Digital image analysis: improving accuracy and reproducibility of radiographic measurement

Objective. To assess the accuracy and reproducibility of a digital image analyser and the human eye, in measuring radiographic dimensions. Design. We experimentally compared radiographic measurement using either an image analyser system or the human eye with digital caliper. Background. The assessment of total hip arthroplasty wear from radiographs relies on both the accuracy of radiographic images and the accuracy of radiographic measurement. Methods. Radiographs were taken of a slip gauge (30 ± 0.00036 mm) and slip gauge with a femoral stem. The projected dimensions of the radiographic images were calculated by trigonometry. The radiographic dimensions were then measured by blinded observers using both techniques. Results. For a single radiograph, the human eye was accurate to 0.26 mm and reproducible to ±0.1 mm. In comparison the digital image analyser system was accurate to 0.01 mm with a reproducibility of ±0.08 mm. In an arthroplasty model, where the dimensions of an object were corrected for magnification by the known dimensions of a femoral head, the human eye was accurate to 0.19 mm, whereas the image analyser system was accurate to 0.04 mm. Conclusions. The digital image analysis system is up to 20 times more accurate than the human eye, and in an arthroplasty model the accuracy of measurement increases four-fold. We believe such image analysis may allow more accurate and reproducible measurement of wear from standard follow-up radiographs.