CN111246916B - Multi-channel real-time phase modulation for EMI reduction in ultrasound devices - Google Patents

Abstract

A method for reducing electromagnetic interference (particularly inside the human body inside a magnetic resonance imaging apparatus) due to the use of a plurality of ultrasound transducers in an array. The electrical signals driving the transducers are shifted in phase relative to each other in order to achieve a maximum shift in the electric and magnetic fields caused by such signals and transducers. The phase offset is dynamically adjusted to respond to changes in drive amplitude and frequency in order to maintain an optimal reduction in electromagnetic interference.

Description

Multi-channel real-time phase modulation for EMI reduction in ultrasound devices

Technical Field

The present application relates generally to ultrasound therapy systems and more particularly to methods for improving the effectiveness of ultrasound therapy by reducing electromagnetic interference (EMI) caused by ultrasonic transducers by using phase modulation on the electrical signals sent to such transducers.

Background

Ultrasound transducers have been used in ultrasound therapy systems to achieve therapeutic heating of diseased and other tissues. An ultrasound transducer array operative to form a beam of ultrasound energy converts sound into thermal energy in an affected tissue region or treatment volume (treatment volume) and subsequently causes a beneficial temperature rise in the treatment volume.

In image-guided ultrasound therapy systems, the patient and the ultrasound therapy device are typically arranged in an imaging volume, such as a Magnetic Resonance Imaging (MRI) device, which makes it possible to guide the applicator placement and, in addition, to monitor the treatment effect of the tissue by providing real-time data from which a temperature map can be calculated. The clinical operator may then monitor the progress of the therapy within the treated volume or diseased tissue, and may make manual or automatic changes to the ultrasound power signal based on the input from the results and the progress of the treatment. By properly monitoring the heating effect, the ultrasound therapy system can be used to treat harmful cells and controllably destroy tumors while minimizing damage to healthy tissue.

Work has been performed in men to demonstrate the treatment of diseases such as prostate cancer using MRI-guided transurethral ultrasound therapy systems. See, for example, Chotra et al, "MRI-compatible transient aqueous system for the treatment of localized state registration using rotational control", Med Phys 35(4): 1346-. See also U.S. publication 2007/0239062; U.S. Pat. No. 6,589,174, "Technique and apparatus for ultrasound therapy", 2003; U.S. Pat. No. 4, 7,771,418, "Treatment of disused tissue using controlled ultrasonic heating", 2010; U.S. Pat. No. 5, 8,998,889, "System and method for control and synthesis of control thermal therapy", 2015; U.S. Pat. No. 9,707,413, "Ultrasonic therapy applicator", 2017. Such systems (including accumulated published and patented work by or for the applicant, all of which are hereby incorporated by reference) teach the use of transurethral ultrasound energy on a diseased prostate to achieve a desired target temperature in the diseased tissue to achieve a clinical outcome, typically necrosis of diseased tissue cells in the prostate. Real-time MRI guidance and temperature monitoring of treatment enables control of the power supplied to the therapeutic ultrasound transducer, but also the rotation of such transducer arrays disposed axially along an elongate applicator inserted into the patient's urethra near the diseased prostate.

As known to those skilled in the art, an ultrasound transducer is constructed and operated to acquire electrical power and generate ultrasound energy waves from the surface of the transducer element in a process generally referred to as transduction. The nature and extent of transduction depends on the materials used to construct the transducer, the transducer geometry, and the electrical input to the transducer. A common material used to construct ultrasonic transducers is piezoelectric transducer crystalline material (lead zirconate titanate, PZT) in several forms.

In the systems for ultrasound thermal therapy disclosed in the above-referenced sources, a Radio Frequency (RF) electrical output is typically generated in the frequency bands 4-4.5MHz and 13-14.4MHz, and used to drive up to 10 piezoelectric elements that convert the electrical energy to acoustic pressure (e.g., ultrasound). The element can be driven at relatively high power, about 4W and 2W at low and high frequency bands, respectively, and it is known that PZT materials can produce nonlinear responses. The effect of this is to generate electromagnetic interference (EMI) in the form of harmonics that can contaminate the MRI image and, when the transducer is used inside an MRI device, can interfere with MRI thermometry, which makes its operation dependent on RF electromagnetic signals. Such EMI is caused by electric fields generated in the transducer elements, as well as magnetic fields generated by currents flowing into and out of the transducer elements, because time-varying electric and magnetic fields can generate electromagnetic radiation, as is well known in the art.

Accordingly, there is a need to improve the accuracy and effectiveness of ultrasound thermal therapy by reducing EMI. When sources of EMI are deployed in confined spaces inside the human body (e.g., in the male urethra and prostate), traditional shielding methods are not always feasible or have limited efficacy. Thus, there is a need for additional methods of reducing EMI without increasing the body of the deployed device, and thus without the risk of injury to the patient. The present disclosure is directed to a method for reducing EMI due to an ultrasound transducer array by adjusting the phase angle of electrical signals sent to different transducers in the array.

Disclosure of Invention

The disclosure herein is directed to methods for improving the effectiveness of the use of ultrasound in therapeutic and other procedures by reducing electromagnetic interference due to an ultrasound transducer array and wires connected to such transducers using phase modulation techniques. The proposed method determines and implements phase offset angles for the signals sent to the respective transducers in order to achieve an optimal degree of reduction of EMI by mutual offset of the electric fields and currents generated by the respective transducers in the array.

