CBCT Scattering Correction Based On EGSnrc

Cone beam CT has many advantages,such as high radiation efficiency and large field.Information on tumor and crisis organs can be obtained before treatment.So it's widely used in radiation therapy for cancer.However,due to collimator cone angle increases and the lack of flat panel detector,a large number of scattering signal reaches the flat-panel detector.There are a large number of scattered noises in the CBCT images.Scattering noise leads to serious image distortion caused by a cup-shaped artifact which seriously affects the image quality of CBCT.It is difficult to achieve effective protection of surrounding organs and tissues in the process of killing tumor accuratelyand cannot guarantee postoperative quality of life.Therefore,scattering correction for CBCT effective removal of scattering artifacts and improving the quality of CBCT images are the current research hotspots.At present,the methods of scattering correction are mainly divided into three kinds: the hardwarebased scatter rejection method,software based scattering modeling method,and the software and hardware combination measurement method.These three methods have their own advantages and disadvantages.The method of scattering rejection is an optimal method in theory,but because of the limitation of flat panel detector the method can only get very limited effectand it need to change the equipment that is more difficult.The method of software modeling requires the balance between correction time and correction effect to meet the requirements of clinical real time.Compared with the first two methods,the scattering correction based on software and hardware combination measurement method although it has its own shortcomings such as(1)system stability problem,(2)the problem of reconstruction error introduced by interpolation but because of its superior scattering removal effect more and more people pay attention to it.In this paper,based on the research work of predecessors a scattering measurement method a software and hardware combination measurement method is proposed in this paper which is a scattering correction method based on a rotating collimator.This method consists of a rotating collimator,a KV ray source,a flat panel detector,a finite arc projection algorithm based on orthogonal moments,and an improved FDK reconstruction method.The cone beam is restricted to a number of fan-shaped beams through the rotation collimator and through the rotation of the collimator,the sector beam rays are scanned along the axis of the accelerator frame to obtain the projection information of the entire volume image and then the image information of the blocked region is estimated by the orthogonal moment method.Finally,the improved FDK reconstruction algorithm is used to reconstruct the image.In order to verify the effectiveness of the proposed method,we use the Monte Carlo simulation toolbox EGSnrc to simulate a simplified CBCT system with a rotating collimatorand build a practical CBCT system for the rotation collimator.The results of Monte Carlo simulation experiment show that compared with the CBCT image,image after scattering correction the mean square error of the reconstruction error decreases from 16.003% to 1.16%,the cupping artifacts from 14.001% to 0.65%,and the peak signal-to-noise ratio increase from 16.9589 to 31.55.The actual experimental results show that compared with the CBCT image,the contrast degree of the CBCT image after the scattering correction is increased from 0.0098 to 0.0399 and the cup artifact is decrease from 20.89 to 5.02,and the contrast-to-noise ratio is increased from 0.4409 to 2.3502.Both the simulation experiment and the actual experimental results show that the scattering correction method based on the rotating collimator can obviously restrain the influence of the scattering on the CBCT image and improve the image quality.Thus,when the tumor target area is not missed the normal tissue around the tumor can be better protected,so that the patient can get the maximum benefit in radiotherapy.