Clinical history
The owner first presented the dog for dyschezia of one week's duration. On examination it was noted that the anus was sore and on rectal palpation the prostate seemed large. The dog also had many rotten teeth, was a little under weight, possibly due to old age and had no other remarkable clinical symptoms. The dog was booked in for radiography and possible castration, the owners' having signed a consent form. The dog was starved overnight with access to water until 2 hours prior to premedication.

Restraint for radiography
This was achieved with general anaesthesia. 

Choice of film/screen/grid
Agfa CP B Blue 100 NIF film was used as it is the standard general purpose film used at this clinic. Rare earth screens were used with a large cassette and a stationary parallel grid was used as the tissue depth was more than 10 cms.

Positioning
Two radiographs were taken, one plain and the other a pneumocystogram. Both were positioned in the same way in right lateral recumbency. This right lateral projection was achieved in the following way:

• The dog was placed in right lateral recumbency with the abdomen on the cassette and grid.
• A foam pad was placed between the hind legs in order to bring them parallel with each other and the table top. They were extended caudally and tied in position.
• A foam wedge was placed under the abdomen to avoid axial rotation and the front legs were pulled cranially and tied.
• A visual check was made to ensure there was no rotation.

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

Collimation

Plain radiograph
The primary beam was collimated to include the caudal abdomen, ie L3 to caudal border of ischium. Dorsally the beam was collimated to the spinous process of L6 and ventrally to include the prepuce.

Pneumocystogram
This radiograph was collimated more to the caudal abdomen, ie L4 to the ischium. The dorsal/ventral collimation remained the same.

Centering
Plain radiograph The primary beam was centred over the bladder by palpation.

Pneumocystogram
Again the bladder was used as a centring point for the primary beam, by palpation.

Preparation for a negative contrast pneumocystogram
The contrast agent used for this study was room air. Filling of the bladder with air was achieved in the following way:

After the plain radiograph had been taken, an enexposed cassette had been replaced under the grid and the dog had been repositioned, an 8 fg dog urinary catheter was aseptically placed into the bladder and a three way tap attached to the end of the catheter, with a 20 ml syringe then also attached to the three way tap. Then 53 mls of urine was aspirated from the bladder into a kidney dish. Following this 110 mls of room air was syringed into the bladder using the three way tap to ensure that no air escaped. Sometimes air can leak around the catheter and it may be necessary to clamp the prepuce around the catheter to prevent this. In this case it was not necessary, which might be due to the fact that there was some resistance when passing the catheter through the urethra at the point of the prostate, indicating constriction of the urethra by the prostate which acted as a seal to stop air passing back down the urethra. The bladder is palpated whilst injecting the air, to give a guide as to how much will be needed to expand the bladder to a reasonable size. The amount of urine, compared with the size of the bladder on the plain radiograph can also give a guide as to how much air will be needed. Under inflation of the bladder will give the impression of thickened walls. Over inflation will have obvious disastrous con- sequences, as would pushing the catheter through the bladder wall. After taking the radiographs, and checking that they were satisfactory, the air was removed from the bladder by aspiration and the catheter taken out.

Exposures
The following exposures were used: 

As with any radiograph of the abdomen the exposure should be taken at expiration when the abdomen is at it's largest, allowing organs to spread out.  

Radiographic appraisal
Positioning
For both the radiographs the hind legs are not equally extended. This was because the left hip would not extend as far as the right, possibly due to arthritis. On the plain radiograph there is some slight axial rotation - the transverse processes of the vertebrae are not superimposed and neither is the pelvis. The pneumocystogram is virtually straight - the pelvis is nearly superimposed and there will always be some magnification of the side of the pelvis furthest from the cassette and also some distortion due to the pelvis being at the edge of the radiographic field, where the primary beam is at it's most divergent from 90 degrees.

Centering
Centring the primary beam is always linked to collimation, as the cross wires always remain in the centre of the collimation area. Therefore the centring should be done first and the extent of the beam altered afterwards. Both of these radiographs were centred over the bladder. In the plain radiograph this was probably the best place to centre the beam. In the pneumocystogram it probably would have been better to try and centre the beam more over the prostate as this was the centre of interest. This is important, as the centre of the primary beam is where the least distortion of structures takes place, due to the photons travelling at 90 degrees to the cassette at this point. One of the qualities required of a good radiograph is to represent the structures with the minimum distortion and magnification, ie as true to reality as possible.

Collimation
On both radiographs there are 4 unexposed film edges which demonstrates the extent of the primary beam. This is important both for safety of personnel and to reduce the amount of scatter which improves film quality. On the plain radiograph a general view of the caudal abdomen was required as well as the prostatic area and pelvis. Therefore the collimation was reasonably good. All of the pelvis was required in order to check for metastasis from any prostatic tumour. The large bowel also needed to be included in order to rule out any other reason for the dyschezia. The pneumocystogram could have been collimated a little closer as it was decided that the area of interest was now the bladder and prostate, although over collimation can lead to areas of interest being excluded from the field and radiographs needing to be repeated.

