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Designing a new x-ray facility

Any new radiation facility design must be checked, and the final assembly inspected, by a certified Radiation Protection Adviser. However you can do some preparatory work and make budget estimates for buildings, with the data provided here.

If you sketch your room plan in a Word document, you can email it to alan@radiologyphysics.com for checking. Include the full postal address of the premises and any relevant phone and fax numbers: we will prepare the necessary documents for notifying the Health and Safety Executive.

For a conventional small animal facility (vertical beam onto a lead-topped table) the primary design consideration is clinical convenience: can you work round the table with sufficient room for the anaesthetic machine, surgical trolley, monitors, and of course the x-ray unit itself (rarely a problem if you use a ceiling-mounted monobloc unit). Can you prevent people standing or entering within 2 m of the patient during the x-ray exposure? Any additional radiation protection requirement can then be designed and calculated very quickly.

Equine and large animal facilities require a little more thought. Consider first how the patient will be restrained and positioned, then consider matters of access and exclusion for staff and owners. You are likely to be using a horizontal x-ray beam, so we need to take account of the stable wall materials and possibly add more shielding to give you the maximum flexibility of use. A small mobile or portable x-ray machine will cope with legs and hoofs but chest and abdominal films may be blurred by movement during the long exposure: contact alan@radiologyphysics.com to consider a high power mobile or even a ceiling-mounted static unit.

For outdoor work, or mobile use in stables, a laser pointer may be more use than a conventional light beam. You will probably need additional portable signs, barrier chains and cones.

Specialist "small furries", avian, reptile or research work can often be done most conveniently in a custom-designed x-ray cabinet. This can sit on a laboratory bench so that x-ray exposures will not interfere with other work in the same room. We can adapt a low-kV unit such as a dental or mammographic x-ray machine to optimise the images of low-contrast animals.





Layout and electricals

By careful positioning of the electrical system, you can minimise many radiation protection and operational problems. Click electricals for small animal x-ray.doc to download some proven suggestions.


The darkroom should be a palace, not a dungeon

Practically everything that ever goes wrong with a radiograph, goes wrong in the darkroom. Nobody who had spent a thousand pounds on a new hi-fi system would dream of putting the loudspeakers in a cardboard box, but small clinics often spend 5000 or more on an x-ray unit and then develop the films in a dusty toilet! Here are a few tips, culled from several hundred awful darkrooms and one or two good textbooks.

White walls and ceiling will show the dirt before it starts falling into the cassettes. Never paint a darkroom black (the film will see any stray light before you do)  blue, or green (x-ray film is more sensitive than the human eye to blue and green light).

A toilet ventilator fan costs a lot less than a box of film. Keeping the air clean and dry  will extend the life of your film stock.

If you take less than 500 films a year, consider not using the smallest film size. 18 x 24 cm film is the least popular for veterinary work, so it's likely that the film currently in your cassette is stale anyway.

Stray light mostly leaks round the door. If you add a draught excluder strip that is thick enough to stop the light, the door probably won't close at all. Click here for some printable sketches of effective modifications to the door, and a simple light baffle for a toilet fan.

Conventional safelights are fine if they aren't disturbed, but the metal ones often leak white light round the filter after you change the bulb, "beehive" saflight gels seem particularly prone to damage, and Paterson-type photographic safelights need a very low wattage bulb for use with x-ray film. Fluorescent safelights are fine when new, but old ones tend to  flicker and the starter emits white light when they do so! So far, there are no reported problems with red LED safelights. You can buy a strip of red LEDs (a brake light repeater bar) from a boy-racer car accessory shop, and run it from a 9 volt "battery eliminator" plug - the cheapest and most reliable source of red light currently available.

Don't position the safelight near a manual developer tank. "Development by inspection" always leads to underdevelopment (the blue tinge of the film base makes the image look much darker and more contrasty in a red light) and overexposure  (to compensate for the "thin, flat" image that results from underdevelopment).

Waste photochemicals are a problem. Your local water and sewage company may allow you to pour small quantities down the drain, if you remove the silver from the waste fixer - your chemical supplier can sell you a silver trap. spent devOtherwise, beware! Fresh developer and fixer are considered innocuous materials that can be sent by  post or common courier, but the European Union in its wisdom has ruled that spent dev and  fix are Hazardous Waste, and you need a licence to move them by road. The licence form runs to several pages and costs around 30 per trip, regardless of quantity or distance, so it is uneconomic to dispose of small quantities....so in the name of public safety, store as much as you can in the garage to get your money's worth from a silver recovery or chemical scrap dealer.



Exposure log

A daily log of exposure factors is the key to radiographic quality assurance and staff safety.

Click here to download a printable log sheet template. 

At a glance, you can estimate your annual workload (is it within the design load of the facility? should you upgrade the protection or alter your work procedures? should you use fewer film sizes to keep the stock fresh?) and review your exposure chart.

At least once a month, conmpare the exposure factors and comments for, say, an average cat VD spine, with a month and a year ago. If the mAs has increased, there may be a fault in the developer thermostat. If it has decreased, have you altered the focal distance? A change in kV may indicate a fault in the x-ray machine.

Scan the comments column for rejects and retakes. If more than 5% of the films are not diagnostic "first time", you have a problem to resolve. Wrong cassette, perhaps? Inexplicit exposure chart? X-ray generator fault?




Radiation monitoring badges

The British Veterinary Association guidance notes suggest that personal monitoring badges should be worn

on the trunk

and under any protective apron.

Whilst this reflects human and industrial radiological practice, it is worth reconsidering in the case of small animal radiography on a table.

The dose limit for a member of the public is 1 millisievert per year. It is desirable that vets and nurses should be considered as members of the public rather than employees (who are subject to a higher dose limit) for the purposes of radiation protection.

The incident dose to the patient from a typical small animal radiograph (50 kV, 5 mAs, 1 m focal distance, vertical beam) is about 50 microgray. If you take 1000 radiographs per year (roughly the workload of four or five full-time vets) the total patient dose would be 50 milligray.

Suppose you were to stand at arm's length from the edge of the table, for every film. The scattered radiation dose at chest height, 1 m from the patient, is about 0.3% of the incident dose, that is, 0.15 milligray per year. So even without a lead apron, your received dose would be less than the "public" dose limit.

A "0.25 mm" lead apron transmits 0.2% of the incident radiation at 50 kV, so the worst-case dose received by a badge worn on the chest, under an apron, would be 0.0003 milligray. At waist level, say 0.5 m from the patient and just above the tabletop, the dose would be about the same. No badge can reliably measure less than 0.2 milligray per year.

Thus in most practices, no purpose is served by wearing a badge under a lead apron, for small animal radiography.

The scattered radiation to your unprotected face and thyroid region would be about 0.1 mGy/yr. It is, therefore, sensible to wear a suitably sensitive badge outside the apron, at collar level, if you stand close to the patient during radiography. At present, the only suitable devices commercially available in the UK appear to be Luxel badges from Landauer Inc (see "contacts").

It is almost unknown for anyone to hold a small animal for radiography nowadays. However if it is absolutely necessary, it is worthwhile monitoring your hand, finger or wrist exposure with an "extremity" badge. Consult your radiation protection adviser or badge supplier.

Referral units and large animal practices should consult their RPA. "Body" badges may be appropriate if high kV radiography, fluoroscopy, or horizontal beam radiographic studies are undertaken. They should certainly be used in scintigraphy or other radionuclide procedures.





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