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The importance of magnetic resonance imaging (MRI) for radiotherapy has increased steadily over the last few years. In contrast to CT images, MR images offer superior soft-tissue contrast, which is a decisive advantage, for example during treatment planning of brain tumors. While CT images provide geometrically exact images of the patient’s anatomy, there are many possible error sources that can influence the geometric accuracy of MRI images. These error sources are largely based on fundamental physical processes that are also responsible for the signal generation. Therefore, these errors can never be eliminated completely. However, errors can be reduced to a minimum by careful selection of the acquisition parameters. The first part of this thesis gives a detailed overview of the requirements for MRIs to be used for radiotherapy planning. In addition, parameters are introduced whose optimization results in a significant improvement in geometric accuracy. Also, the possible advantages and methods of generating synthetic CTs from MR images are briefly presented, thus creating a workflow based only on MR images. In the second part, a common source of error is examined in more detail, which is susceptibility-induced distortions. For this purpose, an additional sequence was used to measure the distortions in treatment planning MRIs for brain irradiation. These were evaluated for different organs at risk (OARs). The influence of magnetic field strength and type of shimming on the distortions was also investigated. Based on the distortions found, recommendations for the creation of planning organ at risk volumes (PRVs) are developed. In the last part, a guide for the implementation of a dedicated radiotherapy treatment planning MRI is given. Optimized protocols for pelvis and brain measurements as well as optimized positioning and coil setups are presented.

Due to its superior soft tissue contrast, magnetic resonance imaging (MRI) is essential for many radiotherapy treatment indications. This is especially true for treatment planning in intracranial tumors, where MRI has a long-standing history for target delineation in clinical practice. Despite its routine use, care has to be taken when selecting and acquiring MRI studies for the purpose of radiotherapy treatment planning. Requirements on MRI are particularly demanding for intracranial stereotactic radiotherapy, where accurate imaging has a critical role in treatment success. However, MR images acquired for routine radiological assessment are frequently unsuitable for high-precision stereotactic radiotherapy as the requirements for imaging are significantly different for radiotherapy planning and diagnostic radiology. To assure that optimal imaging is used for treatment planning, the radiation oncologist needs proper knowledge of the most important requirements concerning the use of MRI in brain stereotactic radiotherapy. In the present review, we summarize and discuss the most relevant issues when using MR images for target volume delineation in intracranial stereotactic radiotherapy.