U.S. patent application number 09/953740 was filed with the patent office on 2003-03-20 for dual-frequency ultrasonic array transducer and method of harmonic imaging.
Invention is credited to Lin, Gregory Sharat.
Application Number | 20030055337 09/953740 |
Document ID | / |
Family ID | 25494471 |
Filed Date | 2003-03-20 |
United States Patent
Application |
20030055337 |
Kind Code |
A1 |
Lin, Gregory Sharat |
March 20, 2003 |
DUAL-FREQUENCY ULTRASONIC ARRAY TRANSDUCER AND METHOD OF HARMONIC
IMAGING
Abstract
An ultrasonic transducer, method, and system are disclosed for
performing ultrasonic harmonic imaging in a medium or a living
body. The ultrasonic transducer consists of a linear array of
alternating long and short elements. A first set of transducer
elements is for transmitting and receiving at a fundamental
frequency, and a second set of transducer elements is for receiving
second harmonic or subharmonic echoes, each set operating at their
respective center frequencies. This dual-frequency ultrasonic
transducer is coupled to an ultrasound system wherein transmit
beamforming is done at the fundamental frequency, and receive
beamforming is done at the second harmonic or subharmonic
frequency. When receive beamforming at the fundamental frequency is
added, the method enables parallel fundamental, harmonic, compound,
and difference imaging. These methods may be utilized to improve
ultrasonic harmonic imaging of hard-to-image patients by optimizing
the transmission of fundamental-frequency ultrasound beams and the
receiving of second harmonic or subharmonic echoes, while
minimizing harmonic distortion and signal losses.
Inventors: |
Lin, Gregory Sharat;
(Fremont, CA) |
Correspondence
Address: |
G. Sharat Lin, Ph.D.
33808 Cassio Circle
Fremont
CA
94555-2016
US
|
Family ID: |
25494471 |
Appl. No.: |
09/953740 |
Filed: |
September 14, 2001 |
Current U.S.
Class: |
600/459 |
Current CPC
Class: |
G01S 15/8915 20130101;
G01S 7/52074 20130101; B06B 1/0622 20130101; G01S 7/52038 20130101;
A61B 8/0833 20130101; A61B 8/481 20130101 |
Class at
Publication: |
600/459 |
International
Class: |
A61B 008/14 |
Claims
What is claimed is:
1. A dual-frequency ultrasonic array transducer comprising: a first
set of piezoelectric elements operable at their center resonant
frequency to transmit ultrasonic pulses at a fundamental frequency;
a second set of piezoelectric elements operable at their center
resonant frequency to receive ultrasonic pulses at a second
frequency which is either twice the fundamental frequency (second
harmonic) or half the fundamental frequency (primary subharmonic),
wherein said first set of piezoelectric elements and said second
set of piezoelectric elements are positioned in a linear
alternating sequence; an acoustic-damping backing layer; electrical
contacts bonded to the front and back of each piezoelectric
element; and an acoustic lens.
2. A dual-frequency ultrasonic transducer of claim 1 wherein said
first set of piezoelectric elements and said second set of
piezoelectric elements have a uniform center-to-center element
separation and element pitch of less than or equal to the
wavelength of the higher of their said center resonant
frequencies.
3. A dual-frequency ultrasonic transducer of claim 1 wherein the
anterior surface is coated with two or a multiple of two acoustic
impedance matching layers of one-quarter wavelength of the higher
of their said center resonant frequencies.
4. A dual-frequency ultrasonic transducer of claim 1 wherein a
standoff pad is attached to said acoustic lens.
5. A dual-frequency ultrasonic transducer of claim 1 wherein said:
first set of piezoelectric elements are operable at their said
center resonant frequency to transmit and receive ultrasonic pulses
at said fundamental frequency; and said ultrasonic pulses received
at said fundamental frequency and said ultrasonic pulses received
at said second frequency are simultaneously processed by a
beamformer and scan converter for simultaneous and/or compound
image viewing.
