SPECT with submegabecquerel amounts of tracer or subsecond time resolution would enable a wide range of new imaging protocols such as screening tracers with initially low yield or labeling efficiency, imaging low receptor densities, or even performing SPECT outside regular radiation laboratories. To this end we developed dedicated ultra-high-sensitivity pinhole SPECT. Methods: A cylindric collimator with 54 focused 2.0-mm-diameter conical pinholes was manufactured and mounted in a stationary

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... ll-animal SPECT system. The system matrix for image reconstruction was calculated via a hybrid method based on both 99m Tc point source measurements and ray-tracing analytic modeling. SPECT images were reconstructed using pixel-based ordered-subsets expectation maximization. Performance was evaluated with phantoms and low-dose bone, dynamic kidney, and cardiac mouse scans. Results: The peak sensitivity reached 1.3% (13,080 cps/MBq). The reconstructed spatial resolution (rod visibility in a micro-Jaszczak phantom) was 0.85 mm. Even with only a quarter megabecquerel of activity, 30-min bone SPECT scans provided surprisingly high levels of detail. Dynamic dual-isotope kidney and 99m Tc-sestamibi cardiac scans were acquired with a time-frame resolution down to 1 s. Conclusion: The high sensitivity achieved increases the range of mouse SPECT applications by enabling in vivo imaging with less than a megabecquerel of tracer activity or down to 1-s frame dynamics.