[Quantitative assessment of scattered photons considering skull bone in brain SPECT].

A Monte Carlo simulation was performed to estimate scattered photons generated in skull bone in brain SPECT. In the simulation, we used cylindrical phantoms which consisted of water and calcium. We calculated energy spectra of planar images and the scatter fractions, which were defined by the ratio of the scattered photons to primary photons, within the energy windows of 20% centered at the photopeak energy. For radionuclides [99mTc (141 keV), 123I (159 keV)], we changed the following parameters: the thickness of the calcium layer (0.0, 0.5, 1.0, 1.5 cm) and its density (1.55, 1.70, 1.85 g/cm3). The results showed that the shape of the energy spectra depended on the photon energy, the thickness of skull and its density. Consequently, the scatter fractions within the energy windows varied from 0.23 to 0.25 (without skull) and from 0.28 to 0.42 (with skull).