Aiming energy transmission method using one or more ultrasound transducers on an implantable device

Abstract

A method of targeting energy transmission using an ultrasonic transducer array (101, 102) on an implantable device (131) is disclosed. The method includes generating, by at least one ultrasonic transducer (111), a first ultrasonic wave having at least one frequency and at least one amplitude, transmitting the first ultrasonic wave to the implantable device (131), transmitting an echo signal from the implantable device (131) back to the ultrasonic transducer array (101, 102), picking up the echo signal by the ultrasonic transducer array (101, 102) such that one or more ultrasonic transducers (111) acquire a plurality of received signals, and coupling the received signals to a processor to produce a time reversed signal; and modulating the one or more ultrasonic transducers (111) to produce a time reversed wave in accordance with the time reversed signal to transmit ultrasonic energy to the implantable device (131), wherein the time reversed wave experiences constructive interference at a location from which the echo signal was transmitted.

Description

[0001] Cross-reference to related applications

[0002] This application claims the benefit of U.S. Provisional Patent Application No. 62 / 795,607, filed January 23, 2019, the entire contents of which are incorporated herein by reference. Technical Field

[0003] This disclosure generally relates to a method for targeted energy transfer using one or more ultrasonic and / or acoustic transducers on an implantable device. In particular, this disclosure relates to a method for focusing ultrasonic energy and transferring that energy to an implantable device. Background Technology

[0004] With advancements in implantable smart technologies, particularly the use of implantable autonomous sensors and actuators, the demand for developing better implantable medical devices is increasing. One research area focuses on improving the lifespan of implantable medical devices. Traditional implantable devices are limited by power capacity, and the power supply is typically bulky and requires frequent replacement. Implanting batteries in the human body can also be dangerous. Compared to other components in the electronic circuitry of implantable devices, batteries require more frequent replacements. Therefore, battery replacement leads to unnecessary infection risks and discomfort from invasive procedures.

[0005] Ultrasonic power transfer has been used to transfer energy from ultrasonic transducers and recharge implantable devices. Such ultrasound-driven implantable devices offer significant medical benefits, including miniaturization and a safer power source. Typically, the implantable device includes a rechargeable battery and an ultrasonic transceiver that efficiently converts ultrasonic power into electrical energy for power. This approach provides an efficient and relatively risk-free energy source, as well as a bidirectional data transfer method to and from the implantable device.

[0006] A research team at the University of California, Berkeley, has proposed a system called "neural dust" in US Patent No. US10118054B2. This system provides a method for acquiring in vivo biological conditions from an implantable device. According to the method, one or more ultrasound transducers are configured to transmit a carrier signal of ultrasound waves to an implantable device. The implantable device receives the carrier signal, modulates the ultrasound waves, and transmits the modulated signal back to the ultrasound transducers via backscattering. This method is a passive monitoring mode triggered by an external interrogator and does not store electrical energy in the implantable device for autonomous monitoring of deep tissue biological conditions. Furthermore, no specific methods are mentioned for focusing the ultrasound waves or improving the energy transmission efficiency from the transducers.

[0007] To achieve higher efficiency of ultrasound energy delivery, ideally the ultrasound waves should be focused on the implantable device. This can be accomplished by using a single bowl-shaped ultrasound transducer. However, such a bowl-shaped transducer has a natural focus spot at a fixed depth. Since the focus spot location is not flexible, neither is the target of the ultrasound waves. The same focusing problem is encountered when using transducer array elements.

[0008] In view of the problems set forth above, there is a need in the art for an improved method for performing targeting of energy delivery on an implantable device using rigid or flexible ultrasound transducer arrays. SUMMARY

[0009] Provided herein is a method for delivering ultrasound energy from an ultrasound transducer array having one or more ultrasound transducers to an implantable device by time reversal focusing. The method comprises the steps of: generating, by the at least one ultrasound transducer, a first ultrasound wave having at least one frequency and at least one amplitude; transmitting the first ultrasound wave to the implantable device, whereby a return signal is reflected from the implantable device to the ultrasound transducer array; receiving, by the ultrasound transducer array, the return signal such that the one or more ultrasound transducers acquire a plurality of received signals based on the return signal; coupling the received signals to a processor to generate a time reversed signal; modulating the one or more ultrasound transducers to generate a time reversed wave according to the time reversed signal, wherein the time reversed wave experiences constructive interference at the location from which the return signal was transmitted; and transmitting the time reversed wave to the implantable device to deliver ultrasound energy from the time reversed wave to the implantable device.

[0010] According to certain aspects of the present disclosure, the method further comprises the step of reflecting the first ultrasound wave as the return signal, wherein the implantable device is a device having a significantly higher acoustic impedance than adjacent tissue.

[0011] According to certain aspects of the present disclosure, the method further comprises iterating the steps of receiving, by the one or more ultrasound transducers, the return signal, and coupling the received signals to the processor to generate the time reversed signal to improve targeting of ultrasound energy.

[0012] According to certain aspects of the present disclosure, the first ultrasound wave is a plane wave or a random time delay wave generated by two or more ultrasound transducers, whereby there is no natural focus spot.

[0013] According to certain aspects of the present disclosure, the processor is configured to perform time reversal focusing on each return signal received by the one or more ultrasound transducers. The processor is configured to perform post-processing filtering of noise and correction of linearity and frequency response of ultrasound generated by the ultrasound transducers.

[0014] According to certain aspects, the ultrasonic energy from the time-reversed wave is converted into an electrical current by a piezoelectric element, a capacitive micromachined ultrasonic transducer (CMUT), an optical-based transducer, or other material.

[0015] According to certain aspects of the disclosure, the implantable device includes a material or substance having nonlinear ultrasonic properties such that the echo signal has a frequency different from the first ultrasonic wave.

[0016] According to certain aspects of the disclosure, the step of coupling the received signal to a processor to produce a time-reversed signal further comprises the steps of: converting the received signal to a digital signal using an analog-to-digital converter (ADC); writing the digital signal to a memory; performing a time-domain flip on the digital signal to obtain a flipped digital signal; and converting the flipped digital signal to a time-reversed signal using a digital-to-analog converter (DAC).

[0017] According to certain aspects of the disclosure, the step of coupling the received signal to a processor to produce a time-reversed signal further comprises the steps of: coupling the received signal to a Fourier transform circuit and an envelope detector arranged in parallel with the Fourier transform circuit; producing frequency components of the received signal using the Fourier transform circuit; producing amplitude components of the received signal using the envelope detector; flipping the received signal by obtaining a conjugate of a Fourier transform of the echo signal; and performing an inverse Fourier transform to obtain the time-reversed signal.

[0018] Also provided herein is a method for simultaneously transmitting ultrasonic energy to a plurality of implantable devices within a living body by time-reversed focusing. Each of the plurality of implantable devices is configured to receive ultrasonic energy from an array of ultrasonic transducers. Time-reversed waves from the array of ultrasonic transducers experience constructive interference at a plurality of locations where the plurality of implantable devices are positioned.

[0019] According to certain aspects of the disclosure, each of the plurality of implantable devices selectively responds to a range of frequencies to selectively activate one or more implantable devices.

