Monte Carlo determination of emerging energy spectra for diagnostically realistic radiopharmaceutical distributions

Abstract

In order to realistically define the internal organs of a representative human, 150 transverse CT scans of an (average) male patient were acquired from head to mid-thigh on the GE 9800 Quick scanner. The reconstructed transverse slices were read into a microVAX 3500 and members of the medical staff outlined 42 separate internal organs contained in the transverse slice. This digitized human phantom serves as an input to a Monte Carlo program which models photoelectric absorption and scatter processes of gamma-rays in matter. The organs can be “filled” with variable amounts of radiopharmaceuticals and the simulation computes the emerging energy spectra for a given source distribution and detector position. The simulation follows gamma-ray histories out to a maximum of 32 scatter events. Scatter spectra are histogrammed into energy distributions of gamma-rays which have undergone a specific number of scatter events before emerging from the phantom. A sum of all these scatter spectra yields the simulated total spectra. Simulated total spectra of diagnostically relevant human distributions are compared to spectra acquired from nuclear medicine clinical patients.