"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.
A smart contrast agent
Photoacoustic images of biological tissue can be made without the use of contrast agents, particularly if tissues are pigmented by molecules like hemoglobin or melanin.
Still, photoacoustic images of melanomas are fuzzy and vague around the edges. To improve the contrast between the malignant and normal tissue, Xia loads the malignant tissue with gold.
"Gold is much better at scattering and absorbing light than biological materials," Xia says. "One gold nanocage absorbs as much light as a million melanin molecules," says Xia.
Xia's contrast agent consists of hollow gold cages, so tiny they can only be seen through the color they collectively lend to the liquid in which they flo
|Contact: Diana Lutz|
Washington University in St. Louis