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

X射线计算机断层扫描（CT）是一种非常重要的三维成像模式，在现代医学中得到广泛应用。从20世纪70年代初开始到20世纪90年代，CT的主要发展旨在增加图像的对比度以便于诊断。当时空间分辨率没有显著提高，图像增强技术的软件开发也没有更新，更大的对比度主要是通过增加注入患者体内的X射线剂量来获得的。在千年之交，研究指出了剂量的不利影响，在扫描后观察到癌症，并怀疑基因修饰。几种方法，硬件和软件，然后已经开发出减少剂量，同时保持对比度。CT对比度明显受到源自偏离直线路径的光子（所谓的散射光子）的噪声的影响。当前的CT图像重建算法假设光子沿直线行进，因此检测到的散射光子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中的剂量，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），以确定它们是否沿直线（弹道光子）传播，或者它们是否散射。其原理是，散射光子到达探测器所需的时间比直线传播的时间要长。这使得可以基于它们的ToF区分弹道光子和散射光子。目前，我们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，利用ToF可以将剂量降低4倍。我们共同奋进，最终目标是建立一个完整的ToF-CT扫描仪。利用ToF的CT图像重建尚未得到解决。目前的发现补助金申请专注于开发图像重建算法，由于该领域的新奇，这些算法尚未存在，并且对于构建全面运行的扫描仪至关重要。除了我们的初步结果预测的剂量减少，这个应用程序解决了CT中的另一个基本问题。除了贡献噪声之外，散射的不利影响是使得当前方法不可能以绝对单位进行吸收的真实定量测量，这对于医学诊断是非常期望的。对于同一患者，这种定量准确性的缺乏导致在比较来自不同扫描仪（特别是来自不同供应商）的结果时测量不均匀。这些不均匀性目前通过近似校准的方式部分减轻，可能对诊断准确性产生影响。除了减少剂量，拟议的工作将提供一种途径，以提高定量准确性的CT，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.