Künzel Roseli, Levenhagen Ronaldo Savarino, Herdade Silvio Bruni, Terini Ricardo Andrade, Costa Paulo Roberto
Area de Ciências Naturais e Tecnológicas, Centro Universitário Franciscano, Santa Maria, Rio Grande do Sul, Brazil.
Med Phys. 2008 Aug;35(8):3539-45. doi: 10.1118/1.2953566.
The protective shielding design of a mammography facility requires the knowledge of the scattered radiation by the patient and image receptor components. The shape and intensity of secondary x-ray beams depend on the kVp applied to the x-ray tube, target/filter combination, primary x-ray field size, and scattering angle. Currently, shielding calculations for mammography facilities are performed based on scatter fraction data for Mo/Mo target/filter, even though modern mammography equipment is designed with different anode/filter combinations. In this work we present scatter fraction data evaluated based on the x-ray spectra produced by a Mo/Mo, Mo/Rh and W/Rh target/filter, for 25, 30 and 35 kV tube voltages and scattering angles between 30 and 165 degrees. Three mammography phantoms were irradiated and the scattered radiation was measured with a CdZnTe detector. The primary x-ray spectra were computed with a semiempirical model based on the air kerma and HVL measured with an ionization chamber. The results point out that the scatter fraction values are higher for W/Rh than for Mo/Mo and Mo/Rh, although the primary and scattered air kerma are lower for W/Rh than for Mo/Mo and Mo/Rh target/filter combinations. The scatter fractions computed in this work were applied in a shielding design calculation in order to evaluate shielding requirements for each of these target/filter combinations. Besides, shielding requirements have been evaluated converting the scattered air kerma from mGy/week to mSv/week adopting initially a conversion coefficient from air kerma to effective dose as 1 Sv/Gy and then a mean conversion coefficient specific for the x-ray beam considered. Results show that the thickest barrier should be provided for Mo/Mo target/filter combination. They also point out that the use of the conversion coefficient from air kerma to effective dose as 1 Sv/Gy is conservatively high in the mammography energy range and overestimate the barrier thickness.
乳腺摄影设备的防护屏蔽设计需要了解患者和图像受体组件产生的散射辐射。二次X射线束的形状和强度取决于施加到X射线管的千伏峰值（kVp）、靶/滤过组合、一次X射线野大小以及散射角。目前，乳腺摄影设备的屏蔽计算是基于钼/钼靶/滤过的散射分数数据进行的，尽管现代乳腺摄影设备设计有不同的阳极/滤过组合。在这项工作中，我们给出了基于钼/钼、钼/铑和钨/铑靶/滤过产生的X射线光谱，在25、30和35 kV管电压以及30至165度散射角下评估的散射分数数据。对三个乳腺摄影体模进行了照射，并用碲锌镉探测器测量散射辐射。一次X射线光谱用基于用电离室测量的空气比释动能和半值层的半经验模型计算。结果指出，钨/铑的散射分数值高于钼/钼和钼/铑，尽管钨/铑的一次和散射空气比释动能低于钼/钼和钼/铑靶/滤过组合。在这项工作中计算的散射分数应用于屏蔽设计计算，以评估这些靶/滤过组合中每一种的屏蔽要求。此外，通过最初采用从空气比释动能到有效剂量的转换系数为1 Sv/Gy，然后采用所考虑的X射线束特定的平均转换系数，将散射空气比释动能从mGy/周转换为mSv/周，对屏蔽要求进行了评估。结果表明，对于钼/钼靶/滤过组合应提供最厚的屏蔽。结果还指出，在乳腺摄影能量范围内，将从空气比释动能到有效剂量的转换系数设为1 Sv/Gy保守性过高，会高估屏蔽厚度。
