Source of Support: None, Conflict of Interest: None
A method of determining the effective electron density of hip prostheses using a scanning liquid ionization chamber (SLIC) electronic portal imaging device (EPID) is described. The method involves establishing an empirical relationship between the pixel intensity values (extracted from a portal image) and the corresponding equivalent thickness of an inhomogeneity such as a hip prosthesis. This is done using a composite phantom made of polystyrrene with embedded inhomogeneities of Aluminum, Copper and Lead. Results for 6 and 10 MV photons show that the relationship depends on the characteristics of the imaging device and on the photon energy, provided the atomic number (Z) of the inhomogeneity is sufficiently low such that the Compton effect remains the dominant photon interaction. The portal images of a pelvic phantom with prosthesis were then used to provide X-ray attenuation information, which allowed the determination of the equivalent path length containin9 the inhomogeneity and, therefore, its effective electron density.