You can measure the distance between a manually drawn symmetry line and a visible midline shift for brain images.
1. From the Viewer, select an image.
2. In the Graphics tab of the toolbar, click Midline Shift 
in the Neural ribbon.
3. Click on the image at the start point and then click again at the end point of the symmetry line. The symmetry line is automatically drawn between these two points.
4. Click the apparent location of the midline shift. The distance between the symmetry line and this point is measured and displayed on the image.
Tilt the line by clicking and dragging either end.