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Segmentation of Quantitative Ultrasonographic Images of the Calcaneus Using Elastic Deformation of the Flexible Fourier Contour

Laboratoire d’Imagerie Paramétrique, Unité Mixte de Recherche, Centre National de la Recherche Scientifique 7623, Université Paris VI, Paris, France (P.L., F.L., C.C.); and Centre d’Evaluation des Maladies Osseuses, Hôpital Cochin, Paris, France (C.R.).

Address correspondence and reprint requests to Pascal Laugier, PhD, Laboratoire d’Imagerie Paramétrique, Unité Mixte de Recherche, Centre National de la Recherche Scientifique 7623, Université Paris VI, 15 rue de l’Ecole de Médecine, 75006 Paris, France. E-mail: laugier{at}lip.bhdc.jussieu.fr.

Objective. This study investigated a new technique for automatic model-based segmentation of broadband ultrasound attenuation (BUA) images of the calcaneus. We determined whether this technique was able to improve osteoporotic fracture discrimination. Methods. The segmentation process included 2 major steps: a model-building stage and the automatic segmentation of new image data sets via an elastic deformation of contour models. Broadband ultrasound attenuation was then averaged within the final contour (BUA_whole). The results of the segmentation were validated on a database of 256 patients by comparison of the clinical results obtained with the automatic circular region of interest (BUA_circ) currently implemented on a commercially available ultrasonography unit. All patients were selected by the same physician, who assessed that the fractures were caused by bone fragility on the basis of the circumstances under which fractures occurred and radiologic data. Results. Short-term reproducibility assessed in 49 women was 3.5% and 3.98% for BUA_circ and BUA_whole, respectively. Both BUA_circ (age-adjusted T score,–3.78; P < .0005; age-adjusted odds ratio, 1.92; 95% confidence interval, 1.34–2.75; area under the receiver operating characteristic curve, 0.70) and BUA_whole (age-adjusted T score,–2.73; P < .01; age-adjusted odds ratio, 1.57; 95% confidence interval, 1.12–2.21; area under the curve, 0.67) performed equally well in discriminating healthy postmenopausal patients (n = 150) from those with fractures (n = 60). Conclusions. Fully automatic segmentation by parametrically deformable elastic models for contour using Fourier descriptors can be achieved with reasonable reproducibility and fracture risk prediction. The method is similar to existing methods (automatic circular region of interest); however, the new contour-based region of interest allows more flexible region of interest geometries and placement and potential adaptation to individual anatomy. The method could also possibly be extended to quantitative ultrasonographic imaging at different skeletal sites.

Key Words: calcaneus • image segmentation • osteoporosis • quantitative ultrasonography

Abbreviations: AUC, area under the receiver operating characteristic curve • BMD, bone mineral density • BUA, broadband ultrasound attenuation • CI, confidence interval • CV, coefficient of variation • OR, odds ratio • QUS, quantitative ultrasonography • ROI, region of interest • UBIS, ultrasonographic bone imaging system