基于蒙特卡罗和CT数据体素模型的双能X射线吸收法腰椎骨密度成像参数优化

目的 研究双能X射线吸收法（DXA）腰椎成像过程并优化成像参数，为提高双能X射线骨密度仪的成像质量提供理论依据。方法 采用蒙特卡罗方法，构建基于计算机断层扫描数据（CT）的体素模型，利用模拟产生的高低能X射线能谱，分别照射体素模型腰椎部位得到骨密度图像，计算每幅图像的品质因数（FOM）。根据FOM大小优化高低能管电压组合、过滤铜片厚度以及入射粒子数比值。结果 FOM在管电压组合为75与200 kVp达到最低值1.59×10^-2。当过滤铜片厚度由0 mm增加至3 mm时，FOM数值从6.30×10^-2下降到1.87×10^-2，且下降趋势逐渐变缓。随着低能与高能的入射粒子数比从1∶3增加到19∶1，FOM先降低后升高，在3∶1时达到最低值1.40×10^-2。结论 低能管电压为70~85 kVp，高能管电压为160~200 kVp，产生高能能谱时额外添加过滤铜片厚度0.3 mm，入射粒子数比值（低能/高能）为1~5时，能在获得较好的DXA腰椎图像质量的同时使患者受到较小的辐射剂量。

Optimization of DXA lumber spine bone imaging parameters based on Monte Carlo method and CT voxel phantom

Objective To study the lumber spine imaging process of dual-energy X-ray absorptiometry (DXA) and parameters used to optimize the image quality. Methods A computational voxel phantom was constructed from patient computed tomography (CT) data. Using the Monte Carlo radiation transport method, a dual energy x-ray beam was simulated to scan the phantom of lumbar spine to generate a bone density image. The Figure of Merit (FOM) of each image was claculated. Parameters including the combination of the high and low energy tube voltage, the thickness of Cu filter, and the ratio of two beam energy incident photon number were optimized, which based on FOM. Results FOM reaches a minimum of 1.59×10^-2 with the tube voltage combination of 75 and 200 kVp. With the thickness of the Cu filter from 0 mm to 3 mm, FOM decreases from 6.30×10^-2 to 1.87×10^-2, showing a gradually slow-down trend. With the incident photon number ratio (low energy/high energy) increasing from 1:3 to 19:1, FOM decreases firstly and then increases, reaching a minimum of 1.40×10^-2 at 3:1. ConclusionsAccording to the simulation results, the combinations of low tube voltage from 70 kVp to 85 kVp and high tube voltage from 160 kVp to 200 kVp, 0.3 mm Cu filter and beam incident photon number ratio from 1 to 5 can yield the best lumbar spine image quality with the lowest patient dose.