A Monte Carlo study of multiple scatter effects in Compton scatter densitometry

The contribution of multiple scatter to the measured signal in x- and gamma-ray Compton scatter densitometry has been investigated theoretically by the use of Monte Carlo techniques to follow individual photon life histories. A three component phantom was employed in the computer model to simulate the patient at three examination sites; the radius/ulna, the femoral neck, and the lumbar spine. Monoenergetic radiation beams of 60- and 100-keV photons and polyenergetic x-ray spectra of 100 and 140 kVp were used. Scattered events were detected over 360 degrees and classified according to their origin and frequency of scatter. The single scatter in bone to multiple scatter ratio was studied as an indication of the signal-to-noise ratio and this was found to vary with phantom size but was independent of photon energy. Correction factors to be used in a clinical densitometer to account for the inclusion of multiple scatter events were computed. These were found to be 0.65-0.58 at the optimum scattering angles for the phantoms considered.