Utilizing high-energy γ-photons for high-resolution ²¹³Bi SPECT in mice

The combined α-, γ-, and x-ray emitter ²¹³Bi (half-life, 46 min) is promising for radionuclide therapy. SPECT imaging of ²¹³Bi is challenging, because most emitted photons have a much higher energy (440 keV) than common in SPECT. We assessed ²¹³Bi imaging capabilities of the Versatile Emission Computed Tomograph (VECTor) dedicated to (simultaneous) preclinical imaging of both SPECT and PET isotopes over a wide photon energy range of 25-600 keV. Methods: VECTor was equipped with a dedicated clustered pinhole collimator. Both the 79 keV x-rays and the 440 keV γ-rays emitted by ²¹³Bi could be imaged. Phantom experiments were performed to determine the maximum resolution, contrast-to-noise ratio, and activity recovery coefficient for different energy window settings. Additionally, imaging of [²¹³Bi-DOTA,Tyr³]octreotate and ²¹³Bi-diethylene triamine pentaacetic acid (DTPA) in mouse models was performed. Results: Using 440 keV γ-rays instead of 79 keV x-rays in image reconstruction strongly improved the resolution (0.75 mm) and contrast-to-noise characteristics. Results obtained with a single 440 keV energy window setting were close to those with a combined 79 keV/440 keV window. We found a reliable activity recovery coefficient down to 0.240 MBq/mL with 30-min imaging time. In a tumor-bearing mouse injected with 3 MBq of [²¹³Bi-DOTA,Tyr³]octreotate, tumor uptake could be visualized with a 1-h postmortem scan. Imaging a nontumor mouse at 5-min frames after injection of 7.4 MBq of ²¹³Bi-DTPA showed renal uptake and urinary clearance, visualizing the renal excretion pathway from cortex to ureter. Quantification of the uptake data allowed kinetic modeling and estimation of the absorbed dose to the kidneys. Conclusion: It is feasible to image ²¹³Bi down to a 0.75-mm resolution using a SPECT system equipped with a dedicated collimator.