U.S. patent application number 09/809680 was filed with the patent office on 2002-01-17 for ultrasonic wave transducer system and ultrasonic wave transducer.
Invention is credited to Adachi, Hideo, Wakabayashi, Katsuhiro.
Application Number | 20020007118 09/809680 |
Document ID | / |
Family ID | 26587613 |
Filed Date | 2002-01-17 |
United States Patent
Application |
20020007118 |
Kind Code |
A1 |
Adachi, Hideo ; et
al. |
January 17, 2002 |
Ultrasonic wave transducer system and ultrasonic wave
transducer
Abstract
There is disclosed an ultrasonic transducer system for harmonic
imaging, comprising: an ultrasonic transducer comprising a
transmitting ultrasonic vibrator for transmitting a fundamental
ultrasound having a center frequency f.sub.0, and a receiving
ultrasonic vibrator for receiving a harmonic signal having a center
frequency nf.sub.0 (n is an integer of 2 or more); and control
means for controlling the ultrasonic transducer, wherein the
transmitting ultrasonic vibrator comprises a transmitting
piezoelectric resonator, the receiving ultrasonic vibrator
comprises a receiving piezoelectric resonator, the transmitting and
receiving piezoelectric resonators are superposed in layers and
disposed, and the control means supplies a drive signal to the
transmitting piezoelectric resonator only for a time t.sub.1, holds
a state between electrodes of the receiving piezoelectric resonator
in a low resistance state including a short circuit for a
predetermined time t.sub.2 (>t.sub.1) after the drive signal is
supplied, and holds the state between the electrodes of the
transmitting piezoelectric resonator in a high resistance state
including an open circuit after an elapse of the predetermined time
t.sub.2, until the next drive signal is supplied to the
transmitting piezoelectric resonator.
Inventors: |
Adachi, Hideo; (Iruma-Shi,
JP) ; Wakabayashi, Katsuhiro; (Hachioji-shi,
JP) |
Correspondence
Address: |
Scully, Scott, Murphy & Presser
400 Garden City Plaza
Garden City
NY
11530
US
|
Family ID: |
26587613 |
Appl. No.: |
09/809680 |
Filed: |
July 9, 2001 |
Current U.S.
Class: |
600/443 |
Current CPC
Class: |
G10K 11/30 20130101;
B06B 1/0681 20130101; G01S 15/8922 20130101; G01S 15/895 20130101;
B06B 1/0611 20130101; G10K 11/32 20130101 |
Class at
Publication: |
600/443 |
International
Class: |
A61B 008/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 15, 2000 |
JP |
2000-072854 |
Feb 23, 2001 |
JP |
2001-048579 |
Claims
What is claimed is:
1. An ultrasonic transducer system for harmonic imaging,
comprising: an ultrasonic transducer comprising a transmitting
ultrasonic vibrator for transmitting a fundamental ultrasound
having a center frequency f.sub.0, and a receiving ultrasonic
vibrator for receiving a harmonic signal having a center frequency
nf.sub.0 (n is an integer of 2 or more); and control means for
controlling the ultrasonic transducer, wherein said transmitting
ultrasonic vibrator comprises a transmitting piezoelectric
resonator, said receiving ultrasonic vibrator comprises a receiving
piezoelectric resonator, the transmitting piezoelectric resonator
and the receiving piezoelectric resonator are superposed and
disposed in layers, and said control means supplies a drive signal
to the transmitting piezoelectric resonator only for a time
t.sub.1, holds a state between electrodes of the receiving
piezoelectric resonator in a low resistance state including a short
circuit for a predetermined time t.sub.2 = (>t.sub.1) after the
drive signal is supplied, and holds the state between the
electrodes of the transmitting piezoelectric resonator in a high
resistance state including an open circuit after an elapse of the
predetermined time t.sub.2, until the next drive signal is supplied
to the transmitting piezoelectric resonator.
2. The ultrasonic transducer system according to claim 1, wherein
said control means comprises an inductance circuit for holding the
state between the electrodes of the receiving piezoelectric
resonator in a low resistance state including the short circuit
state for a specific time.
3. The ultrasonic transducer system according to claim 1, wherein
said control means comprises an on/off control device, disposed in
a final stage of a circuit for supplying the drive signal to the
transmitting piezoelectric resonator, for holding the high
resistance state including the open circuit.
4. The ultrasonic transducer system according to claim 1, wherein
said receiving piezoelectric resonator comprises a polymeric
piezoelectric material.
5. The ultrasonic transducer system according to claim 1, wherein
said receiving piezoelectric resonator comprises a composite
piezoelectric material.
6. The ultrasonic transducer system according to claim 4, wherein
said polymeric piezoelectric material is directly disposed on the
surface of the transmitting piezoelectric resonator on an
ultrasonic emission side.
7. The ultrasonic transducer system according to claim 4, wherein
said polymeric piezoelectric material is disposed on the surface of
the transmitting piezoelectric resonator on an ultrasonic emission
side via an acoustic matching layer.
8. The ultrasonic transducer system according to claim 6, wherein
said polymeric piezoelectric material comprises a polymeric film in
which a piezoelectric property is spontaneously generated by
polarizing a surface energy.
9. The ultrasonic transducer system according to claim 7, wherein
said polymeric piezoelectric material comprises a polymeric film in
which a piezoelectric property is spontaneously generated by
polarizing a surface energy.
10. The ultrasonic transducer system according to claim 1, wherein
said transmitting ultrasonic vibrator and said receiving ultrasonic
vibrator comprise a damping layer (backing layer) disposed on the
back surface of the transmitting piezoelectric resonator, and an
acoustic lens disposed on the front surface of the receiving
piezoelectric resonator in common.
11. The ultrasonic transducer system according to claim 10, wherein
said acoustic lens comprises an acoustic matching function.
12. The ultrasonic transducer system according to claim 10, wherein
said acoustic lens comprises an opening surface having a constant
curvature radius, and the curvature radius of the opening surface
has a value equal to an average value of an acoustic focal length
for the ultrasound having the center frequency f.sub.0 and an
acoustic focal length for the ultrasound having the center
frequency nf.sub.0 (n is an integer of 2 or more).
13. The ultrasonic transducer system according to claim 10, wherein
said acoustic lens comprises an opening surface having a curvature
radius which partially differs.
