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Free AccessCBCT SPECIAL ISSUE: Review Article

Effective dose of dental CBCT—a meta analysis of published data and additional data for nine CBCT units

Published Online:https://doi.org/10.1259/dmfr.20140197

Abstract

Objectives:

This article analyses dose measurement and effective dose estimation of dental CBCT examinations. Challenges to accurate calculation of dose are discussed and the use of dose–height product (DHP) as an alternative to dose–area product (DAP) is explored.

Methods:

The English literature on effective dose was reviewed. Data from these studies together with additional data for nine CBCT units were analysed. Descriptive statistics, ANOVA and paired analysis are used to characterize the data.

Results:

PubMed and EMBASE searches yielded 519 and 743 publications, respectively, which were reduced to 20 following review. Reported adult effective doses for any protocol ranged from 46 to 1073 µSv for large fields of view (FOVs), 9–560 µSv for medium FOVs and 5–652 µSv for small FOVs. Child effective doses from any protocol ranged from 13 to 769 µSv for large or medium FOVs and 7–521 µSv for small FOVs. Effective doses from standard or default exposure protocols were available for 167 adult and 52 child exposures. Mean adult effective doses grouped by FOV size were 212 µSv (large), 177 µSv (medium) and 84 µSv (small). Mean child doses were 175 µSv (combined large and medium) and 103 µSv (small). Large differences were seen between different CBCT units. Additional low-dose and high-definition protocols available for many units extend the range of doses. DHP was found to reduce average absolute error for calculation of dose by 45% in comparison with DAP.

Conclusions:

Large exposure ranges make CBCT doses difficult to generalize. Use of DHP as a metric for estimating effective dose warrants further investigation.

Introduction

The intent of this article is to review the literature on dosimetry of maxillofacial CBCT imaging in dentistry. A discussion of dose outside the context of biological harm has little relevance to patient care; therefore, this report focuses on effective dose, a quantity with direct correlations to biological risk. A number of approaches may be taken to measure dose and calculate effective dose, and some of the advantages and disadvantages of these are explored. Because children are at greater risk from exposure to ionizing radiation, child and adult doses are explored separately. Tables of published effective dose data from all protocols and equivalent dose data from standard CBCT protocols are presented together with previously unreported data for nine CBCT units. Finally, dose–area product (DAP) is contrasted with dose–height product (DHP) as a potential surrogate for estimating effective dose.

Is dose from maxillofacial radiographic imaging a relevant risk?

Dentistry had an early awareness of the dangers of exposure to ionizing radiation, as pioneers of radiographic imaging such as Edmund Kells suffered carcinoma of the hands resulting from repeated unprotected exposures during imaging of their patients.1 We were warned about the dangers of radiography to both practitioners and patients by other pioneers such as Rollins.2 But not all practitioners were convinced that there was a significant risk. In an exchange of letters published in successive February 1901 issues of the weekly predecessor to the New England Journal of Medicine, Rollins assertion that “X-light kills” was rebutted by a prominent surgeon, Ernest Codman, stating that “in careful hands, there is no danger from the use of the X-ray to the patient and very little to the operator.”3 Although the scientific debate about the level of risk associated with diagnostic imaging continues, there is substantial evidence for a cumulative dose-related response to ionizing radiation in the form of cancer developing years after initial exposure. Some of this evidence comes from the Life Span Study of atomic bomb survivors, a well-documented cohort of 105,427 people exposed to a range of doses.4 Analysis of these data and data from several other cohorts provides good support for an increased risk of cancer from acute exposures in a range of 10–50 mSv and chronic exposures in a range of 50–100 mSv.5 This has prompted support by the National Commission on Radiation Protection and Measurements for a linear extrapolation of higher dose-associated cancer risk to lower levels of exposure: “Although other dose–response relationships for the mutagenic and carcinogenic effects of low-level radiation cannot be excluded, no alternate dose–response relationship appears to be more plausible than the linear-non-threshold model on the basis of present scientific knowledge.”6 The Life Span Study data also indicate a significant radiation-associated increase in the risk of cancer occurring in adolescence and young adulthood.4 Diagnostic imaging contributes to individual and population exposures to ionizing radiation, and it has been suggested that as many as 1.5–2.0% of cancers in the USA may be related to X-ray exposure from CT imaging.7 Recent studies have confirmed that cancer risk extends to X-ray exposure from diagnostic imaging of the maxillofacial complex. In a Great Britain cohort of approximately 175,000 subjects who were children at the time of CT head scan exposures, cumulative doses of about 50 mGy almost tripled the risk of leukaemia and doses of about 60 mGy almost tripled the risk of brain cancer.8 Similar findings were seen in an Australian cohort of 10.9 million people aged 0–19 years, where a 24% increase in cancers, including brain cancers and leukaemia, were noted following CT exposure. The incidence was associated with increasing dose and young age at the time of exposure.9

CBCT is a form of CT that has been adapted to maxillofacial imaging and has been enthusiastically embraced by dentistry. Since its introduction in the European market in 1996 and in the US market in 2001, 15 different manufacturers have offered 24 CBCT models in the USA and many more worldwide.10 CBCT use has found its way into many aspects of general and speciality practice, including adolescent orthodontics. Public concern about this particular application has prompted questions from both patients and practitioners about safe use and best practice.11 While the risk from dentomaxillofacial imaging is small for an individual, when multiplied by the large population of patients who are exposed to diagnostic imaging, radiation risk becomes a significant public health issue.

Measures of exposure to ionizing radiation

Exposure is the simplest measure of radiation dose. A variety of radiation detection devices, including ionization chambers, radiosensitive films, thermo or optical light-stimulated luminescent dosemeters, and metal oxide-semiconductor field-effect transistor devices may be used to measure ionization caused by radiation. Because calcified tissues absorb X-rays more effectively than do soft tissues and because the absorption of X-rays varies substantially with the size and shape of imaged anatomy as well as the distribution of tissues of different densities within that anatomy, exposure in air provides a limited and sometimes misleading indication of the energy imparted to different tissue types and thicknesses.

Measurement of absorbed dose in specific tissues or organs permits estimation of potential harm to a tissue of interest. Because the absorption efficiency of the radiation detection device may be different than the organ of interest, adjustment of measured values to compensate for this is necessary. For example, absorption efficiency of bone may be 2–4 times greater than that of soft tissues for average photon energies in the diagnostic spectrum. Absorbed dose is expressed in the international unit, gray and more commonly for diagnostic imaging in milligray.

Equivalent dose is absorbed dose adjusted for the attenuation characteristics of the radiation that is involved. The attenuating quality of radiation has a significant impact on biological effectiveness for cancer induction and genetic effects. Alpha particle radiation produces numerous ionization events over a short distance in comparison with the energy that is transferred by an X-ray photon over the same distance. The calculation of equivalent dose (HT) is the product of the absorbed dose (DT) and a radiation-weighting factor (wR), which accounts for relative biologic effectiveness of the radiation. While wR for α particle radiation is 20, the wR for X-rays is 1. It is convenient, but sometimes confusing, that X-ray absorbed dose and equivalent dose are the same value. Equivalent dose is expressed in international units, sievert and again, for diagnostic imaging, more commonly in millisievert.

Although in some instances it is useful to consider the biological response to an equivalent dose for a particular tissue of interest, it is often desirable to evaluate a variety of exposures of different types and different body areas for a collective outcome such as cancer. The International Commission on Radiological Protection (ICRP) has recommended a calculation called effective dose as the preferred method for comparing risks from different exposures to ionizing radiation. Effective dose is a calculation that considers the most radiosensitive tissues and organs of the body and provides a fractional weighting reflecting the degree of sensitivity for each of those organs. Effective dose is reported in sieverts and for diagnostic imaging is more commonly expressed in millisieverts or microsieverts. Effective dose is calculated using the equation: E=wT×HT, where E is the summation of the products of the tissue weighting factor (wT) and the absorbed dose within that tissue HT.12 Because estimation of the risk of the stochastic effects of genetic mutation and cancer formation has evolved with additional data reported from observations of a variety of exposed populations, the ICRP has changed the calculation of effective dose several times. The most recent change was in 200713 and is noteworthy because weights of several tissues located in and around the maxillofacial region were changed, and several other tissues within this region were added to the calculation.14 Changes in tissue weights have resulted in a 10% increase in weight of tissues located in the maxillofacial area and a 28% increase in weight after adjusting for the distribution of tissues. Newly added tissues for effective dose calculation that are entirely within the maxillofacial area include oral mucosa, salivary glands and the extrathoracic airways.

Approaches to measuring dose—do “all roads lead to Rome”?

