Patent Grant
7678054
The present invention relates to an ultrasonic probe for transmitting and receiving an ultrasonic wave between itself and a patient, and an ultrasonic diagnosing apparatus including the probe. More specifically, the present invention relates to an ultrasonic probe that can change an aperture in the minor-axis direction.
In general, an ultrasonic transducer includes a pair of electrodes sandwiching a layer including a piezoelectric material (hereinafter referred to as a piezoelectric layer), and an ultrasonic probe includes a plurality of the ultrasonic transducers, where the ultrasonic transducers are one-dimensionally arrayed, for example. Further, a predetermined number of transducers of the transducers arrayed in the major-axis direction are determined to be an aperture, the plurality of transducers belonging to the aperture is driven, and an ultrasonic beam converges to a part to be measured in a patient so that the part is irradiated with the ultrasonic beam. Further, the plurality of transducers belonging to the aperture receives an ultrasonic reflective echo or the like emitted from the patient and the ultrasonic reflective echo is converted to an electrical signal.
On the other hand, as for the minor-axis direction perpendicular to the above-described major-axis direction, an aperture-width is modified by changing the frequency of an ultrasonic wave so that the beam-width of the ultrasonic beam decreases and the resolution increases (Patent Document 1: JP7-107595A). In an ultrasonic probe according to Patent Document 1, the thickness of a piezoelectric layer at the center in the minor-axis direction is small and gradually increases toward the end thereof. Therefore, the response to a high frequency at the center is high and the response to a low frequency at the end in the minor-axis direction is high, so that a wide-band frequency characteristic is obtained. As a result, the aperture-width in the minor-axis direction of the ultrasonic probe varies inversely with a frequency, whereby a fine beam-width is achieved over an area ranging from a shallow depth to a deep depth.
However, according to the ultrasonic probe disclosed in Patent Document 1, the low-frequency responses at both ends in the minor-axis direction become higher than that at the center part and the sound pressure at each of the ends is higher than that at the center part, whereby a nonuniform sound-pressure distribution is obtained. Subsequently, the resolution of the ultrasonic probe decreases.
The present invention has been achieved for making the frequency response of an ultrasonic probe to a minor-axis-direction frequency uniform.
The present invention solves the above-described problems through the following means.
According to the present invention, in an ultrasonic probe including an array of a plurality of ultrasonic transducers, where each of the ultrasonic transducers has a piezoelectric layer and a couple of electrodes sandwiching the piezoelectric layer therebetween, the piezoelectric layer has a first piezoelectric layer provided on the ultrasonic-wave emission side, a second piezoelectric layer provided on the other side of the first piezoelectric layer, and a common electrode provided therebetween. The ultrasonic probe has a low-frequency-response distribution that is uniform for an entire aperture in the minor-axis direction perpendicular to a direction in which the ultrasonic transducers are arrayed and a high-frequency-response distribution that is high at the center part in the minor-axis direction.
The above-described frequency-response distributions can be achieved by the following means shown in (1) to (9).
(1) The thickness of the end in the minor-axis direction of the first piezoelectric layer is smaller than the thickness of the center part of the first piezoelectric layer and the thickness of the end of the second piezoelectric layer is larger than the thickness of the center part of the second piezoelectric layer,
(2) each of faces of the first and second piezoelectric layers, the faces being in contact with the couple of electrodes, is plane and a boundary surface between the first piezoelectric layer and the second piezoelectric layer is formed, as a curved face depressed to the second-piezoelectric-layer side,
(3) each of the faces of the first and second piezoelectric layers, the faces being in contact with the couple of electrodes, is plane and the boundary surface between the first piezoelectric layer and the second piezoelectric layer is formed, as a crest whose ridge line corresponds to the center part in the minor-axis direction,
(4) each of the faces of the first and second piezoelectric layers, the faces being in contact with the couple of electrodes, is plane and the boundary surface between the first piezoelectric layer and the second piezoelectric layer has a plane part that is provided at the center part in the minor-axis direction and that is projected to the second-piezoelectric-layer side, and a plane part that is provided at each of both the ends, where the plane parts are projected to the first-piezoelectric-layer side,
(5) the face of the first piezoelectric layer on the ultrasonic-wave emission side is concave, the face of the second piezoelectric layer on the ultrasonic-wave non-emission side is convex, and the boundary surface between the first piezoelectric layer and the second piezoelectric layer is depressed to the second-piezoelectric-layer side with a curvature larger than the curvature of the face of the first piezoelectric layer on the ultrasonic-wave emission side,
(6) the face of the first piezoelectric layer on the ultrasonic-wave emission side is concave, the face of the second piezoelectric layer on the ultrasonic-wave non-emission side is convex, and the boundary surface between the first piezoelectric layer and the second piezoelectric layer is formed, as the crest whose ridge line corresponds to the center part in the minor-axis direction,
(7) each of the first and second piezoelectric layers has a predetermined thickness, where the density of a piezoelectric material used for the first piezoelectric layer decreases from the center part in the minor-axis direction toward the end, and where the density of a piezoelectric material used for the second piezoelectric layer increases from the center part in the minor-axis direction toward the end, and
(8) in addition to the configuration shown in (1) to (7), an adjustment layer including a material whose acoustic impedance is nearly equivalent to the acoustic impedance of the piezoelectric material used for the piezoelectric layer is provided on the ultrasonic-wave non-emission side of the second piezoelectric layer, where the thickness in the minor-axis direction of the adjustment layer gradually increases from the center part to the end.
