Low cerebellar tonsils

• Protrusion of tonsils < 5 mm below foramen magnum
• Tonsils are rounded inferiorly
• Fourth ventricle is in normal position
• Twining’s line –  horizontal line  from tuberculum sella to internal occipital protuberance. This line should traverse midway through the fourth ventricle

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Elster, Allen D., and M. Y. Chen. “Chiari I malformations: clinical and radiologic reappraisal.” Radiology 183.2 (1992): 347-353.

Hofkes, Shawn K., et al. “Differentiation between symptomatic Chiari I malformation and asymptomatic tonsilar ectopia by using cerebrospinal fluid flow imaging: initial estimate of imaging accuracy.” Radiology 245.2 (2007): 532-540.