1. Diagnostic Center for Sonography, Hokkaido University Hospital, N14, W5, Kita-ku, Sapporo 060-8648, Japan
2. Department of Cardiovascular Medicine, Faculty of Medicine and Graduate School of Medicine, Hokkaido University, N15, W7, Kita-ku, Sapporo 060-8638, Japan
3. Division of Cardiology, Hakodate Municipal Hospital, 1-10-1, Minatocho, Hakodate 041-8680, Japan
4. Department of Cardiovascular Medicine, Gunma University Graduate School of Medicine, 3-39-22 Showa-machi, Maebashi, Gunma 371-8511, Japan
5. Division of Cardiovascular Medicine, National Defense Medical College, 3-2 Namiki, Tokorozawa, Saitama 359-8513, Japan
6. Faculty of Health Sciences, Hokkaido University, N12, W5, Kita-ku, Sapporo 060-0812, Japan