RADIATION PROTECTION
Radiation protection refers to the prevention of the occurrence of deterministic effects and reduces the likelihood of stochastic effects
What is a deterministic effect?
A deterministic effect is defined as any somatic effect that increases in severity as a function of radiation dose after a threshold has been reached. e.g.cataracts, skin erythema, fibrosis, and abnormal growth and development following exposure in utero
What is a stochastic effect?
A stochastic effect is defined as one whose probability rather than severity is a function of radiation dose without a threshold. e.g.cancers and genetic factors
DOSE LIMITS
The National Council on Radiation Protection (NCRP) and International Commission on Radiological Protection (ICRP) have laid down the following guidelines for limiting the amount of radiation exposed to people: Occupational dose limit: 50mSv annual effective dose limit Nonoccupational dose limit: 5mSv annual effective dose limit. All unnecessary radiation exposure should be avoided. It is based on the principle of ALARA (As low as reasonably possible), which recognizes the possibility that no matter how small the dose, some stochastic effect may result. The dosage for individuals occupationally exposed in the operation of dental x-ray equipment is 0.2mSv.
Nominal lifetime probability coefficients for stochastic effects Detriment (10-2 Sv-1) Fatal cancer 5.0 Non-fatal cancer* 1.0 Severe hereditary effects 1.3 Total 7.3
Risk in relation to age
Age Group (years) Multiplication factor for risk <10 x3 10-20 x2 20-30 x 1.5 30-50 x 0.5 50-80 x 0.3 80+ Negligible risk
PATIENT EXPOSURE AND DOSE
Patient dose from dental radiography is usually reported as the amount of radiation received by a target organ.
Mean active bone marrow dose
It is specific tissue dose relevant to a particular stochastic effect, leukemia. It is the dose averaged over the entire active bone marrow. The mean active bone marrow dose resulting from an intraoral full-mouth survey of 21 films exposed with round collimation is 0.142 mSv, same survey exposed with rectangular collimation results in a dose of only 0.06mSv. Panoramic radiography results in a dose of 0.01 mSv. Chest x-ray results in a dose of 0.03 mSv.
Thyroid dose
The dose to the thyroid gland from oral radiography is fairly low. A 21-film complete mouth examination results in a thyroid dose of 0.94 mGy, one sixth that resulting from radiographic examination of the cervical spine which is 5.5 mGy. The thyroid dose from panoramic radiography is about 0.074 mGy.
Gonad dose
Dental x-ray examinations result in a genetically insignificant dose of only 0.001mGy.
METHODS OF EXPOSURE AND DOSE REDUCTION
Patient selection
Radiographs are indicated when a reasonable probability exists that they will provide valuable information about a disease that is not evident clinically
Guidelines for justification of x rays
All X-ray examinations must be justified on an individual patient basis by Demonstrating that the benefits to the patient outweigh the potential Detriment. No radiographs should be selected unless a history and clinical examination should be performed. When referring a patient for a radiographic examination, the dentist should supply sufficient clinical information (based upon a history and clinical examination) to allow the practitioner taking clinical responsibility for the X-ray exposure to perform the justification process. Prescription of bitewing radiographs for caries diagnosis should be based upon caries risk assessment. Alternative methods to using ionising radiation in caries diagnosis should be considered once their diagnostic validity has been clearly established. Specialist guidelines on orthodontic radiography should be consulted as an aid to justification in the management of the developing dentition in children. Radiographs should be used in the management of periodontal disease if they are likely to provide additional information that could potentially change patient management and prognosis
It is recommended that radiographic examinations are carried out at the following stages of endodontic treatment: 1. Pre-operative assessment 2. Working length estimation 3. Post-operative 4. At 1-year review or if symptomatic
Pre-extraction radiography may be indicated in the following situations
A history of previous difficult extractions A clinical suspicion of unusual anatomy A medical history placing the patient at special risk if complications were encountered Prior to orthodontic extractions Extraction of teeth or roots that are impacted, buried or likely to have a close relationship to anatomical structures (i.e.mental/inferior dental nerve, the maxillary antrum and/or tuberosity andthe lower border of the mandible).
Other justifications
There is no justification for radiography of edentulous patients without a specific indication such as implant treatment or clinical signs or symptoms. Imaging is essential in implantology in preoperative planning and to review the fixture. Informed consent should be obtained from patients prior to radiography in accordance with national requirements.
CONDUCT OF EXAMINATION
Choice of equipment:
It includes selection of the image receptor, focal spot-to-film distance, x-ray beam collimation,filtration,and the type of leaded apron and collar.
Receptor selection
In 1920,the Eastman Kodak company introduced the regular dental x-ray film However it was too slow.The radiograph of the maxillary area of an adult required 9 seconds. Beginning with the letter designation “A” , film speed has almost doubled with the introduction of each new speed group. Currently, intraoral dental x-ray is available in three speed groups-D,E and F. Faster films are desirable from the standpoint of exposure reduction. F-speed film requires about 755 the exposure of E-speed film and only about 40% that of D-speed
Intensifying screens
Contemporary intensifying screens use the rare earth elements gadolinium and lanthanum. These rare earth phosphors emit green light on interaction with x rays. They decrease exposure by as much as 55% in panoramic and cephalometric radiography. The use of a T-grain film further reduces the patient exposure.
