If light passed through objects, rather than bouncing off them, people might now talk to each other on “photophones”. Alexander Graham Bell demonstrated such a device in 1880, transmitting a conversation on a beam of light. Bell's invention stemmed from his discovery that exposing certain materials to focused, flickering beams of light caused them to emit sound—a phenomenon now known as the photoacoustic effect.

It was the world's first wireless audio transmission, and Bell regarded the photophone as his most important invention. Sadly its use was impractical before the development of optical fibres, so Bell concentrated instead on his more successful idea, the telephone. But more than a century later the photoacoustic effect is making a comeback, this time transforming the field of biomedical imaging.

A new technique called photoacoustic (or optoacoustic) tomography, which marries optics with ultrasonic imaging, should in theory be able to provide detailed scans comparable to those produced by magnetic-resonance imaging (MRI) or X-ray computerised tomography (CT), but with the cost and convenience of a hand-held scanner. Since the technology can operate at depths of several centimetres, its champions hope that within a few years it will be able to help guide biopsy needles deep within tissue, assist with gastrointestinal endoscopies and measure oxygen levels in vascular and lymph nodes, thereby helping to determine whether tumours are malignant or not. There is even scope to use photoacoustic imaging to monitor brain activity and gene expression within cells.

To create a photoacoustic image, pulses of laser light are shone onto the tissue being scanned. This heats the tissue by a tiny amount—just a few thousandths of a degree—that is perfectly safe, but is enough to cause the cells to expand and contract in response. As they do so, they emit sound waves in the ultrasonic range. An array of sensors placed on the skin picks up these waves, and a computer then uses a process of triangulation to turn the ultrasonic signals into a two- or three-dimensional image of what lies beneath.

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