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. 2015 Aug 7;42(9):5084–5099. doi: 10.1118/1.4927786

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© 2015 American Association of Physicists in Medicine.

0094-2405/2015/42(9)/5084/16/$30.00

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FIG. 17. — (a) Monte Carlo simulations of the input ESF (ideally a step function) produced by irradiating a 0.5 mm Ta edge phantom with a 6 MV beam. Results are shown as a function of EID (0, 5, 10 cm). For an EID = 0 cm (i.e., edge resting on the front cover of a Varian AS1000), the input ESF has slightly sloping tails that can negatively bias the LSF baseline after differentiation (LSFs not shown). (b) and (c) The corresponding MTFs. The ideal MTF response of the simulation (ideal detector pixel pitch = 0.392 mm) is given by the formula, sinc (0.392 f), where f is the spatial frequency in units of mm⁻¹. If the LSF is not baseline corrected (b), the results suggest that the EID should be larger than 5 cm. As shown in (c), baseline correction of the LSF may remove any restrictions on the EID. For each geant4 simulation (penelope Physics), 2 × 10¹¹ gamma photons were launched using a parallel source representing an infinite SID so that source blurring did not affect the result. The irradiated area was 78.4 × 78.4 mm², the edge phantom area was 78.4 × 39.2 mm², and edge angle was 2.5°.