A review article by Shengping Qin, Charles Caskey, and Kathy Ferrara is the 7th most cited article in Physics & Medicine in Biology (PMB) over the past 2 years. Ultrasound contrast microbubbles in imaging and therapy: physical principles and engineering (review), published in Phys. Med. Biol 54(2009):R27-R57 has been cited 25 times. The paper is available to download for free until 12/31/11.