Radiology in Birds: Imaging the Possibilities

Key Points

  • High detail film together with appropriate single screen is essential for proper imaging.
  • A grid is generally contraindicated in most avian patients (
  • Develop technique charts utilizing the lowest kilovoltage, the highest milliamperes per second, and the shortest exposure time possible.
  • For the ventrodorsal (VD) view, place the bird in dorsal recumbency with the keel perpendicular to the cassette. Extend the wings bilaterally 90 degrees from the body and extend the legs caudally.
  • For the lateral view, place the bird in right lateral recumbency. Extend the wings dorsally and retract the legs caudally, with the dependent limb slightly cranial to the non-dependent limb.
  • The normal avian lung has a honeycomb appearance on the lateral view. Parts of the air sac space are visible as dark, air-filled areas on VD, and to a lesser extent, lateral views.
  • The cardiohepatic silhouette in the parrot should resemble an hourglass on the VD view. If a vertical line is drawn from the scapula to the acetabula on the VD view, the silhouette should be medial to this line.
  • The proventriculus is seen most clearly on the lateral view. This spindle-shaped structure sits dorsal to the liver on the lateral view. The ventriculus often contains grit and is located caudoventral to the proventriculus.
  • The kidney is seen most readily on the lateral view. The cranial lobe of the kidney sits just below the lumbar spine, cranial to the acetabulum. The gonads lie cranioventral to the kidney, and gonadal swelling may be confused with renomegaly.
  • On occasion, the spleen may be seen on the lateral view as a spherical object, cranial to the femur, just above the level of the proventriculus. When the spleen is visible, this is typically an indication of splenomegaly.
  • Contrast radiography may help to identify the cause of a space-occupying lesion within the coelom.

Introduction

Radiography is a non-invasive, informative tool, to be used alongside other diagnostic information. A specific diagnosis is rarely made with one element of information alone. Perform radiology in all sick patients using at least two views at right angles. Sick birds often suffer from concurrent diseases and some diagnoses may be missed without a comprehensive work-up. A parrot presented off color and weak with polyuria/polydipsia, discolored droppings, and an abnormal fecal gram stain might be considered to have bacterial gastroenteritis, while a radiograph might reveal evidence of heavy metal toxicity.

Preparation: Getting the most out of your radiographs

Equipment

High detail film is essential, although faster film/screen combinations may cause a loss of resolution due to larger crystal size. Use masking tape, blue painter’s tape, or sandbags to position patients.

Technique

Use short exposure time (< 1/60th second), a standard focal film distance of 1.05 m, and be sure to collimate. As avian patients are generally less than 9 cm deep, use of grids to reduce scatter is generally contraindicated. The benefit is outweighed by the loss of definition created by the grid.

Avian radiographs have good natural contrast, so peak kilovoltage (kVp) should be relatively higher and milliamperes per second (mAs) lower. If you increase kVp by 10, you must halve mAs. If contrast is too great (very black and white), increase kVp and decrease mAs. If the image is too ‘grey’, as with ascites, then reduce kVp and increase mAs.

Develop technique charts utilizing the lowest kVp, the highest mAs, and the shortest exposure time possible, and maintain a file of normal radiographic anatomy. Avian veterinarians may deal with more than 9,000 species, and even the most experienced clinician will not be familiar with the finer points of all species.

Positioning

Consistent positioning is vital; therefore most birds should be anesthetized when radiographed. A restraint device may also be useful.

For the ventrodorsal (VD) view, place the bird in dorsal recumbency, with the carina or keel of the sternum perpendicular to the cassette. Position the keel over the spine. Extend the wings bilaterally 90º from the body and extend the legs caudally (Fig 1). If wings and legs are abducted equally, it is likely that positioning will be straight.

For the lateral view, place the bird in right lateral recumbency. Extend the wings dorsally and retract the legs caudally, with the dependent limb slightly cranial to the non-dependent limb. If positioned correctly, the acetabula, ribs, coracoid and kidneys are all superimposed contralaterally (Fig 2).

For the lateral view, place the bird in right lateral recumbency.

Fig 1. To position a bird for the ventrodorsal (VD) view, align the carina or keel of the sternum perpendicular to the cassette and position the keel directly over the spine. Extend the wings bilaterally 90º from the body and extend the legs caudally. Secure the carpus (arrow) with masking tape or blue painter’s tape. Click on the image above to enlarge

Secure the conscious bird from cranial to caudal. First, extend and tape the neck.

Figure 2. Place the bird in right lateral recumbency for the lateral view. Secure the conscious bird from cranial to caudal. First, extend and tape the neck. Extend the wings dorsally and retract the legs caudally, with the dependent limb slightly cranial to the non-dependent limb. If positioned correctly, the acetabula, ribs, coracoid and kidneys are all superimposed contralaterally. Click on image to enlarge.

 

Radiographic evaluation of body systems

Musculoskeletal

The avian skull is fused with numerous interconnecting air-filled diverticula. The largest diverticulum is the infraorbital sinus, which sits rostroventral to the eyes. Scleral ossicles form a bony ring around the orbit, which is visible radiographically. The upper beak forms a joint with the frontal bone via the articular and quadrate bones.

