isometric scale

n.
  1. A radiopaque strip of metal calibrated in centimeters, placed between the buttocks of an individual to be x-rayed, used to measure anteroposterior diameters of the pelvis.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.