| dc.description.abstract | Nuclear medicine imaging of preclinical small animal studies needs to use imagers with higher spatial resolution so the reconstructed images of small animal organs can have the same accuracy as that of human organ reconstructions. In this thesis, the single-photon emission microscope (SPEM) is used as the imaging device. This system is a high-spatial-resolution specialized branch of the single-photon emission computed tomography (SPECT). The SPEM system consists of a 7-pinhole collimator, a thallium-doped cesium iodide crystal [CsI(Tl)], an electrostatic demagnifying tube (DM tube) and an electron-multiplying charge-coupling device (EMCCD).
In the field of computed tomography, in order to obtain high-quality reconstructed object images, an accurate and high-resolution imaging system matrix (H matrix) is required. So, in order to obtain a more accurate imaging system matrix of SPEM, this study has improved the old method of creating imaging system matrix comes from our laboratory. The content includes the fitting method of system geometric parameters, which describe the spatial relationship between the system rotation axis and the EMCCD, and the imaging models for generating the H matrix.
In the past, we conducted the geometric calibration experiment and grid-scan experiment in order to obtain the geometric parameters and to create the imaging model, respectively. In the geometric calibration experiment, a three-point source phantom is placed on a rotary stage, which is located on the three-dimensional translation stages, and each time after the phantom is rotated by a preset angle we capture a projection image. In the grid-scan experiment, a single-point source is placed on the rotary stage, and each time after the source is translated by a preset grid spacing we capture a projection image. From the projection images of these point sources, the point response functions (PRFs) of the original object space can be obtained and fitted by two-dimensional Gaussian functions. Each 2D Gaussian function has six parameters: the amplitude as the luminous flux, the x and y coordinates of the projection centroid on the detector plane, the principal angle of the elliptical contours of the 2D Gaussian and the variances along the major and minor axes of the elliptical contours.
In this study, the x and y coordinates of the projection points from the two previous experiments were used together to fit the geometric parameters. We can obtain the spatial relationship between the phantom and the rotary stage, the spatial relationship between the phantom and the translation stages, the spatial relationship between the rotation axis and the EMCCD, and the spatial positions of pinholes. After that, an imaging model with non-circularly-symmetric PRFs was conceived by analyzing the relationship between the geometric parameters of the system and the Gaussian parameters of the projection images from the grid-scan experiment. The imaging model coefficients were obtained by nonlinear least-squares fitting, including the flux model, width models along the major and minor axes, and principal angle model.
Finally, the calculated geometric parameters and the created imaging model were used to generate the image system matrices of various grid spacing. The iterative algorithm Ordered-Subset Expectation Maximization is used to reconstruct the object images. The sampling method of image acquisition takes the sampling completeness into account, and is divided into circular and helical trajectories. The accuracy of the imaging system matrix is judged by analyzing the reconstructed object images. In the final reconstruction of three-dimensional object images sampled by the circular and helical trajectories, compared with the reconstruction results of the old method used in our laboratory for creating H matrices, the improved method have obtained more reasonable and clearer reconstructed images. | en_US |