A startling change in medical ultrasound is working its way through hospitals and physicians’ offices. The long-standing, state-of-the-art ultrasound machine that’s pushed around on a cart, with cables and multiple probes dangling, is being wheeled aside permanently in favor of handheld probes that send images to a phone. These devices are small enough to fit in a lab coat pocket and flexible enough to image any part of the body, from deep organs to shallow veins, with sweeping 3D views, all wit...