WO2020151179A1

WO2020151179A1 - Targeted energy transmission method using one or more ultrasonic transducers on an implantable device

Info

Publication number: WO2020151179A1
Application number: PCT/CN2019/093038
Authority: WO
Inventors: Tsung Kwan SHEA; Yongping Zheng; Yan To LING; Shuai LI; Md. Monzurul ALAM
Original assignee: Hong Kong Polytechnic University HKPU
Current assignee: Hong Kong Polytechnic University HKPU
Priority date: 2019-01-23
Filing date: 2019-06-26
Publication date: 2020-07-30
Anticipated expiration: 2021-07-23
Also published as: CN113613708B; CN113613708A

Abstract

A targeted energy transmission method using an ultrasonic transducer array (101,102) on an implantable device (131) is disclosed. The method includes generating a first ultrasonic wave having at least one frequency and at least one amplitude by at least one ultrasonic transducer (111), 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 the one or more ultrasonic transducers (111) acquire a plurality of received signals, coupling the received signals to a processor for generating time reversed signals; and modulating the one or more ultrasonic transducers (111) to generate time reversed waves according to the time reversed signals for transmitting the ultrasonic energy to the implantable device (131), wherein the time reversed waves undergo constructive interference at a location where the echo signal is transmitted from.

Description

TARGETED ENERGY TRANSMISSION METHOD USING ONE OR MORE ULTRASONIC TRANSDUCERS ON AN IMPLANTABLE DEVICE

Inventors: Tsung Kwan SHEA, Yongping ZHENG, Yan To LING, Shuai LI, MD. Monzurul ALAM

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Patent Application No. 62/795,607, filed on January 23, 2019, which is incorporated by reference herein in its entirety.

TECHNICAL FIELD

The present disclosure generally relates to a targeted energy transmission method using one or more ultrasonic and/or acoustic transducers on an implantable device. In particular, the present disclosure is directed to a method for focusing ultrasonic energy and delivering energy to the implantable device.

BACKGROUND

With the advancement in implantable smart technologies, particularly the use of implantable autonomous sensors and actuators, the need for developing a better implantable medical device has increased. One area of research focuses on the improvement in the lifetime of an implantable medical device. A conventional implantable device is limited by the capacity of the power source, which is often bulky and requires replacement from time to time. Having battery implanted within the human body may also be hazardous. The battery requires more frequent replacement than other components in the electronic circuitry of the implantable device. Therefore, the replacement of batteries causes unnecessary risk of infection and discomfort from the intrusive operations.

Ultrasonic energy transmission has been utilized to transmit energy from an ultrasonic transducer and recharge the implantable device. Such an ultrasonically powered implantable device has the significant medical benefit of miniaturizing the implantable device and providing a safer power source. Typically, the implantable device comprises a rechargeable battery and an ultrasonic transceiver which can efficiently convert the ultrasonic power into electrical energy for supplying power. This method provides an efficient and relatively risk-free energy source, as well as a mean of bi-directional data transmission to and from the implantable device.

A research team in the UC Barkley proposed a system called “neural dust” in the US patent number US10118054B2. The system provides a method for acquiring an internal body biological condition from an implantable device. According to the method, one or more ultrasonic transducers are configured to transmit a carrier signal of ultrasonic waves to an implant device. The implant device receives the carrier signal, modulates the ultrasonic wave, and transmits the modulated signal to the ultrasonic transducer by backscattering. This method is a passive mode of monitoring triggered by an external interrogator, and there is no electric power stored in the implantable device for performing autonomous monitoring of the deep tissue biological condition. Furthermore, methods for focusing the ultrasonic wave and improving the efficiency of energy transmission from the transducer are not particularly addressed.

In order to achieve higher efficiency on ultrasonic energy delivery, the ultrasonic wave should ideally be focused on the implantable device. This may be done by using a single bowl-shaped ultrasonic transducer. However, such a bowl-shaped transducer has with a natural focus at a fixed depth. As the position of the focus is not flexible, the target of the ultrasonic wave is also not flexible. The same problem of focusing is also encountered when transducer array elements are used.

In light of the issues raised above, there is a need in the art to have an improved method for performing targeted energy transmission using rigid or flexible ultrasonic transducer array on an implantable device.

SUMMARY

Provided herein is 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 comprises the steps of generating a first ultrasonic wave having at least one frequency and at least one amplitude by at least one ultrasonic transducer; transmitting the first ultrasonic wave to the implantable device, thereby an echo signal is reflected to the ultrasonic transducer array from the implantable device; receiving the echo signal by the ultrasonic transducer array such that the one or more ultrasonic transducers acquire a plurality of received signals based on the echo signal; coupling the received signals to a processor for generating time reversed signals; modulating the one or more ultrasonic transducers to generate time reversed waves according to the time reversed signals, wherein the time reversed waves undergo constructive interference at a location where the echo signal is transmitted from; and transmitting the time reversed waves to the implantable device for transmitting the ultrasonic energy from the time reversed waves to the implantable device.

According to certain aspects of the present disclosure, the method further comprises the step of reflecting the first ultrasonic wave as the echo signal, wherein the implantable device is a device having significantly higher acoustic impedance than adjacent tissues.

According to certain aspects of the present disclosure, the method further comprises iterating the steps of receiving the echo signal by the one or more ultrasonic transducers and coupling the received signals to the processor for generating time reversed signals for improving an ultrasonic energy targeting.

According to certain aspects of the present disclosure, the first ultrasonic wave is a plane wave or a random time-delayed wave as generated by two or more ultrasonic transducers such that a natural focus spot is not present.

According to certain aspects of the present disclosure, the processor is configured to perform time reversal focusing on each echo signal received by the one or more ultrasonic transducers. The processor is configured to perform post-processing filtering of noise and correcting a linearity and a frequency response of ultrasound generation by the ultrasonic transducers.

According to certain aspects, the ultrasonic energy from the time reversed waves is converted into electrical currents through piezoelectric elements, capacitive micromachined ultrasonic transducers (CMUTs) , optical based transducers, or other materials.

According to certain aspects of the present disclosure, the implantable device comprises materials or substances with a non-linear ultrasonic property such that the echo signal has a frequency different from the first ultrasonic wave.

According to certain aspects of the present disclosure, the step of coupling the received signals to a processor for generating time reversed signals 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 a time-domain flip of the digital signals to obtain flipped digital signals; and converting the flipped digital signals to obtain the time reversed signals using a digital-to-analog converter (DAC) .

According to certain aspects of the present disclosure, the step of coupling the received signals to a processor for generating time reversed signals further comprises the steps of coupling the received signals to a Fourier transform circuit and an envelope detector which is arranged in parallel to the Fourier transform circuit; generating a frequency component of the received signals using the Fourier transform circuit; generating a magnitude component of the received signals using the envelope detector; flipping the received signals by obtaining a conjugate in Fourier transform of the echo signal; and performing inverse Fourier transform to obtain the time reversed signals.

Also provided herein is a method for transmitting ultrasonic energy to a plurality of implantable devices in a living body simultaneously by time reversal focusing. Each of the plurality of implantable devices is configured to receive the ultrasonic energy from an ultrasonic transducer array. The time reversed waves from the ultrasonic transducer array undergo constructive interference at multiple locations where the plurality of implantable devices is positioned.

According to certain aspects of the present disclosure, each of the plurality of implantable devices is selectively responsive to a range of frequencies for selectively activating the one or more implantable devices.

