We report on the timing resolution and stability of the MicroPET R4 PET scanner. Its detectors have energy resolutions in the range of 25% and previously reported timing resolutions 3.2 ns. Our preliminary evaluation of this instrument showed that artefact-free normalization sinograms could only be obtained with a timing window of 10 ns or more in spite of a timing resolution of 3.2 ns. This instrument uses high-speed electronics albeit with 2-ns timing clock. We performed sham transmission

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... s with nothing in the field of view, and a range of timing windows from 2 to 14 ns and used a 14-ns timing blank scan to generate effective attenuation sinograms as a function of timing window. These showed trues count-rates which fit well to a ERF ( ) function. However, the effective attenuation value, which should be 1.0, changes from block to block and becomes very high in some blocks ( 3.5 at 6 ns) suggesting the need for timing alignment. A method was devised to measure the timing stability to a precision better than the timing bin width of 2 ns.