Clinical history.
The cat was presented with a purulent left nasal discharge and sneezing. No other clinical symptoms were present. Two 10 day courses of oxytetracycline 50 mg (Oxycare, Animal Care) one twice daily were prescribed, but after completion of each course the discharge returned. A further course of ampicillin 25 mg/ml (Ampicillin, Kent) 2mls twice daily also failed to resolve the discharge and 5 weeks after treatment commenced the cat was booked for radiography of the nasal chambers.

Restraint for radiography.
This was achieved by general anaesthesia. 

Choice of film/screen.
Agfa Dentus M2 5cm x 7cm non-screen dental film was selected, firstly for it's high definition which is necessary for examining any changes in the nasal turbinate bones. Secondly, this is a non-screen film which is supplied in a light proof plastic envelope, enabling it to be inserted into a cat's oral cavity.

No grid was necessary for this radiography as the tissue depth was not greater than 10cms.

Area radiographed and positioning.
Intra-oral dorso-ventral nasal chambers.

This projection was achieved by placing the cat in sternal recumbency. The head and neck were extended and the head placed on a foam pad, in order to bring the hard palate parallel to the table and perpendicular to the vertical primary beam, therefore minimising any distortion of structures on the radiograph. The forelegs were pulled caudally and positioning of the head along it's longtitudinal axis was checked in order to achieve a true dorso-ventral projection.

The envelope containing the film was carefully placed, one corner first, into the oral cavity dorsal to the endotracheal tube and pushed as far caudally as possible. This film, despite not being in a cassette, has a back and front and must have the front of the envelope presented to the primary beam.

A right marker, X-ray ref. no., animal/owner details and date were placed on the film using X-RITE tape.

All personnel safety precautions taken during radiography are described in Appendix 1.

Centering and collimation.
The primary beam was collimated to the edge of the film envelope and centred midway along the maxilla above the palatine fissure.

The following exposure was used:

Radiographic appraisal.

Positioning
This is not a true dorso-ventral projection, as the skull is slightly rotated to the right - the right nasal chamber appears slightly smaller than the left. This cat proved very difficult to position visually as the cat's head was not symmetrical, due to two previous road traffic accidents both involving injuries to the head. The first resulting in loss of teeth and left eye enucleation, the second resulting in loss of further teeth, fracture of the mandibular symphysis and luxation of the mandibular joints.

Care has to be taken when placing the film into the oral cavity as it is sensitive to pressure and teeth marks will result in unexposed lines across the radiograph.

Centering
The primary beam was actually centered rostrally over the left nasal chamber. Correct centering for this projection should have been on the midline of the dorsum nasi at the level of the third premolar. However, to include the markers and collimate only to the edge of the film it was necessary to centre to this position.

Collimation
When using film in cassettes collimation should always result in four film edges unexposed in order to demonstrate the extent of the primary beam. It is important to collimate as closely as possible to the area of interest, both for safety of personnel and maximum definition. Non-screen dental film radiography is an exception to this rule, as the size of the film and oral cavity necessitate collimating to the edge of the film in order to include as much of the nasal chambers as possible. However, the primary beam should never extend beyond the film edge and either a lead mat or, as in this case, a lead lined table should be used to absorb as many X-ray photons as possible. In this case the primary beam was collimated exactly to the film edge.

Exposure
As no intensifying screen is used with this film, exposure is entirely dependant on X-ray photons rather than by X-ray photons plus visible light emitted during fluorescence of the phosphor crystals in an intensifying screen (when exposed to X-ray photons). Therefore this film required greatly increased exposure factors. The overall exposure of this radiograph is good. Contrast is good, with a large range of well defined densities present within the bones. Soft tissues can also be seen. Detail is also good, due to the characteristics of this fine grained film, lack of movement blur and good contrast. This film has a wide latitude, and therefore a wide range of exposures would result in an image on the film, although to obtain the best definition the optimium exposure must be selected.

Processing
Non-screen dental film has a thicker emulsion layer than standard films and therefore needs longer developing and fixing times. For this reason it is not suitable to be put through an automatic processor. This film was developed manually in dishes with chemicals from the automatic processor, and was developed for 5 minutes at approximately 20 C, rinsed in water, fixed for 40 minutes and washed for 30 minutes. This film appears to be processed correctly. Under development would have resulted in grey background density, unless grossly under exposed. Gross over development would have an overall blackening effect on the film.

General Comments
Accurate positioning is extremely important when taking radiographs of the nasal chambers, as the slightest rotation can cause distortion of the structures which make it very difficult to see subtle changes within these structures. General anaesthesia or deep sedation is necessary for intra-oral radiography, both to enable good positioning and insertion of the film into the oral cavity.

Despite slight rotation this was a diagnostic radiograph showing patchy increased soft tissue densities in the mid left nasal chambers with loss of turbinate definition, consistent with chronic rhinitis. The cat was given a 3 week course of cephalexin syrup 25 mg/ml (Ceporex, Glaxol) to be given 2 mls twice daily orally. Ideally a swab could have been taken for bacterial culture. Two months later the discharge returned and the cat was re x-rayed. See radiograph 2. Note the same problems were encountered in positioning straight. There was little change seen and the cat was put back on Ceporex and prednisolone 5mg one every other day. The discharge and sneezing resolved and the cat has been free of symptoms for 7 months.

Dental film was used for this radiograph as non screen film is now unobtainable and there was no other alternative available within the practice at the time. However, thin flexible cassettes with ultra vision screens are now available (Sterling Diagnostic Imaging) which are suitable for this type of radiography and are now in use at this practice. The advantages are: smaller exposures are used (due to intensifying screens) and the film can be put through an automatic processor.

General Assessment of Radiography
Radiography was useful in this case for the following reasons:

• It is a non-invasive procedure.
• It enables sight of boney changes necessary for diagnosing nasal tumours which occur fairly commonly in cats.
• This is not a radiographic procedure which could have been undertaken without heavy sedation or general anaesthesia.

References:
Atlas of Radiographic Anatomy of the Dog and Cat, Schebitz & Wilkens, 5th Ed, Paul Parey. 1985 p. 161 - 163
Principles of Veterinary Radiography, Douglas, Herrtage, Williamson, 4th Ed, Bailliere Tindall. 1987. Ch 3, 4