Multi-isotope SPECT imaging of the225Ac decay chain: feasibility studies

Effective use of the decay chain in targeted internal radioimmunotherapy requires the retention of both and progeny isotopes at the target site. Imaging-based pharmacokinetic tests of these pharmaceuticals must therefore separately yet simultaneously image multiple isotopes that may not be colocalized despite being part of the same decay chain. This work presents feasibility studies demonstrating the ability of a microSPECT/CT scanner equipped with a high energy collimator to simultaneously image two components of the decay chain: (218 keV) and (440 keV). Image quality phantoms were used to assess the performance of two collimators for simultaneous and imaging in terms of contrast and noise. A hotrod resolution phantom containing clusters of thin rods with diameters ranging between 0.85 and 1.70 mm was used to assess resolution. To demonstrate ability to simultaneously image dynamic and activity distributions, a phantom containing a generator from was imaged. These tests were performed with two collimators, a high-energy ultra-high resolution (HEUHR) collimator and an ultra-high sensitivity (UHS) collimator. Values consistent with activity concentrations determined independently via gamma spectroscopy were observed in high activity regions of the images. In hotrod phantom images, the HEUHR collimator resolved all rods for both and images. With the UHS collimator, no rods were resolvable in images and only rods ⩾1.3 mm were resolved in images. After eluting the generator, images accurately visualized the reestablishment of transient equilibrium of the decay chain. The feasibility of evaluating the pharmacokinetics of the decay chain in vivo has been demonstrated. This presented method requires the use of a high-performance high-energy collimator.