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Essay on Exposure to radiation in Intraoral Radiography
Introduction
Dental radiography is one of the most frequent types of radiological procedures performed. One of the projections that have been done in dental radiography is intraoral radiography, which means the film is put inside the patient’s mouth. The intraoral can be divided into four sections which are bitewing view, periapical view, occlusal view, and full mouth series. There are conventional intraoral radiography and another one, with the advances in technology, digital intraoral radiography is been developed.
In this new imaging modality, the radiographic film is replaced by a sensor for X-rays. The signal temporarily stored within the sensors is transferred to the computer, which displays an image that may be filed, interpreted, manipulated and quantified. Digital intraoral radiography is divided into two, which are two fundamentally different concepts for direct digital image acquisition, the CCD-based (charge-coupled device) and the Storage Phosphor systems.
The intraoral radiography is said to have a low exposure dose received by the patients. According to several sources that can be trusted, the exposure dose received by the dental patient is equivalent to a few days’ worth of background radiation environmental radiation exposure or similar to the dose received during a cross-country airplane flight.
Digital dental radiography is preferable because although film has been an inexpensive and reliable image receptor in dental radiography for a long time, the advantages of digital dental radiography over film include providing a lower radiation dose, swift availability of radiographs, the possibility of image enhancement and no need for film processing chemicals.
DOSES IN INTRAORAL RADIOGRAPHY
In digital intraoral radiography, the mean exposure time and radiation surface dose for the PSP is greater than that for the CCD system by a factor of 2.45. However, there was also a significantly higher repeat rate using the CCD system compared to the PSP system. Therefore, despite the CCD system requiring more repeat exposures, the radiation received by the patient is less. CCD systems showed a larger dose reduction in comparison to PSP imaging plates.
Another study reported that the dose reduction as a result of shorter exposure times exceeded the increase in doses as a result of the greater number of radiographs with both digital systems. However, with the CCD sensors, the dose reduction per exposure was almost canceled out by the increase in the number of radiographs taken.
Although the patient exposure associated with dental radiography is relatively low, intraoral radiography should be optimized in order to keep the radiation risk “as low as reasonably achievable”, something that is widely known as the ALARA principle but at the same time, produce the best quality of the image. Any radiological procedure should be justified and modified in order to keep the radiation risk as low as reasonably achievable, especially for children.
Compared to adults, children are more sensitive to radiation exposure. Dose assessment is recommended to be performed on a regular basis to ensure that patient exposure is always kept within the recommended levels and at the same time, the malfunction of the equipment also can be detected. All radiological procedures carried out on children must adapt to special radiation protection measures, which aim at recognizing and implementing possible dose reduction strategies in order to eliminate unnecessary and therefore unjustified radiation exposure.
Over the past 20 years, both the X-ray units and the X-ray receptors used in dental radiology have evolved. Modern dental X-ray units incorporate high frequency generators, operate at higher tube potentials and produce X-ray spectra that have higher mean energy and therefore are more penetrating compared to those produced by older dental X-ray units. These improvements have contributed in the reduction of the radiation dose to the entrance skin surface of the patient and the enhancement of image quality.
According to Hart (2009), the new adult reference dose for intra-oral radiographs (2.3 mGy) is 40% lower than the 1999 value (4 mGy), probably owing to the use of faster film-screen and digital systems. This is the first time that a national reference dose for intra-oral radiographs on children has been recommended (1.5 mGy), and it is, 35% lower than the corresponding adult value.
Some studies show that there is a large dose variation between different X-ray units used for the same radiographic projection and it is relatively low. However, although radiation exposure from intraoral radiography is considered to be low, the patient may have a chance to undergo repeated dental radiological procedures. Therefore, the accumulated effect of radiation exposure should be taken into consideration. The salivary gland and the thyroid gland are among the organs at risk in dental radiology. The salivary gland, which often lies within the primary beam in intraoral radiographic projections has been shown to receive doses from 0.02 mGy up to 0.1 mGy per examination.
As stated by Looe et al. (2006), the dose received by the thyroid gland, mainly due to scattered radiation, is comparably less than those received by the salivary glands. On the other hand, the thyroid gland is one of the most radiosensitive organs for children and the dose imparted on the thyroid gland should be minimized whenever possible.
Diagnostic reference levels (DRLs) have been introduced by the European Union in the Medical Exposure Directive (MED) (97/43/Euratom). The directive requires the member states to promote the establishment and the use of DRLs and to ensure that implementation guidance is available (Poppe et al., 2006). A good practice is established when the required levels are not exceeded.