The avian spine shows major variation from mammals. It may be divided into cervical (parrots 12, swans 25), notarium (fused thoracic vertebrae, lumbar vertebrae 1-3), free lumbar (lumbar 4), synsacrum (fused caudal lumbar and pelvic), and the pygostyle (free caudal). Parrots have 8 thoracic vertebrae with complete ribs. The region of the one free lumbar vertebrae (typically lumbar 4) is a natural hinging point and the most common area to suffer trauma or spondylosis. The thoracic girdle consists of the clavicle, coracoid and scapula. These three bones form the triosseum, onto which the proximal humerus articulates. The sternum meets to form the carina (or keel) in the ventral midline (Fig 3). On the wing, the humerus articulates with the ulna and radius. The secondary feathers of the wing insert on the caudal periosteum of the ulna. The ulna and radius articulate with ulnar and radial carpal bones at the carpus, which lead to the major and minor metacarpal bones. The manus has three digits (I – the alula, II – the major digit, III – the minor digit (Fig 4, Fig 5).(Note: Depending on the source, the alula may be referred to as a carpal bone, metacarpal bone, or digit.)

The pelvic girdle comprises the fused ilium and ischium and the unfused pubis. The femur has a small but recognisable femoral head, which sits neatly in the acetabulum. Most birds have four digits. The most medial digit is numbered first. Digit 1 has 2 phalanges, digit 2 has 3 phalanges, digit 3 has 4, and digit 4 has 5 (Fig 6). Hand-reared parrots, particularly African grey parrots (Psittacus erithacus), can suffer from metabolic bone disease.

When radiographing extremities, always image and compare normal and abnormal limbs. With so much interspecies variation, even experienced clinicians can be misled by normal structures. Thoroughly check survey radiographs, especially limbs and ribs. Osteomyelosclerosis (also known as hyperostosis or polyostosis) is defined as increased medullary bone density of the long bones. Minor changes occur in egg laying females, while major changes are seen in birds suffering from hyperestrogenism (Fig 7).

When radiographing extremities, always image and compare normal and abnormal limbs. With so much interspecies variation, even experienced clinicians can be misled by normal structures.

Figure 3. Lateral survey radiograph illustrating the bones of the thoracic girdle (clavicle, coracoid, scapula) as well as the humerus. The sternum meets to form the carina (or keel). Also labeled are the proventriculus, ventriculus, heart, liver, kidney, air sac, and lung. Click on image to enlarge.

The wings are in essentially the same position in the ventrodorsal (VD) and lateral views.

Figure 4.The wings are in essentially the same position in the ventrodorsal (VD) and lateral views. To obtain two radiographic views of the wing, utilize the VD view and the anterior-posterior (AP) view shown here. Click on image to enlarge.

Positioning of the wing

Figure 5. Positioning of the wing for the AP view. Click on image to enlarge.

Survey radiograph from a bird with severe bumblefoot

Figure 6. Survey radiograph from a bird with severe bumblefoot or pododermatitis. The portion of the pelvic limb shown here from top to bottom is tibiotarsus, hock joint, tarsometatarsus, and finally digits. Click on image to enlarge.

Yellow arrows illustrate increased medullary bone density.

Figure 7. Polyostotic hyperostosis is defined as increased medullary bone density of the long bones. Minor changes occur in egg laying females, while major changes are seen in birds suffering from hyperestrogenism due to gonadal tumors or cysts. Red arrows indicate normal bony cortex and medulla. Yellow arrows illustrate increased medullary bone density. Click on image to enlarge.

Cardiopulmonary system

Survey radiography of the trachea shows complete rings. There is marked species variation in tracheal position and length as well as syringeal shape. The syrinx is the avian “voice box” situated at heart base.

On the lateral view the normal avian lung has a honeycomb appearance (Fig 8). Loss of this honeycomb pattern or focal densities indicates likely pulmonary pathology.

Dark, air-filled areas represent parts of the air sac space and are visible on VD, and to a lesser degree, lateral body views (Fig 3, Fig 9).Air sac walls should never be evident radiographically, and the presence of air sac lines is indicative of air sacculitis (past or present).

On the VD view, the avian heart lies between the 2nd and 6th thoracic ribs (Fig 9). In psittacines the heart base is best measured at the level of the 5th thoracic vertebrae, and should not exceed 50% of the width of the thoracic cavity. On the lateral view, heart length should not exceed approximately 50% of sternal length in psittacines. Atherosclerosis may appear as mineralization in the great vessels cranial to the heart.

The lateral view of the normal avian lung has a honeycomb appearance (large arrows)

Fig 8. On the lateral view the normal avian lung has a honeycomb appearance (large arrows). Dark, air-filled areas represent parts of the air sac space (small arrows). Click on image to enlarge.

Liver

The heart – liver silhouette should resemble an “hourglass” or figure eight on the VD view (Fig 9).If a vertical line is drawn from the scapula to the acetabula on the VD view, the heart – liver silhouette should lie on or medial to this line. If the silhouette exceeds this line, this is indicative of organomegaly (Fig 10). Macaws have a relatively small liver when compared to other psittacines.

