Abstract
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Introduction: Versatile Emission Computed Tomography (VECTorTM, MILabs B.V., The Netherlands (Beekman et al., 2021) offers simultaneous multi-isotope PET-SPECT and PET-PET with a uniform resolution down to 0.6 mm (PET) and 0.4 mm (SPECT), the unique capability to eliminate both randoms and positron range blurring, and to drastically reduce scatter contamination compared to coincidence PET. The culmination of these capabilities leads to high effective sensitivity detection at unmatched image resolutions. Often, an increase in PET sensitivity at high resolution is helpful to maintain high-quality images at low injected doses. Here we present the first results of a new line of ultra-sensitive collimators for confocal VECTor PET/SPECT imaging, suitable for imaging in the 1 microCi range.
Methods: We have introduced two new high energy (HE) mouse collimators, one with 93 pinholes of a 3.8-mm diameter (HE-UHS-M) and one with 105 pinholes of a 4.4-mm diameter (HE-XUHS-M) with estimated system random-free sensitivities of 10.0% and 12.3%, respectively, for PET with 19 mm thick crystals. The collimators were evaluated for a VECTor7/CT system, with Derenzo phantoms containing either 99mTc or 18F at several activity levels, down to 1 µCi (0.037 MBq). System matrices were generated using ray-tracing for both SPECT and PET. Images were reconstructed with an iterative method (DM-SROSEM (Goorden et al., 2020)) that corrects for collimator penetration and detector blur during image reconstruction.
Results: Reconstructed images of our new ultra-sensitive collimators show achievable resolutions of 1.1 mm for 18F and 0.8-0.9 mm for 99mTc (Figure 1-2). In comparing the two collimators, we can conclude that the best resolution at activity levels above 0.25 MBq is achieved by the HE-UHS-M, while the 23% more sensitive HE-XUHS-M performs best at the lowest dose. With a total activity of only 1 µCi inside the phantom, the imaging still produced resolutions of 1.4 mm for 18F and 1.0 mm for 99mTc. Comparisons of our simulations with physical measurements confirm the accuracy of our simulations (Figure 3).
Conclusions: The presented collimators will be helpful when extremely low doses are used, which may be the case during the discovery process of new molecules or required for longitudinal studies. On a VECTor/CT system, this ultra-low-dose imaging capability for PET, SPECT, simultaneous PET-PET and PET-SPECT, plus theranostic imaging, will pave the way for many new preclinical research protocols.
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