Sight and sound

One of their key projects is photoacoustic imaging, a technology that combines light and ultrasound. Conventional ultrasound scanners send sound into the body and measure the echo. Photoacoustic imaging systems, however, measure the ultrasound generated when molecules absorb light and heat up.

Photoacoustic images show how molecules in the body change and are particularly useful for measuring blood oxygen levels. “This is a useful feature for studying cancer,” says Thomas Else, a research associate in Bohndiek’s team. “Tumors tend to have lower oxygen levels than healthy tissues because they grow so quickly.”

Most commonly used methods for detecting and monitoring cancer – such as magnetic resonance imaging or X-ray mammography – are large, expensive machines that, in the case of mammography, use ionizing radiation and painful pressure.

“The lack of ionizing radiation makes photoacoustic imaging safe and suitable for long-term monitoring, and it can produce high-contrast images at the bedside, like ultrasound,” Else said. “Photoacoustic systems are also less expensive and more portable than MRI machines, making them a valuable technique for cancer monitoring in a wider range of medical settings.”

Photoacoustic imaging has already been CE marked in Europe and FDA-approved in the United States for the detection of breast cancer, offering a cost-effective alternative to traditional mammography.

But one of the challenges with using photoacoustic imaging, as with several light-based medical technologies, is that it doesn’t work as well on people with darker skin tones. Melanin, the main pigment that affects skin tone, absorbs light, meaning less light can get through the skin to make a measurement.

Typically, photoacoustic imaging uses light from the red end of the visible spectrum to the infrared—wavelengths of about 700 to 900 nanometers. In this range, hemoglobin in the blood is the main source of contrast. However, in darker-skinned patients, melanin can overwhelm the hemoglobin signal.

“Melanin in the skin blocks some of the light and prevents it from reaching deep into the tissue and into the tumor or organ we want to image,” Else said. “We wanted to look at the physics behind the poor performance of light-based imaging technologies for people of color, which could help us solve this problem.”