An ultra-high sensitivity sensing collimation imager: design, modelling, and assessment

In this work, we discuss the design and development of an ultra-high sensitivity imaging system, Sensing Collimation Imager (SCI), dedicated to guide and optimize application of alpha particle-emitting radiopharmaceutical therapy (αRPT). Alpha particles are highly potent and unlike conventional external beam therapy can be used in metastatic settings. Currently, the lack of information of radionuclide distribution hinders estimated internal absorbed dose from αRPT, which calls for ultrasensitive SPECT technologies that can report reliable activity distribution under ultra-low count rate imaging conditions. The proposed SCI-SPECT is based on an innovative design that incorporates the geometry of both conventional pinhole collimator and coded aperture. Aimed to decouple the dependence of resolution and sensitivity, the heavy metal collimator is replaced by scintillator crystals to serve dual purposes of increasing detected events and providing spatial information. The detector module consists of an 8×8 array of crystal elements. Each crystal presents an alternating pattern of GAGG scintillator and acrylic. The depth of interaction (DOI) information is obtained through dual-ended readout system. Our experimental results are based on a working prototype SCI module that is fully functional under a broad range of energy (70 keV – more than 511 keV), which is well-suited for alpha particle emitter imaging applications. Furthermore, we modeled the system response function based on analytical derivation and validated it by comparing with Monte Carlo simulations. We carried out a preliminary imaging study of three rod sources filled with Tc99m. The image was successfully reconstructed with the derived system matrix, and the rods at different locations can be clearly resolved. We also compared the performance with Siemens Symbia Intevo scanner for high γ-ray energy imaging with Cu-64, the reconstructed image shows better resolution while potentially achieving 2 orders of magnitude higher sensitivity.