Combining Light and Sound For Accurate, Painless 3-D Mammograms

Startup is planning clinical trials for a new photoacoustic technique that produces high-resolution images

Photo: OptoSonics and Canon

Breast cancer screening today requires exposure to X-ray radiation. The X-ray images are difficult to interpret, causing anxiety-inducing false alarms and extensive follow-up tests that include biopsies. Besides, getting a mammogram is quite literally a pain, requiring an uncomfortable compression of the breast tissue.

Startup OptoSonics in Oriental, N.C. aims to make breast imaging more accurate, quick, painless, and radiation-free. The company’s technology relies on the photoacoustic effect—the generation of sound from light absorption—to create high-resolution 3-D images of the network of blood vessels inside the breast. “The idea is that because tumors induce the creation of additional blood vessels, you’d see a brighter spot and individual vessels feeding into the mass,” says Robert Kruger, the company’s president and co-founder. Kruger and his colleagues presented details of the technique at the Acoustical Society of America meeting last week.

For a mammogram with the OptoSonics device, the patient lays face down on a padded table with the breast placed through an aperture in a clear plastic cup that is partially filled with water. Around the cup is a hemispherical array of 500 ultrasonic detectors with a near-infrared laser source at the bottom. The laser zaps the tissue with 2048 pulses in 1.7 minutes. The light is tuned to a wavelength that is easily absorbed by hemoglobin in blood.

“Where the light gets absorbed, it creates heat and expands the tissue,” explains Kruger. “This expansion pushes on surrounding tissue and creates an acoustic wave. It’s less than a microsecond of light, so only one-thousandth of a degree of heating, but it’s enough to induce a sonic wave. Blood vessels absorb more light than other tissue, and when the optical absorption is stronger, the sound wave is stronger.”

The waves have a frequency of 1–5MHz, falling at the low end of the ultrasound range. These ultrasonic waves travel through breast tissue and surrounding water to the detector array and then on to a computer for synthesis. The resulting images look like angiograms, showing the blood vessel network with a resolution of 0.6mm.

OptoSonics has licensed their breast screening technology to Canon in Japan and is working with the company to commercialize the instrument. Kruger says that the OptoSonics team has already imaged four patients at the University of North Carolina at Chapel Hill, where they expect to start clinical trials later in the summer. Clinical trials will also start around the same time at Kyoto University in Japan, which has already received a prototype.

OptoSonics and Canon are also integrating new high-definition ultrasound technology into the existing photoacoustic device. The idea is that the device would produce both photoacoustic and HD-ultrasound images to give doctors more data.

The HD-ultrasound technique relies on the same hemispherical detector array but uses added instrumentation to send ultrasonic waves into the breast tissue. During the spiral scan, ultrasound waves reflected from breast tissue hit the detector array, producing 3D images that have higher signal-to-noise ratio and more resolution than conventional ultrasound images, which are gathered linearly as opposed to spirally. The researchers were able to spot 200-micrometer calcifications in a dummy breast using this HD-ultrasound approach. Such micro-calcifications can be cancer indicators, making them valuable to detect during a mammogram.

There is a lot of interest in the medical community to find alternatives to traditional X-ray mammograms. Ultrasound and MRI are already used as follow-up tests. But, says Kruger, they come with their own issues: ultrasound suffers from noise and MRI is expensive and requires injecting contrast agents. Other emerging technologies that are being studied include 3-D mammograms based on computed tomography (combining many 2-D X-ray images into 3D using computer software) as well as molecular breast imaging, which uses short-term radioactive agents delivered intravenously.

Kruger says OptoSonics' approach is a hybrid between ultrasound and computed tomography and should be a strong contender in the mammography market.

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