A theoretical model for respiratory motion artifacts in free-breathing CT scans

Successful radiotherapy treatment depends heavily upon the accuracy of patient geometry captured during treatment simulation using computed tomography (CT) scans. Radiotherapy patients are often scanned under free breathing, and respiratory motion can cause severe artifacts in CT scans, including shortening, elongation or splitting of the shapes and shifting of the midpoint positions of the tumor and organs. This paper presents a theoretical model that explains the source of motion artifacts and the relationship between motion artifacts and the motion parameters of the scanner, treatment couch and tumor/organ. It is shown that an understanding of the relationship between the translational table velocity and the maximum tumor/organ velocity might enable one to mitigate certain types of motion artifacts. We show that splitting artifacts can be eliminated if the scanning speed is above the maximum tumor/organ velocity. Slow scanning speeds are shown to be useful for obtaining accurate internal target volumes (ITVs), and fast scanning speeds are shown to be useful for obtaining accurate tumor/organ shapes. In both cases, an upper bound on the maximum possible error is calculated as a function of the scanning speed. A set of special scanning speeds which allow for an accurate representation of tumor/organ length along the craniocaudal direction is obtained, and a relationship between the maximum displacement of a tumor/organ image's midpoint position and the magnitude of its length distortion is derived.