Displacement Imaging During Focused Ultrasound Median Nerve Modulation: A Preliminary Study in Human Pain Sensation Mitigation

Functional Cerebral Neurovascular Mapping During Focused Ultrasound Peripheral Neuromodulation of Neuropathic Pain

BACKGROUND

Nociceptive pain is required for healthy function, yet, neuropathic pain (disease or injury) can be severely debilitating. Though a wide-array of treatment options are available, they are often systemic and/or invasive. As a promising neuromodulation treatment, Focused ultrasound (FUS) is a noninvasive and highly spatially-targeted technique shown to stimulate neural activity, yet, effects on pain signaling are currently unknown.

OBJECTIVE

Develop and validate a method for studying FUS nerve stimulation modulation of pain-evoked neural responses in vivo.

METHODS

We developed a high-resolution functional ultrasound (fUS) method capable of mapping cortical responses in healthy and neuropathic pain mice in response to FUS neuromodulation treatment.

RESULTS

FUS-evoked hemodynamic responses are correlated with the intensity of peripheral neuromodulation. We confirm functional connectivity is altered in neuropathic mice and demonstrate that FUS can modulate neuropathic pain-evoked hemodynamics.

CONCLUSIONS

The findings presented herein provides evidence for an FUS-based nerve pain method and validates the fUS technique developed for monitoring pain-evoked hemodynamics.

SIGNIFICANCE

We anticipate that the findings presented herein describe a noninvasive and flexible nerve modulation technique for pain mitigation, furthering evidence for clinical translation.

Compound Acoustic Radiation Force Impulse Imaging of Bovine Eye by Using Phase-Inverted Ultrasound Transducer

In general, it is difficult to visualize internal ocular structure and detect a lesion such as a cataract or glaucoma using the current ultrasound brightness-mode (B-mode) imaging. This is because the internal structure of the eye is rich in moisture, resulting in a lack of contrast between tissues in the B-mode image, and the penetration depth is low due to the attenuation of the ultrasound wave. In this study, the entire internal ocular structure of a bovine eye was visualized in an ex vivo environment using the compound acoustic radiation force impulse (CARFI) imaging scheme based on the phase-inverted ultrasound transducer (PIUT). In the proposed method, the aperture of the PIUT is divided into four sections, and the PIUT is driven by the out-of-phase input signal capable of generating split-focusing at the same time. Subsequently, the compound imaging technique was employed to increase signal-to-noise ratio (SNR) and to reduce displacement error. The experimental results demonstrated that the proposed technique could provide an acoustic radiation force impulse (ARFI) image of the bovine eye with a broader depth-of-field (DOF) and about 80% increased SNR compared to the conventional ARFI image obtained using the in-phase input signal. Therefore, the proposed technique can be one of the useful techniques capable of providing the image of the entire ocular structure to diagnose various eye diseases.

Optimization of the Tracking Beam Sequence in Harmonic Motion Imaging

Harmonic motion imaging (HMI) is an ultrasound elastography technique that estimates the viscoelastic properties of tissues by inducing localized oscillatory motion using focused ultrasound (FUS). The resulting displacement, assumed to be inversely proportional to the tissue local stiffness, is estimated using an imaging array based on RF speckle tracking. In conventional HMI, this is accomplished with plane-wave (PW) imaging, which inherently suffers from low lateral resolution. Coherent PW compounding (PWC) leverages spatial and temporal resolution using synthetic focusing in transmit. In this study, we introduced focused imaging with parallel tracking in HMI and compared parallel tracking of various transmit F-numbers (F/2.6, 3, 4, and 5) qualitatively and quantitatively with PW and PWC imaging at various compounded angle ranges (6°, 12°, and 18°). An in silico model of a 56-kPa spherical inclusion (diameter: 3.6 mm) embedded in a 5.3-kPa background and a 5.3-kPa elastic phantom with cylindrical inclusions (Young's moduli: 22-56 kPa, diameters: 2.0-8.6 mm) were imaged to assess different tracking beam sequences. Speckle biasing in displacement estimation associated with parallel tracking was also investigated and concluded to be negligible in HMI. Parallel tracking in receive (Rx) resulted in 2%-7% and 8%-12% increase compared to PW imaging ( ) in HMI contrast and contrast-to-noise ratio in silico and phantoms. Focused imaging with parallel tracking in Rx was concluded to be most robust among PW and PWC imaging for displacement estimation, and its preclinical feasibility was demonstrated in postsurgical human cancerous breast tissue specimens and in vivo murine models of breast cancer.

