An X-ray is electromagnetic radiation, similar to radio/TV waves and visible light. However, the wavelength is shorter and the energy higher, allowing X-rays to penetrate matter, dependent upon density. Some X-rays will be absorbed, others pass through unchanged.

A radiograph is an image of the number and distribution of X-rays passing through a patient, recorded on photographic film, consisting of silver-containing photographic emulsion bound to a transparent base. X-rays react directly with the emulsion, resulting in precipitation of metallic silver (black colour) when the film is processed. In practice, however, most silver precipitation is usually produced by light emitted from an intensifying screen that fluoresces when struck by X-rays. These intensifying screens are used because the film emulsion is more sensitive to visible light than X-rays.

The degree of blackness of a radiograph is directly related to the number of X-rays that reach the film/intensifying screen. Film areas lying beneath body parts that absorb many X-rays (e.g., bone) appear white; those beneath parts that absorb few X-rays (e.g., lungs and air sacs) appear black; between these extremes are a range of grey tones. Many people use jargon terms such as 'X-ray', 'film', 'plate' and 'picture' when referring to a radiograph. This practice is incorrect and should be avoided.

Preparation for Taking a Radiograph

It is essential to practise good radiographic technique, but not always possible to obtain perfect films. The aim is to ensure all films are of diagnostic quality, reducing the need for additional exposures and the associated radiation hazard to the patient and operator. Consider the following:

Exposure Factors

 Subject--depth/type of tissue to be penetrated

 Film-focus distance--influences image size

 Film/intensifying screen combination within the cassette

 kV setting--influences penetration power

 mA setting--influences quantity of X-rays produced

 Exposure time--influences movement blur and quantity of X-rays produced

The last two are often combined as a single mAs factor. Increasing the mA means you can use a shorter exposure time.

The following features are generally required for avian radiography:

 Tissue depth is invariably <10 cm. No grid required

 A standard film-focus distance of 40 inches (101.6 cm). (In small birds, a film-focus distance of 30 inches and halving the mAs may improve the radiographic image.)

 High detail, film-screen combination--mammography is generally preferable

 X-ray machine, capable of >300 mA

 Short exposure times (<1/60th second)

Although mammography combinations (single emulsion film/single screen) result in finer detail, they do require increased exposure, compared to double emulsion/double screen combinations. Utilise the lowest kV, the highest mA and the shortest time possible. Compile a standard exposure chart.

Restraint Technique

Manual restraint should be avoided due to human health and safety issues. The only radiographic view that might require manual positioning is the caudocranial (posterior-anterior) view of the wing. Appropriate protective clothing must be worn and all parts of the human body kept out of primary beam. Conscious restraint involves using devices can be used to position or restrain conscious birds. Anaesthetic restraint is almost always recommended.

Positioning Aids

 Sheet of perspex

 Tape--masking tape (not sellotape or zinc oxide)

 Foam blocks/wedges

 Loosely filled sandbags

Film Processing

 Manual development

 Automatic development

 Digital processing (very useful as the image can be magnified digitally to help evaluate small lesions)

Viewing Requirements

 Radiograph viewer (preferably at eye level)

 Focal bright light source

 Darkened room

 Black-out cards

Film Identification

 Lead tape

 Light marking

 Paper labels

Positioning Birds for Radiography

Whole body lateral and ventrodorsal (VD) views are an accepted standard and part of the routine examination of most ill birds.

VD, Whole Body View

Wings are extended and taped laterally at 90 degrees to the body; legs are extended and taped caudally (tapes attached at tarsometarsus); sternum (keel) is aligned directly over spine, at 90 degrees to the cassette.

Lateral, Whole Body View

Usually right recumbency is used. Both wings are extended and taped dorsally, with the left (non-dependent) wing restrained slightly caudal to the right; both legs are extended and taped caudo-ventrally, secured individually, to reduce body rotation, with the right (dependent) leg slightly cranial to the left leg. When positioned correctly, the acetabula, coracoid, ribs and kidneys should be superimposed. Beware that in old, caged birds it is possible to cause injury when drawing the wings into the correct position for a lateral radiograph. A small foam block between the humeri prevents overextension of the left wing.

