The trick of photoacoustic tomography is to convert light absorbed at depth to sound waves, which scatter a thousand times less than light, for transmission back to the surface. The tissue to be imaged is irradiated by a nanosecond-pulsed laser at an optical wavelength.
Absorption by light by molecules beneath the surface creates a thermally induced pressure jump that launches sound waves that are measured by ultrasound receivers at the surface and reassembled to create what is, in effect, a photograph.
Light, unlike X-rays, which also penetrate deeply, poses no health hazard. Moreover, photoacoustic images have much higher contrast than X-ray images because there are many highly colored molecules in the body that serve as "endogenous" contrast agents. These include hemoglobin, which changes color as it gains or loses oxygen, but also melanin, the pigment that makes moles dark, and DNA, which in its "condensed" form in the cell nucleus is "darker" than the cell cytoplasm.
With a little help from "exogenous" (introduced) contrast agents, such as organic dyes or genes engineered to express colorful products, photoacoustic tomography can also image tissues, such as lymph nodes, that otherwise blend in with their surroundings. Wang also has been experimenting with "reporter genes," genes that encode a colored product, which shows up well in photoacoustic images.
Putting down the scalpel
Sentinel node biopsy provides a good example of the improvement photoacoustic imaging promises over current imaging practice. Sentinel nodes are the nodes nearest a tumor, such as a breast tumor, to which cancerous cells would first migrate.
In a sentinel node biopsy, a surgeon injects a radioactive substance, a dye, or both near a tumor. The bo
|Contact: Diana Lutz|
Washington University in St. Louis