Embodiments are directed to a method of reducing electromagnetic interference due to a set of ultrasound transducers as part of an ultrasound transducer array in a thermal treatment device, the set comprising N transducers, each transducer in the set corresponding to an active channel and being electrically driven with a drive signal of an amplitude, frequency and phase angle, the frequency of the drive signal being the same for all transducers in the set, the method comprising determining and setting the phase angle Θ of each drive signal₁，Θ₂，....，Θ_NThe determination and setting of such phase angles includes: determining the amplitude A of the respective drive signal for each transducer₁，A₂，...，A_NEach amplitude is a non-negative real number; determining the amplitude A_mIs greater than or equal to one other than A_mSum of all amplitudes other than A₁+A₂+...A_m-1+A_m-1+A_m+1+...+A_N(ii) a If A is_mGreater than or equal to the sum of such other amplitudes, then for all i not equal to m, set Θ_m180 ° and set Θ_i0 °; if A is_mLess than the sum of such other amplitudes: determining vectors

Such a vector comprises N elements, each such element being either 1 or-1, such that

And a vector [ A ] comprising all amplitudes₁，A₂，...，A_N]Is non-negative and is not greater in magnitude than the magnitude of the scalar product of any other possible vector comprising N elements, each such element being either 1 or-1, and the vector [ a₁，A₂，...，A_N]Including all amplitudes; for i ═ 1,2, …, N, a vector is defined that includes N elements

So that each element O_iIs equal to P_iAnd A_iThe product of (a); determining

O_aIs not less than

The first positive element of any other element of (a); determining

O_bIs not less than

By removing O from_aA second positive element of any other element than the first; defining the quantity gamma as

By removing O from_aAnd O_bAbsolute value of the sum of any element other than; will theta_aAnd Θ_bThe method comprises the following steps:

and for all Θ_iExcept that_aAnd Θ_bIn addition, setting Θ_i＝cos^-1O_i。

Another embodiment is directed to a method for reducing electromagnetic interference when operating an electrically driven ultrasonic thermal treatment device, the method comprising: positioning the device comprising an ultrasound array of the device relative to a designated treatment region; determining, in a computer-based host unit coupled to the therapy device, a common drive frequency and an amplitude with which to drive each of a plurality of transducer elements of the array; driving each of the plurality of transducer elements with a respective drive signal generated by a respective voltage source, the drive signal for each element comprising the common drive frequency, the amplitude, and a respective phase angle; and modifying a phase angle of at least one drive signal to reduce a net electromagnetic output of the thermal treatment device.

The treatment region may be a lumen, orifice, or other natural or artificial volume within the body of the patient, including the urethra, rectum, or other organ or cavity. In such cases, the treatment is delivered internally (e.g., transurethrally). In other cases, the treatment is delivered to the body from outside, and the ultrasound energy is directed into the body, e.g., through the patient's skin, external organs, and tissue layers. The treatment device may thus comprise an array in line (along a line or axis of the linear apparatus) or may be a geometrically focused array with a curved, contoured or other geometric arrangement.

Drawings

For a fuller understanding of the nature and advantages of the present invention, reference should be made to the following detailed description of the preferred embodiment taken in conjunction with the accompanying drawings in which:

FIG. 1 illustrates an exemplary system for providing image-guided ultrasound therapy to a patient;

FIG. 2 illustrates an exemplary ultrasound array;

FIG. 3 is a schematic circuit diagram illustrating a simplified model of a two transducer array, by which certain principles underlying the method disclosed herein are demonstrated;

FIG. 4 is a schematic diagram of a two-tone mixing model further illustrating some of the principles underlying the methods described herein;

FIG. 5 is an illustrative plot showing the results of two transducers driven, both with and without the methods disclosed herein, demonstrating the results obtained by using such methods;

6A, 6B, 6C, and 6D are vector diagrams that geometrically demonstrate certain aspects of the methods disclosed herein; and

fig. 7A, 7B, 7C, and 7D illustrate flow charts showing the operation of the methods disclosed herein, according to embodiments.

Detailed Description

The disclosure herein is directed to methods for reducing EMI due to a multi-element array of ultrasound transducers in an apparatus for ultrasonic thermal therapy using phase modulation techniques.

Fig. 1 illustrates an exemplary system 10 for providing image-guided ultrasound therapy to a patient. The simplified illustration shows a host computer 100, such as a laptop PC, workstation, or other processing apparatus having a processor, memory, and coupled to some input/output device. The host computer 100 may include a display and may support a user interface 110 to facilitate control and observation of the thermal therapy treatment process.

The host computer 100 is adapted to be coupled to other systems and components through a computer interface connector 120. The connection device 120 carries data and information to and from the host computer 100 and may include a standard or proprietary electrical wiring connection cable, such as a serial connection cable or the like. Furthermore, the connection means 120 may be implemented wirelessly as known to those skilled in the art of wireless communication, and may further be implemented by multiple connections, over a network, or by another suitable method.

