Purpose: Primary transmission properties of 2D antiscatter grids (2D-ASG) play an important role on the image quality of CBCT images. To optimize 2D-ASG design and evaluate the effect of 2D-ASG’s primary transmission on image quality, a numerical 2D-ASG simulation platform was developed.
Methods: Our platform simulates 2D-ASG projections in a CBCT scan by accounting for system blurring, primary transmission through 2D-ASG via modeling of effective septal thickness and grid pitch, Poisson noise, electronic noise, and effects of gantry flex. System blurring was characterized by iteratively optimizing the system point spread function to match the image signal gradients in simulated and experimentally acquired projections by using 2D-ASG prototypes with different grid geometry parameters. Likewise, Poisson and electronic noise were optimized, such that simulated noise mimicked the noise properties of experimental CBCT images. Displacements in grid shadows due to gantry flex were characterized by measuring subpixel grid shadow displacement as a function of CBCT gantry angle.
Results: Effective septal thickness of 190µm and grid pitch of 1.2mm in 2D-ASG provided about 73% primary transmission, which was the main factor affecting low-contrast visualization. Larger grid pitches and wall thicknesses less than 150µm lead to primary transmission values beyond 85% and improvement in low-contrast visualization. Increased wall thickness, combined with low primary fluence in grid shadows, increases image noise, due to the prominence of electronic noise in grid shadows. Undersampling artifacts varied as a function of wall thickness and grid pitch. Gantry flex-induced ring artifacts also varied as a function of reduced grid pitch and increased wall thickness.
Conclusion: Proposed simulation platform allowed successful evaluation of CBCT image quality variations as a function of 2D-ASG primary transmission properties and CBCT system characteristics. This platform can be potentially used for optimizing 2D-ASG design properties based on the imaging task and properties of the CBCT system.
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.