  Liquid xenon offers several features, which make it suitable for applications
in nuclear medicine, such as high scintillation yield and fast scintillation
decay time. Moreover, being a continuous medium with a uniform response, liquid
xenon allows one to avoid most of the geometrical distortions of conventional
detectors based on scintillating crystals. In this paper, we describe how these
properties have motivated the development of a novel concept for positron
emission tomography scanners with Time-Of-Flight measurement, which combines a
liquid xenon scintillating volume and silicon photomultipliers as sensors. A
Monte Carlo investigation has pointed out that this technology would provide an
excellent intrinsic time resolution, down to 70 ps. Also, the transparency of
liquid xenon to UV and blue wavelengths opens the possibility of exploiting
both scintillation and Cherenkov light for a high-sensitivity positron emission
tomography scanner with Time-Of-Flight capabilities. Monte Carlo simulations
point to a time resolution of 30-50 ps obtained using Cherenkov light. A
prototype is being built to demonstrate the high resolution in energy, time and
reconstruction of spatial coordinates of this concept, using a ring of 30 cm
internal diameter and a depth of 3 cm instrumented with VUV-sensitive silicon
photomultipliers.
