Comparison between lucite and water as a phantom material in medical radiology

Abstract

In quality assurance and quality control programs in medical radiology, Lucite (i.e. plexiglas, perspex polymethyl methacrylate (PMMA)) phantoms are often used, for convenience, instead of water to imitate the patient's body. Since Lucite has higher density and lower average atomic number than water, the energy absorption and particularly photon scattering characteristics differ. A comparison is made between water, soft tissue and Lucite phantoms of the same linear dimensions with respect to photon energy absorption and scattering at photon energies between 1 and 150 keV. Analog Monte Carlo methods are used to simulate the photon transport within phantom and receptor. Charged particle equilibrium is assumed since the electron transport is not simulated (kerma approximation). It is also assumed that coherent scattering occurs independently with the atoms of the compounds. The results show that the scatter-to-primary ratio, S/P, of the energy imparted to the receptor, with the receptior located on the rear side of the phantom, is significantly larger with the Lucite than with the water or soft tissue phantom. Also, the single-event distribution of the energy imparted to the receptor by the scattered photons transmitted through the phantom, differs between Lucite and water. The fraction of incident energy imparted to the phantom is at low photon energies (below 80 keV) lower in Lucite than in water. The opposite is true at energies above 80 keV where Compton scattering predominates, and the higher density of Lucite is decisive.