  Total-body PET (TB-PET) scanners, with axial lengths exceeding 1 meter,
provide significantly higher sensitivity than conventional PET scanners due to
their greater acceptance of gamma pairs. However, this increased sensitivity
often comes from gamma rays detected at oblique angles, leading to substantial
multiple scattering along the patient's body. Additionally, the cost of TB-PET
scanners scales with their extended detector length, often costing 5 to 10
times more than conventional scanners. Therefore, optimizing TB-PET performance
requires crystals that enhance energy resolution and control costs.
  Cesium iodide (CsI), though historically less favored for PET due to its
lower stopping power and light yield compared to crystals like LYSO, shows
remarkable improvement when operated at cryogenic temperatures ($\sim$100 K).
Under these conditions, CsI light yield rises dramatically to about 100
photons/keV, providing excellent energy resolution and good coincidence time
resolution at a lower cost - typically 3 to 5 times cheaper than other crystals
at parity of radiation length.
  In our study, cryogenic CsI crystals achieved an energy resolution better
than 7% FWHM at 511 keV and a coincidence time resolution of less than 2 ns.
These results demonstrate the potential of cryogenic CsI as a cost-effective,
high-performance material for TB-PET scanners.