The process of measuring equivalent dose and calculating effective dose requires a real or virtual device known as a phantom. There are numerous design variations described in the literature or commercially available that include differences in phantom size, material composition and number of dosemeter locations. While all phantoms simulate human morphology and radiation attenuation characteristics to a varying extent, the gold standard method of obtaining dosimetry for calculating effective dose utilizes an anthropomorphic phantom. Alternate techniques for calculating dose that do not use anthropomorphic phantoms include CT dose index volume (CTDIvol), dose linear product, air kerma-area product and DAP. In a previous study comparing an anthropomorphic phantom and a standard acrylic cylinder with a single ion chamber used to calculate (CTDIvol), we demonstrated that the standard acrylic cylinder underestimates effective dose by 38–62%.15 This underestimation is in part owing to the failure to account for scatter dose to tissues outside of the scan region. Kerma-area product is another method that has recently been used to calculate dose.16 Values reported in the referenced study underestimate effective dose by 90–300% when compared with effective dose calculated from anthropomorphic phantom data.17 DAP has also been suggested as a simple approach for calculating dose. However, our experiments with the SCANORA® 3D (Soredex, Helsinki, Finland) unit revealed an approximately three-fold change in effective dose between various locations of the small field of view (FOV) with no change in DAP.18 By contrast, anthropomorphic phantoms made from materials that have similar X-ray attenuation characteristics as human tissue and have multiple dosemeters allow for accurate measurement of absorbed dose. In a recent study, we confirmed that an anthropomorphic phantom using bone equivalent material in place of a human skeleton could provide reliable measures of effective dose.19 Virtual phantoms and Monte Carlo simulation of exposure have been used in assessment of organ dose and effective dose for a number of studies. Dose correspondence with anthropomorphic phantom studies is dependent on the virtual phantom that is used as well as imaging geometry and technical parameters. In one study comparing four different phantoms developed from CT data, a 70% difference in effective dose was noted for a large FOV scan depending phantom choice.20 When a cephalometric analysis was performed on ICRP adult male and female phantoms, a 17° downward rotation difference of the Frankfort plane was seen in the female. This downward rotation of the chin effectively moves the thyroid closer to the radiation field and results in an average of a 3.7-fold increase in female thyroid dose for four large FOVs that were evaluated in one study.21 Similar increases are seen in oesophageal dose in this phantom. Although the ICRP phantoms were developed for general dosimetric applications, the inability to adjust phantom posture to establish a Frankfort horizontal plane or any orientation other than the original orientation used to acquire axial CT slices limits the accuracy of these phantoms in simulations of dental CBCT diagnostic protocols. Although virtual phantoms with Monte Carlo simulation hold a great deal of promise, further attention must be given to virtual phantom development consistent with positioning for standard dental diagnostic protocols before this becomes a reliable replacement for anthropomorphic phantoms.

Even anthropomorphic phantom systems are subject to variations in dosimetry from a variety of sources. Intended differences in subject size and organ location are seen when comparing child and adult phantoms. Child phantom effective doses are approximately 36% greater than adult phantom doses for the same imaging protocol.22 This difference is largely related to the proximity of the thyroid gland to the lower border of the mandible. Because of reduced distance between the thyroid and the mandible in a child, direct exposure of the thyroid is more likely and the intensity of scatter radiation from jaw structures to the thyroid is greater.22

Use and abuse of effective dose

Effective dose was developed to provide a measure of stochastic risks from exposures to low doses of ionizing radiation. While developed for use in radiation protection, it should not be applied to estimations of individual patient risks. There are several reasons for this. Foremost, effective dose represents risk to a reference subject who is an average of characteristics, including age, gender and genetic radiation sensitivity. Another reason is that effective dose estimation is subject to numerous sources of uncertainty.23 Among these is the need to extrapolate stochastic outcomes associated with higher doses to the low doses associated with dental diagnostic imaging. X-ray beam shape and imaging geometry for organs partially in or just outside the beam is also estimated to account for as much as ±40% variation. Despite these limitations, effective dose is a useful metric for comparing alternative imaging modalities or examination protocols in terms of relative risk. Employing the same methods of dose measurement for different examinations, units or protocols, we can evaluate which produces a greater or lower risk. The caveat that this risk may be greater or less depending on individual patient characteristics makes the comparison no less valid.

Biological parameters that influence dose

A number of physical and biological parameters influence individual dose and risk. Age has a significant impact on both. Children are physically smaller, which places peripherally located brain and thyroid tissues closer to the dental area that is being imaged. Even if not directly exposed, these organs will receive increased scatter radiation with increased proximity to the location of the scanned volume. But children are not simply small adults. They are also at increased risk from any exposure to ionizing radiation owing to cellular growth and organ development, which increases radiosensitivity of tissues. In conjunction with a longer life expectancy in which cancer can develop, children may be two times or more sensitive to radiation carcinogenesis than are mature adults.24,25 Physical differences associated with gender are also associated with differences in risk. Females are at significant risk for breast cancer, while males are not. Females are at risk for ovarian cancer, while males are at risk for prostate cancer. Because these organs are distant from the maxillofacial area, gender differences do not impact dose and risk estimation for maxillofacial imaging.

Technical parameters that influence dose

Receptor technology and field of view

Multiple technical parameters influence patient dose. Two types of receptor technologies are used to acquire image data. Image intensifiers utilize a round receptor and produce a spherical FOV. Square or rectangular flat panel detectors are incorporated in many CBCT units, and these produce a cylindrical FOV. In general, the cylindrical field is more efficient at capturing the anatomy of the maxillofacial complex when the top of the field includes the temporomandibular joint areas. A cylindrical volume diameter, which captures both temporomandibular joint and chin anatomy will require a spherical volume diameter that is approximately 25% larger to cover the same anatomy.

mAs

X-ray tube current (mA) and exposure time (s) are directly proportional to dose when other factors remain constant. The product of mA and s (mAs) is also directly proportional. For instance, doubling mAs doubles dose. It should be mentioned that exposure time may not be the same value as scanning time. Some CBCT units produce continuous output of radiation during scanning. For these units, scan time is equal to exposure time. However, most detectors are unable to record X-ray exposure during the period when the image detector integrates the X-ray energy absorbed in individual receptor pixels and transfers this signal to the computer. Continued X-ray exposure during signal integration contributes to patient dose but adds nothing to image formation. To eliminate this unnecessary patient exposure, many CBCT units utilize a pulsed X-ray source, where X-ray emission is intermittently turned off during the image acquisition process.

kVp and beam filtration

X-ray beam quality has long been noted as a factor associated with patient dose from diagnostic imaging. Increasing filtration of the X-ray beam reduces patient exposure to lower energy X-ray photons that are more likely to contribute to patient dose without contributing to image formation.26 While higher beam energies (kV) are associated with loss of contrast in film-based imaging, digital imaging affords the possibility of post-acquisition contrast enhancement. Use of 0.4 mm of additional copper filtration in conjunction with increased kVp was demonstrated to reduce patient dose by an average of 43% with one unit.10 In a study of a different manufacturer's unit, the effective dose for a standard exposure with an 8 × 8-cm FOV was reduced 57% using 0.5 mm of additional copper filtration when compared with the dose produced by an earlier version of the unit.15,19

Resolution

In order to maintain adequate signal-to-noise level, exposure must be increased as the voxel size is reduced to create higher resolution images. This can take place as an increase in mA or an increase in the number of basis images that are acquired. With some CBCT units, this choice is under operator control, but, at other times, the unit dictates which exposure factors may be used with different resolutions. Automatic doubling of dose when switching from standard to high resolution has been reported for one unit.22

Methods and materials

Systematic review of literature on CBCT and effective dose

A systematic review of the literature concerning CBCT dosimetry in the maxillofacial region was performed. A PubMed (MEDLINE) database (National Library of Medicine, NCBI) search was performed on 11 November 2013 and updated on 26 May 2014. An EMBASE search was also performed on 26 May 2014 by a senior librarian at the University of North Carolina Health Science Library, Chapel Hill, NC. The strategy to search for publications in English language indexed in the MEDLINE database was as follows: {“Cone-Beam Computed Tomography” [(Mesh)] OR CBCT (tw) OR CBVT (tw) OR Cone beam computed tomography (tw) OR Cone beam volumetric tomography (tw)} AND [“Radiation Monitoring” (Mesh) OR “Radiation dosage” (Mesh) OR absorbed dos* (tw) OR equivalent dos* (tw) OR effective dos* (tw) OR dosimetry (tw)]. The strategy to search for publications in English language indexed in the EMBASE database was as follows: (“cone beam computed tomography”/exp OR “cone beam computed tomography scanner”/exp OR CBCT:ti,ab OR CBVT:ti,ab OR “Cone beam computed tomography”:ti,ab OR “Cone beam volumetric tomography”:ti,ab) AND (“radiation monitoring”/exp OR “radiation dose”/exp OR “dosimetry”/exp OR “absorbed dose”:ti,ab OR “absorbed doses”:ti,ab OR “absorbed dosage”:ti,ab OR “absorbed dosages”:ti,ab OR “equivalent dose”:ti,ab OR “equivalent doses”:ti,ab OR “equivalent dosage”:ti,ab OR “equivalent dosages”:ti,ab OR “effective dose”:ti,ab OR “effective doses”:ti,ab OR “effective dosage”:ti,ab OR “effective dosages”:ti,ab). The PubMed search yielded 519 articles and the EMBASE search yielded 743 articles. Inclusion criteria for this review were all articles published in English in the scientific literature related to CBCT dosimetry in the maxillofacial region. Only articles utilizing tissue weights from the 2007 ICRP recommendations for calculating effective dose were included. The articles had to include pertinent information regarding the scanner used, FOV size and location, exposure technique, phantom type and dosemeter used. Articles not meeting the inclusion criteria were excluded after downloading the references in EndNote® (Thompson Reuters, Rochester, NY) and reviewing the abstracts. 65 articles were initially included and the PDF of the articles were downloaded in EndNote and reviewed in more detail. 43 more articles were excluded leaving 22 articles. Data from these studies were placed in a spreadsheet for analysis. Data identified as outliers led to the identification of methodological errors resulting in the removal of two studies and reassessment of the data. Data from 20 studies and additional unpublished data otherwise meeting the inclusion criteria were ultimately tabulated and presented in this article. Reasons for exclusion of the 43 manuscripts are catalogued in Table 1.

Table 1 Reasons for exclusion of 45 of 65 citations resulting from literature search

Reason for exclusionNumber
Dosimetry based on 1990 International Commission on Radiological Protection calculation of effective dose rather than 2007 recommendations6
Image-guided radiation therapy device or other non-dental unit10
Incomplete dosimetry—multiple organs in the head and neck area have been omitted including remainder tissues6
Not a dosimetry article, a review article, letter or other secondary source7
Technique article that does not provide information about specific or identifiable units3
Effective dose not reported9
Animal study or anatomy other than maxillofacial area2
Foreign language with English abstract1
Other—reported results violate dose/mAs linearity by ×3 indicating an unrecognized calculation error1

Additional dosimetry

Previously unreported data are included for nine additional CBCT units: 3D Accuitomo (J Morita, Osaka, Japan), CS 9000 (Carestream Dental, Atlanta, GA), CS 9300 (Carestream Dental), Orthophos XG 3D (Sirona Dental Systems, Bensheim, Germany), Galileos Comfort Plus (Sirona Dental Systems), ProMax Mid (Planmeca Oy, Helsinki, Finland), NewTom VGi (Cefla Dental Group, Imola, Italy) and OP 300 Maxio (Instrumentarium, Helsinki, Finland). Scanning protocols for these units are found in Table 2.