In some embodiments, host computer 100 is coupled to power control unit 130 through connection device 120. Power control unit 130 may be implemented as a separate hardware device, but may be implemented as part of host computer 100, for example, by being built into a special card in a computer or server system that houses such hardware components.

Power control unit 130 may specifically include at least a processor adapted to process machine or program instructions that may be provided to the processor from another component of system 10 and may be stored on a memory device in power control unit 130. Circuitry including analog and/or digital circuitry may be operated within the power control unit 130 to determine the output power supplied to one or more ultrasound therapy transducer elements in the ultrasound therapy device 150.

In some embodiments, power control unit 130 may deliver controlled electrical drive signals to a plurality of ultrasound transducer elements (e.g., PZT array elements) in ultrasound therapy device 150. The drive signals may be controlled to deliver a programmed amount of power to each element or group of elements of the therapeutic device 150. The drive signal may also be controlled to provide a determined drive voltage, current, amplitude, waveform or frequency to the ultrasound transducer of the therapeutic device 150. As discussed herein, the relative phase of the drive signals may also be controlled, for example, to reduce EMI. Such electrical drive signals are carried from the power control unit 130 to the ultrasound therapy device 150 by a suitable wire, cable or bus 140. Suitable card interfaces or connectors may be included to mate with various ends of the connector or bus to and from their associated components.

In operation, the ultrasound therapy device 150 includes a portion 155 that is inserted into a portion of a patient's body to deliver an appropriate dose of ultrasound energy to tissue in a diseased region of the patient's body.

The patient and the ultrasound therapy device 150 are typically disposed in an imaging volume 160, such as a Magnetic Resonance Imaging (MRI) device, and the imaging volume 160 may provide real-time images of the relevant portion of the patient (e.g., the treatment volume) to the host computer 100 or display and user interface 110. In some embodiments, real-time monitoring of thermal therapy is performed so that a clinical operator can monitor the progress of therapy within the treated volume or diseased tissue. Manual or automatic changes may be made to the power signal from power control unit 130 based on input from the results and the progress of the treatment.

The feedback and coupling of treatment system components to the control components in system 10 may be used to ensure that an optimal Radio Frequency (RF) power signal is provided to each element of the ultrasound array 155 used in the treatment of diseased tissue. Some examples include the treatment of prostate cancer tumors in male patients using MRI-guided ultrasound therapy applications.

The RF power control unit 130 may comprise a separate circuit card with a single processor, amplifier, filter, and other components to achieve the desired drive power output to the elements of the ultrasound array 155 of the ultrasound treatment device 150. Alternatively, a single processor may be used to control the behavior of the various power channels for each array element.

Fig. 2 illustrates an ultrasound array 200, the ultrasound array 200 may be the same as the ultrasound array 155. Each ultrasonic transducer 205 in the ultrasonic array 200 is driven by a separate voltage source 210 via at least one wire 220. Thus, the voltage and relative phase of each drive signal for each transducer 205 may be controlled. The voltage source 210 is included in the RF power control unit 230, and the RF power control unit 230 and the RF power control unit 130 may be the same. Note that RF power control unit 230 may include additional components such as circuits, processors, amplifiers, filters, and other components as discussed above with respect to RF power control unit 130.

Each transducer 205 has 2 electrical terminals, a first of which is electrically connected to one terminal of its respective voltage source 210 via an electrical line known as a drive line, and a second of which is electrically connected to the other terminal of the voltage source 210, in common with all other such second transducer terminals, via an electrical line known as a return line or ground line. The drive line (one for each ultrasonic transducer in the array 200) and the common return line are connected to the proximal end of the applicator and extend to a voltage source 210. When the device is used for thermal treatment, in a typical arrangement, the applicator will be inserted inside the body of the patient to be in the vicinity of a part of the body receiving the treatment, for example transurethrally to the vicinity of the diseased male prostate, when the patient is inside the MRI machine. The drive and return lines, which are typically bundled together inside a single sheath, extend from the applicator to the exterior of the MRI machine where a voltage source is located, along with various means for controlling the device, such as an RF power control unit 230.

The use of a linear array of transducers (each driven by a separate voltage source) makes it possible to more accurately control the application of ultrasonic energy to the tissue being treated, thereby improving the effectiveness of thermal therapy compared to when the transducers are all driven at the same voltage. Where separate voltage sources are used for each transducer, the voltage amplitude of each transducer may be dynamically adjusted in response to feedback according to the treatment plan to maintain an optimal level of ultrasound output for the position and orientation of each transducer at any given time. Systems and methods for controlling and Monitoring Thermal Therapy using Ultrasound are described, for example, in U.S. patent application publication No.2011/0270366 entitled "RF Power Controller for ultrasonic Therapy System" and U.S. patent No.8,998,889 entitled "System and Method for Control and Monitoring of Conformal Thermal Therapy," which are hereby incorporated by reference.

Each voltage source sends an Alternating Current (AC) signal (typically a sinusoidal signal) of a given frequency (referred to as the drive frequency) to one of the transducers at a given voltage amplitude (which amplitude at any given time is determined by the treatment plan and the control algorithms and mechanisms of the system). The AC voltage signal induces an oscillating electric field within the transducer, which in turn induces mechanical oscillations in the transducer by means of piezoelectric induction; these mechanical oscillations transmit ultrasonic acoustic energy into the tissue being treated, where such energy is ultimately converted to thermal energy to achieve the desired therapeutic effect.

