Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
1999-12-03
2001-09-11
Ramirez, Nestor (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
Reexamination Certificate
active
06288477
ABSTRACT:
This invention relates to ultrasonic diagnostic transducer arrays and, in particular, to ultrasonic transducer arrays operating in the k
31
mode.
Ultrasonic transducer arrays are used as the transmitting and receiving elements in medical ultrasonic imaging probes or scanheads. Such arrays are formed by cutting or dicing a plate of piezoelectric material into individual transducer elements forming an array. The elements of the array are coupled to a beamformer which, through timed excitation and reception of signals from the array elements, causes the array to transmit steered and focused beams of ultrasonic energy and receives coherent echo information from along those beams. The piezoelectric material may comprise a polymer or a ceramic with ceramic material such as PZT being preferred for many medical imaging applications.
In order for the transducer array to exhibit good efficiency in response to an excitation signal and good sensitivity to low level echo signals, it is desirable to closely match the electrical impedance of the array elements to the electrical circuitry to which they are connected. Such electrical circuitry generally comprises cables and passive and active electronic components. However the transducer elements are generally designed to exhibit certain desired performance characteristics such as frequency of operation, aperture size, and interelement pitch. These criteria in turn define certain dimensions of the transducer elements which in large measure establish the electrical impedance of the elements for a given piezoelectric material with certain dielectric properties. When a relatively high frequency of operation is desired or elements are to be produced in a 1.5D or 2d array, the dimensions of the array elements become relatively small, which in turn results in relatively high electrical impedances for the transducer elements, often in the range of hundreds or thousands of ohms. Cable impedances are generally in the range of 20-300&OHgr; and the impedance of electrical circuitry connected to the transducer elements can be significantly less than 100&OHgr;. Hence, an undesirable impedance mismatch between the transducer elements and the cable or circuitry often arises.
Numerous approaches have been taken to reduce impedance mismatches by reducing the impedance of the array elements. One is by developing piezoelectric material with an inherent low impedance. But these materials are largely experimental at the present time and are often inferior to standard piezoelectric ceramics with respect to parameters such as electromechanical coupling or temperature dependence. Another approach is to form a transducer element as a stack of thin layers of ceramic which are electrically connected in parallel. Since each thin layer exhibits a relatively low impedance and the stack of thin layers exhibits a larger effective area compared to a full thickness of the piezoelectric material, the electrical impedance of the multilayer ceramic will be relatively low. However, fabricating such thin multilayer transducers in commercial quantities and at commercially reasonable costs has not yet been satisfactorily accomplished. Furthermore, the ability to provide electrical connections to the multiple layers of such a transducer, particularly for 1.5D and 2D arrays, can be very limited. Hence a need for an efficient, low cost, low impedance array transducer continues to exist.
In accordance with the principles of the present invention, a low impedance transducer array is provided by operating the array elements in the k
31
mode. In this mode of operation the element electrical impedance is reduced by the favorable height to thickness ratio of the transducer elements. Electrical connections are easily made to the sides instead of the top and bottom of the transducer elements. In accordance with one aspect of the present invention the elements are constructed from two subelements with an electrode of one polarity applied to the opposing sides of the subelements an electrode of another polarity applied to the nonopposing sides of the subelements. In accordance with another aspect of the present invention a conductive filler is used to form the electrodes of the elements. In a preferred embodiment a conductive material provides both the composite filler structure and the electrode structure of the array.
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Mills et al., “Combining Multi-Layers and Composites to Increase SNR for Medical Ultrasound Transducers,” at pp. 1509-1512, 1996 IEEE Ultrasonics Symposium.
Fraser John D.
Gilmore James Michael
ATL Ultrasound
Medley Peter
Ramirez Nestor
Yorks, Jr. W. Brinton
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