Table 2 Units and protocols used to produce unreported dosimetry included in this manuscript

CBCT unitManufacturerPhantom typeProtocolField of view
3D AccuitomoJ Morita (Osaka, Japan)Atom child, adultStandard, high fidelity, high resolution, high speed/360°, 180° scans/child, average adult, large adultLarge, medium, small
CS 9000Carestream Dental (Atlanta, GA)Rando adultStandardSmall
CS 9300Carestream DentalAtom child, adultStandardMedium, small
Orthophos XG 3DSirona Dental Systems (Bensheim, Germany)Atom child, adultStandard, high definition, endo/child, teen, small adult, average adult, large adultSmall
Galileos® Comfort PlusSirona Dental SystemsAtom child, adultStandard, high definition/child, teen, small adult, average adult, large adultMedium, small
ProMax® MidPlanmeca Oy (Helsinki, Finland)Atom child, adultNormal, low dose, high definition/child, adolescent, small adult, average adult, large adultMedium
NewTom VGiCefla Dental Group (Imola, Italy)Rando adultStandard, high resolutionMedium, small
NewTom 3GCefla Dental GroupAtom childStandardLarge, medium
OP 300 MaxioInstrumentarium (Helsinki, Finland)Atom child, adultStandard, low dose, high definition, endo/child, small adult, regular adult, large adultMedium, small

Following previously published protocols that utilize 24 dosemeters placed in and on anthropomorphic phantoms, dosimetry was acquired for standard imaging protocols and additional protocols when available. Head and neck phantoms replicating the radiation attenuation characteristics of human tissues and anatomy were used.15,22 The child phantom (Atom Model 706 HN; CIRS Inc., Norfolk, VA) simulated characteristics of a 10-year-old child. An adult phantom simulated an average adult male (Atom Max Model 711 HN; CIRS Inc.).

Optically stimulated luminescent dosemeters (nanoDot™, Landauer, Inc., Glenwood, IL) were cleared of ambient charge prior to use, using a minimum of 12 h of exposure to light from a florescent tube, dental radiographic film view box. Dosemeters were read to record residual baseline energy level using a portable reader (MicroStar; Landauer, Inc.). The reader was calibrated before use using a set of 80-kVp reference dosemeters supplied by the manufacturer. After adjusting for individual dosemeter energy sensitivity, photon counts were converted to dose and automatically recorded in a database by the reader. After placement in a phantom and exposure to the CBCT scan, dosemeters were read three times with the reader. The modal value of the three readings was selected as the dose of the dosemeter. Doses were exported from the database as an Excel® (Microsoft, Redmond, WA) spreadsheet and adjusted for response to the estimated mean energy of the X-ray beam using a third-order polynomial calibration curve derived from side-by-side comparison of recorded doses from an ion chamber and optical stimulated luminescent dosemeters over a range of 80–120 kVp using an adjustable kVp source. Beam energy adjustments ranged from 0.97 for an 84-kVp source (mean kV = 56) to 0.78 for a 120-kVp source (mean kV = 80). 2–20 exposures were utilized for each dosemeter run to provide a more reliable measure of radiation in the dosemeters. Smaller FOVs require more exposure repetition because more dosemeters are outside the field of direct exposure and absorb only small quantities of scatter radiation. For every scan, a scout view was also acquired. Dosemeter values were divided by the number of scans to determine the “exposure per examination” for each dosemeter.

Absorbed dose for a tissue or organ used in the estimation of effective dose was calculated by averaging doses for dosemeters located within that tissue and are reported in micrograys (µGy).22 In instances where tissues were not fully contained within the head and neck area, an estimation of the proportion of this tissue within this area was used to calculate organ absorbed dose. For skin surface, lymph nodes and muscle, an estimate of 5% was used. For the oesophageal tract, an estimate of 10% was used. Calculations for bone surface and bone marrow were adjusted for calvarial, jaw or spine location as well as phantom type (child, adult) using estimations of Underhill et al27 for bone distribution and Christy28 for marrow distribution. For bone, a correction factor based on experimentally determined mass energy attenuation coefficients for bone and muscle irradiated with monoenergetic photons was applied. Effective beam energy estimated to be two-thirds of the peak beam energy of the CBCT unit was used to determine bone/muscle attenuation ratios. A linear fit (R2 = 0.996) of ratios from 40 to 80 kV from published data29 was used to calculate bone/muscle ratio for the CBCT unit kVp setting. Calculated values provided bone/muscle attenuation ratios from 3.46 at 54.0 kV (84 kV peak) to 1.97 at 80 kV (120 kV peak) for the units and protocols investigated in this study. The products of absorbed dose and the percentage of a tissue or organ irradiated in the CBCT examination were used to calculate equivalent dose in microsieverts (µSv). Effective dose (E), expressed in µSv, was calculated using ICRP 2007 tissue weighting factors.13

Data analysis

Effective doses for various exposure parameters and protocols are reported in a tabular format together with equivalent doses from standard or default CBCT imaging if these were included in the publication. Means and variance for data grouped by child or adult phantom and small, medium and large FOVs are reported in summary tables. For this manuscript, small FOVs are defined as any field with a height ≤10 cm. Medium FOVs include a range of volume heights from 10 to 15 cm. Large FOVs have volume heights >15 cm. ANOVA is used to distinguish differences in equivalent doses or effective doses owing to the variables of phantom and FOV. An additional ANOVA examines the effect of maxillary or mandibular position on effective dose for small FOVs. An α level of 0.05 was selected for statistical significance.

The product of salivary gland dose and the dimensions of FOV (H × W) are used as a surrogate for DAP to calculate conversion coefficients for effective dose estimation. Similarly, the product of salivary gland dose and volume height alone (DHP) is calculated to investigate the possible use of this metric for calculating effective dose conversion coefficients. This is analogous to the product of CTDI and scan length (dose–length product) as a dose metric in CT imaging. The absolute error between the estimated effective dose derived from DAP and the phantom dose measurement was compared with absolute error between DHP-derived dose and phantom measurement in a matched pairs analysis.

Results

Table 3 displays exposure parameters and doses for an adult phantom and large FOVs. Reported effective doses from standard protocols ranged from 46 to 916 µSv. Table 4 lists doses for medium FOVs. Reported effective doses from standard protocols ranged from 47 to 560 µSv. Tables 5 and 6 provide doses for maxillary and mandibular small FOVs, respectively. Standard protocol doses ranged from 5 to 140 µSv for maxillary views and from 18 to 488 µSv for views including the mandible. Table 7 catalogues temporomandibular joint FOVs. Table 8 combines large and medium FOVs for child phantoms. Doses from standard protocols ranged from 39 to 430 µSv. Tables 9 and 10 list child doses for maxillary and mandibular small FOVs, respectively. Maxillary effective doses from standard protocols ranged from 16 to 177 µSv, while FOVs including the mandible ranged from 24 to 331 µSv. Reported adult effective doses for any protocol in Tables 36 ranged from 46 to 1073 µSv for large FOVs, 9–560 µSv for medium FOVs and 5–652 µSv for small FOVs. Child effective doses from any protocol in Tables 710 ranged from 13 to 769 µSv for large or medium FOVs and 5–582 µSv for small FOVs. Although standard protocols were the focus of this study, the included reports and additional data provided a total of 41 large FOV protocols, 81 medium FOV protocols and 249 small FOV protocols for adult phantom imaging. For child imaging protocols, the totals were 8, 35 and 103 for large, medium and small FOVs, respectively.

Table 3 Adult phantom equivalent and effective doses for standard or default exposures for large field of view (FOV) CBCT units (>15 cm height)

Unit nameManufacturerFOV size H × W (cm)kVpmAsEffective dose (µSv)Bone marrow (µSv)Bone surface (µSv)Skin (µSv)Oesophagus (µSv)Brain (µSv)Thyroid (µSv)Salivary glands (µSv)Remainder (µSv)Study
ProMax Mid-stichedPlanmeca Oy (Helsinki, Finland)16 × 1690(108, 127, 145)
271, 325, 380
“361, 433, 506”
(95, 112, 128)
223, 283, 339
“304, 365, 426”
345142823514524363455049189Current study
SkyView®Cefla Dental Group (Imola, Italy)17 × 179051.587134125587194741582224Pauwels et al30
iCAT NGImaging Sciences (Hatfield, PA)17 × 2312018.7464016060206605085086Morant et al21
3D eXam®Imaging Sciences17 × 2312037156Rottke et al31
3D eXamImaging Sciences17 × 2312018.572Schilling and Geibel32
iCAT® FLXImaging Sciences17 × 2312018.5, 3769, 1368420250396683011293195Ludlow and Walker22
iCAT NGImaging Sciences17 × 2312018.5, 3767a, 12913927739158881591158165Grunheid et al33
iCAT NGImaging Sciences17 × 2312018.57414729452339501831250186Ludlow and Ivanovic15
iCAT NGImaging Sciences17 × 2312018.578Davies et al34
iCAT NGImaging Sciences17 × 2312037182Roberts et al 200935
Alphard VEGAAsahi Roentgen (Kyoto, Japan)18 × 208068, 102123, 18342719822922815305334090532Kim et al36
CS 9500Carestream Dental (Atlanta, GA)18 × 20901081362062159212055852676380Pauwels et al30
CS 9500Carestream Dental18 × 2080, 85, 9086.4, 108, 10893, 163, 26021874712313116408352645389Ludlow10
CS 9500Carestream Dental18 × 2090108151Rottke et al31
CB MercurayHitachi (Tokyo, Japan)19 × 19100, 120100, 150569, 10736923211389393396763335467828Ludlow and Ivanovic15
CB MercurayHitachi19 × 19120b150b9161726801984636510,100320013,9001976Librizzi et al37
CB MercurayHitachi19 × 1980, 100, 100, 120100, 100, 150, 150256, 466, 683, 932Jadu et al38
IlumaImtec (Ardmore, OK)19 × 19120b76, 15294, 157Vassileva and Stoyanov16
IluminaImtec19 × 191202098161745825012673501661248Ludlow and Ivanovic15
IluminaImtec19 × 19120b152b49883438694212336267173384001265Ludlow and Ivanovic15
NewTom 3GCefla Dental Group19 × 191108.1681255816257700333956140Ludlow and Ivanovic15
NewTom 9000Cefla Dental Group19 × 19110Auto957819060601807751550273Qu et al39
DCT PROVATECH (Seoul, Korea)19 × 20901052543911260160143174018953210518Qu et al40

H, height; kVp, kilovolt peak; mAs, milliampere per second; W, width.