Choice of film/screen/grid
The film used for these radiographs is the standard film used at this clinic. A rare earth screen is used when a grid is required, in order to reduce the exposure time and mA. Rare earth screens are more efficient at turning x-ray photons into light, therefore fewer are needed to expose the film correctly. This will reduce the exposure time which reduces movement blur and increases image quality. The rare earth compound used in these screens is Lanthanum Oxybromide which is predominantly a blue light emitting compound. This allows the CP B Blue film to be used either with a rare earth screen or the calcium tungstate screens which are fitted in the other cassettes in use at this clinic, as calcium tungstate also emits blue light. Ideally, for better image quality green light sensitive film and the rare earth compound Gadolinium screen (predominanatly green light emitting) could be used. This is particularly noticable when radiographing joints which need good detail radiographs to reach a diagnosis. A stationary parallel grid was used for these radiographs, as the tissue was more than 10 cms in depth. As the primary beam is attenuated while passing through the tissues scatter is produced (photons which are of longer wave length than the primary beam and no longer follow the path of the primary beam). These photons blur the image and produce a fog which decreases contrast. The thicker the tissue the more scatter is produced. Also at higher kV's the scatter can be harmful to personnel. A grid absorbs photons that are divergent from the primary beam and only allows photons travelling in the same direction as the primary beam to pass through. As the grid is constructed of slats some of the useful beam will be absorbed by the slats, so increased mAs are needed to compensate for this. Back scatter produced when a photon reaches the table also decreases film quality and presents a hazard to personnel. The table used here has a lead mat on the table top to absorb any photons reaching it.

Exposure
Plain radiograph.
There are two reasons for taking a plain radiograph before commencing contrast studies. Firstly an overview of the area is necessary to see if any abnormalities can be seen which might enable a diagnosis to be made without the time, effort and expense of contrast studies. Some contrast studies, such as myelography, also increase the risk of complications for the patient. Having decided that contrast studies are necessary to making a diagnosis, the second reason for taking a plain radiograph first is to ensure that the optimum exposure for the area has been selected. In certain studies there is only one opportunity to take a radiograph after administration of the contrast agent, for example barium to study the oesophagus or intravenous urography. Therefore if a radiograph is not suitably exposed the whole study will have been wasted. In this case the plain radiograph had an overall increased density, indicating slight overexposure. However the main area of interest was the prostate which was lying under the hindleg muscles (which increased the tissue depth and exposure needed). This resulted in that particular area being exposed correctly. The general abdomen lacks really good contrast, ie does not have a large range of densities overall, making it harder to distinguish adjacent tissues. Above a certain point increasing the mAs does not improve contrast but blackens the whole radiograph, which is what appears to have happened here. Therefore the exposure time was reduced for the contrast radiograph which would also reduce movement blur a little.

Pneumocystogram radiograph
The reduction of length of exposure has improved the contrast of this radiograph - the range of densities has increased, enabling individual organs to be seen more clearly. The definition (sharpness) appears to be better on the plain radiograph than on the contrast one, when comparing the spleen for example. This would imply that the contrast radiograph was taken during a respiratory movement which has produced movement blur. Good detail relies on good contrast and definition so neither radiograph has the optimum detail possible. However, the detail was good enough to make a diagnosis in this case. As the physical densities of tissues within the abdomen are similar it is usually better to select a lower kV and higher mAs when examing an abdomen - as long as the kV is sufficient to penetrate the tissues. The rule of thumb is that either increasing or reducing the kV by 10 and either halving or doubling the mAs will result in the same overall exposure.

Preparation of the pneumocystogram
It is important to inflate the bladder with the correct amount of air. In this case it appears to be the correct amount. The bladder is well inflated but not over distended. The urinary catheter can just be seen lying against the cranial bladder wall. There is no distortion of the wall at this point, indicating that there is not undue pressure on the bladder wall, but it would have been better to have the catheter end slightly more caudal to reduce the risk of perforating the bladder. Perhaps more careful measuring by palpating the bladder and laying the catheter (within it's packet) along the line of the urethra outside the body could have avoided this. Occasionally air can travel up the ureters into the renal pelvis of the kidneys. There are no ureters visible on this radiograph. Overall the technique used for producing a suitable pneumocystogram seems to have been satisfactory.

Processing
This was achieved by automatic processor. The film appears to have been processed correctly, which is the advantage of using a processor when repeating exposures of radio- graphs, as the processor should always process consistently (unless broken). One of the disadvantages is damage to the film caused by the rollers. This is a problem with these radiographs and new rollers are currently on order.

Artefacts
Unfortunately there are several on these radiographs.

• Firstly, grid lines can be seen on both radiographs. This is unavoidable when using a stationary grid.
• There are roller scratches running the length of the films, caused by the processor. This is permanent damage and cleaning the rollers has no effect.
• There are suspicious marks not within the emulsion, which appear dorsocranial to the bladder, a small circular area above the ischium and several around the prostate area. They appear to have been present at the time of exposure, possibly due to a dirty screen or grid - they appear in the same place on the film on both radiographs, but not in the same place anatomically. The screens and grid should be more frequently cleaned to avoid this, as a misdiagnosis can be made due to artefacts.

Labelling
This should have been done at the time of exposure using X-rite tape (or similar). It is not provided at this clinic, and a rule is adopted that radiographs are labelled as soon as they emerge from the printer. This is not entirely satisfactory, as it is still possible to forget to label a radiograph at the time.

Diagnosis
The radiographs showed an enlarged prostate which was constricting the rectum, which can be clearly seen. There was no metastasis to the pelvic bones. The dog was susequently castrated in order to help reduce the size of the prostate.

General comments on radiography
On plain radiographs it can sometimes be difficult to see an enlarged prostate or it can be confused with the bladder. A negative contrast pneumocystogram can aid in identifying a large prostate. It is difficult to tell the cause of enlargement from a radiograph. Ultrason- ography is very useful in assessing the prostate. It can also be performed on conscious animals, whereas a pneumocystogram needs to be performed under general anaesthesia. Boney changes in the pelvis could not be assessed by ultrasound, however, so there is a place for both radiography and ultrasonography in assessment of the prostate.  

References:
Diagnostic Radiology, Kealy, Saunders 2nd Ed
Manual of Sm An Diagnostic Imaging, BSAVA 1995
Radiography Diploma Notes 1999