Every year nearly one million unnecessary biopsies are performed in the United States, while tens of thousands of tumors go undetected. This is because diagnostic imaging techniques like x-ray mammography and ultrasound can’t resolve features smaller than about five millimeters. Now biomedical engineers Vasilis Marmarelis and Tae-Seong Kim at the University of Southern California have developed an ultrasound system with 10 times better resolution. It uses an array of dozens of transmitters, each .4 millimeters on a side, to send ultrasonic pulses through tissue to a matching array of sensors on the opposite side. A computer generates images of the tissue based on the time delay and shape of the received sound waves. Resolution is high because the hardware elements are so small, and because transmitted signals carry more information than the reflected signals used in conventional ultrasound. The system will enter human trials this fall, says Marmarelis, and the researchers hope to partner with medical-imaging firms.