Purpose: To review the application of MRI images in the radiation treatment planning,to discuss the advantages and disadvantages of MR imaging with respect to treatment planning, and to investigate the geometric distortion.
Methods: Humanoid therapy phantom was used for MRI and CT scanning, and distances between markers inside and on the surface of the phantom were measured in order to quantify the geometric distortion. The procedure of MRI/CT image fusion, which makes it possible to use the data of both imaging modalities for treatment planning, was described.
Results: At small volumes (head phantom) the geometric distortion was negligible (<2 mm), but at large volumes (eg. pelvis) remarkable geometric inaccuracies were observed. For example, the width of the pelvis measured in the MRI images was 7 mm less than the real distance, which corresponds to 2% inaccuracy. Geometric distortion was observed not only in the axial, but also in the sagittal and coronal planes. We have found that the geometric error increases with the distance measured from the magnetic isocenter. When the geometric distortion is not significant, the MRI/CT image fusion can be carried out reliably with the use of surface markers.
Conclusions: At small volumes the MRI images can be used for treatment planning after their fusion with CT images. At larger volumes the geometric distortion without any correction may preclude the MRI images from using them in the treatment planning. A detailed assessment of geometric distortion must be carried out before the introduction of MRI images into the radiation treatment planning.