Long axial crystals for PET applications: The AX-PET demonstrator and beyond

The usage of long, axially oriented scintillator crystals in a PET scanner has been shown by the AX-PET Demonstrator as a possible solution for a high resolution and high sensitivity PET detector. In the AX-PET implementation, arrays of wavelength shifting (WLS) strips, placed orthogonally behind every crystal layer, are used to define the axial coordinate. After extensive characterization measurements, the AX-PET Demonstrator has been successfully used for the reconstruction of several phantoms and a few rodents. Possible extensions of the AX-PET concept towards Time Of Flight capabilities have been investigated, using Philips digital SiPMs as alternative photodetector. Promising CRT values equal to 211 ps have been demonstrated, using long crystals with dual sided readout and mean timing method. Finally, we report about an alternative way to reconstruct the axial coordinate: exploiting the light sharing of 100 mm long crystals with special surface treatment resulted in axial resolutions of the order of 4 mm FWHM.