This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. Other aspects and advantages of the present invention are disclosed as illustrated by the embodiments hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

The appended drawings, where like reference numerals refer to identical or functionally similar elements, contain figures of certain embodiments to further illustrate and clarify various aspects, advantages and features of the targeted energy transmission method as disclosed herein. It will be appreciated that these drawings and graphs depict only certain embodiments of the invention and are not intended to limit its scope. The targeted energy transmission method as disclosed herein, will be described and explained with additional specificity and detail through the use of the accompanying drawings in which:

FIG. 1A depicts the reflection of ultrasound by spinal vertebrae when a rigid ultrasonic transducer array is placed on a body. FIG. 1B depicts the reflection of ultrasound by spinal vertebrae when a flexible ultrasonic transducer array is placed on a body.

FIG. 2 is a photo of an exemplary flexible ultrasonic transducer array.

FIG. 3A depicts the focusing of ultrasonic waves onto one or more implantable devices using a rigid ultrasonic transducer array. FIG. 3B depicts the focusing of ultrasonic waves onto one or more implantable devices using a flexible ultrasonic transducer array.

FIG. 4A depicts transmitting a wavefront from an ultrasonic transducer through a scattering medium to an implantable device (reflector) . FIG. 4B depicts reflecting an ultrasonic wave from the implantable device back to the ultrasonic transducers. FIG. 4C depicts transmitting a time reversed wave from the ultrasonic transducers through the scattering medium to the implantable device with a focus generated to the implantable device.

FIG. 5A depicts the acoustic intensity distribution of ultrasonic wave sent out by a concave-shaped transducer array with no field modulation by electronic time delay. FIG. 5B depicts the acoustic intensity distribution of ultrasonic wave sent out by a concave-shaped transducer array with plane wave transmission. FIG. 5C depicts the acoustic intensity distribution of ultrasonic wave sent out by a concave-shaped transducer array with random time-delay field. FIG. 5D depicts the acoustic intensity distribution of ultrasonic wave sent out by a single ultrasonic transducer in a concave-shaped transducer array.

FIG. 6 depicts a system diagram of the passive time reversal focusing method for achieving a maximum focusing on the implantable device in accordance with an exemplary embodiment of the present disclosure.

FIG. 7A depicts an exemplary structure of a system for performing time reversal focusing. FIG. 7B depicts another exemplary structure of the system for performing time reversal focusing.

FIG. 8 depicts a system diagram of the active time reversal focusing method using an active implantable device in accordance with an exemplary embodiment of the present disclosure.

FIG. 9 depicts a circuit diagram for the AC time delay circuit for the active implantable device in accordance with an exemplary embodiment of the present disclosure.

FIG. 10A depicts a brightness mode (B-mode) ultrasound image reconstructed from a plane wave activation using a rigid ultrasonic transducer array. FIG. 10B depicts a time reversal field generated from the B-mode ultrasound image shown in FIG. 10A.

FIG. 11A depicts the signal of an instantaneous impulse at the focused location at a depth of 10cm using time reversal focusing method. FIG. 11B depicts the signal of an instantaneous impulse at the focused location at a depth of 10cm using time-delayed focusing method.

FIG. 12A depicts the signal of a tone burst of 8 cycles at 1MHz at the focused location at a depth of 10cm using time reversal focusing method. FIG. 12B depicts the signal of a tone burst of 8 cycles at 1MHz at the focused location at a depth of 10cm time-delayed focusing method

FIG. 13A depicts the acoustic wavefront and the respective simulated acoustic intensity distribution for focusing the acoustic wave at a specific location from an arc-shaped transducer array. FIG. 13B depicts the acoustic wavefront and the respective simulated acoustic intensity distribution for activating a plane wave of ultrasound from an arc-shaped transducer array.

FIG. 14 depicts a typical voltage produced per unit of acoustic pressure of an ultrasonic transducer.

FIG. 15A depicts an exemplary focusing of ultrasound to an implantable device by activating the ultrasonic transducers with time-delay. FIG. 15B depicts an exemplary focusing of ultrasound to an implantable device by activating an optimal number of ultrasonic transducers.

FIG. 16 depicts a system diagram of the active time-delay focusing method using an active implantable device in accordance with an exemplary embodiment of the present disclosure.

FIG. 17 depicts the frequency spectrums of a fundamental signal and a first harmonic signal.

FIG. 18A depicts the image reconstructed from a known convex ultrasonic transducer array. FIG. 18B depicts the image reconstructed from a known S-shaped ultrasonic transducer array. FIG. 18C depicts the image reconstructed from a known concave ultrasonic transducer array.

FIG. 19A depicts an exemplary ultrasound image of a metal plate implanted in a chicken breast tissue. FIG. 19B depicts the intensity distribution of FIG. 19A when using time reversal method.

Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been depicted to scale.

DETAILED DESCRIPTION

The present disclosure generally relates to a targeted energy transmission method. More specifically, but without limitation, the present disclosure relates to a method for focusing ultrasonic energy and communicating with the implantable device using one or more ultrasonic transducers, such as a rigid or flexible ultrasonic transducer array. An objective of the present disclosure is to maximize the efficiency of the ultrasonic energy transmission by focusing the ultrasonic wave at the implantable device.

In the following embodiments, the targeted energy transmission method is merely exemplary in nature and is not intended to limit the disclosure or its application and/or uses. It should be appreciated that a vast number of variations exist. The detailed description will enable those of ordinary skill in the art to implement an exemplary embodiment of the present disclosure without undue experimentation, and it is understood that various changes or modifications may be made in the function and arrangement of the method described in the exemplary embodiment without departing from the scope of the present disclosure as set forth in the appended claims. In view of this disclosure, different embodiments described herein may be combined to form other 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.

The benefits, advantages, solutions to problems, and any element (s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential features or elements of any or all of the claims. The invention is defined solely by the appended claims including any amendments made during the pendency of this application and all equivalents of those claims as issued.

For simplicity and clarity, relational terms such as “first, ” “second, ” “third, ” and the like, if any, are used solely to distinguish one from another entry, item, or device, without necessarily requiring any actual such relationship or order between such entries, items, or devices.

As used herein, the terms “coupled” or “connected, ” or any variant thereof, covers any coupling or connection, either direct or indirect, between two or more elements, unless otherwise indicated or clearly contradicted by context.

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 identify identification, sensing the physiological conditions, such as temperature, pressure, pH, pulse rate, oxygen, analyte, strain, glucose, or any combination thereof. The implantable device may comprise multiple channels, sensors, transmitters, or detectors. The implantable device may perform other functions such as releasing drugs or chemicals into the body, stimulating nerves and tissues, or treating heart issues. The implantable device is implanted in a living subject, wherein the subject may be a human or an animal. The term “ultrasonic energy, ” as used herein, generally refers to the energy transmitted by acoustic waves having a frequency between 20kHz and 1GHz. The term “acoustic wave” is referring to a broadband acoustic wave having a frequency between 10kHz and 1GHz.

In certain embodiments, the implantable device comprises one or more sensors for detecting the amount of the analyte, strain, or pH.

In certain embodiments, the implantable device comprises an optical detector for detecting blood pressure, blood oxygenation, melanin level, glucose, a pulse rate, any other spectral change signature relayed to health, or any optical absorption change related to health.

In certain embodiments, the implantable device comprises a temperature sensor, such as a thermistor or a thermocouple, for detecting temperature.

In certain embodiments, the implantable device comprises a pressure sensor, such as a micro-electro-mechanical system (MEMS) sensor, for measuring blood pressure, intracranial pressure, pulse rate, or other pressures in the body.

In certain embodiments, the implantable device comprises a potentiometric chemical sensor or an amperometric chemical sensor for detecting oxygen level, pH, or glucose.

