Scott Schoen Jr –
Costas Arvanitis –

Georgia Tech
901 Atlantic Dr
Atlanta, GA 30318

Popular version of 2pBAc – Spatial Characterization of High Intensity Focused Ultrasound Fields in the Brain
Presented Tuesday afternoon, December 8, 2020
179th ASA Meeting, Acoustics Virtually Everywhere

The pitch and size of a sound are quite intrinsically connected. This is why, for instance, low-register instruments (such as a tuba or double bass) are large, while higher pitched ones may be very small (like a piccolo or triangle). Sound travels in waves, and the product of the length of the wave (wavelength) and its pitch (frequency) is a constant (namely, the speed of sound).

Consequently, the wavelength of sounds we can hear may be between about 15 m and 0.2 cm. But just as there are wavelengths of light we cannot see (such as ultraviolet and X-rays), there exists sound with much smaller wavelengths. Ultrasound, so called since its frequency is above our hearing range, is able to travel through human tissue and enables noninvasive imaging with millimeter resolution.

Since sound is pressure, it also carries energy. And, much like sunlight through a magnifying glass, sound energy may be focused to a small area to cause heating. This technique has allowed noninvasive and minimally invasive therapy, where focused ultrasound (FUS) creates small regions of high heat or forces to burn or manipulate the tissue. This is especially important for brain diseases, where surgery is particularly challenging.

Fig. 1 – Human cells are sensitive to sound frequencies from about 20 Hz to 20 kHz (left). However, focusing sound to a small area requires small wavelengths—and thus much higher frequencies (right). Not to Scale

Interestingly, it turns out that at very high pressures, so-called nonlinear acoustic effects become important, and the sound begins to interact with itself. One consequence is that if the FUS has to very high frequencies, say 995 kHz and 1005 kHz, the focal spot will a few millimeters, similar to 1 mm. However, the high pressure interaction will also generate energy at 1005 kHz – 995 kHz = 10 kHz—within the audible and tactile range.

This work describes our use of simulations and experiments to understand how this low frequency energy might be realized for FUS through the skull. Understanding the strength and distribution of low frequency energy generated with high frequency FUS may open a new range of therapeutic and diagnostic capabilities in one of the most complex and medically imperative organs: the brain.

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