Assessing the imaging calibration factor for quantitative SPECT/CT of actinium-225

The main aim of this study was to evaluate the effect of quantitative imaging protocol parameters on the imaging calibration factor (ICF) value for ²²⁵Ac.

All measurements were conducted with two measurement geometries: point source (activity of 14.9 MBq) and cylinder filled uniformly with radionuclide (total injected activity of 41.4 MBq). The data were acquired as follows: planar acquisition of a point source (duration 20 min) and tomographic (SPECT/CT) acquisition of a cylindrical phantom (total acquisition time 32 min or 56 min, matrix size 128×128). Nine different combinations of primary and scatter energy windows for ²²⁵Ac quantitative imaging were studied. All acquired data were re-analyzed for a 64×64 matrix size. The aforementioned tomographic experiment was repeated when the total activity in the cylindrical phantom dropped to 4.2 MBq, and for selected combinations of energy windows. ICFs for planar images were determined by taking the total number of counts in the whole field of view of the camera. ICFs for SPECT/CT images were determined with four different methods of segmentation. Additionally, the homogeneity of the reconstructed SPECT/CT images was evaluated.

The ICF values obtained using planar/tomographic geometries and different configurations of acquisition settings were presented. Based on the results obtained, for the ²²⁵Ac quantitative acquisition protocol, it could be recommended to define two main energy windows (for ²¹³Bi and ²²¹Fr, daughter nuclei of ²²⁵Ac) with accompanying lower and upper energy windows for TEW scattering correction. The benefits of a 64×64 matrix should also be considered.