Purpose:X-ray scatter photons result in significant degradation of cone-beam CT(CBCT)image quality.Measurement based algorithms using beam block directly acquire the scatter samples and achieve significant improvement on the quality of CBCT image.In existing algorithms,single scan and stationary beam block design proposed before is promising due to its simplicity and practicability.Although demonstrated effectively on tabletop system,the block fails to correct for the shading artifacts induced by scatter on clinical CBCT machine mainly due to inaccurate scatter estimation from gantry wobble in the scan.In this work,we investigate the motion behavior of the beam block in each projection and estimate the variation of block trajectory from the gantry wobble.An accurate scatter estimation is then achieved from the wobble modeling and high-quality images are reconstructed from the corrected projections.Methods:A "crossing-finger" shaped block was mounted on the exit of the x-ray tube of a Varian onboard imager(Fig.1&2).To estimate the gantry wobble,we track the block in equal-angular distributed projections.Then the block trajectory is estimated using the measured trajectory samples by interpolation in each projection(Fig.3).The correction of gantry wobble was implemented in scatter estimation.Scattercorrected CT images are obtained using a modified half-fan reconstruction algorithm.Results:The proposed method is evaluated using Catphan@500 phantom.Using gantry wobble estimation,our approach reduces the reconstruction error from 86 Hounsfield unit(HU)to 15 HU in the selected regions of interest(Figs.4&5),and improves the image contrast by a factor of 1.3 in the high-contrast regions(Table I).Conclusions:In this work,we propose a practical scatter correction solution with gantry wobble estimation for clinical CBCT system.The method is practical and attractive for CBCT guided radiation therapy.