14. An ultrasonic transducer for harmonic imaging, comprising: a
transmitting ultrasonic vibrator including a transmitting
piezoelectric resonator, for transmitting a fundamental ultrasound
having a center frequency f.sub.0; and a receiving ultrasonic
vibrator including a receiving piezoelectric resonator, for
receiving a harmonic signal having a center frequency nf.sub.0 (n
is an integer of 2 or more), wherein one of said transmitting
piezoelectric resonator and said receiving piezoelectric resonator
has an annular band shape, the other one of said transmitting
piezoelectric resonator and said receiving piezoelectric resonator
has a disc shape, and the disc-shaped piezoelectric resonator is
disposed inside the annular band shaped piezoelectric
resonator.
15. The ultrasonic transducer according to claim 14, wherein said
transmitting piezoelectric resonator comprises the annular band
shaped piezoelectric resonator, and said receiving piezoelectric
resonator comprises the disc shaped piezoelectric resonator.
16. The ultrasonic transducer according to claim 14, further
comprising an acoustic lens disposed in front of the transmitting
piezoelectric resonator and the receiving piezoelectric resonator
and provided with an acoustic matching function, wherein said
transmitting ultrasonic vibrator and said receiving ultrasonic
vibrator partially include the acoustic lens.
17. The ultrasonic transducer according to claim 14, wherein said
transmitting ultrasonic vibrator further comprises a damping layer
disposed on the back surface of the transmitting piezoelectric
resonator, said receiving ultrasonic vibrator further comprises a
damping layer disposed on the back surface of the receiving
piezoelectric resonator, and these damping layers have different
thickness values.
18. The ultrasonic transducer according to claim 14, wherein said
acoustic lens comprises an opening surface having a curvature
radius which partially differs.
19. The ultrasonic transducer according to claim 18, wherein said
acoustic lens comprises a first portion positioned in front of the
transmitting piezoelectric resonator and a second portion
positioned in front of the receiving piezoelectric resonator, the
first portion of the acoustic lens comprises the opening surface
having a constant curvature radius R.sub.t, and the second portion
of the acoustic lens comprises the opening surface having a
constant curvature radius R.sub.r smaller than the curvature radius
R.sub.t.
20. The ultrasonic transducer according to claim 19, wherein said
receiving piezoelectric resonator projects forward from the
transmitting piezoelectric resonator.
21. The ultrasonic transducer according to claim 19, wherein said
curvature radius R.sub.t of the opening surface of the first
portion of the acoustic lens has a value equal to an acoustic focal
length for the ultrasound having the center frequency f.sub.0, and
said curvature radius R.sub.r of the opening surface of the second
portion of the acoustic lens has a value equal to an acoustic focal
length for the ultrasound having the center frequency nf.sub.0 (n
is an integer of 2 or more).
22. The ultrasonic transducer according to claim 19, wherein said
first portion of the acoustic lens has a thickness which is 1/4 of
a wavelength corresponding to the frequency f.sub.0 on average, and
said second portion of the acoustic lens has a thickness which is
1/4 of a wavelength corresponding to the frequency nf.sub.0 on
average.
23. The ultrasonic transducer according to claim 14, wherein said
acoustic lens comprises an opening surface having a constant
curvature radius.
24. The ultrasonic transducer according to claim 23, wherein said
acoustic lens comprises a first portion positioned in front of the
transmitting piezoelectric resonator and a second portion
positioned in front of the receiving piezoelectric resonator, the
first portion of the acoustic lens has an average thickness T.sub.t
equal to 1/4 of a wavelength of the ultrasound having the
fundamental frequency f.sub.0, and the second portion of the
acoustic lens has an average thickness T.sub.r equal to 1/4 of a
wavelength of the ultrasound having the fundamental frequency
nf.sub.0 (n is an integer of 2 or more).
25. The ultrasonic transducer according to claim 24, wherein an
opening surface of the acoustic lens has a curvature radius equal
to a radius of a spherical surface which circumscribes a circle
obtained by connecting a point of the average thickness T.sub.t of
the first portion of the acoustic lens, and a circle obtained by
connecting a point of the average thickness T.sub.r of the second
portion of the acoustic lens.
26. An ultrasonic transducer for harmonic imaging, comprising: a
plurality of transmitting ultrasonic vibrators for transmitting a
fundamental ultrasound having a center frequency f.sub.0; and a
plurality of receiving ultrasonic vibrators for receiving a
harmonic signal having a center frequency nf.sub.0 (n is an integer
of 2 or more), wherein said transmitting ultrasonic vibrators and
said receiving ultrasonic vibrators are alternately disposed in a
radial form.
27. The ultrasonic transducer according to claim 26, wherein each
of said transmitting ultrasonic vibrators comprises a transmitting
piezoelectric resonator, and an acoustic lens disposed in front of
the transmitting piezoelectric resonator, each of said receiving
ultrasonic vibrators comprises a receiving piezoelectric resonator,
and an acoustic lens disposed in front of the receiving
piezoelectric resonator, and the acoustic lens of the receiving
ultrasonic vibrator has a curvature radius smaller than a curvature
radius of the acoustic lens of the transmitting ultrasonic
vibrator.
28. The ultrasonic transducer according to claim 27, wherein said
transmitting ultrasonic vibrator further comprises a damping layer
disposed on the back surface of the transmitting piezoelectric
resonator, said receiving ultrasonic vibrator further comprises a
damping layer disposed on the back surface of the receiving
piezoelectric resonator, and these damping layers have different
thickness values.
29. An ultrasonic transducer for harmonic imaging, comprising: a
transmitting ultrasonic vibrator for transmitting a fundamental
ultrasound having a center frequency f.sub.0 in response to input
of an electric signal; and a receiving ultrasonic vibrator for
receiving a harmonic signal having a center frequency nf.sub.0 (n
is an integer of 2 or more) generated in an object by the
fundamental ultrasound, wherein said transmitting ultrasonic
vibrator comprises a transmitting piezoelectric resonator, said
receiving ultrasonic vibrator comprises a receiving piezoelectric
resonator, the transmitting piezoelectric resonator and the
receiving piezoelectric resonator are disposed on the same plane,
and the transmitting piezoelectric resonator and the receiving
piezoelectric resonator satisfy (g.sub.33r.multidot.V.sub.r.mul-
tidot.Q.sub.r)/(g.sub.33t.multidot.V.sub.t.multidot.Q.sub.t).gtoreq.n.mult-
idot.(1+R), in which g.sub.33t and V.sub.t denote a voltage output
coefficient and a sound velocity of the transmitting piezoelectric
resonator, g.sub.33r and V.sub.r denote a voltage output
coefficient and a sound velocity of the receiving piezoelectric
resonator, n denotes a harmonic order, R denotes an opening area
ratio (an opening area of the receiving piezoelectric resonator/an
opening area of the transmitting piezoelectric resonator), and
Q.sub.t and Q.sub.r denote resonance sharpness of the transmitting
ultrasonic vibrator and the receiving ultrasonic vibrator,
respectively.
