Voxel Level S-values
Voxel S-Values (VSVs) allow to compute the absorbed dose at voxel level starting from a heterogeneous radioactivity distribution, like a PET or a SPECT, following the MIRD schema (Bolch et al., J. Nucl. Med. 50(3) 2009). In a nutshell, the dose-rate at the image acquisition time in a target voxel rt can be computed as a convolution between the PET or SPECT and a kernel of VSVs, which are the mean absorbed dose in rt per decay event from each source voxel rs. If images acquired at multiple times are available, a cumulative activity, obtained integrating the activity over a time window T, can be used to yield the absorbed dose in T. VSVs depend on incident radiation and crossed material and can be simulated by Monte Carlo codes (such as Geant4) in voxelized virtual volumes. Dose Point Kernels (DPKs) are the spherical equivalent of VSVs and are calculated in concentric shells.