System calibration and image reconstruction for a new small-animal SPECT system

A novel small-animal SPECT imager, FastSPECT II, was recently developed at the Center for Gamma-Ray Imaging. FastSPECT II consists of two rings of eight modular scintillation cameras and list-mode data-acquisition electronics that enable stationary and dynamic imaging studies. The instrument is equipped with exchangeable aperture assemblies and adjustable camera positions for selections of magnifications, pinhole sizes, and fields of view (FOVs).; The purpose of SPECT imaging is to recover the radiotracer distribution in the object from the measured image data. Accurate knowledge of the imaging system matrix (referred to as H) is essential for image reconstruction. To assure that all of the system physics is contained in the matrix, experimental calibration methods for the individual cameras and the whole imaging system were developed and carefully performed.; The average spatial resolution over the FOV of FastSPECT II in its low-magnification (2.4X) configuration is around 2.4 mm, computed from the Fourier crosstalk matrix. The system sensitivity measured with a 99mTc point source at the center of the FOV is about 267 cps/MBq. The system detectability was evaluated by computing the ideal-observer performance on SKE/BKE (signal-known-exactly/background-known-exactly) detection tasks.; To reduce the system-calibration time and achieve finer reconstruction grids, two schemes for interpolating H were implemented and compared: these are centroid interpolation with Gaussian fitting and Fourier interpolation. Reconstructed phantom and mouse-cardiac images demonstrated the effectiveness of the H-matrix interpolation.; Tomographic reconstruction can be formulated as a linear inverse problem and solved using statistical-estimation techniques. Several iterative reconstruction algorithms were introduced, including maximum-likelihood expectation-maximization (ML-EM) and its ordered-subsets (OS) version, and some least-squares (LS) and weighted-least-squares (WLS) algorithms such as the Gauss-Seidel (GS) iteration and the algebraic reconstruction technique (ART). These algorithms were compared in terms of their computational cost, data-agreement measures and subjective assessment of image quality.; The spatial resolution of the imaging system was visualized through a miniature Derenzo hot-rod phantom. The smallest rods with 1-mm diameters and 3-mm center-to-center distance were clearly resolved. Mouse bone, kidney and cardiac images illustrated the ability of FastSPECT II to provide high-quality small-animal images. The dynamic-imaging capability was demonstrated via rat myocardial studies.; FastSPECT II can be modified to achieve higher angular sampling and higher magnification. Fourier crosstalk analysis and synthetic phantom studies showed that higher angular sampling improved the spatial resolution and image quality along two transverse axes. Line-phantom and mouse-femur images demonstrated the sub-millimeter resolution of FastSPECT II in the high-magnification (18X) configuration.