30. The ultrasonic transducer according to claim 29, wherein in
said transmitting ultrasonic vibrator, a mechanical resonance
sharpness Q in the center frequency is between 1 and 5.
31. The ultrasonic transducer according to claim 30, wherein a
material of the transmitting piezoelectric resonator has a
piezoelectric constant d.sub.33 which satisfies d.sub.33>200
.times.10.sub.-12 [m/V], and a mechanical quality factor Qm which
satisfies 70<Qm<1000.
32. The ultrasonic transducer according to claim 30, wherein said
transmitting ultrasonic vibrator further comprises a backing layer
disposed on the back surface of the transmitting piezoelectric
resonator, and the backing layer has an ultrasonic attenuation
ratio larger than 5 dB/cm/MHz, and an acoustic impedance Zd which
is 1/3 or less of an acoustic impedance Zp of the transmitting
piezoelectric resonator.
33. The ultrasonic transducer according to claim 30, wherein said
transmitting piezoelectric resonator comprises an energy trapped
electrode.
34. The ultrasonic transducer according to claim 29, wherein said
receiving ultrasonic vibrator has a center frequency of 2f.sub.0,
and a mechanical resonance sharpness Q in the center frequency is
between 1 and 5.
35. The ultrasonic transducer according to claim 29, wherein said
receiving ultrasonic vibrator has a center frequency of 3f.sub.0,
and a mechanical resonance sharpness Q in the center frequency is
between 1 and 5.
36. The ultrasonic transducer according to claim 34 or 35, wherein
a piezoelectric material of the receiving piezoelectric resonator
has a high voltage output coefficient g.sub.33, and a high
longitudinal wave sound velocity.
37. The ultrasonic transducer according to claim 36, wherein the
material having the high voltage output coefficient g.sub.33 and
the high longitudinal wave sound velocity is a piezoelectric single
crystal represented by a chemical formula
K(Nb.sub.1-xTa.sub.x)O.sub.3, 0.ltoreq.x .ltoreq.0.2.
38. The ultrasonic transducer according to claim 36, wherein the
material having the high voltage output coefficient g.sub.33 and
the high longitudinal wave sound velocity is a lead titanate based
piezoelectric ceramic.
39. The ultrasonic transducer according to claim 36, wherein the
material having the high voltage output coefficient g.sub.33 and
the high longitudinal wave sound velocity is a bismuth layer
structure ferroelectric material (BLSF) represented by a chemical
formula Bi.sub.4Ti.sub.3O.sub.12 or
Ma.sub.1-xMb.sub.xBi.sub.2McO.sub.8, 0.ltoreq.x .ltoreq.0.2, in
which Ma and Mb are alkaline earth metal elements such as Sr and
Ba, and Mc is a +5 valence metal element such as Ta and Nb.
40. The ultrasonic transducer according to claim 34, wherein said
receiving piezoelectric resonator comprises an energy trapped
electrode.
41. The ultrasonic transducer according to claim 34, wherein said
receiving ultrasonic vibrator comprises a backing layer disposed on
the back surface of the receiving piezoelectric resonator, and a
material of the backing layer has an ultrasonic attenuation ratio
larger than 5 dB/cm/MHz, and an acoustic impedance Zd which is 1/3
or less of an acoustic impedance Zp of the receiving piezoelectric
resonator.
42. An ultrasonic transducer system for harmonic imaging,
comprising: an ultrasonic transducer comprising a transmitting
ultrasonic vibrator for transmitting a fundamental ultrasound
having a center frequency f.sub.0 in response to input of an
electric signal and a receiving ultrasonic vibrator for receiving a
harmonic signal having a center frequency nf.sub.0 (n is an integer
of 2 or more) generated in an object by the fundamental ultrasound,
said transmitting ultrasonic vibrator comprising a transmitting
piezoelectric resonator, said receiving ultrasonic vibrator
comprising a receiving piezoelectric resonator, said transmitting
piezoelectric resonator and said receiving piezoelectric resonator
being disposed on the same plane, said transmitting piezoelectric
resonator and said receiving piezoelectric resonator satisfying
(g.sub.33r.multidot.V.sub.r.multidot.Q.sub.r)/(g.sub.33t.multi-
dot.V.sub.t.multidot.Q.sub.t).gtoreq.n.multidot.(1+R), in which
g.sub.33t and V.sub.t denote a voltage output coefficient and a
sound velocity of the transmitting piezoelectric resonator,
g.sub.33r and V.sub.r denote a voltage output coefficient and a
sound velocity of the receiving piezoelectric resonator, n denotes
a harmonic order, R denotes an opening area ratio (an opening area
of the receiving piezoelectric resonator/an opening area of the
transmitting piezoelectric resonator), and Q.sub.t and Q.sub.r
denote resonance sharpness of the transmitting ultrasonic vibrator
and the receiving ultrasonic vibrator, respectively; and drive
control means for driving/controlling the ultrasonic transducer,
wherein said drive control means generates an ultrasound in which
at least a component of 2f.sub.0 is inhibited in the transmitting
ultrasonic vibrator.
43. The ultrasonic transducer system according to claim 42, wherein
said drive control means supplies a drive pulse signal having a
frequency characteristic such that the center frequency is in
f.sub.0 and a first dip frequency is in 2f.sub.0 to the
transmitting ultrasonic vibrator.
44. The ultrasonic transducer system according to claim 43, wherein
said drive control means supplies the drive pulse signal of a burst
wave to the transmitting ultrasonic vibrator.
45. The ultrasonic transducer system according to claim 42, wherein
said transmitting ultrasonic vibrator comprises means for imparting
a functionally gradient characteristic concerning at least one of a
piezoelectric constant and a permittivity to the transmitting
piezoelectric resonator.
