Purpose: Prevalent PET imaging reconstructs 2γ photon pairs from electron-positron annihilations. Due to QED, ~60% of annihilations occur directly and 40% occur through positronium (Ps) process. The ratio of triplet Ps (3S1, ortho-Ps, decaying to 3γ) to singlet Ps (1S0, para-Ps, decaying to 2γ) is 3:1 in vacuum. Although 3S1 to 1S0 transitions are forbidden, most ortho-Ps convert to para-Ps through “Pick-off process”, hence 2γ events predominated. The energy difference between 3S1 and 1S0 (hyperfine splitting or HFS) is 203.4GH at ground state and 25.4GHz at first excited state. With magnetic fields, excited energy levels are split (Zeeman Effect), hence transitions between 23S1 to 21S0 become possible. Human body provided such conditions through biological tuning. Absorbing 25.4GHz microwave, many 23S1 converts to 21S0 and decays to 3γ rather than 2γ. This downgrades imaging quality because contemporary PET imaging rejects 3γ events. Such loss of para-Ps becomes crucial in PET-guided radiotherapy since minimal tracer was administrated. Hence shielding against 25.4GHz source is necessary.
Methods: Typical PET facility shielding is to protect area outside imaging room, this microwave shielding is inverse shielding because it tends to isolate patient against external microwave radiations. Microwaves could penetrate through 2-3 feet concrete walls, but will be stopped at thin metal layer, e.g., aluminum of 1-2mm thickness. For this reason, a metal layer should be always put into PET facility shielding in all directions of the vault, since pure concrete shielding is insufficient.
Results: Microwaves of HFS energy 25.4GHz (or other higher excited states) drop PET imaging quality significantly.
Conclusion: In PET-guided radiotherapy, PET imaging at a fractional basis serves for target localization rather than diagnosis, thus minimal injected dose is administrated to reduce exposures. Losing para-Ps is unaffordable because image quality might downgrade to unacceptable level. Hence shielding microwaves of HFS energy levels are necessary.