Zr-89 mouse Compton imaging with the developed next-generation whole gamma imaging system

Introduction: Whole gamma imaging (WGI) is a new imaging concept that combines PET and Compton imaging. Using dual detector rings each for the scatterer (inner) and the absorber (outer), not only positron annihilation 511 keV gamma ray pairs but also single gamma rays can be measured by the Compton imaging scheme without a physical collimator. ⁸⁹Zr (often used as an immuno-PET tracer) is a good radioisotope for WGI because it emits 909 keV gamma rays about 4 times more frequently than positrons. Compton imaging, which is not influenced by the long positron range of ⁸⁹Zr, may have a chance to outperform the PET spatial resolution. Our final goal is developing a clinical WGI system because there are many potential applications in nuclear medicine: however, we found that better energy resolution and better spatial resolution are needed for scatterer detectors based on the experience of developing three WGI prototype systems. Therefore, the aim of this work is to complete new detector technologies for next-generation WGI and to demonstrate them by developing the 4th WGI prototype.

Methods: To get better energy resolution, we have developed a new scatterer detector using a modified GAGG scintillator (1.45×1.45×15 mm³), for which the Ga:Al ratio (for the band gap control) was optimized, and Mg was co-doped (for the energy transfer control). With this scintillator, our recently proposed crosshair light-sharing (CLS) detector, which can measure both time-of-flight and depth-of-interaction information, was developed. The outer absorber detectors were composed of 3.1×3.1×20 mm³ fast-LGSO scintillators. The diameters of scatterer and absorber detector rings were 79 mm and 304 mm, respectively. We measured an ⁸⁹Zr point source at some offset positions of the field-of-view for angular resolution measure (ARM) evaluation. We then demonstrated ⁸⁹Zr-oxalate mouse imaging in the Compton mode. Image reconstruction was performed with list-mode OSEM (50 iterations and 8 subsets).

Results: The ⁸⁹Zr point source could be visualized at all positions, and the full-width at half-maximum of ARM was 8.5°–10.3° for 909 keV gamma rays. In the mouse Compton images, distributions of the ⁸⁹Zr-oxalate matched well with bone structures in the CT.

Conclusions: We have developed the WGI 4th prototype system based on the new detector technologies and succeeded in ⁸⁹Zr-oxalate Compton imaging of a normal mouse. In the next step, we will fine-tune the WGI 4th prototype and explore the potential of WGI ⁸⁹Zr imaging to push the PET spatial resolution limit by comparing PET and Compton images.

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