Transcallosal approach to the third ventricle: Normative morphometric databased on magnetic resonance imaging scans, with special reference to the fornix and forniceal insertion

OBJECTIVE: The ability to visualize median-sagittal brain structures by mag netic resonance imaging (MRI) improves the planning for surgical removal of lesions located in and around the third ventricle. The transcallosal appro ach is the most appropriate path to the anterior part of the third ventricl e. The present study was undertaken to obtain normative morphometric data, derived from sagittal MRI scans, which are necessary for operation planning that takes into account the surgical microanatomy and landmarks encountere d during this approach. METHODS: The morphometric evaluation was performed on 72 median-sagittal MR I scans. The surface landmarks for the corridor were the two points, P5 and P7, located 5 and 7 cm anterior to the central sulcus, respectively. With these two points on the cortical surface as references, a variety of measur ements were made to provide quantitative information about distances betwee n brain structures encountered during the surgical approach. In addition, v arious parameters were determined to characterize the different shapes of t he fornix and the different types of forniceal insertion. RESULTS: The following measurements (means) were obtained: 1) the distance between P5/P7 and the cingulate sulcus was 25.76 mm (range, 17.113-42.73 mm ) with reference to P5, and 25.41 mm (range, 12.91-36.29 mm) with reference to P7; 2) the distance between the cingulate sulcus and the corpus callosu m was 12.91 mm (range, 7.19-22.60 mm) with reference to P5, and 12.92 mm (r ange, 6.75-23.37 mm) with reference to P7; 3) the height of the corpus call osum was 6.22 mm (range, 3.07-9.00 mm) with reference to P5, and 6.92 mm (r ange, 3.50-13.57 mm) with reference to P7; 4) the distance between the ante rior commissure and the foramen of Monro was 6.78 mm (range, 1.86-14.57 mm) , independent of P5 and P7; 5) the distance between the lower margin of the corpus callosum and the upper insertion point of the fornix was 12.44 mm ( range, 2.71-26.13 mm) with reference to P5, and 13.34 mm (range, 3.74-27.58 mm) with reference to P7; 6) the distance between the lower margin of the corpus callosum and the lower insertion point of the fornix was 18.08 mm (r ange, 9.47-29.71 mm) with reference to P5, and 18.58 mm (range, 10.48-30.40 mm) with reference to P7; and 7) the distance between the lower margin of the corpus callosum and the anterior commissure was 23.46 mm (range, 11.98- 32.70 mm) with reference to P5, and 22.89 mm (range, 11.05-33.04 mm) with r eference to P7. Four different insertion types between the fornix and the c orpus callosum were noted and classified. CONCLUSION: Morphometric data concerning the surrounding structures of the third ventricle have received very little attention in the literature. This morphometric study permitted definition of the surgical corridor to the th ird ventricle by preserving important anatomic structures such as the motor strip, genu of the corpus callosum, forniceal commissure (hippocampal comm issure), anterior commissure, and forniceal columns. The detailed morphomet ric data obtained on median-sagittal MRI scans of the brain structures invo lved in the transcallosal interforniceal and/or transcallosal transforamina l approach allow for exact planning of the surgical approach.