Computed Tomography-Magnetic Resonance Image Registration in Radiotherapy Treatment Planning

Magnetic resonance imaging (MRI) is being increasingly used in radiotherapy treatment planning (RTP). MRI has the potential to provide improved localisation of target volumes, leading to better tumour control rates and reduced normal tissue complications, due to capabilities including excellent soft-tissue discrimination and the ability to provide scans in which the image contrast is weighted according to different tissue properties. When computed tomography (CT)-MRI image registration is deployed, MR's advantages are combined with CT's geometrical security and its ability to provide electron density information. The quality of CT-MRI image registration can be favourably influenced by aspects of scan acquisition, including patient positioning/immobilisation and scan protocols. Appropriate protocols can ameliorate the possible presence of MR spatial distortions and other artefacts, but quality assurance of scanning remains essential. Here, the methods and quality assurance of CT-MR image registration are discussed. Developments in MRI scanner technology are progressively offering advantages for RTP, in terms of the possibility of better matching of patient positioning versus CT in a greater range of anatomical regions, while allowing thinner slices for better image quality in reformatted orthogonal planes.