The Parasagittal Line: An Anatomic Landmark for Axial Imaging

No validated imaging landmark exists for characterizing the medial-lateral position of abnormalities at the high convexity-parasagittal region. Our understanding of the courses and deflections of the upper cerebral sulci is limited. Our purpose, therefore, was to define a frontooccipital line with reproducible anatomic relations to the upper cerebral gyri and sulci and to validate that line for use as an anatomic landmark by specific analysis of the gyral-sulcal relationships along it. In 100 subjects of all ages, the gyri and sulci visualized on serial axial CT sections of the upper brain were traced onto a single flat surface to delineate the anatomic relationships among the midline interhemispheric fissure, the paramedian superior frontal sulci (SFS) and intraoccipital sulci (IOS), the medial surface sulci, the high convexity sulci, and the inner table of the skull. These tracings provided a template for drawing a straight, best-fit parasagittal line from the SFS to the IOS and for assessing how reproducibly key anatomic structures align along the parasagittal line. To assure the applicability of the line to MR imaging, selected relationships were retested on serial axial MR sections in the same subjects. The parasagittal line could be drawn in each case and showed reproducible alignment with the SFS, hand-motor area, partes marginales, pars deflections, postcentral "parentheses," distal intraparietal sulci, and IOS. In supraventricular sections, the parasagittal line separated the sulci arising along the medial surface from those arising along the convexity. Because the anatomic relationships of the parasagittal line are reproducible, it may serve as a reference line or landmark. The tendency of this line to demarcate medial sulci from convexity sulci suggests immediate application to the definition of vascular territories and vascular watersheds, a topic under active investigation.