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 Rmax because of photon scatter in the front plate. Because the decrease of MTF with backplate thickness ceases for backplate thickness greater than Rmax, the MTF is influenced more by the backscatter electrons than the backscatter photons.
Falco
T
T
Medical Physics Unit, McGill University, Montreal General Hospital, Canada. tfalco@medphys.mgh.mcgill.ca
Fallone
B G
BG
D055592:Biophysical Phenomena; D001703:Biophysics; D003037:Cobalt Radioisotopes / Q000627:therapeutic use; D003720:Densitometry; D006801:Humans; D008670:Metals; D017785:Photons / Q000627:therapeutic use; D011874:Radiometry / Q000295:instrumentation* / Q000706:statistics & numerical data; D011880:Radiotherapy Planning, Computer-Assisted; D011882:Radiotherapy, High-Energy / Q000295:instrumentation* / Q000706:statistics & numerical data; D012542:Scattering, Radiation; D014962:X-Ray Film
D003037:Cobalt Radioisotopes; D008670:Metals