[0020] This summary is provided to introduce a selection of concepts that are further described below in the detailed description. This summary is not intended to determine key or essential features of the claimed subject matter or the scope of the claimed subject matter. Other aspects and advantages of the present application are disclosed in the following detailed description. BRIEF DESCRIPTION OF DRAWINGS

[0021] The drawings, in which like numerals refer to like or functionally similar elements throughout the several views, include diagrams of certain embodiments to further illustrate and clarify various aspects, advantages, and features of the alignment energy delivery method as disclosed herein. It should be understood that these drawings and diagrams are only meant to depict certain embodiments of the present application and are not intended to limit the scope thereof. The alignment energy delivery method disclosed herein will be described and explained with additional characteristics and details by use of the drawings in which:

[0022] Figure 1A Depiction of spine reflection of ultrasound when rigid ultrasound transducer array is placed on body. Figure 1B Depiction of spine reflection of ultrasound when flexible ultrasound transducer array is placed on body.

[0023] Figure 2 Photo of example flexible ultrasound transducer array.

[0024] Figure 3A Depiction of focusing ultrasound waves onto one or more implantable devices using rigid ultrasound transducer array. Figure 3B Depiction of focusing ultrasound waves onto one or more implantable devices using flexible ultrasound transducer array.

[0025] Figure 4A Depiction of wavefront transmission from ultrasound transducer through scattering medium to implantable device (reflector). Figure 4B Depiction of reflection of ultrasound waves from implantable device back to ultrasound transducer. Figure 4C Depiction of time reversal wave transmission from ultrasound transducer through scattering medium to implantable device, where a focal point of the implantable device is created.

[0026] Figure 5A Depiction of sound intensity distribution of ultrasound waves emitted by concave transducer array without field modulation by electronic time delay. Figure 5B Depiction of sound intensity distribution of ultrasound waves emitted by concave transducer array with plane wave transmission. Figure 5C Depiction of sound intensity distribution of ultrasound waves emitted by concave transducer array with random time delay field. Figure 5D Depiction of sound intensity distribution of ultrasound waves emitted by single ultrasound transducer in concave transducer array.

[0027] Figure 6 Depiction of system diagram for passive time reversal focusing method to achieve maximum focusing on implantable device according to example embodiments of the present disclosure.

[0028] Figure 7A Depiction of example structure of system for performing time reversal focusing. Figure 7B Depiction of another example structure of system for performing time reversal focusing.

[0029] Figure 8 A system diagram of an active time reversal focusing method using an active implantable device is depicted in accordance with example embodiments of the present disclosure.

[0030] Figure 9 A circuit diagram of an AC time delay circuit for an active implantable device is depicted in accordance with example embodiments of the present disclosure.

[0031] Figure 10A A brightness mode (B-mode) ultrasound image reconstructed from plane wave activation using a rigid ultrasound transducer array is depicted. Figure 10B A time reversal field generated from a B-mode ultrasound image is depicted. Figure 10A

[0032] Figure 11A A signal of an instantaneous pulse at a focus location at a depth of 10 cm using a time reversal focusing method is depicted. Figure 11B A signal of an instantaneous pulse at a focus location at a depth of 10 cm using a time delay focusing method is depicted.

[0033] Figure 12A A signal of an 8-cycle tone burst at a frequency of 1 MHz at a focus location at a depth of 10 cm using a time reversal focusing method is depicted. Figure 12B A signal of an 8-cycle tone burst at a frequency of 1 MHz at a focus location at a depth of 10 cm using a time delay focusing method is depicted.

[0034] Figure 13A A sound wave front and corresponding simulated sound intensity distribution for focusing a sound wave at a specific location from an arc transducer array is depicted. Figure 13B A sound wave front and corresponding simulated sound intensity distribution for activating a plane wave of ultrasound from an arc transducer array is depicted.

[0035] Figure 14 A typical voltage produced per unit of sound pressure of an ultrasound transducer is depicted.

[0036] Figure 15A An example of focusing ultrasound to an implantable device by activating ultrasound transducers with a time delay is depicted. Figure 15B An example of focusing ultrasound to an implantable device by activating an optimal number of ultrasound transducers is depicted.

[0037] Figure 16 A system diagram of an active time delay focusing method using an active implantable device is depicted in accordance with example embodiments of the present disclosure.

[0038] Figure 17 A frequency spectrum of a fundamental signal and a first harmonic signal is depicted. ​

[0039] Figure 18A Images reconstructed from a known convex ultrasound transducer array are depicted. Figure 18B Images reconstructed from a known S-shaped ultrasound transducer array are depicted. Figure 18C Images reconstructed from a known concave ultrasound transducer array are depicted.

[0040] Figure 19A An exemplary ultrasound image of a metal plate implanted in chicken breast tissue is depicted. Figure 19B Intensity profiles when using the time reversal method are depicted. Figure 19A

[0041] The skilled person will appreciate that the elements in the figures are shown for simplicity and clarity and are not necessarily drawn to scale. DETAILED DESCRIPTION

[0042] The present disclosure relates generally to a method of targeted energy transmission. More particularly, but not by way of limitation, the present disclosure relates to a method for focusing ultrasound energy using one or more ultrasound transducers (e.g., rigid or flexible ultrasound transducer arrays) and communicating with an implantable device. One object of the present disclosure is to maximize the efficiency of ultrasound energy transmission by focusing ultrasound waves at an implantable device.

[0043] In the following embodiments, the method of targeted energy transmission is merely exemplary and is not intended to limit the disclosure or applications and / or uses of the disclosure. It is to be understood that numerous variations can exist and that the example embodiments described herein can be practiced in a variety of ways without deviating from the spirit or scope of the disclosure as set forth in the appended claims. The different embodiments described herein can be combined to form further embodiments of the invention. The headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described.

[0044] Benefits, advantages, solutions to problems, and any element(s) that might cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential feature or element of any or all the claims. The application is defined solely by the appended claims including any amendments made during the pendency of this application and all equivalents of the claims as issued. It will be appreciated that details are given by way of example only and modifications can be made by those skilled in the art without departing from the scope of the application.

[0045] For simplicity and clarity, related terms not otherwise specifically defined herein are intended to have their ordinary and accustomed meanings in the field of the present application, unless otherwise indicated. For example, the terms "first", "second", "third", etc. are used merely as identifiers, and are not intended to be construed as indicating any actual physical, chronological or other relationship between the items so identified.​

[0046] As used herein, the term "coupled" or "connected" or any variant thereof, means any direct or indirect coupling or connection between two or more elements, unless stated otherwise or clearly contradicted by context.

[0047] The present disclosure generally describes ultrasonic energy transmission and communication to and from one or more implantable devices. The term "implantable device" is used to describe a device in a subject for identifying markers, sensing physiological conditions such as temperature, pressure, pH, pulse rate, oxygen, analytes, strain, glucose, or any combination thereof. The implantable device can include multiple channels, sensors, transmitters, or detectors. The implantable device can perform other functions such as releasing drugs or chemicals into the body, stimulating nerves and tissues, or treating heart problems. The implantable device is implanted in a living subject, where the subject can be a human or an animal. As used herein, the term "ultrasonic energy" generally refers to energy transmitted by acoustic waves with frequencies between 20 kHz and 1 GHz. The term "acoustic wave" refers to a broadband acoustic wave with frequencies between 10 kHz and 1 GHz.

