Development of a new multielement detector system for megavoltage photons

Currently employed megavoltage photon detector systems in radiotherapy suffer from their low detection efficiency. This paper presents a new, more efficient approach. A dense converter material is placed along the direction of the photon beam together with the interspersed active medium. The high-energy electrons, generated in the converter material, create a detectable signal in the active medium. The characteristics of single detector element prototypes consisting of different brass tubes filled with either air or xenon were investigated. The significance of the wall thickness and material of a two-dimensional array of such elements was analyzed by Monte Carlo studies. Saturation curves and the signal as a function of the gas pressure illustrate the equivalence of the prototypes to conventional ionization chambers. A high-density gas like xenon at a high pressure is required to get a sufficiently large signal-to-noise ratio. Acquired MVCT slice images revealed contrasts of - 2.4 percent at a dose of 40 cGy. Monte Carlo calculations suggest an optimal wall thickness of a few hundred micrometers for an element width of 1.5 mm. The superiority of high density and high atomic number materials like tungsten in terms of the amount of primary signal recorded and the extent of crosstalk is demonstrated.© (2002) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.