As noted above, EMI inside the MRI apparatus may result from magnetic fields generated by currents flowing through the drive and return lines, and from electric fields generated in the transducer. When multiple transducers are driven at a common drive frequency by separate voltage sources, EMI from these sources can be reduced by having the voltage signals out of phase with each other. Thus, by the principle of superposition, electric field components having directions opposite to each other will be superposed and offset from each other, as will currents having directions opposite to each other. When multiple ultrasound transducers are deployed adjacent to each other on a linear array, the electric fields generated by the transducers will partially overlap spatially. Similarly, the magnetic fields generated by wires (such as drive lines) in close proximity to each other will also spatially overlap. In both cases, the fields in opposite directions will be offset, thereby reducing EMI. And the current in the return line is a superposition of the currents from all the transducers, such currents will be directly offset to the extent that they are in opposite directions, thereby reducing any resultant magnetic field caused by such currents.

The principle behind the phase modulation method disclosed herein can be demonstrated by considering the simple case of an array comprising 2 ultrasound transducers driven with a common frequency. Fig. 3 is a simplified schematic diagram showing 2 transducers 301 and 302, each represented as a capacitor, each connected to and powered by one of sinusoidal voltage sources 311 and 312, connected by drive lines 321 and 322, and connected by a shared return line 324. The electric fields in and around transducers 301 and 302 are shown with field lines 331 and 332, respectively.

The sinusoidal voltage signals derived from sources 311 and 312 may be described as a function of time as follows:

V₁(t)＝A cos(ω₀t+φ₁)

V₂(t)＝B cos(ω₀t+φ₂)

where A and B are the voltage amplitudes of the respective signals, ω₀Is based on a common drive frequency, phi, measured in radians per second or alternatively in degrees per second over a period of 2 pi radians or 360 degrees₁And phi₂Is a phase angle or phase offset measured in radians or alternatively in degrees.

Note that a sinusoidal signal of a given frequency ω can be thought of as a phase vector or phasor rotating counterclockwise in the complex plane at a rate of ω radians (or degrees) per second or one full cycle per 2 π radians (or 360 degrees); the length or magnitude of the vector is the amplitude of the signal, and the signal at any given time is equal to the projection of the phasor onto the horizontal (real) axis (i.e., the real part of the phasor). The phase offset or phase angle of the signal is the angular position of the phasor relative to the true real axis measured counterclockwise from axis to phasor at any time designated as the start of the cycle (which time is the same for all phasors being considered) at time t-0. When multiple AC signals of a common frequency are expressed as phasors, vector arithmetic may be used to calculate the combined effect of such signals. The methods disclosed herein use such computational techniques.

Returning to FIG. 3, the currents flowing through transducers 301 and 302, respectively, are designated as I₁And I₂Wherein the current in the return line 324 is designated as I_TIn which I_T＝I₁+I₂. If the phase angle phi₁And phi₂Is set so that V₁And V₂For opposite phase, e.g. by setting phi₁0 DEG and phi₂180 ° and a B, i.e. the amplitude of the signals is equal, then V₁＝-V₂And I₁＝-I₂And I_T＝I₁+I₂Will be added to zero, i.e. the net current flowing through the transducer is zero and the current in the return line is zero.

This situation is depicted in fig. 3, where in fig. 3, electric field lines 331 and 332 are shown pointing in opposite directions; the direction of the electric field in the transducer will depend on the meaning of the applied voltage. It is assumed that transducers 301 and 302 are physically parallel to each other, as they will be on a linear transducer array. It can be seen that the electric fields from the respective transducers 301 and 302 overlap in the space 333 between the transducers. Such overlapping fields in the same space will be offset from each other to the extent that they point in opposite directions, according to the superposition principle. Such field cancellation reduces RF emissions caused by such fields. In addition, the radiation from the return line is reduced because the return line carries less current and therefore produces a smaller magnetic field. Furthermore, since the direction of such currents is opposite, the magnetic fields generated by the currents in the drive line will be opposite to each other, and therefore such magnetic fields will also be superimposed and offset from each other, further reducing RF emissions.

Piezoelectric materials used in ultrasound transducers, such as PZT, have been observed to exhibit nonlinear behavior in response to applied signals. In the simple case of two contributing elements, a two-tone nonlinear hybrid model can be used to understand the resulting harmonic and intermodulation content:

in the above equation, V₁And V₂Indicating that the force is applied to both driven (in this case, at ω)₀Common frequency operation). The power level is free to vary. V₀Is the output of a "mixer" that contains the new harmonic and intermodulation content, here represented by a power series approximation of the PZT transfer function. Fig. 4 shows the hybrid model in schematic form. Input signals 410 and 420 are fed into mixer 400, mixer 400 outputting signal 430. For simplicity, the piezoelectric-specific model parameters have been ignored. At a ═ B and the signals are in opposite phase (e.g., phi)₁0 DEG and phi₂180 °), then V₁＝-V₂And the summation term cancels out, thereby eliminating harmonic content.

