Figure.

Common Data Element: measurement of cerebellar tonsil position. CDE instructions are as follows: Define foramen magnum using midline image. Draw line from basion to opisthion. Define basion as the confluence of cortical bone (most inferior point) making up the anterior foramen magnum. Define opisthion as the confluence of cortical bone (most inferior) of the posterior aspect of the foramen magnum (ventral most point). Measure tonsillar position perpendicular to that line. Tonsillar measurement may be taken parasagittally to assure the largest possible number is recorded. Record negative number for tonsil position above line, based onT1 sagittal or coronal MRI. Recognizing the tonsils is not normally midline structure; we define tonsillar position by a midline or near-midline image because they are displaced.