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We developed a new cardiac SPECT system (X-ACT) with a low dose volume CT transmission-based attenuation correction (AC). Three solid-state detectors are configured to form a triple-head system for emission scans and reconfigured to form a large 27Â¿ (69 cm) field-of-view (FOV) detector arc for transmission scans. The transmission line source is the collimated fluorescence X-ray emitted from a lead target when the lead target is fluxed by a narrow X-ray beam from an X-ray tube. High quality transmission scans can be completed in as short as one minute with insignificant patient dose (~5 Â¿Sv). For evaluation, we scanned an anthropomorphic phantom with a uniform cardiac insert and the same anthropomorphic phantom with one 60Â° full defect and one 45Â° 50% defect in the cardiac insert. We also scanned an ACR phantom and a uniform cylindrical phantom. Results showed that image uniformity and defect contrast were improved by AC in the anthropomorphic phantom studies as compared to without AC (NAC). The inferior to anterior wall ratio and the septal to lateral wall ratio were 0.99 and 1.16 before and 1.02 and 1.00 after AC. The defect contrast of the full and 50% defect was 0.528 and 0.156 before and 0.628 and 0.173 after AC. The ACR phantom images with AC showed correction of the bowing effect due to attenuation in the NAC images. The reconstructed attenuation coefficient of water at 140 keV was 0.154Â±0.003/cm and 0.157Â±0.002/cm in the liver and cardiac regions of the anthropomorphic phantom, and 0.150Â±0.003/cm in the uniform region of the ACR phantom and the uniform cylindrical phantom. In conclusion, the X-ACT system generated accurate attenuation maps with one-minute transmission scans. Phantom studies showed that AC improved image quality and quantification over NAC.