Effective doses from panoramic radiography and CBCT (cone beam CT) using dose area product (DAP) in dentistry

Abstract

Objectives:

We compared the effective dose from panoramic radiography with that from cone beam CT (CBCT) using dose area product under adult and child exposure conditions.

Methods:

The effective doses of the cephalo, panorama, implant and dental modes of Alphard 3030 (Asahi Roentgen Ind., Co. Ltd, Kyoto, Japan) CBCT and the Jaw, Wide, Facial and temporomandibular joint modes of Rayscan Symphony (RAY Co., Ltd, Hwaseong, Republic of Korea) CBCT were compared with those of CRANEX^® 3+ CEPH (Soredex Orion Corporation, Helsinki, Finland) panoramic radiography equipment under adult and child exposure conditions. Each effective dose was calculated using a conversion formula from dose area product meter measured values (VacuTec Messtechnik GmbH, Dresden, Germany). The conversion formulae used were suggested by Helmrot and Alm Carlsson and Batista et al, and they were applied with the tube voltage taken into consideration.

Results:

The maximum effective doses from the Alphard 3030 and Rayscan Symphony were 67 and 21 times greater than that from panoramic radiography, respectively. The ratios of the effective dose under the child setting to that under the adult condition were 0.60–0.62 and 0.84–0.95, and the maximum differences in effective doses between the adult and child exposure settings were equivalent to 27 and 4 times greater than a panoramic examination in the Alphard 3030 and Rayscan Symphony, respectively.

Conclusions:

The effective CBCT doses were higher than those of panoramic radiography. The differences in effective doses between the adult and child CBCT settings were dependent on equipment type and exposure parameters. Therefore, adequate mode selection and control of exposure as well as further research are necessary to minimize the effective dose to patients, especially for radiosensitive children.

Introduction

Cone beam CT (CBCT) presents a series of technical differences (algorithms, acquisition modes, beam formats and detectors) in comparison with those of conventional single or multislice CT, but it is actively used in the maxillofacial area owing to several advantages. CBCT takes up less space, costs less to set up, has a quicker scanning time, produces less radiation and is easier to measure exact dimensions on.¹ Its applied fields include almost all dental specialities, including extraction of supernumerary and impacted teeth, evaluation of proximity between the third molar and the mandibular canal, and examination of cysts, tumours, sinusitis and the temporomandibular joint (TMJ), as well as orthognathic surgery and implantation. CBCT has undergone an increasing number of evaluations over the past 10 years as it provides more information than does existing radiographic equipment. The use of CBCT in the Severance Dental Hospital (Seoul, Republic of Korea) has increased steadily by 12.5% from 4471 cases in 2010–2011 and by 31.6% in 2012. Paediatric CBCT dental cases have also increased by 13.1% in 2010–2011 and by 22.8% in 2012.

Although exposure dose from CBCT is relatively low compared with that of conventional CT, it is up to 10 times higher than that of intraoral and extraoral radiography used in dentistry.² Thus, the increase in patient dose during a CBCT examination is a matter of concern. In particular, paediatric patients are at a potential risk of radiation, as they are more radiosensitive than adults, have longer lifetimes and might receive a higher radiation dose than necessary if the exposure settings are not considered for their small body size.³ Therefore, justification for an examination and optimization of the radiographic technique are strongly needed.^4,5 Risk in patients aged less than 10 years old is three times greater than at the age of 30 years.³ Most studies on dental CBCT dosimetry have focused on effective doses to adults and less on those to children. In a study on effective doses for paediatric dental CBCT using an anthropomorphic phantom,⁶ the paediatric effective doses were higher than those for conventional dental radiographic imaging and similar to those for adult CBCT. However, there are insufficient studies in the maxillofacial field to achieve effective dose reduction for paediatric patients.

When the human body is exposed to radiation, the radiation interacts with tissue, and its influence is measured as exposure dose, absorbed dose, equivalent dose and effective dose.⁷ An effective dose is a general method to evaluate the influence of radiation. It is calculated by integrating individual organ dose values, the type of radiation energy, the correlation effect of the energy and the probable health risk on specific tissue considering the respective tissue-weighting factor based on the probable biological effect. Therefore, effective doses can be used as a scale to determine the increased possible health risk to people who are exposed to radiation. Several methods are available to measure effective doses:

1. 1. calculated using software tools

2. 2. calculated using dose free in air on the axis of rotation (D_air), weighted CT dose index and dose–length product

3. 3. calculated using energy delivered to a phantom

4. 4. calculated using a dose area product (DAP) meter.