[0048] In certain embodiments, the implantable device includes one or more sensors for detecting the amount of an analyte, strain, or pH.

[0049] In certain embodiments, the implantable device includes an optical detector for detecting blood pressure, blood oxygenation, melanin levels, glucose, pulse rate, any other spectroscopic change signal related to health, or any light absorption change related to health.

[0050] In certain embodiments, the implantable device includes a temperature sensor, such as a thermistor or thermocouple, for detecting temperature.

[0051] In certain embodiments, the implantable device includes a pressure sensor, such as a microelectromechanical system (MEMS) sensor, for measuring blood pressure, intracranial pressure, pulse rate, or other pressure in the body.

[0052] In certain embodiments, the implantable device includes a potentiometric or amperometric chemical sensor for detecting oxygen levels, pH, or glucose.

[0053] In certain embodiments, the implantable device includes a drug release dispenser for releasing drugs or chemicals into the body.

[0054] In certain embodiments, the implantable device includes a microstimulator or electrode for stimulating nerves or tissues or treating heart problems.

[0055] In certain embodiments, the implantable device can emit electromagnetic or mechanical waves to stimulate tissues, nerves, or organs.

[0056] In some embodiments, the implantable device can cause temperature changes at localized or large locations.

[0057] In some embodiments, the implantable device may use components that require real-time pressure changes, such as actuators or clamps, to introduce pressure changes into the body system.

[0058] In some embodiments, the implantable device can introduce or detect magnetic fields, such as sensors or coils.

[0059] A. Ultrasound transducer array

[0060] Figure 1A and Figure 1B A rigid ultrasonic transducer array 101 and a flexible ultrasonic transducer array 102, placed on a living organism, are shown respectively. The ultrasonic transducer array includes multiple ultrasonic transducers 111, which may be piezoelectric elements, capacitive micromechanical ultrasonic transducers (CMUTs), optically based transducers, or other materials. In some embodiments, the flexible ultrasonic transducer array 102 may include a single ultrasonic element, such as a CMUT called "Sonic Paper". In some other cases, a single ultrasonic transducer 111 may be used instead of an array. Each ultrasonic transducer 111 in the array is controlled by a processor, which can configure each ultrasonic transducer to receive or transmit ultrasonic waves. This allows one or more ultrasonic transducers 111 to transmit ultrasonic waves with different time delays, phase shifts, pulse frequencies, amplitudes, and / or wavelengths. In some embodiments, one or more ultrasonic transducers 111 in the array may be placed with regular spacing, irregular spacing, or sparse arrangement.

[0061] One or more ultrasonic transducers 111 are connected to a processor or computing system configured to selectively communicate with each or all of the ultrasonic transducers 111 to transmit or receive ultrasonic waves. The processor may be configured as one or more central processing units (CPUs), microcontroller units (MCUs), general-purpose microprocessors, application-specific integrated circuits (ASICs), field-programmable logic arrays (FPGAs), programmable I / O devices, or other equivalent integrated or discrete electronic circuits. In some embodiments, the processor or computing system may also include other components such as analog-to-digital converters (ADCs), digital-to-analog converters (DACs), memory, a display panel, a power supply, and I / O ports.

[0062] The flexible ultrasound transducer array 102 can be considered to have one or more ultrasound transducers 111 attached on a flexible and stretchable material, a flexible printed circuit board (PCB), or a Kapton polyimide film, which allows relative movement of the ultrasound transducers 111 position and orientation to adjacent. Thus, compared to the rigid ultrasound transducer array 101, the flexible ultrasound transducer array 102 can allow better flexing and fitting to the complex geometry of the living body surface. An exemplary flexible ultrasound transducer array 102 includes an array of 128 ultrasound transducers 111, as shown in Figure 2

[0063] Ultrasound transmitted from the ultrasound transducers 111 interacts with the body tissue after entering the living body. This interaction can cause reflection, scattering, and refraction of the ultrasound waves. This happens at the tissue boundary where the tissue or medium has different acoustic impedance. Absorption can also cause attenuation to occur, which can happen in the same medium. The acoustic impedance deviation determines the amount of reflection, while the acoustic impedance (Z) is determined by the tissue density (p) and the speed of sound (c), as follows:

[0064] Z = pc (1)

[0065] A more significant change in density between two tissues will result in a greater change in acoustic impedance. The impedance change between two media is called an acoustic impedance mismatch. This acoustic impedance difference between two tissues explains the amount of reflection that occurs at the tissue boundary.

[0066] Since the spine 121 has a higher density than the adjacent tissue, the spine 121 can reflect the incident ultrasound waves from the ultrasound transducers 111. The reflected ultrasound waves can be picked up by the ultrasound transducers 111 on the array. Since the flexible ultrasound transducer array 102 can provide better angular coverage to the living body, it can collect more acoustic information than the rigid ultrasound transducer array 101. Thus, in most applications, the flexible ultrasound transducer array 102 is more preferable than the linearly arranged rigid ultrasound transducer array 101.

[0067] By using an ultrasound transducer array (rigid or flexible), one or more ultrasound transducers 111 can be configured to transmit ultrasound waves each having different time delays, phase modulations, and waveform modulations. By applying time delays, the phases of the ultrasound waves can add up at the target ultrasound focusing region, so that the ultrasound energy can have maximum intensity at a defined focal point.

[0068] Reference Figure 3A ​The rigid ultrasound transducer array 101 focuses the ultrasound beam onto the implantable device 131 inside the living body by applying different time delays to each ultrasound transducer 111. Since the rigid ultrasound transducer array 101 cannot be in good contact with the surface of the living body, there is acoustic coupling between some of the ultrasound transducers 111 and the living body. Similarly, as shown in Figure 3B the flexible ultrasound transducer array 102 also focuses the ultrasound beam onto the implantable device 131 inside the living body by applying different time delays to each ultrasound transducer 111.

[0069] In order to aim the ultrasound at the implantable device 131, it is necessary to dynamically monitor the position of the implantable device 131. Dynamic monitoring means capturing real-time information related to the position of the implantable device 131, which is then used to calculate the aiming of the ultrasound transmission. The method of monitoring the position of the implantable device 131 and focusing the ultrasound is described in more detail below. The method can also be applied in the case of implanting two or more ultrasound-driven implantable devices 131 in the body.

[0070] One method for focusing ultrasound energy to an aiming position is to perform time reversal focusing. The concept of time reversal focusing is a reciprocity-based method. In order to aim energy onto an implantable device 131 inside the living body, ultrasound waves originating from a sound source are first propagated to the implantable device 131, where the implantable device 131 reflects or transmits the ultrasound waves. In the case of reflected waves, time reversal is achieved using a passive method. For the case of transmitted waves, an active method is used instead.