Bold values represent parameters used to produce standard or default scans.

aAverage of 0.3 and 0.4 voxel scan data.

bInitial manufacturer recommended exposure—subsequently reduced.

Table 4 Adult phantom equivalent and effective doses for standard or default exposures for medium field of view (FOV) CBCT units (10–15 cm height)

Unit nameManufacturerFOV size H × W(cm)kVpmAsEffective dose (µSv)Bone marrow (µSv)Bone surface (µSv)Skin (µSv)Oesophagus (µSv)Brain (µSv)Thyroid (µSv)Salivary glands (µSv)Remainder (µSv)Study
3D Accuitomo 170J Morita (Osaka, Japan)10 × 109045, 79, 87.5, 154132, 232, 257, 45326783115214017614985487776Current study
Alphard VEGAAsahi Roentgen (Kyoto, Japan)10 × 108068, 13685, 1842171009147434968726233788Kim et al36
CB MercurayHitachi (Tokyo, Japan)10 × 1012015040746621613441102950130090061355Ludlow and Ivanovic15
CB MercurayHitachi10 × 1080, 100, 120100, 100, 150148, 261, 421Jadu et al38
Alphard VEGAAsahi Roentgen10 × 108068, 13669, 1461969073081710194034544659Kim et al36
CB MercurayHitachi10 × 1080, 100, 120100, 100, 15060, 97, 145Jadu et al38
CS 9300Carestream Dental (Atlanta, GA)10 × 109025765720844371233591855257Current study
3D Accuitomo 170J Morita10 × 149045, 79, 87.5, 154138, 242, 269, 47328689717115029213725861843Current study
3D Accuitomo 170J Morita10 × 149087.5188Theodorakou et al41
NewTom VGQR (Verona, Italy)10 × 1511010.483115163502513541690281Pauwels et al30
iCAT NGImaging Sciences (Hatfield, PA)10 × 1612010, 18.532, 5330120504019070115098Morant et al21
DCT PROVATECH (Seoul, Korea)10 × 169010524917055011016529027003780556Qu et al40
NewTom VGCefla Dental Group (Imola, Italy)11 × 15110Auto81Theodorakou et al41
iCAT NGImaging Sciences11 × 1612010, 18.536, 58401506040310801230110Morant et al21
iCAT® FLXImaging Sciences11 × 1690/1206/10, 18.5, 379/43, 79, 1597917641432383531859256Ludlow and Walker22
CS 9300Carestream Dental11 × 179025.6, 51.5101, 2042107351581168209304445626Current study
NewTom VGiQR12 × 151106.2103100269113548784772076301Current study
3D Accuitomo 170J Morita12 × 179045, 79, 87.5, 154154, 260, 325, 532330105229520044619515843894Current study
3D Accuitomo 170J Morita12 × 179087.5216Theodorakou et al41
OP300 MaxioInstrumentarium (Helsinki, Finland)13 × 159029, 36, 45, 7266, 82, 102, 16412241472521804272096342Current study
i-CAT ClassicImaging Sciences13 × 1612018.56995145014930567267145053Ludlow and Ivanovic15
iCAT NGImaging Sciences13 × 1612018.5871052118245808283183667Ludlow and Ivanovic15
iCAT NGImaging Sciences13 × 1612010, 18.540, 66502008040590801270123Morant et al21
iCAT NGImaging Sciences13 × 1612018.583116124543753551830260Pauwels et al30
3D eXam®Imaging Sciences13 × 1612018.5107Schilling and Geibel32
iCAT FLXImaging Sciences13 × 1690/1206/10, 18.5, 3711/54, 85, 1718518574483804051898265Ludlow and Walker22
iCAT NGImaging Sciences13 × 1612018.577Davies et al34
iCAT NGImaging Sciences13 × 1612018.5111Roberts et al35
iCAT NGImaging Sciences13 × 1612018.582Theodorakou et al41
SCANORA® 3DSoredex (Helsinki, Finland)13.5 × 14.58548688694552552961568221Pauwels et al30
CS 9300Carestream Dental13.5 × 179045.21841595721501037899074006575Current study
Iluma EliteImtec (Ardmore, OK)14 × 21120763686606672773415123072251034Pauwels et al30
Alphard VEGAAsahi Roentgen15 × 158085, 153158, 28860928274213628348216931892Kim et al36
CB MercurayHitachi15 × 15a1201505488745110569100895010758760337Librizzi et al37
CB MercurayHitachi15 × 15a12015056094043606411775933170010,561379Ludlow and Ivanovic15
CB MercurayHitachi15 × 15a100962274854872255030324733673586Lukat et al42
CB MercurayHitachi15 × 15a80, 100, 120100, 100, 150153, 275, 435Jadu et al38
Galileos ComfortSirona Dental Systems (Bensheim, Germany)15 × 15a8521, 4270, 128823824037267233160657Ludlow and Ivanovic15
Galileos ComfortSirona Dental Systems15 × 15a8528848283551243802104292Pauwels et al30
Galileos ComfortSirona Dental Systems15 × 15a8521, 4251, 95Rottke et al31
Galileos Comfort PlusSirona Dental Systems15 × 15a988, 10, 12 (20, 25, 30)38, 47, 56 (106, 130, 154)4313535291652451011144Current study
NewTom VGiQR15 × 151107.8971072991015110024401852269Current study
NewTom VGiQR15 × 151108.81941861849860520452855436Pauwels et al30

H, height; kVp, kilovoltage peak; mAs, milliampere per second; W, width.

Bold values represent parameters used to produce standard or default scans.

aSpherical FOV.

Table 5 Adult phantom equivalent and effective doses for standard or default exposures for small field of view (FOV) CBCT units (<10 cm height) maxillary views

Unit nameManufacturerFOV size H × W(cm)kVpmAsEffective dose (µSv)Bone marrow (µSv)Bone surface (µSv)Skin (µSv)Oesophagus (µSv)Brain (µSv)Thyroid (µSv)Salivary glands (µSv)Remainder (µSv)Study
Veraviewepocs 3DJ Morita (Osaka, Japan)4 × 4a8047.5216517111890Al-Okshi et al43
3D Accuitomo 170J Morita4 × 4a9087.532Theodorakou et al41
CS 9000Carestream Dental (Atlanta, GA)4 × 5a701075522473013020Current study
CS 9000Carestream Dental4 × 5b701071012494126022038Current study
CS 9000Carestream Dental4 × 5a7010719212725183052374Pauwels et al30
CS 9000Carestream Dental4 × 5a7085.624Theodorakou et al41
ProMax 3DcPlanmeca Oy (Helsinki, Finland)4 × 5a84120104012113650Al-Okshi et al43
3D eXam®Kavo (Bieberach, Germany)4 × 1612018.5, 3733, 68Schilling and Geibel32
Alphard VEGAAsahi Roentgen (Kyoto, Japan)5 × 5a80102, 15320, 22431997331159936498Kim et al36
Alphard VEGAAsahi Roentgen5 × 5b80102, 15320, 25411921336169104494105Kim et al36
CS 9300Carestream Dental5 × 5a8460, 10035, 59401701315561461884219Current study
CS 9300Carestream Dental5 × 5b8460, 10048, 80652901421801912399303Current study
OP300 MaxioInstrumentarium (Helsinki, Finland)5 × 5a9011.7, 14.7, 18.7, 23.412, 16, 20, 2540128175204231868Current study
PaX-Uni3DVATECH (Seoul, Korea)5 × 5a8512044474955282091073146Pauwels et al30
Orthophos XGSirona Dental Systems (Bensheim, Germany)5 × 5.5a8536, 51, 66, (72, 86, 101)21, 30, 39, (45, 53, 60)261201682876896108Current study
Orthophos XGSirona Dental Systems5 × 5.5b8536, 51, 66, (72, 86, 101)25, 36, 47, (58, 70, 81)33156181031901036131Current study
ProMax 3DcPlanmeca Oy5 × 884192131983417019863333865514Qu et al19
3D Accuitomo 170J Morita5 × 109087.55411211262189148213885Pauwels et al30
3D Accuitomo 170J Morita5 × 109045, 79, 87.5, 15458, 102, 113, 198224668102271472491951382Current study
CS 9300Carestream Dental5 × 1090255636141813521161264171Current study
3D Accuitomo 170J Morita5 × 149045, 79, 87.5, 15470, 123, 136, 240256767106342012992477474Current study
3D Accuitomo 170J Morita5 × 149087.570Theodorakou et al41
3D Accuitomo 170J Morita5 × 179045, 79, 87.5, 15468, 119, 132, 23224072185362052982452460Current study
Pan eXam Plus 3DKavo6 × 49023, 4940, 79Schilling and Geibel32
3D Accuitomo 170J Morita6 × 6a9027, 45, 52.5, 79, 87.5, 15419, 32, 37, 56, 62, 109107325107181041631126210Current study
3D Accuitomo 170J Morita6 × 6b9027, 45, 52.5, 79, 87.5, 15420, 33, 39, 58, 65, 11410432678151231401068250Current study
NewTom VGiQR (Verona, Italy)6 × 6b11070.11401914861014710283552763478Current study
NewTom VGiQR6 × 6a11065131173442132429823322612443Current study
OP300 MaxioInstrumentarium6 × 89011.7, 14.7, 18.7, 23.425, 31, 40, 50872752794679621134Current study
Pan eXam Plus 3DKavo6 × 89047, 7979, 125Schilling and Geibel32
iCAT® FLXImaging Sciences (Hatfield, PA)6 × 1690/1206/10, 18.5, 374/20, 32, 6532731112131101719119Ludlow and Walker22
iCAT NGImaging Sciences6 × 1612018.5, 3732, 60Davies et al34
iCAT NGImaging Sciences6 × 1612010, 18.522, 35Morant et al21
iCAT NGImaging Sciences6 × 1612018.5, 3737, 68Roberts et al35
iCAT NGImaging Sciences6 × 1612018.533Theodorakou et al41
iCAT NGImaging Sciences8 × 1612010, 18.529, 47Morant et al21
SCANORA® 3DSoredex (Helsinki, Finland)7.5 × 10853046425030451481285178Pauwels et al30
Galileos Comfort PlusSirona Dental Systems8.5 × 15988, 10, 12 (20, 25, 30)27, 34, 41 (84, 103, 122)411273413164103869118Current study
Alphard VEGAAsahi Roentgen10 × 108068, 13669, 1461969073081710194034544659Kim et al36
CB MercurayHitachi (Tokyo, Japan)10 × 1080, 100, 120100, 100, 15060, 97, 145Jadu et al38

