Exhibit Hall | Forum 8
Purpose: 2D antiscatter grids (2D-ASG) have been shown to improve both quantitative accuracy and low contrast visualization in CBCT. However, gantry flex may challenge suppression of 2D-ASGs shadows in projections using flat-field correction methods, which may lead to artifacts in CBCT images. Thus, we developed a novel model to study the effect of gantry flex on 2D-ASGs shadows, and its impact on CBCT image quality.
Methods: In a linac-mounted CBCT, gantry flex displaces 2D-ASG shadows in projections in subpixel range, which is challenging to characterize due to undersampling by detector pixels. Hence, two methods were developed to characterize subpixel displacement in grid shadows: 1) Displacement was characterized by co-registering projections in a CBCT scan using cross-correlation-based affine transform. 2) Grid shadow was first characterized by rearranging pixels in a pixel neighborhood, referred as Oversampled Shadow Profile (OSP). Subsequently, the change in grid shadow width, amplitude, and position was characterized by analyzing OSPs in consecutive projections of a CBCT scan. Finally, the effect of grid shadow displacement on CBCT image quality was evaluated in an analytical 2D-ASG simulation platform.
Results: Shadow displacement in a CBCT scan was up to 150µm and 100µm in transverse and axial directions. Displacement measurements using two methods agreed well. The difference in shadow displacement in two CBCT scans is only 5-10µm in both directions, indicating gantry flex characteristics are largely reproducible. However, 5-10µm displacement uncertainty was large enough to cause ring artifacts in CBCT simulations. Image noise due to artifacts increased from 3.9-8.8 HU to 6.2-14.6 HU, depending on grid pitch, septal thickness, and image spatial resolution.
Conclusion: Our approach successfully characterized the effect of gantry flex on 2D-ASG’s imaging performance. This approach can be used for the optimization of 2D-ASG hardware and CBCT system parameters to minimize 2D-ASG induced artifacts caused by gantry flex.
Funding Support, Disclosures, and Conflict of Interest: This work was funded in part by grants from NIH/NCI R21CA198462 and R01CA245270. No Conflict of Interest.