浅田 恭生

This study aimed to assess fetal radiation exposure in pregnant women undergoing computed tomography (CT) and rotational angiography (RA) examinations for the diagnosis of pelvic trauma. In addition, this study aimed to compare the dose distributions between the two examinations. Surface and average fetal doses were estimated during CT and RA examinations using a pregnant phantom model and real-time dosemeters. The pregnant model phantom was constructed using an anthropomorphic phantom, and a custom-made abdominal phantom was used to simulate pregnancy. The total average fetal dose received by pregnant women from both CT scans (plain, arterial and equilibrium phases) and a single RA examination was ~60 mGy. Because unnecessary repetition of radiographic examinations, such as CT or conventional 2D angiography can increase the radiation risk, the irradiation range should be limited, if necessary, to reduce overall radiation exposure.

PURPOSE: Organ dose evaluation is important for optimizing cone beam computed tomography (CBCT) scan protocols. However, an evaluation method for various CBCT scanners is yet to be established. In this study, we developed scanner-independent conversion coefficients to estimate organ doses using appropriate peak dose (f(0)) indices. METHODS: This study included various scanners (angiography scanners and linear accelerators) and protocols for the head and body (thorax, abdomen, and pelvis) scan regions. f(0) was measured at five conventional positions (center position (f(0)c) and four peripheral positions (f(0)p) at 90° intervals) in the CT dose index (CTDI) phantom. To identify appropriate measurement positions for organ dose estimation, various f(0) indices were considered. Organ doses were measured by using optically stimulated luminescence dosimeters positioned in an anthropomorphic phantom. Thereafter, the conversion coefficients were calculated from each obtained f(0) value and organ or tissue dose using a linear fit for all scanners, and the coefficient of variation (CV) of the conversion coefficients was calculated for each organ or tissue. The f(0) index with the minimum CV value was proposed as the appropriate index. RESULTS: The appropriate f(0) index was determined as f(0)c for the body region and a maximum of four f(0)p values for the head region. Using the proposed conversion coefficients based on the appropriate f(0) index, the organ/tissue doses were well estimated with a mean error of 14.2% across all scanners and scan regions. CONCLUSIONS: The proposed scanner-independent coefficients are useful for organ dose evaluation using CBCT scanners.

Effective dose is sometimes used to compare medical radiation exposure to patients and natural radiation for providing explanations about radiation exposure to patients, but its calculation is lengthy and requires dedicated measuring devices. The purpose of this study was to identify the most suitable conversion coefficient for conversion of easily measurable dose to effective dose in posterior-anterior chest radiography, and to evaluate its accuracy by direct measurement. We constructed an examination environment using Monte Carlo simulation, and evaluated the variation in conversion coefficients from incident air kerma (IAK), entrance-surface air kerma (ESAK), and air kerma-area product (KAP) to effective dose when the irradiation field size and radiation quality were changed. Effective doses were also measured directly using thermoluminescence dosimeters and compared with the effective dose obtained from conversion coefficients. The KAP conversion coefficient most effectively suppressed the effect of irradiation field size, and was then used to set conversion coefficients for various half-value layers. The optimal conversion coefficient was 0.00023 [mSv/(mGy·cm2)] at 120 kVp (half-value layer = 5.5 mmAl). Evaluation of the direct measurements obtained with various radiation qualities revealed that the accuracy of the conversion coefficient was maintained at ≤ 11%. The proposed conversion coefficient can be easily calculated even in facilities that do not have equipment for measuring effective dose, and might enable the use of effective dose for providing explanations about radiation exposure to patients.

We aimed to verify whether the image quality of large low-contrast objects can be improved using visual model-based iterative reconstruction (VMR) while maintaining the visibility of conventional filtered back projection (FBP) and reducing radiation dose through physical and visual evaluation. A 64-row multi-slice CT system with SCENARIA View (FUJIFILM healthcare Corp. Tokyo, Japan) was used. The noise power spectrum (NPS), task-based transfer function (TTF), and signal-to-noise ratio (SNR) were physically evaluated. A low contrast object as a substitute for a liver mass was visually evaluated. In the noise measurement, STD1 showed an 18% lower noise compared to FBP. STR4 was able to reduce noise by 58% compared to FBP. The NPS of VMR was similar to those of FBP from low to high spatial frequency. The NPS of VMR reconstructions showed a similar variation with frequency as FBP reconstructions. STD1 showed the highest 10% TTF, and higher 10% TTF was observed with lower VMR level. The SNR of VMR was close to that of FBP, and higher SNR was observed with higher VMR level. In the results of the visual evaluation, there was no significant difference in visual evaluation between STD1 and FBP (p = 0.99) and between STD2 and FBP (p = 0.56). We found that the NPS of VMR images was similar to that of FBP images, and it can reduce noise and radiation dose by 25% and 50%, respectively, without decreasing the visual image quality compared to FBP.