Introduction
Medical professionals, including nurses, are at risk of radiation exposure during patient diagnosis and treatment. However, the radiation protection education provided by training institutions for medical professionals is insufficient. Additionally, medical professionals have few opportunities to receive radiation protection education during their employment [1]. This results in a lack of understanding of radiation protection among medical professionals, increasing the risk of radiation exposure [1]. Scattered radiation is the most common source of exposure for medical professionals. Therefore, understanding the distribution of scattered radiation is important for radiation protection education.
The radiation flux and dose are invisible, making them difficult to imagine. In recent years, augmented reality (AR) technology has been actively introduced in medical education, including to visualize radiation [2–4]. AR can facilitate understanding by displaying objects that cannot usually be seen. AR is a technology that overlays digital elements, such as computer graphics (CG), onto the real world to create an interactive experience [3].
Previous studies have reported the usefulness of AR in radiation protection education [5–7]. In one of these studies, radiation flux was displayed using AR [7]. The use of AR allows users to learn by observing the three-dimensional (3D) distribution of scattered rays, thereby improving their understanding of scattered radiation. Indeed, this approach has been shown to improve educational effectiveness [8]. As the radiation dose decreases in inverse proportion to the square of the distance from the source of scattered radiation, understanding the relationship between the distance from the radiation source and the radiation dose is important for radiation protection. However, previous AR-based radiation protection educational materials have made it difficult to understand the distance relationship with the source of scattered radiation, owing to the characteristics of AR.
The purpose of this study was to develop an educational application for radiation protection that can visualize the behavior of scattered radiation and display the radiation dose at the user’s position. Therefore, this study developed an AR application with a function to display the radiation dose at the user’s position using results simulated by Monte Carlo calculation. This study aims to provide a more realistic understanding of radiation exposure at a given distance from the source of scattered radiation, which is the patient. This study focused on mobile X-ray imaging, which involves many medical professionals.
Materials and Methods
Mobile radiography in hospital rooms is a typical example of radiation work in which many medical professionals engage. In this study, the behavior of scattered radiation in hospital room radiography was simulated using a Monte Carlo approach. In addition, the results of the Monte Carlo calculations were validated using the results of radiation measurements under the same conditions. Information on the scattered radiation dose to be displayed in the AR was based on the results of this validated Monte Carlo calculation.
1. Simulation of the Behavior of Scattered Radiation in Mobile Radiography in Hospital Rooms
Particle and Heavy Ion Transport Code System (PHITS) version 3.280 [9] was used to simulate the behavior of scattered radiation when using mobile radiography in hospital rooms. PHITS was validated for photon and electron transport by Iwamoto et al. [10], ensuring the reliability of the calculation results. For the calculations, the photon and electron cutoff energies were set to 1×10−3 MeV and 1×10−1 MeV, respectively. A history number of 2,000,000 particles ×93 batches was employed to achieve a statistical error of 0.05% in the calculation results.
1) Simulation geometry
The behavior of scattered radiation during chest imaging in a seated position and during abdominal imaging in a supine position in a hospital room was simulated. The bed, mobile radiography unit, patient, and radiographer were arranged in a virtual room filled with air. The mobile radiography unit was simulated as an iron box. The patient was simulated using the male human body voxel phantom provided by the International Commission on Radiological Protection (ICRP) [11]. The mobile radiography unit was fabricated from 5 mm thick iron and measured 50 cm×50 cm×130 cm. The radiographer was simulated as a rectangular water phantom with a height of 170 cm, thickness of 20 cm, and width of 40 cm. These dimensions were designed to conform to the standard body size of Japanese people. Although the radiographer is generally not located here due to radiation leakage from the X-tube, they were stationed here to provide ample space for training with the AR application. The hospital room was constructed as a 500 cm×400 cm×300 cm air-filled enclosure, with surrounding walls and floors made of 25 cm thick concrete. The patient bed was fabricated from 5 mm thick iron and measured 50 cm×20 cm×130 cm. The detailed geometric configuration is shown in Fig. 1.
2) Beam parameters
This study aimed to enhance radiation protection for nurses. Nurses typically operate near patients and are positioned farther away from the X-ray tube, which minimizes their exposure to leaked X-rays. Consequently, the study design did not incorporate the X-ray tube and leaked X-rays.
The exposure conditions for chest imaging in the seated position were set to 80 kVp tube voltage, 120 cm source-to-detector distance, 22 mAs current time product, and 43 cm× 35.4 cm field size. The exposure conditions for abdominal imaging in the supine position were set to 60 kVp tube voltage, 120 cm source-to-detector distance, 22 mAs current time product, and 43 cm×35.4 cm field size. In this study, mobile radiography conditions, including tube voltage, current-time product, and field size, were replicated from those used at a collaborating medical center. A continuous energy spectrum at 80 kVp and 60 kVp was calculated using the software X-ray- Spectrum-2 (Laboratory of Radiological Technology); this software employs Birch’s approximation formula to estimate diagnostic X-ray spectra [12]. A radiation source for the PHITS was created using these data. In the PHITS code, the energy bin size for the photon beam was configured to be 0.5 keV.
3) Collected physical quantities
The ambient dose equivalent and the absorbed dose were calculated using the PHITS. The ambient dose equivalent, which is a value close to the effective dose and indicates cancer risk according to the definition by the ICRP, was used for display in AR. It was calculated using the T-cross function. Additionally, the ambient dose equivalent was derived by multiplying the conversion coefficient with the flux collected at T-cross using the ‘multiplier’ functionality in the PHITS. The absorbed dose, which represents the amount of energy absorbed by materials exposed to radiation, was used to verify the calculation results and calculated using the T-deposit function.
