The Study Of X-ray Phase Contrast Computed Tomography And Its Application On Microstructure Visualization In Root Canals

X-ray phase-contrast imaging?XPCI?has high resolution,high contrast,and high image quality,and has been widely used in various fields.XPCI is based on the Fresnel diffraction,which utilizes the phase shift to form an image.For low-Z samples,such as the biological soft tissue,the phase shift is 1000 times larger than that of absorption.Therefore,XPCI has been widely used in soft tissue imaging research,and its resolution can reach sub-micron level,which can clearly image the microstructure.Combined with computed tomography?CT?,phase-contrast computed tomography?PCCT?can reveal the three-dimensional?3D?structural information inside the microstructure.However,during the imaging process,the offset of center of rotation?COR?is caused by the inaccuracy of instrument,and it produces artifacts in the image reconstruction,which makes the image information unclear and has a greater impact on the imaging of the microstructure.Therefore,this study firstly corrected the COR to ensure that the reconstruction information was complete and clear,and then used PCCT to image the root canal microstructure in order to provide an imaging evidence for evaluating the effect of root canal treatment.The research contents are as follows:?1?According to the symmetric of the sinogram projection data,a novel COR correction algorithm based on the translation and match of the 0°and 180°projection data was proposed.The OTSU threshold segmentation was applied to reduce the influence of noise and the offset of COR was quantified by the L₁-norm to improve accuracy of correction.The proposed method was applied to the Sheep-Logan simulation model with random gradients and Gaussian noise,and its ability of anti-noise and accuracy correction was evaluated by relative error?RE??peak signal to noise ratio?PSNR?and universal quality index?UQI?.Then,to measure the correction accuracy of the proposed method,the homogenous sample?liver?and heterogeneous sample?tooth,filled and pretreated root canal?were imaged by PCCT,and the root canal was imaged by absorption-based CT.Finally,the method was applied to sparse data to measure the stability of accuracy correction.Compared with the correlation and center of mass methods,the algorithm only needs to calculate a pair of data,which greatly reduces the calculation workload,and has high correction precision,strong anti-noise performance and accuracy stability?when the sampled data is from 10%to 50%of the full projection data,the COR value can still be measured accurately using the proposed algorithm?.?2?This paper applied the clinical CT to select the root canal with the isthmus and record the area of isthmus.Then,the area was imaged by PCCT to visualize the morphology and tissue distribution of the isthmus and select the samples with tissues.The selected samples were pretreated and imaged by clinical CT,micro-CT and PCCT in order to compare the ability of imaging clearly and completely in the smear layer,tissues accumulation and micro-tissues in root canal.According to the contrast of the tissues in PCCT,the soft and hard tissues were distinguished,and the values of SNR and CNR were measured for different tissues obtained by different imaging methods.The results demonstrated the ability of anti-noise and strength of PCCT.After the pretreatment,the root canal containing the tissue was cleaned by endo-shaper and ultrasound,and then imaged by PCCT.The cleaning effect was evaluated by observing the residual tissue,and the study showed that PCCT could provide an imaging evidence for the evaluation of therapeutic effects.