Observation of Shift Phenomena when Using 3T MRI Scanners in Stereotactic Radiosurgery

Background: 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 still imaging (mainly MRI). The new generation of MRI scanners, with field strengths up to 3.0 T, deliver promising image quality with regard to anatomical resolution and contrast, but critical features are the sensitivity to susceptibility and chemical shift effects which both create artifacts. Changes in the magnetic properties of the ferromagnetic plates can cause additional problems in the z direction. Methods: All sequences from all scanners to be investigated were defined automatically and manually (upper, middle and lower block) in light-guided plates. After the procedure of definition the geometric imaging quality of 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 were performed in three steps: (1) the evaluation of the magnitude of error was performed within transversal slices in two orientations (axial/coronal) 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, and (3) distortions caused by chemical shift and/or susceptibility effects were analyzed in a head-phantom. In-house developed software was used for data analyses. Results: For all scanners tested, the 3D-volume scan is the most sensitive to a z shift. For scan times > 10 min the shift can increase to several millimeters in the z direction. SE sequences are rather insensitive but show up with a small shift in the y direction. The 3.0-T MRI-Scanner was analyzed using sequences in axial and coronal orientations. The mean deviation was < 0.3 mm in the axial and < 0.4 mm in the coronal orientation. For the known targets the maximum deviation came up to 1.18 mm (far from the center). Due to inhomogeneities an additional shift in the z direction up to 1.5 mm was observed for a dataset which was shown compressed by 1.2 mm. By optimizing the parameters in the protocol these inaccuracies could be reduced to < 1.1 mm. Conclusions: The 3.0-T scanner showed sophisticated anatomical contrast and resolution in comparison to the established 1.0- 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. Long scan times especially for the 3D-volume sequences lead to a warming up and to a change in the magnetic properties of the ferromagnetic plates which cause an additional effect on the main-field B0 thus disturbing the z gradient.