Studies of the angular reproducibility of positioning patients adjacent to an x‐ray tube: 1. Stent‐rod based and extra‐oral systems

The usefulness of subtraction radiography for detecting small changes in crestal bone is dependent upon achieving reproducible geometry between x‐ray beam and patient structures when serial radiographs are taken. This study evaluates 2 methods currently employed to maintain geometric correspondence: 1) a stent‐based system which rigidly fixes a custom‐made stent to the x‐ray tube by the use of a rod and 2) an extra‐oral 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 was used to measure the change of the orientation of the reference at 2 different measurement times. The rod‐stent system was able to maintain a discrepancy of less than 2 75% of the time over a time period of 6 months. For the extra‐oral system this ranged from 72% to 92% during a 1‐month period.