[0071] B. Time reversal focusing

[0072] Reference is now made to Figures 4A-4C , which demonstrates the concept of time reversal focusing. The array of ultrasound transducers 100 can be arranged linearly or in other ways into other shapes. As shown in Figure 4AAs shown illustratively, one of the ultrasound transducers 111 generates ultrasound waves having at least one frequency and at least one amplitude and transmits the ultrasound waves into the living body having the implantable device 131 therein. In certain embodiments, the ultrasound waves can be transmitted into the living body having a plurality of implantable devices 131 therein, as long as all of the implantable devices 131 are within the field of view. The frequency generated by the ultrasound transducers 111 refers to a frequency higher than audible sound. In other alternative embodiments, two or more ultrasound transducers 111 can be used to generate plane waves or random time delay waves instead of using a single ultrasound transducer 111. In certain embodiments, the center frequency of the ultrasound transducers 111 is about 5 MHz. The transmitted ultrasound can be pulsed or burst. Between the ultrasound transducers 111 and the implantable device 131, there is a scattering medium 141 of body tissue. Refraction occurs when the ultrasound waves are deflected from a straight path of entry into the scattering medium 141 to a direction having a deflection angle. Generally, ultrasound waves are refracted at a medium having a different acoustic impedance. The ultrasound energy can also be absorbed, reflected, or scattered by the tissue. After passing through the scattering medium 141, the ultrasound waves are incident on the implantable device 131. In one embodiment, the implantable device 131 is a device having a significantly higher acoustic impedance than the adjacent tissue because of a significantly higher density. The implantable device 131 can act as a reflector and reflect the ultrasound waves back as an echo signal using a passive method. In alternative embodiments, the implantable device 131 can receive the ultrasound energy and convert it to electrical energy; thus, the ultrasound transducers in the implantable device 131 generate ultrasound waves in the opposite direction. Since the ultrasound waves are generated by the implantable device 131, this method is an active method using more complex electronic components.

[0073] As shown illustratively, the echo signals or generated ultrasound waves from the implantable device 131 travel through the scattering medium 141 in a scattering path back to the ultrasound transducer array 100. The scattering medium 141 has the same scattering and transmission properties when the ultrasound waves travel through in a forward or backward manner. Thus, by using a time reversal method, the focal point at the reflection point can be located. Figure 4B

[0074] One or more ultrasound transducers 111 acquire the echo signals or generated ultrasound waves, each having different amplitude and time delay properties. The received signals 151 are coupled to a processor to generate a time reversed signal 152. As shown illustratively, the time reversed signal 152 is coupled to the ultrasound transducers 111 to generate ultrasound waves having the same amplitude and time delay properties as the echo signals. The ultrasound waves are transmitted into the living body having the implantable device 131 therein. The ultrasound waves are transmitted in a forward or backward manner through the scattering medium 141. The scattering medium 141 has the same scattering and transmission properties when the ultrasound waves travel through in a forward or backward manner. Thus, the focal point at the reflection point is located. Figure 4C ​As shown, the time reversal signal 152 is coupled from the processor to the one or more ultrasonic transducers 111 to produce time reversal waves that are transmitted into the living body and dynamically focused at the implantable device 131. After passing through the scattering medium 141, the time reversal waves from the one or more ultrasonic transducers 111 can undergo constructive interference, where the phase is increased at the location where the reflection originally came from; and a focal point of ultrasonic energy is created at the reflection point. In the case of a single ultrasonic transducer 111, the time reversal wave and any scattered or refracted waves thereof can undergo constructive interference at the location where the reflection originally came from. For example, if an ultrasonic transducer 111 in the ultrasonic transducer array 100 receives a pressure field of p(x, t), where x is the location of the ultrasonic transducer 111 and t is time, then the corresponding time reversal signal will be p'(x, T - t), where T is a total delay constant required by the system. The focused intensity at each location is proportional to the reflection intensity plus an attenuation factor.

[0075] For the purpose of initial activation, the ultrasonic field generated from the ultrasonic transducer array 100 can be modulated by different time delays to achieve dynamic focusing onto the implantable device 131. If a flexible ultrasonic transducer array 102 is used and the ultrasonic transducers 111 are not linearly arranged (e.g., in a concave shape), the modulation of the ultrasonic can improve the transmission quality. A concave array 100 of ultrasonic transducers naturally has a focal point with the highest ultrasonic intensity compared to other adjacent regions, as shown in the sound intensity distribution in Figure 5A This is particularly problematic for the time reversal focusing method because any reflected ultrasonic waves from the region with the natural focal point have higher intensity compared to reflected ultrasonic waves from other regions.

[0076] To overcome this problem, the present disclosure provides a method of applying time delay focusing to different locations within the field of view by sweeping the ultrasonic beam through all locations. However, this method can require long delays, which takes a significant amount of time to sweep the ultrasonic beam. In addition, accurate information of each ultrasonic transducer 111 in the ultrasonic transducer array 100 is required, which will be difficult or even impossible for a flexible ultrasonic transducer array 102.

[0077] In one embodiment, as shown in Figure 5B and Figure 5C The natural focal point can be removed by configuring the ultrasonic transducers 111 to modulate the ultrasonic field and produce a plane wave or a random time delay field. A plane wave is a flat wavefront produced by an ultrasonic transducer array 100 with parallel and non-focused transmission. A random time delay field is produced by activating each ultrasonic transducer 111 with a random delay. In another embodiment, as shown in Figure 5DAs shown, the natural focal point can also be removed by transmitting a single ultrasound wave with only one single ultrasound transducer 111 configured. This method has a weak or no focusing effect.

[0078] In addition to the reflection of ultrasound waves by the implantable device 131, undesired reflections can also occur when the ultrasound waves interact with body tissues inside the living body that have acoustic impedance mismatches. In particular, body structures such as bones and muscle tissue can reflect ultrasound. This reflection is unavoidable. Therefore, it is important to ensure that the implantable device 131 can reflect more ultrasound than other structures in the body. One simple method is to select a material with a higher impedance mismatch than the surrounding medium.

[0079] Because the reflection of ultrasound is the result of an acoustic impedance mismatch between two media. The fraction R of the energy reflected at the boundary between the two media can be described by the following equation:

[0080]

[0081] where z1 and z2 are the acoustic impedance of medium 1 and medium 2.

[0082] As can be seen from equation (2), the greater the acoustic impedance mismatch, the stronger the reflected echo that can be produced. Therefore, by making at least a portion of the implantable device 131 from a material with a higher acoustic impedance mismatch than the human tissue, the implantable device 131 can produce a stronger reflection of the echo signal. For example, an implantable device 131 for monitoring or stimulating muscle tissue is implanted in an area with target human muscle tissue. The acoustic impedance of human muscle tissue is 1.68 x 10 6 kg / (sec.m 2 ). The material used for the implantable device 131 should have a higher acoustic impedance difference, for example, stainless steel with an acoustic impedance of 46.02 x 10 6 kg / (sec.m 2 ). With this arrangement, the implantable device 131 can reflect ultrasound waves with a strong echo signal.