In certain embodiments, the implantable device comprises drug release dispenser for releasing drugs or chemicals into the body.

In certain embodiments, the implantable device comprises micro-stimulators or electrodes for stimulating nerves or tissues or treating heart issues.

In certain embodiments, the implantable device can to send out electromagnetic waves or mechanical waves for stimulation of tissues, nerves, or organ.

In certain embodiments, the implantable device can induce local or large location temperature change.

In certain embodiments, the implantable device can introduce pressure change in the body system, using components that require real-time change in pressure, such as actuators or clamps.

In certain embodiments, the implantable device can introduce or detect a magnetic field, such as inductors or coils.

A. ULTRASONIC TRANSDUCER ARRAY

FIG. 1A and FIG. 1B illustrate a rigid ultrasonic transducer array 101 and a flexible ultrasonic transducer array 102 placed on a living body respectively. The ultrasonic transducer array comprises a plurality of ultrasonic transducers 111, which can be piezoelectric elements, capacitive micromachined ultrasonic transducers (CMUTs) , optical based transducers or other materials. In certain embodiments, the flexible ultrasonic transducer array 102 may comprise one single ultrasonic element, such as a CMUT named “Sonic Paper” . In some other cases, a single ultrasonic transducer 111 can be used instead of an array. Each of the ultrasonic transducers 111 in the array is controlled by a processor, which can configure each ultrasonic transducer to receive or transmit ultrasonic waves. This allows the one or more ultrasonic transducers 111 to transmit ultrasonic waves with different time delay, phase shift, pulse frequency, amplitude, and/or wavelength. In some embodiments, the one or more ultrasonic transducers 111 in the array can have regular spacing, irregular spacing, or be sparsely placed.

The one or more ultrasonic transducers 111 are connected to a processor or a computation system, which is configured to be selectively communicable with each or all ultrasonic transducers 111 for transmitting or receiving ultrasonic wave. The processor may be formed as one or more central processing units (CPU) , microcontroller units (MCU) , general purpose microprocessors, application specific integrated circuits (ASIC) , field programmable logic arrays (FPGA) , programmable I/O devices, or other equivalent integrated or discrete electronic circuitry. In certain embodiments, the processor or the computation system may further include other components such as analog-to-digital converter (ADC) , digital-to-analog converter (DAC) , memory, display panel, power supply, and I/O ports.

The flexible ultrasonic transducer array 102 can be considered of having one or more ultrasonic transducers 111 attached on flexible and stretchable materials, flexible printed circuit board (PCB) , or Kapton polyimide film, which allow a relative movement of location and orientation of adjacent ultrasonic transducers 111. Therefore, the flexible ultrasonic transducer array 102 can allow better bending and fitting onto a complex geometry of the surface of a living body than the rigid ultrasonic transducer array 101. An exemplary flexible ultrasonic transducer array 102 including an array of 128 ultrasonic transducers 111 is shown in FIG. 2.

The ultrasound transmitted from the ultrasonic transducers 111 interacts with the body tissues after entering the living body. The interaction may cause reflection, scattering, and refraction of the ultrasonic wave. This occurs at the tissue boundaries in which the tissues or medium has different acoustic impedance. Attenuation may also occur from absorption, which may happen within the same medium. The deviation in the acoustic impedance determines the amount of reflection, while the acoustic impedance (Z) is determined by the density of the tissues (ρ) and the speed of the acoustic wave (c) , with the following equation:

Z = ρc (1)

A more significant change in density between two tissues will result in a larger change of acoustic impedance. The change in impedance between two mediums is called an acoustic impedance mismatch. Such a difference in acoustic impedance between two tissues accounts for the amount of reflection that occurs at the tissue boundary.

As the spinal vertebra 121 has a higher density than the neighboring tissues, the spinal vertebra 121 can, therefore, reflect the incident ultrasonic waves from the ultrasonic transducers 111. The reflected ultrasonic waves can be picked up by the ultrasonic transducers 111 on the array. As the flexible ultrasonic transducer array 102 may provide better angular coverage on the living body, which can collect more acoustic information than the rigid ultrasonic transducer array 101. Therefore, in most applications, the flexible ultrasonic transducer array 102 is more preferred than the linearly arranged rigid ultrasonic transducer array 101.

With the use of an ultrasonic transducer array (either rigid or flexible) , the one or more ultrasonic transducers 111 can be configured to transmit ultrasonic waves each with different time delay, phase modulation, and waveform modulations. By applying time delay, the phases of the ultrasonic waves can add up in the target ultrasonic focusing region, such that the ultrasonic energy can have a maximum intensity at a defined focus.

Referring to FIG. 3A, the rigid ultrasonic transducer array 101 focuses the ultrasonic beams onto an implantable device 131 inside a living body by applying different time delay to each ultrasonic transducers 111. As the rigid ultrasonic transducer array 101 cannot make good contact to the surface of the living body, there is acoustic coupling between some of the ultrasonic transducers 111 and the living body. Similarly, as shown in FIG. 3B, the flexible ultrasonic transducer array 102 also focuses the ultrasonic beams onto an implantable device 131 inside a living body by applying different time delay to each ultrasonic transducers 111.

In order to target the ultrasonic wave to an implantable device 131, dynamic monitoring the location of the implantable device 131 is needed. Dynamic monitoring means capturing real-time information in relation to the location of the implantable device 131, which will then be used to calculate the targeted ultrasound delivery. The method of monitoring the location of the implantable device 131 and focusing the ultrasound are described in more details below. The method can also be applied to situations when two or more ultrasonically powered implantable devices 131 are implanted to a body.

One method for focusing the ultrasonic energy onto a targeted location is to perform time reversal focusing. The concept of time reversal focusing is a method based on reciprocity. In order to target the energy onto an implantable device 131 in a living body, an ultrasonic wave originated from an acoustic source is first propagated to the implantable device 131, in which the implantable device 131 reflects or transmits the ultrasonic wave. In the case of reflecting the wave, a passive method is used to achieve time reversal. For the case of transmitting the wave, an active method is used instead.

B. TIME REVERSAL FOCUSING

Now refer to FIGS. 4A-4C, the concept of the time reversal focusing is demonstrated. An array of ultrasonic transducers 100 may be arranged linearly or otherwise in other shapes. As exemplarily shown in FIG. 4A, one of the ultrasonic transducers 111 generates an ultrasonic wave having at least one frequency and at least one amplitude, and transmits the ultrasonic wave into a living body with an implantable device 131 therein. In certain embodiments, the ultrasonic wave can be transmitted into a living body with multiple implantable devices 131 therein, as long as all the implantable devices 131 are within the field-of-view. The frequency generated by the ultrasonic transducers 111 refers to the frequencies above the audible sound. In other alternative embodiments, two or more ultrasonic transducers 111 may be used to generate a plane wave or a random time-delayed wave instead of using one single ultrasonic transducer 111. In certain embodiments, the center frequency of the ultrasonic transducer 111 is approximately 5MHz. The ultrasound transmitted can be an impulse or a tone burst. Between the ultrasonic transducer 111 and the implantable device 131, there is a scattering medium 141 of body tissues. Refraction occurs when the ultrasonic wave is deflected from a straight path entering the scattering medium 141 to a direction with an angle of deflection. Generally, the ultrasonic wave is refracted at a medium with different acoustic impedance. The ultrasonic energy may also be absorbed, reflected, or scattered by the tissues. After passing through the scattering medium 141, the ultrasonic wave hits the implantable device 131. In one embodiment, the implantable device 131 is a device having significantly higher acoustic impedance than the adjacent tissues as the density is substantially higher. The implantable device 131 can act as a reflector, and the ultrasonic wave is reflected back as an echo signal using a passive method. In an alternative embodiment, the implantable device 131 can receive and convert the ultrasonic energy into electrical energy; thereby an ultrasonic transducer in the implantable device 131 generates an ultrasonic wave in an opposite direction. As the ultrasonic wave is generated by the implantable device 131, this method is an active method using more complex electronic components.

