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

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.

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.

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.