A review of dental cone-beam CT dose conversion coefficients

Abstract

Objective:

The purpose of this study was to review the literature to examine the usage and magnitude of effective dose conversion factors (DC_E) for dental cone beam CT (CBCT) scanners.

Methods:

A PubMed literature search for publications relating to radiation dosimetry in dental radiography was performed. Papers were included if they reported DC_E, or reported ICRP 103 effective dose and dose-area product. 71 papers relating to dental CBCT dosimetry were found, of which eight reported effective dose conversion factors or provided enough information to calculate dose conversion factors. Scanner model, effective dose, dose-area product, tube voltage, field of view size and DC_E were extracted from the papers for analysis.

Results:

DC_E values ranged from 0.035 to 0.31 µSv/mGy-cm² with a mean of 0.129 µSv/mGy-cm² (SD = 0.056). When categorized into small (<100 cm²), medium (100–225 cm²) and large (>225 cm²) fields of view (FOV), linear fits to the effective dose and dose-area product yielded slopes of 0.129, 0.111 and 0.074 µSv/mGy-cm² for small, medium and large FOVs respectively.

Conclusion:

The range of reported DC_E values and spread with respect to field of view category suggests that DC_E values that depend on FOV would provide more accurate effective dose estimates. Tube voltage was found to be a smaller factor in determining DC_E. Reasonable values for DC_E taking into account FOV size were obtained. There is considerable room for more work to be done to examine the behaviour of DC_E with changes to patient age and dental CBCT imaging parameters.

Introduction

Dental cone-beam CT (CBCT) has been available as a dental imaging modality since the late 1990s.¹ Modern dental CBCT scanners are based on flat panel detectors and offer the potential for multi modality imaging (2D panoramic and cephalometric imaging in addition to 3D CBCT imaging). Dental CBCT scanners are available in a wide variety of gantry geometries, scan technique options, detector sizes and fields of view.² Some units operate at a single fixed X-ray tube voltage while others offer a range of selectable X-ray techniques. Multiple field sizes are often available allowing scans to cover the entire head or small enough to image an individual tooth. Some scanners have an integrated chair or table for the patient to sit or lay on, while others require the patient to stand.

There have been a number of studies investigating the radiation doses to patients from dental CBCT scans since the modality was introduced. The majority of these studies have used thermoluminescent (TLD) or optically stimulated (OSL) dosimeters with an anthropomorphic phantom to measure organ absorbed doses and effective doses from CBCT studies. More recent studies have used Monte-Carlo simulations to calculate effective doses for some dental CBCT scans with some studies also using TLD, OSL or radiochromic film-based dosimetry to validate the simulation results. Depending on the scanner, scan protocol and field of view, effective doses can range from a few tens of µSv to several hundreds of µSv.^3,4 The general tendency has been to report dental CBCT effective doses stratified by field of view (FOV) size, either by FOV height, or FOV diameter. In general, the larger the FOV, the higher the effective doses with median effective doses ranging from under 100 µSv for the smallest FOVs (<10 cm) to just over 1000 µSv for the largest FOVs (>15 cm).^4–8 The range of published effective doses is quite large and determining an effective dose for an arbitrary scan protocol is difficult using the currently published data.

Effective dose

Effective dose is a radiation protection quantity often used to compare radiation doses for different imaging procedures or between different types of imaging modalities. The computation of effective dose is a weighted sum of the absorbed dose to a defined list of tissues and is measured in units of Sievert (Sv). The weighting factors used take into account the effect of different types of radiation on tissue, and the sensitivity of different tissues to radiation. The effective dose calculation, tissues used and weighting factors are defined in ICRP Report 103.⁹

Of the tissues used in the ICRP 103 effective dose calculation, the following are exposed to either the direct beam or scatter radiation during dental CBCT scans: salivary glands, thyroid, brain, bone surface and oral mucosa (remainder tissue).

Kerma area product (sometimes referred to as dose-area product) is a measurable radiation quantity that is generally interpreted as the total amount of radiation directed at a point. The air kerma area product (P_KA) is defined as the air kerma emitted by the X-ray tube integrated over the area of the X-ray beam. A common method for measuring P_KA is to use a dose-area product (DAP) meter, which is simply an ionization chamber, placed at exit port of the collimator assembly. The ionization chamber integrates the total air kerma emitted by the tube during the exposure.

