FAst timing for Positron Emission Tomography

The use of time-of-flight (TOF) information in positron emission tomography (PET) significantly improves the effective sensitivity, enabling better image quality and reduction of scan times, dose and costs. PET manufacturers have recently released systems with 325-400 ps FWHM coincidence resolving time (CRT). Moreover, CRT’s well below 100 ps FWHM have been achieved using short (< 5 mm) scintillation crystals coupled to silicon photomultipliers (SiPMs). Unfortunately, these results are difficult to translate to full systems, which need much larger crystal lengths (> 20 mm) to efficiently capture the annihilation photons emitted from the patient. The optical transport of the scintillation photons, which is also influenced by the varying depth-of-interaction (DOI) of the annihilation photons, gives rise to significant timing fluctuations within such crystals.

The Delft PET group has developed and experimentally validated statistical models to quantitatively predict the time resolution of scintillation detectors as a function of the pertinent photosensor and scintillator properties. These models are useful to guide the development of photosensors and scintillation materials. Moreover, we have shown how time blurring due to optical transport can be mitigated. Based on these results, it is expected that CRTs < 100 ps will be feasible in future PET systems, without sacrificing other crucial performance parameters, such as spatial resolution, energy resolution, and system sensitivity.

An intriguing question is whether it will be possible to achieve CRTs < 20 ps FWHM. This corresponds to TOF positioning resolutions < 3 mm FWHM and would cause a paradigm shift in PET by eliminating the need for image reconstruction. Unfortunately, based on the above models it is not clear that such CRTs will be feasible with conventional inorganic scintillators. Therefore, new approaches need to be investigated. This talk will discuss ultrahigh-time-resolution PET from a practical as well as a theoretical point of view, with the aim to provide input to the search for new conversion materials for TOF-PET.