Scatter correction for cone-beam computed tomography using moving blocker strips: a preliminary study

Abstract

Purpose: One well-recognized challenge of cone-beam computed tomography (CBCT) is the presence of scatter contamination within the projection images. Scatter degrades the CBCT image quality by decreasing the contrast, introducing shading artifacts, and leading to inaccuracies in the reconstructed CT number. The authors propose a blocker-based approach to simultaneously estimate scatter signal and reconstruct the complete volume within the field of view (FOV) from a single CBCT scan.

Methods: A physical strip attenuator (i.e., "blocker"), consisting of lead strips, is inserted between the x-ray source and the patient. The blocker moves back and forth along the z-axis during the gantry rotation. With such a design, the data required for the filtering step of the Feldkamp-Davis-Kress (FDK) algorithm are complete in the unblocked region and the entire volume within the FOV has the measurements at different projection views. The two-dimensional scatter fluence is estimated by interpolating the signal from the blocked regions. A modified FDK algorithm and an iterative reconstruction based on the constraint optimization are used to reconstruct CBCT images from unblocked projection data after the scatter signal is subtracted. A simulation study and an experimental study are performed to evaluate the performance of the proposed scatter correction scheme.

Results: The scatter-induced shading/cupping artifacts are substantially reduced in CBCT using the proposed strategy. In the simulation study, the mean relative error is reduced from 25% to 3% and 2% in the images reconstructed by the modified FDK and constraint optimization, respectively. In the experimental study using a CatPhan 600 phantom, CT number errors in the selected regions of interest are reduced from 256 to less than 20.

Conclusions: An effective scatter correction scheme is proposed for CBCT. A moving blocker consisting of lead strips is inserted between the x-ray source and the patient during CBCT acquisition. The proposed method allows the authors to simultaneously estimate the scatter signal in projection data, reduce the imaging dose, and obtain complete volumetric information within the FOV.