PHWR Public Health Weekly Report

Recently, radiation exposure of patients from imaging examinations has received growing public attention. Patient radiation dose varies depending on the imaging conditions. Generally, image quality tends to improve with an increased radiation dose. Despite the wide availability of equipment for digital radiography (DR), image quality is minimally affected even via DR under overexposure conditions, thus making it difficult to evaluate radiation exposure. Therefore, guidelines and reference dose for radiation are needed in medical institutions. Although medical radiation can expose patients to radiation, it does provide direct benefits to patients and hence no dose limits have been set. However, to protect patients from unnecessary radiation, the principle of “as low as reasonably achievable (ALARA)” is followed. To adhere to the ALARA principle, justification and optimization principles should be implemented; the diagnostic reference level (DRL) is the most widely applied principle in the optimization process. Establishing the DRL was proposed to achieve optimization by the International Commission on Radiological Protection (ICRP) Publications 60 and 73 and the European Commission Council Directive 97/43/Euratom [1,2]. In 2014, the International Atomic Energy Agency stated in “The International Basic Safety Standards: Requirement 34” its recommendation that the government set its own DRL. Furthermore, in the ICRP Publication 135, it is recommended that the DRL be revised as frequently as needed, especially when there are changes in technology, such as in the case of cone-beam computed tomography (CBCT) [3]. In dentistry, the image receptors have changed from film (analog) to digital sensors in the past 20 years. The use of panoramic radiography and CBCT has increased rapidly, further necessitating periodic resetting of the DRL. In the Republic of Korea (ROK), DRL has been set for each imaging device since 2007. In dentistry, the DRL for intraoral and panoramic imaging was established through a nationwide survey in 2009, while that for CBCT was established for the first time through a survey in 2018. This study aimed to establish the national DRL for dental imaging in 2023 by targeting dental university hospitals, dental hospitals, and dental clinics across the country.

1. Current Status of the Diagnostic Radiation Equipment in Dentistry

To ensure the reliability of the survey results for DRL revision in dental imaging, we investigated the current installation status of the diagnostic radiation equipment for dental imaging in ROK by region and medical institution using the equipment status statistics recorded by the Korea Disease Control and Prevention Agency (KDCA). As of 2022, the number of diagnostic radiation equipment used in dental medical institutions nationwide included 25,252 intraoral imaging devices, 5,567 panoramic imaging devices, and 16,102 CBCT machines. Combined CBCT-panoramic imaging devices are classified as CBCT devices during registration, which may have led to an underestimation of the number of panoramic imaging devices. Based on the medical equipment status, 300 imaging devices of each type of diagnostic radiation equipment were used for the investigation in this study (Supplementary Table 1; available online).

2. Data Collection and Questionnaire Development

To revise the national DRL for dental imaging, intraoral radiography, panoramic radiography, and CBCT were selected as the DRL inspection items based on those used in previous studies [4]. The European Guidelines on Diagnostic Reference Levels for Pediatric Imaging No. 185 states that when setting the pediatric DRL, the age ranges for head and neck imaging are divided into 0–3 months, 3 months to 1 year, 1–6 years, and ≥6 years [5]. The items investigated in this study are shown in Table 1. Irradiation dose (i.e., DRL quantity) was measured as the dose area product (DAP; mGy‧cm²). The DAP is a radiation dose calculated by multiplying the absorbed dose and the irradiated area, and it was measured by a DAP meter using an ionization chamber. For intraoral imaging, the DAP meter was placed at the end of the dental X-ray tube to measure the radiation dose. For panoramic radiography and CBCT, the DAP meter was placed in front of the collimator, and the DAP was measured over the entire exposure time. The exposure conditions used were those actually used for adult and pediatric patients at each medical institution. All DAP meters used for measurement were calibrated, and their validity and reliability as measuring equipment were ensured. To increase the accuracy of the national DRL and efficiency of data collection, this study designed a method wherein the questionnaire (Supplementary Figure 1; available online) developed for inspecting diagnostic radiation equipment registered with the KDCA and radiation protection facilities was used to enter the imaging conditions for each inspection item, conduct dosimetry under the respective conditions, and investigate the DRL. Additional information such as the region, device expiration date, exposure amount indicated by each device, device system, manufacturer, and model name were also recorded.

