JP2005005923A - Ultrasonic transmitter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an ultrasonic transmitter which is improved in transmission efficiency. <P>SOLUTION: The ultrasonic transmitter has a structure composed of a plurality of piezoelectric vibrators 1 arranged in a one-dimensional manner and an acoustic concave lens 3 stacked up on the vibrators 1, and the thinnest part of the acoustic concave lens is shorter than the wavelength of signals driving the piezoelectric vibrators and having the maximum frequency inside the acoustic concave lens. An acoustic layer 4 has an acoustic impedance intermediate between an acoustic impedance possessed by the acoustic concave lens and an average acoustic impedance possessed by a work as an object of testing, and is half or below as thick as the wavelength of the signals having the maximum frequency. The layer is laminated on the transmission surface of the acoustic concave lens 3. Electrodes 2-1 and 2-2 for driving the piezoelectric vibrators 1 are provided on both the sides of the piezoelectric vibrators 1. Lamb waves can be restrained from occurring in the acoustic concave lens and the piezoelectric vibrators. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an ultrasonic transmitter that irradiates a treatment site or an imaging target site in a subject with ultrasonic waves.
[0002]
[Prior art]
A method of realizing the control of the focal position along the direction perpendicular to the transmission surface of the ultrasonic transmitter with a phased array is well known, without mentioning a conventional example. An acoustic concave lens is also well known (for example, Non-Patent Document 1, page 200, FIG. 3.3.10). A matching layer for increasing the transmission efficiency from the acoustic concave lens is also well known (for example, Patent Document 1).
[0003]
[Non-Patent Document 1]
G. S. Kino, "Acoustic Waves (devices, imaging, & analog signal processing)" (Prentice-Hall, 1987), pp. 200
[Patent Document 1]
Japanese Patent Laid-Open No. 2002-153383
[Problems to be solved by the invention]
In the following description, the acoustic energy density at the focal point with respect to the driving voltage (acoustic energy per unit area) is defined as “wave transmission efficiency”.
[0005]
When using an ultrasonic transmitter with a variable focal position to control the transmission of ultrasonic waves in a compact device configuration, control the focal position in the azimuth direction (the direction along the ultrasonic wave transmission surface). Is achieved by moving the relative position of the ultrasonic transmitter and the desired focal point, and controlling the focal position in the depth direction (perpendicular to the ultrasonic wave transmission surface) with a phased array, achieving variable focus. it can. In this case, by arranging strip-shaped piezoelectric transducer elements with a very small width in the width direction (long axis direction), each piezoelectric transducer element is controlled with a delay time or phase determined by the distance from each piezoelectric transducer element to the focal position. Control the distance from the ultrasonic transmitter to the focal point. An acoustic lens is placed in the length direction (short axis direction) of the piezoelectric vibrator element so that the ultrasonic waves can be converged in the short axis direction, and the ultrasonic waves can be converged in the short axis direction. Compared to the case of using a concentric annular array, an ultrasonic transmitter that uses an acoustic lens in the short axis direction is more suitable for imaging from the intercostal space, or in the rectum or vagina. This is suitable for transmission under conditions where the width of one side of the two-dimensional transmission aperture of ultrasonic waves is limited, such as when transmission is necessary. For this reason, since the aperture in the minor axis direction is generally smaller than the aperture in the major axis direction, the convergence of the ultrasonic wave in the minor axis direction is loose, and the focal range determined by the acoustic lens used in the minor axis direction is It is configured to cover as much as possible the range in which the focus can be made variable.
[0006]
When it is desirable to be small in terms of usability, such as a medical ultrasound probe, or perpendicular to the direction of insertion into the subject, such as an ultrasound transmitter for transrectal or laparoscopic When the thickness of the acoustic concave lens is reduced, such as when the cross-sectional area on the surface is to be as small as possible, or when including an array of lenses, the ultrasonic wave does not converge to the designed focal position. There was a problem that the side lobe was larger than the expected prediction and the transmission efficiency was not good.
