Radiation propagation modelling and image reconstruction for X-ray time-of-flight computed tomography

X射线计算机层析成像(CT)是一种非常重要的三维成像方式，在现代医学中得到了广泛的应用。从20世纪70年代初的S开始，到90年代的S，CT的主要发展方向是提高图像的对比度，以便于诊断。在当时空间分辨率没有显著提高，图像增强技术也没有最近的软件开发的情况下，通过增加注入患者的X射线剂量在很大程度上获得了更大的对比度。在世纪之交，研究指出了这种剂量的不良影响，在扫描后观察到癌症，并怀疑存在基因修改。然后，人们开发了几种方法，包括硬件和软件，以在保持对比度的同时减少剂量。CT对比度明显受到来自偏离直线路径的光子(即所谓的散射光子)的噪声的影响。目前的CT图像重建算法假定光子沿直线运动，因此检测到的散射光子会在重建图像中产生噪声。我们的团队已经建立了一个雄心勃勃的研究计划，以减少CT的剂量，同时提高图像质量和定量准确性。我们的方法依赖于测量X光子的飞行时间(ToF)来确定它们是沿直线传播(弹道光子)，还是散射。其原理是，与直线运动相比，散射光子到达探测器所需的时间更长。这使得根据它们的TOF区分弹道光子和散射光子成为可能。我们目前在这一领域处于世界领先地位。在模拟和初步实验中，我们已经证明了利用TOFS可以将剂量减少到原来的1/4。我们已经进行了一项联合努力，其最终目标是建立一台完整的TOF-CT扫描仪。利用TOFS进行CT图像重建尚未得到解决。目前的Discovery Grant应用侧重于开发由于该领域的新颖性而尚不存在的图像重建算法，这些算法对于建立完全可操作的扫描仪至关重要。除了我们的初步结果预测的剂量减少外，这项应用还解决了CT中的另一个基本问题。除了造成噪声外，散射的一个不利影响是使目前的方法不可能以绝对单位对吸收进行真正的定量测量，这对于医学诊断是非常必要的。在比较不同扫描仪(特别是不同供应商)的结果时，这种缺乏定量准确性的情况会导致同一患者的测量结果不一致。这些不一致性目前通过近似校准的方式得到了部分缓解，可能会影响诊断的准确性。除了减少剂量，拟议的工作将提供一种途径，以提高CT的定量准确性，导致更可靠的诊断。

X-ray computed tomography (CT) is a very important 3D imaging modality finding widespread use in modern medicine. From its inception in the early 1970's to the 1990's, major developments in CT aimed at increasing the contrast in images to ease diagnostic. Without significant improvements in spatial resolution at that time and the more recent software developments in image enhancement techniques, greater contrast was largely obtained by increasing the X-ray dose injected into the patient. At the turn of the millennium, studies have pointed out the dose's adverse effects, where post-scan cancers were observed, and genetic modifications were suspected. Several approaches, both hardware and software, have then been developed to reduce the dose while preserving contrast. CT contrast is notably affected by the noise originating from photons that are deviated from a straight-line path, so-called scattered photons. Current CT image reconstruction algorithms assume that photons travel in straight lines, and thus detected scattered photons contribute noise in the reconstructed image. Our group has set up an ambitious research program for reducing the dose in CT, while increasing image quality and quantitative accuracy. Our approach relies on measuring the time of flight (ToF) of X photons to determine if they traveled in straight line (ballistic photons), or if they scattered. The principle is that a scattered photon takes more time to reach a detector than if it travels in straight line. This makes it possible to discriminate ballistic from scattered photons based on their ToFs. We are currently the world leaders in this field. In simulations and preliminary experiments, we have demonstrated that exploiting ToFs allows reducing the dose by a factor of 4. We have undertaken a joint endeavor whose ultimate goal is to build a complete ToF-CT scanner. CT image reconstruction taking advantage of ToFs has not yet been addressed. The present Discovery Grant application focuses on developing image reconstruction algorithms that do not yet exist owing to the novelty of the field, and which are essential to build a fully operational scanner. In addition to dose reduction predicted by our preliminary results, this application addresses another fundamental problem in CT. Apart from contributing noise, an adverse effect of scattering is to make impossible with current approaches the true quantitative measurement of absorption in absolute units, which is highly desirable for medical diagnostics. This lack of quantitative accuracy leads, for the same patient, to measurement non-uniformities when results from different scanners are compared (especially from different vendors). These non-uniformities are currently partially mitigated by way of approximate calibrations, with possible consequences on diagnostic accuracy. Additionally to dose reduction, the proposed work will provide an avenue to improve the quantitative accuracy of CT, leading to more reliable diagnosis.