6. A dual-frequency ultrasonic transducer of claim 1 wherein: a
transmitter/receiver coupled to said dual-frequency ultrasonic
transducer is operable to transmit focused beams at one said center
resonant frequency and receive focused beams at the other said
center resonant frequency; a transmit beamformer coupled to said
transmitter/receiver is operable to focus transmit pulses by
applying appropriate delays to all channels across any given active
transmit aperture; a receive beamformer coupled to said
transmitter/receiver is operable to focus on echoes received from a
sequence of focal points within said medium or body by applying
appropriate delays to all channels in any given active receive
aperture; a scan converter coupled to said receive beamformer is
operable to convert the geometric coordinate system of the
ultrasound vectors into Cartesian coordinates (raster display
format); and a display subsystem and monitor coupled to said scan
converter is operable to display an ultrasound image.
7. A dual-frequency ultrasonic transducer of claim 6 wherein: said
receive beamformer is further operable to independently focus on
echoes received at both said center resonant frequencies; said scan
converter is further operable to convert the geometric coordinate
system of the ultrasound vectors received at both said center
frequencies into Cartesian coordinates (raster display format); and
a frame processor coupled to said scan converter is operable to
assemble a fundamental-frequency image, a second harmonic or
subharmonic image, a compound image being a combination of said
fundamental-frequency image and said second harmonic or subharmonic
image, and/or a difference image being a subtraction of said
fundamental-frequency image from said second harmonic or
subharmonic image.
8. A method of performing ultrasonic harmonic imaging comprising
the steps of: (a) generating electrical pulse sequences at a
fundamental frequency; (b) applying said electrical pulse sequences
to a first set of piezoelectric elements with center resonant
frequency equal to said fundamental frequency, thereby converting
them into acoustic pulses--said first set of piezoelectric elements
being arranged in alternating positions with respect to a second
set of piezoelectric elements in a dual-frequency ultrasonic linear
array transducer; (c) introducing said acoustic pulses into an area
of a medium or body to be ultrasonically imaged; (d) receiving
echoes at a second harmonic or subharmonic frequency in said second
set of piezoelectric elements with center resonant frequency equal
to said second harmonic or subharmonic frequency, thereby
converting them into a first set of received electrical pulses; (e)
beamforming said first set of received electrical pulses so as to
focus on second harmonic or subharmonic echoes originating from a
specified depth zone within said medium or said body; (f)
assembling a second harmonic or subharmonic image by scan
converting all image vectors comprised of intensities mapped from
said first set of received electrical pulses; and (g) displaying
said second harmonic or subharmonic image.
9. A method according to claim 8 further comprising the steps of:
(h) receiving echoes at said fundamental-frequency in said first
set of piezoelectric elements, thereby converting them into a
second set of received electrical pulses; (i) beamforming said
second set of received electrical pulses so as to focus on
fundamental-frequency echoes originating from a specified depth
zone with said medium or said body; (j) assembling a
fundamental-frequency image by scan converting all image vectors
comprised of intensities mapped from said second set of received
electrical pulses; and (k) displaying said fundamental-frequency
image.
10. A method according to claim 9 wherein said
fundamental-frequency image and said second harmonic or subharmonic
image are summed or averaged together to generate a compound
image.
11. A method according to claim 9 wherein said
fundamental-frequency image and said second harmonic image are
subtracted one from another to produce a difference image.
12. A method according to claim 8 wherein acoustic wavefronts are
transmitted and received by said first set of piezoelectric
elements and/or said second set of piezoelectric elements whose
center-to-center spacing and element pitch is less than or equal to
the wavelength of the higher of their said center resonant
frequencies.
13. A method according to claim 8 wherein said first set of
piezoelectric elements and said second set of piezoelectric
elements are coated with two or a multiple of two acoustic
impedance-matching layers whose thickness is one quarter of the
wavelength of the higher of their said center frequencies.
14. An apparatus comprising: a first plurality of ultrasonic
transducer element means for transmitting acoustic energy at a
first set of frequencies into an area of a subject being scanned; a
second plurality of ultrasonic transducer element means for
receiving acoustic energy at a second set of frequencies from an
area of a subject being scanned, wherein said first plurality of
ultrasonic transducer element means is laid out in an alternating
sequence with respect to said second plurality of ultrasonic
transducer element means; an acoustic-damping backing layer means;
electrical connection means; and an acoustic lens means.
15. An apparatus of claim 14 wherein: said first plurality of
ultrasonic transducer element means have a center frequency equal
to a fundamental frequency; and said second plurality of ultrasonic
transducer element means have a center frequency equal to either
twice said fundamental frequency (second harmonic) or half said
fundamental frequency (primary subharmonic).