According to the above-descried (1) to (7), the piezoelectric layer includes two layers and the minor-axis-direction frequency characteristic and sound-pressure characteristic of the first piezoelectric layer and those of the second piezoelectric layer complement one another. Subsequently, responses to low frequencies in the minor-axis direction are made uniform. That is to say, the thickness of the second piezoelectric layer gradually increases from the center part thereof in a direction perpendicular to a direction in which the ultrasonic transducers are arrayed (hereinafter referred to as a minor-axis direction) toward the ends. Therefore, the high-frequency response at the center part becomes high. On the other hand, the thickness of the first piezoelectric layer decreases from the center part in the minor-axis direction toward the ends, so that the low-frequency response at the center part becomes high. Since the frequency-response characteristic of the first piezoelectric layer is added to that of the second piezoelectric layer, the minor-axis-direction response characteristic for a low frequency becomes uniform. Subsequently, according to the ultrasonic probe of the present invention, it becomes possible to obtain a high response to a high frequency at the center part in the minor-axis direction of the transducers and a uniform low-frequency response for each of the entire aperture, whereby it becomes possible to obtain a small ultrasonic beam-width over an area ranging from a small depth to a large depth, so that a high resolution is achieved.
Further, since the acoustic impedance of the adjustment layer according to configuration (8) is nearly equivalent to that of the piezoelectric material, there is a large difference between the acoustic impedance of the adjustment layer and that of the backing layer provided on the anti-piezoelectric-layer side of the adjustment layer. Subsequently, an ultrasonic wave is effectively reflected by the adjustment layer and the frequency characteristic of the reflective ultrasonic wave depends on the thickness. As a result, the response characteristic in the minor-axis direction of the transducer for a low frequency becomes more uniform than in the past. Further, a high-frequency component of an ultrasonic wave emitted from the transducer to the back-face side is reflected by the adjustment layer that is thin at the center of the transducer and transmitted back to the ultrasonic-wave emission side. Subsequently, the sound pressure of a high frequency emitted from the center of the ultrasonic probe in the minor-axis direction to the patient increases, whereby a high-frequency response is obtained at the center of the transducer in the minor-axis direction.
Here, the backing layer includes a material whose acoustic impedance is significantly smaller than that of the piezoelectric layer. Further, the attenuation rate of the material is higher than that of the piezoelectric layer. Subsequently, it becomes possible to change the frequency characteristic in the minor-axis direction and achieve the function for changing an aperture according to a frequency. Further, the distribution of the thickness of the adjustment layer in the minor-axis direction is determined to be a frequency characteristic for achieving a predetermined high-frequency response distribution.
In place of the above-described configurations (1) to (8), there is provided configuration (9), wherein each of the first and second piezoelectric layers has a predetermined thickness, the adjustment layer including the material whose acoustic impedance is nearly equivalent to the acoustic impedance of the piezoelectric material used for the piezoelectric layer is provided on a back face of the electrode in contact with the second piezoelectric layer, and the thickness of the adjustment layer gradually increases from the center part of the ultrasonic transducer in the minor-axis direction toward the end.