The effect of phase modulating the signal with two elements can be seen in fig. 5, which fig. 5 depicts an illustrative plot 500 based on experimental results obtained when 2 elements are driven at the same amplitude and frequency (alternatively in phase and 180 degrees out of phase). The horizontal axis 510 represents frequency and the vertical axis 520 represents power of the combined signal. Trace 530 shows in-phase (phi)₁＝φ₂) The combination of the drive signals, while trace 540 shows out of phase (phi)₁+180°＝φ₂) A combination of drive signals. The peak 550 represents harmonics that occur at integer multiples of the natural frequency of the piezoelectric element being driven. It can be easily seen that when the drive signals are out of phase, the peaks are much lower, confirming that the harmonic content caused by the drive signals being out of phase with each other is largeAnd (4) reducing.

The RF voltage signals may be advanced or retarded, that is, they may have their phase angle adjusted without greatly affecting the delivery of ultrasonic acoustic energy to the target. The ultrasonic energy produced by a transducer depends on the amplitude and frequency of the voltage signal driving the transducer, while it does not depend on the phase of the signal; and such energy is transmitted in a direction that depends on the position and geometry of the transducer and the surrounding tissue, but not on the phase of the signal. Thus, the methods disclosed herein (which involve selecting and implementing different phase angles for the various signals used to drive the ultrasound transducer array) are effective to reduce EMI from such signals while maintaining the effectiveness of such signals for driving the transducers to achieve the desired therapeutic goal.

The two-element model discussed above can be easily extended to multiple elements where one or more subsets of such elements are driven at a common frequency. A simple way of assigning phase angles would be to set the alternating elements of the common frequency to 0/180/0/180 … degrees. However, this method is only optimal when the power output on all elements is the same. In many ultrasound applications, it is desirable to drive different transducer elements on an array at the same frequency, but at different power levels, and during a procedure, dynamically adjust such power levels in response to feedback in order to achieve a therapeutic or other goal of the procedure. Better algorithms are needed in terms of power levels between transducer elements and varying over time.

The method disclosed herein is directed to determining and setting the phase angle of a set of sinusoidal voltage signals driving an ultrasound transducer array such that, for each subset of signals of a given drive frequency, the vector sum of all signals in such subset is minimized. In calculating the vector sum, each voltage signal of a given frequency is expressed as a phasor, that is to say as an amplitude in the complex plane having a length, or equal to the amplitude of the signal, and measured counter-clockwise from the positive real axis, equal to the phase angle or phase of such a signalA vector of angles of offset. That is, such as V₁(t)＝A₁cos(ω₀t+φ₁) Will be represented in phasor representation as a length a₁From the true (i.e., right-pointing) axis, pointing counterclockwise by an angle phi₁The vector of (2). Alternatively, as a vector, a phasor may be represented by its components (i.e., its real and imaginary parts) as [ A ]₁ cosφ₁，A₁ sinφ₁]Or as a single complex number

Wherein j²Is-1. When so represented, the phasor may be multiplied by

To obtain a voltage signal as a function of time:

. Since all phasors for a given frequency are multiplied by the same factor to obtain the resulting signal, the combined effect of two or more signals for a given frequency can be calculated by simply adding the phasors of such signals. Such representations of AC signals are well known in the electrical arts.

The vector sum of all signals at a given frequency will be the net signal representing all transducers being driven at that frequency being transmitted into the array. By minimizing this net signal, the net current in the drive and return lines (which current is proportional to the voltage signal carried by such lines) will also be minimized. A reduction in the net current in the wire will reduce the magnetic field caused by such current and will therefore reduce the RF emission. Likewise, a reduction in the net voltage applied to ultrasound transducer elements disposed on a linear array, and thus physically parallel to each other, will result in a greater shift in the electric field generated by such transducers, and thus a reduction in RF emissions from such fields.

For a given subset of transducers being driven at a given common frequency, as disclosed hereinAn open algorithm may be used to optimize the respective phase angle of the signal sent to each transducer in the set. Let N be the number of transducers in the subset, and let A₁、A₂Wait until A_NIs the respective amplitude (in voltage, or alternatively in power) of the signal to be sent to the transducers in the set; such amplitudes will be determined and selected by any means for such purposes in order to achieve therapeutic and/or other goals of the ultrasound procedure. Note that all A' s_iAre all non-negative and will generally be positive for all active channels. (inactive channels will be represented by zero amplitude). The goal is to find the phase angle θ₁,θ₂,…,θ_NTo minimize the vector sum of all phasors.

The algorithm proceeds as follows: defining N vectors (that is, an ordered set of N scalar quantities) consisting of amplitudes

As a first pass or rough approximation, we consider only phase angles of 0 degrees and 180 degrees, and what combination of such angles will minimize the total signal. A phase angle of 180 degrees is equivalent to multiplying the signal by-1, simplifying the calculation of the stage. Calculate the sum of the amplitudes and determine the maximum amplitude:

A_max＝max[A₁，A₂，…，A_N]

determining

Whether or not the determination is true, that is,

is greater than or equal to the amplitude of the largest element in (1)

Is greater than or equal to the sum of all other amplitudes. Note that this will always be the case when N is 2. The maximum shift of the signal that can be achieved by phase modulation is then obtained by setting the phase of the signal with the largest amplitude to 180 degrees and the phase of all other signals to 0 degrees. If it is not

The signal will be fully shifted and if so

The signal will not shift completely but will shift to the maximum extent possible for such amplitudes.

