Research And Implementation Of Beam Hardening Calibration In Medical Cone-beam Computed Tomography

X-ray Computed Tomography (CT) is a relatively new technology since1970’s, which utilizes the attenuation of the X-ray passing through objects. Gray image of the object could be obtained by projecting in different angles and reconstructing these projections, thus enabling the nondestructive visualization of the internal structure and other properties of objects. This technology has been widely used in medical diagnosis and industrial nondestructive detection.With the development of CT technology, and owing to the advantages as high spatial resolution, low radiation and fast imaging, flat panel detector based Cone-Beam CT (CBCT) has been rapidly applied in medical diagnosis, like breast CBCT, dental CBCT, etc. However, a series of problems and challenges of CBCT need to be solved in the procedure of detection. Beam hardening effect is one of the key factors which lead to image distortion. Therefore, some researches on beam hardening calibration and Metal Artifact Reduction (MAR) were promoted in this thesis, which mainly include as follows:Beam hardening effect could cause cupping and streak artifacts in reconstructed images. The new concepts of full-forward projection and energy compensation are proposed for correcting the two artifacts, respectively. For cupping artifact calibration, a corrector was calculated by full-forward projecting the cupping-involved images. Image was calibrated by the corrector as a weighting function. For streak artifact calibration, the high-attenuation projections in sinogram domain were obtained by forward projecting the regions with high CT values in image domain. Then the energy-based calibration model was built for compensating the beam hardening involved projections. Finally, the streak artifacts were reduced by reconstructing the model.It’s complicated to explain the causes of metal artifacts, even beam hardening effect is one of them. Here a fast and stable MAR method was implemented, which is sinogram-based. Metal regions in the image were segmented and forward projected to get the regions affected by the metal in the sinogram. Then an appropriate linear interpolation was applied to reduce the influence of metal fillings. At last, a comparison with the streak artifact calibration method was investigated and both of whose advantages and disadvantages were presented while applying on the metal and streak artifacts, respectively.Finally, the above calibration algorithms are implemented in both simulated data and the Cone-beam CT system to testify their feasibilities.