βγ imaging: simultaneous detection of single-gamma and position-annihilation with CZT-based PET scanner

Introduction: Gamma-Positron emission tomography (β⁺ &gamma; imaging) is a hybrid form of emission tomography that simultaneously detects 511 keV photon pairs originating at the site of positron-electron annihilation and singly-emitted high-energy gamma rays emitted directly from the parent isotope (e.g., Copper-64 and Zr-89). Reconstruction of combined b⁺gprovides the incredible potential for high-resolution imaging of ligand and receptor labelled with different PET isotopes for dynamic imaging of brain or head and neck cancer. Cross-strip cadmium zinc telluride (CZT) is an optimal detector technology for β⁺&gamma; coincidences imaging because: (i) high stopping power based on the edge-on geometry that enables detection of multiple interaction photon events (MIPEs), (ii) its 3D positioning minimizes parallax, (iii) its high-energy resolution (<4% for CZT vs >10% for LSO scintillator crystals) and high spatial resolution enable single-photon reconstruction and annihilation-photon-pair quantum-entanglement analysis, (iv) it has a good dynamic range up to 1.2 MeV in maximum photon energy deposited per interaction. It operates at room temperature and the cross-strip readout with inherent depth of interaction (DOI) capabilities enable simple system development and scaling. In this work, we demonstrate overall peak sensitivity of a prototype dual-panel system based on a cross-strip CZT detectors and present the higher sensitivity compared to when the same system is operated using only a positron-emitting isotope and only a high-energy gamma emitter in Compton-camera mode.

Methods: Figure 1 illustrates a model of the proposed prototype system of cross-strip CZT detectors with a 23 detectors around the circumference and 30 detector along Z axis. The prototype is based on a CZT modular design previously developed for a dedicated head and neck system at Univeristy of Califirnia, Santa Cruz [1]. A point source in the center of the FOV was used to assess ideal β⁺&gamma; system sensitivity. For the simulation we have utilized the GPU-based Monte-Carlo modeling software developed at the University of Arizona that offers several advantages over conventional Geant4-based solutions [2].

Results: Table 1 shows the sensitivity of the various interactions of both annihilation photons and the prompt gamma. As can be seen in table 1, there are different possible combinations of interactions for the two annihilation photons and the prompt gamma. The total sensitivity of the PET alone system is 5.67%. When the annihilation photons and prompt gamma are detected, if the prompt gamma undergoes Compton scatter it yields a cone of response and is not subject to positron-range effects. In addition, quantum entanglement can be utilized to categorize the scatter events due to positron -annihilation photons. If only the prompt-gamma is detected via Compton scatter because one or more positron annihilation photons misses the detector or does not deposit its full energy, the cone of response from the prompt gamma can be utilized for the reconstruction. The overall sensitivity of such a system including the PET mode and Compton camera mode increases to 9.14 %.

Conclusions: We proposed and investigated a β⁺&gamma; imaging system based on CZT telluride. Simulation studies indicate that this hybrid approach enables more sensitivity and spatial resolution than the same system operated using only a positron-emitting isotope and only a high-energy gamma emitter in Compton-camera mode.

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