From the Departments of *Radiology, †Health Services, ‡Medicine, §Biostatistics, ¶Epidemiology and ∥Neurological Surgery; the **Center for Cost and Outcomes Research, University of Washington, Seattle, Washington; and ††Veterans Affairs Puget Sound Health Care System and Seattle Epidemiologic Research and Information Center (ERIC), Seattle, Washington.

Supported by the Department of Veterans Affairs-ERIC Grant and in part by Grants HS-08194, HS-094990 from the Agency for Healthcare Research and Quality and P60-AR48093-01 from National Institute for Arthritis and Musculoskeletal and Skin Diseases.

Acknowledgment date: May 12, 2004. First revision date: July 23, 2004. Acceptance date: August 16, 2004.

The manuscript submitted does not contain information about medical device(s)/drug(s).

Federal funds were received in support of this work. No benefits in any form have been or will be received from a commercial party related directly or indirectly to the subject of this manuscript.

Address correspondence to Jeffrey G. Jarvik, MD, MPH, Department of Radiology, University of Washington, Box 357115, 1959 NE Pacific St., Seattle, WA 98195; E-mail: jarvikj@u.washington.edu