Standardization of DQE(f) Measurements at MV Energies
E Abel1*, A Meyer2, M Sun1, D Sawkey1, R Fahrig3, D Morf 2, J Star-Lack1, (1)Varian Medical Systems, Palo Alto, CA (2) Varian Medical Systems Imaging Laboratory, Baden, Switzerland (3) Stanford University, Stanford, CATH-A-141-11 Thursday 8:00AM - 9:55AM Room: 141
The detective quantum efficiency (DQE(f)) is a critical metric to evaluate detector performance. For diagnostic imagers operating at keV energies, there are clear standards from the International Electrotechnical Commission (IEC) using patient equivalent filters to appropriately harden the beam. Such standards do not exist for MeV imaging. Through a systematic investigation comparing simulated and measured DQE(f) we have demonstrated both the need and plausibility for such a standard at MV energies.
The DQE(f) of a Varian AS1000 EPID imaged at 6 MeV by a Varian TrueBeam linac was both measured and simulated. The spectra necessary were extracted from the TrueBeam phase space files available from MyVarian.com website, then hardened through 20 cm of water. From this an equivalent thickness of aluminum was calculated, and the DQE(f) simulated and measured. The Geant4 simulation replicated the Varian AS1000 portal imager geometry, with optical blur in the Gd2O2S scintillator modeled using isotropic scattering. For the measurement of DQE, a narrow, angled slit was formed using two parallel tungsten blocks placed immediately before the imager from which the Fujita over-sampled MTF was calculated. The NPS was generated from an ensemble of gain and offset corrected flood fields per the IEC guidelines.
Simulation indicates a 40% attenuation of the un-flattened 6 MeV spectrum by 20 cm of water, the equivalent aluminum thickness of which is 85 mm. Simulation and measurement show, with 10% agreement, as much as a 70% decrease in DQE(f) for the hardened beam
We have demonstrated, through simulation and experiment, as much as a 70% reduction in DQE for a 6 MeV un-flattened beam through 20 cm of water. We believe it plausible, therefore, to establish a measurement standard similar to that used for keV imaging to fairly evaluate imagers under realistic imaging conditions.
Funding Support, Disclosures, and Conflict of Interest: This research was funded in part by Varian Medical Systems, Inc. and by NIH grant R01 CA138426