Dose conversion factors

The concept of an effective dose conversion factor (DC_E) has been in use in diagnostic radiology for many years.¹⁰ DC_E values allow an easily measured quantity such as kerma area product (P_KA) or entrance surface dose to be converted to an effective dose value for combinations of X-ray tube voltage, filtration/half value layer, X-ray field size and anatomical region exposed. Since organ and tissue doses used in the effective dose calculation can be difficult to measure, a common method to obtain DC_E values is through Monte Carlo simulations. Monte Carlo simulations using mathematical or voxel-based phantoms makes obtaining organ/tissue absorbed doses under different X-ray beam conditions fairly easy, so DC_E tables such as those published by Hart et al can be produced for commonly performed radiographic procedures.^11,12

Using an appropriate DC_E value, effective dose for a study can be estimated using Equation 1

$${\displaystyle E=DC{E}_E\times P_{KA}}$$ (Equation 1)

A review of the current literature was undertaken to investigate the use and magnitude of effective dose conversion factors being reported for dental CBCT scanners.

Methods and materials

A literature search was performed using PubMed to search the MEDLINE database for published literature on radiation dosimetry for dental CBCT machines. The search terms selected, “dental radiography radiation dosimetry”, were intentionally broad to capture as many results as possible. This was expressed in Pubmed as (“radiography, dental”[MeSH Terms] OR (“radiography”[All Fields] AND “dental”[All Fields]) OR “dental radiography”[All Fields] OR (“dental”[All Fields] AND “radiography”[All Fields])) AND (“radiometry”[MeSH Terms] OR “radiometry”[All Fields] OR (“radiation”[All Fields] AND “dosimetry”[All Fields]) OR “radiation dosimetry”[All Fields]).

Required selection criteria for papers were scanner model and dose conversion factors (DC_E). Optional selection criteria for papers were ICRP 103 effective dose (E), kerma area product (P_KA), tube voltage, or field of view size. Papers that reported effective dose and P_KA but not DC_E were included, but papers that only reported either effective dose or P_KA but not DC_E were excluded. Papers that reported only ICRP60 effective doses were also excluded.

Of the 1618 results returned by the Pubmed search as of April 2020, approximately 71 papers were retrieved that directly related to dental CBCT dosimetry. Eight of these papers met the inclusion criteria and gave DC_E or provided sufficient information (effective dose and P_KA) to calculate a DC_E. Scanner model, tube voltage, effective dose, P_KA, DC_E and field of view size were extracted from the papers when available. Some papers reported both ICRP 60 and ICRP 103 effective doses but most of the more recent papers reported ICRP 103 effective doses. ICRP 103 added some additional tissues and modified tissue weighting factors from those published in ICRP 60.¹³ Only ICRP 103 effective doses were used in the analysis here. Data obtained from the papers were analysed to see if any trends in the response of DC_E with changes in tube voltage or field of view could be identified. Data analysis was performed using R v3.6.4 (https://www.r-project.org/).¹⁴

Results

From the eight papers used for this review, 2010 was the publication year of the earliest paper found from which DC_E could be calculated. Publication year of the other papers were 2013 (1), 2014 (2), 2017 (1) and 2018 (3). Four papers reported DC_E values in their results.^15–18 One of these papers provided only DC_E. Four other papers provided effective dose and P_KA from which DC_E values were calculated.^19–22 A total of 78 DC_E measurements were obtained, covering a range of field sizes and imaging modes for nine different dental CBCT scanners (Table 1). For the majority of the papers, effective dose estimates were obtained using TLD, OSL, or radiochromic film with an anthropomorphic head phantom. Dose-area product in most papers was measured using an external dose-area product meter. Several of the papers reviewed also used Monte Carlo simulations to obtain effective dose estimates.^{16,17,19,21,22}

Table 1.

Effective dose, dose area product (P_KA) and dose conversion factors (DC_E) from published literature

