Fossa posterior measurements: significance of sella to floor of 4th ventricle measurements (normal position of floor of 4th ventricle).

Two proportional methods are introduced to determine the normal position of the floor of the 4th ventricle expressed by two preventricular ratios: (see article); where ds = dorsum sellae, Ts = tuberculum sellae and Tw = Twining's line. T4 is the intersection between Tw and the floor of the 4th ventricle. The methods give information of the position of the 4th ventricle, the diameter of pons and the a-p diameter of the pituitary fossa.