[0083] Assuming that the implantable device 131 can produce a significantly stronger ultrasound reflection than other physiological structures in the living body, most of the acoustic energy reflected to the ultrasound transducer array 100 comes from the reflection at the implantable device 131. Time reversal focusing is then performed using the received echo signals. Most of the acoustic energy transmitted from the ultrasound transducer array 100 thereafter can be aimed back to the reflection point at the implantable device 131. Advantageously, variations due to the position and orientation of each ultrasound transducer 111 can be compensated. Thus, the precise physical position and orientation of the ultrasound transducers 111 are not necessary; therefore, this method is able to use a flexible ultrasound transducer array 102 without any details of each ultrasound transducer 111.

[0084] Figure 6 A system diagram for a passive time reversal focusing method to achieve maximum focusing on the implantable device 131 is shown. This is an optional iterative method for optimizing focusing. Given that the implantable device 131 reflects more ultrasound than other structures in the medium, this passive time reversal focusing method can iteratively increase the acoustic energy focused onto the implantable device 131 after each iteration cycle.

[0085] The passive time reversal focusing method can be used to transmit acoustic energy to the implantable device 131 using an ultrasound transducer array 600 placed on the living body. The ultrasound energy from the time reversal wave is converted to an electric current through piezoelectric elements, CMUTs, optical-based transducers, or other materials. The ultrasound transducer array 600 can be a rigid ultrasound transducer array 101 or a flexible ultrasound transducer array 102. The ultrasound transducer array 600 can be configured to transmit a plane wave, a random time delay field, or a single ultrasound wave. In the case of transmitting ultrasound energy to a living body with multiple implantable devices 131 simultaneously, the time reversal wave experiences constructive interference at multiple locations where the multiple implantable devices 131 are located. In certain embodiments, each of the multiple implantable devices 131 selectively responds to a specific frequency range to selectively activate a specific one or more implantable devices 131.

[0086] In one embodiment, two or more ultrasound transducers 111 are activated simultaneously to produce a plane wave in the form of a flat wavefront. The transmission of the ultrasound signal has no focal point and does not produce natural focusing.

[0087] In one embodiment, two or more ultrasound transducers 111 are activated, and each ultrasound transducer 111 transmits an ultrasound signal with a random and different time delay. The acoustic intensity distribution is not focused at any point, and a random time delay field can be transmitted.

[0088] In one embodiment, only one signal ultrasound transducer 111 is activated to transmit ultrasound signals. The other ultrasound transducers 111 in the ultrasound transducer array 600 are still enabled as receivers and acquire echo signals to determine the location of the implantable device 131.

[0089] Ultrasound transmitted from the ultrasound transducer array 600 (Tx) enters the body tissue medium 610 and can experience refraction, scattering, attenuation, and reflection. Due to the implantable device 131 having a higher acoustic impedance difference, the reflected echo signals are primarily from the implantable device 131. The echo signals are picked up by the ultrasound transducer array 600, and the multiple received signals are acquired by one or more ultrasound transducers 111 of the ultrasound transducer array 600 (Rx) for analysis.

[0090] The received signals are coupled to the processor 620. Each ultrasound transducer 111 can provide direct information of the phase and intensity of the received echo signals at its location and orientation and compensate for any changes thereof. The processor 620 is configured to perform time reversal focusing on each echo signal. In certain embodiments, the processor 620 is further configured to perform post-processing filtering of noise and correct for linearity and frequency response of the ultrasound produced by the ultrasound transducers 111. The time reversed signals are coupled back to the ultrasound transducer array 600 (Tx) to transmit ultrasound waves into the body tissue medium 610 according to the time reversed echo signals and dynamically focus at the implantable device 131. During this transmission, the previously receiving ultrasound transducers 111 are now configured to transmit ultrasound. Since the transmission is based on the time reversed echo signals, the acoustic energy transmitted from the ultrasound transducer array 600 can be advantageously aimed back to the implantable device 131.

[0091] An iterative method can be used in conjunction with the passive time reversal focusing method, whereby the accuracy of the aiming can be improved. Since the implantable device 131 will reflect more ultrasound than other structures in the medium 610, iteratively applying the time reversal method can increase the focused ultrasound energy at the implantable device 131 with each iteration. During each iteration, the processor 620 determines whether the ultrasound energy aiming is improved. The iterations are repeated until the energy aiming is not further improved. This iterative method can advantageously enhance the accuracy of the focusing to achieve maximum focusing on the implantable device 131.

[0092] Figure 7AA system for performing time reversal focusing is shown according to one embodiment. The echo signal received by the ultrasound transducer 111 is converted to a digital signal using an ADC 711. The ADC 711 is typically implemented using a mix of integrated circuit devices and discrete components or integrated into the processor 620. The digital signal is then written to a memory 712 of the processor 620 for processing time domain flipping 713. The time domain flipping 713 is performed by time flipping the digital signal from left to right to obtain a flipped digital signal. By coupling the flipped digital signal to a DAC 714, a time reversed signal can be obtained. Preferably, the ADC 711 and the DAC 714 have the same number of bits in the digital domain. In certain embodiments, the time reversed signal can perform post-processing filtering of noise and correct for linearity and frequency response of the ultrasound generation by the ultrasound transducer 111.

[0093] Figure 7B Another system for performing time reversal focusing is shown according to one embodiment. The echo signal received by the ultrasound transducer 111 is coupled to a Fourier transform circuit 721 and an envelope detector 722 disposed in parallel with the Fourier transform circuit 721. The Fourier transform circuit 721 is configured to produce frequency components of the echo signal, and the circuit can include components such as multipliers and computation units. Alternatively, the Fourier transform circuit 721 can be implemented as an algorithm or software executable by the processor 620 or a computer system. The envelope detector 722 is configured to receive the echo signal, track the voltage envelope of the echo signal, and produce an amplitude component of the echo signal.

[0094] A digital signal processing (DSP) circuit 723 receives the frequency components and the amplitude component of the echo signal from the Fourier transform circuit 721 and the envelope detector 722 to flip the echo signal. The DSP circuit 723 can be implemented as an integrated circuit and operates to obtain a conjugate of the Fourier transform of the echo signal. By using an inverse Fourier transform circuit 724, a time domain of the time reversed signal can be obtained. Preferably, the Fourier transform circuit 721 and the inverse Fourier transform circuit 724 have the same number of data samples. Alternatively, the inverse Fourier transform circuit 724 can be implemented as an algorithm or software executable by the processor 620 or a computer system.

[0095] Figure 8A system diagram of an active time-reversal focusing method using an active implantable device 810 transmitting ultrasound is shown. The active time-reversal focusing method is similar to, but reverses, a passive time-reversal focusing method. Initially, at least one ultrasound transducer 111 is activated and transmits ultrasound waves into the living body. The active implantable device 810 includes a built-in ultrasound transducer for receiving ultrasound waves. The ultrasound waves transmit electrical energy to charge the active implantable device 810. In some embodiments, the ultrasound transducer 111 may perform synchronization with the active implantable device 810. In some embodiments, electrical energy for charging the active implantable device 810 may be collected from other sources, such as heat, ions, dynamic motion, or magnetic fields.

