SU‐E‐J‐43: Scatter and Beam Hardening Corrections for MV‐CBCT Using a Few MLC Leaf‐Pairs and a Cylindrical Water Equivalent Phantom

Purpose: MV‐CBCT has multiple advantages over KV‐CBCT for online IGRT, but its imaging contrast is lower than that of KV‐CBCT, partly because of the scatter and the beam hardening effects. 1. To correct the scatter artifacts in megavoltage CBCT using a few MLC leaf‐pairs as beam stoppers and after that; 2. To correct the beam hardening effect using fan beam projections acquired from a water equivalent cylindrical phantom. Methods: Two or three evenly spaced MLC leaf pairs were used as beam stoppers when acquiring the MV CBCT projections, and then the pixel readings under the leaf‐blocked area was largely due to scatter. These pixel readings, plus the pixel readings just outside the field edges, can help to estimate the 2D scatter map by interpolation, which can then be subtracted from the original projections. After the scatter correction, the beam hardening correction was made by using the fan beam projections taken from a water equivalent cylindrical phantom with little scatter. In this study, 180 full MV‐projections for a Catphan500 phantom with a bone equivalent annulus were acquired over 360 degree using 6 MV photon beams, with 0–180 and 180‐360 degree scans using different leaf‐pair as beam stoppers. The fan beam projections for beam hardening correction were acquired from a phantom of uniform density with a diameter of 25 cm. Results: Scatter and beam hardening induced cupping and shading artifacts were substantially reduced in MV‐CBCT. Also, the reconstructed Catphan images were complete because we used different leaf‐pairs as beam stoppers for 0‐180 and 180‐360 degree scans. Conclusion: An efficient and effective scatter correction scheme was proposed and applied for MV‐CBCT. After the scatter correction, the MV‐CBCT image quality matched that from fan‐beam MVCT. Beam hardening correction further improved the imaging contrast of MV‐CBCT.