Specific Imaging Findings

Callosal dysgenesis (CD) is a spectrum of congenital defects of the corpus callosum (CC), which can be complete or partial. In complete CD, the CC, hippocampal commissure, and in half of the cases the anterior commissure are absent. In partial CD, the anterior commissure is always present as well as portions of CC. Multiplanar MR imaging is necessary to assess the CD abnormalities. With complete CC agenesis the morphology of the lateral ventricles is modified, as they become widely separated and lose the medially converging configuration, assuming an abnormal parallel appearance. The occipital horns are also enlarged. The interhemispheric fissure extends downwards to the roof of the third ventricle, which bulges upward. The midline cortical pattern is altered with the cingulate gyrus appearing absent and radially arrayed gyri converging to the roof of the third ventricle. The parietal and occipital sulci are shallow, and the hippocampi show round configuration. A parallel bundle of parasagittal white matter tracts is seen in the medial superior aspect of the lateral ventricles with relatively high T1 and low T2 signal intensity (bundle of Probst). On coronal images the constellation of findings leads to “trident” or “Viking helmet” appearance. Associated other abnormalities are frequently found with CD (in about three-quarters of cases), the most common ones being lipoma, interhemispheric cyst, and cortical dysplasia.