Modelling image formation and simplification of image aquisition for Talbot-Lau x-ray phase contrast CT

使用Talbot-Lau方法的相位对比度成像除了提供通常的吸收图像之外，还提供了物体的差分相位图像和暗场图像。近10年来的研究表明，差分位相图像和暗场图像对吸收图像具有重要的补充作用。虽然该方法适用于普通的X射线管，但多色光谱会导致图像中出现伪影。一个重要而特殊的方面与暗场有关。暗视野可能是由颗粒状结构如肺泡或乳腺肿瘤中微米大小的钙化引起的。这种非常有趣的贡献几乎完全可以被射束硬化或边缘的贡献所覆盖。此外，射束硬化还会影响相位像和吸收像。应该注意的是，这种影响对于暗场来说可能是极端的：由于光束硬化，暗场图像可能只包含结构。这不能发生在相图和吸收图像中。Talbot-Lau方法需要获取所谓的相移曲线。这一过程可以将CT成像的采集时间延长到可能导致不想要的伪影甚至意味着无法忍受的采集时间的量。在应用项目中，将分别制定迭代CT重建策略，并进一步发展。重要的是，吸收像、差分相位像和暗场像这三幅图像不会被独立重建。取而代之的是，通过对对象的物理属性进行建模，将考虑三幅图像中的相关性，以便减少噪声和消除伪影。应用迭代重建的主要目的是研究相关性，最初并不打算包括用于提高图像质量的其他最先进的措施。此外，还将研究是否可以省去CT成像中的相步，从而实现更容易的采集和更短的采集时间。甚至有可能放弃获取参考相移曲线。

Phase contrast imaging using the Talbot-Lau method delivers in addition to the usual absorption image the differential phase image and the darkfield image of an object. During the recent 10 years it has been proven that the differential phase image and the darkfield image contain valuable and complementary information to the absorption image. While the method is applicable at usual X-ray tubes, the polychromatic spectrum leads to artifacts in the images. An important and special aspect is related to the darkfield. The darkfield can be caused by granular structures like lung alveoli or micrometer-sized calzifications in breast tumors. This very interesting contribution can almost completely be overlayed by contributions from beam hardening or edges. Also the phase image and the absorption image may be affected by beam hardening. It should be noticed that the effect can be extreme for the darkfield: a darkfield image may exclusively contain structures due to beam hardening. This cannot occur in the phase and the absorption image. The Talbot-Lau method requires the acquisition of so-called phase stepping curves. This procedure can extend the acquisition time for CT imaging to an amount which may induce unwanted artifacts or even imply intolerable acquisition times. Within the applied project an iterative CT reconstruction strategy will be developed respectively further developed. The important ingredient is that the three images - absorption image, differential phase image and darkfield image - will not be reconstructed independently. Instead, by modeling the physics properties of the object the correlations in the three images will be taken into account in order to reduce noise and to eliminate artifacts. The applied iterative reconstruction primarily aims at the investigation of the correlations and is initially not intended to include other state-of-the-art measures for enhancing the image quality. Further, it will be investigated whether the phasestepping can be omitted for CT imaging thus reaching easier acquisition and shorter acquisition times. It might even be possible to waive the acquisition of the reference phasestepping curve.