Quantitative SPECT imaging of 155Tb and 161Tb for preclinical theranostic radiopharmaceutical development

Background Element-equivalent matched theranostic pairs facilitate quantitative in vivo imaging to establish pharmacokinetics and dosimetry estimates in the development of preclinical radiopharmaceuticals. Terbium radionuclides have significant potential as matched theranostic pairs for multipurpose applications in nuclear medicine. In particular, Tb-155 (t(1/2) = 5.32 d) and Tb-161 (t(1/2) = 6.89 d) have been proposed as a theranostic pair for their respective applications in single photon emission computed tomography (SPECT) imaging and targeted beta therapy. Our study assessed the performance of preclinical quantitative SPECT imaging with Tb-155 and Tb-161. A hot rod resolution phantom with rod diameters ranging between 0.85 and 1.70 mm was filled with either Tb-155 (21.8 +/- 1.7 MBq/mL) or Tb-161 (23.6 +/- 1.9 MBq/mL) and scanned with the VECTor preclinical SPECT/CT scanner. Image performance was evaluated with two collimators: a high energy ultra high resolution (HEUHR) collimator and an extra ultra high sensitivity (UHS) collimator. SPECT images were reconstructed from photopeaks at 43.0 keV, 86.6 keV, and 105.3 keV for( 155)Tb and 48.9 keV and 74.6 keV for Tb-161. Quantitative SPECT images of the resolution phantoms were analyzed to report inter-rod contrast, recovery coefficients, and contrast-to-noise metrics. Results Quantitative SPECT images of the resolution phantom established that the HEUHR collimator resolved all rods for Tb-155 and Tb-161, and the UHS collimator resolved rods >= 1.10 mm for Tb-161 and >= 1.30 mm for Tb-155. The HEUHR collimator maintained better quantitative accuracy than the UHS collimator with recovery coefficients up to 92%. Contrast-to-noise metrics were also superior with the HEUHR collimator. Conclusions Both Tb-155 and Tb-161 demonstrated potential for applications in preclinical quantitative SPECT imaging. The high-resolution collimator achieves < 0.85 mm resolution and maintains quantitative accuracy in small volumes which is advantageous for assessing sub organ activity distributions in small animals. This imaging method can provide critical quantitative information for assessing and optimizing preclinical Tb-radiopharmaceuticals.