As the general method of measuring, the effective dose is complicated and time consuming, and a DAP meter is often used to measure effective doses within diagnostic reference levels (DRLs)⁸ that are recommended by the International Commission on Radiological Protection. The Health Protection Agency, previously known as the National Radiological Protection Board, in the UK recommends the use of DAP for national DRLs for CBCT.^3,9 The DAP is relatively easy to measure and is a better indicator of risk than an entrance dose because it uses entrance dose and field size. The DAP correlates well with the total energy imparted to a patient, which is related to the effective dose and therefore to the overall health risk.⁹

This study was carried out to measure DAP using a DAP meter, which is an easy to use tool, and to calculate effective doses from DAP values using conversion coefficients suggested by Helmrot and Alm Carlsson¹⁰ and Batista et al.¹¹ Effective doses for both adult and child exposure conditions of the two types of CBCT were compared with the effective dose for panoramic radiography.

Methods and Materials

Dental panoramic radiography and cone beam CT

We used CRANEX^® 3+ CEPH (Soredex Orion Corporation, Helsinki, Finland) for the panoramic radiography equipment and Alphard 3030 (Asahi Roentgen Ind., Co. Ltd, Kyoto, Japan) and Rayscan Symphony (RAY Co., Suwon, Republic of Korea) for the CBCT equipment, which enabled selection of four different modes and fields of view (FOV) depending on the purpose of the examination.

Reproducibility of exposure dose

The reproducibility of exposure dose and output from the two types of CBCT was confirmed using RaySafe™ Xi (Unfors RaySafe AB, Billdal, Sweden). The most frequently used CBCT modes, the I mode of the Alphard 3030 and Jaw mode of Rayscan Symphony, were evaluated.

Tube voltage, exposure time and radiation dose were repeatedly examined 10 times. As a result, Alphard 3030 and Rayscan Symphony showed a within 10% variation between the pre-determined value and the measured value for tube voltage and exposure time. The coefficient of variation for reproducibility of exposure dose was 0.004 for the Alphard 3030 and was 0.025 for the Rayscan Symphony (Table 1), both of which were <0.05 as suggested by IEC 60601-2-7 (Particular requirements for the safety of high-voltage generators of diagnostic X-ray generators).¹²

Table 1.

Reproducibility of exposure dose on Alphard 3030 (Asahi Roentgen Ind., Co. Ltd, Kyoto, Japan) and Rayscan Symphony (RAY Co., Ltd, Hwaseong, Republic of Korea)

Parameters	Alphard 3030			Rayscan Symphony
	Tube voltage (kV)	Exposure time (s)	Radiation dose (mGy)	Tube voltage (kV)	Exposure time (s)	Radiation dose (mGy)
Setup	80	17	–	90	19.5	–
Measurement (mean ± standard deviation)	78.2 ± 0.21	17.2 ± 0.00	5.6 ± 0.02	85.2 ± 0.44	20.6 ± 0.00	2.4 ± 0.02
Error (%)	2.3	1.2	–	5.3	5.6	–
Coefficient of variation	0.003	0.000	0.004	0.005	0.000	0.025

Dose area product measurement

DAP was measured by attaching an ion chamber of a DAP meter (VacuDAP™; VacuTec Meßtechnik GmbH, Dresden, Germany) to the centre of the radiographic tube. The active area of the rectangular measuring chamber was 177 × 182 mm. CBCT exposure conditions for adult and children for each mode were determined so as to not affect image quality. The exposure conditions for cephalo (C), panorama (P), implant (I) and dental (D) modes of the Alphard 3030 are shown in Table 2. The exposure conditions for Facial, Wide, Jaw and TMJ modes of the Rayscan Symphony are shown in Table 3. The exposure conditions for panoramic radiography were 69 kV, 10 mA and 19 s. The DAP was measured first with the panoramic radiography equipment, once in each mode for the two types of CBCT and repeated 10 times.

Table 2.