Especially when a large number of transducers (e.g., 10) are driven at a given frequency, it may often be the case that the maximum amplitude will not be greater than the sum of all other amplitudes, i.e.,

in this case, the next step is to determine the combination of 0 degrees and 180 degrees phase angle (or, respectively, equal to 1 and-1) that will result in the maximum shift of the signal. For this purpose, the "phase offset vector" is defined as an N vector consisting of 1 and-1. (Note that this vector is different from the phase vector or "phasor" discussed above.) there is 2^NA possible such phase offset vector representing all possible permutations of 1 and-1. For a given phase offset vector

By application of

The net or residual amplitude resulting from the represented phase angle (0 degrees and 180 degrees) may be calculated as

And

is also referred to as an inner product or dot product. The product is computed by multiplying N pairs of corresponding elements in 2 vectors and then summing the resulting N products to obtain a scalar result:

the next step in the algorithm is to determine the phase shift vector of R that will result in a residual with the smallest magnitude or absolute value

Here, p_kIs all possible 2^NA phase offset vector. For example, if N is 3, then

To

The following will be (note that the order here does not matter):

can be obtained by

All the possibilities

And comparing the results, or by other methods known in the art, such as by using a look-up table, or an iterative minimization algorithm whose cost function computes the residual from some penalty metric. Note that, in terms of symmetry, for each

There will be another one in which all elements are retained

For example, in the above list

So 2 are

R values of equal magnitude but opposite sign will result. Therefore, because we are only interested in the magnitude of the residual, a maximum of 2^N-1This possibility needs to be checked to determine R_min。

Is provided with

Get R ═_minIs/are as follows

One of (1):

there may be more than one such

But any one may be selected. According to symmetry with respect to

Is negative for each possibility

Will also exist

Another one of positive inner product and same amplitude

I.e. the first of the sign reversal

Without loss of generality, if R_min>0, then we can choose and

to obtain a positive inner product

In some embodiments, the same R, if present, is obtained_minBased on other criteria, such as trying to maximize the number of pairs of physically adjacent transducers driven in opposite phase to each other, a phase offset vector is selected from these possible candidates

If R is_min0, the entire shift of the voltage signal can be determined by

Setting the phase angle of the signal; that is, if

Then if P _i1, then each signal from channel i (with amplitude a)_i) Is set to have a phase of 0 degrees, or if P_iSet to have a phase of 180 degrees-1.

If R is_min>0, then setting the phase angle in this way (i.e., according to a coarse approximation step) will not result in the entire shift of the voltage signal, but will reduce the net signal to an amplitude R_minOne of them. In this case, the algorithm continues by taking two of the voltage signals and further adjusting their phase angle to achieve the entire shift from all signals.

We will vector N

Is defined as

And

element by element multiplication of (1), wherein

Is selected as described above:

suppose that

Has been selected as described above such that

That is to say that the first and second electrodes,

is positive, then let alpha be

Wherein a is the index of its position and let β be

Where b is the index of its position:

note that for R_min>0 is at

There will always be at least 2 positive elements present. (if this is not the case, the one positive element will be greater than the sum of the magnitudes of all other elements, as discussed above

The situation of (a) is not consistent. ) Let gamma be

The sum of the remaining elements in (a):

note that R_min＝α+β+γ。

Note that γ is not always negative. (if γ is positive, then R_minWill not be the smallest possible residual since it can be determined by subtracting P from P_aAnd P_bThe signs of both become smaller in turn. ) Also note that the magnitude of γ will be less than the sum of α and β (if γ is not, we will have R_min<0). The magnitude of γ will be greater than the difference between α and β:

γ＜0

α-β＜|γ|＜|α+β|

the quantity γ represents the phasor of the resulting signal from the voltage signals of all channels except those represented by α and β. The next step is to set the phase angles for the two channels so that they deviate from the resulting signal. These angles can be found geometrically by forming triangles with sides of lengths α, β, and | γ |. Such a triangle can be formed because the three quantities satisfy the above "triangle inequality".

The cosine theorem states that for any triangle having an angle C opposite to the side of length C, where the lengths of the other sides are a and b:

c²＝a²+b²-2ab cos C

by rearranging the bits, the cosine theorem is used to calculate the phase angles a and B for the channels represented by α and β, respectively:

channel a is assigned a phase angle-B and channel B is assigned a phase angle a, the remaining channels being assigned in a coarse approximation step, i.e. respectively, on the basis of

Is either +1 or-1, is assigned either 0 degrees or 180 degrees.

The same methods described above can also be used for R_min<0. In this case, the calculation is the same except that α and β are chosen to be

Is the largest (in magnitude) negative element, and γ is positive; and the phase angles of channels a and B are assigned 180 deg. -B and 180 deg. + a, respectively.

The method of selecting the phase angle may be illustrated by numerical and graphical examples. Consider the case where N is 4, i.e., 4 active channels, with amplitudes of 1, 3, and 2, respectively, i.e.,

the smallest possible residual may be determined to have a magnitude of 1.