For AR and validation, the calculation results for the hospital room were divided into 10 cm×10 cm×10 cm voxels, and the average physical quantities in each voxel were calculated. To minimize clutter and ensure application stability, the dose collection volume was set to 103 cm3.
2. Validation of the Simulation Results
The simulation accuracy was validated by comparing the calculated and measured results obtained under identical irradiation conditions.
1) Measurement system
The measurements and calculations of chest imaging in the seated position were compared for verification. Commercial X-ray equipment IME-100A (Toshiba Medical) was used for X-ray exposure. The absorbed dose of scattered radiation was measured using a calibrated ion chamber RC1800 (Radcal Corp.). A CT Torso Phantom CTU-41 (Kyoto Kagaku) was used to simulate the patient. The exposure conditions of the measurements were the same as those used in the Monte Carlo calculations described earlier. This measurement system was also used to align the calculation results with the measured values. Calibration was performed using the physical dose at the point in front of the patient.
The X-ray dosimeter was placed at a distance of d (50, 100, 150, and 200 cm) from the center of the phantom and at an angle of θ (45°, 90°, and 135°) with respect to the beam (Fig. 2). Measurements were performed thrice at each measurement point, and the average value was used as the measurement value.
2) Comparison of measurement and calculation results
The error (Err) between the calculated dose (Dcalc) and the measured dose (Dmeas) was obtained using the following Equation (1), and the accuracy of the scattered radiation simulation was verified, as:
3. Development of the AR Application
An AR application was developed with the following features. (1) Utilizing the camera functionality of tablet devices, dose information was overlaid onto the real world, with the QR code position serving as the origin. (2) A box displayed in the real world showed a color map of the ambient dose equivalent based on the calculated scattered radiation dose. (3) A panel of the CG was displayed at the user’s position. The panel showed the ambient dose equivalent and distance from the scattered radiation source, which was the patient.
Unity version 2021.3.9f1 (https://unity.com) was used as the developmental application. The tablet device’s operating system was Android 11. The radiographer, patient, radiation generator, and direct radiation were created using CG-3D graphics.
Results
1. Scattered Radiation Simulation
Fig. 3 shows a color map of the radiation flux distribution in the hospital room for chest and abdominal radiography. In both cases, the ambient dose equivalent tended to decrease with increasing distance from the patient, who was the source of scattered radiation. In addition, the radiographer and mobile radiography unit acted as shielding devices, and the color corresponding to the ambient dose-equivalent changed. The dose behind the radiographer was approximately 10% lower than the dose at the same distance from the patient.
2. Simulation Accuracy Verification
Table 1 lists the values of error obtained from the measurement and simulation results. These results demonstrate that the calculated values were approximately within 10% of the measured values.
3. AR Application Usage
Fig. 4 shows how the scattered radiation dose is displayed in the real world when an AR application is used. The color of the box is displayed as a color map, allowing the user to visually understand the distribution of the ambient dose equivalent of the scattered rays. Users can observe from any position while holding the tablet device. In addition, a CG panel, such as that shown in Fig. 5, can be displayed at any position to display the accurate dose at the observation position.
Discussion
1. Calculation Accuracy of Scattered Ray Distribution
The calculation results were validated, and the error between the displayed radiation dose values and the measured radiation dose values was generally less than 10%. This difference could have been caused by the differences in the human phantoms used for the calculation and measurement. The human phantom used for the calculations was a simulated model of the European standard body type. In contrast, the human phantom used for the measurement was a simulated model of the Japanese standard body type. This difference may have caused the distribution of the scattered radiation to change. The CTU-41 phantom used for measurement had shoulders that were approximately 5 cm smaller than the male human-body voxel phantom used for calculation. Therefore, in the measurements, the reduced absorption of scattered radiation within the phantom likely increased the scattered radiation dose. In practice, an error of 10% for 10 μSv is 1 μSv, which is not considered to be a significant error in radiation protection.
This discrepancy arises from the differences in the phantom sizes used for the calculation and measurement. Similarly, in clinical practice, the size of the computational phantom often differs from that of the patient, and the irradiation conditions used in calculations may not match the actual irradiation conditions. Consequently, the dose displayed in the AR application might differ from the actual radiation field dose. Informing users of this potential discrepancy when using the application is crucial.
2. Usability of and Challenges Associated with the Scattered Radiation Visualization AR Application
AR applications offer a distinct advantage over virtual reality for visualizing scattered radiation in real space. By overlaying radiation dose information onto the physical environment in the AR application, users can readily appreciate the relationship between distance and dose, gaining a deeper understanding of scattered radiation behavior.
Despite these advantages, applications have some potential challenges that require further attention. The main challenge lies in initially understanding the meaning of the displayed numerical values. For instance, proximity to a patient results in a higher peripheral dose equivalent, with a decrease in dose occurring as the distance increases. Comprehensive prior education on radiation protection, encompassing an understanding of radiation dose levels and the associated health risks, can effectively address this concern and equip users with the skills to interpret the data accurately.
Nevertheless, despite this challenge, there are several advantages of using AR applications. First, AR applications can be easily used on tablet devices without specialized equipment. This makes them well-suited for use in radiation protection education at training institutions and medical facilities. Second, AR applications allow users to simulate diverse imaging scenarios, supporting training aimed at minimizing exposure to scattered radiation across different contexts.
Conclusion
In this study, an AR application was developed to visualize the behavior of scattered radiation during mobile imaging for radiation protection education. The application allows users to experience and understand the relationship between the distance from the patient and the dose of scattered radiation. The practical use of this AR application in radiation protection education can enhance medical professionals’ understanding of radiation protection.
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