1. Department of Radiology University of California, San Francisco; Department of Mathematics, South China Normal University, Guangzhou, China
2. Department of Radiology, Box 0946, University of California, San Francisco, San Francisco CA 94143-0946.
3. Osteoporosis and Arthritis Research Group,University of California, San Francisco
4. Department of Epidemiology and Biostatistics, University of California, San Francisco
5. Department of Medicine, University of Maryland, Baltimore
6. Department of Radiology, Osteoporosis and Arthritis Research Group, University of California, San Francisco