Will generate

Wherein

(other substitutions will also result in R_minIs-1 or R _min1, but 1 is sufficient for the calculation. ) Thus, the coarse approximation step yields [180 °,180 °,0 °,180 ° ]]The phase angle of (c). This is depicted in FIG. 6A by a vector, where

Is shown at the top, with negative elements pointing to the left and a single positive element pointing to the right. The residual of-1 (R _ min) is depicted below.

Since the residual is negative, α and β are chosen as the two largest vectors in the negative direction, with lengths α -2 and β -1, corresponding to channels a-4 and b-1, respectively. Fig. 6B shows the remaining vectors after these 2 vectors are removed, with the new residual γ ═ 2 depicted below.

The next step is to adjust the phases of the vectors a and β so as to offset γ, i.e. so that 3 vectors will add up to zero on the vectors. The use of the cosine theorem is proposed,

we therefore set the phase of channel a to 180 ° + 28.96 ° -151.04 °, and the phase of channel B to 180 ° + a to 180 ° +75.52 ° -255.52 °. In fig. 6C, vectors α and β are shown placed at these angles, where γ is placed at 0 °. Angles a and B are indicated in the triangle formed by vectors α, β and γ. It can be seen that α is 180 ° + B from the (zero phase) position pointing to the right, and β is 180 ° + A from such position.

The final phase angles obtained by the method are therefore [255.52 °,180 °,0 °,155.04 ° ]. In fig. 6D, the phasors represented by the individual channels are shown as having these phase angles. The vector sum of the 4 phasors is zero, resulting in zero net current in the drive and return lines and a maximum excursion of the electric field generated by the transducer.

If the ultrasonic transducers on the array are being driven at more than one drive frequency, the methods disclosed herein are performed for each subset of transducers being driven at a given drive frequency. Because the amplitude and drive frequency are adjusted and recalibrated during the course of the ultrasound procedure, the phase angle calculation is repeated each time there is a change in any amplitude and/or frequency for any of the channels. For optimal EMI reduction, every time there is an update in frequency and/or amplitude as determined by the control algorithm and/or treatment plan, the phase angle corresponding to the new set of amplitudes and frequencies is calculated as disclosed herein before the change in amplitude and/or frequency is implemented in the signal sent to the transducer. Then, when these signals are changed, the new frequency and amplitude are implemented simultaneously with the new phase angle determined as disclosed herein.

The methods disclosed herein are illustrated in the flow chart 70 depicted in fig. 7A-7D. The method begins at step 702, where an N vector is generated

Is defined, the vector includes each of the amplitudes of the N voltage signals of a given drive frequency to be transmitted to the respective ultrasound transducers in the array. Such amplitudes are determined according to the objectives of the ultrasound procedure and according to the received feedback and other considerations. These amplitudes are not determined by the methods disclosed herein, but rather are determined from inputs to such methods. The output of the method is a vector

The vector includes the phase angle of the voltage signal to be used for transmission to each transducer.

The method continues to step 704 where, in step 704, a calculation is made

With each possibility

The dot product (or scalar product) of vectors (that is, every possible N vector whose elements are either +1 or-1). Then, in step 706, R is determined from the result of step 704_minFor the smallest dot product in magnitude calculated, and determining and selecting a vector

For the vector

May be present to give R_min(or-R)_min) More than one possibility of

Vectors, in this case, such possibility

One of the vectors is chosen arbitrarily or based on other considerations, such as having adjacent channels whose phases are offset by opposite amounts to maximize electric field cancellation. Note that for purposes of this flow chart, R_minMay be positive or negative. As noted elsewhere herein, in some embodiments, R is_minAnd

can be passed through in pairs

Not involving calculation in case all possible permutations are madeAll possible dot products are determined by other methods.

Following step 708, in step 708, the maximum amplitude is determined (that is,

the largest element) is greater than half the sum of all amplitudes. If so, then at step 710 (see FIG. 7D), based on

The phase angle is set, that is,

where the inverse cosine is taken element by element such that the phase angle for each channel is either 0 deg. or 180 deg., depending on whether the corresponding element of P is +1 or-1. In some embodiments, step 708 occurs before step 704, such that steps 704 and 706 are not necessary if step 710 is reached. In such a case, it is preferable that,

will be set so that if channel m is the channel with the largest amplitude, θ ≠ m for all i ≠ m_m180 deg., and θ_i＝0°。

If step 708 results in "no," the flow diagram continues to placeholder A. FIG. 7B begins with placeholder A, which then proceeds to step 712, computing

And at step 714, the flowchart is according to R_minNon-negative ("yes" branch), or negative ("no" branch), split into 2 branches. The scalar quantity Θ used in calculating the phase angle is set to either 0 ° or 180 ° at steps 720 and 730. Steps 720 and 730 then proceed to placeholders C and D, respectively.

