Can You Hear the Light?

The Marvelous World of Photoacoustic Imaging

Photo by Hike Shaw on Unsplash

Around a decade ago, researchers found a way to create 3D images of a hand by getting a camera to “see” reflected light with a red laser.

Now, recent papers suggest the use of ultrasound detectors to “hear” light to produce them.

The technique that these scientists used to accomplish this feat is known as photoacoustic imaging, and it has the potential to revolutionize the world of biomedical imaging.

Photoacoustic Imaging

In short, photoacoustic imaging, also known as optoacoustic imaging is a technique that leverages the photoacoustic effect of light, dating back to the time of Alexander Graham Bell.

Simply put, the photoacoustic effect tells us that when light interacts with any material (scattering medium), it causes rapid heating, expansion, and the release of sound waves.

Originally, this discovery was not put to good use, as the sound waves produced from regular interactions between light and materials would be essentially undetectable. This was the case until a decade ago when scientists showed that the more powerful the light source, the more powerful the resulting sound wave would be.

Papers showed that the addition of a highly sensitive ultrasound detector produced images of hand veins that showed nano-scale resolutions, as well as the ability to track blood flow in real-time — all for roughly the same cost as current imaging systems.

Producing Images

So, how does this effect apply to medical imaging?

Once a nanosecond laser is pulsed onto the skin, it produces multiple sound waves from different sources, with the intensity and frequency of each sound wave providing insight into the material that the light interacted with.

Photoacoustic imaging of vein patterns and their oxygen saturation. Scientists are using this to monitor blood flow in the brain and its activity in real-time

In the case of one point, for example, knowing the speed of sound and the time it takes for each wave to be received, the intersections between three sources of ultrasound can collectively give us an idea of that point’s location in the body.

This is the essence of the image processing for photoacoustic imaging, where the above principle is applied to localize millions of photoacoustic points in the region of interest to form an image.

Applications

Current tools for diagnostic imaging, such as CT or MRI all pose risks such as radiation and danger to patients with medical implants. Photoacoustic imaging provides people with the opportunity to get an accurate image without the risk of DNA damage or hefty price.

Even today, your doctor prefers using ultrasound to see soft tissues, but an MRI for your brain. Using different wavelengths of light, photoacoustic imaging can isolate different parts of the body by changing a single setting on a laser, making the device completely modular and versatile.

Photoacoustic imaging of a patient’s hand veins at different wavelengths. At different wavelengths, different features become visible.

Photoacoustic imaging technology is being used to detect concentrations of hemoglobin in the brain — letting scientists see areas of high and low activity, as well as potential stroke sites, blood clots, and tumours in real-time.

With the expected fall in the price of laser components and transducers in the future, photoacoustics can be tightly packed into a handheld device for in-home use. As manufacturers gain experience in these devices, photoacoustics will move from an enterprise to a consumer market — democratizing medical imaging for billions across the globe.

Disadvantages

Despite the apparent advantages of this technique over similar methods, researchers have been facing numerous roadblocks. One of these challenges is calibration and noise — commercial ultrasound transducers are sensitive to fluctuations in the surrounding area, and scans have to be performed in a highly controlled environment.

An image showing a small fraction of the complexity of a photoacoustic setup.

Photoacoustics is still in its infancy, with manufacturers not receiving enough consumer demand for these products to invest in research and development for dedicated components. With the widespread adoption of photoacoustics in diagnostic imaging, optoacoustic imaging is bound to experience breakthroughs in accuracy and sensitivity.

Takeaways

Photoacoustic imaging is a drastically different method of imaging the body — providing real-time, modular, and high-resolution models of our inner workings. This system shows great promise, and its implementation in society could lead to a new era of medical imaging.

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