Tang Xiao

Computed laminography and reconstruction algorithm

Computed laminography (CL) is an alternative to computed tomography if large objects are to be inspected with high resolution. This is especially true for planar objects. In this paper, we set up a new scanning geometry for CL, and study the algebraic reconstruction technique (ART) for CL imaging. We compare the results of ART with variant weighted functions by computer simulation with a digital phantom. It proves that ART algorithm is a good choice for the CL system.

Beam hardening correction for a cone-beam CT system and its effect on spatial resolution

In this paper, we present a beam hardening correction (BHC) method in three-dimension space for a cone-beam computed tomography (CBCT) system in a mono-material case and investigate its effect on the spatial resolution. Due to the polychromatic character of the X-ray spectrum used, cupping and streak artifacts called beam hardening artifacts arise in the reconstructed CT images, causing reduced image quality. In addition, enhanced edges are introduced in the reconstructed CT images because of the beam hardening effect. The spatial resolution of the CBCT system is calculated from the edge response function (ERF) on different planes in space. Thus, in the CT images with beam hardening artifacts, enhanced ERFs will be extracted to calculate the modulation transfer function (MTF), obtaining a better spatial resolution that deviates from the real value. Reasonable spatial resolution can be obtained after reducing the artifacts. The 10% MTF value and the full width at half maximum (FWHM) of the point spread function with and without BHC are presented.

Monte Carlo simulation of glandular dose in a dedicated breast CT system

A dedicated breast CT system (DBCT) is a new method for breast cancer detection proposed in recent years. In this paper, the glandular dose in the DBCT is simulated using the Monte Carlo method. The phantom shape is half ellipsoid, and a series of phantoms with different sizes, shapes and compositions were constructed. In order to optimize the spectra, monoenergy X-ray beams of 5–80 keV were used in simulation. The dose distribution of a breast phantom was studied: a higher energy beam generated more uniform distribution, and the outer parts got more dose than the inner parts. For polyenergtic spectra, four spectra of Al filters with different thicknesses were simulated, and the polyenergtic glandular dose was calculated as a spectral weighted combination of the monoenergetic dose.

基于观测、模拟和同化数据的PM2.5污染回顾分析

基于观测数据空间插值、数值模拟以及最优插值同化方法构建了京津冀地区PM2.5（空气动力学当量直径小于等于2.5 μm的颗粒物，即细颗粒物）空间插值数据、模拟数据和同化数据，并首次比较分析了三种数据在PM2.5污染回顾分析上的应用潜力和优缺点。针对2014年2月19~28日京津冀地区PM2.5污染过程的分析发现：（1）观测空间插值数据难以完整表征PM2.5污染的时空演变特征，在没有观测覆盖区域误差较大，容易出现虚假的高低值中心；（2）模拟数据具有较高时空分辨率，对PM2.5污染时空演变特征描述更加细致，但在这次污染过程中仍存在较大不确定性，其均方根误差大于100 μg/m 3 ；（3）同化数据不仅能对PM2.5空间分布特征进行细致描述，其数据精度在独立验证站点也显著高于模拟数据，其均方根误差比模拟数据低约50%，与站点观测数据的相关系数也比模拟数据高0.2以上。基于PM2.5同化数据，对这次京津冀PM2.5污染过程的时空演变特征进行了详细回顾分析，发现这次污染过程存在自京津冀南部PM2.5污染累积并向北输送发展的生成特点，消亡过程为风向转换下自北向南清除，造成京津冀南部城市先污染后清除，北部城市后污染先清除，并且有慢累积、快清除的特征。从发展演变过程中污染所占空间面积来看，25日PM2.5污染范围最大，覆盖模式第三区域60.5%面积。

Tuning of the cosmic-ray test system of the BESIII drift chamber

The BESIII drift chamber and its subsystems need a cosmic-ray test after the chamber construction to check the chamber construction quality, testing the joint operation of the whole system and the performance of the chamber. The noise performance, drift time and charge measurements, and the scanning of channels were examined specifically. The preliminary results of the test indicate that the whole system works well.