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  4. Low Dose Megavoltage Cone-Beam CT Acquisition Using a Novel Multi-Layer Imager (MLI)

Low Dose Megavoltage Cone-Beam CT Acquisition Using a Novel Multi-Layer Imager (MLI)

M Myronakis1*, P Huber2 , R Fueglistaller2 , A Wang3 , Y Hu1 , P Baturin3 , M Shi1,4 , D Morf2 , J Star-Lack3 , R Berbeco1 , (1) Department of Radiation Oncology, Brigham and Women's Hospital, Dana-Farber, Boston, MA, (2) Varian Medical Systems, Baden, Baden, (3) Varian Medical Systems, Palo Alto, CA, (4) University of Massachusetts Lowell, Lowell, MA,

Presentations

(Tuesday, 7/31/2018) 7:30 AM - 9:30 AM

Room: Room 205

Purpose: We demonstrate low dose megavoltage cone-beam acquisition (MVCBCT) using a novel, high DQE multi-layer imager (MLI). The specific objectives of this work are a) acquisition of MVCBCT of a pelvis phantom using clinical 6MV and 2.5MV beams and b) optimization of MVCBCT acquisition to balance reconstructed image quality and delivered imaging dose.

Methods: The MLI is composed of four-layers, each containing a stack of copper filter/converter, gadolinium oxysulfide (GOS) scintillator and a-Si detector array. A total of 720 projections of a pelvis phantom were acquired over 360 degrees at 6MV and 2.5MV beam energies on a Varian TrueBeam linear accelerator. The dose per projection was 0.0167 MU and 0.0025 MU for 6MV and 2.5MV, respectively. MVCBCT images were reconstructed with varying projections to achieve a total of 1.55 to 12 MUs at 6MV and 0.22 to1.8 MUs at 2.5MV. The contrast-to-noise (CNR) between the bony areas and soft tissue in the reconstructed image was calculated.

Results: In general, CNR decreased with decreasing total MUs in both beam energies (6MV and 2.5MV), as expected. Reconstructed images with 6MV and 2.5MV beams and the MLI were compared with 6MV with the standard EPID AS1200) at 1.7 MU. At this dose, the CNR for the MLI was roughly twice that of the AS1200 at 6MV and nearly 7 times greater for the 2.5MV beam with the MLI. The bony anatomy is easily discernable in all images and an implanted gold fiducial is more easily visualized in the MLI images.

Conclusion: MVCBCT imaging with a novel MLI prototype mounted on a clinical linear accelerator was demonstrated. Reconstructed image quality was dependent on total delivered dose. At 1.7 MU total delivery, both the 6MV and 2.5MV beams provided major improvements in CNR. It is estimated that using the 2.5 MV beam will reduce the required delivered dose by a factor of 8-10 times.

Funding Support, Disclosures, and Conflict of Interest: This work was supported, in part, by award number R01CA188446 from the National Institutes of Health and a research grant from Varian Medical Systems, Inc.

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