As shown in FIG. 4B, the echo signal or the generated ultrasonic wave from the implantable device 131 travels in a scattering path through the scattering medium 141 back to the array of ultrasonic transducers 100. As the scattering medium 141 has the same scattering and transmission properties when the ultrasonic wave travels through in a forward or backward manner. Therefore, by using a time reversal method, a focus at the point of reflection can be located.

The one or more ultrasonic transducers 111 acquire the echo signals or the generated ultrasonic waves, each with different properties of amplitude and time delay. The received signals 151 are coupled to the processor for generating the time-reversed signals 152. As shown in FIG. 4C, the time-reversed signals 152 are coupled from the processor to the one or more ultrasonic transducers 111 for generating time reversed waves, which are transmitted into the living body and dynamically focused at the implantable device 131. After passing through the scattering medium 141, the time reversed waves from the one or more ultrasonic transducers 111 can undergo constructive interference with the phase added up at the location where the reflection originally came from, and generate a focus of ultrasonic energy at the point of reflection. In the case of one single ultrasonic transducer 111, the time reversed wave and any scattered or refracted waves thereof may undergo constructive interference at the location where the reflection originally came from. For example, if an ultrasonic transducer 111 in the array of ultrasonic transducers 100 receives pressure field of p (x, t) , where x is the location of the ultrasonic transducer 111 and t is the time, the corresponding time reversal signal will be p’ (x, T-t) where T is the overall delay constant necessary in the system. The intensity of focus at each location is directly proportional to the reflection intensity plus attenuation factors.

For the purpose of initial activation, the ultrasound field generated from the array of ultrasonic transducers 100 can be modulated by different time-delay to achieve a dynamic focusing onto the implantable device 131. The modulation of ultrasound may improve the quality of transmission if a flexible ultrasonic transducer array 102 is used and the ultrasonic transducers 111 are not in arranged linearly, such as in a concave shape. A concave-shaped array of ultrasonic transducers 100 would naturally have a focus where ultrasonic intensity is highest compared to the other neighboring regions, as demonstrated in the acoustic intensity distribution in FIG. 5A. This would be particularly problematic for time-reversal focusing method as any reflected ultrasonic wave from the region with a natural focus has a higher intensity when comparing to the reflected ultrasonic wave from other regions.

In order 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 ultrasound beams across all locations. However, this method may require a longer delay, which takes a lot of time for sweeping the ultrasound beams. Furthermore, accurate information of each ultrasonic transducer 111 in the array of ultrasonic transducers 100 is required, which would be difficult or even impossible for a flexible ultrasonic transducer array 102.

In one embodiment, as shown in FIG. 5B and FIG. 5C, the natural focus spot can be removed by configuring the ultrasonic transducers 111 to modulate the ultrasound field and generate a plane wave or a random time-delay field. The plane wave is a straight and flat wavefront generated by the array of ultrasonic transducers 100 with parallel and unfocused transmissions. The random time-delay field is generated by activating each of the ultrasonic transducers 111 with random delay. In another embodiment, as shown in FIG. 5D, the natural focus spot can also be removed by configuring only one single ultrasonic transducer 111 to transmit a single ultrasonic wave. This method has weaker to no focusing effect.

Apart from the reflection of ultrasound by the implantable device 131, undesired reflection may also occur when the ultrasonic wave is engaging body tissues inside the living body having acoustic impedance mismatched. In particular, the body structures, such as the bone and muscle tissues, can reflect the ultrasound. This reflection is not possible to be avoided. 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 by selecting a material with higher impedance mismatch than the surrounding medium.

As a reflection of ultrasound is a result of the mismatch of acoustic impedance between two mediums. The fraction of energy being reflected R at a boundary between two mediums can be described as by the following equation:

wherein z ₁ and z ₂ are the acoustic impedance of medium 1 and medium 2.

From Equation (2) , it is apparent that the larger the acoustic impedance mismatch, the stronger the reflection echo can be generated. Therefore, the implantable device 131 can generate a stronger reflection of echo signal by having at least part the implantable device 131 made of a material of higher acoustic impedance mismatch with the human tissues. For example, an implantable device 131 for monitoring or stimulating muscle tissues is implanted at the region with the target human muscle tissues. The human muscle tissues have an acoustic impedance of 1.68x10 ⁶ kg/ (sec. m ²) . The material used in the implantable device 131 should have a higher acoustic impedance difference, such as stainless steel with an acoustic impedance of 46.02x10 ⁶ kg/ (sec. m ²) . With this arrangement, the implantable device 131 can reflect the ultrasonic wave with a strong echo signal.

Assuming that the implantable device 131 can generate a significantly stronger reflection of ultrasound than other physiological structures in the living body, then most of the acoustic energy reflecting to the array of ultrasonic transducers 100 comes from the reflection at the implantable device 131. The received echo signals are then used to perform time reversal focusing. Most of the acoustic energy thereafter transmitted from the array of ultrasonic transducers 100 can be targeted back onto the point of reflection at the implantable device 131. Advantageously, the variations incurred as a result of the position and orientation of each ultrasonic transducer 111 can be compensated. Therefore, the measurement and details of the precise physical position and orientation of the ultrasonic transducers 111 are not necessary; thereby this method enables the use of a flexible ultrasonic transducer array 102 without any details of each ultrasonic transducer 111.

FIG. 6 shows a system diagram of the passive time reversal focusing method for achieving a maximum focusing on the implantable device 131. This is an optional iterative method for optimizing the focus. Considering 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 cycle of iteration.

The passive time reversal focusing method can be used to deliver acoustic energy to an implantable device 131 using an ultrasonic transducer array 600 placed on a living body. The ultrasonic energy from the time reversed waves is converted into electrical currents through piezoelectric elements, CMUTs, optical based transducers, or other materials. The ultrasonic transducer array 600 can be a rigid ultrasonic transducer array 101 or a flexible ultrasonic transducer array 102. The ultrasonic transducer array 600 can be configured to transmit a plane wave, a random time-delay field, or a single ultrasonic wave. In the case of transmitting ultrasonic energy a living body having a plurality of implantable devices 131 simultaneously, the time reversed waves undergo constructive interference at multiple locations where the plurality of implantable devices 131 is positioned. In certain embodiments, each of the plurality of implantable devices 131 is selectively responsive to a particular range of frequencies for selectively activating particular one or more implantable devices 131.

In one embodiment, two or more ultrasonic transducers 111 are activated simultaneously to generate a plane wave in the form of a straight and flat wavefront. There is no focal point for transmission of the ultrasonic signal and natural focus is not resulted.

In one embodiment, two or more ultrasonic transducers 111 are activated, and each ultrasonic transducer 111 transmits the ultrasonic 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.

In one embodiment, only one signal ultrasonic transducer 111 is activated to transmit the ultrasonic signal. Other ultrasonic transducers 111 in the ultrasonic transducer array 600 are still enabled as a receiver and acquire the echo signals for determining the position of the implantable device 131.

The ultrasound transmitted from the ultrasonic transducer array 600 (Tx) enter the medium 610 of body tissues and may undergo refraction, scattering, attenuation, and reflection. As the implantable device 131 has a higher acoustic impedance difference, the reflected echo signal primarily comes from the implantable device 131. The echo signal is picked up by the ultrasonic transducer array 600 and a plurality of received signals are acquired by the one or more ultrasonic transducers 111 of the ultrasonic transducer array 600 (Rx) for analysis.

