Double Exposure Dental Radiography

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  1. Dental Radiography Guidelines
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Dental Sensors can be underexposed if the exposure switch is not activated for the indicated or correct length of time. In other words, the clinician let go of the exposure button too soon. When you set your x-ray generator to a set time say.20 seconds, when you press the button you need to make sure the button is being held down for the.

  1. Dental radiographs are commonly called X-rays. Dentists use radiographs for many reasons: to find hidden dental structures, malignant or benign masses, bone loss, and cavities. Double exposure which may occur when two images superimposed as a result of the receptor being used twice.
  2. However low the risk from the examination, it is imperative that each radiographic exposure is fully justified and the doses optimised to reduce the detrimental effects to the lowest possible level. Categories to indicate the health detriment. 68 The risk from dental radiography falls into the lowest risk category. Even the risk from medical CT.

2.1 Radiation doses and risks in dental practice
Nicholas Drage and Anne Walker

2.1.1 Introduction
Radiography is an invaluable tool for the clinician, providing information that is impossible to obtain by clinical examination alone. Of all the x-ray examinations carried out in the UK, around 26% are taken by general dental practitioners.1 In 2008, it was estimated that 20.5 million dental radiographs were taken by dentists in NHS and private practice, and of these 2.7 million were panoramic radiographs.1 Consequently dentists, equipment manufacturers, medical physics experts and radiation protection advisers need to work to keep radiation doses and risks as low as reasonably practicable. If selection criteria are used properly, the collective dose to the population is reduced, since unnecessary or unproductive x-ray examinations are eliminated.

X-rays are a type of electromagnetic (EM) radiation. EM radiation also includes visible light, radiowaves, microwaves, cosmic radiation and several other varieties of ‘rays’. All can be considered as ‘packets’ of energy, called photons, which have wave properties, most importantly a wavelength and frequency. X-rays are shortwavelength, high-frequency EM radiation. The importance of this is that high frequency means high energy. When x-rays hit atoms this energy can be transferred, causing ionisation of the atoms.

2.1.2 Radiation damage
An x-ray beam consists of millions of high-energy photons. These can damage molecules by ionisation of atoms, but damage to the DNA in the chromosomes is of particular importance. Most DNA damage is successfully repaired, but rarely a portion of a chromosome may be permanently altered (a mutation). This may lead to uncontrolled cell replication, ultimately leading to the formation of a tumour. The latent period between exposure to x-rays and diagnosis of a tumour may be many years. The probability of a tumour being produced is related to the radiation dose, so knowledge of the doses delivered is important. Such effects where the magnitude of the risk is related to dose can be considered as ‘chance’ (stochastic) effects. For these effects there is no clear evidence of the existence of a safe level of radiation dose,2,3 ie. it is currently assumed that any level of dose could lead to tumour induction. However, the lower the radiation dose, the lower the risk of radiation-induced tumours.

There is strong, well documented epidemiological evidence that exposure to radiation at doses above some tens of millisieverts is associated with an increased risk of cancer.4 Studies have shown increased cancer risk associated with CT scans in childhood and raised levels of exposure to background radiation.5,6 Dose levels associated with dental radiology are even smaller; however, a number of epidemiological studies have provided evidence of a possible increased risk of brain, salivary gland and thyroid tumours related to dental radiography.7–16

Another stochastic effect is that of heritable damage seen in the children of irradiated parents. As the radiation dose to the reproductive organs is so low in dental examinations, the risk of heritable effects is negligible.17R

There are other known damaging effects of radiation, such as skin erythema, hair loss and effects on fertility, that definitely have threshold doses below which they will not occur. As dental radiography would never exceed these thresholds which are some thousands of millisieverts, except in extraordinary circumstances, these tissue reactions or deterministic effects are given no further consideration. Cataract formation was, until recently, believed to have a similar threshold; however, the evidence has indicated a threshold of about 500 mSv, a factor of three lower than previously thought.2,18 This level is still well above that observed in dental radiography, but risk of cataract induction could become a concern if many repeated cone beam computed tomography (CBCT) or CT examinations which included the orbits were undertaken.

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2.1.3 Radiation dose

The term ‘dose’ is widely used but often misunderstood. There are three common terms used to describe dose: absorbed dose, equivalent dose and effective dose.

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Absorbed dose (D)
Of the three dose quantities, this is the only one that can be directly measured. The absorbed dose is the mean energy imparted to a unit mass of matter (eg. tissue) by the ionising radiation.2 The units of measurement are joules per kilogram (J/kg), which are called grays (Gy).

