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Shifting Multiple EPID Imager Layers to Improve Image Quality and Resolution in MV CBCT

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H Chen

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


SU-E-J-27 (Sunday, July 12, 2015) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: Vertical stacking of four conventional EPID layers can improve DQE for MV-CBCT applications. We hypothesize that shifting each layer laterally by half a pixel relative to the layer above, will improve the contrast-to-noise ratio (CNR) and image resolution.

Methods: For CNR assessment, a 20 cm diameter digital phantom with 8 inserts is created. The attenuation coefficient of the phantom is similar to lung at the average energy of a 6 MV photon beam. The inserts have attenuations 1, 2...8 times of lung. One of the inserts is close to soft tissue, resembling the case of a tumor in lung. For resolution assessment, a digital phantom featuring a bar pattern is created. The phantom has an attenuation coefficient similar to soft tissue and the bars have an attenuation coefficient of calcium sulfate. A 2 MeV photon beam is attenuated through these phantoms and hits each of the four stacked detector layers. Each successive layer is shifted by half a pixel in the x only, y only, and x and y (combined) directions, respectively. Blurring and statistical noise are added to the projections. Projections from one, two, three and four layers are used for reconstruction. CNR and image resolution are evaluated and compared.

Results:When projections from multiple layers are combined for reconstruction, CNR increases with the number of layers involved. CNR in reconstructions from two, three and four layers are 1.4, 1.7 and 1.99 times that from one layer. The resolution from the shifted four layer detector is also improved from a single layer. In a comparison between one layer versus four layers in this preliminary study, the resolution from four shifted layers is at least 20% better.

Conclusion: Layer-shifting in a stacked EPID imager design enhances resolution as well as CNR for half scan MV-CBCT.

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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