Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
2002-01-07
2003-11-11
Jaworski, Francis J. (Department: 3737)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
Reexamination Certificate
active
06645150
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to technology and designs of efficient ultrasound bulk wave transducers for wide frequency band operation, and also transducers with multiple electric ports for efficient operation in multiple frequency bands, for example frequency bands with a harmonic relation, where it is possible to receive the 1
st
, and/or 2
nd
, and/or 3
rd
, and/or 4
th
harmonic frequency bands of the transmitted frequency band.
2. Description of the Related Art
In medical ultrasound imaging, one uses a variety of center frequencies of the transmitted pulse to optimize image resolution for required image depth. To image deep organs one can use frequencies down to ~2 MHz, while for shallow depths one can use frequencies higher than 10 MHz.
In many cases one also transmits an ultrasound pulse in one band of frequencies, and receive the back scattered signal in a second band of frequencies. This is for example done in 2
nd
harmonic imaging of tissue, where the receive band is centered around the 2
nd
harmonic frequency of the transmit pulse band. Nonlinear elasticity in the tissue distorts the forward propagating pulse, which increases the higher harmonic content in the pulse with depth. This method considerably reduces noise in the ultrasound image.
Second harmonic imaging is also used for the detection of ultrasound contrast agent. As the nonlinear elasticity of the contrast agent is very strong, it is also interesting to use a receive band centered around higher than the 2
nd
harmonic band, for example the 3
rd
or 4
th
harmonic component of the transmit frequency band.
It is also useful to transmit an ultrasound burst with two separate frequency bands, both for imaging of soft tissue and ultrasound contrast agents. The non-linear effects will then introduce new frequency bands in the scattered signal, centered around sums and differences of the transmitted center frequencies. When the center frequencies of the transmitted frequency bands coincide, the difference frequency is referred to as a sub harmonic frequency component produced by the non-linearity of the tissue or contrast agent elasticity.
Traditional ultrasound transducers for medical imaging have limitations for such applications in that they are efficient over a limited band of frequencies. The active material in the transducers, is usually a plate of piezoelectric ceramic that vibrates in thickness mode. Other piezoelectric materials like the crystal LiNbO
3
, or the polymer PVDF, are also sometimes used. In the following we mainly refer to ceramic materials while it is understood that other piezoelectric materials can be used in the same manner.
The ceramic has much higher characteristic mechanical impedance (Z
x
~34MRayl) than the tissue (Z
x
~1.5MRayl), and the energy coupling between the tissue and the ceramic plate is therefore by nature very low. To improve this energy coupling, the plate is operated around &lgr;/2 resonance when the backing mount has a lower characteristic impedance than the piezoelectric plate, or &lgr;/4 resonance when the backing mount has a higher characteristic impedance than the piezoelectric plate. The resonance increases the amplitude of the thickness vibrations, hence improving the tissue/ceramic energy coupling at the resonance frequency. The resonance, however, gives a limited bandwidth of the energy coupling, limiting the minimal pulse length transmitted through the transducer.
To increase the bandwidth of the energy coupling, impedance matching layers are commonly used between the tissue and the ceramic plate to raise the mechanical impedance seen from the plate towards the tissue. Further improvement in the bandwidth of the tissue/ceramic energy coupling, is obtained with the well known ceramic/polymer composite materials. Such materials are made by dicing grooves in the ceramic plate and filling the grooves with softer polymer, a process that produces a composite ceramic/polymer material with mechanical impedance Z
x
~12-20MRayl, substantially lower than for the whole ceramic.
Even with these techniques, it is difficult to produce efficient energy coupling bandwidths larger than ~80% of the center resonance frequency, limiting the bandwidth to ~35% for 2
nd
harmonic imaging, and making it impossible to use higher than the 2
nd
harmonic component of the back scattered signal for imaging. The reason for this is that the transducer plate is the dominant resonant layer in the structure, and the electrodes are placed on the surface of the piezoelectric layer so that the electrode distance becomes large at high frequencies.
For improved bandwidth with 2
nd
harmonic imaging, there has been presented transducer structures with two piezoelectric layers with electrodes on the surfaces that gives a dual band performance. Through coupling of the electrodes one is able to transmit selectively in a low and a high frequency band, and receive selectively in the same low and high frequency bands. However, the presented patents make less than optimal use of the multilayer design for widest possible bandwidth, and the flexibility for selecting transduction in different frequency bands is limited.
The present invention presents a new layered transducer structure including optimized examples of the design that provides wider transduction bandwidths than previous designs, allowing transmission and reception of ultrasound pulses over two octaves, i.e. from a 1
st
to a 4
th
harmonic component of the lowest frequency band. The invention also provides details of efficient manufacturing of the layered structure. The method to increase the bandwidth is also useful for single piezoelectric layer transducers, increasing the relative bandwidth of such transducers to above 100%. This makes single piezoelectric layer transducer efficient for 2
nd
harmonic imaging and also for 1
st
harmonic imaging in different frequency bands.
The invention further presents methods for electronic selection of a wide variety of combinations of electro-acoustic ports in multi-layered transducers, for electronic selection of the efficient transduction bands of the transducer. This allows the transmit ultrasound pulses with frequency components in multiple bands, say both a 1
st
and a 2
nd
harmonic band, with transmitter amplifiers that switches the drive voltage between +V, −V, and zero. The invention further devices methods of combining the received signals from multiple electric ports for parallel reception of signals over two octaves of frequencies, or in a 1
st
, 2
nd
, 3
rd
, and even 4
th
harmonic component of the transmitted frequency band.
SUMMARY OF THE INVENTION
The invention presents solutions to the general need for ultrasound transducers that can efficiently operate over a large frequency band, or in separated frequency bands both for transmit and receive, so that: 1) one can use the same transducer to operate with several ultrasound frequencies to select the optimal frequency for the actual depth, 2) one can obtain wider transmit and receive bands with 2
nd
harmonic measurements and imaging, 3) one can receive higher than the 2
nd
harmonic component of the transmitted pulse, for measurement and imaging of objects with high non-linear elastic properties, and 4) one can transmit a complex ultrasound burst containing frequencies in more than one frequency band, and receive signals in frequency bands centered around sums and differences of the transmitted center frequencies.
According to the present invention, such wide band or multi band operation of the transducer is achieved through three design attributes:
1. Overall structure: The total transducer is composed of a set of piezoelectric and purely elastic layers, mounted on a backing material with so high absorption that reflected waves in the backing material can be neglected. The layers are grouped into: 1) a core, high impedance section that contains the piezoelectric layers, 2) a load matching section of elastic impedance matching layers between the high impedance section
Angelsen Bjorn A. J.
Johansen Tonni F.
Cohen & Pontani, Lieberman & Pavane
Jaworski Francis J.
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