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Combination of Multiple EPID Imager Layers Improves Image Quality and Tracking Performance of Low Contrast Objects

S Yip

S Yip1*, J Rottmann1 , H Chen1 , D Morf2 , R Fueglistaller2 , J Star-Lack2 , G Zentai2 , R Berbeco1 , (1) Brigham and Women's Hospital, Dana-Farber Cancer Institute and Harvard Medical School, Boston, MA, (2) Varian Medical Systems, Palo Alto, CA,


TH-EF-BRB-2 (Thursday, July 16, 2015) 1:00 PM - 2:50 PM Room: Ballroom B

Purpose: We hypothesize that an electronic portal imaging device (EPID) consisting of multiple layers, each containing a scintillator coupled to amorphous silicon flat panel imager (AMFPI), increases photon detection efficiency, improving image quality and tracking accuracy of low contrast targets during radiotherapy.

Methods: The prototype 4-layer imager evaluated in this study contained four individually programmable layers each with a copper-Gd₂O₂S scintillator coupled to an AMFPI with a complete set of readout electronics. The imager was placed on a Varian TrueBeam linac. A Las-Vegas phantom programmed with sinusoidal motion (Peak-to-peak amplitude=20cm, Period=3.5sec) was imaged at a frame rate of 10Hz with one to four layers activated (activation of only the top layer is equivalent to using a conventional EPID). Image quality was assessed by counting the number of visible circles, and computing the contrast-to-noise ratio (CNR) of the least visible circle (depth=0.5mm, diameter=7mm). A previously validated tracking algorithm was employed for auto-tracking. Tracking error was defined as the difference between the programmed and tracked positions of the circle. The Pearson correlation coefficient of the CNR and tracking error was also computed.

Results: Motion-induced EPID image blurring significantly reduced circle visibility. During one cycle of phantom motion, the number of visible circles ranged from 11–23, 13–24, 15–25, and 16–26 when imaged with one, two, three and four layer imagers, respectively. Compared with using only a single layer, combining 2, 3, and 4 layers increased the median CNR by factors of 1.19, 1.42, and 1.71, respectively and reduced the average tracking error from 3.32mm to 1.67mm to 1.47mm, and 0.74mm, respectively. Significant correlations (p~10⁻⁹) were found between the tracking error and CNR.

Conclusion: Combination of four amorphous silicon layers, instead of single layer, significantly improves the EPID image quality and tracking accuracy for a poorly visible object that replicates lung tumor radiotherapy conditions.

Funding Support, Disclosures, and Conflict of Interest: The project described was supported, in part, by a grant from Varian Medical Systems, Inc., and Award 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.

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