The first step of photoacoustic imaging involves using a laser to send light into tissue. Photons from this light are absorbed by molecules in the tissue, like red blood cells, melanin and even water.
"It's basically compressing one second's worth of summer-noon sunlight over a fingernail area in a single nanosecond," says Yao. “When the laser hits a cell, the energy causes it to heat up a tiny bit and expand instantaneously, creating an ultrasonic wave. It's analogous to striking a bell to cause it to ring."
Researchers use algorithms, image processing tools and deep learning software to translate these ultrasound signals into high-resolution, highly colorful images. And the colors aren’t just for show––instead they are used to highlight certain characteristics of the tissue, such as absorption properties.
“If you were to look at an ultrasound image, you can see the outline of the tissue but the contrast is in different shades of grays, and it can be difficult to decipher small details,” says Yao. “Photoacoustic imaging provides a much clearer contrast, because different molecules in the body absorb and react to different wavelengths of light. They have specific chemical structures which act like natural contrast agents.”
For example, hemoglobin that is carrying oxygen will absorb light at one wavelength while deoxygenated hemoglobin does a better job of absorbing light at a different wavelength. These differences cause the tissue to emit ultrasonic waves at specific frequencies, resulting in varied colors in the final photo.
“The colors in our images can tell us a lot about the tissue characteristics, like the amount of oxygen in a tissue or the amount of blood flow to an area,” says Yao. “Photoacoustic imaging can also show small details like the DNA and RNA inside of cells, which appear darker than the surrounding cell.”
Yao and his team have already used photoacoustic imaging to study topics like the temperature of deep tissues for thermal-based cancer therapies. They have also formed partnerships with physicians at Duke University Medical Center to image and study blood vessels affected by different skin diseases.
Now, Yao and his lab are pursuing the use of photoacoustic imaging to track oxygen levels throughout the brain. In their first project, they’re investigating how the brain uses oxygen as it heals after a stroke.
“In general, there is a lack of information about how the blood vessels throughout the brain respond to different levels of oxygenation,” says Yao. “We want to see how small and large blood vessels are affected and how age may play a role. We’re hoping we can use this technique to illuminate how vessels repair themselves and begin to restore functions.”
Their second project explores how the drug epinephrine affects brain function when given to a patient after a cardiac arrest. Epinephrine essentially jumpstarts the heart after a heart attack, and it increases blood pressure by constricting blood vessels throughout the body. Already, the team has used photoacoustic imaging to see that the use of epinephrine in animal models reduces oxygen levels in certain sections of the brain. Now, the team is exploring how they can resolve this issue to optimize use of the drug.
Beyond these projects, Yao says his team is always looking for ways to push photoacoustic imaging forward, with the goal of making the technology faster, smaller and more colorful.