gms | German Medical Science

The inter-individual complexity of cortical geometry often impedes studies that involve comparison of functional loci in different individuals. The presentation of functional data in 2D slices or even rendered three-dimensionally is not possible for this task. Compensation strategies involve explicit surface reconstructions (e.g. extensively smoothed surfaces, spherical representations and cortical flat maps). Mapping the cortical surface onto a well-defined geometrical body permits a comparison of different brain surfaces by establishing a 2D coordinate system. In this study we used this technique to align different brain surfaces in order to establish a quality control system for motor functional MRI.

Functional MRI studies of 5 patients during standardised paradigms for hand-, foot-, and tongue movements were analysed with the Brain Voyager software tool (Brain Innovation, Maastricht, NL) on a standard PC. Statistical maps for the time course of activated voxels and corresponding 3D surfaces were made. Surfaces were reconstructed as native 3D surfaces, extremly smoothed surfaces, spheres and flat maps. After distortion correction, the spherical visualizations were standardized and matched.

The functional motor areas were clearly visible in each of the reconstructed surfaces. After alignment of the normalized spheres, an inter-individual comparison of the functional data sets was possible. By overlaying all data reconstructions, a group analysis of the functional loci reveals all the areas activated by the motor paradigm in each individual.

Morphing cortical surfaces to well-defined geometrical structures makes is possible for functional data sets to be displayed while surface topology is respected (in contrast to slice representations or flat maps). Orientation on the spherical reconstructions may be slightly cumbersome. The proposed approach allows for the creation of cortical surface mapping coordinates (“latitude and longitude” of spherical coordinates), the display of data of group analyses and permits “quality control” for the activation potential of different paradigms. Although distortions are unavoidable, they are not a major methodological drawback of the method.