Scanner	kV	E (µSv)	PKA (mGy-cm2)	DCE (µSv/mGy-cm2)	FOV (cm)	Reference
ILUMA Ultra	120	157	1840	0.085	18.5 × 23.4	Vassileva and Stoyanov22
	120	94	1100	0.086	18.5 × 23.4
	120	46	540	0.085	18.5 × 23.4
I-CAT NG 360°	120	66	556	0.119	16 × 13	Morant et al17
	120	58	476	0.122	16 × 11
	120	53	415	0.128	16 × 10
	120	47	361	0.130	16 × 8
	120	35	276	0.127	16 × 6 maxilla
	120	39	270	0.144	16 × 6 mandible
	120	25	181	0.138	16 × 4
	120	29	214	0.136	8 × 8
	120	46	443	0.104	23 × 17 full
I-CAT NG 180°	120	40	303	0.132	16 × 13
	120	36	260	0.138	16 × 11
	120	32	226	0.142	16 × 10
	120	29	197	0.147	16 × 8
	120	22	151	0.146	16 × 6 maxilla
	120	24	147	0.163	16 × 6 mandible
	120	16	98	0.163	16 × 4
	120	18	117	0.154	8 × 8
	120	24	241	0.100	23 × 17 full
Alphard VEGA	80	183.07	3704	0.049	20 × 17.9	Kim et al15
	80	123.02	2485	0.049
	80	303.66	4499	0.068	15.4 × 15.4
	80	163.23	2508	0.065
	80	288.48	4499	0.064	15.4 × 15.4
	80	158.49	2508	0.063
	80	145.85	1910	0.076	10.2 × 10.2
	80	68.51	956	0.072
	80	184.33	1910	0.096	10.2 × 10.2
	80	85.1	956	0.089
	80	22.34	644	0.034	5.1 × 5.1
	80	20.1	429	0.047
	80	25.26	644	0.039	5.1 × 5.1
	80	20.2	429	0.047
	80	93.67	644	0.146	5.1 × 5.1
	80	61.51	429	0.145
Alphard 3030	80	428.3	3349	0.128	20 × 20	Shin et al18
	80	255.9	2001	0.128	20 × 20
	80	350.7	2743	0.128	15.4 × 15.4
	80	210.1	1643	0.128	15.4 × 15.4
	80	273.7	2140	0.128	10.2 × 10.2
	80	171	1337	0.128	10.2 × 10.2
	80	81.46	637.4	0.128	5.1 × 5.1
	80	50.77	395.8	0.128	5.1 × 5.1
Rayscan Symphony	90	158	1109	0.142	14.2 × 14.2
	90	133.4	937	0.142	14.2 × 14.2
	90	160.3	1126	0.142	14.2 × 14.2
	90	143.2	1006	0.142	14.2 × 14.2
	90	153.9	1081	0.142	14.2 × 14.2
	90	146.3	1028	0.142	14.2 × 14.2
	90	154.5	1085	0.142	9.7 × 9.7
	90	141.9	996.7	0.142	9.7 × 9.7
3D Accuitomo 170	90	48	401	0.120	4 × 4	Ernst et al19
	90	97	818	0.119	8 × 5
	90	250	2160	0.116	14 × 10
	90	49	401	0.122	4 × 4
	90	105	818	0.128	8 × 5
	90	240	2160	0.111	14 × 10
Cranex3D × 180°	90			0.180	5 × 5	Kralik et al16
	90			0.230	6.1 × 7.8
	90			0.230	7.8 × 7.8
	90			0.190	7.8 × 15
	90			0.220	13 × 15
Cranex3D × 360°	90			0.190	5 × 5
	90			0.300	6.1 × 7.8
	90			0.310	7.8 × 7.8
	90			0.230	7.8 × 15
	90			0.280	13 × 15
ProMax3D	90	88	510	0.173	4 × 5 (TLD)	Kadesjö et al20
NewTom5G	110	172	1080	0.159	6 × 6 (TLD)
NewTom5G	110	166	1080	0.154	6 × 6 (Film)
Alphard VEGA	80	174	3568.1	0.049	20 × 17.9 (MC)	Kim et al21
	80	289	4336.4	0.067	15.4 × 15.4 (MC)
	80	152	1837.8	0.083	10.2 × 10.2 (MC)
	80	216	3568.1	0.061	20 × 17.9 (TLD)
	80	366	4336.4	0.084	15.4 × 15.4 (TLD)
	80	187	1837.8	0.102	10.2 × 10.2 (TLD)

The effective doses presented by Vassileva et al were calculated using Monte Carlo simulations (PCXMC, STUK, Finland) modelling a single scanner and three imaging protocols (standard adult, low dose adult, paediatric).²² DC_E calculated from effective dose and P_KA values provided in the paper was 0.085 µSv/mGy-cm² for all three protocols. Only 12 projections at 30 degree intervals. As a result, it is possible that effective doses were underestimated somewhat.

