Automated measurement of radiographic hip joint-space width

Radiographic joint-space narrowing (JSN) is the principle indicator of cart ilage loss in osteoarthritis (OA). JSN is usually assessed qualitatively by visual inspection or in clinical research, is measured manually with a gra duated handheld lens directly applied to the x-ray film, or from digitized radiographs by hand tracing the joint margins with a mouse. The minimum joi nt-space width (mJSW) and joint-space area (JSA) are recorded as the indice s of OA progression in epidemiological studies and clinical drug trials. We present a computerized method that automatically finds the articular margi ns of the hip to improve determination of mJSW and JSA. The algorithm requi res that three seed points are manually identified on the femoral head and uses three steps to process each digitized hip x-ray. First, a Hough transf orm finds the center and radius (R) of a circle that approximates the femor al head. Finding R indicates whether magnification differences must be corr ected on repeat exams. Second, a gradient algorithm finds the edge of the f emoral head and acetabulum. Third, the mid-line of the femoral neck is auto matically found and used to define the joint portion (theta) that is assess ed for narrowing. theta is fixed for follow-up exams of the same subject. T he algorithm was evaluated in three ways to determine its performance chara cteristics. First, the inter-reader and intra-reader variability for mJSW a nd JSA associated with the selection of the seed points was found to be neg ligible (<1%) compared to the variability associated with manual scoring wi th a lens or by tracing the joint margins with a mouse. Second, from duplic ate hip x-rays of 19 subjects with OA, the Root Mean Square Standard Deviat ion and coefficient of variation for mJSW and JSA defined by the algorithm was determined to be better than manual techniques by at least a factor of 2. Third, the algorithm correctly identified the joint margin in more than 85% of the 105 cases tested. Automated measures of radiographic hip joint-s pace narrowing is less subjective than manual methods and may be applicable for monitoring OA progression in clinical research. (C) 2001 American Asso ciation of Physicists in Medicine.