Table 1 Selected criteria for the 2024 national DRLs in dental radiography

Examination type	Patient size	Protocol
Intraoral radiography	Adult	Mandibular fist molar
	Child	Mandibular fist molar
Panoramic radiography	Adult	-
	Child	-
CBCT	Adult	Implant planning for maxillary fist molar
CBCT	Child	Evaluation of mesiodens or impacted canine

DRL=diagnostic reference level; CBCT=cone-beam computed tomography; -=not available..

1. Data Measurement Targets

The medical institutions selected as data measurement targets were among those that requested inspection institutions to inspect their diagnostic radiation instruments. The medical institutions participating in this study to revise the national DRL for dental imaging included 331 medical institutions for intraoral imaging, 328 institutions for panoramic imaging, and 301 institutions for CBCT. The information on the radiation dose for radiography in adults and children at each institution was investigated. The results of the expiration date analysis of the investigated devices based on the year of manufacture showed that the average remaining expiration period was 7.6 years for intraoral imaging devices, 5.7 years for panoramic imaging devices, and 4.6 years for CBCT devices. Among the 331 intraoral imaging devices, 170 were fixed and 161 were mobile devices. The DAP values were displayed in 4 out of 331 (1.2%) intraoral imaging devices, 200 (61%) out of 328 panoramic imaging devices, and 210 (70%) out of 301 CBCT devices.

2. Analysis of Dental Radiography Conditions

Radiography conditions for intraoral imaging, panoramic imaging, and CBCT were analyzed according to the adult and pediatric ages. Descriptive statistical analysis of the tube voltage (kVp), tube current (mA), and exposure time (second) was performed for each imaging type and age group. Table 2 shows the confirmed mean, minimum, maximum, median, mode, first quartile (25% level), and third quartile (75% level) values.

Table 2 Distribution of exposure conditions

Examination type	Technique factors	Patient size	Mean	Minimum	Maximum	Median	Mode	25%	75%
Intraoral radiography	kVp	Adult	64.2	60	70	65	60	60	70
		Child	64	60	70	60	60	60	65
	mA	Adult	4.2	1	10	3	2	2	6
		Child	4.3	1	10	2.5	2	2	6
	sec	Adult	0.4	0.03	3.15	0.28	0.2	0.18	0.4
		Child	0.2	0.03	1.18	0.2	0.1	0.1	0.3
Panoramic radiography	kVp	Adult	76.5	63	94	74	74	73	80
		Child	70.6	60	94	67	67	67	72
	mA	Adult	10.7	2	15	10	12	10	12
		Child	8.6	5	12	10	10	7	10
	sec	Adult	13.9	7.15	24	13.5	13.5	13.5	14.1
		Child	13.3	6	24	13.5	13.5	11.5	14
CBCT	kVp	Adult	90.4	74	120	94	94	85	94
		Child	87.7	65	100	90	94	80	94
	mA	Adult	10.5	2.8	13	8	8	7.4	8.3
		Child	7.1	2	13	7.3	7	6.5	7.7
	sec	Adult	18.6	5	24	18	18	18	24
		Child	18.3	5	24	18	18	18	22

CBCT=cone-beam computed tomography..

3. Dose Measurement Results by Imaging Type in Dental Radiology

Dosimetry for establishing the DRL for dental imaging was conducted by placing the ionization chamber of the DAP meter in front of the respective device collimator. The distribution of the DAP by age group according to the dental imaging type is shown in Table 3. Figure 1 shows the graphs comparing the irradiation doses in panoramic imaging according to the medical institution type, expiration date, country of manufacture, and combined or non-combined imaging device. The comparison of the dose by medical institution type showed that the dose in dental clinics and dental hospitals was almost twice as high as that in dental university hospitals. The graphs comparing the dose in CBCT by medical institution type, expiration date, and country of manufacture are shown in Figure 2.

Figure 1. Third quartile DAP values in panoramic radiography by medical institution type, device age, manufacturing country, and device type
DAP=dose area product.

Figure 2. Third quartile DAP values in CBCT by medical institution type, device age, manufacturing country
DAP=dose area product; CBCT=cone-beam computed tomography.

