IMAGING FINDINGS The sagittal T1weighted MRI image (1A) reveals agenesis of the corpus callosum (ACC). This is the largest white matter commissure in the brain, lying in the midline, where it connects the two cerebral hemispheres (1E, arrow 1). Normal development of the corpus callosum is associated with inversion of the cingulate gyrus forming the cingulate sulcus, which parallels the corpus callosum superiorly (1E, arrow 2). Thus, in ACC, the cingulate sulcus is absent, causing the medial hemispheric sulci to radiate into the roof of the 3rd ventricle (1A, arrow). The axial T1-weighted MRI (1B), reveals the resultant high-riding 3rd ventricle extending superiorly between parallel lateral ventricles (arrow). The trigones and occipital horns of the lateral ventricles are often dilated in ACC, termed colpocephaly (i).