Equivalent dose (HT)
Some types of ionising radiation are more damaging to tissue than others. This is taken into account when making dose calculations by weighting the absorbed dose depending on the type of radiation used. For instance, alpha particles will potentially cause much more biological damage than x-rays for the same absorbed dose, and so they are given a weighting 20 times that of x-ray photons. For dental radiography, which only uses x-rays, the absorbed dose and the equivalent dose are numerically the same. Equivalent dose is still measured in J/kg, but its unit is given the special name of sievert (Sv).

Effective dose (E)
Different tissues of the body are more susceptible to the effects of ionising radiation than others. This is taken into account when calculating the effective dose. The International Commission on Radiological Protection (ICRP) has published revised tissue weighting factors for the most radiosensitive tissues of the body.2 The modifications were introduced mainly on the basis of new epidemiological evidence of cancer induction in the survivors of the Japanese atomic bombs. The main changes relevant to the calculation of effective dose from dental radiography are the addition of the salivary glands as an individual weighted tissue and the inclusion of the oral mucosa in the remainder tissues. Consequently, effective doses calculated using the current ICRP recommendations for dental exposures are significantly higher than using the previous weighting factors.19–21 Therefore, caution must be taken when trying to compare effective doses calculated using different weighting factors.

In dentistry, the effective dose is often small, so it is more appropriate to use subunits such as the millisievert (mSv) which is one thousandth of a Sv, or the microsievert (μSv), which is one millionth of a Sv.

2.1.4 Factors influencing effective dose

The radiation dose delivered for a specific imaging requirement can be determined by a range of factors, some related to the equipment type and design, others to the operator’s use of the equipment. They can be grouped into three categories:

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  • The sensitivity of the image receptor – using a high-speed (ie. more sensitive) system will reduce the dose required.
  • The area exposed to the primary beam – reducing the volume exposed and ensuring that more sensitive organs are in areas of lower dose will limit the effective dose.
  • The exposure factors selected – selecting equipment settings that give lower dose while maintaining adequate image quality.

Correct selection of equipment and technique can significantly reduce the patient dose. However, dose should not be reduced to such a level that the image quality is not adequate for the clinical purpose. There is a balance between radiation dose and image quality, and the operator must be trained to understand how the factors interact to optimise the imaging process.

Intraoral radiography
It is recommended that intraoral dental x-ray equipment should have the following design features:22

  • Equipment should operate at a voltage between 60–70kV.
  • Constant potential units are preferred to generators with pulsating waveforms.
  • The beam must be adequately filtered using an appropriate thickness of aluminium.
  • An open-ended spacer cone should be used in conjunction with rectangular collimation.
  • A long focus-to-skin distance should be used (200mm for sets operating between 60–70 kV).
  • A range of available exposure times of sufficiently fine graduations to produce optimally exposed radiographs over the full range of possible patient sizes, anatomical views and speeds of imaging system.
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In addition, a sensitive image detector system should be used, such as F-speed film or a digital detector. The use of modern equipment combined with good technique and sensitive detectors can make a tenfold difference to the effective dose.19 A national review of dose in dental practice in the UK found that the average dose for an adult molar setting had decreased by 57% since 2017 as a result of the adoption of new equipment and more sensitive image detectors.23

A number of studies have compared doses in digital and film intraoral radiography in which the dose reduction offered by digital over conventional film ranged between 20–70%.24,25 However, Berkhout et al 24 put forward the caveat that in practice it is likely that the overall dose reduction is less than 25%, owing to the tendency to take more images when using digital systems. In addition, owing to the greater range of dose that will give acceptable images, care must be taken to use the level of dose that will give an acceptable image as opposed to the best image, which is likely to be at a significantly higher dose.26

Panoramic and cephalometric radiography
There are also marked differences in the doses associated with different panoramic machines, primarily related to the speed of the detector and the selection of beam sizes available.27 Modern film screen systems have speeds very similar to current digital detectors, and it is unlikely that the introduction of digital technology will see the same dose reduction in either panoramic or cephalometric radiography as is currently being experienced in intraoral radiography.27

Cone beam computed tomography
CBCT is the most significant development in dental and maxillofacial imaging in recent years. The first commercially available machine on the market was introduced in the late 1990s, and there are now a large number of manufacturers producing CBCT machines. High-resolution, three-dimensional images of teeth and jaws are produced. Consequently, there are several applications of CBCT in dentistry including endodontics, orthodontic diagnosis and the assessment of the jaw prior to implant placement.28–30 The European Commission has produced evidence-based referral criteria to clarify those clinical situations in which CBCT may be useful.31R CBCT units are significantly cheaper than medical CT machines and have a much smaller footprint. These features, combined with the potential for increased diagnostic yield, make these systems attractive to dentists working outside the hospital sector.

