Study On X-ray Spectral CT Based On MARS System

X-ray is one of three vital physics discoveries (X-ray discovered in1895,Radiation discovered’ in1896and Electron discovered in1897) at the end of the19thcentury, and the three discoveries mean the naissance of modern physics. X-ray is a kindof electromagnetic radiation, whose wavelength range is between ultraviolet and gammarays, it has a high penetrating ability, and penetrate many substances which visible lightcould not penetrate. Thus x-ray could be used for medical diagnosis and treatment, ornon-destructive inspection of materials in industry or other fields. Especially, X-CT(X-ray Computed Tomography) makes use of x-ray, by means of reconstructionalgorithm, to reconstruct a slice image of three-dimensional object, X-CT technique hasbeen widely used in biomedicine, aerospace, geology&archaeology, weapons,bridge&dam, radioactive contamination fields.X-ray is a continuous energy spectrum, similar to visible light, due to the differentwavelengths or frequencies, x-ray can be divided into different energy spectrums.Recently based on x-ray energy-discriminative photon-counting detector (similar tovisible light prism), a new type of X-CT technology--x-ray spectal CT technique wasdeveloped. It is a x-ray computed tomography technique using the information carriedby different energy x-ray interacted with the object, which could display richer x-rayattenuation characteristics to present the interior structure of the object, and providemore substances properties information to identify categories of material. X-ray spectralCT is a milestone for X-CT development, which is an advanced CT technique and aresearch hotspot in the current CT field.Author completed thesis study work with some domestic and overseas professors’supervision in the biomedical imaging laboratory, Virginia Tech.,USA and thedepartment of Physics and Astronomy, University of Canterbury, New Zealand, whoreceive a scholarship “Chinese Building for High-level University graduateprograms”granted by China Scholarship Council (Admission No.2010605056, File No.[2010]3006). The study work focuses on a typical x-ray spectral CT system--MARS(Medipix All Resolution System) CT, which was partially supported by U.S.NIH/NIBIB Grant EB011785, National Natural Science Foundation of China (No.60172074) and Chinese Deep Exploration Program Foundation (No.SinoProbe-03-01-4F). Thesis study content mainly contains:①Study for MARS spectral CT system structure and geometrical alignment. Weproposed a sinogram center line alignment method to determine the horizontal plane ofcone-beam. There is no need to take into account system axis and sample position, thismethod is more efficient to determine the horizontal plane of cone-beam, which isfoundation for exact reconstruction of the x-ray spectral CT image.②Study for x-ray spectral CT projection data characteristics and the imagereconstruction algorithm. X-ray spectral CT often detect x-ray photon within the limitedenergy range, while the x-ray photon number is limited in the limited energy range,resulting in a mount of quantum noise in the projection image. In addition, there aresome deficiencies for x-ray spectral CT detective system (energy-discriminativephoton-counting detector), which could generate some noise and artifacts in theacquired projection data. In order to improve reconstruction image quality, we usedsome pre-processing method to deal with the projection data, moreover, we utilize basedon image total variation (TV) minimum ordered subset simultaneous algebraicreconstruction algorithm (OS-SART) to reconstruct x-ray spectral CT image. Thealgorithm is a combination of TV-based optimization algorithm and OS-SARTreconstruction algorithm, which could accelerate the iterative convergence and suppressnoise and artifacts of reconstructed images.③Study for Substance identification method based on K-edge characteristics.Physically, different materials have different x-ray absorption characteristics, especiallythe K-edge characteristics could be used for identifying different materials. Unlikeconvertional X-CT, x-ray spectral CT is able to analyze the x-ray spectrumcharacteristics. In order to analyze the characteristics of the x-ray spectrum, we used theMARS x-ray spectral CT to identify the K-edge characteristics of some contrast agentsand metal materials, which could demonstrate energy-discriminative performance of thespectral CT.④Study for x-ray spectral CT data evaluation and color CT image reconstruction.To demonstrate material-discriminative potential of the spectral CT, we used principalcomponent analysis (PCA) method to analyze and evaluate the spectral CT data, andextract and quantify these spectrum data, finally we considered the first threeeigenimages corresponding to the first three maximum eigenvalues as RGB(Red/Green/Blue) components of the color image, and reconstructed the true-color CTimage, which could display much richer x-ray attenuation information. ⑤Further research on K-edge imaging technique. K-edge imaging is a vitalapplication for x-ray spectral CT. Before and after K-edge, x-ray attenuation coefficientis very different, so imaging with two energy bins on both sides of a K-edge couldimprove the image contrast of the known materials. This paper proposed a K-edgeimaging optimization model to determine how to select two energy bins for optimalK-edge imaging. This model used the signal difference to noise ratio (SDNR) as acriterion to optimize the energy bin widths on both sides of the K-edge in the K-edgeimaging. The experiment results demonstrate that this model could improve the contrastof ROI of the two reconstructed imaging and minimize noise level.Thesis studies lie in a frontier field, but also in a typical interdisciplinary, involvingx-ray detection and application in the nuclear field, precision mechanical scanningsystem calibration in the optical engineering field, material K-edge characteristics in thequantum physics field, digital image processing and CT image reconstruction in thecomputer field, optimization model analysis in the mathematics field, etc. Therefore, thestudy results integrated interdisciplinary research essence, and highlighted theadvantage of interdisciplinary research. At present, x-ray spectral CT technique is still atthe research and development stage, but it has more advantages than the conventionalX-CT technique, thus, carrying out the study on x-ray spectral CT has vital andprofound theoretical significance, which has a great application potential.