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

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, ¹⁵⁵Tb (t_1/2 = 5.32 d) and ¹⁶¹Tb (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 ¹⁵⁵Tb and ¹⁶¹Tb. A hot rod resolution phantom with rod diameters ranging between 0.85 and 1.70 mm was filled with either ¹⁵⁵Tb (21.8 ± 1.7 MBq/mL) or ¹⁶¹Tb (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 ¹⁵⁵Tb and 48.9 keV and 74.6 keV for ¹⁶¹Tb. Quantitative SPECT images of the resolution phantoms were analyzed to report inter-rod contrast, recovery coefficients, and contrast-to-noise metrics.

Quantitative SPECT images of the resolution phantom established that the HEUHR collimator resolved all rods for ¹⁵⁵Tb and ¹⁶¹Tb, and the UHS collimator resolved rods ≥ 1.10 mm for ¹⁶¹Tb and ≥ 1.30 mm for ¹⁵⁵Tb. 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.

Both ¹⁵⁵Tb and ¹⁶¹Tb 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.