The Simulation And Design Of Dynamic Bowtie Filter For Cone-beam CT

In computed tomography?CT?,a pre-patient attenuator?bowtie filter or bowtie?can be used to balance the photon flux on a detector array by selectively attenuating X-ray according to the shape and size of the patient's body.And it can reduce the radiation dose suffered by the patient and the effects of scattering and beam hardening on imaging.As the attenuating profile of the traditional static stays fixed as the CT gantry rotates,the static bowtie can not effectively balance the photon flux on a detector array throughout the CT scan.To this end,Prof.Liu Fenglin of Chongqing University in China and Prof.Wang Ge of the Rensselaer Polytechnic Institute jointly proposed a dynamic bowtie that can modulate X-ray in real time and is suitable for cone-beam CT scans.In this paper,it is called the dynamic bowtie for cone-beam CT.The bowtie consists of a highly attenuating bowtie?HB?filled in with heavy liquid and a weakly attenuating bowtie?WB?immersed in the liquid of the HB.The HB targets a balanced flux distribution on a detector array when no object is in the field of view?FOV?.When an object is placed in the FOV,WB replaces a portion of the high attenuation liquid to compensate linearly for the X-ray attenuation through the object so that the photon flux on the detector is balanced.As the CT gantry rotates,WB rotates synchronously with the object to maintain the linear compensating relationship so that the photon flux on the detector is balanced in real time.This paper focuses on the Monte Carlo simulation study and design of the dynamic bowtie,and based on the simulation design,the effects of the dynamic bowtie on the radiation dose suffered by a simulation phantom,the reconstructed image quality and the scattering and beam hardening were analyzed.In addition,the application of the dynamic bowtie was studied.The main contents of this paper include:?1?The principle of the dynamic bowtie was deduced.Based on the midplane for cone-beam CT scan,the design theory of dynamic bowtie was deduced whem the X-ray are attenuated by the dynamic bowtie and the object with a uniform attenuation coefficient.?2?The Monte Carlo simulation framework for the CT system was constructed.Based on the Monte Carlo simulation software MCNP5,a CT system simulation framework containing the reflected target,a target phantom and the dynamic bowtie was constructed,and he CeCl₃ solution was selected as the HB attenuation liquid.?3?The dynamic bowtie was designed based on Monte Carlo simulation.To balance the photon flux reaching the detector,the shape of HB was designed and the 33.6%CeCl₃solution was determined.The results show,for the design phantom,the dynamic range of X-ray is compressed by 1293 times,the radiation dose suffered by design phantom is reduced by 76%and the standard deviation of CT reconstructed image is reduced from0.086 to 0.059,and for the target phantom,the dynamic range is compressed by 111 times and the additive zero-mean gaussian noise variance of the CT reconstructed image was reduced from 0.079 to 0.027.?4?The effects of beam hardening and scattering under the action of the dynamic bowtie were analyzed.The results show that the dynamic bowtie can reduce beam hardening effects and the number of scattering photons that reach the detector.?5?The applications of the dynamic bowtie are prospected.The dynamic bowtie for the patient's abdomen was designed,and its applications in both the circumferential and linear scanning modes are prospected.