The received signals are coupled to the processor 620. Each ultrasonic transducer 111 can provide direct information of the phase and intensity of the received echo signal at its location and orientation, and compensate for any variation thereof. The processor 620 is configured to perform time reversal focusing on each echo signal. In certain embodiments, the processor 620 is also configured to perform post-processing filtering of noise and correcting the linearity and frequency response of ultrasound generation by the ultrasonic transducers 111. The time reversed signal is coupled back to the ultrasonic transducer array 600 (Tx) for transmitting ultrasonic wave in accordance with the time reversed echo signals to the medium 610 of body tissues and dynamically focused at the implantable device 131. During this transmission, the previous receiving ultrasonic transducers 111 are now configured to transmit the ultrasound. As the transmission is based on the time reversed echo signals, the acoustic energy transmitted from the ultrasonic transducer array 600 can advantageously target back onto the implantable device 131.

An iterative method can be used in conjunction with the passive time reversal focusing method such that the precision of the targeting can be improved. As the implantable device 131 can reflect more ultrasound than other structures in the medium 610, applying the time reversal method iteratively can increase the focused ultrasonic energy at the implantable device 131 in each iteration. During each iteration, the processor 620 determines whether the ultrasonic energy targeting is improved. The iteration is repeated until there is no further improvement on the energy targeting. This iterative method can advantageously enhance the accuracy of focusing for achieving maximal focusing onto the implantable device 131.

FIG. 7A shows a system for performing time reversal focusing in accordance with one embodiment. The echo signal received by the ultrasonic transducer 111 is converted to a digital signal using an ADC 711. The ADC 711 is generally implemented using a mix of integrated circuit devices and discrete components or integrated to the processor 620. The digital signal is then written to the memory 712 of the processor 620 for processing time-domain flip 713. The time-domain flip 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 may perform post-processing filtering of noise and correcting the linearity and frequency response of ultrasound generation by the ultrasonic transducer 111.

FIG. 7B shows another system for performing time reversal focusing in accordance with one embodiment. The echo signal received by the ultrasonic transducer 111 is coupled to a Fourier transform circuit 721 and an envelope detector 722 which is arranged in parallel to the Fourier transform circuit 721. The Fourier transform circuit 721 is configured to generate a frequency component of the echo signal, and the circuit may comprise components such as multipliers and computational units. Alternatively, the Fourier transform circuit 721 can be implemented as an algorithm or a 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 generate a magnitude component of the echo signal.

The digital signal processing (DSP) circuit 723 receives the frequency component, and magnitude component of the echo signal from the Fourier Transform circuit 721 and the envelope detector 722 for flipping the echo signal. The DSP circuit 723 can be realized as an integrated circuit and operates to obtain the conjugate in Fourier transform of the echo signal. By using the inverse Fourier transform circuit 724, the time domain of the time reversed signal can be obtained. Preferably, the Fourier transform circuit 721 and the inverse Fourier transform circuit 724 possess the same number of data samples. Alternatively, the inverse Fourier transform circuit 724 can be implemented as an algorithm or a software executable by the processor 620 or a computer system.

FIG. 8 shows a system diagram of the active time reversal focusing method using an active implantable device 810, which transmit ultrasound. The active time reversal focusing method is similar to the passive time reversal focusing method but in a reversed manner. Initially, at least one ultrasonic transducer 111 is activated and transmits ultrasonic wave into the living body. The active implantable device 810 comprises a built-in ultrasonic transducer for receiving the ultrasonic wave. The ultrasonic wave delivers electrical energy to charge up the active implantable device 810. In certain embodiments, the ultrasonic transducer 111 can perform synchronization with the active implantable device 810. In certain embodiments, the electrical energy for charging the active implantable device 810 can be harvested from other sources such as heat, ions, kinetic motion, or magnetic field.

Once the active implantable device 810 is charged up with sufficient electrical energy, the built-in transducer of the active implantable device 810 can generate a second ultrasonic wave in the opposite direction to the incidental ultrasonic wave from the ultrasonic transducer 111. In certain embodiments, the built-in transducer can send out the second ultrasonic wave in all directions with roughly the same intensity in order to reach all ultrasonic transducers 111 in the ultrasonic transducer array 800 for maximizing the efficiency. The second ultrasonic wave travels through the tissues of the living body, then picked up by the ultrasonic transducer array 800. It is not necessary to have all ultrasonic transducers 111 in the ultrasonic transducer array 800 picking up the second ultrasonic wave. The ultrasonic transducer array 800, which can be in rigid or flexible shape, can then perform time reversal focusing back to the active implantable device 810 with a more focused and higher energy. In certain embodiments, the one or more ultrasonic transducers 111 acquire a plurality of received signals, which is coupled to the processor 820. The processor 820 is configured to perform time reversal focusing on each received signals. The time reversed signals are coupled back to the ultrasonic transducers 111 (Tx) for transmitting ultrasonic wave accordingly, such that the acoustic energy can advantageously dynamically focus at and target back onto the active implantable device 810.

An iterative method can be used in conjunction with the active time reversal focusing method such that the precision of the targeting can be improved. During each iteration, the built-in transducer of the active implantable device 810 can transmit ultrasonic wave according to the incidental ultrasonic wave. By applying the time reversal method iteratively, the ultrasonic energy can be more focused on the active implantable device 810. During each iteration, the processor 820 determines whether the energy targeting is improved. The iteration is repeated until there is no further improvement on the energy targeting. This iterative method can advantageously enhance the accuracy of focusing for achieving maximal focusing onto the active implantable device 810.

When the first ultrasonic wave is picked up by the built-in transducer, the ultrasonic wave is converted into electric current and coupled to an alternating current (AC) time delay circuit. An exemplary circuit diagram for the AC time delay circuit is shown in FIG. 9. Other electronic circuits comprising resistor-capacitor (RC) pair, transistors, FPGA, ASIC, flip-flop, or other electronic components may be applied for introducing the time delay without departing from the spirit of the present disclosure. The purpose of using the AC time delay circuit is to introduce a time delay to the received ultrasonic wave, such that the ultrasonic wave originally transmitted by the ultrasonic transducers 111 during a transmission period is dissipated. Therefore, the active implantable device 810 can transmit a second ultrasonic wave back to the ultrasonic transducer array 800 during a detection period without interference by the ultrasound from the transmission period or any reflection thereof. In certain embodiments, the second ultrasonic wave comprises a synchronization signal having information on the time delay, which is received by the ultrasonic transducer array 800 for the processor 820 to determine the time required for the ultrasound to travel to the active implantable device 810.

In the case of transmitting ultrasonic energy a living body having a plurality of active implantable device 810 simultaneously, the time reversed waves undergo constructive interference at multiple locations where the plurality of active implantable device 810 is positioned. In certain embodiments, each of the plurality of active implantable device 810 is selectively responsive to a particular range of frequencies for selectively activating particular one or more active implantable device 810.

