Time-of-flight PET with BaF2

The available TOFPET (time-of-flight positron-emission tomography) scanners with BaF₂ have a TOF resolution of about 500 ps. The possible reasons for this can be several, including spread in the time of arrival of photons to the photocathode, transit time spread in the photomultiplier, jitter in the timing from the discriminator and the time-to-digital converters, and systematic channel-to-channel shifts. The authors have developed a Monte Carlo program to study the contribution of each of these effects to the TOF results. They simulate the interactions of 511 KeV gammas with the detector (Compton and photoelectric absorption), the emission and propagation of UV light in the crystal, and the pulse formation in the photomultiplier. The time spread due to each effect is calculated, as well as the time spread due to all these effects combined. Experimental evidence indicates that the length of the crystal has a strong effect on the light collection time spread, both due to reflections and to the exponential distribution of interaction points inside the crystal. To quantify the length effect, the authors calculated the FWHM (full-width half maximum) of the time of production of the first photoelectron on the photocathode