Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
2002-06-24
2004-07-13
Jaworski, Francis J. (Department: 3737)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
C310S334000
Reexamination Certificate
active
06761692
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to technology and design of efficient ultrasound transducers for high frequencies, and also transducers with multiple electric ports for efficient operation in multiple frequency bands, for example frequency bands with a harmonic relation. The invention has special advantages where the highest frequencies are above 10 MHz, but has also applications for transducers at lower frequencies.
2. Description of the Related Art
Medical ultrasound imaging at frequencies above ~10 MHz, has a wide range of applications for studying microstructures in soft tissues, such as the composition of small tumors or a vessel wall. In many of these situations it is also desirable to use ultrasound pulses with frequencies in several frequency bands, for example to
1. use a pulse with frequencies in a low frequency band of for example 30-40 MHz to get larger image depth for an overview image, and then be able to switch to or use simultaneously a pulse with frequencies in a high frequency band, say 60-80 MHz, for high resolution imaging of close structures in a shorter depth of the image, or
2. to transmit an ultrasound pulse with frequencies in a low frequency band, say around 30 MHz, and receive back scattered signal components at a harmonic of the transmit band, say a 2
nd
harmonic band around 60 MHz, a 3
rd
harmonic band around 90 MHz, or even a sub-harmonic band around 15 MHz.
Ultrasound transducers for medical applications are currently based on ferro-electric, ceramic plates as the active material, that vibrates in thickness mode. When polarized, the materials show piezoelectric properties with efficient electromechanical coupling. However, the characteristic impedance of the ceramic material (Z
x
~33 MRayl) is much higher than that of the tissue load material (Z
L
~1.5 Rayl). In order to get adequate thickness vibration amplitude of the plate for efficient power coupling into the tissue load material, one must operate the plates at thickness resonance, typically L
x
=&lgr;/2 resonance. Here L
x
is the plate thickness, &lgr;=c
1
/f is the wavelength of longitudinal waves normal to the plate with wave velocity c
1
and frequency f. The resonance makes the transducer efficient in a band of frequencies around a center frequency f
0
=c
1
/&lgr;
0
=c
1
/2L
x
. Acoustic matching plates between the ceramic plate and the load are used to improve the power coupling to the load, a technique that increases the bandwidth of the transducer resonance.
With the well known composite technique, where the ceramic plate is diced into small posts, and the interpost space is filled with epoxy, the efficient characteristic impedance is reduced to ~15 MRayl, which is still around 10 times higher than the characteristic impedance of the load, such as soft tissue or water. Transducers of composite material must therefore also operate in thickness resonant mode, albeit one can obtain some wider bandwidth than with the transducers of whole ceramic.
Hence, both with whole and composite ceramic, the resonant operation requires that the plate thickness is inversely proportional to the center frequency of the operating transducer band.
This requires thicknesses in the range of 200-20 &mgr;m for center frequencies in the range of 10-100 MHz. Today, lapping of the ceramic plate is the common technology to manufacture plates with correct thickness, which becomes difficult and expensive at thicknesses in ranges below 50-60 &mgr;m, corresponding to frequencies above 30-40 MHz. Composite ceramic/epoxy material is also difficult to make for frequencies above 15 MHz, and it is hence a general need for efficient methods to manufacture transducers with a functioning high frequency band above 15 MHz.
SUMMARY OF THE INVENTION
The invention presents a new design of ultrasound transducers where the active electromechanical coupling material is ferroelectric, ceramic films that are made piezoelectric through electric polarization. The piezoelectric film layers are arranged into a transducer plate composed of multiple film layers, possibly also non-piezoelectric layers, where all the film layers have close to the same characteristic impedance. For coupling of the vibrations to an electric port, electrodes are placed inside the plate structure with a piezoelectric layer between the electrodes to form an electric port of the transducer, which interacts with the acoustic port of the transducer plate surface. By placing the electrodes inside the plate, the distance between the electrodes can be made substantially shorter than the total thickness of the transducer plate, which is an important aspect for high frequency operation of the transducer according to the invention.
The electromechanical coupling of the electrode port is highest at the frequencies where the thickness vibrations of the piezoelectric layer between the electrodes, is maximum. Due to reflections inside the transducer, one obtains a standing wave vibration pattern within the plate. The maximal vibration amplitude in the plate is found at the plate resonances, and to transform the resonant vibration amplitude to a large thickness vibration of the material between the electrodes, one must also place the electrodes at antinodes with opposite vibration direction in the standing wave vibration pattern. This gives a distance between the electrodes ~&lgr;/2, where &lgr; is the wave length in the material. Hence, the highest sensitivity of the transducer is found when the transducer plate is at a thickness resonance and the distance between the electrodes is ~&lgr;/2 with correct placement at antinodes in the standing wave pattern. With very high backing impedance, the back interface is a node, and it can pay to put one electrode at the back interface and the other at the antinode at &lgr;/4 distance in front of the back electrode.
The close to constant characteristic impedance within the transducer plate implies that the mechanical thickness resonances of the transducer are determined by the total plate thickness, not by the thicknesses of the individual film layers that composes the plate. By placing electrodes inside the transducer plate, the transducer plate can operate over a larger range of resonances, from &lgr;/2 to multiple &lgr; resonances, while the electrodes at the center frequency are placed at antinodes with distance ~&lgr;/2 internal in the transducer plate, maximizing the electromechanical coupling of the electrodes over the actual frequency band. This allows the use of thicker transducer plates than the standard &lgr;/2 transducer plates, which provides manufacturing advantages as described below.
The plate is hence so much thicker than the active material between the closest electrode layers, that the phase angle of the wave propagation through the film layers outside these active layers has a substantial, non-negligible effect on the mechanical thickness resonances of the whole plate. We shall say that a layer has a thickness substantially larger than another layer when the difference between the two layers of the wave propagation phase angle is non-negligible in the determination of resonances. Similarly, a layer has non-negligible thickness when the propagation phase angle is non-negligible in the determination of resonances
Film layers outside the active ceramic material can be made of other types of material with similar characteristic impedance as the ferroelectric, ceramic film, for example layers of conductive film. Conducting layers can have a combined function as electrodes, and as vibrating layer with non-zero thickness for the definition of the transducer plate thickness resonances. One simple design of the transducer according to the invention, is an active ferroelectric ceramic layer with a thin electrode with negligible propagation phase angle on the back side, and a conducting layer on the front side which both functions as a front electrode and an elastic layer that makes the total plate substantially thicker than the active piezoel
Angelsen Bjorn A. J.
Johansen Tonni F.
Østgård Jarle
Cohen & Pontani, Lieberman & Pavane
Eagle Ultrasound AS
Jaworski Francis J.
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