[0007]
In addition, it is composed of a piezoelectric vibrator and a concave lens, and the thickness of the thinnest part of the concave lens, that is, the thickness of the center line part of the concave surface, is less than the wavelength in the concave lens at the center frequency of the signal driving the piezoelectric vibrator. In the waver, there are problems that the side lobe is large and the degree of convergence of the ultrasonic power to the focal point is low.
[0008]
An object of the present invention is to provide an ultrasonic wave transmitter with improved wave transmission efficiency.
[0009]
[Means for Solving the Problems]
The ultrasonic transmitter of the present invention has a structure in which an acoustic concave lens is laminated on a plurality of piezoelectric vibrators arranged in one dimension, and the thickness of the thinnest part of the acoustic concave lens is a signal for driving the piezoelectric vibrators. In the ultrasonic wave transmitter that is equal to or less than the wavelength within the acoustic concave lens at the maximum frequency among the frequency components of the above, having an acoustic impedance of a value between the acoustic impedance of the acoustic concave lens and the average acoustic impedance of the subject, It is characterized in that an acoustic layer having a thickness of half or less of the wavelength at the maximum frequency is laminated on the transmission surface of the acoustic concave lens. The thickness of the acoustic layer has a value between one-fifth and one-third of the wavelength at the maximum frequency, and the acoustic impedance of the acoustic layer is the acoustic impedance of the piezoelectric vibrator and the acoustic of the subject. It has a value between half and twice the geometric mean of impedance.
[0010]
The ultrasonic wave transmitter according to the present invention is an ultrasonic wave transmitter in which an acoustic concave lens is laminated on a plurality of piezoelectric vibrators arranged in one dimension, and the piezoelectric vibrator is driven on the side opposite to the acoustic concave lens of the plural piezoelectric vibrators. It is characterized by laminating films in which metal and polymer layers are alternately laminated with a period of half the wavelength at the frequency.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
1A and 1B are diagrams for explaining the configuration of an ultrasonic transmitter according to an embodiment of the present invention. FIG. 1A is a perspective view and FIG. 1B is a cross-sectional view. In the ultrasonic transmitter, the acoustic concave lens 3 is laminated on a plurality of strip-shaped piezoelectric vibrators 1, and the thickness of the thinnest part of the acoustic concave lens 3, that is, the center line part of the concave surface of the acoustic concave lens 3 is the piezoelectric vibrator 1. Is equal to or less than the wavelength within the acoustic concave lens 3 at the center frequency of the signal for driving the acoustic lens, has an acoustic impedance between the acoustic impedance of the acoustic concave lens 3 and the average acoustic impedance of the subject, and has a thickness of the above An acoustic layer 4 having a half or less of the wavelength at the center frequency is laminated on the transmission surface of the acoustic concave lens 3. Electrodes 2-1 and 2-2 for driving the piezoelectric vibrator are formed on both surfaces of the piezoelectric vibrator 1.
[0012]
The configuration shown in FIG. 1 makes it possible to avoid excitation of unnecessary modes such as Lamb waves in the piezoelectric vibrator 1 and the acoustic concave lens 3. The thickness of the acoustic layer 4 is most preferably about one-fourth of the wavelength at the center frequency, and the acoustic impedance is a geometric average of the acoustic impedance of the concave lens material and the acoustic impedance of the subject. It is desirable to be close to.
[0013]
In the embodiment of the present invention, the acoustic layer 4 made of a polymer material is laminated on the transmission surface of the acoustic concave lens 3 in order to reduce the “cuttering” of acoustic energy in the transmitter that causes Lamb waves. In the following description, the acoustic layer 4 is referred to as a Lamb wave suppression layer 4 because of its function.
[0014]
FIG. 2 is a diagram showing a calculation (simulation) result of a beam pattern from the ultrasonic transmitter in the embodiment of the present invention. FIG. 2 shows a beam pattern representing the angular distribution of ultrasonic waves in the minor axis direction when ultrasonic waves are transmitted at an angle (horizontal axis) of 0 °. FIG. 2A is a diagram illustrating a calculation result when the Lamb wave suppression layer is not used, and FIG. 2B is a diagram illustrating a calculation result when the Lamb wave suppression layer is used.