Exposure parameters of Alphard 3030 (Asahi Roentgen Ind., Co. Ltd, Kyoto, Japan)

Exposure setting	Mode	Parameters	Fields of view (mm2)	Voxel size (mm)
Adult	C	80 kV, 5 mA, 17 s	200 × 200	0.39
	P	80 kV, 5 mA, 17 s	154 × 154	0.30
	I	80 kV, 8 mA, 17 s	102 × 102	0.20
	D	80 kV, 8 mA, 17 s	51 × 51	0.10
Child	C	80 kV, 3 mA, 17 s	200 × 200	0.39
	P	80 kV, 3 mA, 17 s	154 × 154	0.30
	I	80 kV, 5 mA, 17 s	102 × 102	0.20
	D	80 kV, 5 mA, 17 s	51 × 51	0.10

C, cephalo; D, dental; I, implant; P, panorama.

Table 3.

Exposure parameters of Rayscan Symphony (RAY Co., Ltd, Hwaseong, Republic of Korea)

Exposure setting	Mode	Parameters	Fields of view (mm2)	Voxel size (mm)
Adult	Facial	90 kV, 10 mA, 19.5 s	142 × 142	0.38
	Wide	90 kV, 10 mA, 19.5 s	142 × 97	0.38
	Jaw	90 kV, 10 mA, 19.5 s	142 × 97	0.38
	Temporomandibular joint	90 kV, 10 mA, 19.5 s	97 × 97	0.38
Child	Facial	90 kV, 8 mA, 19.5 s	142 × 142	0.38
	Wide	90 kV, 8 mA, 19.5 s	142 × 97	0.38
	Jaw	90 kV, 8 mA, 19.5 s	142 × 97	0.38
	Temporomandibular joint	90 kV, 8 mA, 19.5 s	97 × 97	0.38

Effective dose from dose area product

The DAP was used as it is easier to measure the effective dose than to use complicated and time-consuming conventional methods. The effective dose was calculated from the measured DAP value multiplied by a conversion coefficient. We adopted the conversion formulae suggested by Helmrot and Alm Carlsson¹⁰ and Batista et al¹¹ taking into consideration the tube voltage condition.

Effective dose from panoramic radiography equipment with voltages of 60–75 kV was calculated with the conversion coefficient using Equation (1) suggested by Helmrot and Alm Carlsson.¹⁰ They derived a conversion coefficient for typical panoramic radiographic tube voltage and have shown that the use of a DAP meter with their formula for determining patient dose during intraoral and panoramic examination is feasible:

where E is the effective dose and P_KA the DAP.

Although the conversion coefficient of Helmrot and Alm Carlsson¹⁰ was obtained for the tube voltages 60–75 kV, it has been widely used for voltages >75 kV. In which case, the effective dose might be underestimated. Therefore, CBCT effective dose with voltages >75 kV was calculated using Equation (2) suggested by Batista et al:¹¹

where kV is the tube voltage, E is the effective dose and P_KA the DAP.

Results

The average DAP value for the panoramic radiography equipment was 79.9 mGy cm². The average effective dose was 6.39 μSv after applying conversion coefficients to the equation. The average DAP value and effective dose from the four modes of Alphard 3030 (C, P, I and D modes) and the four modes of Rayscan Symphony (Facial, Wide, Jaw and TMJ modes) at adult and child exposure conditions are shown in Tables 4 and 5, respectively.

Table 4.

Comparison of effective dose and dose area product from adult and child modes in Alphard 3030 (Asahi Roentgen Ind., Co. Ltd, Kyoto, Japan)

Mode (fields of view; mm2)	Exposure setting	kV	mA	Dose area product (mGy cm²)	Effective dose (μSv)	Effective dose ratio (child:adult)
C (200 × 200)	Adult	80	5	3349 ± 2.25	428.3 ± 0.29	0.60
	Child		3	2001 ± 5.21	255.9 ± 0.66
P (154 × 154)	Adult	80	5	2743 ± 2.95	350.7 ± 0.38	0.60
	Child		3	1643 ± 1.83	210.1 ± 0.23
I (102 × 102)	Adult	80	8	2140 ± 6.77	273.7 ± 0.87	0.62
	Child		5	1337 ± 0.84	171.0 ± 0.11
D (51 × 51)	Adult	80	8	637.4 ± 0.97	81.46 ± 0.13	0.62
	Child		5	395.8 ± 1.40	50.77 ± 0.18
Panorama	Adult	69	10	79.9 ± 0.57	6.39 ± 0.01

C, cephalo; D, dental; I, implant; P, panorama.

