Purpose: To develop an indirect scatter estimation method for beam-blocker based cone beam CT (CBCT) scatter correction to overcome the influence of the inserted beam blocker on scatter signal intensity.
Methods: A beam blocker made of lead strips was mounted between the X-ray source and object for scatter estimation. In the traditional beam-blocker based scatter correction method, the scatter signal in the blocked region was multiplied by an amplification factor to directly represent the scatter in the unblocked region. The multiplied scatter signal was then subtracted from the total signal in the unblocked region to obtain the primary signal for CBCT reconstruction. The factor, however, must be optimized for individual patients, which is a tedious process and lacks objective stop criterion. In our proposed method, however, the primary signal calculated in the blocked region was used to derive the scatter signal in the unblocked region. The primary signal after scatter correction in the unblocked region was applied to CBCT reconstruction. The CBCT images corrected with the proposed and traditional method were compared by imaging a Catphan@600 phantom.
Results: Using a fan beam CT as a reference, the CT number error was 4±2 HU and 5±3 HU in the CBCT corrected with the proposed method and traditional method, respectively. The traditional method, however, required iterative optimization of the scatter multiplication factor which was not necessitated in the proposed method. Moreover, the stop criterion was also determined subjectively in the traditional method.
Conclusion: The proposed method effectively avoids the influence of the inserted beam blocker itself on the scatter intensity estimation, and proves a more practical and robust way for beam-blocker based scatter correction application.