Radiation Force as a Physical Mechanism for Ultrasonic Neurostimulation of the Ex Vivo Retina
Mike D. Menz, Patrick Ye, Kamyar Firouzi, Amin Nikoozadeh, Kim Butts Pauly, Pierre Khuri-Yakub, Stephen A. Baccus
The Journal of Neuroscience 2019, 39, 6251-6264 · 10.1523/jneurosci.2394-18.2019
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. SIGNIFICANCE STATEMENT Ultrasonic neurostimulation is a promising noninvasive technology that has potential for both basic research and clinical applications. The mechanisms of ultrasonic neurostimulation are unknown, making it difficult to optimize in any given application. We studied the physical mechanism by which ultrasound is converted into an effective energy form to cause neurostimulation in the retina and find that ultrasound acts via radiation force leading to a mechanical displacement of tissue. We further show that standing waves have a strong modulatory effect on activity. Our quantitative model by which ultrasound generates radiation force and leads to neural activity will be important in optimizing ultrasonic neurostimulation across a wide range of applications.
Abstract via europepmc.
Exposures
Exposure 1: Ultrasonic stimulation of isolated retina across carrier frequencies (0.5-43 MHz)
Target: retinal explant — “isolated retina on multielectrode array (ganglion cell layer)”
Device: Olympus / Panametrics · custom (43 MHz); Panametrics (15 MHz); Olympus (2.25 MHz operated at 1.9/2.9 MHz; 0.5 MHz) · 43 MHz custom transducer; A319S (15 MHz); V305 (2.25 MHz); V301 (0.5 MHz) ✓
| Waveform | continuous | |
|---|---|---|
| Fundamental frequency (kHz) | 500, 1,900, 2,900, 15,000, 43,000swept | ✓✓✓⚑ |
| Pulse duration (ms) | not applicable | ⚑ |
| Pulse repetition frequency (Hz) | not applicable | |
| Duty cycle (%) | 100 | ✓✓✓ |
| Sonication duration (s) | 0.1 | ✓✓✓ |
| Free-field pressure (kPa) | not reported | |
|---|---|---|
| Free-field Isppa (W/cm²) | 1.6, 40, 60, 95, 155swept | ✓✓✓⚑ |
| Free-field Ispta (W/cm²) | 1.6, 40, 60, 95, 155swept | ✓✓✓ |
| In-situ estimate | not applicable | |
| In-situ pressure (kPa) | not applicable | |
| In-situ Isppa (W/cm²) | not applicable | |
| In-situ Ispta (W/cm²) | not applicable |
CW is used for all experiments. A 100 ms CW pulse duration was found optimal at 43 and 15 MHz and used as the default pulse duration at other frequencies (repeated every 5 s to minimize adaptation). For 43 MHz, pulse intensity and duration were additionally varied across a wide range (Fig. 6a,b) to characterize response; those swept values are given only in figure form. Imaging of retinal displacement at 43 MHz used a separate 1 s ON/1 s OFF stimulus protocol at ISP = 40 W/cm2.
Consistency checks: f0 out of range.
Flags from extraction
exposures[0].fundamental_frequency_khz— Frequencies 0.5-43 MHz were tested as a frequency-response sweep at the same retinal target; kept as a single exposure per the frequency-sweep exception, though pulse duration/intensity combinations differed somewhat by frequency (mostly given only in figures, not text).exposures[0].free_field.isppa_w_cm2— Specific intensities are stated in text for particular sub-experiments (e.g., ISP=40 W/cm2 at 43 MHz for imaging; ISP=1.6 W/cm2 maximum achievable at 500 kHz for electrophysiology) but a single per-frequency intensity list for the main stimulation/threshold sweep is only given in figures (Fig. 6, 7); left not_reported to avoid conflating figure-only values with text-stated ones.n_subjects— Paper gives numbers of retinas/salamanders per sub-experiment (imaging, 43 MHz, 15 MHz, lower frequencies) but never a single combined total exposed to ultrasound; recorded as the list of group sizes and subject_unit set to 'preparation' since one retina is the functional unit per animal.device— Four different transducers from different manufacturers (one custom-built) were used across the frequency range; device fields combine all into one entry with a flag rather than four near-duplicate exposures.exposures[0].timing.pulse_duration_ms— For 43 MHz, pulse duration was swept over a wide range (Fig. 6a,b, figure-only values); only the 100 ms default value used for other frequencies is stated in the text.