In contrast, low-intensity ultrasound (depicted as blue), targets only cancer cells (depicted in red), leaving healthy ones (green) intact. High-intensity ultrasound (left, depicted as red) kills all cells. Which do will depend on their physical properties. Not all cells respond equally to this standing wave. That then boosts the ultrasound energy deposited on the cells, Mittelstein says. The team suspects the standing wave brings microbubbles closer together. And that extra oscillation proved essential to killing cancer cells. In them, he says, “the pressure goes up and down at twice the amplitude of the traveling wave.” In the end, bubbles in the standing wave oscillate more than do those in a normal wave. Some other stationary spots, called “anti-nodes,” also develop. And this wave has some “special stationary spots called ‘nodes,’” he explains. The colliding waves combine to form a special pattern known as “a standing wave,” Mittelstein notes. But when those waves hit a surface of some type, they can reflect back - into the oncoming traveling waves.
They move out from the machine that produces them. The initial ultrasound waves are known as traveling waves. More damage occurred when the ultrasound waves bounced back to hit the cancer cells more than once. “But only the cancer cells,” he notes, “were vulnerable to certain frequencies of ultrasound.” To kill cancer cells, Mittlestein reports, “microbubble oscillation was necessary - but not sufficient.” Microbubbles oscillated in both healthy and cancer cells.
The oscillation caused these microbubbles to grow, then violently collapse. The ultrasound waves caused these bigger bubbles to oscillate (move back and forth). The treatment caused super-small microbubbles - likely tiny bubbles of air present in the fluid - to merge. It also left more than eight in every 10 immune cells unharmed. One minute of 500,000 hertz ultrasound, delivered in 20-millisecond bursts, killed nearly every cancer cell. They also tested different pulse durations (from 2 to 40 milliseconds). The team tested different ultrasound frequencies (ranging from 300,000 to 650,000 hertz). Ultrasound waves travel much faster and occur at a higher frequency than sounds we can hear. Then the scientists directed short pulses of low-intensity ultrasound at this suspension. The cells were all suspended in a liquid. So his team set out to do that.įirst, they mixed cancer cells with healthy blood cells and immune cells. The process, Mittelstein explains, is “similar to how a trained singer can shatter a wine glass by singing a specific note.” Explainer: What is a computer model? These models suggested that low-intensity ultrasound might kill those cells. This other Caltech team created computer models of cancer cells. Mittelstein’s team wanted to try something different.Īnother Caltech lab had studied effects of low-intensity ultrasound on cancer cells. The down side: It kills healthy ones, too. Targeted cells and their neighbors can reach 65° Celsius (149° Fahrenheit) in just 20 seconds. The waves vibrate water inside cells within that area. These sound waves send lots of energy to a small, focused area. Explainer: Understanding waves and wavelengthsĭoctors had already used high-intensity ultrasound to kill cancer cells. (That’s also what makes it “ultra” sound.) Medical imaging relies on very short pulses of this low-intensity ultrasound. The treatment sends out pulses of sound waves - energy - that have a frequency above 20,000 hertz (cycles per second). Low-intensity ultrasound, he says, “may allow physicians to target cancer cells based on their unique physical and structural properties.” Any spillover of the energy should cause little harm to healthy tissue. Mittelstein is a biomedical engineer at the California Institute of Technology, in Pasadena. It’s exciting, says David Mittelstein of his team’s findings. Healthy cells should suffer little if any harm from it. It limits the ultrasound energy’s damage to only the cancer cells. Even this treatment, however, can sometimes damage healthy tissue. One new idea would destroy cancer cells with ultrasound energy. So researchers are looking for new approaches that spare the healthy cells. Because they tend to take out healthy cells along with cancerous ones, these treatments can leave patients tired, hurting and more. Most cancer treatments involve surgery, chemical poisons or toxic radiation.
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