Metal Artifact Reduction (MAR) in Image-Guided Radiation Therapy (IGRT)

• Applicant: Professor Dr. Marc Kachelrieß
• Subject Area: Medical Physics, Biomedical Technology
• Term: from 2014 to 2018
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 250538804

Image quality in x-ray computed tomography (CT) often suffers from artifacts arising from metal implants. For example these artifacts can be due to prostheses, due to dental fillings, due to fixations, clips or markers. The metal artifacts may impair diagnosis or they may impair quantitative evaluations of the images such as it is required for radiation treatment planning. The type and intensity of the metal artifacts depends on the type, size, density, position and number of the metal implants, and on the scan parameters of the CT scan. Additional factors such as intended or unintended patient motion play a significant role, too. This particularly applies for image-guided radiation therapy (IGRT) where patient data are acquired under free breathing using flat detector cone-beam CT technology which is much more prone to artifacts than diagnostic CT systems. Current methods are not able to reduce or remove such moving metal artifacts, as they arise in IGRT.Several methods for metal artifact reduction (MAR) are described in the literature. Some of them replace the projection values of rays that run through a metal object by some surrogate information. Others are not based on such inpainting techniques but rather use the information in the metal shadow and a physical model of the measurement process to come up with corrected rawdata. In most cases MAR is dedicated to clinical (diagnostic) CT. Less work has been published dedicated to flat detector CT systems, although those are much more sensitive to scattered radiation and therefore to metal artifacts. Approaches dedicated to image-guided radiation therapy that cope with patient motion are not known, apart from one manual method, although IGRT has to fulfill special requirements to be able to adapt the treatment plan to the current patient situation. On the one hand image quality should be high to be able to reliably delineate the tumor margins. In case of the prostate, for example, surrounding hip prostheses may generate metal artifacts that extend into the target volume and that potentially prevent an accurate adaptation of the treatment plan. In other cases tumors are marked with metal markers that also generate metal artifacts. On the other hand free patient breathing is typical for those scans. Patient motion, however, changes or amplifies the metal artifacts and blurs the metal object. This blurring, however, may prevent a reliable segmentation of the implants but this segmentation is a prerequisite for metal artifact reduction.This project aims at developing a dedicated MAR algorithm for image-guided radiation therapy. We want to consider the particular properties of flat detectors on the one hand, and want to develop a dedicated motion compensation approach that allows to apply MAR also to the case of moving metal. The feasibility shall be demonstrated using simulations, phantom measurements, and patient measurements.

• DFG Programme: Research Grants