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We have developed a CsI(Tl)/PIN detector module for high resolution SPECT and low dose photon-counting CT imaging. Using the detector modules with 6.1mm pixels, we built a cardiac SPECT system with three detector heads. The detector heads form a triple-head (each 20×15cm), geometry for emission scans and reconfigure to form a large transaxial field-of-view (FOV) geometry for transmission scans using an x-ray based transmission source. Anthropomorphic phantom and patient data was used to evaluate the performance of the new cardiac SPECT camera. We then developed modules with 2.8mm pixel size to further improve the spatial resolution, improving energy resolution to 7.5%. Improved energy resolution was achieved by improving the crystal arrays, electronics and packaging of the module. Using the 2.8mm modules, we built a large FOV planar imager (39×31cm), and evaluated its emission performance with clinical studies and its CT performance with an anthropomorphic phantom. The count rate capability was 20kcps per detector pixel. The combined SPECT/CT scan was completed in 5 min, resulting in high quality and accuracy attenuation maps (μH20@140keV was 0.151+/-0.003/cm at ~5μSv CT dose). The large FOV imager showed excellent clarity and high resolution for emission bone studies and demonstrated reconstructed CT image quality suitable for attenuation correction or image fusion and localization (~150μSv CT dose).