The stack ruler shows where the current image is located with respect to other images in the series. This is most useful for tomographic series which do not include relative position information for localizer lines, such as breast tomography series.
The stack ruler appears as an overlay along the edge of the image frame and shows the relative position of the currently displayed image within the series. The ruler displays the current image number and the total number of images in the stack.
Toggle the stack ruler overlay on and off as follows:
Select an image frame containing a series consisting of one or more images.
From the Presentation toolbar, click the Show Stack
Ruler button, 
. The stack ruler appears in
the default location on the left side of the image.
To reposition the stack ruler to the other side of the image frame, press the Show Stack Ruler button again.
To hide the stack ruler, continue pressing the Show Stack Ruler button until the overlay disappears from the image frame.