46. The ultrasonic transducer system according to claim 45, wherein
said means for imparting the functionally gradient characteristic
comprises a heater for imparting a temperature gradient to the
transmitting piezoelectric resonator along a thickness direction of
the transmitting piezoelectric resonator.
47. The ultrasonic transducer system according to claim 42, wherein
said transmitting piezoelectric resonator has an functionally
gradient characteristic in at least one of a piezoelectric constant
and a permittivity.
48. The ultrasonic transducer system according to claim 47, wherein
said transmitting piezoelectric resonator has an inclination
piezoelectric material in which at least one of the piezoelectric
constant and the permittivity monotonously changes along a
thickness direction.
49. The ultrasonic transducer system according to claim 47, wherein
said transmitting piezoelectric resonator comprises a plurality of
piezoelectric thin plates being laminated on one another and having
the functionally gradient characteristic in which at least one of
the piezoelectric constant and the permittivity gradually differs.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is based upon and claims the benefit of
priority from the prior Japanese Patent Applications No.
2000-072854, filed Mar. 15, 2000; and No. 2001-048579, filed Feb.
23, 2001, the entire contents of which are incorporated herein by
reference.
BACKGROUND OF THE INVENTION
[0002] The present invention relates to an ultrasonic transducer
and ultrasonic transducer system for use in harmonic imaging
ultrasonic diagnosis, and particularly to an ultrasonic transducer
which transmits a fundamental ultrasound having a center frequency
f.sub.0, and detects the reflected ultrasound having a center
frequency nf.sub.0 (n: integer of 2 or more), generated by the
propagation of the fundamental ultrasound.
[0003] In recent years, harmonic imaging ultrasonic diagnosis has
attracted attention. A diagnosis method is roughly classified into
a contrast harmonic imaging using a contrast medium, and tissue
harmonic imaging of detecting the non-linearity of an elastic
property of a living tissue, and displaying the non-linearity in an
image. The situation is described in detail in "special issue on
electronics clinical medicine ultrasound--Latest Ultrasound:
distributed text of 1999 academic lecture by the Japan Society of
Ultrasound in Medicine".
[0004] The tissue harmonic imaging is a technique of transmitting
an ultrasonic pulse having a center frequency f.sub.0 to a living
tissue without using the ultrasonic contrast medium, extracting a
high order harmonic component nf.sub.0 (n being an integer of 2 or
more) included in a returned echo signal, and displaying a relation
between an amplitude of the component and an echo signal receiving
time in a tomographic image to obtain a diagnosis image.
[0005] For an in vitro purpose, a diagnosis apparatus with the
aforementioned function mounted thereon is already on the market.
In the tissue harmonic imaging diagnosis method, heart structures
such as a left chamber wall can be relatively clearly observed,
even in an overweight person, an aged person or a person who
smokes, whose echo image has been frequently blurred because of
mixed noise.
[0006] The ultrasonic diagnosis method is at present used only for
the in vitro purpose, and a second order high harmonic wave (n=2),
that is, the ultrasound having a center frequency of 2f.sub.0 is
used. In a conventional ultrasonic transducer, transmission of the
ultrasound having a center frequency f.sub.0 and reception of the
ultrasound having a center frequency 2f.sub.0 are performed by the
same ultrasonic vibrator. Therefore, the ultrasonic vibrator used
needs to have a remarkably broad band.
[0007] Moreover, to further enhance the resolution, utilization of
a third order harmonic signal is expected, but an ultra-broad band
ultrasonic vibrator which can detect an ultrasound having a center
frequency of 3f.sub.0, that is, a third order harmonic signal has
not been realized yet.
[0008] It is usually said that the sensitivity of a second order
harmonic signal is deteriorated by 15 to 20 dB, and a third order
harmonic signal is further deteriorated by 15 to 20 dB as compared
with the fundamental frequency signal. Therefore, the
aforementioned sensitivity deterioration with the broadened band
disadvantageously causes further deterioration of the diagnosis
image.
[0009] Furthermore, since the transmission of the ultrasound having
the center frequency f.sub.0 and the reception of the ultrasound
having the center frequency 2f.sub.0 are performed by the same
ultrasonic vibrator, a fundamental wave and various unnecessary
vibrations are unavoidably superimposed onto a received ultrasonic
signal.
[0010] To improve such disadvantages, Jpn. Pat. Appln. KOKAI
Publication No. 11-155863 discloses an ultrasonic transducer which
has a transmitting piezoelectric resonator and receiving
piezoelectric resonator in one case which can efficiently receive
the high order harmonic component. A constitution of the ultrasonic
transducer is shown in FIG. 31.
[0011] As shown in FIG. 31, an ultrasonic transducer 1000 has a
transmitting piezoelectric resonator 1002, and a receiving polymer
piezoelectric resonator 1004 disposed in front of the transmitting
piezoelectric resonator. The receiving polymer piezoelectric
resonator 1004 and transmitting piezoelectric resonator 1002 are
layered and disposed via an acoustic matching layer 1006.
[0012] Front electrodes of the transmitting piezoelectric resonator
1002 and receiving polymer piezoelectric resonator 1004 are both
connected to a grounding lead wire 1008 and are kept at a ground
potential. A back-side electrode of the transmitting piezoelectric
resonator 1002 is connected to a transmitting shielding wire 1010,
and a drive signal is supplied via the wire. A back-side electrode
of the receiving polymer piezoelectric resonator 1004 is connected
to a receiving shielding wire 1012, and a received signal is
extracted via the wire.
[0013] The transmitting piezoelectric resonator 1002 has a resonant
frequency or an antiresonant frequency which agrees with a resonant
frequency of the ultrasonic contrast medium or a frequency having a
specific relation with respect to the ultrasonic contrast medium.
On the other hand, the receiving polymer piezoelectric resonator
1004 is a non-resonating piezoelectric resonator, and can receive
even the high order harmonic component generated based on the
nonlinear behavior of the ultrasonic contrast medium.
[0014] Since the acoustic matching layer 1006 is disposed between
the transmitting piezoelectric resonator 1002 and the receiving
polymer piezoelectric resonator 1004 in the ultrasonic transducer
1000, only a portion with the ultrasonic contrast medium present
therein, such as a blood vessel in a human body and a cancer tissue
with capillary concentrated on a peripheral portion thereof, can be
depicted more clearly than other portions.