16. An apparatus of claim 14 wherein the center-to-center element
spacing between ultrasonic transducer element means is no more than
the wavelength of the higher of the center resonant frequencies of
said first plurality of ultrasonic transducer element means and
said second plurality of ultrasonic transducer element means.
17. An apparatus of claim 15 wherein acoustic impedance-matching
layer means consists of two or a multiple of two layers of
thickness equal to one quarter of the wavelength of the higher of
the center resonant frequencies of said first plurality of
ultrasonic transducer element means and said second plurality of
ultrasonic transducer element means.
18. An apparatus of claim 14 further comprising acoustic stand-off
pad means for eliminating near-field acoustic artifacts.
19. An apparatus of claim 14 further comprising: transmitter means
for generating and beamforming transmit pulses at said first set of
frequencies to be applied to said first plurality of ultrasonic
transducer element means; a first receiver means for digitizing and
beamforming pulses generated by said second plurality of ultrasonic
transducer element means received from echoes at said second set of
frequencies; scan converter means for mapping pulses at said second
set of frequencies into intensities and arranging the data into a
first raster image; and display means for visualizing said first
raster image.
20. An apparatus of claim 19 further comprising: switching means
for switching said first plurality of ultrasonic transducer element
means between said transmitter means and a second receiver means;
said second receiver means for digitizing and beamforming pulses
generated by said first plurality of ultrasonic transducer element
means received from echoes at said first set of frequencies; scan
converter means for mapping pulses at said first set of frequencies
into intensities and arranging the data into a second raster image;
compound imaging means for averaging said first raster image and
said second raster image; difference imaging means for subtracting
said second raster image from said first raster image; and display
means for visualizing said second raster image, the compound image,
and the difference image.
Description
FIELD OF THE INVENTION
[0001] The present invention generally relates to ultrasonic
transducers, and more specifically to ultrasonic array transducers
capable of transmitting and receiving ultrasonic pulses at two
different frequencies.
BACKGROUND OF THE INVENTION
[0002] Ultrasonic imaging technology has become an important tool
for examining the internal structure of living organisms. In the
diagnosis of various medical conditions, ultrasonic imaging is
often useful to examine soft tissues within the body to show the
structural detail of internal tissues and fluid flow. An important
application of ultrasonic imaging is in the detection and
identification of various internal structural abnormalities, such
as cysts, tumors, abscesses, mineral deposits, blood vessel
obstructions, and anatomical defects without physically penetrating
the skin.
[0003] Ultrasonic images are formed by producing very short pulses
of ultrasound using an electro-acoustic transducer, sending the
pulses through the body, and measuring the properties (e.g.,
amplitude and phase) of the echoes from tissues within the body.
Focused ultrasound pulses, referred to as "ultrasound beams", are
targeted to specific tissue regions of interest in the body.
Typically, an ultrasound beam is focused at small lateral and depth
intervals within the body to improve spatial resolution. Echoes are
received by the ultrasound transducer and processed to generate an
image of the tissue or object in a region of interest. The
resulting image is usually referred to as a B-scan image.
[0004] The echoes from soft tissues and from contrast agents, such
as various microbubbles, consist of ultrasound signals at the
transmitted frequency (the fundamental frequency) as well as
signals at various multiples of the transmitted frequency
(harmonics). Apart from the fundamental frequency, the strongest
harmonic signal is generally at the second harmonic or twice the
fundamental frequency.
[0005] Ultrasonic beams are subject to random scattering and
distortion as they travel through soft tissues, particularly where
there are acoustic interfaces such as between muscle and fat.
Collectively referred to as tissue aberrations, these tend to
degrade the clarity of the B-scan image. However, harmonic echoes
generally exhibit less distortion and diffraction than echoes at
the fundamental frequency. Thus, an ultrasound image constructed
out of harmonic echoes is often sharper, less hazy, and less
distorted.
[0006] Recently, harmonic ultrasound imaging has come into
widespread use, particularly in viewing deep abdominal organs and
the heart. In large patients with thick aberrating layers of fat
and muscle, or gastric air pockets, harmonic imaging has been found
to provide diagnostically superior ultrasonic images of the liver,
kidneys, stomach, uterus, ovaries, and other abdominal organs.