[0096] Once the active implantable device 810 is charged with sufficient electrical energy, its internal dummy transducer can generate a second ultrasonic wave in the opposite direction to the incident ultrasonic wave from the ultrasonic transducer 111. In some embodiments, the internal dummy transducer can emit a second ultrasonic wave of approximately the same intensity in all directions to reach all ultrasonic transducers 111 in the ultrasonic transducer array 800, thereby maximizing efficiency. The second ultrasonic wave travels through the tissue of a living organism and is then picked up by the ultrasonic transducer array 800. It is not necessary for all ultrasonic transducers 111 in the ultrasonic transducer array 800 to pick up the second ultrasonic wave. The ultrasonic transducer array 800, which may be of a rigid or flexible shape, can then perform time-reversal focusing of the active implantable device 810 with a more focused and higher energy return. In some embodiments, one or more ultrasonic transducers 111 acquire multiple received signals, which are coupled to a processor 820. The processor 820 is configured to perform time-reversal focusing on each received signal. The time-reversed signal is coupled back to the ultrasonic transducer 111 (Tx) for corresponding transmission of ultrasonic waves, so that the acoustic energy can be advantageously and dynamically focused on the active implantable device 810 and aimed back onto the active implantable device 810.

[0097] The iterative method can be combined with an active time-reversal focusing method to improve aiming accuracy. During each iteration, the internal dextrinsic transducer of the active implantable device 810 transmits ultrasonic waves according to the incident ultrasonic waves. By iteratively applying the time-reversal method, the ultrasonic energy can be more concentrated on the active implantable device 810. During each iteration, the processor 820 determines whether the energy aiming has been improved. The iteration is repeated until the energy aiming is no longer further improved. This iterative method can advantageously improve focusing accuracy to achieve maximum focusing on the active implantable device 810.

[0098] When the first ultrasonic wave is picked up by the internal dextrinsic transducer, the ultrasonic wave is converted into an electric current and coupled to an alternating current (AC) time delay circuit. An exemplary circuit diagram of the AC time delay circuit is shown below. Figure 9Other electronic circuits including resistor-capacitor (RC) pairs, transistors, FPGAs, ASICs, flip-flops, or other electronic components can be used to introduce time delays without departing from the spirit of the disclosure. The purpose of using the AC time delay circuit is to introduce a time delay for the received ultrasound waves so that the ultrasound waves that the ultrasound transducer 111 would have otherwise transmitted during the transmission period are dissipated. Thus, the active implantable device 810 can transmit a second ultrasound wave back to the ultrasound transducer array 800 during the detection period without interference from the ultrasound or any reflections thereof from the transmission period. In certain embodiments, the second ultrasound wave includes a synchronization signal having information about the time delay, which is received by the ultrasound transducer array 800 for the processor 820 to determine the time required for the ultrasound to travel to the active implantable device 810.

[0099] In the case of transmitting ultrasound energy to a living body having multiple active implantable devices 810 at the same time, the time-reversed wave experiences constructive interference at multiple locations where the multiple active implantable devices 810 are located. In certain embodiments, each of the multiple active implantable devices 810 selectively responds to a specific frequency range to selectively activate a specific one or more active implantable devices 810.

[0100] Figure 10A A brightness mode (B-mode) ultrasound image reconstructed from a plane wave activation is provided. The experiment is performed on a piece of chicken breast sample with a stainless steel surface embedded to simulate an implantable device. From the experimental results, it can be seen that the device with a higher acoustic impedance difference can be seen on the upper right side of the strong reflection ultrasound image. Figure 10B A time-reversed field generated from a B-mode ultrasound image shown in Figure 10A is shown. Although the figure is shown in grayscale (color not shown), the colors of the figure are blue (darker regions) and yellow (brighter regions). The blue regions represent low ultrasound intensity, while the yellow regions represent high ultrasound intensity. The intensity on the time-reversed field is generated from a computer simulation at a homogeneous medium, and the intensity shows the time-averaged intensity throughout the burst and the acoustic wave travel period.

[0101] The time-reversed focusing method of the present invention can also take into account the inhomogeneity of the body tissue medium. In contrast to the conventional time-delay focusing method that focuses ultrasound waves by assuming a uniform and known velocity across the medium, the time-reversed focusing method can advantageously take into account any effects, such as scattering and sound velocity variations when propagating through the medium. Thus, using the time-reversed focusing method can mitigate the noise and distortion of the ultrasound signal.

[0102] To demonstrate the difference, signals of (1) an instantaneous pulse and (2) an acoustic source with a 1 MHz tone burst of 7 cycles were used and focused at a location inside a tissue medium at a depth of 10 cm. The results of the time reversal focusing method and the time delay focusing method were compared. Simulations were performed using the k-Wave simulation toolbox. The acoustic speed of the medium was assumed to be 1540 m / s and the density to be 1000 kg / m 3 and the speed of the scattering medium was 1800 m / s and the density was 1500 kg / m 3 The scattering medium was randomly distributed in about 1 / 4 of the medium space. The time reversal focusing method was performed by simulating focusing of acoustic waves at a location inside the medium at a depth of 10 cm. The reflected acoustic waves were picked up and acquired by 216 linearly aligned ultrasound transducers in the simulation space. Time reversed ultrasound waves were emitted from each ultrasound transducer by performing time reversal focusing on the received ultrasound signals. For time delay focusing, the signals emitted by the 216 ultrasound transducers had time delays corresponding to the distance to the focal point. By assuming an acoustic speed of 1540 m / s, the time delays could be set so that the ultrasound signals would arrive at the focal point at the same time.

[0103] Figure 11A and Figure 11B A comparison between the time reversal focusing method using an instantaneous pulse and the time delay focusing method of is shown. Figure 12A and Figure 12B A comparison between the time reversal focusing method using an acoustic source with a 1 MHz tone burst of 7 cycles and the time delay focusing method is shown. The signal-to-noise ratio (SNR) can be obtained by calculating the root mean square of the time between the signals and dividing by the maximum signal strength. For the instantaneous pulse case, the SNR using the time reversal focusing method was 3.01% while using the time delay method was 5.61%. The SNR for the tone burst was 5.01% for the time reversal focusing method while it was 12.92% for the time delay method.

[0104] C. Image formation and targeting energy delivery

[0105] The position of the implantable device 131 can also be monitored by forming an ultrasound image. This is done by reflecting ultrasound waves from the surface of the implantable device 131. Due to the acoustic impedance mismatch of the implantable device 131 and the adjacent tissue, the ultrasound waves travelling to the surface are reflected at the boundary. Advantageously, the reflection of the acoustic waves can provide information of the position and structure of the implantable device 131, as well as other physical properties such as stiffness, temperature and speed of sound.

[0106] Reference is made to Figure 13AThe flexible ultrasound transducer array 102 operating as a sound wave source is configured to generate sound waves. The sound waves generated at each ultrasound transducer 111 are determined based on the relative position, orientation, and directivity of each ultrasound transducer 111, so that time delay focusing and / or electronic beamforming can be performed. The time delay focusing and electronic beamforming are performed by controlling the timing and / or phase of the ultrasound wave generation by the processor. In certain embodiments, the time delay focusing is performed by calculating the relative center position from the flexible ultrasound transducer array 102 to the focal point 1301, determining the delay for each ultrasound transducer 111, and generating multiple ultrasound waves each with the delay by the ultrasound transducers 111, so that a sound wave front 1302 propagating to the focal point 1301 can be obtained by constructive interference of the ultrasound waves at the focal point 1301. The constructive interference of the ultrasound waves can produce stronger energy at the focal point 1301. By repeating and iteratively performing the ultrasound focusing method at different positions and depths, a complete scan of the entire field of view can be performed. The echoes received from each position can be combined with the reflection times to produce a complete ultrasound image on the sound beam intensity distribution.

