The representation of patterns of structural brain asymmetry in normal individuals

Hemispheric asymmetries were examined in 20 normal individuals who underwent MR scanning. The asymmetry indices were represented for a series of contiguous, consecutive coronal images through the occipital and frontal lobes, along the dorso-ventral [DV] axis. A monotonic decrease in size of the parieto-occipital hemispheric areal asymmetry was observed, from a marked left predominance to a greater degree of symmetry. When frontal and occipital lobe asymmetries were compared, the familiar frontal-right vs occipital-left counterclockwise torque was observed, but was found to be more prominent in the dorsal aspect of the frontal cuts and the ventral aspect of the occipital cuts. At the individual level, a within-subject linear model for predicting cerebral area as a function of cut, hemisphere and DV, revealed this marked DV effect. The use of the within-subject linear model further enabled the individual assessment of structural asymmetries, and provided criteria by which subjects could be grouped. We further show how a deviation of the occipital falx will engender different hemispheric asymmetries, depending on the orientation of the coronal imaging plane. Given the latter, we recommend planning the coronal cuts based on a low-ventricular transaxial scout, wherein the deviated falx, if present, can be taken into account.