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  • Photoacoustic imaging penetrates deeply into skin

BioPhotonics
May 2012

ST. LOUIS – By combining the best qualities of ultrasound and light absorption, photoacoustic tomography (PAT) provides clinicians with multicontrast images of biological tissue several inches below the skin’s surface.

The technique, developed in 2003 by Dr. Lihong V. Wang at Washington University, achieves the imaging depth by combining the spatial resolution of ultrasound with the high contrast resulting from light absorption by colored molecules such as hemoglobin or melanin.

“Since then, the field doubled in size approximately every three years,” Wang told BioPhotonics.

He is working with physicians at Washington University School of Medicine to move four applications of PAT to clinical trials: visualizing sentinel lymph nodes important in breast cancer staging; imaging melanomas; monitoring early response to chemotherapy; and imaging the gastrointestinal tract. Their findings were detailed in the March 23 issue of Science (doi: 10.1126/science.1216210).


Major embodiments of PAT, with representative in vivo images: a) Optical-resolution photoacoustic microscopy of sO2 in a mouse ear; b) acoustic-resolution photoacoustic microscopy of normalized total hemoglobin concentration, (hemoglobin), in a human palm; c) linear-array photoacoustic CT of normalized methylene blue concentration, (dye), in a rat sentinel lymph node (SLN); d) circular-array photoacoustic CT of cerebral hemodynamic changes, Δ(hemoglobin), in response to one-sided whisker stimulation in rat; e) photoacoustic endoscopy of a rabbit esophagus and adjacent internal organs, including the trachea and lung. UST = ultrasonic transducer. Courtesy of Dr. Lihong V. Wang.


Among the most exciting advances for PAT is its ability to reveal the use of oxygen by tissues. Excessive oxygen burning, or hypermetabolism, is a hallmark of cancer. Almost all diseases, especially cancer and diabetes, cause abnormal oxygen metabolism. PAT would provide an early-warning diagnostic test that does not require a contrast agent, which is a potential game-changer, Wang said.

The trick of PAT is to convert light absorbed at depth to sound waves – which scatter a thousand times less than light – for transmission back to the surface. In the technique, a nanosecond-pulsed laser at an optical wavelength is directed at the tissue of interest. The thermoelastic expansion of the tissue converts photons to sound waves, which are used to form images with a resolution associated with the ultrasound wavelength at tissue depths never before possible.

“Conventional optical microscopy uses unscattered light, which is substantially attenuated beyond the optical diffusion limit (~1 mm in the skin),” Wang said. “Sound scattering is about a thousand times weaker than light scattering. But detecting light-induced sound through the photoacoustic effect, the tissue transparency is equivalently improved by about a thousandfold for detection. We listen to optical structures illuminated by scattered light, which penetrates much deeper than unscattered light.”

Because scattering does not destroy photons, the technique makes it possible to reach a depth of about 7 cm.

“It is more challenging to reach depths much greater than 7 cm, although we demonstrated 8 cm already in the lab,” Wang said. His lab is now exploring a technology called “time-reversed ultrasonically encoded (TRUE) optical focusing” to break through the 7-cm limit. “Even at 7 cm, PAT can find reasonably broad biomedical applications,” he said.

In current practice, optical microscopy is used to examine organelles and cells, and nonoptical imaging techniques such as x-ray tomography are used for tissues and organs. The clinical imaging technologies do not provide a strong contrast such as optical techniques do, so between the micro and macro domains, a large divide exists, where the image acquired at one length scale cannot be related to those acquired at another.

“Existing modalities do not image the same contrast at micro- and macro-length scales, but PAT does,” Wang said. “Using the same contrast allows the same landmark features to appear in images acquired across length scales with different spatial resolutions.”

With help from exogenous (introduced) contrast agents, PAT can image tissues such as lymph nodes that otherwise would blend with their surroundings. Wang also is experimenting with “reporter genes,” which encode a colored product and show up well in photoacoustic images.

Sentinel node biopsy provides a good example of the improvement that PAT promises over current imaging practices. Sentinel nodes are the nodes nearest a tumor, to which cancerous cells first migrate. To perform a biopsy, a surgeon injects a dye, a radioactive substance or both near a tumor. The body treats both substances as foreign, so they flow to the first draining node to be filtered and flushed from the body.

“A gamma probe or a Geiger counter is used to locate the radioactive particles, but [either one] gives only a rough idea of the node’s location,” Wang said. To find the node, the surgeon must cut open the area and follow the dye visually to the sentinel lymph node.

Instead, Wang proposes injecting an optical dye that shows up clearly in photoacoustic images. A hollow needle can be guided directly to the node and a tissue sample taken through the needle. In the current clinical trial, the surgeon is not taking tissue but rather deploying a tiny metal clip through the needle. The patient then undergoes a lymph node dissection, and the dissected lymph node is x-rayed to see whether it contains the clip.

“If this technique proves accurate, we will be converting a surgical procedure into a needle biopsy possible on an outpatient basis,” he said. “In the US alone, 100,000 of these surgical biopsies are done every year, so the new procedure would spare many patients injury – not to mention expense.”

PAT is currently being studied in clinical trials on humans.

“We need to validate the technology in the clinic and get FDA approval,” Wang told BioPhotonics.

For more on Wang’s research, tune into Photonics Media’s live Advances in Biomedical Photonics webinar July 19 at 1 p.m. He will speak on “Photoacoustic Tomography: Ultrasonically Breaking Through the Optical Diffusion Limit.” For more information, visit: www.photonics.com/webinars.


GLOSSARY
spatial resolution
In a vision system, the linear dimensions (X and Y) of the field of view, as measured in the image plane, divided by the number of pixels in the X and Y dimensions of the system's imaging array or image digitizer, expressed in mils or inches per pixel.
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