The presence of the molar tooth sign on axial MRI through the malformed pontomesencephalic junction (isthmus). The molar tooth sign consists of the following triad: (1) deepening of the interpeduncular fossa (*), (2) thick and straight superior cerebellar peduncles (arrows), and (3) hypoplastic vermis.

Molar tooth sign on axial T1-weighted section through the superior cerebellar peduncles. Joubert syndrome is characterized by two features in particular: enlarged superior cerebellar peduncles that do not decussate in the isthmus portion of the brain stem and cerebellar vermis dysplasia. This axial section illustrates the thickened superior cerebellar peduncles (arrows). Since they do not decussate, isthmus is then reduced and the interpeduncular fossa is secondarily enlarged (*).