[0015] Since the ultrasonic transducer 1000 has separate
transmitting and receiving piezoelectric resonators, the band is
easily broadened, and properties suitable for harmonic imaging are
expected to be displayed, as compared with the conventional
ultrasonic transducer for general use for performing
transmission/reception with the single piezoelectric resonator.
[0016] However, in the conventional ultrasonic transducer shown in
FIG. 31, the transmitting and receiving ultrasonic vibrators are
superposed and disposed. Therefore, when a transmitted ultrasonic
wave is passed through the receiving ultrasonic vibrator, the
ultrasonic wave excites the receiving ultrasonic vibrator and is
modulated by the vibration. As a result, undesired vibration of the
resonant frequency of a receiving ultrasonic vibrator film is mixed
in with the transmitted ultrasonic wave. This means that it is
impossible to judge whether the signal detected by the receiving
ultrasonic vibrator is the high order harmonic signal from the
ultrasonic contrast medium or the signal mixed during transmission.
Therefore, the mixture of the undesired vibration causes a large
deterioration of the resolution.
[0017] Moreover, for use in a so-called tissue harmonic imaging
(THI) for detecting a nonlinear ultrasonic wave generated with
propagation of the fundamental ultrasonic wave in the living
tissue, the high order harmonic wave needs to be securely selected
and detected, because a sound pressure level of the nonlinear
ultrasonic wave generated with the propagation of the fundamental
ultrasound in the living tissue is as small as about -20 dB, as is
well known. However, in the conventional ultrasonic transducer
shown in FIG. 31, since the receiving ultrasonic vibrator has a
non-resonating broad-band property, also for the received signal,
the high order harmonic signal level is -20 dB lower with respect
to the fundamental wave and such a situation is unchanged.
BRIEF SUMMARY OF THE INVENTION
[0018] An object of the present invention is to provide a technique
of an ultrasonic transducer which has a transmitting piezoelectric
resonator and receiving piezoelectric resonator contained in the
same case, but which can detect a harmonic signal with a high
sensitivity without being adversely affected by resolution
deterioration caused by residual vibration.
[0019] Additional objects and advantages of the invention will be
set forth in the description which follows, and in part will be
obvious from the description, or may be learned by practice of the
invention. The objects and advantages of the invention may be
realized and obtained by means of the instrumentalities and
combinations particularly pointed out hereinafter.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING
[0020] The accompanying drawings, which are incorporated in and
constitute a part of the specification, illustrate presently
preferred embodiments of the invention, and together with the
general description given above and the detailed description of the
preferred embodiments given below, serve to explain the principles
of the invention.
[0021] FIG. 1 shows a side section of an ultrasonic transducer in a
first embodiment.
[0022] FIG. 2 shows a side section of a first modification of the
ultrasonic transducer in the first embodiment.
[0023] FIG. 3 shows a side section of a second modification of the
ultrasonic transducer in the first embodiment.
[0024] FIG. 4 shows a side section of a third modification of the
ultrasonic transducer in the first embodiment.
[0025] FIG. 5A is a graph showing a relation between a curvature
radius of an acoustic lens disposed in front of a disc
piezoelectric resonator, and a focal length F in an ultrasonic
propagation medium of water, and
[0026] FIG. 5B is a graph showing a relation between a focus and
the acoustic lens curvature radius for 5 MHz and 10 MHz.
[0027] FIG. 6A schematically shows a constitution of a control
system for controlling transmission/reception of the ultrasonic
transducer shown in FIG. 1, and
[0028] FIG. 6B shows a timing chart of control signals V.sub.t and
V.sub.r inputted to an on/off control device and selector shown in
FIG. 6A.
[0029] FIG. 7A schematically shows a constitution of a modification
of the control system for controlling transmission/reception of the
ultrasonic transducer shown in FIG. 1, and
[0030] FIG. 7B shows a timing chart of the control signals V.sub.t
and V.sub.r inputted to the on/off control device shown in FIG.
7A.
[0031] FIG. 8 shows a listing of simulation results obtained by
calculating an effect of a surface charge generated in the
piezoelectric resonator through which an ultrasound is
transmitted.
[0032] FIG. 9A shows a side section of the ultrasonic transducer of
a second embodiment, and
[0033] FIG. 9B shows an enlarged middle portion of the acoustic
lens shown in FIG. 9A.
[0034] FIG. 10 shows a side section of a first modification of the
ultrasonic transducer of the second embodiment.
[0035] FIG. 11 shows a side section of a second modification of the
ultrasonic transducer of the second embodiment.
[0036] FIG. 12 shows a side section of a third modification of the
ultrasonic transducer of the second embodiment.
[0037] FIG. 13A shows an upper surface of a fourth modification of
the ultrasonic transducer of the second embodiment, and
[0038] FIG. 13B shows a side section of the ultrasonic transducer
taken along line 13B-13B of FIG. 13A.
[0039] FIG. 14 is a sectional view of an in-plane separated
ultrasonic transducer of a third embodiment.
[0040] FIG. 15A is a front view showing models of the transmitting
piezoelectric resonator and receiving piezoelectric resonator shown
in FIG. 14, and
[0041] FIG. 15B is a sectional view taken along line 15B-15B of
FIG. 15A.
[0042] FIG. 16A is a front view of the piezoelectric resonator of
the ultrasonic transducer for detecting only a fundamental
ultrasound as a comparison object of the third embodiment, and
[0043] FIG. 16B is a sectional view taken along line 16B-16B of
FIG. 16A.
[0044] FIG. 17A is a front view showing models of the transmitting
piezoelectric resonator and receiving piezoelectric resonator in
the in-plane separated ultrasonic transducer according to the
modification of the third embodiment, and
[0045] FIG. 17B is a sectional view taken along line 17B-17B of
FIG. 17A.
[0046] FIG. 18 is a plan view of the transmitting piezoelectric
resonator having an energy trapped electrode structure according to
the modification of the third embodiment.
[0047] FIG. 19 is a diagram corresponding to a portion surrounded
by a dashed line of FIG. 18, and shows a layout of an electrode
plate on a front side of the energy trapped electrode
structure.
[0048] FIG. 20 is a diagram corresponding to the portion surrounded
by the dashed line of FIG. 18, and shows a layout of the electrode
plate on a back side of the energy trapped electrode structure.