Because the heart is surrounded by the lungs which contain
aberrating pockets of air, harmonic ultrasound imaging frequently
provides clearer images of the cardiac chambers and valves.
[0007] Conventional ultrasonic transducers consist of
electro-acoustic elements of a particular resonant center
frequency. Because lower frequency ultrasonic signals are more
penetrating and higher frequency ultrasonic signals enable higher
resolution, the choice of a transducer's center frequency is an
optimization trade-off between penetration and resolution,
depending on the clinical application. Thus, a transducer intended
for abdominal use has a lower center frequency of 2.5-5.0 MHz to
achieve deep penetration to 18-25 centimeters at lower resolution,
while a transducer intended for breast imaging has a higher center
frequency of 7-14 MHz to achieve a resolution of 0.2-0.5 mm at
reduced penetration. These transducers perform harmonic imaging by
having a wide bandwidth such that the transmitted pulses are at the
low end of this bandwidth and the harmonic echoes are received at
the high end of the same bandwidth. Because a conventional
transducer used for harmonic imaging neither transmits nor receives
signals at its center frequency, it does not perform efficiently in
either transmit or receive. Moreover, transmitting far from a
wideband transducer's center frequency introduces undesirable
harmonic distortion. These facts limit both the transmitted signal
quality and the quality of the resulting harmonic image. They also
limit the electro-acoustic efficiency of the transducer, and,
hence, penetration.
[0008] The present invention, an ultrasonic array transducer for
harmonic imaging, consists of alternating elements of different
center frequencies--fundamental and harmonic--for transmit and
harmonic receive, respectively. Lower-frequency elements are
optimally matched to the transmitted fundamental frequency, and
higher-frequency elements are optimally matched to harmonic echoes.
The transmit elements can also be used for receiving echoes at the
fundamental frequency. Thus, this transducer has the novelty of
being able to optimally receive echoes at both the fundamental and
harmonic frequencies simultaneously. Alternatively, the transducer
design may be modified to optimally transmit at a fundamental
frequency and receive at a subharmonic frequency (half the
fundamental frequency).
SUMMARY OF THE INVENTION
[0009] An ultrasonic array transducer is described for performing
harmonic imaging. The transducer consists of alternating elements
with center frequency at a fundamental frequency and center
frequency at twice the fundamental frequency. The former elements
are used to transmit and receive at the fundamental frequency,
while the latter elements are used to receive harmonic echoes.
Center-to-center element spacing is constrained to less than one
quarter of the fundamental wavelength.
[0010] A method is further described for step-by-step fabrication
of said dual-frequency transducer. Alternating elements are milled
to half height from the back side before addition of electrical
contacts and the acoustic-damping backing layer. Surface layers are
then laid on the front side in the usual manner.
[0011] A method and system are further described for operating said
ultrasonic transducer. In one embodiment of the present invention,
the low-frequency elements are used for transmitting and the
high-frequency elements are used for receiving. This has the added
benefit of eliminating electronic noise generated by
transmit-receive switching.
[0012] In a preferred embodiment of the present invention, the
low-frequency elements are used for both transmitting and receiving
at the fundamental frequency, and the high-frequency elements are
used for receiving harmonics echoes. This system has the added
benefit of being able to generate fundamental, harmonic, compound,
and difference images in real time.
[0013] In each embodiment, the high-frequency elements may also be
used for transmitting (or transmitting and receiving) at a
fundamental frequency and the low-frequency elements may be used
for receiving subharmonic echoes.
[0014] Other features of the present invention will be apparent
from the accompanying drawings and from the detailed description
which follows.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] The present invention is illustrated by way of example and
not limitation in the figures of the accompanying drawings, in
which like references indicate similar elements, and in which:
[0016] FIG. 1A illustrates a dual-frequency ultrasonic array
transducer transmitting a pulse at the fundamental frequency and
receiving a second harmonic echo from a tissue reflector.
[0017] FIG. 1B illustrates a dual-frequency ultrasonic array
transducer transmitting a pulse at the fundamental frequency and
receiving a subharmonic echo from a tissue reflector.
[0018] FIG. 2 is a graph of the optimal bandwidths of the
transmitted signal, received harmonic signals, and the bandwidth of
a conventional wideband transducer.
