放射診斷低能量X射線範圍的空氣克馬標準

Low Energy X-ray Air Kerma Standard in Diagnostic Radiology

朱健豪(Chien-Hau Chu) ； 林威廷(Uei-Tyng Lin) ； 蘇水華(Shi-Hwa Su)

《中華放射線醫學雜誌》 30卷4期 (2005/08) Pp. 217-223

摘要

核能研究所國家游離輻射標準實驗室自行設計及製作完成，一個改良自Attix型式的自由空氣游離腔，並應用於量測低能量範圍的X射線。臨床上，此能量範圍的X射線適用於放射診斷乳房攝影，該游離腔主要執行空氣中X射線曝露的直接量測。此研究以單筒活塞式設計取代過去兩筒式設計。物理修正量方面部份以實驗量測評估，包括有效體積、離子再結合及空氣衰減等修正因子，部份則以蒙地卡羅法計算評估，包括孔徑衰減、腔壁衰減、光子散射及電子損失等修正因子。本研究並建置一組與國際度量衡局(BIPM)相同的X射線射質條件，以校正一支追溯至德國聯邦物理技術研究院(PTB)的游離腔。比較雙方的校正結果，核能研究所與德國PTB差異小於0.6%，在95%信賴水準的基礎上，其量測不確定度小於0.6%，此代表雙方的校正結果具一致性。本研究的各項測試與比對結果顯示，核能研究所自製的自由空氣游離腔所建立之10-50kV X射線空氣克馬率原級標準合乎國際水準，可作為國內低能量X射線校正之追溯標準，並可用以規劃未來建立量測乳房攝影X射線空氣克馬率的原級標準。

關鍵字

自由空氣游離腔 ； 空氣克馬 ； 乳房攝影 ； 蒙地卡羅法

並列摘要

The National Radiation Standard Laboratory (NRSL) of the Institute of Nuclear Energy Research (NRSL) designed and constructed an improved Attix style free-air ionization chamber for measurements of low energy x-rays. Clinically, x-rays of this energy range are used in mammography radiology and the chamber is used to perform in-air exposure measurements. The original Attix two-section design was replaced by the piston design. The correction factors were determined experimentally for volume estimation, ion recombination and air attenuation. The correction factors of aperture transmission, wall transmission, electron loss and photon scatter were determined by using Monte Carlo calculations. INER established the BIPM x-ray beam code and calibrated a transfer chamber that had been calibrated by the Physikalisch Technische Bundesanstalt (PTB, Germany). The experimental results of the INER-PTB comparison using a transfer chamber yielded a difference of less than 0.6% at the 95% confidence level in calibration factors. Also, the overall uncertainty for the x-ray measurement in terms of air kerma was less than 0.6% at the 95% confidence level. These results indicate that the improved free-air ionization chamber can serve as a primary standard as well as a trace standard in low energy x-ray calibration service in Taiwan and even a basis of the future mammography x-ray air kerma primary standard.

並列關鍵字

Free-air ion chamber ； Air kema ； Mammography ； Monte Carlo calculations