Magnetic resonance imaging (MRI): considerations and applications in radiotherapy treatment planning

The emerging utilisation of conformal radiotherapy (RT) planning requires sophisticated imaging modalities. Magnetic resonance imaging (MRI) has introduced several added imaging benefits that may confer an advantage over the use of computed tomography (CT) in RT planning such as improved soft tissue definition, unrestricted multiplannar and volumetric imaging as well as physiological and biochemical information with magnetic resonance (MR) angiography and spectroscopy. However, MRI has not yet seriously challenged CT for RT planning in most sites. The reasons for this include: (1) the poor imaging of bone and the lack of electron density information from MRI required for dosimetry calculations; (2) the presence of intrinsic system-related and object-induced MR image distortions; (3) the paucity of widely available computer software to accurately and reliably integrate and manipulate MR images within existing RT planning systems. In this review, the basic principals of MRI with its present potential and limitations for RT planning as well as possible solutions will be examined. Methods of MRI data acquisition and processing including image segmentation and registration to allow its application in RT planning will be discussed. Despite the difficulties listed, MRI has complemented CT-based RT planning and in some regions of the body especially the brain, it has been used alone with some success. Recent work with doped gel compounds allow the MRI mapping of dose distributions thus potentially providing a quality assurance tool and in a manner analogous to CT, the production of dose-response information in the form of dose volume histograms. However, despite the promise of MRI, much development research remains before its full potential and cost-effectiveness can be assessed.