Acoustic imaging arrays

Details Acoustic imaging arrays Acoustic imaging arrays

1998-08-24

2001-05-22

Budd, Mark (Department: 2834)

Electrical generator or motor structure

Non-dynamoelectric

Piezoelectric elements and devices

C310S316020

Reexamination Certificate

active

06236144

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to acoustic imaging arrays.
It is well known to use a row of acoustic elements in an acoustic imaging system. These are in the form of a one-dimensional (1D) array such as a series of individual elements spaced along a single row or column. Conventionally the acoustic elements are formed from a piezoelectric material. The elements generate an acoustic signal which propagates through a medium and is reflected by an object in the medium which is to be detected. Signals reflected by the object are detected by the elements and electrical signals are generated which can then be processed. An acoustic imaging system using a 1D array is used in ultrasound imaging to provide internal images of the human body or to image underwater objects. A two-dimensional (2D) image is generated by physically sweeping the 1D array over the region to be imaged.
It has been proposed to use 2D arrays to generate 2D and 3D images. A 2D array may be a plurality of individual elements arranged in a grid. Compared with a 1D array, a 2D array provides improved resolution and better quality image and also eliminates the need for physical focussing or sweeping allowing real time images to be obtained. However, the use of 2D arrays has been limited by the difficulty in processing large amounts of data which would be generated by an array of any useful size, for example 10,000 elements in an array of 100×100. Furthermore, it is difficult to make connections to such a large number of elements on a scale of several centrimeters squared.
Although the difficulties in data processing can now be tacked by using high power computers having faster processing speeds it does not deal with the problem of the large number of connections. The connections and associated connecting tracks are supported by an interconnect layer. However, the interconnect layer often has an acoustic impedance which differs to that of the elements and acoustic reflections are caused by the mismatch. Although reflections can be minimized using materials having a better match of acoustic impedance, problems are encountered as the number of elements in an array is increased. As the number of elements in the array increases the thickness of the interconnect layer must also be increased which increases the amount of acoustic reflections. This degrades the sensitivity of the system.
Another problem is cross coupling between elements in the array. This becomes more severe as the scale of the array is reduced and the elements are closer together.
SUMMARY OF THE INVENTION
According to a first aspect the invention provides an acoustic imaging system comprising an array of elements for receiving acoustic energy and converting the energy into electrical signals; a track layer having a plurality of electrically conductive tasks; processing means for receiving and processing the electrical signals and a plurality of electrically conductive paths electrically connecting the elements to the processing means wherein the array comprises a plurality of physically separate sub-arrays which are assembled together.
With such a modular arrangement of sub-arrays, little or no redesign of the track layer is required to produce larger arrays.
Preferably the elements also generate acoustic energy. Preferably the acoustic energy is in the range 0.1 to 20 MHz.
Preferably the elements comprise piezoelectric material. Most preferably the piezoelectric material is ceramic. It may be lead zirconate titanate (PZT). The array may comprise a 1-3 composite of elongate members of piezoelectric material embedded in a matrix. Preferably the matrix is a polymer material, for example epoxy resin.
An advantage of using a filled matrix is that an element may comprise a plurality of elongate members. Therefore, the size (and particularly the width) of the elongate members can be much smaller than the usual size of an element. If the elongate members are reduced to a size much smaller than the wavelength of acoustic waves generated and/or detected by the system, acoustic coupling between the members is reduced. Embedding the elongate members in a matrix means that they can be smaller, that is more fragile, because they are supported. In addition the matrix filler reduces cross-coupling and also reduces acoustic impedance of the array of elements. This avoids the need to use additional techniques to reduce cross-coupling, for example supporting individual elements on a diced backing layer.
Preferably a plurality of elongate members are in electrical contact with each electrically conducting path. The size and shape of the cross section of an electrically conducting path can be chosen to define a particular group of elongate members as an individual element. The elements may be all identical or may differ, for example for different purposes. In one embodiment one element comprises nine elongate members in a mini-array or matrix of 3×3.
In one embodiment a sub-array comprises 100 elements. Preferably outputs from elements of a sub-array are fanned down onto a readout chip. Outputs from a plurality of readout chips, each connected to a sub-array, may be multiplexed together to provide an overall array output.
The sub-arrays may be designed to transmit and/or receive at different operating frequencies so that in total the array may have multi-frequency characteristics. The operating frequency of each sub-array is determined by its thickness. Preferably a plurality of sub-arrays have a plurality of different thickness to produce a plurality of different operating frequencies.
Preferably an absorbing layer for absorbing acoustic energy is present in the acoustic imaging system. It may be disposed between the array and the track layer. Preferably it comprises the plurality of electrically conductive paths, for example a polymer having conducting paths in a matrix. The matrix and/or the paths may be loaded to provide suitable properties, for example conductivity and absorption of acoustic energy.
Preferably a common electrode is in electrical contact with a front face of the array. Preferably groups of elongate members are connected to a respective contact pad at a rear face of the array. The contact pads may have shapes to define the shapes of the elements.
Preferably the common electrode supports a quarter wave matching layer which is matched acoustically minimized reflections of acoustic waves. The acoustic impedance of the matching layer preferably will be between that of the medium, for example water, and that of the piezoelectric material.
The array may be shaped to provide quasi-optical effects, for example focussing.
The array may be a linear 1D array or a 2D array. A 2D is an arrangement having m rows and n columns where n and m are both greater than unity.


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Hall, D.D.N., et al., “Theoretical And Experimental Evaluation of a Two-Dimensional Composite Matrix Arrayl”, IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control., vol. 40, No. 6, Nov. 1, 1993, pp. 704-709.

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