Fig. 1

Topographic measurement of the beta zone. In the figure, point O is the center of the disc, and point A and B is the vertices of the beta zone, the point M and C is the maximum radial extent on the beta zone margin and disc margin. The beta zone is the area inside the white line, circumferential angle of the beta zone was defined as the angle between point A and B (angle 1), location of the beta zone was defined as the angle between maximum radial extent and vertical line from the center point of disc (angle 2), distance from C to M was defined as maximum radial extent of the beta zone