• There can be apparent superior migration of the dens to the foramen magnum.

• It's apparent because it's due to cranial settling with the skull base moving down rather than the odontoid moving up.

• To assess this on imaging, there are a number of lines that can be used.

• However McRae's line is relatively easy to perform.

• McRae's line is a line drawn from the anterior to the posterior margin of the foramen magnum on the lateral x-ray or in the mid sagittal plane on CT or MRI. 

• McRae's Line is considered abnormal, and there is basilar invagination,  if the tip of the odontoid extends above this line.

Tip of the odontoid process (Pink arrow) lies above just above McRae's line (Orange dashed line).