Quantitative whole-body dynamic planar scintigraphy in mice with 99mTc and 161Tb.

Abstract

Background: Planar scintigraphy remains commonplace in clinical practice and has been used for quantification and dosimetry estimation over an expanding range of gamma-emitting radionuclides in recent years. Applications of planar scintigraphy, in combination with SPECT/CT imaging, can add value to radiopharmaceutical development in preclinical models and in translation to human use. The aim of this study was to demonstrate whole-body quantitative accuracy in mice using pinhole collimated planar scintigraphy on a preclinical SPECT/CT system, following corrections to sensitivity variations across the field of view.

Results: Planar projections were acquired using short imaging time frames, thus allowing for dynamic biodistribution data to be collected and compared to the known injected activity and whole-body SPECT data. Encapsulation of [^99mTc]TcO₄^- in a supramolecular cage was used to demonstrate the visual and quantitative changes in biodistribution over time, as compared to [^99mTc]TcO₄^- alone. For these radiopharmaceuticals, whole-body quantification was 98.7 ± 7.3% of the decay-corrected true injected activity, as opposed to 74.8 ± 7.5% when calculated without a sensitivity correction. Similarly, the final planar scintigraphy frame acquired at 1-hour post-injection quantitatively agreed with activity values returned from the whole-body SPECT: 99.5 ± 10.6% (final frame, planar) vs. 99.1 ± 5.5% (SPECT). Regions of interest (ROIs) over selected organs between planar scintigraphy and SPECT were also in good agreement. Quantitative accuracy of planar scintigraphy was further validated in a preclinical tumour model of prostate cancer using [¹⁶¹Tb]Tb-PSMA-617. In this case, the whole-body planar value was 94.6 ± 3.6% of the recorded injected activity and, consistent with ^99mTc findings, was underestimated without sensitivity correction (76.6 ± 3.1%). Tumour uptake values were equivalent between corrected planar scintigraphy (5.2%IA) and SPECT (5.3%IA) at 1-hour post-injection.

Conclusions: Using a common radionuclide and one of emerging radiotherapeutic interest, whole-body injected activity and organ-specific ROI values obtained by planar scintigraphy strongly correlated to the true injected activity and values obtained by SPECT following sensitivity-based corrections. The addition of quantitative dynamic planar scintigraphy into the preclinical workflow followed by SPECT imaging adds value to pharmacokinetic and dosimetry assessments of novel gamma-emitting radiopharmaceuticals in imaging and therapeutic applications.