Contrast-enhanced ultrasonography for the diagnosis of spontaneous necrosis of hepatocellular carcinoma: A report of 2 cases oleh Yu Ota, MD, PhD, Kazunobu Aso, MD, Shin Otake, MD, Mitsuyoshi Okada, MD, Kounosuke Shukuda, MD, Koji Sawada, MD, PhD, Hideki Yokoo, MD, PhD, Mishie Tanino, MD, PhD, Mikihiro Fujiya, MD, PhD, Toshikatsu Okumura, MD, PhD