New Acoustic Lens Harbinger of Noninvasive Cancer Treatments
Medical use of ultrasonic waves is not a novel concept. Low frequency ultrasound is used to treat kidney stones and thousands of expectant mothers who benefit from sonic imaging. However, a new acoustic lens being developed by California Institute of Technology's Aeronautics and Applied Physics team, headed by
Chiara Daraio, may apply ultrasonic waves to noninvasive cancer treatments.
Linear Sine Wave
Nonlinear Wave
An acoustic lens focuses wave propagation, or how a wave moves, much like an optic lens focuses light in a telescope. Acoustic lenses are commonly used in high-end audio equipment to focus treble (the “tweeter”) waves as they decay faster than bass (low end) frequencies. These consumer market lenses operate only within the range of human hearing, which spans from about 20 to 20,000 vibrations per second (or Hz, Hertz). Beyond the range of human hearing we find the ultrasonic range, which becomes useful in cancer treatment at about 1 Million Hertz.
Cal Tech's Nonlinear Acoustic Lens Fires "Sound Bullets"
Daraio's team is developing a Nonlinear Acoustic Lens, or a lens that focuses the propagation of nonlinear waves. Linear waves have a regular propagation; they move in a regular way like a sine wave. Conversely, nonlinear waves are waves (acoustic, oceanic or electromagnetic) with a widely varying propagation. Nonlinear waves account for most of the sounds we hear on a daily basis. This is why we use linear waves as alarms or warnings (police sirens, alarm clocks and even computer error code beeps are synthesized linear waves).
Cal Tech's lens is a 21x21 vertical matrix of steel spheres. The team uses fishing wire to compress certain spheres so when one of the spheres is excited by a controlled hammer strike the resultant sound waves hit the compressed spheres at just the right time so as to coalesce into a “sound bullet” which strikes an absorbent material. The lens is not dissimilar to a Newton's Cradle, the popular corporate desk mate. It is a series of suspended steel spheres and begins its pleasing motion when one ball strikes a static chain and transfers the force to the final ball, which swings up before descending to restart the process. The genius of the non linear acoustic lens becomes clear when you imagine a Newton's Cradle with only two steel spheres suspended with unequal wire- if you swung them at each other the result would be brief, unfocused chaos.
Sound Bullets In Action
How Will Sound Bullets Treat Cancer?
While the sun's rays are powerful and abundant, an ant crossing the sidewalk is unlikely to burst into flames of its own accord. However, if a questionably moral youth takes out a magnifying glass, also known as an optic lens, and brings the suns scattered rays to a fine point on the unsuspecting ant, the poor insect will experience a burning doom. An acoustic lens brings sound waves into focus and can theoretically burn tumors away just like the focused sunrays burn an ant.
Unlike an optic laser, sound bullets fired by Cal Tech's acoustic lens can propagate through barriers. A properly calibrated lens could place the terminal focal point within a patient allowing tumors to be fired upon with laser accuracy- without the need for invasive surgery.
Acoustic Lens
All healthy tissue could be easily be avoided with advancements in sonic imaging technology. In 2009 researchers with the US Department of Energy's Lawrence Berkeley National Laboratory developed an acoustic 'hyperlens' for imaging purposes. Through use of meta-materials and clever physics, the Berkeley team is able to turn evanescent waves into propagating waves. Evanescent waves are much smaller, and so are more detailed than those used in sonic imaging. Berkeley created a lens which works much like a bullhorn for evanescent waves, allowing sonic imaging devices to read information on objects that are smaller than before.
With advances in lens technology we are approaching a time when we can safely break apart tumors using sound bullets without harming healthy tissue or invasive surgery.
The Time Is Right for Noninvasive Ultrasonic Cancer Treatment
In 2003 University of Washington researchers developed a device for non invasive cancer treatment using low level ultrasonics. UW's device was met with mixed reactions from the medical community that were cautious at best and critical at worst. Daraio's and Berkely's teams do not go so far. Both teams are using solid research and practical experiments to create technology that may in our lifetimes change our daily lives. They're doing the unsung academic work of physicists. And doing it very well.