The DRL is very important because inadequate techniques or machine malfunctions in the case where they are consistently exceeded can be detected during the examination, so that appropriate corrective action could be undertaken. Patients increased their chance to be subjected to unnecessarily high-radiation doses due to unsatisfactory equipment or inadequate techniques.
To establish the DRLs, entrance surface dose (ESD), dose area product (DAP) or other dose-related quantities may be used. In intraoral radiography, DAP has been chosen as the measurement quantity as it could be measured without the patient in place and the field size of the beam is directly reflected on the measured value. Rectangular collimator is preferable compared to the cylindrical collimator because the rectangular one can fit the size and shape of the film better, even though most X-ray units in intraoral radiography are equipped with cylindrical collimators.
Furthermore, the introduction of DRLs has led to DAP meters being installed as an integral part of radiology equipment used for the automatic registration of patient doses. The DAP meters could also be a possibility for panoramic units. It is completed with the advent of digital radiography and the use of automatic exposure control for these examinations, so, such equipment would allow easy monitoring and follow-up of individual patient doses. It has been suggested that this dose area products are closely correlated with effective doses under specific circumstances because DAP is directly measurable or indirectly accessible from exposure.
For instance, Poppe et al. (2006) stated that the measured DAP values for maxillary molar examinations range from 3.8 to 134.8 mGy cm2. The minimum dose measured for non-digital systems was 17.4 mGy cm2 and the maximum value measured was 134.8 mGy cm2. The highest third quartile value was calculated for occlusal examinations, whereas the lowest value was calculated for mandibular incisor examination.
Moreover, there is a large difference between patient exposures among different dental facilities. Sometimes, the differences are up to a factor of 35 for the same examination. This s due to the inconsistencies in radiological practices performed in clinical routines such as different X-ray units, exposure techniques, film speed, or even inadequate exposure setting and film development procedures, and further. It can be seen that many dentists do not prefer using dose-optimized programs for faster films. In addition, the correlation between DAP and tube loading may also be used as a rule of thumb in determining the imparted dose on patients. That’s why it is necessary to have the DRLs laid out as guidelines.
In intraoral radiography, periapical is the most commonly performed and usually, two to four teeth are shown on the image providing full tooth structure, including pulp, root, and gum anatomy. On the other side, bitewings are taken to show the upper and lower teeth together on a single image while occlusal radiography demonstrates the dental arches at right angles to the occlusal plane.
Although DRLs are useful in optimizing radiological procedures by identifying inadequate exposure techniques, they still got a disadvantage. The problem is, that they are lacking information on the risk associated with the radiological procedure. So, the other alternative is by using the conversion coefficient. It is used to estimate effective doses from DAP values that have been published for common radiological procedures, including intraoral radiography.
In intraoral radiography, basically, the thyroid gland and brain tissue receive only a small fraction of the dose caused by scattered radiation within the phantom except for occlusal examination of the maxilla where a high dose was measured at the brain tissue. So, we can say that the overall skin dose is also relatively low as only a small fraction of skin was exposed directly to the x-ray beam.
Moreover, the salivary gland which often lies within the primary beam is exposed to high doses during intraoral examinations. This gland received the highest dose followed by the red bone marrow. Then, the mandibular angle also was exposed to a high exposure during most of the dental examinations. Doses measured at the thyroid gland and brain tissue were only attributed to scattered radiation and therefore considerably low. However, only a low dose was recorded at the third cervical vertebra mainly due to scattered radiation.
The other factor that determines the exposure dose in intraoral radiography is the type of film. It is remarkable that several facilities using an E or F-speed film have higher doses than other facilities using the less sensitive D-speed films. For example, in the category of dentists using X-ray units operating at 65 kV, the lowest dose measured using D-speed film is 3.5 times lower than the highest dose measured using E or F speed film. However, the exposure time for the digital radiographic systems was set to 10-50% of that of E-speed film in most cases of intraoral radiography.
However, in some clinics and hospitals, working with a faster film type or higher tube voltage is not always associated with lower exposure. Many precaution measures could be taken at no cost to reduce patient exposure by choosing the appropriate exposure parameters. Operators of X-ray units shall pay special attention to ensuring that the right radiological equipment and techniques are used when performing radiological procedures on the dental patient.
When using the faster film types, the operator should reduce the exposure time, so that the level of radiation exposure received is not beyond the acceptable level of radiation exposure in intraoral radiography. Still, it is very important for the technologists to be informed about the necessity and importance of reducing the exposure times when working with the faster film types
The quantitative aspects of radiation doses are needed to observe and determine the necessary radiation protection measures and at the same time, can help the general public to allay radiation fear in dental radiography. Because, it is afraid to develop certain diseases and the highest risk in intraoral radiography is leukemia and thyroid cancer, even in doses as low as 500 mSv.