[0107] Reference is made to Figure 13B An alternative method for forming an ultrasound image is depicted. Either the rigid transducer array 101 or the flexible transducer array 102 can be used. For the flexible transducer array 102, the method is performed by calculating the relative position and orientation between the multiple ultrasound transducers 111 in real time, determining the delay for each ultrasound transducer 111, and generating multiple ultrasound waves each with the delay by the ultrasound transducers 111, so that a planar sound wave front 1312 can be obtained. The planar sound wave front 1312 is a flat wave front. For the rigid transducer array 101, the planar sound wave front 1312 can be obtained by generating multiple ultrasound waves simultaneously by the ultrasound transducers 111. By changing the steering angle of the ultrasound transducers 111, the reflecting surfaces inside the living body can form a reflection matrix. The reflection matrix together with the position and orientation information of the transducer array can be used collectively to calculate the physical position of each reflecting surface and thereby form an ultrasound image.

[0108] After the ultrasound image is formed, image analysis is performed to identify the implantable device 131 in the ultrasound image. Information such as brightness, shape, thickness, stiffness, etc. can be used to identify the implantable device 131 within the ultrasound image. Pattern recognition can also be used to determine the location on the implant based on the specific shape and size of the implantable device 131. Furthermore, by manipulating the surface of the implantable device 131 or using different materials, the reflection pattern of the echo signal can be made unique so that the echo signal can be easily distinguished from other echoes from other surrounding tissues. Classification such as big data analysis can be used to classify the object under the ultrasound image and perform accurate identification of the implantable device 131. In addition to pattern recognition, other imaging techniques such as shear wave elastography, phase-contrast method can also be used to improve the determination of the location of the implantable device 131 from the image.

[0109] After the location and orientation of the implantable device 131 are determined, focused energy is delivered to the implantable device 131 with maximum efficiency. The method includes focusing ultrasound energy to the implantable device 131 and converting the ultrasound energy to electric current through piezoelectric elements, CMUT, optical-based transducers, or other materials. The ultrasound transducer in the implantable device 131 typically has a specific frequency response. The typical voltage produced by the ultrasound transducer per unit of acoustic pressure is shown as Figure 14 The energy transmission efficiency can be optimized by selecting the resonant frequency of the ultrasound transducer to transmit ultrasound waves. In certain embodiments, the available frequency range of the ultrasound transducer can also be selected for transmission if linear conversion of ultrasound energy to electric current is desired.

[0110] Referring now to Figure 15A and Figure 15B After the frequency range is selected, the focused ultrasound field can be generated by calculating the travel time of the acoustic wave from the ultrasound transducer 111 to the focal point. Figure 15A An example of focusing ultrasound to the implantable device 131 by activating all ultrasound transducers with a time delay is shown. Similarly, Figure 15B An example of focusing ultrasound to the implantable device 131 by activating multiple ultrasound transducers 111 is shown. In both cases, when transmitting ultrasound, different time delays are applied to different ultrasound transducers 111 so that the ultrasound from the ultrasound transducers 111 add up to get the acoustic wave front 1501, 1502 propagating to the focal point at the implantable device 131 with maximum energy transmission. The number of ultrasound transducers 111 activated for transmission is determined according to the depth, location, and orientation of the implantable device 131.

[0111] Figure 16A system diagram showing an active time delay focusing method using an active implantable device 1610 that transmits ultrasound is shown. Initially, at least one ultrasound transducer 111 is activated and transmits ultrasound waves into the living body. The active implantable device 1610 includes a built-in ultrasound transducer for receiving the ultrasound waves. The ultrasound waves convey electrical energy to charge the active implantable device 1610. In certain embodiments, the ultrasound transducer 111 can perform synchronization with the active implantable device 1610. In certain embodiments, electrical energy for charging the active implantable device 1610 can be collected from other sources, such as heat, ions, kinetic motion, or magnetic fields.

[0112] Once the active implantable device 1610 is charged with sufficient electrical energy, the built-in transducer of the active implantable device 1610 can emit second ultrasound waves. The second ultrasound waves travel through the tissue of the living body and are then picked up by the ultrasound transducer array 1600. It is not necessary for all of the ultrasound transducers 111 in the ultrasound transducer array 1600 to pick up the second ultrasound waves. The ultrasound transducer array 1600, which can be rigid or flexible in shape, can then perform reconstruction to obtain information of the location and / or orientation of the active implantable device 1610. In certain embodiments, a plurality of ultrasound transducers 111 acquire a plurality of received signals, which are coupled to a processor 1620. The processor 1620 is configured to perform image analysis and image focusing to reconstruct an image of the active implantable device 1610. The steps of image focusing include time delay focusing and analog scatter correction focusing. A phase delay focusing equation can be applied to all of the ultrasound transducers 111 such that ultrasound energy can be focused onto the active implantable device 1610 in a desired waveform. The phase delay focusing equation is formulated such that the phase delay at each ultrasound transducer 111 can be set such that all waves constructively interfere when the sound waves are incident on the focal point. More particularly, a time delay or phase delay is introduced by taking into account the time of flight of the sound waves from each ultrasound transducer 111 to the focal point. Typically, the phase delay focusing equation is based on the relative location and orientation between one or more ultrasound transducers 111.

[0113] In the case of transmitting ultrasound energy to a living body having multiple active implantable devices 1610 at the same time, the phase delay at each ultrasound transducer 111 can be set such that constructive interference occurs at multiple locations where the multiple active implantable devices 1610 are located. In certain embodiments, each of the multiple active implantable devices 1610 selectively responds to a specific frequency range to selectively activate a particular one or more active implantable devices 1610.

[0114] An advantage of the active time delay focusing method is that the waveform delivered to the active implantable device 1610 can be fully controlled. The waveform can be fully modulated in a manner that facilitates signal transduction as well as providing electrical energy to activate the active implantable device 1610 and communicate with it.

[0115] The iterative method can be combined with an active time-delay focusing method to improve aiming accuracy. During each iteration, the internal transducer of the active implantable device 1610 can transmit ultrasonic waves to the ultrasonic transducer array 1600, and the processor 1620 can determine whether the energy aiming has been improved. The iteration is repeated until the energy aiming is no longer further improved. This iterative method can advantageously enhance focusing accuracy to achieve maximum focusing on the active implantable device 1610.

[0116] D. Additional features and experimental results

[0117] In some embodiments, the second ultrasonic wave emitted by the active implantable device 1610 may carry other data, such as temperature, pressure, pH, glucose level, or other sensing data. Electrical energy received from the ultrasonic wave can power the sensor. Although ultrasound has been used for signal conversion, the sensing data is transmitted in a targeted signal to the ultrasonic transducer 111. Sensitivity is significantly improved even when receiving weak second ultrasonic waves carrying sensing data.

