Instead of inserting a needle into the spine and taking an x-ray, the localizer hangs a theoretical plumb line through the target spine segment. The challenge is to make this virtual plumb line visible on x-ray. The two radio-opaque cables act as visible proxies for the plumb line on lateral radiography. The key to accuracy is to make sure that the x-ray source, the proximal cable, the target spine segment, the distal cable, and the x-ray film are all aligned. Misalignment of any of these components would result in erroneous estimation of the location of the plumb line. The lasers serve to align these components.

For a detailed description of the alignment errors that are avoided by use of the localizer, please view Narrated Slideshow #2.

Figure reproduced by permission from Journal of Neurosurgery: Spine.