[0015]
The beam pattern was calculated by the time-domain finite difference method using a wave equation considering piezoelectricity, and included not only a simple longitudinal wave effect but also a transverse wave effect. The conditions for the calculation simulation will be described with reference to FIG.
[0016]
As shown in FIG. 2A, it can be seen that side lobes having a maximum size of −7 dB as compared with the central beam appear on the left and right sides of the ultrasonic beam (center beam) at the angle of 0 degrees. . The side lobe causes a decrease in the signal-to-noise ratio and further causes a false image when the ultrasonic transmitter is used for ultrasonic imaging. In order to use an ultrasonic wave transmitter for heat coagulation treatment, the side lobe has a focal position relative to the energy applied to the transducer due to the cause of temperature rise in the part other than the treatment focus or energy concentration. In this case, sufficient sound pressure cannot be obtained.
[0017]
In FIG. 2B, which shows the calculation result of the beam pattern at the same focal plane as in FIG. 2A, it can be seen that the side lobe is greatly reduced. In this example, in the configuration in which the Lamb wave suppression layer is not used, side lobes appear at about −7 dB at a position where the horizontal axis in FIG. 2A is around ± 20 degrees, but the Lamb wave suppression layer is used. In the configuration, the side lobe of −23 dB is maximum at a position where the horizontal axis in FIG. 2B is around ± 30 degrees. Thus, the use of the Lamb wave suppression layer showed an improvement of 16 dB compared with the peak value of the side lobe. This Lamb wave suppression layer is different from the acoustic matching layer described in Patent Document 1 for enhancing the transmission efficiency. The Lamb wave suppression layer has a function that works very effectively when the acoustic concave lens 3 is thin and the influence of the Lamb wave cannot be ignored.
[0018]
FIG. 3 is a diagram showing the calculation (simulation) result of the sound pressure distribution transmitted from the ultrasonic transmitter in the embodiment of the present invention. FIG. 3A shows a calculation model. FIG. 3B is a diagram illustrating a calculation result when the Lamb wave suppression layer is not used, and FIG. 3C is a diagram illustrating a calculation result when the Lamb wave suppression layer is used. 3 (B) and 3 (C) are diagrams in which the sound pressure distribution at the moment when a certain period of time has elapsed since the start of driving is binarized with positive and negative values, and the black portion indicates a region where the sound pressure is positive. The white portion indicates a region where the sound pressure is negative.
[0019]
In FIG. 3A, a concave lens made of aluminum (acoustic concave lens 3 in FIG. 1) is laminated on a vibrator made of PZT (lead titanium zirconate) (piezoelectric vibrator 1 in FIG. 1). The curvature of the acoustic concave lens is 60 mm, and the thickness of the thinnest part is 0.5 mm. The Lamb wave suppression layer quality (see FIG. 1) is a film containing a polymer designed to have an acoustic impedance of 8 MRay and a thickness that is a quarter of the wavelength at 3.5 MHz that is the driving frequency of the vibrator. It is. In order to approach the conditions for actually using the transmitter, the back surface of the vibrator (piezoelectric vibrator 1 in FIG. 1) is air, and the front face of the concave lens (acoustic concave lens 3 in FIG. 1) is water. The sound pressure distribution was calculated. The housing is not included in the calculation of the sound pressure distribution.
[0020]
As is apparent from the result shown in FIG. 3B, the sound pressure along the direction orthogonal to the traveling direction of the ultrasonic wave transmitted into the piezoelectric vibrator and the acoustic concave lens is not constant, that is, the ultrasonic wave transmitter. Are not driven in phase in the direction along the transmission surface. In other words, vibrations that combine longitudinal and transverse waves such as Lamb waves are complexly excited, and as a result, the transmission surface of the ultrasonic transmitter is not driven in the same phase, so that it does not behave as a geometric optical lens. Large side lobes are generated. As a result, the transmission efficiency is also poor. As shown in FIG. 3 (B), the wavefront transmitted from the ultrasonic transmitter into the water is disturbed.