Since the above-described adjustment layer is provided, the response characteristic for a low frequency in the minor-axis direction of the transducer becomes uniform and a high high-frequency response can be obtained at the center of the transducer in the minor-axis direction, as described above.
Further, the ultrasonic diagnosing apparatus of the present invention uses the ultrasonic probe of the present invention. Transmission means for transmitting an ultrasonic signal for driving the transducers of the ultrasonic probe has the function of transmitting an ultrasonic signal with a frequency according to a control instruction to the ultrasonic probe. A reception-processing means for performing reception processing for a reflective-echo signal received by the ultrasonic probe has the function of selecting a reflective-echo signal with the frequency according to the control instruction and performing the reception processing. Subsequently, a high-frequency response can be obtained at the center of the transducer in the minor-axis direction. Further, since the response characteristic for a low frequency in the minor-axis direction becomes uniform, it becomes possible to obtain the small ultrasonic beam-width over the area ranging from a small depth to a large depth and achieve the high resolution.
Embodiments of the present invention will be described with reference to the attached drawings, as below.
An embodiment of the present invention will be described with reference to
In
The ultrasonic probe 1 of the embodiment includes a piezoelectric layer 2, an acoustic-matching layer 3 provided on the ultrasonic-wave-emission-face side of the piezoelectric layer 2, a backing layer 4 provided on the back-face side of the piezoelectric layer 2, and an acoustic lens 5 provided on the ultrasonic-wave-emission-face side of the acoustic-matching layer 3, as shown in
Here, the acoustic lens 5 is used for performing focusing in the minor-axis direction and includes a material such as silicon rubber whose acoustic impedance is nearly equivalent to that of a body and whose sonic speed is slower than that of the body. The acoustic-matching layer 3 includes two layers. Each of the two layers functions, as a ¼-wavelength plate for a center frequency. Further, the lower layer of the acoustic-matching layer 3 includes a material such as ceramic whose acoustic impedance is lower than that of the piezoelectric layer 2. Further, the upper layer of the acoustic-matching layer 3 includes a material such as resin whose acoustic impedance is nearer to that of the body than in the case of the lower layer. The piezoelectric layer 2 includes piezoelectric-ceramic PZT, PZLT, a piezoelectric single crystal PZN-PT, PMN-PT, an organic piezoelectric material PVDF, and/or a complex piezoelectric layer including the above-described materials and a resin. The backing layer 4 includes a material that has a large ultrasonic attenuation rate and that attenuates an ultrasonic wave emitted toward the back of the piezoelectric layer 2. The separation layers 6 include a material that can significantly attenuate an ultrasonic wave (e.g., a material equivalent to a vacuum).
Here, the first piezoelectric layer 2-1 is formed, so as to have a plane-convex shape, that is to say, the ultrasonic-wave emission face thereof is plane and the back face thereof is convex. Further, the center part thereof has the largest thickness T1max. The thickness of the first piezoelectric layer 2-1 decreases toward each of the ends. Therefore, each of the ends of the first piezoelectric layer 2-1 has the smallest thickness T1min. On the other hand, the second piezoelectric layer 2-2 is formed, so as to have a concave-plane shape, that is to say, the ultrasonic-wave emission face thereof is concave and the back face thereof is plane. Further, the center part thereof has a smallest thickness T2min. The thickness of the first piezoelectric layer 2-2 increases toward each of the ends. Therefore, each of the ends of the second piezoelectric layer 2-2 has the largest thickness T2max. Subsequently, faces that are in contact with the electrodes 7-1 and 7-2 of the piezoelectric layer 2 are formed on planes that are in parallel with each other and a boundary surface between the first piezoelectric layer 2-1 and the second piezoelectric layer 2-2 is depressed to the second-piezoelectric-layer-2-2 side. Incidentally, the piezoelectric layer 2 may be formed so that the expression T1max=T2min and the expression T1min/T2max=1/4 hold, for example.
