JPH04293399A - Array type piezoelectric transducer - Google Patents

Abstract

PURPOSE:To realize excellent resolution in the direction of a short axis while maintaining an enough focal depth by controlling the effective area of the electrode of a piezoelectric element. CONSTITUTION:The effective area of the electrode of piezoelectric elements 1-1 to 1-N is weighted by making the area projected in the direction of a long axis per the unit length in the short axis direction large at the center part of an opening and making it smaller toward the end part. By such weighting in the short axis direction, the transmission sound pressure and receiving sensitivity of an ultrasonic wave in the short axis direction can be made large at the center part of the opening and made smaller toward the end part. At this time, a repetitive pitch in the long axis direction of the shape of the electrode is set below a half wave length. Thus, the formation of grating lobe caused by the weighting in the short axis direction can be prevented.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an array type piezoelectric transducer used in electronic scanning ultrasonic imaging devices for medical diagnosis, flaw detection, underwater exploration, and the like.

0002

BACKGROUND OF THE INVENTION Electronic scanning ultrasonic imaging devices have far superior scanning freedom and high speed compared to manual or automatic mechanical scanning devices, and are therefore useful for medical diagnosis, underwater exploration, and other applications. It is widely used in various fields such as flaw detection. In order to convert ultrasound signals and electrical signals in such an electronic scanning ultrasound imaging device,
Generally, an array type piezoelectric transducer is used in which a large number of electrically independent piezoelectric elements are arranged on a flat or curved surface. That is, by electrically controlling the timing and gain of ultrasonic transmission/reception by each element of the array type piezoelectric transducer, an ultrasonic sound field necessary for imaging is formed.

[0003] In order to flexibly form the ultrasonic sound field for transmission and reception that is ideal for imaging, the electrical independence of the piezoelectric element must not be limited to a specific one-dimensional direction on a flat or curved surface. , originally, independence should be given in two-dimensional directions. However, in order to achieve this, the number of elements per array-type piezoelectric transducer, which used to be about 100 when electrical independence is limited to one dimension, will now need 1000 to 10000 elements, and the number of lead wires required for this will increase. The number and scale of electronic circuits will be enormous. Therefore, ultrasonic imaging using two-dimensional array type piezoelectric transducers is not necessarily practical at present unless high voltage electronic circuits that can be used for piezoelectric transducers become further miniaturized.

[0004] The problem with ultrasonic imaging using a one-dimensional array type piezoelectric transducer whose electrical independence is limited to one-dimensional direction is that it is included in the plane or curved surface of the array arrangement, and the direction of the array arrangement (called the long axis) is the formation of an ultrasonic sound field in the direction of the axis perpendicular to (called the minor axis). In order to obtain the same resolution in the short-axis direction as in the long-axis direction, it is possible to perform focusing by attaching an acoustic cylindrical lens or by adding curvature to the piezoelectric element itself. Many of the array-type piezoelectric transducers used in many ultrasonic diagnostic apparatuses have such a configuration. Due to this short-axis focusing, the resolution in the short-axis direction is significantly improved at a distance that is about the focal length of the short-axis focusing, but sufficient short-axis resolution cannot be obtained at distances significantly away from the focal length. In fact, it may even deteriorate compared to when there is no short-axis focusing. To solve this problem, a short-axis focusing method is needed that achieves good short-axis resolution while maintaining a sufficiently large depth of focus.

[0005] As a general method for solving the above problem, a method is known in which the transmitting sound pressure and receiving sensitivity of ultrasonic waves are weighted such that they are higher in the center of the aperture in the short axis direction and lower toward the ends. ing.

As a possible method for realizing such weighting, a method has been proposed in which the polarization strength of piezoelectric elements constituting an array type piezoelectric transducer is distributed in the minor axis direction. In other words, the polarization intensity is 100% polarized at the center of the aperture, and the degree of polarization is reduced toward the edges to achieve the necessary weighting, and the applied voltage at the time of polarization is A method for obtaining a desired degree of polarization through control has been proposed. This proposal seems to enable weighting relatively easily, but in reality, in the polarization process, which is performed under high temperature and high voltage, polarization is achieved by applying a difference in voltage between adjacent electrodes. It's not an easy thing to do. In addition, the characteristics of a piezoelectric material that is half-polarized are 100%
Since there may be qualitative differences compared to polarized ones, this point also requires attention in practical terms. In addition, as a prior art related to the present invention, Japanese Society of Ultrasonics in Medicine Proceedings 57-
285 ``Basic study of high resolution in short axis direction''.

[0007]

SUMMARY OF THE INVENTION The present invention has been made in view of the above points, and uses a more practical short-axis weighting method to maintain a sufficiently large depth of focus.
The objective is to achieve good resolution in the short axis direction.