[0021]
As apparent from the result shown in FIG. 3C, the distribution of the sound pressure along the transmission surface of the ultrasonic transmitter is improved to be uniform as compared with the result shown in FIG. You can see that As shown in FIG. 3C, the phase of the sound pressure is the same in the piezoelectric vibrator and the acoustic concave lens. In addition, it can be seen that the wavefront transmitted from the ultrasonic transmitter into the water is substantially concentric, and the wavefront disturbance shown in FIG.
[0022]
Hereinafter, the effect of the Lamb wave suppression layer 4 will be discussed in comparison with the case where the conventionally used acoustic concave lens 3 is thick.
[0023]
FIG. 4 is a diagram showing a simulation result showing the dependence of the transmission efficiency on the thickness of the concave lens (acoustic concave lens 3) in the embodiment of the present invention. The conditions for the simulation are the same as those for FIGS. The horizontal axis of FIG. 4 shows the thickness of the thinnest portion of the acoustic concave lens 3, that is, the thickness of the central portion of the acoustic concave lens 3, and shows the relative value with respect to the wavelength of the longitudinal wave of the acoustic concave lens 3 at the driving frequency. The vertical axis of FIG. 4 indicates transmission efficiency = (acoustic energy density at the focal point) / (total acoustic energy), and in the case of no Lamb wave suppression layer, the thickness of the acoustic concave lens is ¼ of the wavelength. It is shown as a relative value to.
[0024]
When there is no Lamb wave suppression layer 4, the acoustic energy at the focal point increases as the thickness of the acoustic concave lens 3 increases. This indicates that the closer to the condition of the acoustic lens conventionally used, the better the transmission efficiency. This is supported by increasing the thickness of the Lamb wave and increasing its influence as the thickness increases. This is because if the wavelength of the Lamb wave is longer than the length of the piezoelectric vibrator element 1 in the length direction (short axis direction), phase disturbance along the transmission surface of the ultrasonic wave transmitter is eliminated. On the other hand, when the Lamb wave suppression layer 4 is used, even if the thickness of the acoustic concave lens 3 is ¼ of the wavelength, the transmission efficiency is higher than when the thickness of the acoustic concave lens 3 is four times the wavelength without the Lamb wave suppression layer. good. This makes it possible to use an acoustic concave lens that is an order of magnitude thinner than the conventional lens compared with the same transmission efficiency.
[0025]
FIG. 5 is a diagram showing the result of simulation showing the dependence of the transmission efficiency on the thickness of the Lamb wave suppression layer 4 in the example of the present invention. The conditions of the simulation are the same as the conditions related to FIGS. 2 and 3, and the thickness of the acoustic concave lens 3 is a quarter of the wavelength. The vertical axis in FIG. 5 indicates transmission efficiency = (acoustic energy density at the focal point) / (total acoustic energy). The horizontal axis of FIG. 5 shows the thickness of the Lamb wave suppression layer 4 and is shown as a relative value to the wavelength at the maximum frequency among the frequency components of the signal that drives the piezoelectric vibrator 1. The point of thickness 0 in FIG. 5 indicates a case where there is no suppression layer.
[0026]
From the results shown in FIG. 5, the thickness of the quarter of the wavelength at the above-mentioned maximum frequency is optimal, and the practical use range is from one-fifth of the wavelength to about one-third of the wavelength. It is thought that. Here, a range in which the sound pressure is reduced to 90% and the acoustic energy is reduced to 80% as compared with the optimum value is assumed as a practically acceptable range.
[0027]
FIG. 6 is a diagram showing a simulation result showing the dependence of the transmission efficiency on the acoustic impedance of the Lamb wave suppression layer 4 in the example of the present invention. The conditions of the simulation are the same as the conditions related to FIGS. 2 and 3, and the thickness of the acoustic concave lens 3 is a quarter of the wavelength. The vertical axis in FIG. 6 shows the transmission efficiency = (acoustic energy density at the focal point) / (total acoustic energy) as a relative value. The horizontal axis of FIG. 6 shows the acoustic impedance (MRay) of the Lamb wave suppression layer 4. The points of acoustic impedance = 1.5 MRay (same value as water) and acoustic impedance = 17 MRay (same value as aluminum) shown for reference in FIG. 6 are equivalent to the case where there is no Lamb wave suppression layer 4. .
