Study Of Synchrotron Radiation-based X-ray Phase-contrast Imaging Methods

Studies of the PhD thesis were performed at the X-ray Imaging and BiomedicalApplication (BL13W) beamline at Shanghai Synchrotron Radiation Facility and focusing onthe basic biomedical applications of X-ray phase-contrast imaging (PCI) techniques. Myresearch includes two aspects: propagation-based PCI (P-PCI) and grating-based PCI(G-PCI).Firstly, the studies are based on the simplest X-ray PCI method, P-PCI method. Weinvestigated the potential of gas-filled microbubbles as contrast agents for P-PCI inbiomedical applications. When imaging parameters are optimized, the microbubbles functionas microlenses that focus the incoming X-rays to form bright spots, which can significantlyenhance the image contrast. In this study, both numerical simulations and ex vivo experimentswere performed to investigate the formation of the contrast and the effectiveness ofmicrobubbles as contrast agents in P-PCI. Subsequently, We proposed a one-shot thicknessmeasurement method for sponge-like structures using P-PCI method. In P-PCI, theair-material interface refracts the incident X-ray. Refracted many times along their paths bysuch a structure, incident X-rays propagates randomly within a small divergent angle range,resulting in a speckle pattern in the captured image. The model experimental results showedthat the structure thickness and contrast of corresponding phase-contrast projection wasdirectly related in images. This relationship can be described by a natural logarithm equation.Thus, from the one phase-contrast view, depth information can be retrieved from its contrast.Preliminary biological experiments indicated promise in its application to measurementsrequiring in vivo and ongoing assessment of lung tumor progression. Besides, we proposed anexperiment scheme for the mouse brain study, achieving both high spatial resolution and improved soft-tissue contrast. This scheme includes two steps: sample preparation andvolume imaging. In the first step, cerebral vessels was heparinized saline to displace the bloodinside. Then, the whole brain is taken out, fixed in formalin solution and dehydrated withgraded ethanol. After sample preparation, X-ray phase-contrast CT is carried out to collect thedata for volume reconstruction. Here, We adopt a phase-retrieval combined filteredback-projection (FBP) method to reconstruct its3D structures instead of using direct FBPreconstruction. The experimental results showed that, by using propagation-based PCItechnique, air-filled blood vessels were highly visible in the reconstructed images andsoft-tissue structures were also unveiled.In the second part, We studied the biomedical imaging applications of G-PCI technique.First of all, we installed an X-ray grating interferometer at the BL13W beamline of SSRF.Compared with imaging results from conventional absorption based micro-computedtomography, this set-up has shown much better soft tissue imaging capability. In particular,using the set-up, the carotid artery and the carotid vein in a formalin-fixed mouse could bevisualized in situ without contrast agents.Compared with traditional X-ray CT systems, the standard data collection procedure,“phase-stepping”(PS), in the grating-based phase-contrast CT (GPC-CT) is time consuming.The imaging time of a GPC-CT scan is usually up to hours. It will cause serious motionartifacts in the reconstructed images. Additionally, the radiation dose delivered to the objectwith the PS-based GPC-CT is several times larger than that by a conventional CT scan. Toaddress these problems, We followed the interlaced PS method and proposed a novel imagereconstruction method, namely the inner-focusing (IF) reconstruction method. With theinterlaced PS method, the sample rotation and the grating stepping in GPC-CT occur at thesame time. Thus, the interlaced GPC-CT scan can have a comparable temporal resolution withexisting CT systems. Without any additional requirements, the proposed IF reconstruction method can prevent the artifacts existing in the conventional interlaced PS method. Bothnumerical simulations and real experiments were carried out to verify the proposed IFreconstruction method. Besides, We also extended the proposed image reconstruction to theX-ray Tomosynthesis method, to achieving a fast and low-dose grating-based X-rayphase-contrast Tomosynthesis method.