H, height; kVp, kilovoltage peak; mAs, milliampere per second; W, width.

Bold values represent parameters used to produce standard or default scans.

aAnterior.

bPosterior.

cUpgraded unit with additional filtration.

Table 6 Adult phantom effective doses for standard or default exposures for small field of view (FOV) CBCT units (<10 cm height) views including mandible

Unit nameManufacturerFOV size H×W (cm)kVpmAsEffective dose (µSv)Bone marrow (µSv)Bone surface (µSv)Skin (µSv)Oesophagus (µSv)Brain (µSv)Thyroid (µSv)Salivary glands (µSv)Remainder (µSv)Study
3D Accuitomo 170J Morita (Osaka, Japan)4 × 4a9087.54337373237195212070Pauwels et al30
Veravieweposcs 3DJ Morita4 × 4a804722361081550900Al-Okshi et al43
CS 9000Carestream Dental (Atlanta, GA)4 × 5b70107222185424204063380Current study
CS 9000Carestream Dental4 × 5a701074078352429025170986Pauwels et al30
CS 9000Carestream Dental4 × 5a7010738371501010201101037141Current study
3D eXam®Kavo (Bieberach, Germany)4 × 1612018.5, 3738, 76Schilling and Geibel32
Alphard VEGAAsahi Roentgen (Kyoto, Japan)5 × 580102, 15362, 949544213117753163570507Kim et al36
CS 9300Carestream Dental5 × 5b8460, 10048, 81562074437324242350243Current study
CS 9300Carestream Dental5 × 5a8460, 10066, 127873325466513843220382Current study
OP300 MaxioInstrumentarium (Helsinki, Finland)5 × 5b9011.7, 14.7, 18.7, 23.416, 20, 26, 3214521615816262183Current study
Orthophos XGSirona Dental Systems (Bensheim, Germany)5 × 5.5b8536, 51, 66 (72, 86, 101)22, 31, 40 (49, 65, 84)2410117161616085092Current study
Orthophos XGSirona Dental Systems5 × 5.5a8536, 51, 66 (72, 86, 101)27, 38, 50, (61, 72, 83)33142192119207951116Current study
ProMax 3DcPlanmeca Oy (Helsinki, Finland)5 × 884192171953293860208554048659Qu et al19
3D Accuitomo 170J Morita5 × 109045, 79, 87.5, 15487, 153, 169, 29786294411124312164206497Current study
CS 9300Carestream Dental5 × 10902575582063038323861910248Current study
3D Accuitomo 170J Morita5 × 149045, 79, 87.5, 154135, 237, 262, 461258810641277412266363865Current study
3D Accuitomo 170J Morita5 × 179045, 79, 87.5, 154121, 212, 235, 414193618621526513745539734Current study
Pan eXam Plus 3DKavo6 × 49023, 4949, 115Schilling and Geibel32
3D Accuitomo 170J Morita6 × 69027, 45, 52.5, 79, 87.5, 15437, 61, 72, 108, 120, 2101113473654366502814381Current study
3D Accuitomo 170J Morita6 × 69027, 45, 52.5, 79, 87.5, 15448, 80, 93, 148, 158, 2521565144475557453150572Current study
NewTom VGiQR (Verona, Italy)6 × 611042.1191147381529125811504570595Current study
NewTom VGiQR6 × 61102913010326872581876673253426Current study
OP300 MaxioInstrumentarium6 × 89011.7, 14.7, 18.7, 23.443, 54, 68, 86862822833183051523221Current study
Pan eXam Plus 3DKavo6 × 89047, 79110, 184Schilling and Geibel32
iCAT® FLXImaging Sciences (Hatfield, PA)6 × 1690/1206/10, 18.5, 378/34, 61, 127611241833512901567198Ludlow and Walker22
iCAT NGImaging Sciences6 × 1612018.54533332546251973172Pauwels et al30
iCAT NGImaging Sciences6 × 1612018.5, 3758, 113Davies et al34
iCAT NGImaging Sciences6 × 1612010, 18.524, 39Morant et al21
iCAT NGImaging Sciences6 × 1612018.5, 3775, 149Roberts et al35
iCAT NGImaging Sciences6 × 1612018.549Theodorakou et al41
Picasso TrioVATECH (Seoul, Korea)7 × 12859181625756395831837254Pauwels et al30
Picasso TrioVATECH7 × 128591, 127, (109)d81, 123, (102)d9410785875672410342Pauwels et al30
Picasso TrioVATECH7 × 12851271231261561131345512982432Pauwels et al30
DCT PROVATECH7 × 169010518076240201502023602280377Qu et al40
SCANORA® 3DSoredex (Helsinki, Finland)7.5 × 10853047343529253521052147Pauwels et al30
SCANORA 3DSoredex7.5 × 10853045373931312401117155Pauwels et al30
3D Accuitomo 170J Morita8 × 89045, 79, 87.5, 15492, 162, 180, 31618056010385839214257564Current study
3D eXamKavo8 × 812018.5, 3762, 122Schilling and Geibel32
CS 9300Carestream Dental8 × 8903275592193332693301935254Current study
iCAT FLXImaging Sciences8 × 890/1206/10, 18.5, 375/23, 44, 85851392342582221172149Ludlow and Walker22
iCAT NGImaging Sciences8 × 812010, 18.518, 29Morant et al21
NewTom VGiQR8 × 81106.3 (38.7)61 (206)5313943261742581494208Current study
OP300 MaxioInstrumentarium8 × 89011.7, 14.7, 18.7, 23.449, 61, 78, 97933083535243351754255Current study
Orthophos XGSirona Dental Systems8 × 88536, 51, 66 (72, 86, 101)48, 67, 91 (117, 144, 166)532402832532981718226Current study
Prexion 3D high resTeraRecon (Foster City, CA)8 × 8901483883251508264133783180093721309Ludlow and Ivanovic15
Prexion 3D standardTeraRecon8 × 89076189164760135533836834761684Ludlow and Ivanovic15
ProMax 3DPlanmeca Oy8 × 88419.6, 16928, 122881211455310212576346Pauwels et al30
ProMax 3DPlanmeca Oy8 × 88419.618Theodorakou et al41
Promax 3DPlanmeca Oy8 × 88472, 96488, 6524682170339120600126712 9391846Ludlow and Ivanovic15
ProMax 3DcPlanmeca Oy8 × 88422.4, 96, 120, 144, 168, 192, 19230, 102, 169, 216, 272, 298, 3062558831639421511016582962Qu et al19
Veravieweposcs 3DJ Morita8 × 870517355576940330195685Pauwels et al30
NewTom VGiQR8 × 121106.1, (39.1)82 (280)8721949412373161942275Current study
Kodak 9500Carestream Dental8 × 159010892858451915412166304Pauwels et al30
OP300 MaxioInstrumentarium8 × 159022.5, 28.4, 36, 4576, 96, 121, 1521274317166585502680409Current study
3D eXamKavo8 × 1612010, 37 (18.5)45, 170 (88)Schilling and Geibel32
iCAT FLXImaging Sciences8 × 1690/1206/10, 18.5, 378/39, 70, 148701502340953291693225Ludlow and Walker22
iCAT NGImaging Sciences8 × 1612018.5, 3765, 1347615133171501831639235Grunheid et al33
Galileos Comfort plusSirona Dental Systems8.5 × 15988, 10, 12 (20, 25, 30)29, 37, 45, (92, 113, 133)40118212623225894121Current study
CS 9500Carestream Dental9 × 1580, 85, 9086.4, 108, 10876, 98, 16611338655542645331680313Ludlow10
Alphard VEGAAsahi Roentgen10 × 108068, 13685, 1842171009147434968726233788Kim et al36
CB MercurayHitachi (Tokyo, Japan)10 × 1012015040746621613441102950130090061355Ludlow and Ivanovic15
CB MercurayHitachi10 × 1080, 100, 120100, 100, 150148, 261, 421Jadu et al38

H, height; kVp, kilovoltage peak; mAs, milliampere per second; W, width.

