Abstract
Introduction: Measurements taken from digital radiographs and corrected for the labelled scale do not correspond to actual clinical dimensions accurately.
Methods: The routine radiographs taken of 30 hip hemiarthoplasty patients were used to make measurements in the region of the hip. 15 patients were compared in two hospitals using different digital imaging processing. The images were scaled to 100% (actual size), three blinded measurements were taken of the prosthetic femoral head and the average calculated. These results were compared to the actual size of the implanted prosthesis. Further analysis was carried out to determine true magnification.
Results: A consistent difference was observed between measured dimensions at 100% scale and known actual prosthesis dimensions. It was identified that the magnification factor ranged from 110 to 115% with at average magnification factor of 113%. No significant difference was observed between the two hospital systems.
Discussion: Many factors influence radiographic magnification, including variations in distances from x-ray source to patient and from x-ray source to x-ray plate. Angulation and degree of divergence of the x-ray beam and rotation of a limb also influence the magnification which occurs. Accurate measurements can only be taken from digital x-rays if appropriate calibration is performed.
Correspondence should be addressed to Mr Richard Wallace at Musgrave Park Hospital, 20 Stockman’s Lane, Belfast BT9 7JB, Northern Ireland.