The avian heart lies between the 2nd and 6th thoracic ribs.

Figure 9. On the ventrodorsal (VD) view, dark, air-filled areas represent parts of the air sac space. The avian heart lies between the 2nd and 6th thoracic ribs. The heart – liver silhouette should resemble an “hourglass”. Click on image to enlarge.

Illustrating widening of the cardiohepatic silhouette (arrows)

Figure 10. Survey ventrodorsal (VD) view illustrating widening of the cardiohepatic silhouette (arrows) Click on image to enlarge.

Digestive system

The crop lies to the right of the midline, cranial to the thoracic inlet on the VD view (Fig 11). The proventriculus is seen most clearly on the lateral view. This spindle-shaped structure sits dorsal to the liver on the lateral view (Fig 12). The ventriculus often contains grit and is situated caudoventral to the proventriculus (except in ostriches in which it is anterior). The intestines occupy the dorsocaudal portion of the abdominal cavity.

If the bird is positioned symmetrically and the cardiohepatic shadow does not appear symmetrical, then a further extension on the left side is indicative of proventricular dilation. Dilation of any part of the gastrointestinal tract is abnormal (unless dilation is due to the presence of a cast or pellet in a raptor).

The crop lies to the right of the midline (arrow)

Figure 11. The crop lies to the right of the midline (arrows), cranial to the thoracic inlet on the ventrodorsal (VD) view. Click on image to enlarge.

The proventriculus is seen most clearly on the lateral view

Figure 12. The proventriculus is seen most clearly on the lateral view where this spindle-shaped structure sits dorsal to the liver on the lateral view (small arrows). The ventriculus often contains grit and is situated caudoventral to the proventriculus (large arrows). Click on image to enlarge.

The proventriculus:keel ratio (PKR) is one method used to differentiate a normal proventriculus from an enlarged proventriculus (Dennison 2010, Dennison 2008). To determine PKR, first identify the pertinent landmarks including the serosal margin of the proventriculus, the thoracosynsacral junction, identified radiographically by following the synsacrum to its end, and the keel of the sternum. Measure proventricular diameter perpendicular to the long axis of the proventriculus just at the level of the thoracosynsacral junction. Measure the height of the keel immediately caudal to the superimposition of the corocoid onto the sternum, perpendicular to the dorsal margin of the sternum at that site (Dennison, 2008). Normal PKR is less than 0.48 and birds with reported proventricular disease have a PKR of 0.52.

Urogenital system

The kidney is seen most readily on the lateral view. The cranial lobe of the kidney is apparent below the lumbar spine, just cranial to the acetabulum. Just cranioventral to the cranial lobe of the kidney lies the gonads. Gonad swelling may be confused with renomegaly (Fig 13). Dehydration causes deposition of radiopaque urates in the kidneys, which can be confused with renal calculi or renal mineralization. If dehydration is suspected, the patient should be given copious fluid therapy and re-radiographed the following day.

The cranial lobe of the kidney is apparent below the lumbar spine, just cranial to the acetabulum (arrows)

Figure 13. The kidney is seen most readily on the lateral view. The cranial lobe of the kidney is apparent below the lumbar spine, just cranial to the acetabulum (arrows). Just cranioventral to the cranial lobe of the kidney lies the gonad, and gonadal enlargement may be confused with renomegaly. Click on image to enlarge.

Spleen

On occasion, the spleen may be seen on the lateral view as a spherical object, cranial to the femur, at mid-abdominal cavity height, just above the level of the proventriculus. The spleen should not be larger than 1.5x the diameter of the femur. When the spleen is visible, it is typically an indication of splenomegaly, the most common cause being chronic antigenic stimulation generally due to psittacosis (Fig 14).

When the spleen is visible on the lateral view, this is typically an indication of splenomegaly.

Figure 14. When the spleen is visible on the lateral view, this is typically an indication of splenomegaly. The spleen sits cranial to the femur, at mid-abdominal cavity height, just above the level of the proventriculus (arrows). The spleen should not be larger than 1.5x the diameter of the femur. Click on image to enlarge.

Organomegaly

A space-occupying lesion will expand at the expense of the air sac space. Loss of coelomic space will be seen as air sac compression, loss of the hourglass cardiohepatic silhouette, and possibly loss of the radiolucent space between the proventriculus and the kidney. An abdominal space-occupying lesion may be physiological (e.g. distended oviduct and ovarian follicles prior to egg laying) or pathological (e.g. egg bound, organomegaly, egg peritonitis, neoplasia).

In any such case, gastrointestinal barium contrast will delineate the position of the gut in relation to other organs. If plain radiographs show organomegaly of unknown cause, measure tubing such as a red rubber catheter and mark the appropriate distance to the proventriculus while the patient is still anesthetized. Pass the tube per os down into the proventriculus. Express 6 ml/kg of barium into the proventriculus, continuing as one gradually removes the catheter. This technique will highlight the proventriculus, distal esophagus, and crop, with minimal risk of aspiration, as would be anticipated if the barium where only placed into the crop.

Note: All photographs provided by Dr. Christal Pollock.

References