Monte Carlo simulations were used by Morant et al to obtain sex-averaged effective dose results.¹⁷ Their simulations made use of the voxel-based ICRP anatomical male and female reference phantoms and modelled a specific scanner. Multiple fields of view as well as full and half rotation scans were simulated. DC_E values ranged from 0.10 to 0.14 µSv/mGy-cm² for the full rotation scans and 0.10–0.16 µSv/mGy-cm² for the half rotation scans. Using a linear fit to their effective dose and dose-area product measurements, they derived a DC_E value of 0.130 µSv/mGy-cm² which was consistent with the DC_E values calculated from the individual measurements.

TLD-based dosimetry and DAP measurements were used by Kim et al to obtain DC_E values for a single dental CBCT scanner at four FOV modes and two preset exposure settings (adult, low dose).¹⁵ The scanner operated at a fixed tube voltage so the low dose mode used a lower X-ray tube current. Scans were also performed at the maxilla and mandible levels as permitted by each FOV mode. DC_E values obtained by them ranged from 0.034 to 0.15 µSv/mGy-cm².

Shin et al used calculated DC_E values based on an empirical formula derived for dental panoramic radiography to get effective doses.^18,23 Their DC_E values of 0.128 and 0.142 µSv/mGy-cm² were consistent with those published by Morant et al. Dental panoramic radiography uses a very different beam geometry than dental CBCT does although (narrow fan beam versus wide rectangular cone beam) and the applicability of the calculated DC_E values to dental CBCT geometries and scan parameters is unknown.

Ernst et al performed TLD-based dosimetry as well as Monte Carlo simulations.¹⁹ Their results reported good agreement in organ/tissue doses between the TLD measurements and Monte Carlo simulations, but they did not calculate DC_E values. DC_E calculated from their data ranged from 0.116 to 0.120 µSv/mGy-cm² based on their TLD measurements and 0.111–0.128 µSv/mGy-cm² based on their Monte Carlo simulations.

Kralik et al used Monte Carlo simulations and DAP measurements, but only DC_E values were provided in their paper.¹⁶ Their simulations investigated full and half rotation scans for different FOVs for a single dental CBCT scanner. The DC_E values they obtained ranged from 0.19 to 0.23 µSv/mGy-cm² for half rotation scans and 0.23–0.31 µSv/mGy-cm² for full rotation scans. These were the highest DC_E values reported out of all of the papers reviewed. Their linear fits to effective dose and DAP values gave DC_E values of 0.21 µSv/mGy-cm² for the half rotation scans and 0.25 µSv/mGy-cm² for the full rotation scans.

Kadesjö et al looked at effective doses for both dental CBCT and 2D dental imaging for small FOV scans using a paediatric head phantom (10 year old).²⁰ Two scanners were investigated using TLD and radiochromic film-based dosimetry. DC_E values calculated based on their effective dose and DAP values ranged from 0.154 to 0.173 µSv/mGy-cm². These were slightly higher than other values published, but consistent with expectations for the paediatric age range.

Kim, Han, et al performed TLD studies to validate Monte Carlo simulations using PCXMCRotation (STUK, Finland).²¹ Three FOVs for a single dental CBCT scanner were investigated. Their DC_E values ranged from 0.049 to 0.083 µSv/mGy-cm² for their Monte Carlo simulations and 0.061–0.102 µSv/mGy-cm² for their TLD results. These values were comparable to the other study by Kim et al on the same brand dental CBCT scanner.¹⁵

The mean effective dose from the publications that reported it was 124.9 µSv (SD = 97 µSv) with a median value of 101 µSv and an inter quartile range (IQR) of 126.7 µSv (n = 68). The mean reported P_KA was 1310 mGy-cm² (SD = 1211 mGy-cm²) with a median value of 956 mGy-cm² (IQR = 1498.5 mGy-cm², n = 68). The mean DC_E was 0.128 µSv/mGy-cm² (SD = 0.056 µSv/mGy-cm²) with a median of 0.128 µSv/mGy-cm² (IQR = 0.058 µSv/mGy-cm², n = 78).

