Phase Contrast Computed Tomography

Abstract

Absorption-based X-ray imaging has been used for medical and industrial applications and become an invaluable tool since German scientist Röntgen’s discovery more than a hundred years ago. By the development of computed tomography (CT), absorption-based X-ray imaging could be further improved and non-destructive three-dimensional (3D) views of internal structures became possible. In particular, for medical diagnostics applications, X-ray CT became an invaluable tool during the last 30 years. However, this kind of techniques provides poor contrast and spatial resolution for weakly absorption materials and structures with low atomic number. For example, since the absorption coefficients of soft tissue are very close to that of water, it is very difficult to distinguish internal features in the soft material. In the quest of better contrast in X-ray imaging, a number of different phase contrast imaging (PCI) techniques have been explored during the last few decades. The phase shift cross section is three orders of magnitude larger than the absorption cross section for materials with low atomic number, which is an indication that PCI is more sensitive to density variation than absorption-based X-ray imaging. As one of the important research topics in the field of PCI, phase contrast CT (PC-CT) has consequently been developed to get the phase shift cross section tomography image of the samples. Unlike absorption-based CT, PC-CT uses the phase shift rather than the absorption as the imaging signal and may provide better image quality in soft tissue and low atomic number samples. In this chapter, an overview of PCI and PC-CT methods was presented. Some of contents are directly adapted from the literatures. We will firstly introduce the concept of X-ray refraction, the complex refractive index and the phase signal. Then we will discuss the imaging principles of several kinds of PCI methods including crystal interferometer, analyzer based imaging, propagation based imaging and grating based imaging. Followed is the description of several PC-CT methods. PC-CT based on gratings and ordinary X-ray source will be the focus of discussion. Finally a description about the future of PC-CT will be presented.