Apparent cerebellar ectopia: a reappraisal using volumetric MRI.

Cerebellar ectopia, once considered rare, is apparently extremely common on magnetic resonance imaging (MRI). However, assessment of cerebellar position relative to the foramen magnum is usually made from thick sagittal sections and therefore may not be as reliable as sometimes believed. Volumetric MRI, acquired in the coronal plane with section thickness 1.5 mm, was used to assess the position of the cerebellum in 144 subjects who also had two-dimensional sagittal imaging using 5.0 mm thick sections. On these images, the frequency of cerebellar ectopia appeared to be 19% (27 cases), but volumetric coronal images indicated that the biventral lobules protruded below the foramen magnum in only 3 (2%), and the cerebellar tonsils lay well above in all cases. Hence the actual frequency of cerebellar tonsillar ectopia in this sample was zero. We concluded that cerebellar ectopia really is rare.