Purpose: Motion of gas bubbles in abdominal region can produce significant artifacts during on-board CBCT scanning, which adversely affects the imaging quality and limits the process of CBCT-based adaptive planning and image-guided radiotherapy. In this study, we aim to test the effectiveness of the simultaneous motion estimation and image reconstruction technique (SMRIE) for improving CBCT image quality and HU accuracy for abdominal scan.
Methods: Two 3D abdominal CT images for a same patient acquired with 3 mins time interval were used in this study and the deformation vector field (DVF) between these two scans were calculated. Based on the calculated DVF, we simulated n (n=0, 2, 4, 6, 8) different DVFs through adding random motion vectors, and the corresponding n different CT images were generated. A n+2 phases 360° CBCT projection was then simulated through combining the projection images from the n+2 CT images, where projections from every continuous 360°/(n+2) are from the same CT. Then we applied SMEIR algorithm onto the generated multi-phase CBCT projection, the motions of gas bubbles between these phases were iteratively predicted through inverse-consistent 2D-3D registration following motion compensated simultaneous algebraic reconstruction technique (SART), and the motion induced artifacts were corrected in the meanwhile.
Results: When using SART for CBCT reconstruction, gas bubble motion artifacts in the simulated CBCT image showed the same pattern to those in the real abdominal CBCT scan. By introducing SMRIE technique, the boundary of structures and HU inaccuracy of gas bubble region are well corrected in the reconstructed CBCT image compared with SART reconstruction result.
Conclusion: We introduced SMRIE technique for gas bubble motion artifact reduction, the improved image quality in the result demonstrated that SMRIE technique is promising for on-board CBCT gas bubble motion artifact reduction.