[0028]
From the results shown in FIG. 6, the acoustic impedance is optimally the geometric mean of the acoustic impedance of the acoustic concave lens 3 and the acoustic impedance of water, but from 3 MRay to 1 MRay, that is, from about half to twice the geometric mean. It is considered that it can be used practically in a wide range.
[0029]
FIG. 7 is a diagram showing an actual measurement result of a transmitted sound field from an ultrasonic transmitter in an embodiment of the present invention, and a result of measuring a sound field from an actually manufactured ultrasonic transmitter by the Schlieren method. It is. In FIG. 7, the measurement intensity | strength on the central axis of the ultrasonic transmission beam of an ultrasonic transmitter is shown among the measurement results by the schlieren method. In FIG. 7, the vertical axis represents the sound pressure (dB), and the horizontal axis represents the distance (mm) from the transmission surface of the ultrasonic transmitter. The measured intensity by the Schlieren method is a value proportional to the square of the integral value of the sound pressure along the optical path. As is clear from FIG. 7, when comparing the width of 3 dB before and after the position where the sound pressure is maximum, the Lamb wave suppression layer 4 is 45 mm when there is no Lamb wave suppression layer 4 (dotted line). In some cases (solid line), the beam is focused in the depth direction of 20 mm, and it has been experimentally confirmed that the effect of suppressing the Lamb wave is effective.
[0030]
FIG. 8 is a cross-sectional view of an ultrasonic transmitter using the frequency selective reflection heat transport layer 5 on the back surface of the piezoelectric vibrator in the embodiment of the present invention. The configuration excluding the frequency selective reflection heat transport layer 5 and the housing 6 shown in FIG. 8 is the same as the configuration of FIG. The frequency selective heat transport layer 5 is formed by periodically laminating a plurality of layers of metal and polymer material, and the well-known Bragg reflection condition (2 d sin θ = λ, d is the period of the thickness of the above-mentioned plurality of layers. Λ is the wavelength at the drive center frequency), and is configured to reflect a specific frequency (in this case, the frequency of the therapeutic ultrasound). Although θ is (90 degrees−incident angle), the condition of Bragg reflection can be described as 2d = λ because it can be considered to be almost 90 degrees under the condition of the flat plate vibrator. By using one of the periodic structures of the frequency selective heat transport layer 5 as a metal, heat conduction is performed more efficiently than when the back surface is air, and the reflection of ultrasonic waves is higher than when the whole is metal. The rate can be kept high, and the transmission efficiency can be kept high. In addition, if the back surface is air, the transmission efficiency will not be stable when water leakage occurs in the housing 6, but if the back surface is solidified, it is stable even if water leakage occurs as a fail-safe mechanism. Can be made to function. The frequency selective heat transport layer 5 acts as a frequency selective reflection film / cooling film.
[0031]
In another embodiment, it is also preferable to use a non-cylindrical acoustic concave lens described below. When the Lamb wave suppression layer enables precise control of the focal position, optimization of the focal depth by optimizing the curvature of the concave surface of the acoustic lens becomes significant. As described above, when electronic variable focus is performed in the long axis direction and fixed focus is performed by the acoustic concave lens in the short axis direction, it is desirable that the focal depth of the fixed focus covers the entire focus range used in the electronic variable focus. Although this depends on the aperture diameter vs. focal length, the depth of focus due to fixed focus in the minor axis direction may be too narrow if only a simple arc is used. By using an acoustic lens having a non-cylindrical surface shape in which the focal length varies depending on the position in the minor axis direction, that is, the curvature of the concave surface of the acoustic lens is continuously changed, the depth of focus can be expanded. When the beam of the acoustic concave lens becomes as designed by using the Lamb wave control layer, such beam optimization of the concave lens is also an effective method.
[0032]
The ultrasonic wave transmitter of the present invention is effective when the wave transmission efficiency becomes a problem as in the ultrasonic heat coagulation treatment. This is because if the wave transmission efficiency is poor, the heat generated by the ultrasonic transmitter cannot be ignored, the heat generated may flow to the outside, and the piezoelectric vibrator may deteriorate during driving due to the heat generated. It is. In another embodiment, as in the case of a high-frequency imaging probe, this is also a solution for the case where attenuation of ultrasonic waves in the acoustic lens leads to a decrease in image sensitivity.
