Objective: The aims of this study were (1) to evaluate the effect of surrounding materials on the iterative reconstruction-based line-source response function (IR-RF) of (18)F, (11)C, (13)N, and (15)O using a preclinical PET system, and (2) to determine whether and how annihilation outside the source can be visualized experimentally.
Methods: We performed all the measurements using the LabPET-8 PET/CT subsystem built-in the Triumph II platform (TriFoil Imaging, Inc., Northridge, CA, USA). IR-RF was measured for (18)F, (11)C, (13)N, and (15)O, and was expressed as full-width at half-maximum (FWHM) and full-width at tenth maximum (FWTM) using a glass capillary phantom mounted in materials of various densities, which were chosen to cover the wide range of real tissues. To determine whether and how annihilation outside the source can be visualized, we designed a concentric ring paper phantom, which consisted of a source at the center with 4 ring-like paper layers.
Results: When the radionuclides were placed in air (material density 0 g/cm(3)), IR-RFs were similar among the radionuclides tested. As the surrounding material density increased, IR-RFs for higher energy-emitting radionuclides ((11)C, (13)N, and (15)O) became worse, whereas those of (18)F remained relatively constant over the range of surrounding material densities (0-2.17 g/cm(3)). Both FWHM and FWTM values were closely correlated with mean energy of radionuclides at middle to high material densities (material density 0.94-2.17 g/cm(3)). The FWTM/FWHM ratio of high energy-emitting radionuclides such as (15)O increased as a function of material density, which was followed by subsequent decrease at high material densities (1.2-2.17 g/cm(3)). Using a concentric ring paper phantom, annihilations outside the source were visible and measurable. The innermost layer was visible with all radionuclides, whereas the outer layers only with high energy positron emitters.
Conclusions: The results indicate that surrounding material affects IR-RF particularly for high energy positron emitters. Furthermore, annihilation outside the radio-active source can be visualized with some circumstances such as those seen with a concentric ring paper phantom.