STUDIES OF THE ANGULAR REPRODUCIBILITY OF POSITIONING PATIENTS ADJACENT TO AN X-RAY TUBE .1. STENT-ROD BASED AND EXTRAORAL SYSTEMS

The usefulness of subtraction radiography for detecting small changes in crestal bone is dependent upon achieving reproducible geometry betw een x-ray beam and patient structures when serial radiographs are take n. This study evaluates 2 methods currently employed to maintain geome tric correspondence: 1) a stent-based system which rigidly fixes a cus tom-made stent to the x-ray tube by the use of a rod and 2) an extra-o ral system which positions a patient in the x-ray unit by means of ear rods. The projection of a light beam from a fixed subject reference w as used to measure the change of the orientation of the reference at 2 different measurement times. The rod-stent system was able to maintai n a discrepancy of less than 2 degrees 75% of the time over a time per iod of 6 months. For the extra-oral system this ranged from 72% to 92% during a 1-month period.