Data given as mean ± standard deviation.

Table 5.

Comparison of effective dose and dose area product from adult and child modes in Rayscan Symphony (RAY Co., Ltd, Hwaseong, Republic of Korea)

Mode (fields of view; mm2)	Exposure setting	kV	mA	Dose area product (mGy cm2)	Effective dose (μSv)	Effective dose ratio (child:adult)
Facial (142 × 142)	Adult	90	10	1109 ± 63.41	158.0 ± 9.03	0.84
	Child		8	937 ± 28.29	133.4 ± 4.03
Wide (142 × 97)	Adult	90	10	1126 ± 50.9	160.3 ± 7.25	0.89
	Child		8	1006 ± 23.7	143.2 ± 3.37
Jaw (142 × 97)	Adult	90	10	1081 ± 44.6	153.9 ± 6.34	0.95
	Child		8	1028 ± 24.1	146.3 ± 3.43
Temporomandibular joint (97 × 97)	Adult	90	10	1085 ± 36.3	154.5 ± 5.16	0.92
	Child		8	996.7 ± 26.06	141.9 ± 3.71
Panorama	Adult	69	10	79.9 ± 0.57	6.39 ± 0.01

Data are given as mean ± standard deviation.

Among the four modes of Alphard 3030, C mode showed the highest DAP value and effective dose at 3349 mGy cm² and 428.3 μSv under the adult condition and 2001 mGy cm² and 255.9 μSv under the child condition respectively. The largest difference in the DAP value and effective dose between the adult and child exposure conditions were 1348 mGy cm² and 172 μSv in C mode, respectively. By contrast, the D mode showed the lowest DAP value and effective dose of 637 mGy cm² and 81.5 μSv under the adult condition and 396 mGy cm² and 50.6 μSv under the child condition, respectively.

Among the four Rayscan Symphony modes, the Wide mode under the adult condition and the Jaw mode under the child condition showed the highest DAP values of 1126 and 1028 mGy cm² and the highest effective doses of 160.4 and 146.3 μSv, respectively. However, the largest differences in the DAP value and effective dose between the adult and child conditions were 172 mGy cm² and 24.6 μSv in the Facial mode, respectively. In addition, the Jaw mode in the adult condition and the Facial mode in the child condition showed the lowest DAP values of 1081 and 937 mGy cm² and the lowest effective doses of 153.9 and 133.4 μSv, respectively.

Figure 1 shows the relative effective doses from the Alphard 3030 and Rayscan Symphony, which were compared with those from the panoramic radiography equipment. When a patient was examined once in the C mode on the Alphard 3030 under either the adult or child exposure condition, the effective doses were, respectively, 67 and 40 times greater than those measured during a panoramic radiography examination. This means that the difference between effective those from the C mode under the adult and child exposure conditions is equivalent to 27 times the doses measured during a panoramic examination. The D mode effective doses were 13 times and 8 times greater than those from panoramic radiography under the adult and child exposure conditions, respectively. In each mode, the ratio of the effective dose under the child setting to that under the adult condition was 0.60–0.62.

Figure 1.

Effective dose ratio compared with panorama. (a) Alphard 3030 (Asahi Roentgen Ind., Co. Ltd, Kyoto, Japan), (b) Rayscan Symphony (RAY Co., Ltd, Hwaseong, Republic of Korea). TMJ, temporomandibular joint. C, cephalo; D, dental; I, implant; P, panorama.

The effective doses under the adult and child exposure conditions for the Rayscan Symphony were, respectively, 24–25 times and 21–23 times greater than those from panoramic radiography. Additionally, the difference in effective doses between the adult and child exposure conditions in each mode was equivalent to 2–4 panoramic examinations. However, the ratio of effective dose under the child setting to that under the adult setting was 0.84–0.95, which was higher than that of the Alphard 3030.

Discussion

Panoramic radiographic and CBCT applications have increased in the dental field, and the patient radiographic dose is attracting more attention. In particular, many studies^3–5 have been conducted on reducing the exposure dose to radiosensitive paediatric patients. The DAP is recommended among several methods to measure effective doses, as it is a simple and time-saving method. However, because DAP uses the units of mGy cm² instead of μSv, it is difficult to compare effective doses derived from DAP. In this study, a conversion coefficient suggested by Batista et al¹¹ was used to calculate the effective dose more conveniently.

