SU-E-I-50: Investigation of the Effect of Anti-Scatter Grid Septa Size and Orientation On Artifacts with High Resolution Detectors When Decreased Pixel Size Approaches Septa Width

Abstract

Purpose: As x-ray detectors with greatly increased spatial resolution become available, their decreased pixel dimensions can approach the size of anti-scatter grid septa resulting in loss of primary x-ray detection and imaging artifacts. We investigate this effect and any potential advantage gained by rotating anti-scatter grids 45 degrees. Methods: We calculated the percent of pixel area unobstructed by grid septa for the maximum single pixel coverage of septa-pixel alignment for both 0 degree and 45 degree grid orientations for a broad range of ratios of pixel to septa sizes. We also calculated the percent benefit gained from a 45 degree grid orientation over the 0 degree orientation. Results: There is some benefit provided by the 45 degree orientation for pixel-to-septa ratios between 0.75 and 1.21 with a peak benefit of 8.6% at a ratio of 1.0 for exact alignment of the centers of the pixel and septa. For ratios below 0.75 total loss of data may occur regardless of grid orientation for some pixels. For ratios above 1.21, the 45 degree orientation corresponds increasingly to septa covering up a greater pixel area along the pixel diagonal. Large pixel-to-septa ratios for most current systems meet this criteria with flat panel detector pixels typically of 150 to 200 um and grid septa of 20 to 30 um implying that the 45 degree orientation is inappropriate. For new and proposed high resolution 25 to 35 um pixel detectors, septa coverage and the consequent image artifacts must be considered. Conclusion: The analysis of grid septa coverage is dependent on the pixel-to-septa ratio. Although some small benefit in using a 45 degree orientation for ratios near 1 is possible, other image processing and/or mechanical grid movement methods will have to be explored to minimize image artifacts resulting from grid usage with high resolution detectors. Support in part by NIH grant R01EB002873