FIG. 10A provides a brightness mode (B-mode) ultrasound image reconstructed from a plane wave activation using a rigid ultrasonic transducer array 101. The experiment was performed on a piece of chicken breast sample, with a stainless steel surface embedded therein to emulate an implantable device. From the experimental results, a device with a higher acoustic impedance difference can be seen at the top right side of the ultrasound image with strong reflection. FIG. 10B shows a time reversal field generated from the B-mode ultrasound image shown in FIG. 10A. The experiment was conducted using Vantage 128TM (Verasonics Inc. ) , a 128 channels ultrasound system, and a 5MHz linear transducer with an array of 128 ultrasonic transducers. Although the figure is depicted in grayscale (not shown in color) , the color of the figure is in blue (darker area) and yellow (brighter area) . The blue area represents low ultrasound intensity, and the yellow area represents high ultrasound intensity. The intensity on the time reversal field was generated from computer simulation at a homogeneous medium and the intensity shows the time average intensity within the entire burst and acoustic wave traveling period.

The time reversal focusing method of the present invention can also take into account the inhomogeneity of the medium of body tissues. As compared to the conventional time-delay focusing method, which focuses ultrasound by assuming a uniform and known speed across the medium, the time reversal focusing method can advantageously take into account any effect such as scattering and change in sound speed when propagating through the medium. Therefore, using the time reversal focusing method can mitigate noises and deformations of the ultrasound signals.

For demonstrating the difference, signals of (1) an instantaneous impulse and (2) an acoustic source of a tone burst of 1MHz with 7 cycles are used and focused at a location inside a medium of tissues at a depth of 10cm. The result for time reversal focusing method and time-delay focusing method are compared. The simulation was performed using the k-Wave simulation toolbox. It was assumed that the medium has an acoustic sound speed of 1540m/s and a density of 1000kg/m ³, and the scattering medium has a sound speed of 1800m/s and a density of 1500kg/m ³. The scattering medium is randomly distributed in the medium occupying approximately 1/4 of the medium space. Time reversal focusing method was conducted by simulating an acoustic wave focused at the position at 10cm depth inside the medium. The reflected acoustic wave was picked up and acquired by 216 ultrasonic transducers aligned linearly in the simulation space. By performing time reversal focusing of the received ultrasound signal, a time reversed ultrasonic wave was sent out from each of the ultrasonic transducers. For time-delay focusing, the 216 ultrasonic transducers sent out signals with time delay corresponding to the distance to the focal spot. By assuming that the speed of sound is 1540m/s, the time delay can be set so that the ultrasound signals would have reached the focal spot at the same time.

FIG. 11A and FIG. 11B show the comparison between time reversal focusing method and time-delay focusing method with an instantaneous impulse. FIG. 12A and FIG. 12B show the comparison between time reversal focusing method and time-delay focusing method with an acoustic source of a tone burst of 1MHz with 7 cycles. The signal to noise ratio (SNR) can be obtained by calculating the root mean square of the time after the signal, and dividing by the maximum signal intensity. The SNR for the case of an instantaneous impulse was 3.01%for using the time reversal focusing method and 5.61%for using the time-delay method. The SNR of tone burst was 5.01%for using the time reversal focusing method and 12.92%for using the time-delay method.

C. IMAGE FORMATION AND TARGETED ENERGY DELIVERY

The location of the implantable device 131 can also be monitored by forming an ultrasonic image. This is done by the reflection of the ultrasonic wave from the surface of the implantable device 131. Due to acoustic impendence mismatch of the implantable device 131 and the neighboring tissues, ultrasonic wave traveled to such surface is reflected at the boundary. Advantageously, the reflection of the acoustic wave can provide a location and structural information of the implantable device 131, as well as other physical properties such as stiffness, temperature, and acoustic speed.

Referring to FIG. 13A, the flexible ultrasonic transducer array 102, operating as an acoustic wave source, is configured to generate acoustic waves. The acoustic wave generated at each ultrasonic transducer 111 is determined based on the relative location, orientation, and directionality of each ultrasonic transducer 111, thereby time-delay focusing and/or electronic beamforming can be performed. Time-delay focusing and electronic beamforming are performed by controlling the timing and/or phase of the ultrasonic wave generation by the processor. In certain embodiments, time-delay focusing is performed by calculating the relative center location from the flexible ultrasonic transducer array 102 to a focus point 1301, determining a delay for each ultrasonic transducers 111, and generating a plurality of ultrasonic waves by the ultrasonic transducer 111 each with the delay, thereby an acoustic wavefront 1302 propagating to the focus point 1301 is obtainable with a constructive interference of the ultrasonic waves at the focus point 1301. The constructive interference of the ultrasonic waves can generate stronger energy at the focus point 1301. By performing the ultrasonic focusing method repeatedly and iteratively at different locations and depths, a complete scanning of the entire field-of-view can be performed. The echo received from each location can be combined with the time of reflection to generate a full ultrasonic image on the acoustic beam intensity distribution.

Referring to FIG. 13B, an alternative method for forming an ultrasonic image is depicted. A rigid transducer array 101 or a flexible transducer array 102 may be used. For a flexible transducer array 102, the method is performed by calculating a relative position and orientation between the plurality of ultrasonic transducers 111 in real-time, determining a delay for each ultrasonic transducer 111, and generating a plurality of ultrasonic waves by the ultrasonic transducer 111 each with the delay, thereby a plane acoustic wavefront 1312 is obtainable. The plane acoustic wavefront 1312 is a straight and flat wavefront. For a rigid transducer array 101, the plane acoustic wavefront 1312 can be obtained by generating a plurality of ultrasonic waves by the ultrasonic transducer 111 simultaneously. By varying the steering angle of the ultrasonic transducers 111, the reflecting surface inside the living body can form reflection matrices. The reflection matrices together with the location and orientation information of the transducer array can collectively be used to calculate the physical location of each reflecting surface and hence form an ultrasound image.

After forming the ultrasound image, image analysis is performed to recognize the implantable device 131 in the ultrasound image. Information such as brightness, shapes, thickness, stiffness, etc. can be used to identify the implantable device 131 within the ultrasound image. Pattern recognition could also be used to determine the location on the implant based on the specific shape and size of the implantable device 131. Additionally, by manipulating the surface of the implantable device 131 or using different materials, the reflection pattern of the echo signal can be made unique, thereby the echo signal can be easily differentiated from other echoes coming from other surrounding tissues. Classification such as big data analysis can be used to classify objects under the ultrasound images and perform accurate recognition of the implantable device 131. Besides from pattern recognition, other imaging techniques such as shear wave elastography, phase contrast method can be used to improve the determination of the location of the implantable device 131 from the image.

After determining the location and orientation of the implantable device 131, focused energy is delivered to the implantable device 131 with maximum efficiency. The method comprises focusing the ultrasonic energy to the implantable device 131, and converting the ultrasonic energy into electrical currents through piezoelectric elements, CMUTs, optical based transducers, or other materials. The ultrasonic transducer in the implantable device 131 generally has a specific frequency response. A typical voltage produced per unit of acoustic pressure of an ultrasonic transducer is shown in FIG. 14. The energy transmission efficiency can be optimized by selecting a resonance frequency of the ultrasonic transducer for transmitting ultrasonic wave. In certain embodiments, the usable frequency range of the ultrasonic transducer can also be selected for transmission if a linear conversion of ultrasonic energy into electrical currents is required.

Now refer to FIG. 15A and FIG. 15B, after selecting the frequency range, a focused ultrasound field can be generated by calculating the traveling time of the acoustic wave from the ultrasonic transducer 111 to the focus. FIG. 15A shows an exemplary focusing of ultrasound to an implantable device 131 by activating all the ultrasonic transducers with time-delay. Similarly, FIG. 15B shows an exemplary focusing of ultrasound to an implantable device 131 by activating a number of the ultrasonic transducers 111. In both case, different time delays are applied to different ultrasonic transducers 111 when transmitting ultrasound so that the ultrasonic waves from the ultrasonic transducers 111 add up to obtain an

acoustic wavefront

1501, 1502 propagating to a focus point with maximum energy transmission at the implantable device 131. The number of ultrasonic transducer 111 activated for transmission is determined according to the depth, location, and the orientation of the implantable device 131.

