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First Experimental Study of On-Board CBCT Imaging Using 2.5MV Beam On a Radiotherapy Linac

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Q Xu

Q Xu1,2*, R Li1 , Y Yang1 , L Xing1 , (1) Department of Radiation Oncology, Stanford University, Stanford, California 94305 (2) Institute of Image Processing and Pattern Recognition, Xi'an Jiaotong University, Xi'an, 710049, China


SU-E-I-72 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Varian TrueBeam version 2.0 comes with a new inline 2.5MV beam modality for image guided patient setup. In this work we develop an iterative volumetric image reconstruction technique specific to the beam and investigate the possibility of obtaining metal artifact free CBCT images using the new imaging modality.

An iterative reconstruction algorithm with a sparse representation constraint based on dictionary learning is developed, in which both sparse projection and low dose rate (10 MU/min) are considered. Two CBCT experiments were conducted using the newly available 2.5MV beam on a Varian TrueBeam linac. First, a Rando anthropomorphic head phantom with and without a copper bar inserted in the center was scanned using both 2.5MV and kV (100kVp) beams. In a second experiment, an MRI phantom with many coils was scanned using 2.5MV, 6MV, and kV (100kVp) beams. Imaging dose and the resultant image quality is studied.

Qualitative assessment suggests that there were no visually detectable metal artifacts in MV CBCT images, compared with significant metal artifacts in kV CBCT images, especially in the MRI phantom. For a region near the metal object in the head phantom, the 2.5MV CBCT gave a more accurate quantification of the electron density compared with kV CBCT, with a ~50% reduction in mean HU error. As expected, the contrast between bone and soft-tissue in 2.5MV CBCT decreased compared with kV CBCT.

On-board CBCT imaging with the new 2.5MV beam can effectively reduce metal artifacts, although with a reduced soft-tissue contrast. Combination of kV and MV scanning may lead to metal artifact free CBCT images with uncompromised soft-tissue contrast.

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