ANALYSIS AND CORRECTION OF GEOMETRIC DISTORTIONS IN 1.5-T MAGNETIC-RESONANCE IMAGES FOR USE IN RADIOTHERAPY TREATMENT PLANNING

The aim of this study is to investigate and correct for machine- and o bject-related distortions in magnetic resonance images for use in radi otherapy treatment planning. Patients with brain tumours underwent mag netic resonance imaging (MRI) in the radiotherapy position with the he ad fixed by a plastic cast in a Perspex localization frame. The imagin g experiments were performed on a 1.5 T whole body MRI scanner with 3 mT m(-1) maximum gradient capability. Image distortions, caused by sta tic magnetic field inhomogeneity, were studied by varying the directio n of the read-out gradient. For purposes of accuracy assessment, exter nal and internal landmarks were indicated. Tubes attached to the cast and in the localization frame served as external landmarks. In the mid sagittal plane the brain-sinus sphenoidalis interface, the pituitary g land-sinus sphenoidalis interface, the sphenoid bone and the corpora o f the cervical vertebra served as internal landmarks. Landmark displac ements as observed in the reversed read-out gradient experiments were analysed with respect to the contributions of machine-related static m agnetic field inhomogeneity and susceptibility and chemical shift arti facts. The machine-related static magnetic field inhomogeneity in the midsagittal plane was determined from measurements on a grid phantom. Distortions due to chemical shift effects were estimated for bone marr ow containing structures such as the sphenoid bone and the corpora of the cervical vertebra using the values obtained from the literature. S usceptibility-induced magnetic field perturbations are caused by the p atient and the localization frame. Magnetic field perturbations were c alculated for a typical patient dataset. The midsagittal head image wa s converted into a susceptibility distribution by segmenting the image into water-equivalent tissues and air; also the Perspex localization frame was included in the susceptibility distribution. Given the susce ptibility distribution, the magnetic field was calculated by numerical ly solving the Maxwell equations for a magnetostatic held. Results wer e shown as magnetic field perturbations and corresponding spatial dist ortions of internal and external landmarks. The midsagittal head image s were corrected for the machine imperfections (gradient non-linearity and static magnetic field inhomogeneity). The locations of the extern al landmarks in the frame were also corrected for susceptibility artif acts. The efficacy of the corrections was evaluated for these external landmarks in the localization frame with known geometry. In this stud y at 1.5 T with read-out gradient strength of 3 mT m(-1), machine-rela ted, chemical shift and susceptibility-induced static magnetic field i nhomogeneity were of the same order, resulting in spatial distortions between -2 and 2 mm with only negative values for the chemical shift e ffect. Both the patient and the localization frame proved to perturb t he magnetic field. The field perturbations were shown to be additive. In total, static magnetic held inhomogeneity led to spatial distortion s ranging from -2 to 4 mm in the direction of the read-out gradient. N on-linearity of the gradients resulted in spatial distortions ranging from -3.5 to 0.5 mm. After correction for the machine imperfections an d susceptibility artifacts, the geometric accuracy of the landmarks in the localization frame was better than 1.3 mm.