Fifty-seventh annual meeting of the American association of physicists in medicine
TH-EF-BRB-01: JUNIOR INVESTIGATOR WINNER — Novel EPID for Enhanced Contrast and Detective Quantum Efficiency
Beams-eye-view imaging applications such as real-time soft-tissue motion estimation are hindered by the inherently low image contrast of electronic portal imaging devices (EPID) currently in clinical use. We introduce and characterize a novel EPID that provides substantially increased detective quantum efficiency (DQE), contrast-to-noise ratio (CNR) and dynamic range without degradation in spatial resolution.
The prototype design features a stack of four conventional EPID layers combined with low noise integrated readout electronics. Each layer consists of a copper plate, a scintillator (GdO2S2:Tb) and a photodiode/TFT-switch (aSi:H). We characterize the prototype in terms of contrast-to-noise ratio (CNR), modulation transfer function (MTF) and DQE. CNR is estimated using a Las Vegas contrast phantom, presampled MTF is estimated using a slanted edge technique and the DQE is calculated from measured normalized noise power spectra (NPS) and the MTF. The prototype has been designed and built to be interchangeable with the current clinical EPID in terms of size and data output specifications.
DQE(0) can be nearly quadrupled to about 4.5% by using the four-layered design instead of only a single layer device. No substantial differences are observed between each layer's individual MTF and the one for all four layers operating combined indicating that defocusing is negligible. Also, using four layers instead of one nearly doubles (factor x1.9) the CNR showing that the device is mostly quantum noise limited.
A layered EPID design allows improving the radiation sensitivity while maintaining spatial resolution and saturation level of a single layer conventional EPID. Experimental characterization of this first 4-layered prototype demonstrates substantially improved DQE and CNR while maintaining high resolution and remaining quantum noise limited. Besides overall improved image quality and dosimetric sensitivity we anticipate this novel detector design to enable more accurate soft-tissue motion estimations during radiation therapy procedures, particularly of the lung.
The project was partially supported by a grant from Varian Medical Systems, Inc. and grant No. R01CA188446-01 from the National Cancer Institute. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Cancer Institute or the National Institutes of Health.