FIG. 16 shows a system diagram of the active time-delay focusing method using an active implantable device 1610 which transmit ultrasound. Initially, at least one ultrasonic transducer 111 is activated and transmits ultrasonic wave into the living body. The active implantable device 1610 comprises a built-in ultrasonic transducer for receiving the ultrasonic wave. The ultrasonic wave delivers electrical energy to charge up the active implantable device 1610. In certain embodiments, the ultrasonic transducer 111 can perform synchronization with the active implantable device 1610. In certain embodiments, the electrical energy for charging the active implantable device 1610 can be harvested from other sources such as heat, ions, kinetic motion, or magnetic field.

Once the active implantable device 1610 is charged up with sufficient electrical energy, the built-in transducer of the active implantable device 1610 can send out a second ultrasonic wave. The second ultrasonic wave travels through the tissues of the living body, then picked up by the ultrasonic transducer array 1600. It is not necessary for to have all ultrasonic transducers 111 in the ultrasonic transducer array 1600 picking up the second ultrasonic wave. The ultrasonic transducer array 1600, which can be in rigid or flexible shape, can then perform reconstruction to obtain information of the location and/or orientation of the active implantable device 1610. In certain embodiments, the plurality of ultrasonic transducers 111 acquires a plurality of received signals, which are coupled to the processor 1620. The processor 1620 is configured to perform image analysis and image focusing for reconstructing an image of the active implantable device 1610. The step of image focusing comprising a time-delay focusing and a simulated scatter-corrected focusing. A phase delay focusing equation can be applied to all ultrasonic transducers 111 such that ultrasonic energy can be focused onto the active implantable device 1610 with a desired waveform. The phase delay focusing equation is formulated such that the phase delay at each ultrasonic transducer 111 can be set such that when an acoustic wave hits on the focus point, all waves add up with a constructive interference. More particularly, a time-delay or a phase-delay is introduced by taking into account the time of flight of the acoustic wave from each ultrasonic transducer 111 to the focus point. Generally, the phase delay focusing equation is based on a relative position and orientation between the one or more ultrasonic transducers 111.

In the case of transmitting ultrasonic energy to a living body having a plurality of active implantable device 1610 simultaneously, the phase delay at each ultrasonic transducer 111 can be set such that constructive interference is resulted at multiple locations where the plurality of active implantable device 1610 is positioned. In certain embodiments, each of the plurality of active implantable device 1610 is selectively responsive to a particular range of frequencies for selectively activating particular one or more active implantable device 1610.

The advantage of the active time-delay focusing method is that there can be a full control on the waveform that is delivered to the active implantable device 1610. The waveform can be fully modulated in a manner to facilitate signal transduction as well as providing electrical energy for activating and communicating with the active implantable device 1610.

An iterative method can be used in conjunction with the active time-delay focusing method such that the precision of the targeting can be improved. During each iteration, the built-in transducer of the active implantable device 1610 can transmit ultrasonic wave to the ultrasonic transducer array 1600, and the processor 1620 can determine whether the energy targeting is improved. The iteration is repeated until there is no further improvement on the energy targeting. This iterative method can advantageously enhance the accuracy of focusing for achieving maximal focusing onto the active implantable device 1610.

D. ADDITIONAL FEATURES AND EXPERIMENTAL RESULTS

In certain embodiments, the second ultrasonic wave sent out by the active implantable device 1610 can carry other data, such as temperature, pressure, pH value, glucose level, or other sensing data. The electrical energy received from the ultrasonic wave can power the sensors. While ultrasound has been used for signal transformation, a targeted signal transfer of the sensed data to the ultrasonic transducers 111. The sensitivity can be significantly enhanced even when receiving a weak second ultrasonic wave with the sensed data.

In certain embodiments, data or signal can be delivered to the active implantable device 1610 using different data modulations of the ultrasound wave, such as magnitude modulations, frequency modulations, and pulse modulations. The sensors in the active implantable device 1610 can receive commands from the processor 1620, such as adjusting the gain of sensor amplifier or enabling a sensor regularly. Since the present invention provides a method of targeting the ultrasonic energy to the active implantable device 1610, high efficiency of communication, and low noise due to multiple scattering can be achieved. Two-way communication between the active implantable device 1610 and the ultrasonic transducer array 1600 can be established.

In certain embodiments, the implantable device 131 (active or not) can have a non-linear effect on ultrasound sonication. This can be done by introducing materials or substances with non-linear ultrasonic property, for example, micro-bubbles are well known to have non-linear effect of ultrasound. These non-linear properties will generate harmonics of the reflected ultrasound wave. These harmonics usually have a frequency of half or an integer multiple of the sonication frequency.

As shown in FIG. 17, the frequency spectrums of a fundamental signal and a first harmonic signal are shown. As the harmonic signals are the frequency spectrums which do not overlap with the fundamental signal (signals originally sent out from the ultrasonic transducer array 100) , the implantable device 131 can be identified from other structures by selecting the frequency spectrum of the harmonic signals. Image reconstructions or time reversal could then be done on that harmonic spectrum to provide more specific monitoring and/or targeting.

For an active

implantable device

810, 1610, the ultrasound generated can have a different frequency of ultrasound than the charging ultrasound wave using a frequency changer. This can allow differentiation between the generated signal and the backscattered charging signal. In certain embodiments, shorter wait time or no wait time is needed for the generation of ultrasound after charging the active

implantable device

810, 1610.

Multiple implantable devices 131 can be present in the field of view of the ultrasound transducer array 100. In order to identify the implantable devices 131, other than analyzing the possible shape and structural features, an active

implantable device

810, 1610 can be coded with different frequencies to make it identifiable. The active

implantable device

810, 1610 with different frequencies transmission can be identified by the ultrasound transducer array 100 by simple frequency filtering of signals.

Frequency encoding could also be used to selectively activate a particular implantable device 131. For example, the ultrasound sensor of each implantable device 131 can have different resonance frequencies which would only be activated with ultrasound wave of specific frequencies. In one embodiment, filtering circuit or computerized designs within the receiving part of the implantable device 131 can be used to allow activation of implantable device 131 only when a certain frequency of ultrasound is received. This can allow different activation time of implantable device 131 within the field of view of the ultrasound transducer array 100 by frequency selection.

FIG. 18A shows an image reconstructed from a known convex ultrasonic transducer array 100. FIG. 18B shows another image reconstructed from a known S-shaped ultrasonic transducer array 100. FIG. 18C shows yet another image reconstructed from a known concave ultrasonic transducer array 100. In all three cases, a plane wave is used for activation.

FIG. 10A and FIG. 10B show an ultrasound image reconstructed of a piece of chicken breast sample with a stainless steel surface embedded. FIG. 19A and FIG. 19B show the B-mode ultrasound image and the time reversal field generated for another similar experience. Time reversal method was used to target the ultrasound back to the stainless steel surface. The intensity distribution at the location of the stainless steel is approximately 2 times larger than the neighboring tissues.