[0019] FIG. 3A shows a longitudinal cross-section of a
dual-frequency ultrasonic array transducer.
[0020] FIG. 3B shows a transverse (elevational) cross-section of a
dual-frequency ultrasonic array transducer.
[0021] FIG. 4A illustrates a piezoelectric transducer material
mounted on a fabrication substrate.
[0022] FIG. 4B illustrates a piezoelectric transducer material
after dicing into individual elements and filling the kerfs.
[0023] FIG. 4C illustrates a piezoelectric transducer array after
milling of alternating elements.
[0024] FIG. 4D illustrates a piezoelectric transducer array with
attachment of conductive flex circuits and acoustic-damping backing
layer.
[0025] FIG. 4E illustrates a piezoelectric transducer array
inverted and with fabrication substrate removed.
[0026] FIG. 4F illustrates a piezoelectric transducer array with
attachment of third conductive flex circuit, acoustic matching
layers, acoustic lens, and standoff pad in cut-away views.
[0027] FIG. 5A is a schematic diagram of an embodiment of the
front-end circuitry needed for harmonic imaging with the
dual-frequency ultrasonic array transducer.
[0028] FIG. 5B is a schematic diagram of a preferred embodiment of
the front-end circuitry for simultaneous fundamental and harmonic
imaging with the dual-frequency ultrasonic array transducer.
[0029] FIG. 6 is a partial block diagram of an ultrasound scanner
for generating fundamental, harmonic, compound, and difference
ultrasound images.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0030] A dual-frequency ultrasonic array transducer for harmonic
imaging, shown in FIG. 1A, comprises an array 100 of individual
piezoelectric elements coupled to an ultrasound scanner. The
piezoelectric elements consist of long elements 102 whose center
frequency is the fundamental transmit frequency, and short elements
104 whose center frequency is twice the fundamental frequency
(second harmonic). A transmit pulse 140 at the fundamental
frequency is applied to each long element. A transmit beamformer
appropriately delays pulses applied across the electronic aperture
so as to focus the acoustic beam 130 emitted by the array of long
elements. Acoustic beam 130 is transmitted at the fundamental
frequency and is reflected by target 120, which may be living
tissue in a body or an inanimate structure within a medium which
allows ultrasound signals to pass and be reflected. A reflected
echo 132 contains pulses at the fundamental frequency, pulses at
the second harmonic, and pulses at other frequencies. The long
transducer elements 102 optimally receive acoustic signals at the
fundamental frequency which is their center frequency. The short
transducer elements 104 optimally receive acoustic signals at the
second harmonic frequency which is equal to their center frequency.
All transducer elements convert the received acoustic signals into
electrical pulses. In particular, the short transducer elements 104
preferentially convert second harmonic echoes into electrical
pulses 142 at the second harmonic frequency.
[0031] This method of harmonic signal transduction is advantageous
over conventional ultrasound transducers because the usually weaker
second harmonic echoes are received by the short transducer
elements at their center frequency where electro-acoustic
conversion efficiency is maximal, hence where transducer
sensitivity is greatest. Similarly, fundamental-frequency pulses
are transmitted by the long transducer elements at their center
frequency, ensuring acoustic beams without contamination by
non-fundamental frequencies and minimal transducer heating. This is
in contrast to a conventional ultrasound transducer used in
harmonic imaging. The wide bandwidth 202 of a conventional wideband
transducer is illustrated in FIG. 2. The center frequency f.sub.T
of the conventional transducer is chosen such that the fundamental
frequency f.sub.0 to be transmitted is within its bandwidth
(defined in this illustration as -3 dB down from the peak at the
center frequency) at the low-frequency end, and the second harmonic
frequency 2f.sub.0 to be received is also within its bandwidth at
the high-frequency end. At f.sub.0 and 2f.sub.0 the transducer is
not only less efficient than at its center frequency, but also
distorts frequency and phase modulations in the signals by
asymmetrically suppressing the lower sideband of the
fundamental-frequency signal and the upper sideband of the second
harmonic signal. The present invention solves this problem by
transmitting fundamental-frequency pulses at the center of the
bandwidth 204 of the long elements and receiving second harmonic
signals at the center of the bandwidth 206 of the short elements.