[0049] FIG. 21 shows an ultrasonic transducer system including the
ultrasonic transducer of FIG. 14 according to a fourth
embodiment.
[0050] FIG. 22A shows a drive voltage waveform of a spike wave as
one example of a drive signal supplied to the transmitting
piezoelectric resonator, and
[0051] FIG. 22B shows a frequency characteristic of the wave.
[0052] FIG. 23 shows a change of a first dip frequency for a fall
time with respect to the spike wave shown in FIG. 22A.
[0053] FIG. 24A shows the drive voltage waveform of a trapezoidal
wave as another example of the drive signal supplied to the
transmitting piezoelectric resonator, and
[0054] FIG. 24B shows the frequency characteristic of the wave.
[0055] FIG. 25A shows the drive voltage waveform of a burst wave as
still another example of the drive signal supplied to the
transmitting piezoelectric resonator, and
[0056] FIG. 25B shows the frequency characteristic of the wave.
[0057] FIG. 26 shows the change of the first dip frequency with
respect to the burst wavelength for the spike wave shown in FIG.
25A.
[0058] FIG. 27 is a sectional view of the in-plane separated
ultrasonic transducer of a fifth embodiment.
[0059] FIG. 28 shows an impedance characteristic of the
piezoelectric resonator having a functionally gradient
characteristic in a piezoelectric constant and the piezoelectric
resonator having no functionally gradient characteristic.
[0060] FIG. 29 is a partial sectional view of the functionally
gradient type piezoelectric resonator replaced with the ultrasonic
transducer transmitting piezoelectric resonator of FIG. 14 in the
modification of the fifth embodiment.
[0061] FIG. 30 shows the whole function characteristic of the
functionally gradient type piezoelectric resonator of FIG. 29.
[0062] FIG. 31 shows a conventional ultrasonic transducer having
transmitting and receiving piezoelectric resonators superposed onto
each other.
DETAILED DESCRIPTION OF THE INVENTION
[0063] [First Embodiment]
[0064] According to a first embodiment, there is provided an
ultrasonic transducer system suitable for harmonic imaging
ultrasonic diagnosis. The system includes an ultrasonic transducer
and a control system of the ultrasonic transducer. First the
ultrasonic transducer and next the control system will be described
hereinafter.
[0065] As shown in FIG. 1, the ultrasonic transducer has a
transmitting piezoelectric resonator 102, receiving piezoelectric
resonator 104, housing 106 for containing these piezoelectric
resonators 102, 104, and acoustic lens 108.
[0066] The transmitting piezoelectric resonator 102 has a
piezoelectric material such as lead zirconium titanate (PZT),
bismuth layer structure and another piezoelectric ceramic material,
and crystal, lithium niobate, PZT and another single crystal
piezoelectric material. The transmitting piezoelectric resonator
also has a pair of electrodes disposed opposite each other via the
piezoelectric material.
[0067] The transmitting piezoelectric resonator 102 and receiving
piezoelectric resonator 104 are layered via an acoustic matching
layer 110, that is, laminated and disposed. A damping layer
(backing layer) 112 is disposed on a back surface of the
transmitting piezoelectric resonator 102.
[0068] In such a laminate structure, the transmitting piezoelectric
resonator 102, acoustic lens 108 and backing layer 112 constitute a
transmitting ultrasonic vibrator, and the receiving piezoelectric
resonator 104, acoustic lens 108 and backing layer 112 constitute a
receiving ultrasonic vibrator. The transmitting and receiving
ultrasonic vibrators include the acoustic lens 108 and backing
layer 112 in common.
[0069] The laminate structure including the transmitting
piezoelectric resonator 102, receiving piezoelectric resonator 104,
acoustic lens 108 and backing layer 112 is fixed into the housing
106 via an insulating layer 114 lined on an inner surface of the
housing 106. The insulating layer 114 insulates the electrodes of
the piezoelectric resonators 102, 104. The acoustic lens 108 is
disposed in front of the receiving piezoelectric resonator 104.
[0070] The electrode on an ultrasonic emission side of the
transmitting piezoelectric resonator 102 and the electrode on a
side opposite to the ultrasonic emission side of the receiving
piezoelectric resonator 104 are electrically connected to the
housing 106 via a wiring 116, and are held at the same potential as
that of the housing 106. A two-core coaxial cable 118 has a lead
wire 122 electrically connected to the electrode on the side
opposite to the ultrasonic emission side of the transmitting
piezoelectric resonator 102, a lead wire 124 electrically connected
to the electrode on the ultrasonic emission side of the receiving
piezoelectric resonator 104, and a shielding wire 120 electrically
connected to the housing 106.
[0071] The transmitting piezoelectric resonator 102 has a resonant
frequency f.sub.0, and the receiving piezoelectric resonator 104
has a resonant frequency nf.sub.0 (n being an integer of 2 or
more). For example, the transmitting piezoelectric resonator 102
has a resonant frequency of 5 MHz, and the receiving piezoelectric
resonator 104 has a resonant frequency of 10 MHz. The resonant
frequencies of the piezoelectric resonators 102, 104 can be
adjusted by controlling a vibrator thickness.
[0072] The receiving piezoelectric resonator 104 is formed by a
method of bonding a piezoelectric polymeric film having a polarized
state with an adhesive beforehand. However, the transmitting
piezoelectric resonator is sometimes cracked or damaged by a
bonding pressure, influence of a bubble, or the influence of an
adhesive layer. There is also a case in which the designed
properties cannot steadily be obtained due to the adhesive layer
having a non-uniform thickness. In this case, it is preferable to
form a piezoelectric polymer layer in which surface energy poling
is possible. This method is described in detail in document "Junya
IDE et al: Jpn. J. Appl. Phys. Vol. 38 (1999) pp. 2049 to 2052",
which is incorporated herein by reference. This method includes:
forming a polymeric material, such as polycyanophenyl sulfide, into
a film on the electrode formed on the surface of the acoustic
matching layer 110, and forming an upper electrode after the film
is cured. After the film is formed, the polarized state can be
spontaneously realized by a surface energy effect without
especially performing a polarizing treatment. Since the film can be
formed by spin coating or the like, a target structure can be
easily realized as compared with the method of bonding the
piezoelectric polymeric film by adhesive. The polymeric material is
dropped on the electrode (not shown) formed on the surface of the
acoustic matching layer 110, the electrode is spin-coated at an
appropriate revolution number, the upper electrode is further
formed after the film is cured, and the receiving piezoelectric
resonator 104 is thus formed.
