Radiation Force as a Physical Mechanism for Ultrasonic Neurostimulation of the Ex Vivo Retina

Abstract

Focused ultrasound has been shown to be effective at stimulating neurons in many animal models, both in vivo and ex vivo. Ultrasonic neuromodulation is the only noninvasive method of stimulation that could reach deep in the brain with high spatial-temporal resolution, and thus has potential for use in clinical applications and basic studies of the nervous system. Understanding the physical mechanism by which energy in a high acoustic frequency wave is delivered to stimulate neurons will be important to optimize this technology. We imaged the isolated salamander retina of either sex during ultrasonic stimuli that drive ganglion cell activity and observed micron scale displacements, consistent with radiation force, the nonlinear delivery of momentum by a propagating wave. We recorded ganglion cell spiking activity and changed the acoustic carrier frequency across a broad range (0.5-43 MHz), finding that increased stimulation occurs at higher acoustic frequencies, ruling out cavitation as an alternative possible mechanism. A quantitative radiation force model can explain retinal responses and could potentially explain previous in vivo results in the mouse, suggesting a new hypothesis to be tested in vivo. Finally, we found that neural activity was strongly modulated by the distance between the transducer and the electrode array showing the influence of standing waves on the response. We conclude that radiation force is the dominant physical mechanism underlying ultrasonic neurostimulation in the ex vivo retina and propose that the control of standing waves is a new potential method to modulate these effects.

Spike-frequency dependent inhibition and excitation of neural activity by high-frequency ultrasound

“…Nonetheless, the shear viscosity/relaxation time in our model is reasonable for a soft, gel-like material, and is comparable to fast relaxation times observed experimentally in brain tissue (Arbogast and Margulies, 1999;Abolfathi et al, 2009;Rashid et al, 2012Rashid et al, , 2013. Moreover, the simulated time course of displacement is consistent with experimental measurements of the tissue displacement in response to ultrasound at 43 MHz and 40 W/cm 2 in the salamander retina, which was found to be complete in less than ms (Menz et al, 2019).…”

“…In conclusion, our results demonstrate that high-frequency ultrasound is a viable and promising modality for neuromodulation applications where frequency is not limited by transmission through the skull, and our insights into the common molecular mechanisms underlying both inhibitory and excitatory effects of high-frequency ultrasound pave the way for rational design and optimization of neuromodulation protocols to consistently produce either inhibitory or excitatory effects. Sources for material properties are as follows: a standard value; b based on typical acoustic properties of plastics (Selfridge, 1985); c following Menz et al (2019), based on (Thijssen et al, 1985); d (Company, 1965); e measured (Prieto et al, 2018); f following Menz et al (2019), based on (de Korte et al, 1994); g based on typical thermal properties of plastics (Gaur and Wunderlich, 1982;Harper, 2006); h typical values for soft tissues (Hand, 1998); i based on typical mechanical properties of plastics (Harper, 2006); j Menz et al (2019), from measurements of ultrasound-induced displacement in the retina; k tissue assumed to be incompressible for small deformations; l see text. ms for the onset and 0.22 mV and 6.7 ms for the offset, for the example shown; mean values (±SEM) were 0.41 ± 0.04 mV and 10.4 ± 1.2 ms for the onset and 0.35 ± 0.04 mv and 9.7 ± 1.7 ms for the offset (N = 15).…”

“…Ultrasound can non-invasively modulate action potential activity in neurons in vivo and in vitro, with improved depth penetration and spatial resolution relative to other non-invasive neuromodulation modalities, and it may therefore become an important new technology in basic and clinical neuroscience (Fry et al, 1958;Gavrilov et al, 1996;Tufail et al, 2010;Bystritsky et al, 2011;Fomenko et al, 2018;Blackmore et al, 2019). Investigation of this phenomenon has predominantly focused on low-frequency ultrasound (defined here as less than 3 MHz, although there is no firmly defined boundary between "high" and "low" frequency in the neuromodulation field), but higher ultrasound frequencies have also been shown to modulate action potential firing in vitro (Menz et al, 2013;Menz et al, 2019) and to directly modulate ion channel activity in heterologous systems (Kubanek et al, 2016;Prieto et al, 2018). The focus on lower frequency ultrasound is understandable, since envisioned clinical applications involving transcranial focused ultrasound have been a primary motivation for research on ultrasound neuromodulation, and loss of ultrasound power due to attenuation in the skull limits these applications to low frequency ultrasound.…”

Ultrasound neuromodulation depends on pulse repetition frequency and can modulate inhibitory effects of TTX

“…The number of neurons exposed to the US is inherently lower in the slice than in vivo given that the slice occupies only a small portion of the US focus as opposed to in vivo, where the entire sound focus may interact with a large population of neurons in brain tissue. A similar explanation relating exposure volume to stimulation is offered in both Ye et al 18 and Menz et al 38 Furthermore, several in vivo studies explore higher pressure regimes for USN. In our model pulses above 350 kPa often resulted in slice motion which limited our ability to explore higher pressure.…”

Ultrasonic Retinal Neuromodulation and Acoustic Retinal Prosthesis

“…Furthermore, they varied the acoustic frequency in a wide range between 0.5 and 43 MHz, finding enhanced retinal activity under higher frequencies. This observation argues against any major direct role played by cavitation in ultrasonic stimulation of the retina, as cavitation tends to diminish when the frequency is increased [ 70 ].…”