On the other hand, even low doses of radiation can cause changes in the DNA of the cell that may not be lethal but that could cause mutations that could lead to cancer. Most of these DNA changes are discovered and repaired before they cause problems, but the repair mechanism is not perfect and some of the changes may persist and accumulate. Non-repairable damage is more likely to occur with higher doses or dose rates but there is a chance that even a single small “hit” of radiation could produce a mutation that could cause cancer.
The more radiation a person is exposed to, the more chances he has of receiving a non-repaired DNA injury. Similar surveys conducted in dental radiographic facilities over the last 10 years have demonstrated a trend for reduction of the ESD, with the use of faster films and digital receptors, as well as with modern x-ray units and rectangular collimation. The emphasis should be increased on the structures located in the oral region, particularly the salivary glands.
In the gonadal areas, the gonads are not in the line of exposure, especially during intra-oral or panoramic radiography. However, the dose to the genetic cells results from scattered radiation in dental radiography. Scatter radiation during dental radiography may result in the exposure of the dental personnel in the area. A dentist or dental auxiliary may accumulate perceptible amounts of radiation doses from his repeated exposure to scatter radiation. The precaution also must be highlighted to the pregnant women because the excessive exposure dose received by pregnant women may result in spontaneous abortion, congenital abnormalities, microcephaly, and decreased mental efficiency.
The study by Brooks (2008), shows that the doses actually used to obtain dental radiographs are frequently higher than what can be obtained in ideal situations. For example, a recent study was done in Spain measured entrance doses in several thousand dental offices and reported that there was frequently no difference in radiation dose with different films and in different locations within the mouth.
According to the revised recommendations for calculating effective dose, dental radiography involves 32% to 422% more risk than previously thought. Therefore, efforts should be made to reduce the dose as much as possible but not at expense of image quality and diagnostic accuracy.
In addition, different groups of teeth need different exposure times for obtaining quality diagnostic information. Moreover, the patient dose is determined not only by the amount of radiation per exposure but also by the number of radiographs taken. A recent study shows that the total number of radiographs taken by dentists using digital radiography was significantly larger than the number of radiographs taken by film users. The number of radiographs taken by dentists using solid-state systems compared to film-users while phosphor plate users took 32% more radiographs.
The main reason when taking more radiographs is to achieve better diagnostics and descriptions of certain conditions of the patient. Even though it provides better diagnosis, positioning errors occurred more often in digital radiography than in film-based radiography. This is due to the stiffness of the digital sensors that are significantly more difficult to position in the patient’s mouth, rather than the positioning film and more uncomfortable for the patient although.
CONCLUSION
In conclusion, digital intra-oral radiography is a well-accepted diagnostic tool in dental practice. However, some of the claims made by manufacturers of digital systems, are not valid to their full extent. For instance, the dose reduction per exposure is real, but it is still to be determined what the actual dose reduction is because of the fact that dentists tend to make more radiographs when using a digital system.
Sometimes, the importance of the level of exposure dose received by the dental patient is underestimated. This could lead to the poor characteristics of x-ray devices, inadequate film processing conditions, and outdated techniques used. Regular quality control of dental x-ray units can eliminate deficiencies related to equipment. Inadequate technique is a more significant problem because dentists and radiology technicians are insufficiently educated in the field of radiation protection.
Because of that, it is best to select the imaging technique that will provide that information with the lowest radiation dose. To obtain that, dental equipment must stay in good condition, including film processing apparatus and solutions, and use good techniques to avoid retakes. In addition, using fast film or digital imaging and small collimation, whenever feasible, will also keep the radiation dose as low as reasonably achievable, or ALARA which is a goal worth pursuing. in general, both entrance and effective doses are reduced when higher film speed (E-speed or F-speed instead of D-speed) or digital imaging is used.
In addition, rectangular collimation of the beam also reduces the effective dose because less tissue is exposed in total. There are no such things as necessary “routine” radiographs the way there are required. Instead, dentists must make radiographs only when they think they are necessary to make an accurate dental assessment or diagnosis to reduce the number of X-rays taken to the minimum needed for dental health. Efficient implementation of the basic principles of radiation protection, particularly the practice optimization through the quality assurance program, is the only adequate way of reducing the patient dose and at the same time, preserving the quality of diagnostic information.
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