FIG. 7C begins with placeholders C and D. In steps 722 and 732, α is defined as

The largest positive (or negative, as the case may be) entry in which the index of the position is i (α ═ O)_i). At steps 524 and 534, β is defined as

The second largest positive (or negative) entry in (b) has a position index of j (β ═ O)_j)。

The branches of the flow chart rejoin at step 540, and in step 540, the residual γ is defined as removing O_iAnd O_jThen,

Of the remaining entries in (a). Step 540 then proceeds to placeholder E, which is also found in fig. 7D. In step 542, angles a and B are calculated from α, β, and γ using the cosine theorem. Specifically, a ═ cos^-1([β²+γ²-α²])/[2×|β|×|γ|]) And B ═ cos^-1([γ²+α²-β²])/[2×|γ|×|α|])。

In step 744, the phase angle is approximated according to the coarse approximation step

Is temporarily set as

Then, in step 746, the phase angles of channels i and j are set according to the angle determined in step 542 (that is, the vector α is O ═ O)_iAnd β ═ O_jSuch that if R) is present, the phase angle of (c) is adjusted to be equal to_minIs not negative, then theta_iIs set to-B, or if R_minIs negative then theta_iIs set to 180-B; and if R is_minIs not negative, then theta_jIs set to A, or if R_minIs negative then theta_jIs set to 180 ° + a. Note that if R is_minIf 0, then in step 542The calculation of (a) will result in (B) being 0, so that the phase angles will all be either 0 ° or 180 °.

From step 746 or step 710, the next step is step 750, where channels 1 through N are based on the vector in step 750

Is assigned a phase angle, and such a phase angle is transmitted to the ultrasonic transducer

Is implemented in the voltage signal of a given amplitude. The flowchart 70 may be repeated (i.e., return to step 502) at each hardware update interval, i.e., each time any one of the transducers has any change in the amplitude and/or frequency at which it is driven, the method shown in the flowchart will be followed in order to determine the optimal phase angle to be used with the drive signal so that such phase angle can be achieved to the greatest extent possible in synchronization with the change in amplitude and/or frequency.

The present invention should not be considered limited to the particular embodiments described above. Various modifications, equivalent processes, as well as numerous structures to which the present invention may be applicable will be readily apparent to those of skill in the art to which the present invention is directed upon review of the present disclosure.

Claims (10)

1. A method of reducing electromagnetic interference caused by a set of ultrasound transducers being part of an array of ultrasound transducers in a thermal treatment device, the set comprising N transducers, each transducer in the set corresponding to an active channel and being electrically driven with a drive signal of an amplitude, frequency and phase angle, the frequency of the drive signal being the same for all transducers in the set, the method comprising determining and setting the phase angle Θ of each drive signal₁，Θ₂，...，Θ_NThe determination and setting of such phase angles includes:

determining the amplitude A of the respective drive signal for each transducer₁，A₂，...，A_NEach amplitude is a non-negative real number;

determining the amplitude A_mIs greater than or equal to one other than A_mSum of all amplitudes other than A₁+A₂+…+A_m-1+A_m+1+…+A_N；

If A is_mGreater than or equal to the sum of such other amplitudes, then for all i not equal to m, set Θ_m180 ° and set Θ_i0 °, where i is 1,2, …, N, Θ_iIs the phase angle of the ith drive signal;

if A is_mLess than the sum of such other amplitudes:

determining vectors

Such a vector comprises N elements, each such element having a value of either +1 or-1, such that

And a vector [ A ] comprising all amplitudes₁，A₂，...，A_N]Is non-negative and is not greater in magnitude than the magnitude of the scalar product of any other possible vector comprising N elements, and vector a₁，A₂，...，A_N]Including all amplitudes;

for i-1, 2, …, N, a vector is defined comprising N elements

So that each element O_iIs equal to P_iAnd A_iThe product of (a);

determining

First positive element of (2)_a，O_aNot less than

Wherein a is

An index of the position of the first positive element of (a);

determining

Second positive element of (2)_b，O_bNot less than

By removing O from_aAny other element than, wherein b is

An index of the position of the second positive element of (1);

defining the quantity gamma as

By removing O from_aAnd Θ_bAbsolute value of the sum of any element other than; and

will theta_aAnd Θ_bThe method comprises the following steps:

and for all Θ_iExcept that_aAnd Θ_bIn addition, setting Θ_i＝cos^-1O_i；

Wherein the content of the first and second substances,

Θ_ais the phase angle, theta, of the driving signal in the active channel a_bIs the phase angle of the drive signal in active channel b.

2. The method of claim 1, wherein the ultrasound transducer array is deployed inside a magnetic resonance imaging device.

3. The method of claim 2, wherein the ultrasound transducer array is deployed for the purpose of applying conformal thermal therapy to a human patient.

4. A method as claimed in claim 1, wherein the amplitude and frequency are updated at intervals, and the determination and setting of such phase angles is repeated at each such interval, based on the new amplitude and frequency resulting from such updating.

5. The method of claim 4, wherein updating the amplitude and frequency comprises adding and/or removing one or more ultrasound transducers in the array to and/or from the set.

6. The method of claim 4, wherein the setting of the phase angle of the drive signal in conjunction with the updating of the amplitude and frequency is implemented in the drive signal in synchronization with the implementation of the new amplitude and frequency in the drive signal.

7. The method of claim 1, wherein the drive signal is a sinusoidal signal.

8. The method of claim 1, wherein the vector is

By calculating said vector [ A ]₁，A₂，...，A_N]And comparing the results, wherein each of the set of possible vectors comprises N elements, each of the N elementsEither 1 or-1 or a subset of such a set.

9. The method of claim 1, wherein the ultrasound transducer array is a linear array.

10. The method of claim 1, wherein the ultrasound transducer array is a focused array.

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