  In the present work we describe the design, construction, and testing of the
optical prototype developed for the BOLDPET project, with the objective of
creating a PET detection module with high spatial and time resolution. The
BOLDPET technology uses an innovative detection liquid, trimethylbismuth, for
detecting 511 keV $\gamma$-quanta resulting from positron annihilation. The
optical signal is exclusively produced through the Cherenkov mechanism, and the
produced photons are detected using Planacon microchannel-plate
photomultiplier. We achieve an excellent time resolution of 150 ps (FWHM)
within a sizable detection volume measuring 55 mm x 55 mm x 25 mm. Through
detailed Geant4 simulations, we examine the limiting factors affecting time
resolution and explore potential avenues for improvement. Furthermore, we
demonstrate the feasibility of coarse 2D localization of interactions using the
optical signal alone, achieving a precision of about 5-8 mm (FWHM) within the
homogeneous detection volume.
