Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
2001-10-15
2003-12-02
Jaworski, Francis J. (Department: 3737)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
Reexamination Certificate
active
06656124
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to ultrasonic imaging transducer arrays designed for medical applications, and, more particularly, to transducer arrays based on multilayer piezoelectric structures which provide improvements in the electrical and acoustic behavior of the transducer arrays and to methods of making such arrays.
2. Description of the Related Art
Imaging human organs by ultrasound is an essential modality in most medical specialties and particularly in the fields of obstetrics, radiology and cardiology. The ultrasound or ultrasonic transducer is the limiting element that determines the quality factor of the diagnostic imaging. Several types of ultrasound transducers exist, ranging from sector moving single elements, to electronic linear arrays and to multidimensional arrays. The latter is the most sophisticated device currently available for imaging applications.
Conventionally, linear arrays comprise elementary transducers arranged along a single axis, while multidimensional devices comprise elements disposed in orthogonal planes to provide either am expanding lateral focus or crossing B-mode planes useful for rendering 3D images. Additionally, the term “1.5D” array is given to a linear array having transducer rows independently addressable in elevation. This arrangement provides the possibility of producing a better focussed ultrasound beam in the depth direction of examination by switching and synthesizing apertures along this direction. Similarly, the term 2D array or matrix array is usually employed for transducers having elements of square shape uniformly distributed in two orthogonal directions of the front plane. This arrangement permits the transducer device to perfectly synthesize the beam pattern or to correct phase aberrations due to the tissue or acoustic aperture when used in a conventional application. Volumetric or multiplane imaging approaches are only feasible with this type of device if movement of the transducer device during the image acquiring process is not desired.
Geometric specifications for linear phased arrays require that each elementary transducer exhibit an angular response of +/−45° which results in an acoustic aperture of about one wavelength for each individual transducer. This requirement enables the array to limit the emergence of grating and side lobes that introduce artifacts into the image. This requirement must be observed for any imaging array device including those designed for 1.5 and 2D use. Because the frequencies of transducer are usually in the range of 3 to 10 MHz, the elementary transducer aperture inherently varies from 0.150 mm to 0.50 mm in width in order to satisfy the acoustic radiation requirements. These dimensions pose severe fabrication difficulties in achieving repeatable elements and interconnections.
Other difficulties that affect the design of such narrow band transducers concern the surface of the transducer element forming the device and the mismatch in electrical impedance created when compared to conventional electric circuits. Inherently, low sensitivity and an oscillating or undamped impulse response can be observed with such transducer elements. This drawback is partially overcome in a linear phased array by increasing the elevation aperture of the array in order to expand the surface area of the transducer element. Unfortunately, this solution is not suitable for 1.5D and 2D array devices where the surface area of each element is completely bounded or predetermined. In this regard, the worst case is that of the 2D array where square shaped element transducers are required.
Considering this point further, as discussed above, considering this point further, a critical problem associated with 2D array construction concerns the electrical and capacitive characteristics of the transducer elements. In this regard, it is noted that the capacitance of a piezoelectric or dielectric element is a function of the dielectric constant, the surface area and the thickness of the material. Specifically, the capacitance is calculated based on the relation:
C
=
ϵ
r
×
S
e
,
where &egr;
r
represents the relative dielectric constant of the piezoelectric material, S the surface area and e the thickness of element. The capacitance of a transducer element directly governs reflection of the incident energy and this effect is emphasized when the capacitance value is substantially lower than that of the transmission line. Furthermore, a lower transducer capacitance also tends to lower the energy storage capability of the transducer device so that sensitivity is significantly affected.
The recent development of high dielectric constant piezoelectric materials such as relaxor based ceramics or single crystals has resulted in high density linear phased arrays which outperform conventional transducers made of standard ceramics. These new transducer constructions employ piezoelectric materials having a relative dielectric constant as high as 5000 in order to minimize capacitance loss. However, this approach to optimizing transducer devices is only obtained at the expense of a severe limitation on the operating temperature to avoid risk of depolarization or premature aging of the device.
Returning to a consideration of transducer arrays of 1.5D and 2D configurations, the excessively small surface area of the transducer element undermines the advantages associated with the materials described above, so that difficulties are encountered by engineers in the development and manufacture of such devices. In order to overcome the capacitance mismatch problem, attempts have been made to integrate active impedance matching into the device and to provide built-in driving circuitry connected to each transducer, with some relative success. However, the heating of such components results in a rapid rise in the temperature of the transducer and therefore results in excess current regulation for medical devices. Other attempts involve the use of multilayer structures in 2D arrays wherein sophisticated manufacturing techniques have been implemented, such as micro via methods and screen printing processes. However, such a fabrication process is not suitable, in practice, for low volume production and thus this technology for transducer fabrication still remains in the laboratory prototype stage. Further, the acoustic performance is yet to be confirmed.
Turning now to 1.5D arrays wherein the obstacles encountered are less important than for 2D arrays, the main problem regarding this kind of transducer is the variation in the surface of the transducer element when viewed in a common elevation plane. The transducer elements, which are conventionally disposed in the azimuth direction are further arranged in parallel rows that are organized in concentric manner along the elevation plane. Indeed, such rows have the elevation dimension thereof shifted in a manner so as to exhibit a wider aperture at the central area of the transducer array and provide the narrowest aperture at the edge of transducer.
In order to optimize the geometric aperture of a 1.5D array, a Fresnel synthetic aperture may be advantageously provided in the elevation plane. However, this approach presupposes that the transducer is equipped with row apertures downshifted or varied according to a specific law of progression designed to reduce the side lobes emanating from such a synthetic aperture construction. Advantages relating to the provision of an elevation synthetic aperture for an imaging linear array include the ability to modify the focal distance in the plane of space in contrast to conventional devices which only provide a fixed focus. However, in the construction of such devices a major obstacle is encountered which has not been fully addressed in the prior art. This obstacle concerns the electrical impedance variation between a transducer element belonging to a given row and that of another row. This varying impedance characteristic leads unavoidably to a dramatic decrease in the sensitivity
Flesch Aime
Nguyen-Dinh An
Jaworski Francis J.
Larson & Taylor PLC
Patel Maulin
Vermon
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