Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
1999-12-20
2002-07-09
Lateef, Marvin M. (Department: 3737)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
Reexamination Certificate
active
06416478
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to transducers and more particularly to phased array transducers for use particularly in the medical diagnostic field.
Ultrasound machines are often used for observing organs in the human body. Typically, these machines contain transducer arrays for converting electrical signals into pressure waves and vice versa. Generally, the transducer array is in the form of a hand-held probe which may be adjusted in position to direct the ultrasound beam to the region of interest.
FIG. 1
illustrates a prior art transducer array
10
for generating an ultrasound beam. Typically, such an array may have 128 transducer elements
12
in the azimuthal direction. Adapted from radar terminology, the x, y, and z directions are referred to as the azimuthal, elevation, and range directions, respectively.
Each transducer element
12
, typically rectangular in cross-section, includes a first electrode
14
, a second electrode
16
and a piezoelectric layer
18
. In addition, one or more acoustic matching layers
20
may be disposed over the piezoelectric layer
18
to increase the efficiency of the sound energy transfer to the external medium. The electrode
14
for a given transducer element
12
may be part of a flexible circuit
15
for providing the hot wire or excitation signal to the piezoelectric layer
18
. Electrode
16
for a given transducer element may be connected to a ground shield return
17
. The piezoelectric layer
18
is metalized on its top and bottom surfaces and the matching layer
20
is also metalized on all surfaces so that electrode
16
which is in physical contact with the matching layer
20
is electrically coupled to a surface of the piezoelectric layer
18
by the metallization.
The transducer elements
12
are disposed on a backing block
24
. The backing block
24
may be highly attenuative such that ultrasound energy radiated in its direction (i.e., away from an object
32
of interest) is substantially absorbed. In addition, a mechanical lens
26
may be placed on the matching layer
20
to help confine the generated beam in the elevation-range plane and focus the ultrasound energy to a clinically useful depth in the body. The transducer array
10
may be placed in a nose piece
34
which houses the array. Examples of prior art transducer structures are disclosed in Charles S. DeSilets,
Transducer Arrays Suitable for Acoustic Imaging
, Ph.D. Thesis, Stanford University (1978) and Alan R. Selfridge,
Design and Fabrication of Ultrasonic Transducers and Transducer Arrays
, Ph.D. Thesis, Stanford University (1982).
Individual elements
12
are electrically excited by electrodes
14
and
16
with different amplitude and phase characteristics to steer and focus the ultrasound beam in the azimuthal-range plane. An example of a phased array acoustic imaging system is described in U.S. Pat. No. 4,550,607 issued Nov. 5, 1985 to Maslak et al. and is specifically incorporated herein by reference. U.S. Pat. No. 4,550,607 illustrates circuitry for combining the incoming signals received by the transducer array to produce a focused image on the display screen. When an electrical signal is imposed across the piezoelectric layer
18
, the thickness of the layer changes slightly. This property is used to generate sound from electrical energy. Conversely, electrical signals are generated across the electrodes in contact with the piezoelectric layer
18
in response to thickness changes that have been imposed mechanically from sound waves reflected back to the piezoelectric layer
18
.
The pressure waves generated by the transducer elements
12
are directed toward an object
32
to be observed, such as the heart of a patient being examined. Each time the pressure wave confronts tissue having different acoustic characteristics, a wave is reflected backward. The array of transducers may then convert the reflected pressure waves into corresponding electrical signals.
For the transducer shown in
FIG. 1
the beam is said to be mechanically focused in the elevation direction. The focusing of the beam in the azimuthal direction is done electronically by controlling the timing of the transmissions of each transducer element. This may be accomplished by introducing appropriate phase delays in the firing signals.
Reflected energy from a particular location in the imaging plane is collected by the transducer elements. The resultant electronic signals from individual transducer elements are individually detected and reinforced by introducing appropriate delays. Extensive processing of such data from the entire imaging phase is done to generate an image of the object. Such an image is typically displayed on a CRT monitor.
Sometimes it is desirable to image particular features to the exclusion of others. For example, it may be desirable to image the flow of blood in a patient to the exclusion of the surrounding organs and muscles. Introducing contrast agents into the patient's bloodstream allows the imaging of the blood stream. Contrast agents may be in the form of a solution or suspension of microbubbles or agents that produce microbubbles. The use of contrast agents provides selective evaluation of the signal components affected by the materials or media which have been introduced. This has the advantage that selective representation of the region filled with those agents is possible without finding the difference between two or more conditions recorded before and after application of the materials or media.
Nonlinear contrast agents are described for example by V. Uhlendorf, et al., in “Nonlinear Acoustical Response of Coated Microbubbles in Diagnostic Ultrasound” (1995) Ultrasonic Symposium, pp. 1559-1562). Such agents possess a fundamental resonant frequency. When they are insonified with high intensity ultrasonic energy at this fundamental frequency, they radiate ultrasonic frequency at a harmonic of the fundamental frequency. Such contrast agents are often used to highlight regions containing blood loaded with the contrast agent. For example, in the case of a blood-filled chamber of the heart, the borders of the chamber can be distinguished more easily when contrast agent is used. Since the contrast agent generates harmonic ultrasound energy, echoes from tissue (containing no contrast agent) at the fundamental frequency may be eliminated or reduced by filtering at the receive beamformer. Because most transducers operate in the half wavelength resonance mode they are not able to effectively receive energy at a second harmonic frequency since at the second harmonic frequency, the transducer elements are approximately one wavelength thick. This causes the charge generated on the two halves of the transducer element to be out of phase with each other which results in a cancellation or a null.
A wideband transducer can be operated to transmit pressure waves at one frequency and receive second harmonic frequency signals reflected back.
FIG. 2
is a graph illustrating the transmit response from a transducer having a wide bandwidth, for example 70%. A bandwidth of 70% means that the bandwidth measured between the lower frequency at which the sensitivity is −6dB with respect to the maximum sensitivity attained over the useful frequency range of the transducer and the upper frequency at which the sensitivity is −6dB with respect to the maximum sensitivity is 70% of the center frequency where the center frequency is defined as the average of the lower and upper −6dB frequencies. Using a transducer with a center frequency f
c
of 4.5 MHz, an ultrasound wave can be transmitted at 2/3 f
c
or 3 MHz and received at 4/3 f
c
or 6 MHz.
While energy may be transmitted and received within the transducer's available bandwidth, there are several disadvantages associated with using a wideband transducer in such a manner. Because transducer bandwidths are typically 75% and less, it is necessary to work near the edges of the transducer's bandwidth in order to transmit at one frequency and receive at another. This results in lower
Acuson Corporation
Brinks Hofer Gilson & Lione
Lateef Marvin M.
Patel Maulin
Summerfield Craig A.
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