The bimastoid line has been described and used to evaluate basilar invagination on frontal skull plain film and coronal reconstructed CT image.
The bimastoid line is drawn between the inferior tips of the of mastoid processes bilaterally. The tip of the odontoid process of C2 normally projects less than or equal to 10 mm above this line. Basilar invagination is present when the tip of the odontoid process projects above 10 mm.
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