X線CT装置の半価層測定における非接続形X線出力アナライザ専用鉛ケースの開発

摘要

2004年，GonzalezらがX線診断の被ばくによってがんの発生する確率を推計した結果，日本は先進15力国の中で最も高いと報告された.更に，2011年には東北地方太平洋沖地震に伴う福島第一原子力究電所事故が起こり，一般公衆の放射線被ばくに対する関心が高まっている.2015年6月には医療被ばく研究情報不ットワーク(Japan Network for Research and Information on Medical Exposures: J-RIME)から，日本としての診断参考レベル(diagnostic reference levels: DRL)が公表され，今後はより一層，医療被ばく管理の重要性が増すと考えられる.%Measurement of the half-value layer (HVL) is a difficult task in computed tomography (CT), because a nonrotating X-ray tube must be used. The purpose of this study is to develop a lead-covered case, which enables HVL measurements with a rotating CT X-ray tube. The lead-covered case was manufactured from acrylic and lead plates, which are 3 mm thick and have a slit. The slit-detector distance can be selected between 14 mm and 122 mm. HVL measurements were performed using a wireless X-ray output analyzer "Piranha." We used the following exposure conditions: tube voltages of 80,100, and 120 kV; a tube current of 550 mA; and an exposure time of 1.0 s. The HVLs were measured by using the following two methods: (a) Nonrotating method-a conventional method that uses the nonrotating exposure mode. (b) Rotating method-a new method that uses the lead-covered case and the rotating exposure mode. As a result, when the slit-detector distance was 58 mm, the HVL values obtained by the nonrotating and rotating methods were 4.38 and 4.24 mmAl at 80 kV, 5.51 and 5.37 mmAl at 100 kV， 6.61 and 6.48 mmAl at 120 kV, respectively. A lead-covered case, which enables the measurement of the HVL in a rotating X-ray tube, was developed. The case is useful in measuring the HVLs at facilities that cannot fix the X-ray tube.