Multi-isotope SPECT imaging of the 225Ac decay chain: feasibility studies

Effective use of the Ac-225 decay chain in targeted internal radioimmunotherapy requires the retention of both Ac-225 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 Ac-225 decay chain: Fr-221 (218 keV) and Bi-213 (440 keV). Image quality phantoms were used to assess the performance of two collimators for simultaneous Fr-221 and Bi-213 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 Fr-221 and Bi-213 activity distributions, a phantom containing a Bi-213 generator from Ac-225 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 Fr-221 and 213Bi images. With the UHS collimator, no rods were resolvable in Bi-213 images and only rods. 1.3 mm were resolved in Fr-221 images. After eluting the Bi-213 generator, images accurately visualized the reestablishment of transient equilibrium of the Ac-225 decay chain. The feasibility of evaluating the pharmacokinetics of the Ac-225 decay chain in vivo has been demonstrated. This presented method requires the use of a high-performance high-energy collimator.