Figure 3.

Manual calculation of target coordinates from axial T2 weighted image with the Leksell frame. The outer fiducials of the lateral plates form a rectangle; the diagonals of this rectangle intersect at the centre of the frame. The geometry of the frame dictates that the Z (vertical) coordinate can be determined by adding 40 mm to the distance between the posterior and middle fiducial markers on the lateral fiducial plates. X (mediolateral) and Y (anteroposterior) axes are defined by lines drawn through the frame centre and parallel to the sides of the rectangle. The geometry of the frame dictates that centre of the frame corresponds to X = 100 mm and Y = 100 mm. The X and Y coordinate of any point within the image can therefore be calculated by measuring its displacement from the centre of the frame. Lateral displacements to the left are added and displacements to the right subtracted from 100 to give the X coordinate. Anterior displacements are added to 100 and posterior displacements subtracted from 100 to give the Y coordinate. In this example, a target was selected in the right STN: Z coordinate = 40 + (Z1 + Z2)/2 mm; X coordinate = (100 − X1) mm, since the target lies to the right of the frame centre. Y coordinate = (100 + Y1) mm since the target lies anterior to the frame centre