The DAP value for panoramic radiography was 79.9 mGy cm² and the effective dose was 6.39 μSv. The DAP values for the Alphard 3030 and Rayscan Symphony CBCT were 396–3349 mGy cm², and the effective doses were 50.6–428.3 μSv, which corresponded to the DRLs.^13,14 The effective dose was decreased as the FOV or the current (mA) was decreased, as reported by other studies.^15,16 The DAP values and the effective doses of all Alphard 3030 modes became low when the FOV was small and the current (mA) was low as in the child exposure condition (Table 4). However, although DAP and effective dose became low when the current was low on the Rayscan Symphony, no difference was observed despite the change in FOV (Table 5). The reason was that the change in FOV in each mode was greater for the Alphard 3030 than that for the Rayscan Symphony. Another reason is that while the filter within the radiographic tube of the Alphard 3030 collimates according to the FOV set by the different modes, the Rayscan Symphony exposes the same dose of radiation irrespective of the mode change, and the FOV is controlled by the detector program.

Although it is difficult to compare the two types of CBCT directly, the ratio of the effective dose for adults and children was different. The Alphard 3030 showed a consistent ratio, whereas the Rayscan Symphony did not. Having the effective dose from panoramic radiography as a reference, the effective dose from the Alphard 3030 was 13–67 times greater under the adult setting and 8–40 times greater under the child setting. The effective dose from Rayscan Symphony was 24–25 times greater in the adult setting and 21–23 times greater in the child setting. The adult setting had a 5–27 times greater dose than that of the child setting for panoramic radiography on the Alphard 3030 and 1–4 times more for the Rayscan Symphony. This result indicates that the effective dose is different between the two types of equipment depending on the exposure condition for image quality.

Effective dose variation according to CBCT type has also been reported by Ludlow and Ivanovic.¹⁶ In their phantom study, the effective doses for the large FOV CBCT ranged from 68 to 1073 μSv, medium FOV CBCT ranged from 69 to 560 μSv and small FOV CBCT ranged from 189 to 652 μSv using the International Commission on Radiological Protection 2007 tissue weighting factors.¹⁷ In the present study, the C mode of the Alphard 3030 was the large FOV group, the P mode of the Alphard 3030 and the Facial, Wide and Jaw modes of the Rayscan Symphony were the medium FOV group, and the I and D modes of the Alphard 3030 and TMJ mode of the Rayscan Symphony were the small FOV group. The measured effective dose in this study was within the range of Ludlow and Ivanovic's results except for the small FOV group. The effective dose of the small FOV group in the present study ranged from 50.8 to 273.7 μSv. Our equipment's effective dose was low because although tube voltage and currents were similar, a smaller FOV, tube current or shorter scan time was used. Because the effective dose with fixed FOV varies depending on technical factors, it is difficult to directly compare our effective dose results to those of Ludlow and Ivanovic.¹⁶ However, the objective of our study was to compare the effective dose between adult and child exposure settings.

It is advantageous that effective dose can be calculated simply from the measured DAP value. However, there are shortcomings to this method in that it is impossible to measure effective dose on separate portions of tissue in the dental field, and there are errors according to the types of equipment and established conditions. Therefore, an additional study on conversion coefficient is required to reduce the range of errors from actual effective dose.

Conclusion

We estimated the effective dose using the DAP value and conversion coefficient from panoramic radiography and CBCT, which are the most frequently used imaging equipment in the dental field. As a result of this study, change in modes and the adult–child exposure conditions for CBCT considerably affected patient radiation dose. Although the effective CBCT dose satisfies DRL, effective dose was higher than that for panoramic radiography and the ratio compared with the panoramic radiography was different between the tested CBCTs. The ratio of the effective dose for adult and child exposure conditions was different between the tested CBCTs. As voltage and current of CBCT equipment becomes controllable, as with other radiographic equipment, proper selection of the mode is necessary when considering the FOV and age of patients. It is necessary to minimize radiation dose to children taking into consideration the trade-off between the diagnostic benefit and the dose detriment. However, guidelines on the recommended exposure dose for adult and child are absent in the dental radiographic field. Therefore, additional study and effort is essential to reduce patient dose whilst considering radiographic image quality.