Operations performed for making an ultrasonic diagnosis by using the above-described ultrasonic probe of the embodiment will now be described. First, the electrode 7-1 and the electrode 7-2 are grounded, and an ultrasonic transmission signal transmitted from the transmission unit 32 is applied to the common electrode 8. Here, the frequency of the transmission signal for driving the ultrasonic probe is controlled by the ultrasonic-pulse generation circuit 31. Further, the focus position of the ultrasonic beam is calculated by the control unit 38 according to the depth of a part to be measured. The part to be measured can be inputted and set by an operator through the input unit 39. An instruction is transmitted from the control means 38 to the ultrasonic-pulse generation circuit 31 and the transmission unit 32 according to the depth of the part to be measured that is set in the above-described manner, and the frequency of the transmission signal and the focus position are set. The control unit 38 transmits an instruction to the reception-processing unit 35, so as to set the frequency and focus position of a reflective-echo signal subjected to reception processing so that the frequency and focus position agree with those of the transmission signal.
Thus, the ultrasonic probe is driven, whereby an ultrasonic wave is generated in the piezoelectric layer 2 and emitted from the face thereof on the electrode 7-1 side. Here, since the piezoelectric layer 2-2 has the concave-plane shape, the piezoelectric layer 2-2 resonates at its ends at low frequencies, as is the case with the known art, and the sound pressure at low frequencies increases. On the other hand, since the piezoelectric layer 2-1 has the plane-convex shape and has a small thickness at each of its ends, the low-frequency sound pressure at each of the ends is low. As a result, by laminating the piezoelectric layer 2-1 on the piezoelectric layer 2-2, the low-frequency sound pressure at the ends can be prevented from being emphasized.
Here, an effect relating to the frequency characteristic of the ultrasonic probe of the embodiment will be described with reference to
As a result, the function for varying an aperture according to a frequency can be obtained, as shown in
According to the adjustment layer 9 of the embodiment, the surface thereof in contact with the piezoelectric layer 2-4 is plane and the opposite surface is concave. That is to say, the thickness of the adjustment layer 9 is minimized at the center thereof in the minor-axis direction and gradually increases toward each of the ends thereof. Thus, according to the embodiment, there is a large difference between the acoustic impedance of the adjustment layer 9 and that of the backing layer 4. Therefore, an ultrasonic wave is effectively reflected in the adjustment layer 9 and a frequency characteristic of the reflection depends upon the thickness. Subsequently, according to the ultrasonic probe of the embodiment, a frequency characteristic depending on the thickness of the adjustment layer 9 in the minor-axis direction can be obtained, and the effect of the frequency characteristics shown in
According to the embodiment, the sound pressure at low frequencies of each of the ends is lower than that of the center part and the sound-pressure distribution is uniform, as is the case with the embodiment shown in
Further, in this embodiment, the adjustment layer 9 shown in
Further, according to the embodiment, the adjustment layer 9 shown in
The concave acoustic lens 11 can be used for other embodiments. Further, in this embodiment, the adjustment layer 9 shown in
According to the embodiment, at low frequencies, the sound pressure at each of the ends is not higher than that at the center part and the sound-pressure distribution is uniform, as is the case with the embodiment shown in
According to the embodiment, it becomes possible to achieve a sound-pressure distribution that is uniform in the minor-axis direction at low frequencies and an aperture-variable function for obtaining a narrow beam in a wide frequency band. Further, in this embodiment, the adjustment layer 9 shown in
Further, the same effect can be obtained by adjusting the elastic constant of the piezoelectric material instead of adjusting the density of the piezoelectric material, as in the above-described embodiment. In that case, the elastic constant of the first piezoelectric layer 13-1 is minimized at the center in the minor-axis direction and gradually increases toward the end. The elastic constant of the second piezoelectric layer is maximized at the center in the minor-axis direction and gradually decreases toward the end.
As has been described, according to each of the embodiments of the present invention, the frequency response characteristic varies from the center part in the minor-axis direction towards the ends so that a wide band ranging from a low-frequency band to a high-frequency band is achieved at the center part and a narrow band wherein a high-frequency response decreases is achieved at the end. Further, at low frequencies, the sound pressure at each of the ends does not increase so that a uniform sound pressure can be obtained in the area ranging from the center part to the end. Further, at high frequencies, a response from the center part increases, so that focus is achieved in the neighborhood of the probe. At low frequencies, focus is achieved at the deep part due to responses for the entire aperture, so that a high-resolution image can be obtained.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2003-014586 | Jan 2003 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2004/000610 | 1/23/2004 | WO | 00 | 7/25/2005 |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO2004/064643 | 8/5/2004 | WO | A |
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| Number | Date | Country | |
|---|---|---|---|
| 20060142659 A1 | Jun 2006 | US |
