular, grinding gizzard normally contains grit. This will show on plain radiography very clearly. The grit can reflux into the proventriculus in a normal anaesthetised bird. Lead is sometimes ingested and in this case it will collect in the gizzard. Birds that have lead in their gizzard will always be suffering from lead poisoning and should be treated but the absence of lead does not rule out lead poisoning. Lead is much more radio-opaque than grit. Galvanised wire, and other metallic objects will also cause poisoning if retained in the gizzard.

Often the intestines are seen as a mass in which only small amounts of detail are visible, occasionally they are air-filled. The cloaca can be demonstrated most obviously on the lateral view and can be seen to contain urates in some individuals.

The spleen is usually only visible on the lateral view, but can also be seen on the ventrodorsal view if very enlarged. A slightly oblique ventrodorsal view is often useful to show the spleen. Avian tuberculosis, lymphoma and chlamydial infection will all dramatically enlarge the spleen.

The kidneys are seen very easily on a lateral radiograph but overlie each other. They can be differentiated into their various divisions on the V-D view but are overlain and somewhat obscured by the intestines. Sometimes they are very distinct on the V-D view and they may well be enlarged and more dense in these cases. Urates may be seen in the kidney as fine white lines.

Occasionally the gonads of either sex are obvious just ventrocranial to the kidney on the lateral view.

Extremities

After an initial survey radiograph it is often useful to examine the area of concern with a small 'coned down' view. The exposure will vary for different areas of the limb in the same bird: the shoulder will require a different exposure to the elbow. Because of the use of fine definition screens there is also a lot of useful soft tissue detail visible on limb radiographs.

Osteopathology

There are several changes that occur in bone that are specific to certain diagnoses.

In cases of osteomyelitis there is usually a clear-cut lysis of bone, as opposed to the more diffuse changes in mammals. The pus is not liquid and does not drain. The abscess enlarges to form an expanded outline within the bone, especially if it has formed within a marrow cavity.

Septic arthritis is sometimes seen in joints, especially the phalangeal or carpal joints. It will cause lysis of the articular surface; the edge of the lesion is not clear cut, having a roughened outline; and the bone beyond the septic arthritis has often disappeared. Joint fluid containing heterophils and monocytes is needed to give a certain diagnosis.

Tumours usually give an indistinct outline of gradual osteolysis as they infiltrate. There is often deposition of bone as well. With bone tumours in birds, as in mammals, the changes do not cross the joint.

Periostitis causes a fuzzy bone proliferation. It appears to be caused by infection as in most cases there is a marked heterophilia and the periostitis slowly resolves following antibiotic therapy.

Osteoarthritis gives the same periarticular changes that are seen in mammals, however this condition is seen very much less frequently and is usually a sequel to joint injury. Some elderly parrots have osteoarthritis in their knee joints.

In young growing birds osteodystrophy is usually accompanied by folding fractures. In older growing birds it will show misshapen bones, especially of the pelvic limb, with an increased tendency to fracture. Juvenile osteodystrophy will cause permanent bony deformities. In adult birds osteoporosis due to malnutrition and often excessive egg production causes loss of bone density, gross irregularity of the cortices of long bones and multiple fractures. Early or mild osteoporosis is much more difficult to diagnose with certainty. Comparison with the surrounding soft tissue and great confidence in both the radiographic technique and the processing are required to make the diagnosis.

Malunion of fractures is usually atrophic and shows as rounding of the ends of the fracture and no periosteal proliferation.

Contrast Techniques

Barium is very useful when given orally, either as a diluted liquid or mixed with food such as liquid baby food. Both these preparations should be given using a cannula directly into the crop or by endoscope into the proventriculus. The passage of a barium meal is best followed using a specially constructed box with a single perch, a plastic front and provision for the cassette to be positioned as the rear wall. The potentially poor positioning is far outweighed by the fact that the bird does not need repetitive anaesthesia during the barium meal. It is always possible to anaesthetise the bird to take a 'better' radiograph if it is needed but great care must be taken to prevent inhalation of the liquid barium still in the bird's gizzard. Within 5 minutes a normal parrot or bird of prey should have barium in its crop, proventriculus, gizzard, and even into the small intestine. A lot of ill birds will take 30 minutes to move the barium this far. It is impossible to differentiate liver enlargement from alimentary tract enlargement without a barium meal. Barium will also show the displacement of viscera and therefore will give some indication as to the reason for this. This technique is very useful for showing tumours of the kidney or gonad and differentiating the other causes of abdominal distension: especially frequent in the budgerigar.

Intravenous Iohexol (Omnipaque) can be used for an intravenous pyelogram -1 ml given iv to an Amazon will give a good flow of contrast through the ureters in 5 to 10 minutes of administration.

References

1.  Atlas of Diagnostic Radiology of Exotic Pets, Rubel G., Isenbugel E., and Wolvekamp P. 1991 Wolfe Publishing Limited, London.

2.  An Atlas of Radiographic Anatomy, Smith S A and Smith B J. 1992 W B Saunders and Co. Philadelphia.

3.  Krautwald-Junghanns M-E and Trinkaus K. (2000) Imaging techniques Chapter 4 In Avian Medicine. Tully, Dorrestein and Lawton eds.) Butterworth Heinemann UK pp. 52-73.