The present disclosure may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiment is, therefore, to be considered in all respects as illustrative and not restrictive. The scope of the disclosure is indicated by the appended claims rather than by the preceding description, and all changes that come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

Claims

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 a first ultrasonic wave having at least one frequency and at least one amplitude by at least one ultrasonic transducer;

transmitting the first ultrasonic wave to the implantable device, thereby an echo signal is reflected to the ultrasonic transducer array from the implantable device;

receiving the echo signal by the ultrasonic transducer array such that the one or more ultrasonic transducers acquire a plurality of received signals based on the echo signal;

coupling the received signals to a processor for generating time reversed signals;

modulating the one or more ultrasonic transducers to generate time reversed waves according to the time reversed signals, wherein the time reversed waves undergo constructive interference at a location where the echo signal is transmitted from; and

transmitting the time reversed waves to the implantable device for transmitting the ultrasonic energy from the time reversed waves to the implantable device.
The method of claim 1, further comprising the step of reflecting the first ultrasonic wave as the echo signal, wherein the implantable device is a device having significantly higher acoustic impedance than adjacent tissues.
The method of claim 1, further comprising iterating the steps of receiving the echo signal by the one or more ultrasonic transducers and coupling the received signals to the processor for generating time reversed signals for improving an ultrasonic energy targeting.
The method of claim 1, wherein the first ultrasonic wave is a plane wave or a random time-delayed wave as generated by two or more ultrasonic transducers such that a natural focus spot is not present.
The method of claim 1, wherein the processor is configured to perform time reversal focusing on each echo signal received by the one or more ultrasonic transducers.
The method of claim 5, wherein the processor is configured to perform post-processing filtering of noise and correcting a linearity and a frequency response of ultrasound generation by the ultrasonic transducers.
The method of claim 1, wherein the ultrasonic energy from the time reversed waves is converted into electrical currents through piezoelectric elements, capacitive micromachined ultrasonic transducers (CMUTs) , optical based transducers, or other materials.
The method of claim 1, wherein the implantable device comprises materials or substances with a non-linear ultrasonic property such that the echo signal has a frequency different from the first ultrasonic wave.
The method of claim 1, wherein the step of coupling the received signals to a processor for generating time reversed signals 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 a time-domain flip of the digital signals to obtain flipped digital signals; and

converting the flipped digital signals to obtain the time reversed signals using a digital-to-analog converter (DAC) .
The method of claim 1, wherein the step of coupling the received signals to a processor for generating time reversed signals further comprises the steps of:

coupling the received signals to a Fourier transform circuit and an envelope detector which is arranged in parallel to the Fourier transform circuit;

generating a frequency component of the received signals using the Fourier transform circuit;

generating a magnitude component of the received signals using the envelope detector;

flipping the received signals by obtaining a conjugate in Fourier transform of the echo signal; and

performing inverse Fourier transform to obtain the time reversed signals.
A method for transmitting ultrasonic energy to a plurality of implantable devices in a living body simultaneously by time reversal focusing, wherein each of the plurality of implantable devices is configured to receive the ultrasonic energy from an ultrasonic transducer array in accordance with the method of claim 1, and wherein the time reversed waves from the ultrasonic transducer array undergo constructive interference at multiple locations where the plurality of implantable devices is positioned.
The method of claim 11, wherein each of the plurality of implantable devices is selectively responsive to a range of frequencies for selectively activating the one or more implantable devices.
A method for transmitting ultrasonic energy from an ultrasonic transducer array having one or more ultrasonic transducers to an active implantable device by time reversal focusing, the method comprising the steps of:

generating a first ultrasonic wave having at least one frequency and at least one amplitude by at least one ultrasonic transducer;

transmitting the first ultrasonic wave to the implantable device, wherein the first ultrasonic wave is received by a built-in transducer in the implantable device;

converting the first ultrasonic wave into electrical energy;

generating a second ultrasonic wave based on the first ultrasonic wave, wherein the second ultrasonic wave is transmitted in an opposite direction to the first ultrasonic wave;

receiving the second ultrasonic wave by the ultrasonic transducer array such that the one or more ultrasonic transducers acquire a plurality of received signals based on the second ultrasonic wave;

coupling the received signals to a processor for generating time reversed signals;

modulating the one or more ultrasonic transducers to generate time reversed waves according to the time reversed signals, wherein the time reversed waves undergo constructive interference at a location where the echo signal is transmitted from; and

transmitting the time reversed waves to the implantable device for transmitting the ultrasonic energy from the time reversed waves to the implantable device.
The method of claim 13, wherein the active implantable device comprises a delay circuit for introducing a time delay to the first ultrasonic wave such that the echo signal is transmitted after dissipating the first ultrasonic wave.
The method of claim 14, wherein the delay circuit is an alternating current (AC) time delay circuit.
The method of claim 14, wherein the second ultrasonic wave comprises a synchronization signal having information on the time delay.
The method of claim 13, wherein the active implantable device comprises a frequency changer such that the second ultrasonic wave has a frequency different from the first ultrasonic wave.
The method of claim 13, wherein the active implantable device is configured to be activated by the first ultrasonic wave having a predetermined frequency for enabling a selection of the active implantable device.
The method of claim 13, wherein the processor is configured to encode commands to the first ultrasonic wave by data modulations such as magnitude modulations, frequency modulations, and pulse modulations.
A method for transmitting ultrasonic energy to a plurality of active implantable devices in a living body simultaneously by time reversal focusing, wherein each of the plurality of active implantable devices is configured to receive the ultrasonic energy from an ultrasonic transducer array in accordance with the method of claim 13, and wherein the time reversed waves from the ultrasonic transducer array undergo constructive interference at multiple locations where the plurality of active implantable devices is positioned.
The method of claim 20, wherein each of the plurality of active implantable devices is selectively responsive to a range of frequencies for selectively activating the one or more active implantable devices.
A method of transmitting ultrasonic energy from an ultrasonic transducer array having a plurality of ultrasonic transducers to an active implantable device by time-delay focusing, the method comprising the steps of:

generating a first ultrasonic wave having at least one frequency and at least one amplitude by at least one ultrasonic transducer;

transmitting the first ultrasonic wave to the implantable device, wherein the first ultrasonic wave is received by a built-in transducer in the implantable device;

converting the first ultrasonic wave into electrical energy;

generating a second ultrasonic wave based on the first ultrasonic wave, wherein the second ultrasonic wave is transmitted in an opposite direction to the first ultrasonic wave;

receiving the second ultrasonic wave by the ultrasonic transducer array such that the plurality of ultrasonic transducers acquires a plurality of received signals based on the second ultrasonic wave;

coupling the received signals to a processor for generating phase delay signals; and

applying a phase delay focusing equation to the plurality of ultrasonic transducers based on the phase delay signals such that the ultrasonic energy can be focused onto the active implantable device with a desired waveform, wherein the ultrasonic energy is focused onto the active implantable device with constructive interference at a location of the active implantable device.
The method of claim 22, wherein the processor is configured to perform an image analysis and an image focusing for reconstructing an image of the active implantable device.
The method of claim 22, wherein the image focusing comprises a time-delay focusing and a simulated scatter-corrected focusing.
The method of claim 22, wherein the phase delay focusing equation is based on a relative position and a relative orientation between the one or more ultrasonic transducers.
The method of claim 22, wherein the ultrasonic energy is converted into electrical currents through piezoelectric elements, capacitive micromachined ultrasonic transducers (CMUTs) , optical based transducers, or other materials.
A method for transmitting ultrasonic energy to a plurality of active implantable devices in a living body simultaneously by time-delay focusing, wherein each of the plurality of active implantable devices is configured to receive the ultrasonic energy from an ultrasonic transducer array in accordance with the method of claim 22, and wherein the ultrasonic energy is focused onto the plurality of active implantable devices with constructive interference at multiple locations where the plurality of active implantable devices is positioned.
The method of claim 27, wherein each of the plurality of active implantable devices is selectively responsive to a range of frequencies for selectively activating the one or more active implantable devices.