Real-time distance measurement between the needle tip and target with PlusToolkit and 3D Slicer

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You can very easily compute distance between two fiducial points in Python, but if you want to display distance between two points in the viewer, like a ruler, then it is probably better to use the annotation ruler. Click on the small down-arrow button next to the fiducial place icon on the toolbar and select placement of ruler, then click on the two points in the image.