Vasospasm after SAH: typically appears at 4-14 days, peaks at 7-10 days.

Vestibular aquaduct enlargement: 1.5 mm midpoint on axial image.

Proptosis: <6 mm in distance between posterior sclera to the interzygomatic line at the level where both lenses are present. Interzygomatic line 0-6 mm anterior to the posterior sclera, mild. Interzygomatic line at the posterior sclera, moderate. Interzygomatic line behind the posterior sclera, severe.

Silicone oil density within posterior segment of eye can be above or below 100HU. Bleeding always less than 100HU. Silicone oil extending into the anterior segment, especially the posterior chamber of the anterior segment can cause glaucoma which is an emergency.

Pituitary stalk thickness: >4 mm or more than the width of basilar artery–abnormal. >2 mm at insertion–abnormal. Loss of tapering–abnormal.

Pituitary gland height normal limit: 6 mm for children, 8 mm for men and postmenopausal women, 10 mm for reproductive age women, 12 mm for women during pregnancy or lactation. Pituitary should be convex upper margin for children and older adult men.

Chiari I: Normal distance that the tonsils can protrude below the margins of foreman magnum. (above which Chiari I malformation is suspected): 0-10 years of age: 6 mm. 10-30 year of age: 5 mm. 30-80 years of age: 4 mm. >80 years of age: 3 mm.

Huntington’s disease: The frontal horn distance/caudate head distance: Normal is 2.2-2.6. Abnormal is defined as <2.2 due to atrophy of caudate heads.

Basilar invagination: Tip of dens above basion to opisthion.

Basilar impression: Tip of dens 3 mm above posterior hard plate to opisthion. or 4.5 mm above posterior hard plate to inferior occiput.

Platybasia : Flattening of skull base. Normal 125-143 degree. Platybasia >143, basilar kyphosis <125 (The angle is from two lines, one connects the nasion to the center of sella, one connects the basion to the center of sella)

Craniocervical junction injury: Basion dens interval <12 mm. Altanto-dens interval <3 mm in adults, <5 mm in kids. Anterior arch Jefferson fracture <7 mm wide. Above these value, think ligamental injury.

Flexion and extension of cervical spine: 30 degree from neutral position to be adequate study. Listhesis of 3.5 mm each way to define instability.

Normal IAC width: 2-8 mm. <2 mm stenotic.