T-GRAIN FILM
The use of a T-grain film further reduces the patient exposure. Mechanism: The film contains silver halide grains that are tabular or flat rather than pebblelike in shape.With their flat surface oriented towards the x-ray source, these grains present a greater cross-section,which increases their ability to gather light from intensifying screens. T-grain films used with rare earth screens is twice as fast as calcium tungstate screen-film combinations and 1 1/3 times as fast as conventional rare earth screen-film combinations with no loss in image quality
Focal spot-to-film distance
Two standard spot-to-film distances are used in intraoral radiography,one 20cm and the other 41cm. Use of an FSFD longer distance results in a 32% reduction in exposed tissue volume. Mechanism: The greater the distance, the x-ray beam is less divergent.A reduction in the exposed tissue volume should be reflected in a reduction in the effective dose. There is a 30% decrease in the effective dose resulting from the use of a 30cm FSFD instead of a 20cm FSFD for a simulated 19-film complete mouth survey using Dspeed film.
Collimation
It is mandatory that the x-ray beam used in intraoral radiography be collimated so that the field of radiation at the patient’s skin surface is “contained in a circle having a diameter of no more than 7cm”when the x-ray is operated above 50kVp. Thus,limiting the size of the x-ray beam is necessary to reduce the patient exposure.
Methods to limit the size of the xray beam
A rectangular position-indicating device(PID) may be attached to the radiographic tube housing. Using a PID having an exit opening of 3.5x4.4cm reduces the area of the patient’s skin surface exposed by 60% over that of around PID. Film holders with rectangular collimators may be used with round PIDs,these holders reduce patent exposure to the same degree as rectangular PIDs.
Filtration
The purpose of filtration is to remove the low energy photons, which have limited penetrating power,and are mainly absorbed by the patient. When an x-ray beam is filtered with 3mm of aluminium, the surface exposure is reduced to about 20% of that with no filtration. Patient exposure may be further reduced by removing both the high energy and low energy x-ray photons from the beam,leaving the mid-range energy photons to expose the film. This can be done by using materials like samarium,erbium,yttrium,niobium,etc. The use of these materials in combination with aluminium filtration may reduce the patient exposure by 20% to 80% compared to conventional aluminium filtration alone.
LEADED APRONS AND COLLARS
Leaded aprons and collars
Leaded aprons are useful because they attenuate as much as 98% of the scatter radiation to the gonads. Similarly, thyroid collars reduce the exposure to the thyroid by 92%. However according to the latest study, Lead aprons do not protect against scattered radiation internally within the body and in the case of panoramic radiography, they may physically interfere with the procedure and degrade the final image
CHOICE OF INTRAORAL TECHNIQUE
The decision as to which technique is used should be based on the diagnostic quality of the resultant radiographs, the efficiency of using radiation,and the convenience of the technique. The more efficient the technique,the fewer radiograph retakes will be required,along with less patient exposure. The Precision instrument with rectangular field collimation and Rinn XCP instrument are used for efficient radiography.
Operating the equipment
Kilovoltage
It is the exposure factor that controls the energy of the x-ray beam. As the kilovoltage is reduced,the effective energy of the x-ray beam decreases and the radiographic contrast increases. High voltage techniques,which produce images of low contrast, also reduce the effective dose delivered per intraoral examination. With an increase in kilovoltage from70 to 90,there is a reduction of 23% of exposure. 65 to 70 kV is recommended as the kilovoltage of choice for dental (intraoral) X-ray sets using AC equipment, with 60kV for those using DC X ray Sets.
Milliampere -seconds
Image density is controlled by the quantity of x rays produced, which in turn is best controlled by the combination of mill amperage and exposure time termed milliampere-seconds. Patient exposure is directly related to mAs. For an average adult when a F-speed film and an operating kilovoltage of 70, 3.5 mAs is suggested.
PROCESSING THE FILM
A major cause of unnecessary patient exposure is the deliberate overexposure of films compensated by the underdevelopment of the film. Care must be exercised in selecting the processing solutions The film manufacturer’s recommendations must be followed and not the solution manufacturer’s directions. Film processors which are used widely may increase patient exposure if not maintained properly
PROTECTION OF PERSONNEL
Every effort must be made so that the operator can leave the room or take a position behind a suitable barrier or wall during exposure of the film. Walls of the operatory must be of sufficient thickness that the exposure to nonoccupationally exposed individuals is no greater than 100mGy per week. In most instances, it is not necessary to line the walls with lead; the walls constructed of gypsum wallboard are good enough.
Radiation scattered from a primary beam
If leaving the room or making use of some other barrier is impossible, strict adherence to the position-and-distance rule is required.
The operator should stand at least 6 feet from the patient, at an angle of 90 to 135 degrees to the central ray of the x ray beam. The operator should never hold films in place.. Neither the operator nor patient should hold the radiographic tube housing during the exposure. The best way to ensure that personnel are following office safety rules is to use personnel-monitoring devices. They are badges which contain either piece of sensitive film or a radiosensitive crystal and a printed report of accumulated exposure at regular intervals.
References
White and Pharoah: Oral RadiologyPrinciples and Interpretation.(Health physics) Radiation protection 136-European guidelines on radiation protection in dental radiology.