Navigation error of update navigation system based on intraoperative MRI

A navigation system based on an intra-operative magnetic resonance imaging (iMRI) is more accurate system compared to a conventional navigation system based on the preoperative images, because an error caused by the intra-operative brain deformation such as a brain shift can be eliminated in the new system. We here present the study about the accuracy of the updated navigation (UN) based on the iMRI. The UN system was installed in the navigation system for the intra-operative irradiation (PRS navigator). The accuracy was estimated retrospectively in 47 cases (94 times) by the assessment of the errors with least-squares method. The maximum errors were more than 5 mm in three cases whose iMR images were distorted. Navigational errors were also compared between two types of the developed fiducial markers. The UN by the new markers whose markers for iMRI and markers for the registration were different showed significantly less errors (maximum 2.36 mm, mean 1.16 mm) compared to the UN by the previous markers (maximum 4.65 mm, mean 1.42 mm). In conclusion, the accuracy of the UN depends not only on the internal error, but also on the image quality including the distortion and the fiducial markers.