Cone-beam computed tomography (CBCT) which provides three-dimensional (3D) images, was developed for the dental use in the late 1990s.1 The advantages of CBCT include a lower entrance dose, higher resolution, and lower cost than conventional computed tomography (CT). Moreover, CBCT provides 3D information via multiplanar and 3D reconstructed images. For this reason, its use in dental practice has rapidly increased and is commonly used in preoperative implant planning, the localization of impacted teeth, the diagnostic and surgical planning of oral and maxillofacial radiology, the evaluation of periodontal and periapical lesions, endodontic problems, and orthodontic treatment planning.2,3,4,5 Although the dose of CBCT is lower than that of conventional CT, it is higher than that of conventional radiography mainly used in dental practice. There have been numerous studies about effective doses of CBCT using thermoluminescent dosemeter (TLD) chips.6,7,8,9,10,11,12 However, this estimation method using TLD chips is laborious and time-consuming.

The establishment of diagnostic reference levels in medical imaging was recommended by the International Commission on Radiation Protection (ICRP) to promote the optimization of patient radiation exposure.13 In Korea, diagnostic reference levels for diagnostic radiology including dental radiographic procedures were reported by the Korea Food and Drug Administration.14,15,16,17 Since the entrance surface dose (ESD) and dose-area product (DAP) are well-defined and easy-to-use methods, they have been frequently used as the adequate dose quantities for DRLs.18,19 Especially, DAP was recommended as a dose quantity for CBCT by Health Protection Agency (HPA) in the United Kingdom.20 Lofthag-Hansen et al21 compared two methods of the CT dose index (CTDI) and DAP to calculate the effective dose of CBCT examination, and they proposed that the DAP measurement was the appropriate method to determine the effective dose. In addition, they commented that the conversion factors should be determined according to the dental regions and radiographic techniques.21

In diagnostic medical radiology, the conversion factors from DAP to effective dose have been suggested by a number of authors.22,23,24 There has been a very few reports about those in dental radiology. Looe et al25,26 suggested the conversion coefficients for the estimation of effective dose of the intraoral, panoramic, and lateral cephalometric radiography. However, the conversion coefficient for CBCT remains unknown.

The objectives of this study were to measure the DAP and effective dose in one CBCT device and to determine the conversion coefficients from the DAP value to effective dose.

The Alphard VEGA (Asahi Roentgen Ind. Co., Kyoto, Japan) CBCT scanner, which has four fields of view (FOVs) including the C mode (200 mm×179 mm), P mode (154 mm×154 mm), I mode (102 mm×102 mm), and D mode (51mm×51 mm), was used in this study.

The DAP was measured using the DIAMENTOR M4-KDK (PTW, Freiburg, Germany) (Fig. 1). In addition, the effective dose was measured using TLD-100 LiF chips (1.8×1.8×0.035 inch; Harshaw Chemical Co., Solon, OH, USA) in an adult male Alderson Radiation Therapy phantom of the head and neck (Radiology Support Devices, Inc., Long Beach, CA, USA) (Fig. 2).

Recently, investigation into the patient dose exposed during CBCT examination has been of increasing interest due to the profound dissemination of CBCT equipment in dental practice. This study showed that the DAP values of the Alphard VEGA CBCT examinations ranged from 644 mGycm² to 4499 mGycm², the effective doses from 22 µSv to 304 µSv at the adult standard exposure. The voxel sizes in the CBCT scanner were 0.39 mm, 0.3 mm, 0.2 mm, and 0.1 mm in the C, P, I, and D modes, respectively. In practice, each mode serves a specific purpose. The C mode (200mm×179 mm) is primarily used for orthodontic analysis; the P mode (154 mm×154 mm) for the evaluation of both impacted third molars or the diagnosis of lesions at the maxillofacial region; the I mode (102mm ×102 mm) for implant treatment planning involving more than three teeth or for the diagnosis of a jaw cyst; and the D mode (51 mm×51 mm) for implant treatment planning involving one or two teeth, one impacted tooth, the diagnosis of a small jaw cyst, or unilateral TMJ evaluation.