[0008]

[Means for Solving the Problems] As mentioned above, it is not necessarily easy to realize weighting by controlling the degree of polarization, so in the present invention, instead of controlling the degree of polarization, the This is achieved by controlling the effective area of That is, the effective area of the electrode is weighted by making the area projected in the long axis direction per unit length in the short axis direction larger at the center of the opening and smaller toward the ends. However, depending on the shape of the electrodes used for weighting, there is a risk of forming a grating lobe, which is a type of unnecessary response. do.

[0009]

[Operation] Weighting in the short axis direction by giving a distribution to the effective area of the electrodes allows the transmission sound pressure and reception sensitivity of ultrasound to be high in the center of the aperture in the short axis direction, and high at the edges, without depending on polarization control. The depth of focus can be lowered as the depth of focus increases, and good short-axis resolution can be achieved while maintaining a sufficiently large depth of focus.

0010

Embodiments Hereinafter, embodiments of the present invention will be explained in detail with reference to FIGS. 1 and 2.

FIG. 1 shows the electrode shape of an array type piezoelectric transducer according to an embodiment of the present invention. Electrical-mechanical conversion is performed by the thickness displacement that occurs when an electric field is applied in the thickness direction of piezoelectric materials such as piezoelectric ceramics or piezoelectric polymers, and the polarization that occurs in the thickness direction when stress is applied in the thickness direction. It transmits ultrasonic waves to the living body or water it comes into contact with through layers, and conversely receives ultrasonic waves from them. Ultrasonic frequency 3 MHz (wavelength 0.5 mm in living body or water), short axis length 10 mm, long axis direction repetition pitch of electrode shape 0.25 mm (equivalent to half wavelength),
The shape of the electrode facing the ground electrode of an array type piezoelectric transducer with an element pitch of 0.5 mm is illustrated on one side of the axis of symmetry with respect to its short axis. The entire array type piezoelectric transducer actually consists of a large number of elements, about 100, but only a portion of them are shown in the figure.

Since the element pitch is larger than half a wavelength in a living body or underwater, the repeating pitch in the long axis direction of the electrode shape is set to the width divided into two, and two adjacent repeating pitches are electrically connected in common. , treated as electrically independent array elements. As a result, the repetition pitch in the long axis direction of the electrode shape can be suppressed to less than half a wavelength, and the grating rotation caused by weighting in the short axis direction can be suppressed.
It was possible to prevent the formation of blisters. On the right side of FIG. 1, regarding the effective area of the electrode, the area projected in the major axis direction per unit length in the minor axis direction is plotted with respect to the coordinate in the minor axis direction. The projected area per unit length is large at the center of the aperture and becomes smaller toward the ends, thereby achieving the desired weighting.

FIG. 2 shows the electrode shape of an array type piezoelectric transducer according to another embodiment of the present invention. The principles of electrical/mechanical conversion, ultrasonic frequency, short axis length, electrode shape repetition pitch in the long axis direction, element pitch, etc. are the same as in the embodiment shown in Fig. 1, and the shape of the electrode facing the ground electrode is One side of the axis of symmetry with respect to its short axis is illustrated for a portion of the figure. In this example, the electrode facing the ground electrode is subdivided into half wavelengths or less in the short axis direction, and the gap between the subdivided electrodes is made smaller at the center of the opening and becomes larger toward the ends. The configuration is such that the desired weighting is performed. For two adjacent repeating pitches in the long axis direction, the electrodes subdivided in the short axis direction and the long axis direction are electrically connected in common and treated as electrically independent array elements. On the right side of FIG. 2, the effective area of the electrode projected in the long axis direction per unit length in the short axis direction is plotted with respect to the short axis direction coordinate. The projected area per unit length is large at the center of the opening and becomes smaller toward the ends.

In this example, a so-called composite piezoelectric structure is created in which PZT-based ceramics or the like is used as the piezoelectric material, and the gap between the subdivided electrodes in FIG. 2 is replaced with a polymeric material that has no piezoelectric activity. You can also take it. When this structure is adopted, the electrodes in the range treated as electrically independent array elements may be continuous electrodes. Even in that case, since an electrode placed on a polymer material with no piezoelectric activity is not counted as the effective area of the electrode, the projected area per unit length plotted on the right side of FIG. 2 will be the same.

[0015]

[Effects of the Invention] As described above, according to the present invention, the transmission and reception sensitivities of the array element can be made high at the center of the aperture in the short axis direction and decreased toward the ends. As a result, it is possible to achieve a depth of focus large enough for practical use while ensuring good resolution in the short axis direction.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the shape of an electrode according to an embodiment of the present invention.

FIG. 2 is a diagram showing the shape of an electrode according to another embodiment of the present invention.

[Explanation of symbols]

1-1 to 1-N...Electrically independent piezoelectric elements constituting an array type piezoelectric transducer.

Claims (1)

[Claims] Claim 1: In an electronic scanning ultrasonic transducer in which a large number of piezoelectric elements of the same shape are arranged in a one-dimensional direction, a distribution obtained by projecting the electrode effective area of the piezoelectric elements in the arrangement direction is determined. An array type piezoelectric transducer characterized in that the distribution density per unit length is greater than that at the ends.