The transmitted signal itself may be of narrow bandwidth 208, thus,
utilizing only a fraction of the available bandwidth of the
transmit elements.
[0032] An ultrasonic transducer for subharmonic imaging, shown in
FIG. 1B, comprises an array 101 of individual piezoelectric
elements coupled to an ultrasound scanner. The piezoelectric
elements consist of short elements 103 whose center frequency is
the fundamental transmit frequency, and long elements 105 whose
center frequency is half the fundamental frequency (primary
subharmonic). A transmit pulse 141 at the fundamental frequency is
applied to each short element. A transmit beamformer appropriately
delays pulses applied across the electronic aperture so as to focus
the acoustic beam 131 emitted by the array of long elements.
Acoustic beam 131 is transmitted at the fundamental frequency and
is reflected by target 121, which may be living tissue in a body or
an inanimate structure within a medium which allows ultrasound
signals to pass and be reflected. A reflected echo 133 contains
pulses at the fundamental frequency, pulses at the primary
subharmonic, and pulses at other frequencies. The short transducer
elements 103 optimally receive acoustic signals at the fundamental
frequency which is their center frequency. The long transducer
elements 105 optimally receive acoustic signals at the subharmonic
frequency which is equal to their center frequency. All transducer
elements convert the received acoustic signals into electrical
pulses. In particular, the long transducer elements 105
preferentially convert subharmonic echoes into electrical pulses
143 at the primary subharmonic frequency.
[0033] A longitudinal cross-section of the dual-frequency
ultrasonic array transducer is shown in FIG. 3A. The drawings are
exemplary of a harmonic imaging transducer, but apply equally to
the design and fabrication of a subharmonic imaging transducer.
Long elements 102 and short elements 104 are arranged in
alternating positions along the length of the array. The
center-to-center spacing of the elements is optimally chosen to be
less than or equal to the wavelength of the upper bandwidth limit
of the second harmonic (or higher of the two frequencies of
interest) to ensure adequate acceptance angles for beam steering
and near-field beam focusing. A common electrical contact 306 is
shown on the anterior surface of the transducer elements, and
individual electrical contacts 308 and 310 are shown on the
posterior surfaces of the long and short elements respectively. The
transducer elements are embedded in a backing layer 320 which
provides both acoustic damping and structural mounting. The forward
surface of the array is coated with two (or more) quarterwave
matching layers 322 and 324 whose function is to match the acoustic
impedance of the transducer elements to the acoustic lens 326. The
thickness of each matching layer is one quarter of the wavelength
of the second harmonic (or the higher of two frequencies). Two
layers grouped together may also effectively serve as a quarterwave
matching layer for signals at the fundamental frequency (or the
lower of two frequencies). Ceramic piezoelectric transducers
typically have a high acoustic impedance of 15-25 MRayls, much
higher than that of soft tissues at approximately 1.5 MRayls.
Quarterwave matching layers having intermediate acoustic
impedances, reduce acoustic reflectance at the interface between
two different materials by reducing the difference in acoustic
impedance at that interface. An optional standoff pad 328 puts the
skin line a few millimeters away from the transducer elements to
eliminate the near-field alternating-line dropout ("picket fence")
artifact due to transmitting and/or receiving through every other
element in the array. If the center-to-center element spacing and
associated element pitch are small enough, the standoff pad may not
be necessary.
[0034] FIG. 3B shows an elevational cross-section of the same
transducer. Flex circuit 334 is attached to anterior electrical
contact 306. Flex circuit 336 is attached to posterior electrical
contact 310 (shown in FIG. 3A). Flex circuit 332 is attached to
posterior electrical contact 308.
[0035] A method of fabricating the dual-frequency ultrasonic array
transducer is illustrated in FIGS. 4A-4F. FIG. 4A shows a mounting
block 412 serving as a substrate to support a ceramic piezoelectric
block 410. The ceramic piezoelectric block is diced into individual
long elements 102 separated by kerfs 414 in FIG. 4B. The kerfs are
inter-element spacings that are filled with non-conductive
acoustic-damping material. Every other element is milled down to
half height to form the short elements 104. Electrical connection
layers 308 and 310 are deposited on the rear surfaces of all
transducer elements, connected to respective flex circuits (e.g.,
332), and then filled in with acoustic-damping backing material 320
as shown in FIG. 4D. The entire block is inverted and the original
mounting block 412 is removed as illustrated in FIG. 4E. The
anterior electrical connection layer 306, quarterwave matching
layers 322 and 324, acoustic lens 326, and optional standoff pad
328 are successively deposited or mounted on top of the array of
exposed transducer elements.
