羽場 友信

There has been a growing concern regarding exposure to superficial radiosensitive organs with the rapid increase of computed tomography examinations. Manufacturers have been developing various dose reduction methods in order to prevent harm to these organs. Our previous study revealed a unique phenomenon in X-ray computed tomography scanning, in which the maximum absorbed dose position shifts to a deeper region from the surface in a water cylindrical phantom. We considered that this result could be applied as a dose reduction method. Therefore, in this study, we investigate the tube voltage dependence of this unique phenomenon. The results show that the peak dose position shifts to a deeper region with increasing tube voltage. The superficial radiosensitive organs can thus be protected from peak dose exposure by adjusting the tube voltage.

近年の X 線 CT 装置には、患者の被ばく低減と画質の均一化を目的として、患者の体厚などに応じて管電流を変化させる管電流変調機構(Auto Exposure Control : AEC)が標準装備されている。そのため、モンテカルロシミュレーションを用いて患 者の被ばく線量について検討を行う場合、AEC の影響を考慮しなければならない。<br /> 本研究室ではこれまで、球形半導体検出器を用いて測定した管電流変調データを、 EGS5 に組み込んで人体ファントムの被ばく線量の取得を試みていたが、主に体表部 に近い位置に設置された検出器で、実測と計算の値が大きく異なる場合があった。<br /> そこで今回は、管球回転開始位置の違いによるスキャン中の回転軌道の違いが影響 しているのではないかと考え、回転開始位置の異なる実測変調データ数種を EGS5 に 組み込み、検証を行った。

The longitudinal dose profile in a computed tomography dose index (CTDI) phantom had been studied by many researchers. The cross-sectional dose profile in the CTDI phantom, however, has not been studied. It is also important to understand the cross-sectional dose profile in the CTDI phantom for dose estimation in X-ray CT. In this study, the cross-sectional dose profile in the CTDI phantom was calculated by use of a Monte Carlo (MC) simulation method. A helical or a 320-detector-row cone-beam X-ray CT scanner was simulated. The cross-sectional dose profile in the CTDI phantom from surface to surface through the center point was calculated by MC simulation. The shape of the calculation region was a cylinder of 1-mm-diameter. The length of the cylinder was 23, 100, or 300 mm to represent various CT ionization chamber lengths. Detailed analyses of the energy depositions demonstrated that the cross-sectional dose profile was different in measurement methods and phantom sizes. In this study, we also focused on the validation of the weighting factor used in weighted CTDI (CTDIw). As it stands now, the weighting factor used in CTDI w is (1/3, 2/3) for the (central, peripheral) axes. Our results showed that an equal weighting factor, which is (1/2, 1/2) for the (central, peripheral) axes, is more suitable to estimate the average cross-sectional dose when X-ray CT dose estimation is performed. © Japanese Society of Radiological Technology and Japan Society of Medical Physics 2013.

In this study, the absorbed dose for organs in infant, child, and adult patients undergoing head computed tomography (CT) scans was determined. The CT examinations were simulated using the Electron Gamma Shower ver. 5 (EGS5) Monte Carlo code. The eye lens dose among child and infant patients was higher than that among adult patients by 15.0% and 69.7%, respectively. The effectiveness of a bismuth shield in reducing the eye lens dose for children undergoing head X-ray head CT scans was also evaluated. Application of a bismuth shield over the eyes of children undergoing head X-ray CT reduced the eye lens dose by 47.6% from the dose value in the case wherein shielding was not used. The use of metallic shields was found effective in reducing the radiation dose to children.

In this study, we validated the effectiveness of using a message passing interface (MPI) method in reducing computation time and enhancing calculation accuracy. A chest and abdominal X-ray computed tomography scan was simulated. Our results indicated that the MPI method was accurately incorporated into the Electron Gamma Shower ver. 5 (EGS5) MC code. The MPI method could reduce computation time from 34 hours to 6 hours. We believe that MC simulation is going to be used widely. Therefore, incorporating the MPI method into EGS5 is very useful for EGS5 users.