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Earlier Melanoma Detection Spurred on by Gold; Alexander Graham Bell
Posted: August 16, 2010
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The imaging technique is based on the photoacoustic effect discovered by Alexander Graham Bell 100 years ago. Bell exploited the effect in what he considered his greatest invention ever, the photophone, which converted sound to light, transmitted the light and then converted it back to sound at the receiver. (The public preferred the telephone to the photophone, by some facetious accounts because they just didn't believe wireless transmission was really possible.)
In Bell's effect, the absorption of light heats a material slightly, typically by a matter of millikelvins, and the temperature rise causes thermoelastic expansion. "Much the same thing happens," says Wang "when you heat a balloon and it expands."
If the light is pulsed at the right frequency, the material will expand and contract, generating a sound wave. "We detect the sound signal outside the tissue, and from there on, it's a mathematical problem," says Wang. "We use a computer to reconstruct an image. We're essentially listening to a structure instead of looking at it," says Wang.
"Using pure optical imaging, it is hard to look deep into tissues because light is absorbed and scattered," Wang explains. "The useful photons run out of juice within one millimeter." Photoacoustic tomography (PAT) can detect deep structures that strongly absorb light because sound scatters much less than light in tissue.
"PAT improves tissue transparency by two to three orders of magnitude," says Wang. Moreover, it's a lot safer than other means of deep imaging. It uses photons whose energy is only a couple of electron-volts, whereas X-rays have energies in the thousands of electron-volts. Positron emission tomography (PET) also requires high-energy photons, Wang says.