Analyzing 3.0T MRI-scanners for implementation in radiosurgery

Introduction: Given the high mechanical accuracy of the Leksell Gamma Knife', the most sensitive technical factor having an influence on the overall precision of radiosurgery is the imaging (mainly magnetic resonance imaging (MRI)) study. The new generation of MRI-Scanners with field strength up to 3.0 T deliver promising image quality concerning anatomical resolution and contrast but critical features are the sensitivity to susceptibility and chemical shift effects both creating artefacts. Methods: The 3.0 T MRI-Scanner (Siemens 'Trio') was analyzed and compared to a Siemens Magnetom 'Expert' 1.0 T and to a Philips 'Gyroscan' 1.5 T. The analyses was performed in three steps: (1) The evaluation of the magnitude of error was performed within transversal slices in two orientations (axial/coronal) by using a cylindrical phantom with an embedded grid. (2) The deviations were determined for 21 targets in a slab phantom with known geometrical positions within the stereotactic frame. (3) Distortions caused by chemical shift and/or susceptibility effects were analyzed in a head-phantom. An in-house developed software was used for data analyses. Results: The 3.0 T MRI-Scanner was analyzed using sequences in axial and coronal orientations. The mean deviation was < 0.3 mm in axial and < 0.4 mm in coronal orientation. For the known targets the maximum deviation came up to 1.16 mm (far from the center). Due to inhomogeneities a shift in z-direction up to 1.5 mm was observed for a data set which was shown compressed by 1.2 mm. By optimizing these parameters in the protocol these inaccuracies could be reduced to < 1. 1 mm. Conclusions: The scanner showed up with sophisticating anatomical contrast and resolution in comparison to the established 1.0 T and 1.5 T scanners. However, due to the high-field strength the field within the head coil is very sensitive to inhomogeneities and therefore 3.0 T imaging data still have to be handled with care. Copyright (c) 2006 S. Karger AG, Basel.