Research On Correction Method Of Ring Artifacts And Beam Hardening Artifacts Of Micro-CT

Micro-CT imaging system has been widely used in the field of disease modeling,drug testing,tumor monitoring,bone parameters measurement and many other fields due to its nature of non-destructive,non-intrusive and high resolution.In order to achieve high spatial resolution,the micro CT system uses an optical coupling high resolution X-ray detector as well as a micro focus X-ray source.Due to the complexity manufacturing process of scintillator in optical coupling high resolution detector,the defects of scintillator would lead to heavy ring artifacts in reconstructed CT images.Furthermore,because of the low power energy of micro focus X-ray source,the artifacts induced by beam hardening effect in Micro-CT will be more serious than that of routine based medical and industrial CT.In order to improve the imaging quality of the Micro-CT system,this paper focuses on the correction of the ring artifacts and the beam hardening artifacts.Detector shifting method is an efficient way to remove the ring artifacts caused by inhomogeneous responses of detector units.In this paper,a displacement correction algorithm is proposed to calibrate the shifting position of detector.It is also used to test the repositioning accuracy and the straightness of the platform which is used to produce detector shifting.By the results of the platform test,the detector shifting method with discrete fixed-position has been employed,and its effectiveness has also been verified by the projection data of carbon,tibia and femur.Experiment results demonstrate that the proposed method can provide a good mitigation of ring artifacts.Further experiment shows that the more fixed-position of detector shift,the better performance can be acheived.When applied in the Micro-CT system in our laboratory,10 fixed-position can meet the requirements for the ring artifacts correction.Beam hardening effect can induce strong artifacts in CT images,which results in severely deteriorated images with incorrect intensities.As the beam hardening artifacts are mainly caused by the polychromatic nature of X-ray source,the spectra measuring method was implemented to verify the influence of different filters on energy spectrum as well as the beam hardening artifacts.By the experiments,both aluminum filter and fluorite filter can pre-hardening the X-ray beam and suppress the beam hardening artifacts to some extent.Other than aluminum filter,fluorite filter with the same thickness can obtain better correction result.Neither filters can correct all kinds of beam hardening artifacts since the incident beam spectral is still polychromatic.Furthurmore,as the flux is reduced,the lower photon count also results in a lowered signal to noise ratio(SNR).Due to the limitation of the correction effect of the filters,this paper develops an effective and efficient beam hardening correction algorithm incorporated in filtered back-projection based maximum a posteriori(BHC-FMAP)to farther eliminate the beam hardening artifacts.In the proposed algorithm,the beam hardening effect is modeled and incorporated into the forward-projection of the maximum a posteriori(MAP)to suppress the beam hardening artifacts,and the image update process is done by FDK based back-projection to speed up the convergence.The proposed BHC-FMAP approach does not require the information about the beam spectrum or the material properties,as well as additional segmentation operation.The proposed method was qualitatively and quantitatively evaluated using both phantoms and animal projection data.Experiment results demonstrate that the BHC-FMAP method can efficiently provide a good correction of beam hardening artifacts.