POSTPROCESSING OF FUNCTIONAL MRI DATA OF MOTOR CORTEX STIMULATION MEASURED WITH A STANDARD 1.5 T IMAGER

Functional magnetic resonance imaging (fMRI) is usually based on acqui sition of alternating series of images under rest and an activation ta sk (stimulus). Brain activation maps can be generated from fMRI data s ets by applying several mathematical methods. Two methods of image pos tprocessing have been compared: (i) simple difference of mean values b etween rest and stimulation, and (ii) Student's t-test. The comparison shows that the difference method is very sensitive to arbitrary signa l fluctuations as seen mainly in large vessels (e.g., in the sagittal sinus), leading to insignificantly activated spots in brain activation maps. In contrary, Student's t-test maps show strongly reduced sensit ivity for fluctuations and have the advantage of giving activation thr esholds by setting significance levels. This allows the comparison of activation strength between patient collectives by using a grid overla y technique leading to an observer independent quantification of the s timulation effects. The method was able to reproduce previous findings of activation differences between healthy volunteers and schizophreni c patients. Moreover, a simple algorithm for the correction of slight head movements during the functional imaging task is presented. The al gorithm is based on shifting the fMRI data set relative to a reference image by maximizing the linear correlation coefficients. This leads t o a further reduction of insignificant brain activation and to an impr ovement in brain activation map quality.