Persons who participate in examinations are exposed to radiation
The radiation exposure of personnel can be reduced through simple measures.
Persons who participate in veterinary X-ray examinations are exposed to radiation. Most of the exposure is caused by radiation scattering off the animal when the person (immobiliser) holds the animal immobile during the imaging. In order to prevent unnecessary radiation exposure, only the persons who are necessary for the examination may be present in the examination room during X-ray examinations. Persons under the age of 18 or who are pregnant are not allowed to hold animals immobile during imaging. When possible, the immobilisation should be carried out using various sandbags and racks.
Exposure can be reduced
Due to the shielding effect of the animal’s body, the scatter off the animal’s body is the strongest back towards the X-ray tube. The amount of scattered radiation is almost directly proportionate with the surface area of the radiation field: reducing a field of 30 cm x 30 cm to a field of 20 cm x 20 cm reduces the amount of scattered radiation by over 50%. If a person who is in the examination room moves double the distance away from the animal being examined, this reduces the radiation dose of the person to one quarter (inverse square law of distance). The immobiliser can reduce the radiation exposure of his/her upper body by leaning as far away from the animal as possible during imaging. Reducing the imaging values (kV and mAs) also reduces the amount of scattered radiation.
Use of protective devices
In order to reduce their radiation exposure, persons working near the animal being examined or the X-ray tube must use protective devices during the irradiation, and no part of them must be exposed to the primary radiation. The most common pieces of protective equipment are lead aprons, coats and gloves and thyroid collars.
During examinations, the persons holding the animal and cassette holder should wear a lead apron and a thyroid collar that provides shielding that corresponds with the shielding provided by a 0.5 mm layer of lead. Shielding that corresponds with the shielding provided by a 0.25 mm layer of lead is usually sufficient for persons working further away. A lead apron that corresponds with the shielding provided by a 0.5 mm layer of lead lets though less than 5% of the radiation.
When working near the radiation beam, the persons holding the animal and the cassette holder should wear lead aprons and lead gloves that provide shielding that corresponds with the shielding provided by a 0.25 mm layer of lead. It is advisable to wear gloves that provide shielding that corresponds with the shielding provided by a 0.5 mm layer of lead. If the imaging volume is great, it may be necessary to shield the head with a windowed shield hanging from the ceiling or safety glasses. The proportion of the scattered radiation from the examination room walls in a worker’s radiation exposure is so minor that it is not significant for protection.
In special situations where it is necessary to hold the imaging cassette manually, such as when imaging the rear knee of a horse, it is safer to use a large imaging cassette. In such cases, the radiation beam is carefully restricted so that it does not hit the holder’s hands. The need to redo the X-rays can be reduced by sedating the animal for the duration of the examination.
Monitoring of exposure
Individual monitoring of exposure must be organised for persons who must regularly work close to the radiation beam. The monitoring is carried out using a personal dosimeter. The dosimeter is attached to the employee’s work uniform – for example on the shirt collar inside the protective apron, on the side closest to the examined animal. The dosimeter must be placed so that it is not shielded by the employee’s body or the protective apron in the direction of the scattered radiation.
Dose limits in radiation work: annual effective dose 20 mSv, annual equivalent dose in the lens of the eye 150 mSv, and annual equivalent dose on the skin, hands or feet 500 mSv. In veterinary X-ray practices, the radiation exposure of employees has been far below the dose limits, and the measures doses are usually around 0.1–0.5 mSv. Nevertheless, it is important to remember to wear protective devices and use safe work practices in order to comply with the principle of optimisation.