DC_E values appear to be poorly correlated with X-ray beam area or FOV, with a very slight trend toward decreasing DC_E with increasing field of view area (Figure 1). A common practice in dental CBCT dosimetry publications has been to categorize dose results by field of view height or diameter. Typical field of view categories were small (height/diameter ≤10 cm), medium (height/diameter between 10 and 15 cm) and large (height/diameter >15 cm).^3–7 Using these divisions, FOVs were categorized as small (<100 cm²), medium (100–225 cm²) and large (>225 cm²). Using these groupings a much more distinct relationship between DC_E and FOV emerged. The mean DC_E for the small, medium and large FOV were 0.148 (SD = 0.062), 0.137 (SD = 0.046) and 0.084 (SD = 0.028) µSv/mGy-cm² respectively (Figure 2). Median and mean effective dose, P_KA and DC_E grouped by FOV are listed in Table 2. Using a two-sided t-test, the mean DC_E for the small and medium FOV were not found to be statistically different (p = 0.44) while the mean DC_E for the small – large and medium – large FOV were statistically different (p < 0.001).

Figure 1.

Change in dose conversion factor with field of view area. The slope of the linear fit line is −1.60 × 10⁻⁴ (µSv/mGy-cm²)/cm².

Figure 2.

DC_E values categorized by small, medium and large FOV areas.

Table 2.

Mean (median) effective dose, P_KA and DC_E grouped by FOV area

FOV	E (µSv)	PKA (mGy-cm2)	DCE (µSv/mGy-cm2)	N
Small	64.2 (48)	515.8 (429)	0.148 (0.143)	31
Medium	125.6 (144.5)	1095.2 (1017)	0.137 (0.131)	28
Large	204.1 (183.1)	2627 (2508)	0.084 (0.084)	19

In the small FOV data, there are several outliers on the high and low end. On the low end, low DC_E (0.034–0.047 µSv/mGy-cm²) comes from scans performed in the small FOV (D-mode, 5.1 × 5.1 cm) in the maxillary region.¹⁵ The low effective dose measured in these scans was likely due to sensitive tissues (salivary glands) not being in the primary beam due to the scan location and small FOV. Monte Carlo simulations performed by Kralik et al produced the highest DC_E values (derived from linear fits to effective dose and P_KA data) out of all the publications reviewed, and can be seen as the high DC_E outliers in the small and medium FOV data.¹⁶

An estimate for reasonable DC_E values for the small, medium and large FOV can be obtained by plotting the effective dose as a function of P_KA (Figure 3) and obtaining the slope of a linear fit line through the data. Constraining the fit to go through the origin gives slopes of 0.129, 0.111 and 0.074 µSv/mGy-cm² for the small, medium and large FOVs respectively. Data from Kralik et al were not included because effective dose and P_KA values were not provided..

Figure 3.

Effective dose as a function of dose area product. The slopes for the small, medium and large FOV are 0.129, 0.111 and 0.074 µSv/mGy-cm², respectively.

All of the papers reviewed provided information on the scan protocols used, which included tube voltage. Table 3 gives the mean and median of the effective dose, P_KA, and DC_E grouped by tube voltage. Figure 4 shows a box plot of DC_E grouped by tube voltage. Figure 5 shows the same data with the addition of FOV categories at each tube voltage. Table 4 gives the linear fit parameters for DC_E as a function of tube voltage for each FOV category with the intercept constrained to go through the origin. Table 5 gives the mean and standard deviation DC_E grouped by tube voltage and FOV. Two-sided t-tests between the small, medium and large FOVs at each tube voltage suggest the mean DC_E are not statistically different at 80 and 90 kV, but are statistically different at 120 kV (p < 0.05).

Table 3.

Mean (median) effective dose, P_KA and DC_E grouped by tube voltage

kVp	E (µSv)	PKA (mGy-cm2)	DCE (µSv/mGy-cm2)	N
80	171.8 (167.1)	2096 (1910)	0.087 (0.080)	30
90	137.9 (143.2)	1042 (1006)	0.175 (0.142)	25
110	169.0 (169.0)	1080 (1080)	0.157 (0.157)	2
120	44.6 (36.0)1	400.6 (270.0)	0.128 (0.132)	21

Figure 4.

DC_E box plot for different tube voltages.

Figure 5.

DC_E box plot for different tube voltages and FOV sizes

Table 4.

Linear fit parameters (DC_E = m*kV) for DC_E as a function of kV with the intercept constrained to go through the origin

FOV	Slope (m)	R2
Small	0.0015	0.85
Medium	0.0014	0.89
Large	0.0009	0.89

Table 5.