Interdependence of Tissue Temperature, Cavitation, and Displacement Imaging During Focused Ultrasound Nerve Sonication

Focused ultrasound (FUS) peripheral neuromodulation has been linked to nerve displacement caused by the acoustic radiation force; however, the roles of cavitation and temperature accumulation on nerve modulation are less clear, as are the relationships between these three mechanisms of action. Temperature directly changes tissue stiffness and viscosity. Viscoelastic properties have been shown to affect cavitation thresholds in both theoretical and ex vivo models, but the direct effect of temperature on cavitation has not been investigated in vivo. Here, cavitation and tissue displacement were simultaneously mapped in response to baseline tissue temperatures of either 30 °C or 38 °C during sciatic nerve sonication in mice. In each mouse, the sciatic nerve was repeatedly sonicated at 1.1-MHz, 4-MPa peak-negative pressure, 5-ms pulse duration, and either 15- or 30-Hz pulse repetition frequency (PRF) for 10 s at each tissue temperature. Cavitation increased by 1.8-4.5 dB at a tissue temperature of 38 °C compared to 30 °C, as measured both by passive cavitation images and cavitation doses. Tissue displacement also increased by 1.3- [Formula: see text] at baseline temperatures of 38 °C compared to 30 °C. Histological findings indicated small increases in red blood cell extravasation in the 38 °C baseline temperature condition compared to 30 °C at both PRFs. A strong positive correlation was found between the inertial cavitation dose and displacement imaging noise, indicating the potential ability of displacement imaging to simultaneously detect inertial cavitation in vivo. Overall, tissue temperature was found to modulate both in vivo cavitation and tissue displacement, and thus, both tissue temperature and cavitation can be monitored during FUS to ensure both safety and efficiency.

An Efficient and Multi-Focal Focused Ultrasound Technique for Harmonic Motion Imaging

Harmonic motion imaging (HMI) is an ultrasound-based elasticity imaging technique that utilizes oscillatory acoustic radiation force to estimate the mechanical properties of tissues, as well as monitor high-intensity focused ultrasound (HIFU) treatment. Conventionally, in HMI, a focused ultrasound (FUS) transducer generates oscillatory tissue displacements, and an imaging transducer acquires channel data for displacement estimation, with each transducer being driven with a separate system. The fixed position of the FUS focal spot requires mechanical translation of the transducers, which can be a time-consuming and challenging procedure. In this study, we developed and characterized a new HMI system with a multi-element FUS transducer with the capability of electronic focal steering of ±5 mm and ±2 mm from the geometric focus in the axial and lateral directions, respectively. A pulse sequence was developed to drive both the FUS and imaging transducers using a single ultrasound data acquisition (DAQ) system. The setup was validated on a tissue-mimicking phantom with embedded inclusions. Integrating beam steering with the mechanical translation of the transducers resulted in a consistent high contrast-to-noise ratio (CNR) for the inclusions with Young's moduli of 22 and 44 kPa within a 5-kPa background while the data acquisition speed is increased by 4.5-5.2-fold compared to the case when only mechanical movements were applied. The feasibility of simultaneous generation of multiple foci and tracking the induced displacements is demonstrated in phantoms for applications where imaging or treatment of a larger region is needed. Moreover, preliminary feasibility is shown in a human subject with a breast tumor, where the mean HMI displacement within the tumor was about 4 times lower than that within perilesional tissues. The proposed HMI system facilitates data acquisition in terms of flexibility and speed and can be potentially used in the clinic for breast cancer imaging and treatment.