Bold values represent parameters used to produce standard or default scans.

aPosterior.

bAnterior.

cUpgraded unit with additional filtration.

dAverage of high- and low-dose protocols.

Table 7 Adult phantom equivalent and effective doses for standard or default exposures for small field of view (FOV) CBCT units (<10 cm height)—temporomandibular joint views

Unit nameManufacturerFOV size H × W(cm)kVpmAsEffective dose (µSv)Bone marrow (µSv)Bone surface (µSv)Skin (µSv)Oesophagus (µSv)Brain (µSv)Thyroid (µSv)Salivary glands (µSv)Remainder (µSv)Study
CS 9000Carestream Dental (Atlanta, GA)4 × 568, 70, 7068, 86, 10810, 14, 2118182182323143Lukat et al42
NewTom VGiQR (Verona, Italy)8 × 811022, 9345, 1294010262150202130Al-Okshi et al43
NewTom VGiQR8 × 1211019.1564812274230202400Al-Okshi et al43
CB MercurayHitachi (Tokyo, Japan)10 × 1012015027924418823085365005384810190Librizzi et al37

H, height; kVp, kilovoltage peak; mAs, milliampere per second; W, width.

Bold values represent parameters used to produce standard or default scans.

Table 8 Child phantom equivalent and effective doses for standard or default exposures for medium and large field of view (FOV) CBCT units (>10 cm height)

Unit nameManufacturerFOV size H × W(cm)kVpmAsEffective dose (µSv)Bone marrow (µSv)Bone surface (µSv)Skin (µSv)Oesophagus (µSv)Brain (µSv)Thyroid (µSv)Salivary glands (µSv)Remainder (µSv)Study
ProMax mid-stichedPlanmeca Oy (Helsinki, Finland)16 × 1690(72.3, 90) 217, 244 “289, 362”(88, 107) 277, 307 “362, 488”3181118222112282921543706145Current study
IlumaImtec (Ardmore, OK)19 × 191202046Vassileva and Stoyanov16
Newtom 3GCefla Dental Group (Imola, Italy)20 × 20110AEC5671182224443059553185Current study
3D Accuitomo 170J Morita (Osaka, Japan)10 × 109045, 79, 87.5, 154160, 281, 311, 54815158035499114128045736825Current study
CS 9300Carestream Dental (Atlanta, GA)10 × 108025864318747301718461489223Current study
3D Accuitomo 170J Morita10 × 149087.5237Theodorakou et al41
3D Accuitomo 170J Morita10 × 149045, 79, 87.5, 154183, 321, 355, 626209804319114169329936440947Current study
Newtom VGCefla Dental Group11 × 15110Auto114Theodorakou et al41
iCAT® FLXImaging Sciences (Hatfield, PA)11 × 1690/1206/10, 18.513, 56, 115115190815339110012045302Ludlow and Walker22
CS 9300Carestream Dental11 × 178025.6, 41.2110, 1781205241118185016482751425Current study
3D Accuitomo 170J Morita12 × 179087.5282Theodorakou et al41
3D Accuitomo 170J Morita12 × 179045, 79, 87.5, 154212, 353, 430, 7692449403072532039426566221004Current study
OP300 MaxioInstrumentarium (Helsinki, Finland)13 × 159029, 36, 45, 7293, 108, 134, 2155822566384077651668254Current study
iCAT FLXImaging Sciences13 × 1690/1206/10, 18.518, 70, 120120211825373110032038303Ludlow and Walker22
iCAT NGImaging Sciences13 × 1612018.5134Theodorakou et al41
CS 9300Carestream Dental13.5 × 178045.21891185091557896417413075458Current study
Galileos Comfort plusSirona Dental Systems (Bensheim, Germany)15 × 15986, 8 (15, 20)39, 52, (122, 160)2691252218538458991Current study
Newtom 3GCefla Dental Group15 × 15110Auto9410026556446798751250190Current study

H, height; kVp, kilovoltage peak; mAs, milliampere per second; W, width.

Bold values represent parameters used to produce standard or default scans.

Table 9 Child phantom equivalent and effective doses for standard or default exposures for small field of view (FOV) CBCT units (<10 cm height) maxillary views

Unit nameManufacturerFOV size H × W(cm)kVpmAsEffective dose (µSv)Bone marrow (µSv)Bone surface (µSv)Skin (µSv)Oesophagus (µSv)Brain (µSv)Thyroid (µSv)Salivary glands (µSv)Remainder (µSv)Study
3D Accuitomo 170J Morita (Osaka, Japan)4 × 4a9087.528Theodorakou et al41
CS 9000Carestream Dental (Atlanta, GA)4 × 5a7085.616Theodorakou et al41
CS 9300Carestream Dental5 × 5a7548, 8041, 63261183612702271930243Current study
CS 9300Carestream Dental5  ×  5b7548, 8047, 79381723319623842165282Current study
OP300 MaxioInstrumentarium (Helsinki, Finland)5 × 5a9011.7, 14.7, 18.7, 23.416, 20, 26, 32621203445343867Current study
3D Accuitomo 170J Morita5 × 109045, 79, 87.5, 15458, 102, 113, 19844163228263554232988433Current study
CS 9300Carestream Dental5 × 108025451880279781611269179Current study
3D Accuitomo 170J Morita5 × 149045, 79, 87.5, 15485, 149, 165, 29065244245375645684423648Current study
3D Accuitomo 170J Morita5 × 179045, 79, 87.5, 15491, 160, 177, 31277289177395085754974701Current study
3D Accuitomo 170J Morita6  × 6b9027, 45, 52.5, 79, 87.5, 15432, 53, 61, 92, 102, 18037138201215313432580406Current study
OP300 MaxioInstrumentarium6 × 89011.7, 14.7, 18.7, 23.427, 34, 43, 5493439510790708108Current study
iCAT® FLXImaging Sciences (Hatfield, PA)6 × 1690/1206/10, 18.55, 23, 3939436911252158889142Ludlow and Walker22
iCAT NGImaging Sciences6 × 1612018.543Theodorakou et al41
Galileos Comfort plusSirona Dental Systems (Bensheim, Germany)8.5 × 15986, 8 (15, 20)21, 32 (72, 98)12412361759249672Current study

H, height; kVp, kilovoltage peak; mAs, milliampere per second; W, width.

Bold values represent parameters used to produce standard or default scans.

aAnterior.

bPosterior.

Table 10 Child phantom equivalent and effective doses for standard or default exposures for small field of view (FOV) CBCT units (<10 cm height) views including mandible

Unit nameManufacturerFOV size H × W(cm)kVpmAsEffective dose (µSv)Bone marrow (µSv)Bone surface (µSv)Skin (µSv)Oesophagus (µSv)Brain (µSv)Thyroid (µSv)Salivary glands (µSv)Remainder (µSv)Study
CS 9300Carestream Dental5 × 5a7548, 8044, 64291352918443321776223Current study
CS 9300Carestream Dental5 × 5b7548, 8056, 86411873623575322144285Current study
OP300 MaxioInstrumentarium (Helsinki, Finland)5 × 5a9011.7, 14.7, 18.7, 23.433, 42, 53, 6722863101732864182Current study
3D Accuitomo 170J Morita (Osaka, Japan)5 × 109045, 79, 87.5, 154152, 266, 295, 5191515916010712733825023660Current study
CS 9300Carestream Dental5 × 10802561301342420464571365192Current study
3D Accuitomo 170J Morita5 × 149087.5214Theodorakou et al41
3D Accuitomo 170J Morita5 × 149045, 79, 87.5, 154152, 267, 296, 5211877397012814233563356645Current study
3D Accuitomo 170J Morita5 × 179045, 79, 87.5, 154170, 299, 331, 5821967739211318633265937833Current study
3D Accuitomo 170J Morita6 × 6a9027, 45, 52.5, 79, 87.5, 15446, 77, 90, 136, 150, 26567259494213710313873469Current study
3D Accuitomo 170J Morita6  ×  6b9027, 45, 52.5, 79, 87.5, 15471, 118, 141, 208, 227, 375108419687220215495558725Current study
OP300 MaxioInstrumentarium6 × 89011.7, 14.7, 18.7, 23.461, 77, 97, 12234135617375761153164Current study
iCAT® FLXImaging Sciences (Hatfield, PA)6 × 1690/1206/10, 18.59, 43, 73739834301065301654224Ludlow and Walker22
iCAT NGImaging Sciences6 × 1612018.563Theodorakou et al41
3D Accuitomo 170J Morita8 × 89045, 79, 87.5, 154128, 225, 249, 4391174541007432422695023687Current study
CS 9300Carestream Dental8 × 8803282421833025807801529219Current study
iCAT FLXImaging Sciences8 × 890/1206/10, 18.57, 34, 60607720201344031401191Ludlow and Walker22
OP300 MaxioInstrumentarium8 × 89011.7, 14.7, 18.7, 23.477, 97, 123, 1534316737231866851416210Current study
ProMax 3DPlanmeca Oy (Helsinki, Finland)8 × 88419.624Theodorakou et al41
iCAT FLXImaging Sciences8 × 1690/1206/10, 18.512, 50, 858512639351926591754248Ludlow and Walker22
Galileos® ComfortPLUSSirona Dental Systems (Bensheim, Germany)8.5 × 15986, 8 (15, 20)29, 44, (99, 136)207111201732740461Current study

H, height; kVp, kilovoltage peak; mAs, milliampere per second; W, width.

Bold values represent parameters used to produce standard or default scans.

aAnterior.

bPosterior.

