Characteristics of metal-plate/film detectors at therapy energies. I. Modulation transfer function

Measurements of modulation transfer function (MTF) for front and back metal-plate/film portal detectors are reported for the Cobalt-60 and 10 MV spectra. The detectors consist of a double-emulsion portal film secured between plates of Al, Cu, brass, or Pb with thicknesses varying from 0 to 4.81 mm. Secondary electrons produced within the front plate generate the main signal, but the MTF decreases with an increase in front plate thickness greater than the maximum range of electrons R-max because of photon scatter in the front plate. Because the decrease of MTF with backplate thickness ceases for backplate thickness greater than R-max, the MTF is influenced more by the backscatter electrons than the backscatter photons. (C) 1998 American Association of Physicists in Medicine. [S0094-2405(98)01212-7].