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The effective dose from CBCT is dependent on many factors, with one of the main ones being the volume of tissue irradiated. The volume of tissue that is irradiated is often referred to as the field of view (FoV). Comparison of effective dose with respect to FoV size is shown in Table 2.1. There is a marked range in the effective doses in each category because of differences in the equipment being used, including the detector type and the scanning parameters selected (tube current, voltage and time of exposure, including the number of projections obtained).

In addition, for small FoV scanners the dose varies depending on the region of the jaw being scanned.47 Some machines allow a choice of exposure factors and offer a wide choice of FoVs, while on other units these features are either fixed or there is a limited choice. These factors must be one of the prime considerations when considering buying such a unit. It is important that the exposure factors and FoV are optimised to the clinical question being investigated.31R,60 There is no need for the routine use of lead aprons for patients undergoing CBCT examinations.31R,60 However, there is some evidence to support the use of a thyroid shield for CBCT, as this may reduce the effective dose by up to 40%.58,61 If a thyroid shield is used, it must be positioned carefully so as not to cause artefacts on the images or obscure areas of interest.60

The effective dose from medical CT is generally higher than that from CBCT. However, scanning parameters can be optimised to reduce the dose considerably,50,62–64 with the advantage that soft–tissue differentiation is still possible even with low exposure factors.

Table 2.1: Effective doses for traditional dental 1: Effective doses for traditional dental radiography, CBCT and CT

examinations – tabular summary of literature review

Radiographic techniqueEffective dose (μSv)References
Intraoral radiograph (bitewing/periapical)

0.3–21.6

19,32,33
Panoramic radiograph2.7–3819,21,32–42
Lateral cephalometric radiograph2.2–1419,36,42–46
CBCT (small field of view*)11–21435,47–53
CBCT (medium field of view**)18–67420,35,37,42,47–55
CBCT (large field of view***)60–510.620,21,35,41,51,52,54–56
CBCT (extended field of view****)30–102520,21,41,42,46,47,54–58
CT scan (mandible)250–141034,36,47,50
CT Scan (mandible and maxilla)430–86035,54,59

* The height of cylindrical volume or spherical diameter of the volume ≤ 5cm

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** The height of cylindrical volume or spherical diameter of the volume > 5cm and ≤ 10cm.

*** The height of cylindrical volume or spherical diameter of the volume > 10cm and ≤15cm

**** The height of cylindrical volume or spherical diameter of the volume > 15cm

2.1.5 The risks

It is useful to understand the magnitude of the risk associated with dental radiography when considering the justification of individual dental exposures and the effect of dental exposures on the whole population. Risk factors are regularly published and are constantly being refined as new data becomes available and more sophisticated modelling is undertaken.2,65

As discussed in Section 2.1.2, above, the risk associated with dental radiography is primarily that of cancer induction. A publication by the UK Health Protection Agency proposes the use of a total lifetime cancer induction risk factor for an average 30-to-39-year-old of 6.8% per Sv for men and 5.5% per Sv for women for radiography of the head.65 Another way of expressing this is to say that there is a 1 in 15,000 (for men) and 1 in 18,000 (for women) risk of a cancer being induced for every 1mSv effective dose received from dental radiography.

Risk is age-dependent, being highest for the young and lowest for the elderly. The tissues of younger people are more radiosensitive and their prospective life span is likely to exceed the latent period. For the very elderly, life expectancy will be less than the latency period, which can be anything from five years upwards and the risk could be considered negligible. In general, young children are at about two to three times the risk of develop radiation-induced cancer than adults in their thirties for the same effective dose,65 hence the importance of using specific paediatric exposure protocols when radiographing children to ensure that the dose is minimised.

