The Ax-Pet Concept: New Developments And Tomographic Imaging

The Axial PET (AX-PET) concept proposes a novel detection geometry for PET, based on layers of long scintillating crystals axially aligned with the bore axis. Arrays of wavelength shifting (WLS) strips are placed orthogonally and underneath the crystal layers; both crystals and strips are individually readout by G-APDs. The axial coordinate is obtained from the WLS signals by means of a Center-of-Gravity method combined with a cluster algorithm. This design allows spatial resolution and sensitivity to be decoupled and thus simultaneously optimized. In this work we present the latest results obtained with the 2-module AX-PET scanner prototype, which consists of 6 radial layers of 8 LYSO crystals each (crystal size: 3 x 3 x 100 mm(3)). The WLS arrays comprise 26 strips (3-mm wide) per layer. The estimated energy resolution from point-like measurements is 11.8% (FWHM at 511 keV). The intrinsic spatial resolution was measured for the two modules in coincidence at two different configurations using point-like sources, showing very little degradation when the modules were placed oblique to each other. The axial spatial resolution was 1.5 mm (FWHM) in all the studied cases. Tomographic data of extended phantoms filled with fluorine-18 have been acquired. Imaging a larger transaxial Field-of-View (when compared to the previous measurement campaign) was possible thanks to implementing secondary motion of one of the modules. We have also developed various reconstruction approaches which take into account the particular nature of AX-PET data, as well as a count rate model which allowed us to develop an acquisition protocol able to compensate for count losses. The reconstructed phantom images confirm the imaging capabilities of AX-PET, and the recent advancements in the DAQ let us expect significant improvements for future campaigns.