Development of quaity control system of mammographic X-ray unit to decrease patiant dose in mammography

我们的目的是直接测量乳腺X射线光谱的准直器和一个低效率的CdZnTe探测器最近开发的，并找出最佳的拟合响应函数的CdZnTe探测器校正测量光谱。分析了由乳腺X射线摄影装置在25-32 kV和240 mAs（= 80 mAs的3倍）下产生并通过0.03 mm Mo或0.025 mm Rh滤光片和物体（0.1 mm Al至0.8 mm Al体模）透射的光子光谱（目标Mo或Rh）。由于探测到的光谱被CdZnTe探测器的响应所扭曲，并且不呈现真实的光子光谱，因此通过剥离程序进行校正。通过评价相互作用（K-逃逸、相干散射和康普顿散射过程）和使用蒙特卡罗方法计算的不完全电荷收集，确定了本程序中使用的检测器的响应函数。我们分别用Cd、Zn和Te的K_<α1>、K_<α2>、K_<β1>、K_<β2>辐射，用不完全电荷收集权函数，并考虑康普顿散射。通过改变影响不完全电荷收集的重要因素（空穴平均路径长度λ h、CZT晶体死层和权重因子W q），继续进行Monte Carlo模拟，直到找到最佳拟合响应函数。校正后的光子光谱与放射卫生局（BRH）使用Ge探测器测量的乳腺X射线光谱数据进行了比较。从校正的光子谱计算了25-32 kV铝的衰减曲线，并与使用电离室测量的衰减曲线进行了比较。使用CdZnTe探测器获得的这些结果与BRH的乳腺X射线光谱数据和电离室获得的衰减曲线一致。

Our purpose is to directly measure mammographic x-ray spectra with collimators and a low-efficiency CdZnTe detector developed recently and to find out the best fit response function of CdZnTe detector to correct the measured spectra. Photon spectra (target Mo or Rh) produced by a mammographic x-ray unit at 25-32 kV and 240 mAs (=3 times of 80 mAs) and transmitted through 0.03 mm Mo or 0.025 mm Rh filter and object (0.1 mm Al to 0.8 mm Al phantoms) have been analyzed. Since detected spectra were distorted by the response of CdZnTe detector and did not present the true photon spectra, the correction was applied by the stripping procedure. The response function of detector used in this procedure has been determined by the evaluation of interactions (K-escape, coherent scattering, and Compton scattering processes) and incomplete charge collection calculated using the Monte Carlo method. We have used K_<α1>, K_<α2>, K_<β1>, K_<β2> radiations of Cd, Zn, and Te, respectively and have used the weight function for the incomplete charge collection and have considered Compton scattering. The Monte Carlo simulations were continued by changing the important factors (mean path length of hole λ_h, dead layer of the CZT crystal and weight factor W_q) of incomplete charge collection until the best fit response function was found out. Corrected photon spectra were compared with the mammographic x-ray spectral data of Bureau of Radiological Health (BRH) measured using a Ge detector. Attenuation curves of aluminum for 25-32 kV were calculated from the corrected photon spectra and compared with the attenuation curves measured using an ionization chamber. These results obtained using the CdZnTe detector agreed with the mammographic x-ray spectral data of BRH and attenuation curves obtained by the ionization chamber.