Construction and Evaluation of Differential Phase-contrast Cone Beam CT Imaging System PDF Download

Author: Jiangkun Liu
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Languages : en
Pages : 114

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Book Description
"When X-rays travel through an object, both the intensity and phase status are varied. The amount of variation depends on the attenuation coefficients and phase coefficients of the materials that the object consists of. In recent years, cone beam breast computed tomography (CBBCT) has emerged as a cutting-edge X-ray imaging modality by reconstructing the attenuation contrast. It is an effective method for screening and diagnosis for breast cancer by providing isotropic three-dimensional images with high resolution and high contrast-to-noise ratio (CNR). However, because the variation of attenuation coefficients among soft tissues is subtle, CBBCT is limited in further characterizing breast lesions. Phase contrast CT technology has been attracting research interests recently. It provides new insight into an object by imaging the phase coefficient, which is more sensitive than the attenuation coefficient. Therefore, it has the potential to overcome the limitations of CBBCT and deliver complementary information. Several phase contrast imaging methods have been developed in the past years. However, the requirement of a coherent X-ray source with a sufficiently small dimension impairs their application as a standard method in hospital-based medical imaging. In this thesis, a grating-based bench-top differential phase contrast cone beam CT (DPC-CBCT) system was designed and constructed. Based on the attenuation-based CBCT imaging system setup, it deploys three more major components: a source grating, a phase grating and an analyzer grating. The source grating enables the system to use a hospital-grade X-ray tube and together they provide sufficient X-ray output power and spatial coherence. The phase grating and the analyzer grating transform the phase shift into intensity contrast based on Talbot interferometry so that a high-resolution detector is not necessary. One of the major challenges of the system construction is grating fabrication because of their high aspect ratio and high precision requirements. This work presents designs for robust recipes that produced gratings meeting our demanding criteria. Another challenge is that the system requires highly precise grating alignment to produce the best contrast effect. An effective method is presented in this work that aligns the phase grating the analyzer grating precisely. The second part of this thesis is to evaluate our DPC-CBCT imaging system in terms of uniformity, CNR, noise property and contrast resolution using a cylinder phantom. As the field of view of the imaging system is limited due to the current grating fabrication technique, it is necessary to investigate the performance of volume of interest (VOI) imaging. The VOI imaging experiment was carried out by scanning a large cylinder phantom. In order to evaluate the performance of DPC-CBCT on actual soft tissues, human breast specimen and small animal experiments were carried out. Phantom experiment results indicate that, compared with attenuation imaging, phase contrast imaging provides higher CNR and contrast resolution. However, in specimen and small animal experiments, phase contrast image quality was greatly degraded. The coherence property of an X-ray beam is critical in phase contrast imaging because the image formation mechanism is based on X-ray beam diffraction. Inhomogeneous objects, such as bones and soft tissues, have a large amount of internal density fluctuations or small structures on a micrometer scale. These small structures produce strong small-angle scattering and greatly reduce the coherence of X-ray beams reaching the detector. In order to evaluate the coherence loss caused by an object quantitatively, the last part of this thesis introduces dark-field imaging, which forms an image by computing small-angle scattering power. Dark-field imaging is also based on grating interferometry and the data can be obtained from the same scan with phase contrast imaging. It can be used to characterize the distribution of micro-structures and to investigate coherence loss in DPC-CBCT imaging. Besides coherence loss, several other factors affect the performance of DPC-CBCT, such as polychromatic X-ray spectrum and imperfect gratings. We believe that phase contrast imaging has the potential to be a more powerful imaging tool and more work will be dedicated to the improvement of DPC-CBCT"--Pages vi-viii.