Aspects of MR image distortions in radiotherapy treatment planning

Background: Registration of computed tomography (CT) and magnetic resonance (MR) images are commonly performed to define the different target regions used in radiotherapy treatment planning (RTTP). The accuracy of target defi nition will then depend on the spatial accuracy of the CT and MR data, and on the technique used to register the images. CT images are usually regarde d as geometrically correct, while MR images are known to suffer from geomet ric distortion. The aim of this paper is to discuss the possible impact of MR image distortions in the radiotherapy treatment planning process. Methods: The origin, magnitude, and relative impact of the different source s of geometric distortions that affect the MR image data at different magne tic fields and for different acquisition settings are described. Techniques for distortion correction are reviewed, and their Limitations are outlined . The sensitivity of image registration techniques to the presence of geome tric distortions in the MR data is discussed. Finally, art overview of imag e registration techniques used and results obtained in clinical radiotherap y treatment planning applications is given. Results: Spatial distortions in MR images vary with field strength and with the image acquisition protocol. The spatial accuracy generally decreases w ith distance from the magnet isocenter. Distortion correction techniques ba sed on phantom evaluations cannot adequately model patient-induced distorti ons. Conclusion: Image protocols with high gradient bandwidths should be used to reduce the spatial distortions in MR images. Correction techniques based o nly on phantom measurements could be sufficient at Low magnetic fields, whi le at higher fields additional corrections of patient-related distortions m ight be needed. Registration techniques based on matching of Landmark point s Located far from the magnet isocenter are especially prone to MR distorti ons.