[0118] In some embodiments, different data modulations of ultrasound, such as amplitude modulation, frequency modulation, and pulse modulation, can be used to transmit data or signals to the active implantable device 1610. Sensors in the active implantable device 1610 can receive commands from the processor 1620, such as adjusting the gain of a sensor amplifier or periodically activating the sensor. Because the present invention provides a method for aiming ultrasonic energy at the active implantable device 1610, highly efficient communication and low noise due to multiple scattering can be achieved. Bidirectional communication can be established between the active implantable device 1610 and the ultrasonic transducer array 1600.

[0119] In some embodiments, the implantable device 131 (active or passive) may have a nonlinear effect on ultrasonic sonication. This can be achieved by introducing materials or substances with nonlinear ultrasonic properties; for example, microbubbles are known to have ultrasonic nonlinear effects. These nonlinear properties will generate harmonics that reflect the ultrasonic waves. The frequencies of these harmonics are typically half or integer multiples of the sonication frequency.

[0120] like Figure 17 As shown, the spectra of the fundamental frequency signal and the first harmonic signal are illustrated. Since the harmonic signal has a spectrum that does not overlap with the fundamental frequency signal (the signal initially emitted from the ultrasonic transducer array 100), the implantable device 131 can be identified from other structures by selecting the spectrum of the harmonic signal. Image reconstruction or time reversal can then be performed on this harmonic spectrum to provide more specific monitoring and / or targeting.

[0121] For active implantable devices 810, 1610, the generated ultrasound can have a different ultrasound frequency than the charging ultrasound waves using a frequency converter. This can allow for a distinction between the generated signal and the backscattered charging signal. In certain embodiments, generating ultrasound after charging the active implantable devices 810, 1610 requires less or no waiting time.

[0122] There can be multiple implantable devices 131 in the field of view of the ultrasound transducer array 100. To identify the implantable devices 131, in addition to analyzing possible shape and structural features, the active implantable devices 810, 1610 can be coded with different frequencies to make them identifiable. The ultrasound transducer array 100 can identify the active implantable devices 810, 1610 with different frequency transmissions by simple frequency filtering of the signal.

[0123] Frequency coding can also be used to selectively activate specific implantable devices 131. For example, the ultrasound sensors of each implantable device 131 can have different resonance frequencies that will only be activated by ultrasound waves of a specific frequency. In one embodiment, a filtering circuit or computerized design within the receiving portion of the implantable device 131 can be used to allow activation of the implantable device 131 only when ultrasound waves of a specific frequency are received. This can allow for different activation times of implantable devices 131 within the field of view of the ultrasound transducer array 100 by frequency selection.

[0124] Figure 18A An image reconstructed from a known convex ultrasound transducer array 100 is shown. Figure 18B Another image reconstructed from a known S-shaped ultrasound transducer array 100 is shown. Figure 18C Another image reconstructed from a known concave ultrasound transducer array 100 is shown. In all three cases, activation was performed using plane waves.

[0125] Figure 10A And Figure 10B A reconstructed ultrasound image of a sample of chicken breast with a stainless steel surface embedded in it is shown. Figure 19A And Figure 19B A B-mode ultrasound image and a time-reversed field generated for another similar experiment are shown. The ultrasound waves were aimed back to the stainless steel surface using the time-reversal method. The intensity distribution at the location of the stainless steel was about 2 times that of the adjacent tissue.

[0126] The present disclosure can be implemented in other specific forms without departing from the spirit or essential characteristics thereof. Accordingly, the present embodiments are to be considered in all respects as illustrative and not restrictive. The scope of the disclosure is, therefore, indicated by the appended claims rather than by the foregoing description, and all changes that come within the meaning and range of equivalents are intended to be embraced therein.

Claims

1. A method for transmitting ultrasonic energy from an ultrasonic transducer array having one or more ultrasonic transducers to an implantable device by time reversal focusing, the method comprising the steps of: generating, by at least one ultrasonic transducer, a first ultrasonic wave having at least one frequency and at least one amplitude; transmitting the first ultrasonic wave to the implantable device, whereby a return signal is reflected from the implantable device to the ultrasonic transducer array; receiving, by the ultrasonic transducer array, the return signal such that the one or more ultrasonic transducers acquire a plurality of received signals based on the return signal; coupling the received signals to a processor to generate a time reversal signal; modulating the one or more ultrasonic transducers to generate a time reversal wave in accordance with the time reversal signal, wherein the time reversal wave experiences constructive interference at a location from which the return signal was transmitted; and transmitting the time reversal wave to the implantable device to transmit ultrasonic energy from the time reversal wave to the implantable device, wherein a natural focal point is removed by configuring two or more ultrasonic transducers to modulate an ultrasonic field such that the first ultrasonic wave generates a plane wave or a random time delay wave.

2. The method of claim 1, further comprising the step of reflecting the first ultrasonic wave as an echo signal, wherein, The implantable device is a device having an acoustic impedance significantly higher than adjacent tissue.

3. The method of claim 1, further comprising iterating the steps of receiving, by the one or more ultrasonic transducers, the return signal and coupling the received signals to a processor to generate a time reversal signal to improve ultrasonic energy targeting.

4. The method of claim 1, wherein, The processor is configured to perform time reversal focusing on each return signal received by the one or more ultrasonic transducers.

5. The method of claim 4, wherein, The processor is configured to perform post-processing filtering of noise and to correct linearity and frequency response of ultrasonic generated by the ultrasonic transducers.

6. The method of claim 1, wherein, The ultrasonic energy from the time reversal wave is converted to an electric current by a piezoelectric element, a capacitive micromachined ultrasonic transducer (CMUT), an optical-based transducer, or other material.

7. The method of claim 1, wherein, The implantable device comprises a material or substance having nonlinear ultrasonic properties such that the return signal has a frequency different from the first ultrasonic wave.

8. The method of claim 1, wherein, The step of coupling the received signals to a processor to generate a time reversal signal further comprises the steps of: converting the received signals to digital signals using an analog-to-digital converter (ADC); writing the digital signals to a memory; performing time domain flipping on the digital signals to obtain flipped digital signals; and converting the flipped digital signals to obtain the time reversal signal using a digital-to-analog converter (DAC).

9. The method of claim 1, wherein, The step of coupling the received signals to a processor to generate a time reversal signal further comprises the steps of: coupling the received signals to a Fourier transform circuit and an envelope detector disposed in parallel with the Fourier transform circuit; generating frequency components of the received signals using the Fourier transform circuit; generating amplitude components of the received signals using the envelope detector; flipping the received signals by obtaining a conjugate of a Fourier transform of the return signal; and performing an inverse Fourier transform to obtain the time reversal signal.

10. A method for transmitting ultrasonic energy simultaneously to a plurality of implantable devices in a living body by time reversal focusing, wherein, Each of the plurality of implantable devices is configured to receive ultrasound energy from an ultrasound transducer array according to the method of claim 1, and wherein the time reversed waves from the ultrasound transducer array experience constructive interference at a plurality of locations where the plurality of implantable devices are located.

11. The method of claim 10, wherein, Each of the plurality of implantable devices selectively responds to a range of frequencies to selectively activate the one or more implantable devices.

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