There is often considerable concern about radiography during pregnancy because of possible risk to the foetus. In dental radiography, it is unusual for an x-ray beam to be pointed at the abdomen (only for vertex occlusal radiographs, which are rarely indicated) and, in those cases where radiography is essential, abdominal lead protection should be used when a foetus lies in the primary beam.22 For all other dental radiographic views, including panoramic and CBCT examinations, there is no requirement to delay radiography until after the birth. However, as the subject of radiography during pregnancy is emotive, it is recommended that practitioners offer pregnant patients the option of delaying non-urgent radiography.66,67

Table 2.1 gives typical doses for radiographic examinations of teeth and jaws, including dental radiographic techniques likely to be used in primary dental care. The doses have been calculated either using the current ICRP recommendations or, in those studies that predate these recommendations, those ones that include weighting factors for the salivary glands. Effective doses are calculated for a reference patient and there are many uncertainties in the calculations. Risk estimates for an individual based on the calculated effective dose may be higher or lower by a factor of five.68 For this reason, risk has been split into broad categories to indicate the health detriment.68 The risk from dental radiography falls into the lowest risk category. Even the risk from medical CT of the jaws is considered very low. However low the risk from the examination, it is imperative that each radiographic exposure is fully justified and the doses optimised to reduce the detrimental effects to the lowest possible level. categories to indicate the health detriment.68 The risk from dental radiography falls into the lowest risk category. Even the risk from medical CT of the jaws is considered very low. However low the risk from the examination, it is imperative that each radiographic exposure is fully justified and the doses optimised to reduce the detrimental effects to the lowest possible level.

2.1.6 Diagnostic reference levels
Surveys of dental radiography practice have consistently shown a significant variation in dose for the same examinations between dental practices. In the 2005 survey of dental practices, the dose for the same intraoral dental radiographic examination varied by a factor of 600 between the lowest and highest doses.27

The concept of diagnostic reference levels (DRLs) has been developed in response to such wide variations in patient doses to provide an audit standard. National
reference levels for conventional dental radiography are based on the third-quartile values for the distribution of doses found in such surveys, ie. 75% of equipment
gives doses below the reference value.69,70 Auditing patient dose and comparison with locally-set DRLs is a legal requirement in the UK.71,72 Those dentists with
equipment giving doses above the local DRL need to investigate the cause and instigate measures to reduce the doses to below the local DRL. For general dental

practice it is recommended that these national reference levels are adopted as the local DRLs, ie. local audit standards,22 unless local measurements of patient doses support the adoption of lower values. This should be determined with the help of your medical physics expert. The current recommended national reference levels for dental radiography are shown in Table 2.2.

Audit data are not yet available for the establishment of a national DRL for CBCT. However, local DRLs should be set up with the help of a medical physics expert.74

The Health Protection Agency (now part of Public Health England) carried out an initial analysis and recommended an achievable dose based on the ability to collimate to the area of clinical interest.74

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Table 2.2: Suggested national diagnostic reference levels for dental radiography

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ExaminationNational Diagnostic Reference Level References
Intraoral (adult molar)1.7 mGy*23
Panoramic (adult)93 mGy cm2 **23
Panoramic (child)67 mGy cm2 **23
Lateral cephalometric
radiograph (adult)
40 mGy cm273
Lateral cephalometric
radiograph (child)
25 mGy cm273
Achievable Dose
CBCT (adult upper first
molar implant)
250 mGy cm2 **73

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* measured as the absorbed dose in air at the end of the spacer cone
** measured as the dose area product (DAP)

2.1.7 Doses and risks in context

The effective dose from a CBCT examination is comparable to between two days' and six months’ additional background radiation, depending on the machine used and the exposure parameters selected. For comparative purposes, a chest x-ray (18 μSv) is equivalent to around three days of additional background radiation, while some medical computed tomography examinations equate to over ten years' background radiation.76–78

Unfortunately, one in four people in the UK die from cancer.79 Radiography increases the chance of developing cancer, and it is estimated that diagnostic radiology (medical and dental) causes 700 cancer cases per year in the UK.80 The collective dose to the population from dental radiography is low so only a small proportion of these cancers can be attributed to dental radiography.81 However, choosing to radiograph a patient and selecting the technique and its frequency should be a matter of balancing the risk against the clinical benefits to that patient.

A

No patient should be expected to receive additional radiation dose and risk as part of a course of dental treatment unless they are likely to benefit from dental radiography.

Notwithstanding the already low individual risk, every effort should be made to undertake the radiography at minimum dose to the patient. the lowest-dose examination that will answer the clinical question should always be undertaken.*

*this important statement is given the highest level of recommendation even though there are no randomised controlled trials to support it. Such a study design would be neither possible nor ethical.