[0036] The dual-frequency ultrasonic array transducer is connected
to the front end of an ultrasound scanner in a manner described in
the block diagram of FIG. 5A. The drawings are exemplary of a
harmonic imaging subsystem, but apply equally to the operation of a
subharmonic imaging subsystem. In this embodiment, the front-end
controller 510 sequences the transmit timing and receive
beamforming events to be performed by the front-end circuitry.
Transmit pulse generators 520 produce precisely-timed pulse
sequences for each active channel in the transmit aperture with a
delay profile necessary for electronic beam focusing. The
transmitter array 522, driven by timed transmit pulse sequences at
the fundamental frequency, sends its output to individual long
elements 102 in the transducer. Harmonic echoes are received by
individual short elements 104, whose outputs are fed into
preamplifiers 504 and analog-to-digital converters 506. Receive
aperture switching array 530 selectively passes signals from those
channels within the active receive aperture to the receive
beamformer 532. The receive beamformer applies specified delays to
each channel in the receive aperture to electronically focus the
received signals from a particular focal depth, sums them together,
and optionally selects those signals within a specified temporal
(depth) window from the transmit event. The resulting second
harmonic signal is then sent to the scan converter of the
ultrasound scanner to be assembled into a viewable image.
[0037] In an alternative preferred embodiment of the invention,
illustrated in FIG. 5B, transmit-receive (T/R) switches 502 are
added to the long elements 102. This enables them to be used for
both transmitting and receiving. Additional preamplifiers 504 and
analog-to-digital converters 506 are connected to the receive side
of each T/R switch. The output of these additional channels is fed
into a separate receive aperture switching array 540 and receive
beamformer 542 for imaging at the fundamental frequency. The
resulting summed fundamental-frequency signal is sent to the scan
converter independently of the summed second harmonic signal
(described above).
[0038] The block diagram of an ultrasound scanner for simultaneous
fundamental-frequency and second harmonic imaging is shown in FIG.
6. Again, the drawings are exemplary of a harmonic imaging system,
but apply equally to the concept of a subharmonic imaging system.
The output of the fundamental-frequency beamformer 542 is fed into
one scan converter 644 to assemble the fundamental-frequency image.
The output of the second harmonic beamformer 532 is fed into a
second scan converter 634 to assemble the second harmonic image.
These images may be displayed individually or side-by-side on the
viewing screen of the ultrasound scanner. In addition, the two
images may be summed together by summing unit 652 to form a
dual-frequency compound image. Compound imaging in general has been
found to be useful in reducing speckle and anisotropic reflection
artifacts, and in improving image smoothness. The two images may
also be subtracted by subtraction unit 654 to form a difference
image. Because fundamental-frequency images contain echoes with
both linearly-propagated and non-linearly-propagated components,
and second harmonic images tend to be comprised primarily of
non-linearly-propagated echoes, subtraction imaging may be expected
to better visualize the spatial patterns of non-linear propagation
and reflection, hence, providing a new parameter for ultrasonic
imaging and potential tissue differentiation.
[0039] In another alternative embodiment of the invention, the
short elements may be used for transmitting at a fundamental
frequency and the long elements may be used for receiving at one
half the fundamental frequency for the purpose of subharmonic
imaging. In addition, the short elements may be used for receiving
at the fundamental frequency so as to enable simultaneous
fundamental-frequency and subharmonic imaging.
[0040] In the foregoing, an ultrasonic array transducer has been
described for transmitting at a fundamental frequency and receiving
harmonic echoes from a medium or body, the transducer consisting of
alternating elements of two different center frequencies. A method
has also been described for fabrication of this type of transducer.
A method and system have further been described for displaying
fundamental, harmonic, compound, and difference images using this
transducer. Although the present invention has been described with
reference to specific exemplary embodiments, it will be evident
that various modifications and changes may be made to these
embodiments without departing from the broader spirit and scope of
the invention as set forth in these claims. Accordingly, the
specification and drawings are to be regarded in an illustrative
rather than a restrictive sense.
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