ToF
Time-of-Flight PET — measures the timing difference between the two coincident 511 keV photons from a single annihilation event, localizing the annihilation along the line of response. The smaller the timing uncertainty, the tighter the localization, and the better the small-lesion conspicuity at matched injected dose.
Why it matters to buyers: ToF capability and ToF timing resolution define the modern PET tier. Discovery 690 (PMT-era) achieved ~500 ps; current digital (SiPM) platforms reach 178–250 ps. Image-quality and dose-reduction differences across that range are clinically visible — particularly for small lesions and theranostic dosimetry. ToF tier should be confirmed in writing on any refurb purchase.
Why it matters to engineers: Requires fast scintillators (LYSO / LSO) and fast electronics. BGO is not ToF-capable — sites with BGO PET (Discovery VCT, Discovery STE) cannot software-upgrade to ToF; the path is a new scanner. Timing calibration drifts; quarterly NEMA IQ phantom testing catches drift before it shows up clinically.