Transcranial focused ultrasound is an emerging form of non-invasive neuromodulation that uses acoustic energy to affect neuronal excitability. The effect of ultrasound on human motor cortical excitability and behavior is currently unknown. We apply ultrasound to the primary motor cortex in humans using a novel simultaneous transcranial ultrasound and magnetic stimulation paradigm that allows for concurrent and concentric ultrasound stimulation with transcranial magnetic stimulation (TMS). This allows for non-invasive inspection of the effect of ultrasound on motor neuronal excitability using the motor evoked potential (MEP). We test the effect of ultrasound on single pulse MEP recruitment curves and paired pulse protocols including short interval intracortical inhibition (SICI) and intracortical facilitation (ICF). In addition, we test the effect of ultrasound to motor cortex on a stimulus response reaction time task. Results show ultrasound inhibits the amplitude of single-pulse MEPs and attenuates intracortical facilitation but does not affect intracortical inhibition. Ultrasound also reduces reaction time on a simple stimulus response task. This is the first report of the effect of ultrasound on human motor cortical excitability and motor behavior and confirms previous results in the somatosensory cortex that ultrasound results in effective neuronal inhibition that confers a performance advantage.

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Transcranial focused ultrasound neuromodulation of the human primary motor cortex

An apparatus for generating focused currents in biological tissue is provided. The apparatus comprises an electric source capable of generating an electric field across a region of tissue and means for altering the permittivity of the tissue relative to the electric field, whereby a displacement current is generated. The means for altering the permittivity may be a chemical source, optical source, mechanical source, thermal source, or electromagnetic source.

Apparatus and method for stimulation of biological tissue

Primarily this report outlines our investigation on utilizing magneto-acousto-electrical-tomography (MAET) to image the lead field current density in volume conductors. A lead field current density distribution is obtained when a current/voltage source is applied to a sample via a pair of electrodes. This is the first time a high-spatial-resolution image of current density is presented using MAET. We also compare an experimental image of current density in a sample with its corresponding numerical simulation. To image the lead field current density, rather than applying a current/voltage source directly to the sample, we place the sample in a static magnetic field and focus an ultrasonic pulse on the sample to simulate a point-like current dipole source at the focal point. Then by using electrodes we measure the voltage/current signal which, based on the reciprocity theorem, is proportional to a component of the lead field current density. In the theory section, we derive the equation relating the measured voltage to the lead field current density and the displacement velocity caused by ultrasound. The experimental data include the MAET signal and an image of the lead field current density for a thin sample. In addition, we discuss the potential improvements for MAET especially to overcome the limitation created by the observation that no signal was detected from the interior of a region having a uniform conductivity. As an auxiliary we offer a mathematical formula whereby the lead field current density may be utilized to reconstruct the distribution of the electrical impedance in a piecewise smooth object.

https://iopscience.iop.org/article/10.1088/0967-3334/29/6/S04/pdf

Magneto-acousto-electrical tomography: a potential method for imaging current density and electrical impedance

A review on the use of magnetic fields and ultrasound for non-invasive cancer treatment.

In this paper, we provide the mathematical basis for three different magneto-acoustic imaging approaches (vibration potential tomography, magneto-acoustic tomography with magnetic induction and magneto-acoustic current imaging) and propose new algorithms for solving the inverse problem for each of them.

/pdf/mathematical-models-and-reconstruction-methods-in-magneto-iz3mpu4k2w.pdf

Mathematical models and reconstruction methods in magneto-acoustic imaging

The ions of solutions exposed to the propagation of ultrasound in the presence of a magnetic field experience Lorentz force Their movement gives rise to a local electric current density, which is proportional to the electric conductivity of the medium In vitro assessment of this current is performed using simple models of biological media A constant magnetic field of 035T and 500kHz pulsed ultrasound are used The sensing electrodes are exposed to neither the pressure wave nor the magnetic field, thus ensuring that the signal is not due to any undesirable electrode effect The experimental results confirm that the current is proportional to the electrical conductivity of the medium The changes in the measured current against the width of the measurement chamber show that the electrodes only collect fraction of the current created within the medium The magnitude of the measured current is 50nA in a saline solution of 05S/m conductivity The technique enabled the determination of the conductivity of a porcine blood sample against haematocrit It is concluded that this type of measurement has the potential to allow the electrical conductivity of a medium to be determined using ultrasound

Electric current generated by ultrasonically induced Lorentz force in biological media.

In the presence of a magnetic field, the propagation of ultrasound in an electrolyte produce a bulk current density due to Lorentz force. The present study describes the experimental set-up used to detect the interaction current in vitro. The used model consisted of a rectangular block of agar gel. A focused transducer produced pulses (6 /spl mu/s, 500 kHz) with a peak pressure up to 1.5 MPa. The magnetic field was applied using a purpose-build permanent magnet. The current was collected using two rectangular brass plates, parallel to the beam axis and placed on opposite sides of the gel block. The collected current was about 100 nA. The passage of the ultrasound pulse through the interfaces gel/surrounding medium produced clearly identified waveforms. The inverse filtering of the time signal enabled the 1-D reconstruction of the interfaces. The conclusion is that the interaction current is detectable and can potentially give rise to a new imaging modality.

Interaction ultrasound-magnetic field: experimental setup and detection of the interaction current

The non-linearity of the electrode-tissue interface impedance gives rise to harmonics and thus degrades the accuracy of impedance measurements. Also, electrodes are often driven into the non-linear range of their polarisation impedance. This is particularly true in clinical applications. Techniques to correct for electrode effects are usually based on linear electrode impedance data. However, these data can be very different from the non-linear values needed. Non-linear electrode data suggested a model based on simple assumptions. It is useful in predicting the frequency dependence of non-linear effects from linear properties. Sauer's treatment is a first attempt to provide a more general and rigorous basis for modelling the non-linear state. The paper reports Sauer's treatment of the non-linear case and points out its limitations. The paper considers Sauer's treatment of a series arrangement of two impedances. The tissue impedance is represented by a linear voltage-current characteristic. The interface impedance is represented by a Volterra expansion. The response of this network to periodic signals is calculated up to the second-order term of the series expansion. The resultant, time-dependent current is found to contain a DC term (rectification), as well as frequency-dependent terms. Sauer's treatment assumes a voltage clamp across the impedances and neglects higher-order terms in the series expansion. As a consequence, it fails adequately to represent some experimentally observed phenomena. It is therefore suggested that Sauer's expressions for the voltage divider should be combined with the non-linear treatments previously published by the co-authors. Although Sauer's work on the non-linear voltage divider was originally applied to the study of the non-linear behaviour of the electrode-electrolyte interface and biological tissues, it is stressed, however, that the work is applicable to a wide range of research areas.

J. Jossinet

Papers

Electric current generated by ultrasonically induced Lorentz force in biological media.

Interaction ultrasound-magnetic field: experimental setup and detection of the interaction current

Sauer's non-linear voltage division.