Tables 1114 provide summary data for adult and child doses and include mean values with standard deviations for standard or default exposure equivalent and effective doses. Lower numbers of units with equivalent doses reflect the practice of some studies that have included fewer weighted tissues in their calculation of effective dose or report only effective dose. Standard adult exposure settings resulting in average adult effective doses of 212 µSv for large FOVs, 177 µSv for medium FOVs and 84 µSv for small FOVs are found in Table 11. Small adult FOVs producing average effective doses of 53 µSv for maxillary views and 102 µSv for mandibular views are seen in Table 12. Because data for few large FOVs were found for child phantoms, large and medium FOVs were combined in Table 13. The average effective dose for large or medium FOVs was 175 µSv. The average child effective dose for small FOVs was 103 µSv. When child phantom small FOVs were analysed by arch location, an average dose of 67 µSv was seen for maxillary views and of 128 µSv was seen for mandibular views, as seen in Table 14.

Table 11 Average equivalent and effective doses (μSv) for an adult using standard exposure settings of dental CBCT units

Field of view sizeBone marrowBone surfaceSkinOesophagusBrainThyroidSalivary glandsRemainderEffective dose
Large
 Units reported161616141616161623
 Mean3591457189122218211303484475212
 SD4282081215125260116163465502212
Medium
 Units reported323232263232323243
 Mean2338441637512117623675442177
 SD24011941535319116572643325137
Small
 Units reported7779785778777974101
 Mean942996243211413225931684
 SD813766138746433202729578

SD, standard deviation.

Table 12 Small field of view (FOV)—average equivalent and effective doses (μSv) for an adult using standard exposure settings of dental CBCT units

FOV locationBone marrowBone surfaceSkinOesophagusBrainThyroidSalivary glandsRemainderEffective dose
Maxilla
 Units reported252727202725272538
 Mean852365318153155137421353
 SD75215401325310494915538
Mandible
 Units reported484848364748484759
 Mean10031661571125692745380102
 SD843886341151477232433788

SD, standard deviation.

Table 13 Average equivalent and effective doses (μSv) by field of view (FOV) size for a 10-year-old child using standard exposure settings of dental CBCT units

FOV sizeBone marrowBone surfaceSkinOesophagusBrainThyroidSalivary glandsRemainderEffective dose
Large or medium
 Units reported131313131313131318
 Mean1304481427996216212918404175
 SD833311196179111482117319115
Small
 Units reported282828282828282834
 Mean6021365341718432388336103
 SD502026733154997170523889

SD, standard deviation.

Table 14 Small field of view (FOV)—average equivalent and effective doses (μSv) by arch for a 10-year-old child using standard exposure settings of dental CBCT units

FOV locationBone marrowBone surfaceSkinOesophagusBrainThyroidSalivary glandsRemainderEffective dose
Maxilla
 Units reported111111111111111114
 Mean3412210017250279207829867
 SD23899213205190155322354
Mandible
 Units reported171717171717171720
 Mean77273424612012072589360128
 SD5723428388211381813251101

SD, standard deviation.

Table 15 provides p-values for an ANOVA of effective dose and equivalent doses for each of the weighted tissues that are typically included in head and neck dosimetry studies. The ANOVA model investigated the effects of FOV and phantom type. Tukey honest significant difference results are provided for FOV for statistically significant factors. With the exception of remainder tissues, all weighted tissues and effective dose demonstrated significantly increased dose with increased FOV size. With the exception of thyroid dose, which was significantly greater in child exposures, no differences were seen in equivalent or effective dose owing to phantom type. A separate analysis investigated dose differences related to maxillary or mandibular location for small FOVs for child and adult phantoms. Significantly higher doses were associated with mandibular field positions for the oesophagus, thyroid, salivary gland and remainder tissues as well as effective dose. Once again, only the thyroid tissue demonstrated significantly higher doses in child phantoms than in adult phantoms.

Table 15 ANOVA: all volume data models include phantom type and field of view (FOV); small volume data model includes phantom type and arch

p-valuesPost hoc statistical testModel/HSD variable levelBone marrowBone surfaceSkinOesophagusBrainThyroidSalivary glandsRemainderEffective dose
  Phantom0.05380.17880.82680.48210.63510.00050.71430.89260.6198
FOV size<0.0001<0.0001<0.0001<0.0001<0.00010.00040.0080.0868<0.0001
All FOVsTukey HSD TestLargeAAAAAAAAA
MediumBBAABAA BAA
SmallCCBBCBBBB
Small FOV Phantom0.04060.29860.64790.32810.30090.00090.67890.74450.1706
FOV location0.12620.11040.384<0.00010.0779<0.00010.00420.01980.0002

HSD, honestly significant difference.

Bold values indicated statistical significance at α < 0.05.

FOV levels not connected by the same letter (A, B, C) are significantly different.

Distribution of mean effective dose components from large FOV CBCT imaging of adult phantoms is displayed graphically in Figure 1. A similar graphic for combined large or medium FOVs is provided for child phantoms in Figure 2. The graphics demonstrate the greater contribution of thyroid exposure to effective dose in the child (37%) than in adult (20%).

Figure 1
Figure 1

Distribution of effective dose components in an adult phantom for large field of view dental CBCT.

Figure 2
Figure 2

Distribution of effective dose components from large or medium field of view CBCT imaging of a child phantom.

Table 16 provides summary statistics for volume DAP and DHP and the ratios of effective dose to each of these values. The coefficient of variation increases with reduction in FOV area or height. The coefficient of variation is consistently lower for E/DHP than for E/DAP ratios regardless of the FOV size. This suggests that FOV height may be a better predictor of effective dose than FOV area.

Table 16 Dose–area product (DAP) and dose–volume height (DHP) calculations and derived conversion coefficients for calculating effective dose derived from phantom data

PhantomFOVDescriptive statisticDAP (mGy cm2)DHP (mGy cm)Conversion coefficient
E/DAP (µSv mGy−1 cm−2)E/DHP (µSv mGy−1 cm−1)
AdultLargeMean1229640.173.44
SD1240660.040.66
c.v.23%19%
MediumMean683460.273.90
SD574360.080.78
c.v.28%20%
Small maxillaMean6171.288.29
SD5550.622.66
c.v.48%32%
Small mandibleMean175180.876.63
SD161180.511.87
c.v.59%28%
ChildLarge and mediumMean529350.355.07
SD362230.100.53
c.v.29%11%
Small maxillaMean121110.856.87
SD12980.420.74
c.v.49%11%
Small mandibleMean153161.008.47
SD126110.432.98
c.v.44%35%

c.v., coefficient of variation; E, effective dose; FOV, field of view; SD, standard deviation.

Analysis by phantom and field of view size.

Table 17 displays average absolute error between phantom-based calculations of the effective dose and E/DAP or E/DHP ratios as coefficients for effective dose estimation. The magnitude of error increases as the FOV size is reduced. The average absolute error for all volumes was 35% for E/DAP and decreased to 19% for E/DHP. This difference was statistically significant (p < 0.0001).

Table 17 Absolute error between phantom effective dose and dose calculated with dose–area product (DAP) and dose–height product (DHP) conversion coefficients

Field of viewPhantomnEphantom − EDAP (µSv)Ephantom − EDHP (µSv)Probability < t
LargeChild
Adult1636.428.80.0372
MediumChild1332.515.20.0183
Adult3239.127.50.0611
Small maxillaChild1141.35.10.0213
Adult3148.223.00.0050
Small mandibleChild1745.638.70.2851
Adult4856.125.9<0.0001
All 16845.725.1<0.0001
% diff from E 35%19%

E, effective dose.

Bold values indicated statistical significance at α < 0.05.

Discussion

Reported dosimetry for standard CBCT exposure settings demonstrated significant reductions in effective dose associated with the use of small FOV sizes. While a trend of dose reduction from large to medium FOVs was seen, this was not statistically significant. The absence of a significant dose–FOV relationship is likely related to two factors. Increasing FOV extends anatomic coverage superiorly increasing the amounts of brain and bone coverage with little increase in exposure of other weighted tissues. The resulting proportional increase in effective dose is smaller than that seen when small fields centred on the dentoalveolar area expand both cranially and caudally in medium FOV volumes. The wide range of effective doses produced using standard settings by different CBCT units is another factor affecting the statistical significance of differences that may be present between medium and large FOVs. Standard deviations associated with dose values for these volumes were on the same order as calculated means indicating substantial variability among devices. Although not assessed in detail in this study, exposure variability also increases substantially when one includes the range of protocol options offered by many manufacturers. Doses for the same FOV may have as much as a 15-fold difference between low-dose and high-resolution protocols.22

This study has focused on standard or default exposures. These are protocols recommended by the manufacturer of the CBCT unit for imaging of average or typical patients. It should be noted that manufacturers may change exposures associated with the standard designation over time. Examples of this are seen with Iluma and CB Mercuray devices. Initially, the highest exposure protocols were recommended for these devices. Later, much lower exposure protocols became the recommended “standard”. This is reflected in study data from different investigators in Table 3. While marketing materials were changed to reflect updated standard protocols, the effect on selection of imaging parameters by end users is unclear. Differences between recommended use of products and actual clinical application are not new to dental radiology. Continued use of round cones and D-speed film by many dental practitioners in the face of many years of recommendations by the American Dental Association and the National Commission on Radiation Protection and Measurements encouraging the use of rectangular collimation and high-speed receptors is a prominent example.44,45

Methodologic errors in dosimetry and effective dose calculation

Regardless of the FOV size, remainder tissues accounted for <10% of the effective dose calculations using ICRP 199012 tissue weights.18 The brain was the only tissue contributing significant dose in the remainder group. Application of ICRP 200713 tissue weights resulted in remainder doses contributing 27–42% to effective dose depending on FOV size and location.18 Studies excluding remainder tissues from their dose calculations significantly underestimate dose, and a number of these were excluded from this report.46,47 A citation that did not include oral mucosa, which was added to the remainder group in the 2007 ICRP calculation of effective dose was also excluded.48 The oral mucosa is directly exposed in any maxillofacial CBCT scan and as a component of the remainder group has a tissue weight of 0.0092. This is nearly the tissue weight of the salivary glands (0.01). Similarly, the extrathoracic region is exposed in most dental CBCT scans. Together, the contribution of these tissues to effective dose exceeds that of the oesophagus, skin, bone surface and brain combined and cannot be overlooked in a calculation of stochastic risk.18