Peripheral focused ultrasound stimulation and its applications: From therapeutics to human-computer interaction

Peripheral focused ultrasound stimulation (pFUS) has gained increasing attention in the past few decades, because it can be delivered to peripheral nerves, neural endings, or sub-organs. With different stimulation parameters, ultrasound stimulation could induce different modulation effects. Depending on the transmission medium, pFUS can be classified as body-coupled US stimulation, commonly used for therapeutics or neuromodulation, or as an air-coupled contactless US haptic system, which provides sensory inputs and allows distinct human-computer interaction paradigms. Despite growing interest in pFUS, the underlying working mechanisms remain only partially understood, and many applications are still in their infancy. This review focused on existing applications, working mechanisms, the latest progress, and future directions of pFUS. In terms of therapeutics, large-sample randomized clinical trials in humans are needed to translate these state of art techniques into treatments for specific diseases. The airborne US for human-computer interaction is still in its preliminary stage, but further efforts in task-oriented US applications might provide a promising interaction tool soon.

Ultrasound Activation of Mechanosensory Ion Channels in Caenorhabditis Elegans

Ultrasound is capable of noninvasive transcranial focusing and activating the targeted neurons in brain regions, receiving increasing attention. Ion channel, acting as a nano-ionic switch, enables to modulate the ion flow across cellular membranes and it is of importance to control the firing frequency of a neuron. In this article, we demonstrate the behavioral response of Caenorhabditis elegans (C. elegans) in response to ultrasound stimulation mediated by the activation of mechanical sensitive MEC-4 and MEC-6 ion channels. By specific mutation of MEC-4 and MEC-6 ion channels, mutant worms show a significant decrease in the percentage of reversal behavior (30% ± 10.5% and 10% ± 6.9%, respectively), compared with wild type (85% ± 8.2%). Furthermore, ALM and PLM neurons expressing MEC-4 and MEC-6 ion channels could be evoked directly by ultrasound stimulation, indicating MEC-4 and MEC-6 may pave a new way for sonogenetics.

Synchronous temperature variation monitoring during ultrasound imaging and/or treatment pulse application: a phantom study

Ultrasound attenuation through soft tissues can produce an acoustic radiation force (ARF) and heating. The ARF-induced displacements and temperature evaluations can reveal tissue properties and provide insights into focused ultrasound (FUS) bio-effects. In this study, we describe an interleaving pulse sequence tested in a tissue-mimicking phantom that alternates FUS and plane-wave imaging pulses at a 1 kHz frame rate. The FUS is amplitude modulated, enabling the simultaneous evaluation of tissue-mimicking phantom displacement using harmonic motion imaging (HMI) and temperature rise using thermal strain imaging (TSI). The parameters were varied with a spatial peak temporal average acoustic intensity (I _spta ) ranging from 1.5 to 311 W.cm^-2, mechanical index (MI) from 0.43 to 4.0, and total energy (E) from 0.24 to 83 J.cm^-2. The HMI and TSI processing could estimate displacement and temperature independently for temperatures below 1.80°C and displacements up to ~117 μm (I _spta <311 W.cm^-2, MI<4.0, and E<83 J.cm^-2) indicated by a steady-state tissue-mimicking phantom displacement throughout the sonication and a comparable temperature estimation with simulations in the absence of tissue-mimicking phantom motion. The TSI estimations presented a mean error of ±0.03°C versus thermocouple estimations with a mean error of ±0.24°C. The results presented herein indicate that HMI can operate at diagnostic-temperature levels (i.e., <1°C) even when exceeding diagnostic acoustic intensity levels (720 mW.cm^-2 < I _spta < 207 W.cm^-2). In addition, the combined HMI and TSI can potentially be used for simultaneous evaluation of safety during tissue elasticity imaging as well as FUS mechanism involved in novel ultrasound applications such as ultrasound neuromodulation and tumor ablation.