Development of fluorescent x-ray detection technique in FXCT

荧光x射线技术用于测定微量的特定元素，但必须在物体表面进行测量。然后，对于生物样品，需要将标本切成薄片，并用垂直于其表面的光束进行扫描。荧光x射线计算机断层扫描（FXCT）已经发展到揭示特定元素在生活状态下的分布。通过将x射线探测器定位于光束90°，同步加速器x射线的线极化有助于显著减少重叠在特定元素的K_α线上的康普顿散射。然而，康普顿散射在大物体中明显增加，高纯度锗探测器无法测量大量的光子。为了解决这一问题，采用滤波法、平行准直法和劳-镜法对康普顿散射进行了抑制实验。滤光片法最初被认为是减少康普顿散射最简单、最有效的方法，但由于滤光片本身产生二次荧光x射线，并且该荧光x射线与碘K_α线重叠，因此它是无用的。强平行准直法是减少康普顿散射和检测少量荧光x射线最有效的方法，但会降低荧光x射线检测的效率。理想的劳-镜法很难建立镜面系统，因此需要进一步改进x射线反射镜及其专用控制。对于体内FXCT成像，准直器或劳-镜方法似乎是一种有用的技术，以减少大量的康普顿散射从大的目标。

Fluorescent x-ray technique is used to evaluate very low contents of specific elements in the order of picograms, however the measurement must be performed in the surfade of object. Then for in biological sample, the specimen is required to be cut into thin slices and scanned with a beam perpendicular to its surface. Fluorescent x-ray computed tomography (FXCT) have being developed to reveal the distribution of specific element in living state. By positioning the x-ray detector at 90° to the beam, the linear polarization of synchrotron x-rays helps to significantly reduce the Compton scattering overlapping the K_α line of specific elements. However, Compton scattering has increased markedly in large object, and highly purified germanium detector cannot measure the huge amounts of photon. To improve this problem, three experiments such as filter method, parallel collimator method and Laue-mirror method, were done to examine the reduction of Compton scattering from object.Filter method, which is firstly thought most easy and highly effective method to reduce Compton scattering, is not useful because secondary fluorescent x-ray was generated by filter itself and this fluorescent x-ray overlaps on the iodine K_α line. Intense parallel collimator method, which reduces the efficacy of fluorescent x-ray detection, is most effective to reduce Compton scattering and to detect small amounts of fluorescent x-ray. Most ideal Laue-mirror method is difficult to set up the mirror system, so further improvement of x-ray mirror and its dedicate control is needed.For in vivo FXCT imaging, collimator or Laue-mirror method appear to be an useful technique to reduce the huge amounts of Compton scattering from large object.