Room: Room 205
Purpose: Spatial resolution in MDCT varies with image location in both the azimuthal and radial directions. Although the root causes of the image quality (IQ) variation have been previously described, a theoretical framework that describes how each physical factor degrades IQ across the image volume has been missing. The purpose of this work was to investigate one of these factors, the impact of the distortion of the focal spot shape across fan angles on radial resolution variation across the image volume.
Methods: First, an analytic model was developed to quantify the relationship between the focal spot shape seen at the iso-center and the effective focal spot shape realized at a given image location. The physical factors under investigation include the focal spot size and shape in two spatial directions, i.e., X and Z, as well as the fan angle and target angle. The derived model was validated using a vendor-developed numerical simulation package developed to simulate CT scanners. This simulation tool allows for independent measurement of each free parameter in the model, which is otherwise not possible on a physical system.
Results: The measured effective focal spot profiles matched the modeled profiles closely across varied focal spot widths, focal spot lengths, fan angles, and target angles. Increasing focal spot length, typically associated with degrading cross-plane resolution only, is shown to also degrade in-plane resolution at fan angles greater than zero degrees.
Conclusion: In this work, a quantitative model to describe the relationship between the variations of the radial resolution due to focal spot shape distortion was developed and then validated with numerical simulation studies. The developed model provides a quantitative tool to understand the variation of off-isocenter resolution degradation and this quantitative understanding offers a new framework to improve clinical scan protocol optimization or simplify development of correction algorithms.
Funding Support, Disclosures, and Conflict of Interest: Research funded by GE Healthcare.