Observations regarding scatter fraction and NEC measurements for small animal PET

The scatter fraction (SF) was measured for a line source, a mouse-sized phantom (25 mm φ×70 mm) and a rat-sized phantom (60 mm φ×150 mm) on the microPET II small animal PET scanner. Measurements were performed for four lower energy thresholds of 150, 250, 350 and 450 keV and a fixed upper energy threshold of 750 keV. Four different methods were used to obtain the SF. Significant scatter fractions were measured with just the line source in the field of view, with the spatial distribution of these events consistent with scatter from the gantry and environment. For mouse imaging, this component dominates over object scatter, and the measured SF is strongly method dependent. The environmental SF rapidly increases as the lower energy threshold decreases and can be more than 20% for an open energy window of 150-750 keV. The object SF originating from the mouse phantom is about 4% and does not change significantly as the lower energy threshold changes. The object SF for the rat phantom ranges from 20-40% for different energy windows and increases as the lower energy threshold decreases. Because the measured SF is highly dependent on the method, and there is as yet no agreed upon standard for animal PET, care must be exercised when comparing NEC rates for small objects between different scanners. Differences may be methodological rather than reflecting the performance of the scanner. Furthermore, this data has implications for scatter correction methods when the majority of the detected scatter does not arise from the object itself.