Lightweight PET Calibration Source

PET scanners are commonly calibrated with cylindrical sources, about 20 cm in diameter, in which positron-emitting radioactivity is mixed. The sources are heavy and may require massive shielding for storage. In this work we ask if scanner setup can instead be based on hollow, lightweight sources based on small beta sources and positron flight across the air, reducing the shielding requirements, and providing a nearly scatter-free source with negligible photon attenuation. A set of two positron-emitting (beta) sources was created by depositing a 0.7-mm layer of polyurethane resin, containing a total of 42.6 MBq
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Ge on two copper plates. We also created a second set of sources by depositing a 6-mm spot of
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Ge DTPA on each of two steel plates. These were positioned inside a hollow polyethylene cylinder with 21.3 cm outer diameter, 32 cm length and 7 mm wall thickness. The sources were at the cylinder's two flat ends. Depending on the positrons' direction after beta decay, the positrons either stopped in the beta source or flew across the air until they encountered the cylinder's walls, causing the emission of annihilation radiation. The cylinder was positioned in an SiPM-based PET/CT scanner. PET scans and a time of flight (TOF) calibration were performed. Fused PET/CT images showed that positrons stopped both in the cylinder's inner wall (33% and 18% of all positrons emitted by the first source and sets of sources, respectively) and in the air inside the cylinder (5% and 2%). Standard software was used to measure 211 ps TOF resolution, comparable to the value derived from traditional extended sources. Because the positron sources are small, they can be easily shielded, eliminating the need for more than 100 kg of shielding. A lightweight, hollow, scatter and attenuation-free source appears to be a practical alternative to the current state of the art.