Sampling is an important component of dosimetry, and the sampling strategy is critical to both internal validity of a dosimetry study and its extensibility to other studies or patient populations. One approach that has been taken is to sample doses over a regularly spaced grid throughout a phantom. This approach requires many dosemeters and results in time consuming and expensive study protocols. An additional complication is that the tissues and organs of particular interest for radiation biology are not uniformly distributed. A uniform grid of dosemeters may not coincide with the location of a tissue of interest. An alternate approach is to place dosemeters only in the weighted tissues used in the calculation of the effective dose. A uniform distribution of dosemeters within the selected tissue is still a resource intensive choice, so efforts to strategically locate dosemeters within a tissue such that the average dose of a limited number of dosemeters reasonably reflects that of a uniform distribution of dosemeters is desirable. We have used this approach in measuring calvarial bone and marrow doses, where a limited number of strategically positioned dosemeters are used to reflect dose to the entire skull. For tissues that are incompletely contained in the maxillofacial area, sampling of the directly exposed portion of the tissue multiplied by the percentage of total tissue that is directly exposed provides a reasonable estimation of organ dose. This approach is taken with skin, muscle, bone, lymph nodes and oesophagus in our studies.10,15,17,22,49 Studies that fail to account for body-wide distributions of tissue produce overestimations of equivalent doses for the bone, bone marrow, oesophagus, lymphoid tissue, muscle and skin.50,51 This error may result in a 10- to 20-fold overestimation of specific organ dose with a concomitant exaggeration of effective dose. The bone marrow, which varies in quantity and distribution by patient age is accorded different percentages for child and adult phantoms.22 The same dosemeters may be used to calculate bone and bone marrow dose; however, it is important to account for differences in X-ray attenuation efficiency when calculating absorbed doses. The higher effective atomic number of bone leads to increased photoelectric interactions and increased dose to this tissue. Mass attenuation coefficients for bone and soft tissue are available from a variety of sources and are usually calculated using mono-energetic photons beams.29 Bone/muscle attenuation ratios can be calculated from these data. A linear fit of ratios from 40 to 80 kV is adequate to cover the mean beam energy range produced by CBCT units. In our studies, the effective beam energy of the highly filtered, low ripple, polychromatic beams used for CBCT is estimated to be two-thirds of the peak beam energy. Using this assumption and an equation developed from the linear fit of monochromatic data, bone muscle attenuation ratios from 1.97 for a peak kilovoltage of 120 kVp to 3.63 for 80 kVp are used. Studies that fail to adjust for attenuation differences of the bone and soft tissues may underestimate bone doses by a factor of 2–4×.30,42 Examining the ratio of the bone to bone marrow doses using the means in Table 11 suggests a range of 3.1–4.1× average underestimation of the bone dose if dosemeter values are not corrected for bone attenuation efficiency. This will lead to a 2–5% underestimation of effective dose depending on the size of the FOV.

Tissues that are completely outside the field of direct exposure are not sampled in our dosimetry protocol. These tissues account for 75% of the weighted tissues in a full body exposure; however, their indirect exposure in maxillofacial examinations accounts for <2% of effective dose.18,51,52 Although the oesophagus is typically outside the field of direct exposure during CBCT scans, it is potentially exposed to scatter radiation. Because the oesophagus surrounds an open air space, caudally directed scatter photons can extend into the upper oesophageal passage exposing the mucosal walls. For this reason, an organ fraction of 10% is used in calculating oesophageal dose. Because of the oesophagus tissue weight of 0.04 in the calculation of the effective dose, this organ is as or more important than skin dose. Figure 1 suggests that studies that fail to measure oesophageal dose underestimate effective dose by as much as 2%.30

While the use of additional dosemeters in indirectly or directly exposed tissues may increase the precision of calculation of organ dose, it does not guarantee an increase in the accuracy of the calculation of effective dose. Patient positioning for CBCT can vary between devices and operators, and this can have a pronounced effect on dose. Duplication of phantom position when comparing dose differences owing to technique is critical if results are to reflect differences owing to protocol rather than confounding owing to differences in patient position. A 10° rotation in phantom position was noted to produce a 92% difference in dose to the thyroid.18 Similarly, while caudal–cranial positioning differences of the FOV by a few millimetres may be clinically acceptable, when it results in direct exposure of the thyroid area, it can lead to 3- to 4-fold increases in dose to the thyroid gland and increases in effective dose by as much as 30%.53

The allure and disappointment of dose–area product

While the use of DAP has been advocated as a measure of dose for CBCT units,54,55 its accuracy as a measure of risk is debatable. In relating DAP to effective dose, a conversion coefficient must be used. Using a single CBCT device, a recent study calculated conversion coefficients for DAP to effective dose for large to small FOVs and found that a 3.8-fold range of values (0.038–0.146 µSv mGy−1 cm−2) was required for different field sizes and anatomic locations.36 A 7.5-fold range of E/DAP values was calculated from the adult phantom data in this study encompassing a large number of measurements on CBCT devices of varying beam energies. A 2.9-fold range of E/DAP values was calculated for a more limited set of child phantom data in this study. The use of volume height in the place of projection area resulted in statistically improved accuracy in the estimation of effective dose but still led to a range in the conversion coefficient of 2.4-fold for adult imaging and a 1.7-fold range for child imaging. A best-case scenario of using conversion factors specific for FOV size, arch location and patient type still resulted in an average absolute error of 35% of the calculated effective dose when using DAP. Improvement in average absolute error to 19% was seen when volume height was substituted for beam area. This improvement is intriguing as beam height may be easily substituted for beam area in dose calculations. While dose to salivary glands was used in this study, dose measured in air or air kerma are typically used in DAP calculations. These result in different numerical values for effective dose conversion factors but could be hypothesized to provide similar variability for DAP and DHP calculations. While this warrants further investigation, it is apparent that development of universal conversion coefficients to translate simple measures of exposure to patient dose with the goal of risk estimation is problematic.

Why does dose–height product correlate more closely than dose–area product to effective dose?

The dimensions of the rectangular X-ray beam are used to calculate the area portion of DAP. For a homogenous object that is at least the size of the X-ray beam cross section, the product of exposure and beam area will provide a good correlation with absorbed dose. For maxillofacial imaging, the imaged structures are non-homogenous with respect to tissue density, the shape of tissues imaged and the distribution of tissues with respect to radiation sensitivity. Beams of wider width may more than cover the horizontal dimension of the face. The portion of the X-ray beam that extends beyond the face increases beam area but adds negligibly to patient dose because it exposes the patient to only a small amount of scatter photons from the beam interaction with air. The vertical dimensions of even large FOVs rarely exceed the dimension of the maxillofacial area. Therefore, an increase in height of the FOV almost always results in an increased volume of exposed tissue. But DHP is a better correlate with effective dose even for small FOVs. This may be explained by the vertical distribution of radiosensitive organs. Small amounts of direct exposure of an organ contribute more to dose than do larger amounts of scatter radiation. For instance, a small amount of direct exposure of the thyroid gland may result in a dramatic increase in both thyroid and effective dose.53 Mandibular locations of small FOVs provide greater exposure of the thyroid and submandibular salivary glands than do same size maxillary views. Increasing height of the FOV brings new and potentially radiosensitive tissues into the area of direct exposure, while increasing width of the beam simply increases dose to tissues already being exposed.

Conclusions

Given a choice, dentists prefer images with technical factors that provide high signal-to-noise ratios and high resolution. Dentists requesting images from an imaging centre or providing examinations in their own offices may not understand the risk implications of using higher doses to obtain image volumes. If “pretty pictures” are being obtained when a “just diagnostic” image is needed, we are doing the patient a disservice.18 As imaging professionals, it is our responsibility to educate our colleagues in other specialities and general dentistry about the risk differences between “diagnostic” and “pretty”. This is reason enough for dosimetry research. Recognizing that diagnostic imaging is the single greatest source of exposure to ionizing radiation for the US population that is controllable, the National Commission on Radiation Protection and Measurements has introduced a modification of the as low as reasonably achievable concept. ALADA represents “as low as diagnostically acceptable”.56 Implementation of this concept will require evidence-based judgments of the level of image quality required for specific diagnostic tasks, and exposures and doses associated with this level of quality. Little research is currently available in this area.

For a dosimetry study reporting effective dose to be comparable with the contemporary studies cited in this review, it should incorporate the following elements: the dose calculation should follow the ICRP 2007 recommendations including new and adjusted tissue weights from all weighted tissues in the head and neck area. The location of dosemeters or points of measurement should be specified. When an entire organ is not exposed, such as bone or skin surface, the strategy for extrapolating dose for the entire organ from incomplete sampling should be described. Adjustment for the mass attenuation differences of the bone and soft tissue should be made and described. Adjustments for sensitivity of the dosimetry system for the mean beam energy of the X-ray source should also be described.

Dosimetry research involves many variables in addition to the examination that is being assessed. Decisions affecting these variables may have a profound impact on accuracy, validity and extensibility of results. Currently, anthropomorphic phantom dosimetry, when properly executed, provides the most accurate estimation of effective dose. Large exposure ranges make CBCT doses difficult to generalize. The use of DAP with average conversion coefficients to calculate dose results in significant inaccuracy. The use of DHP as a metric for estimating effective dose improves accuracy and warrants further investigation.

Conflict of interest

Travel support and honoraria were provided to Dr Ludlow by AFP Imaging, Carestream Dental, Instrumentarium, and Sirona Dental Systems. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health.

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Volume 44, Issue 1January 2015

© 2015 The Authors. Published by the British Institute of Radiology


History

  • RevisedSeptember 03,2014
  • ReceivedJune 10,2014
  • AcceptedSeptember 11,2014
  • Published onlineOctober 15,2014

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Keywords