Accelerated microPET Transmission Imaging

The Siemens microPET family of scanners provides for acquiring a measured attenuation correction using a ⁵⁷Co point transmission source that spirals around the field of view (FOV). The standard transmission source mechanism advances the source 0.5 mm for every spiral about the FOV. The transmission scan measurement requires approximately 10 minutes, so that often in the interest of time it is not done. The aim of this work was to reduce the transmission scan time on the microPET without compromising the accuracy of the measured attenuation map. The transmission source mechanisms of a microPET R4 and Focus 120 were modified by replacing the 0.5 mm pitch source driving rod with rods of pitches 1.0 mm and 1.5875 mm, thus reducing the transmission scanning times by 50% and 68% respectively. The reductions in transmission imaging acquisition times drop to 40% and 56% for the two rods when the time required to configure the system to low energy singles mode is considered. Transmission data were acquired of uniform cylinders and of a euthanized rat. The reconstructed attenuation maps showed no apparent artefacts introduced by the increase in source pitch. Image noise increased according to that expected from Poisson statistics. The effects of increased image noise had little effect on the attenuation map when segmentation methods were used. We conclude that accurate measured attenuation maps can be produced in as little as 1/3 of the normal transmission scan time by modifying the transmission source mechanism to have a more aggressive source pitch.