[0033]
According to the present invention, by suppressing the Lamb wave in the acoustic concave lens and the piezoelectric vibrator, it is possible to suppress the transmission of unnecessary ultrasonic waves to other than the focal point, and in the transmitter Heat generation can be suppressed and the transmission efficiency of the entire ultrasonic transmitter can be improved.
[0034]
The present invention is not limited to the specific embodiments described above, and various modifications can be made without departing from the scope of the technical idea.
[0035]
【The invention's effect】
ADVANTAGE OF THE INVENTION According to this invention, the ultrasonic transmitter which improved the transmission efficiency can be provided.
[Brief description of the drawings]
FIG. 1 is a diagram illustrating a configuration of an ultrasonic transmitter according to an embodiment of the present invention.
FIGS. 2A and 2B show calculation results of a beam pattern from an ultrasonic transmitter in an embodiment of the present invention, FIG. 2A shows a result when a Lamb wave suppression layer is not used, and FIG. 2B shows a Lamb wave suppression layer. The figure which shows the result in the case of using.
FIGS. 3A and 3B are diagrams showing calculation results of a sound pressure distribution transmitted from an ultrasonic transmitter in an embodiment of the present invention, where FIG. 3A shows a calculation model, and FIG. 3B shows Lamb wave suppression; The result when not using a layer, (C) is a figure which shows the result when using a Lamb wave suppression layer.
FIG. 4 is a graph showing the dependence of the transmission efficiency on the thickness of the acoustic concave lens in the embodiment of the present invention.
FIG. 5 is a graph showing the dependence of the transmission efficiency on the thickness of the Lamb wave suppression layer in the example of the present invention.
FIG. 6 is a graph showing the dependence of the transmission efficiency on the acoustic impedance of the Lamb wave suppression layer in the embodiment of the present invention.
FIG. 7 is a diagram showing an actual measurement result of a transmitted sound field from an ultrasonic transmitter in the embodiment of the present invention.
FIG. 8 is a cross-sectional view showing the configuration of an ultrasonic wave transmitter with a frequency selective reflection film and cooling film on the back surface in an embodiment of the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Piezoelectric vibrator, 2-1, 2-2 ... Electrode, 3 ... Acoustic concave lens, 4 ... Acoustic layer (Lamb wave suppression layer), 5 ... Frequency selective reflection film, 6 ... Housing.

Claims (4)

A structure in which an acoustic concave lens is laminated on a plurality of piezoelectric vibrators arranged one-dimensionally, and the thickness of the thinnest portion of the acoustic concave lens is a maximum frequency among frequency components of a signal for driving the plurality of piezoelectric vibrators. In the ultrasonic wave transmitter having a wavelength equal to or less than the wavelength in the acoustic concave lens, the wavelength at the maximum frequency has an acoustic impedance with a value between the acoustic impedance of the acoustic concave lens and the average acoustic impedance of the subject. An ultrasonic wave transmitter characterized by laminating an acoustic layer having a thickness equal to or less than one-half of the acoustic wave transmission surface of the acoustic concave lens. 2. The ultrasonic wave transmitter according to claim 1, wherein the thickness of the acoustic layer has a value between one-fifth and one-third of a wavelength at the maximum frequency. Transmitter. 2. The ultrasonic transmitter according to claim 1, wherein the acoustic impedance of the acoustic layer has a value between half and twice the geometric mean of the acoustic impedance of the piezoelectric vibrator and the acoustic impedance of the subject. Ultrasonic transmitter characterized by. In the ultrasonic transmitter in which an acoustic concave lens is stacked on a plurality of piezoelectric vibrators arranged in one dimension, the half of the wavelength at the driving frequency of the piezoelectric vibrator is opposite to the acoustic concave lens of the plurality of piezoelectric vibrators. An ultrasonic wave transmitter characterized by laminating a film in which metal and polymer layers are alternately laminated in a cycle.

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