Fig 1.

The idealized model of a normal subarachnoid space (A) and a patient with a Chiari I malformation (B). Each model is shown with a comparable sagittal MR image. For the normal model, the sagittal plane is in the midline. For the tonsillar herniation case, it is paramedian and through the nearest tonsil (to show the extent of tonsillar herniation). Blue is used for the CSF in the model. The white reference line indicates comparable levels in the models and MR images, while red lines show the level of model extension cranially and caudally. Anterior is to the reader's left, and posterior is to the right.