[0073] The acoustic lens 108 has an acoustic opening surface having
a concave surface shape. The opening surface has a constant
curvature radius. That is, the opening surface has a part of a
spherical surface.
[0074] Setting of the curvature radius of the spherical surface of
the acoustic lens 108 will next be described with reference to FIG.
5A and FIG. 5B. FIG. 5A shows a relation between a curvature radius
R of the acoustic lens disposed in front of a disc piezoelectric
resonator, and a focal length F in an ultrasonic propagation medium
of water. Here, the abscissa indicates D (=a.sup.2/.lambda.R', a:
opening radius, .lambda.: wavelength in the material in which the
ultrasonic wave is propagated, R': R'=2.25 R at a lens apparent
curvature radius, R: processing curvature radius of the acoustic
lens), and the ordinate indicates F/R'. This is a relation derived
from a well-known Rayleigh equation.
[0075] FIG. 5B further shows a relation between the focal length
and an actual processing curvature radius of the acoustic lens for
5 MHz and 10 MHz. It is seen from FIG. 5B that to adjust a focus in
the same position, for example, a 30 mm position in any frequency,
different curvature radii, for example, 16 mm for 10 MHz and 40 mm
for 5 MHz are preferably set.
[0076] However, since the opening surface for
transmitting/receiving the ultrasonic wave is actually shared, the
curvature radius of the opening surface of the acoustic lens 108
has an intermediate value or an average value between these
curvature radii, for example, a value of 25 mm, and an optimum
focus image forming state is realized. In other words, the
curvature radius of the opening surface of the acoustic lens 108
has a value equal to an average value of an acoustic focal length
for the ultrasound having a center frequency f.sub.0 and an
acoustic focal length for the ultrasound having a center frequency
nf.sub.0 (n being an integer of 2 or more).
[0077] Additionally, the opening surface of the acoustic lens 108
may have a partially different curvature radius. For example, the
curvature radius of an opening surface center portion may have a
value optimum for reception, that is, a value equal to the acoustic
focal length for the ultrasound having the center frequency
nf.sub.0, and the curvature radius of a peripheral portion may have
a value optimum for transmission, that is, a value equal to the
acoustic focal length for the ultrasound having the center
frequency f.sub.0 (vice versa).
[0078] A case in which a harmonic signal of 10 MHz is generated
during transmission has been described above, but in actuality the
harmonic signal is gradually generated with a propagation distance
of the ultrasonic wave by non-linearity of an elastic coefficient
of an organism. Therefore, the focus of the harmonic received
signal is not so remote as described above. However, since the
signal focus becomes far from a focus in a fundamental frequency
with a degree of the harmonic signal, deviation of both focuses
accordingly increases, and improvement of an ultrasonic image
resolution by harmonics imaging is eliminated.
[0079] A control system of transmission/reception of the
aforementioned ultrasonic transducer will next be described with
reference to FIG. 6A and FIG. 6B.
[0080] FIG. 6A schematically shows a constitution of the control
system. As shown in FIG. 6A, the control system has an on/off
control device 150 for controlling the transmitting piezoelectric
resonator 102, and a selector 160 for controlling a signal flow of
the receiving piezoelectric resonator 104. The on/off control
device 150 supplies a high voltage V.sub.d supplied via a terminal
152 to the transmitting piezoelectric resonator 102 in response to
a control signal V.sub.t inputted via a terminal 154. The selector
160 leads the received signal of the receiving piezoelectric
resonator 104 to either a branch 164 connected to an amplifier or a
grounded branch 166 in response to a control signal V.sub.r
inputted via a terminal 162.
[0081] FIG. 6B shows a timing chart of the control signals V.sub.t
and V.sub.r inputted to the on/off control device 150 and selector
160. It is seen from the timing chart that at a period t.sub.3, a
pulse with a pulse width t.sub.1 is inputted to the terminal 154 of
the on/off control device 150, and a pulse with a pulse width
t.sub.2 (>t.sub.1) is inputted to the terminal 162 of the
selector 160.
[0082] In response to an input of the pulse with the pulse width
t.sub.1, a transmission ultrasonic wave is transmitted from the
transmitting ultrasonic transducer. This transmission ultrasonic
wave periodically generates a surface charge with a polarity which
restricts deformation in a receiving ultrasonic transducer when the
ultrasonic wave is transmitted through the receiving ultrasonic
transducer disposed in front of the transmitting ultrasonic
transducer.
[0083] This charge generates an electric field in the piezoelectric
resonator in a direction in which a change of the polarized state
is restricted, and a state in which mechanical displacement does
not easily occur, that is, a stiff state is induced by an inverse
piezoelectric effect. On the other hand, when the surface charge is
discharged by an external circuit, the electric field of the
direction restricting the polarized state change is not generated.
As a result, the stiff state is not induced.
[0084] That is, a difference appears in stiffness of the
piezoelectric resonator with a way of processing the charge
generated between the electrodes of the piezoelectric resonator.
This phenomenon is a peculiar phenomenon which occurs in the
piezoelectric resonator. The stiffness of the piezoelectric
resonator is generally represented by c.sup.E (electric field 0)
and c.sup.D (electric displacement 0). This is a well known
phenomenon having the following relation:
c.sup.E=(1-K.sup.2)c.sup.D(K: electromechanical coupling
coefficient)
[0085] When the piezoelectric resonator with the ultrasound
transmitted therethrough is sufficiently thick compared to the
wavelength of ultrasounds, generation of a surface charge has
little influence. However, a large influence is exerted when the
thickness of the piezoelectric resonator is of the order of
1/4.lambda. as in the present embodiment.
[0086] FIG. 8 lists simulation results obtained by calculating the
influence. In FIG. 8, rows indicate only a transmitted ultrasonic
pulse (left row), only a received ultrasonic pulse (middle row),
and a total transmitted and received ultrasonic pulse (right row),
and lines indicate a charge processing state between the electrodes
of the receiving piezoelectric resonator 104 during transmission,
and a charge processing state between the electrodes of the
transmitting piezoelectric resonator 102 during reception.