Head/Skull

Full assessment will require left-right lateral, right-left lateral, DV, VD and rostrocaudal (frontal sinus) views. The avian neck is very flexible. Inhalation masks and endotracheal tubes may need to be temporarily removed for film exposure.

Wings

VD (mediolateral) view is easy. An extended caudocranial Cd-Cr (posterior-anterior) view is important, but more difficult to achieve. The bird's head needs to be against the cassette and the wing extended parallel to the cassette.

Legs

Cr-Cd and lateral views are required.

Feet

Dorsoplantar and lateral views are required. Individual toes are positioned using tapes.

X-Ray Beam

The X-ray beam must be collimated to the patient size to reduce scatter, and radio-opaque right/left markers appropriately positioned. Following the initial survey radiographs, it is often useful to examine a particular area with further, 'coned down', views. Any particular area of interest should be as close to the cassette as possible. When radiographing the extremities it is vital to radiograph the normal and abnormal limbs, to enable comparison; there is much interspecies variation and normal structures may mislead.

Introduction to Avian Radiology

Important points to remember:

 There are numerous interconnecting sinuses in the skull, the largest of which is the infraorbital sinus (rostroventral to the eyes).

 The avian spine is markedly different to the mammalian spine. The major divisions are:

 Cervical vertebra

 Notarium (fused thoracic and lumbar 1-3)

 Free 4th lumbar vertebra 4

 Synsacrum (fused caudal lumbar and pelvic vertebra)

 Pygostyle (free caudal vertebra)

 The pectoral girdle comprises the bilaterally fused clavicle, the scapula and the coracoid.

 The humerus articulates with the ulna and smaller radius. The ulna and radius articulate with the ulnar and radial carpal bones at the carpus, which leads to the metacarpus. The manus has three digits:

 I, the alula

 II, the major metacarpal

 III, the minor metacarpal

 The pelvic girdle comprises a fused ilium and ischium and an unfused pubis.

 The femur articulates at the stifle with the tibiotarsus, which in turn articulates with the tarsometatarsus at the simple intertarsal joint. Most birds have four digits. Digit 1 has two phalanges, digit 2 has three phalanxes, digit 3 has four phalanges and digit 4 has five phalanges.

 The trachea shows complete cartilage rings. The normal lung shows a honeycomb appearance on lateral views. Most birds have nine air sacs, which surround the heart and viscera. Air sac membranes should not be visible radiographically.

 On VD views, the heart should not exceed 50% of the width of the thoracic cavity and the heart-liver silhouette should resemble a figure-of-eight shape.

 The maximum liver width should not be more than 50% the maximum body width at the same point and the outline should be reasonably symmetrical.

 The crop is just cranial to the thoracic inlet, to the right of midline on VD. The proventriculus is to the left of the liver on VD and dorsal to the liver on lateral. The ventriculus is caudoventral to the proventriculus on lateral views.

 The spleen is often visible on the lateral view, just dorsal to the proventriculus. It is considered enlarged when its diameter exceeds 1.5x the diameter of the femur.

 The kidneys overlie each other and are visible on the lateral view, ventral to the lumbar spine, cranial to the acetabulum. Sometimes, the cranial poles are distinct on the VD view.

 The gonad and cranial pole of the kidneys may be superimposed on the lateral view, or the gonad may be just ventrocranial to the kidney.

Contrast Techniques

Barium studies are very useful when diagnosing gastrointestinal (GI) disease. Barium is delivered by crop tube, or direct into the proventriculus by catheter. The passage of a barium meal may be followed conscious, using a specially constructed box for restraint; positioning may be poor, but this avoids the need for repeated anaesthetics. If a bird is anaesthetised to achieve a better positioned radiograph, extreme care must be taken to avoid inhalation of the barium. In a healthy psittacine or raptor, barium delivered by crop tube will be in the proventriculus, ventriculus and maybe even the small intestine within 5-10 minutes. Barium is also used to highlight displacement of viscera in small birds, therefore differentiating causes of coelomic distension.

Common Problems/Film Faults

The most common problem encountered in avian radiography is poor positioning. At best this produces radiographs that are non-diagnostic and at worst can be very misleading. Film faults are as for radiography of other species.

Health and Safety

Ionising Radiation Regulations and the Radiology Local Rules must always be followed. Personal dosimeters must always be worn.