CNRS, Université Bordeaux, CENBG, UMR5797, F-33170 Gradignan, France.
CNRS, Université Bordeaux, CENBG, UMR5797, F-33170 Gradignan, France.
Phys Med. 2022 Feb;94:85-93. doi: 10.1016/j.ejmp.2021.12.002. Epub 2022 Jan 7.
Proton computed microtomography is a technique that reveals the inner content of microscopic samples. The density distribution of the material (in g·cm) is obtained from proton transmission tomography (STIM: Scanning Transmission Ion Microscopy) and the element content from X-ray emission tomography (PIXE: Particle Induced X-ray Emission). A precise quantification of chemical elements is difficult for thick samples, because of the variations of X-ray production cross-sections and of X-ray absorption. Both phenomena are at the origin of an attenuation of the measured X-ray spectra, which leads to an underestimation of the element content. Our aim is to quantify the accuracy of a specific correction method that we designed for thick samples.
In this study, we describe how the 3D variations in the mass density were taken into account in the reconstruction code, in order to quantify the correction according to the position of the proton beam and the position and aperture angle of the X-ray detector. Moreover, we assess the accuracy of the reconstructed densities using Geant4 simulations on numerical phantoms, used as references.
The correction process was successfully applied and led, for the largest regions of interest (little affected by partial volume effects), to an accuracy ≤ 4% for phosphorus (compared to about 40% discrepancy without correction).
This study demonstrates the accuracy of the correction method implemented in the tomographic reconstruction code for thick samples. It also points out some advantages offered by Geant4 simulations: i) they produce projection data that are totally independent of the inversion method used for the image reconstruction; ii) one or more physical processes (X-ray absorption, proton energy loss) can be artificially turned off, in order to precisely quantify the effect of the different phenomena involved in the attenuation of X-ray spectra.
质子计算机微断层扫描是一种揭示微观样本内部内容的技术。通过质子透射断层扫描（STIM：扫描透射离子显微镜）获得物质的密度分布（以 g·cm 表示），通过 X 射线发射断层扫描（PIXE：粒子感生 X 射线发射）获得元素含量。对于厚样品，由于 X 射线产生截面和 X 射线吸收的变化，化学元素的精确定量变得困难。这两种现象都导致了测量 X 射线谱的衰减，从而导致元素含量的低估。我们的目的是量化我们为厚样品设计的特定校正方法的准确性。
在这项研究中，我们描述了如何在重建代码中考虑质量密度的 3D 变化，以便根据质子束的位置以及 X 射线探测器的位置和孔径角来量化校正。此外，我们使用作为参考的数值体模上的 Geant4 模拟来评估重建密度的准确性。
校正过程成功应用，对于最大的感兴趣区域（受部分容积效应影响较小），磷的精度≤4%（未经校正的差异约为 40%）。
这项研究证明了在厚样品的断层扫描重建代码中实现的校正方法的准确性。它还指出了 Geant4 模拟的一些优势：i）它们产生的投影数据完全独立于用于图像重建的反演方法；ii）可以人为地关闭一个或多个物理过程（X 射线吸收、质子能量损失），以便精确量化参与 X 射线谱衰减的不同现象的影响。
