1. Department of Radiology, University of Washington Medical Center, Seattle, WA 98195_7115, USA
2. Department of Physiology & Biophysics, University of Washington Medical Center, Seattle, WA 98195_7115, USA
3. Department of Bioengineering, University of Washington Medical Center, Seattle, WA 98195_7115, USA
4. Department of Rehabilitation Medicine, University of Washington Medical Center, Seattle, WA 98195_7115, USA
5. Department of Medicine, University of Washington Medical Center, Seattle, WA 98195_7115, USA