In descending order, the DAP values at the adult exposure setting were the highest in the P mode followed by the values in the C, I, and D mode. The differences among the DAP values tended to be proportional to the field size.21 However, the DAP value in the C mode, the largest FOV, was lower than that in the P mode in this study. This may be because the tube current for the C mode was two-thirds of that in the P mode. This result is in agreement with a previous study that observed an increasing DAP value at each mode as the tube current value increased.21

The Health Protection Agency of the United Kingdom proposed the use of the DAP as the dose quantity for the regular assessment of patient dose for dental CBCT.20,29 The Health Protection Agency Working Party on dental CBCT suggested that DRLs should be set for both adult and child radiography and the adult protocol should be that used for the placement of the upper first molar implant in a standard adult patient and the child measurement should be made using the clinical protocol used to image a single impacted maxillary canine in a 12 year old male.20 Given the wide range of DAP measurements recorded for different CBCT models, it was not considered appropriate to derive a National Reference Doses based on the third quartile DAP measurement of a dose survey as this would be little benefit for dose optimization.20 Instead they presented the achievable dose, which was based on the third quartile DAP value of a dose survey, where the X-ray field size had been normalized to an appropriate size to adequately image the two views proposed above, 4 cm diameter × 4 cm height cylindrical volume. 20 And an achievable dose of 250 mGycm² was proposed for the adult procedure based on data by 41 CBCT units.20 The adjusted DAP value at D mode normalized to an area of 16 cm² was 396 mGycm². Since this value was higher than the achievable dose suggested by the Health Protection Agency, it was required to investigate methods to reduce patient dose in the CBCT device used in this study.

The effective doses are different according to the irradiated organs and areas of the body, despite being exposed by radiation with the same DAP value. The effective doses were calculated to determine the conversion coefficients at each mode of different sites. The effective doses at the adult exposure setting ranged from 22 µSv to 304 µSv across all CBCT examinations. These values were similar to those of Pauwels et al9 who reported that the effective doses from 14 CBCT devices ranged from 19 µSv to 368 µSv. In this study, the effective dose for the P mode at the maxilla was the highest. This might be due to the high tube current and large FOV in the P mode as well as the involvement of the radiosensitive salivary glands at the maxilla. In the D mode, a high variation from 22 µSv to 94 µSv was found according to the irradiated regions, and the effective dose at the maxillary incisors was lowest, but that of the mandibular molars was the highest. The high irradiation among these radiosensitive salivary glands may have influenced these results.

Previous studies about the conversion coefficients for other imaging modalities in dental radiology reported 0.009-0.108 µSv/mGycm² for intraoral periapical radiography,25 0.087-0.131 µSv/mGycm² for panoramic radiography,25 and 0.056-0.077 µSv/mGycm² for lateral cephalometric radiography (ICRP 2006, draft).26 In this study, ICRP 103 tissue weighting factors were used to calculate the effective doses. In the D mode, the conversion coefficient for the maxillary incisors was the lowest and that for the mandibular molars was the highest. These results were similar to those from the intraoral periapical radiographic study,25 which showed the lowest conversion factors for maxillary incisor and highest for mandibular molar.25 In addition, the conversion coefficients tended to be dependent on tube voltage.25,26 However, we did not investigate the effect of tube voltage on the conversion coefficient because the CBCT scanner used in this study had a fixed tube voltage. Further research using a CBCT scanner with an adjustable tube voltage is needed.

In summary, the present study demonstrates the conversion coefficients in one CBCT device with fixed 80kV ranged from 0.038 µSv/mGycm² to 0.146 µSv/mGycm² according to the imaging modes and irradiated region and were highest for the D mode at the mandibular molar.