Table 3 Distribution of DAP

Examination type	Patient size	Mean	Minimum	Maximum	Median	25%	75%
Intraoral radiography	Adult	42.8	4.0	270.1	32.0	19.4	48.2
	Child	27.5	2.4	219.4	23.9	13.4	30.8
Panoramic radiography	Adult	333.7	12.1	5,492.4	151.2	101.3	354.3
	Child	217.5	11.4	5,030.0	87.3	55.8	223.6
CBCT	Adult	1,413.4	110.0	9,387.9	913.4	684.1	1,856.1
	Child	1,146.4	46.3	9,290.6	744.2	597.8	1,349.7

Unit: mGy‧cm². DAP=dose area product; CBCT=cone-beam computed tomography..

The DRL value (arbitrary nominal value set at 75% of the distribution of investigated doses) was established based on the third quartile values investigated in this study. The national DRL for dental imaging in 2023 was set at 48 mGy‧cm² for intraoral imaging in adults, 31 mGy‧cm² for intraoral imaging in children, 354 mGy‧cm² for panoramic imaging in adults, 224 mGy‧cm² for panoramic imaging in children, and 1,856 mGy‧cm² for CBCT in adults, and 1,350 mGy‧cm² for CBCT in children.

The newly established DRL for dental imaging showed no significant decrease but rather an increase compared to the DRL announced in 2019. In the last survey, the DRL for intraoral imaging was 46 mGy‧cm² for adults and 29.9 mGy‧cm² for children, indicating a slight increase in the values recorded in this survey. The DRL for adults increased significantly from 227 mGy‧cm² in the last survey to 354 mGy‧cm² in this survey for panoramic imaging but decreased slightly for CBCT from 2,060 mGy‧cm² to 1,856 mGy‧cm² [4]. Notably, the results of analyzing panoramic imaging devices with significantly increased DRL showed that the DAP was high with the latest equipment, domestic equipment, and combination equipment. Considering that the mean expiration date of the domestic equipment and combined equipment investigated in this study was approximately 5 years, the recently purchased panoramic devices are mostly combination devices produced domestically, which could explain the high DAP of these imaging devices. Additionally, the radiation dose in dental clinics and dental hospitals was twice as high as that in dental university hospitals, indicating a significant difference by medical institution type. Dental clinics usually do not employ dental radiologists or radiology technicians, and national-level dose surveys in foreign countries have also shown that dental clinics have a wider distribution of radiation doses than medical institutions. This study is significant because it has a relatively high representativeness owing to the significantly higher number of radiation dose measurements compared to the previous DRL surveys. However, since this study randomly selected target medical institutions to represent all medical institutions, it involved a larger number of dental clinic-level medical institutions than those in previous studies. This could be why the DRL in this study showed an overall increasing trend compared to the previous DRLs in ROK. Since the radiation dose in CBCT varies greatly depending on the field of view (FOV) selection, it is believed that the radiation dose measured was lower on devices that have been used for more than 10 years with relatively smaller FOVs.

Several reports on DRL emphasize that it is not a value intended for dose limitation. The equipment advancements facilitate obtaining images adequate for diagnostic purposes even with doses lower than the first quartile of the overall distribution. Furthermore, if it is justifiable to use a dose higher than the DRL depending on the patient's situation, this should be at the discretion of the medical professional. Additionally, each examination do not be evaluated. The purpose of the DRL is to ensure that the dose used by medical institutions is within normal limits and should be implemented in conjunction with image quality management. We hope that medical institutions will actively utilize the DRL and strive to optimize patient dose during dental imaging, thereby achieving dose reduction effects when the national DRL is revised in the future.

Ethics Statement: Not applicable.

Funding Source: This research was supported by the Korea Disease Control and Prevention Agency, policy research service project (2023-10-003).

Acknowledgments: We would like to thank all the inspection attendants of the inspection agency who conducted the DRL investigation on site and the medical institution officials who responded to the investigation.

Conflict of Interest: The authors have no conflicts of interest to declare.

Author Contributions: Conceptualization: YJM, JWG, JHW. Data curation: JHK. Formal analysis: MSH, JHK. Methodology: MSH, JHK. Project administration: YJM, JWG, JHW. Supervision: MSH. Writing – original draft: JHK. Writing – review & editing: MSH.

Supplementary data are available online.