DC_E mean and standard deviation grouped by tube voltage and FOV

kV	FOV	DCE (µSv/mGy-cm2)	SD	N
80	Small	0.089	0.051	8
	Medium	0.097	0.022	8
	Large	0.081	0.032	14
90	Small	0.184	0.066	13
	Medium	0.167	0.052	12
110	Small	0.157	0.004	2
120	Small	0.146	0.013	8
	Medium	0.132	0.010	8
	Large	0.092	0.009	5

Discussion

The primary purpose for calculating effective doses for imaging procedures is to facilitate the optimization of imaging protocols and compare the risks from different imaging protocols and modalities. Even although effective dose is not intended to be used for patient-specific dosimetry, it is still sometimes used for that purpose by incorporating patient-specific correction factors for age and patient weight. Thus having a method to easily obtain effective dose estimates is useful. Many dental CBCT units provide an indication of the kerma-area product (P_KA) for the selected protocol, so P_KA based DC_E values provide an easy way to obtain effective dose estimates.

Figure 1 suggests poor correlation between FOV and DC_E. Using previously published conventions of categorizing dental CBCT doses by small, medium, or large FOV gives a somewhat different picture with a more visible trend for lower DC_E values as FOV increases. Figure 2 indicates that using DC_E values based on FOV size would be more appropriate than using a single DC_E for all dental CBCT scan protocols. Although the mean DC_E between the small and medium FOV did not seem statistically different, this could be due to the limited number of data points available.

A linear fit to the complete set of effective dose and P_KA data results in a slope of 0.083 µSv/mGy-cm², which is close to the slope obtained for the large FOV, but lower than that for the small and medium FOVs. This indicates that using a DC_E value without taking into account FOV size could significantly underestimate or overestimate the effective dose.

When tube voltage is considered, the behaviour of DC_E appears to be more complex. Figure 4 suggests DC_E increases with increasing kV, while Figure 5 implies a decrease in DC_E with increasing FOV at a given kV. This is reflected in the linear fit slopes given in Table 4.

One area that still requires more investigation is dental CBCT radiation dose to the paediatric population. The carcinogenic risks of radiation exposure to the paediatric population is well known. Paediatric patients may also be subjected to follow-up dental CBCT scans resulting in additional exposure. Of the eight papers reviewed here, only three reported results for paediatric aged phantoms. Vassileva et al²² included a paediatric dental CBCT protocol, but did not mention what age PCXMC phantom was used to obtain the effective dose value for the paediatric protocol. Since their paediatric DC_E has essentially the same value as the adult DC_E values, it seems unlikely their paediatric protocol simulation used a paediatric phantom. The paediatric protocols evaluated by Shin et al appear to be identical to the adult protocols except for using a lower tube current (mA) setting. The same DC_E values were used to calculate effective doses for both the adult and child protocols on the two dental CBCT machines they evaluated.¹⁸ Intuitively, paediatric DC_E values would be expected to be higher than for adults, so their effective doses for the child protocol are likely to be underestimated. The paper by Kadesö was the only one to look at effective doses specifically to paediatric patients, but their work only looked at small FOV scans.²⁰ Their results and those of others show that effective doses to paediatric patients from dental CBCT are higher than for adults.^20,24–26 The DC_E values reported by Kadaesjö were in the higher end of the values in Table 1.

Although a single DC_E value can be calculated from the published effective dose and dose area product data, the range of DC_E values published in the literature suggests that a range of DC_E values would be more useful for calculating dental CBCT effective dose estimates. It has been shown in the literature that dental CBCT effective doses can change significantly depending on field of view size and scan location due to the location of sensitive organs relative to the X-ray beam.^8,16 A single DC_E value also does not take into account patient size or age. Tables that take into account age, scan location and X-ray tube technique would provide more accurate effective dose estimates than a single DC_E value applied to all dental CBCT imaging.

Conclusion

Effective dose conversion factors provide a simple method for obtaining effective dose estimates for dental CBCT from the dose area product that many scanners report. A summary of DC_E values found in the current literature was presented. DC_E values range from 0.035 to 0.31 µSv/mGy-cm² with a median value of 0.128 µSv/mGy-cm² (SD = 0.056). FOV size appears to be a significant factor affecting DC_E with tube voltage being a smaller factor. Linear fits to effective dose vs P_KA for small, medium and large FOVs gave reasonable DC_E values that can be used to obtain effective dose estimates that take into account scan FOV. Further investigation into imaging parameters such as age, field size, tube voltage and beam location would be useful to determine the most significant factors that affect DC_E.