Photoacoustic tomography can 'see' in color and detail several inches beneath the skin, using a combination of light and sound waves. Photographing inside the body is difficult because light photons can penetrate soft tissue to only about a millimeter before they're so scattered it isn't possible create an image. But scattering doesn't destroy the photons, which can reach a depth of about 3 inches. Light absorption by molecules at depth creates a pressure jump that launches sound waves that, in photoacoustic tomography, are measured by ultrasound receivers at the surface and reassembled to create what is, in effect, a photograph. Researchers are working with physicians to move four applications of photoacoustic tomography into clinical trials. One is to visualize lymph nodes that are important in breast cancer; a second to monitor early response to chemotherapy; a third to image melanomas; and the fourth to image the gastrointestinal tract.