Computed Quantitative Planar Imaging for Targeted Alpha Therapy: Model-Based Sparse Reconstruction Validated With a Novel 225 Ac Epoxy Phantom

Targeted Alpha Therapy (TAT), using alpha-emitting radionuclides (AER) such as ²²⁵Ac, shows promise for the treatment of advanced and refractory cancers. Currently, TAT is prescribed on the basis of activity (e.g., MBq, kBq/kg), with no account taken of individual biodistribution or kinetics. The delivery of patient-specific treatment, based on absorbed dose criteria, requires in-vivo imaging of the AER biodistribution, a challenging scenario due to the scarcity of imageable photons. To address this, we present a novel computed quantitative planar (CQP) imaging method that reconstructs a coronal projection of the 3D AER distribution from anterior/posterior scintigraphy coregistered with CT. The model is regularized using maximum a posteriori estimation with sparse ℓ1 tight-framelet transforms and solved via a convergence-guaranteed fixed-point proximity algorithm. To experimentally evaluate our approach, we built a modular slab phantom containing a known distribution of ²²⁵Ac vitrified in epoxy. CQP reconstruction was characterized by significantly reduced bias and noise, improved spatial resolution, and better signal-to-noise ratios, compared to geometric mean methods. The CQP approach is clinically implementable with conventional SPECT/CT systems, without need for hardware additions or modifications, and can assist dosimetry workflows, especially where 3D SPECT/PET is impractical.