- 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
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.