[0087] That is, (a1), (a2), (a3) indicate the transmitted
ultrasonic pulse, received ultrasonic pulse, and actual ultrasonic
pulse while the electrodes of the receiving piezoelectric resonator
104/transmitting piezoelectric resonator 102 are in a short/short
state. Similarly, (b1), (b2), (b3) indicate the transmitted
ultrasonic pulse, received ultrasonic pulse, and actual ultrasonic
pulse in a short/open state, (c1), (c2), (c3) indicate the pulses
in an open/open state, and (d1), (d2), d3) indicate the pulse in an
open/short state.
[0088] It is apparent from FIG. 8 that a residual vibration appears
from (a2) and (d2) in short/short and open/short states, and this
method cannot be said to be a preferable charge processing method
between the electrodes. Additionally, the short state is a low
resistance state including short circuit, and the open state is a
high resistance sate including open circuit.
[0089] Characteristic values of pulse waveforms, that is, a pulse
maximum amplitude (Vpp), center frequency (CF) and -20 dB pulse
width (PW) are shown in Table 1.
1 TABLE 1 Transmitter/receiver Transmitter property Receiver
property total property Transmitter Receiver Force CF PW Vpp CF PW
Vpp CF PW terminal terminal N MHz .mu.s V MHz .mu.s V MHz .mu.s 1
Short Short 7 .times. 10.sup.4 7.44 0.219 0.078 9.91 0.234 0.647
7.81 0.358 2 Short Open 13 .times. 10.sup.4 9.58 0.195 0.086 8.19
0.266 1.89 8.63 0.234 3 Open Open 17 .times. 10.sup.4 9.91 0.328
0.069 7.44 0.297 1.862 9.16 0.398 4 Open Short 17 .times. 10.sup.4
9.91 0.329 0.09 10.59 0.258 1.96 10.13 0.258
[0090] Based on a general view that a large Vpp and small PW of the
transmitter/receiver total property result in a broad band and high
sensitivity, it is seen from the table that the charge processing
method between the electrodes in the short/open state is most
preferable. That is, it is most preferable to control the state
between the electrodes of the receiving piezoelectric resonator 104
to be short during transmission of the fundamental ultrasound, and
to control the state between the electrodes of the transmitting
piezoelectric resonator 102 to be open during receiver. This
phenomenon is basically different in principle from an effect
obtained by disposing the acoustic matching layer 110 in a boundary
of the transmitting piezoelectric resonator 102 and receiving
piezoelectric resonator 104.
[0091] Operation of the present embodiment will next be described
with reference to FIG. 6A and FIG. 6B.
[0092] In the on/off control device 150, a direct-current voltage
V.sub.d is supplied to the terminal 152, and the control signal
V.sub.t with a controlled pulse width is inputted to the terminal
154. The control signal V.sub.t is a rectangular wave, impulse
wave, or the like, and is, for example, a rectangular wave having a
period t.sub.3 and pulse width t.sub.1 as shown in FIG. 6B. The
transmitting piezoelectric resonator 102 generates an ultrasonic
pulse 174 for a pulse waveform of the inputted control signal
V.sub.t.
[0093] The ultrasonic pulse 174 has a center frequency f.sub.0, and
is propagated in a living tissue 170 and reflected by an
acoustically discontinuous boundary surface 172. This pulse forms
an echo signal 176 including a relatively large amount of harmonic
signals having a frequency nf.sub.0 (n being an integer of 2 or
more) due to non-linearity of an elastic property of the organism,
and the echo signal is received by the receiving piezoelectric
resonator 104. Moreover, when a non-linear medium (contrast medium)
is injected beforehand in the living tissue 170, the transmitted
ultrasonic pulse 174 forms an echo signal including a large amount
of harmonic components generated from the non-linear medium. Since
the receiving piezoelectric resonator 104 has a resonant frequency
having frequency nf.sub.0 (n being an integer of 2 or more) as the
center frequency, the harmonic signal is selectively received and
converted to an electric signal.
[0094] The selector 160 leads the received signal of the receiving
piezoelectric resonator 104 to either the branch 164 or the branch
166 in response to the control signal V.sub.r inputted to the
terminal 162. The control signal V.sub.r is synchronized with the
control signal V.sub.t which is a rectangular wave having a pulse
width t.sub.2 with the same period as the period t.sub.3 of the
control signal V.sub.t. The selector 160 leads the received signal
to the grounded branch 166 for time t.sub.2 corresponding to "H",
and leads the received signal to the branch 164 connected to a
subsequent signal processor such as anamplifier for time
t.sub.4=t.sub.3-t.sub.2 corresponding to "L".
[0095] The pulse width t.sub.2 of the control signal V.sub.r is set
to be longer than the pulse width t.sub.1 of the control signal
V.sub.t, and this corresponds to a time when the ultrasonic wave
generated in at least the transmitting piezoelectric resonator 102
is completely transmitted through the receiving piezoelectric
resonator 104.
[0096] The control signal V.sub.r inputted to the terminal 162
changes to "L" at a timing at which the echo signal 176 is received
by the receiving piezoelectric resonator 104, and a received signal
V.sub.out from the receiving piezoelectric resonator 104 is led to
the subsequent signal processor such as the amplifier.
[0097] As described above, it is preferable to control the state
between the electrodes of the transmitting piezoelectric resonator
102 during reception to be an open state or a nearly open state.
While the echo signal 176 is received by the receiving
piezoelectric resonator 104, the control signal V.sub.r is of "L",
and the state between the electrodes of the transmitting
piezoelectric resonator 102 is kept to be substantially in the open
state.
[0098] In the control, instead of transformer coupling, a control
device in which output resistance is small in an on state and large
in an off state and a large output voltage can be obtained is
preferably used in a final stage of a transmission drive circuit.
For example, a high-speed power MOSFET whose output voltage is
large is suitable for the control device.
[0099] Additionally, since a polymeric piezoelectric material
constituting the receiving piezoelectric resonator 104 has a large
voltage output coefficient g.sub.33, reception sensitivity is high.
On the other hand, since a mechanical quality factor Qm is small, a
selection property for the received frequency is small. Then, in
order to improve the selection property, inductance is effectively
connected in parallel with the receiving piezoelectric resonator
104 in some case. Moreover, since a composite piezoelectric
material has a mechanical quality factor Qm larger than the
mechanical quality factor Qm of the polymeric piezoelectric
material, and has a relatively large selection property, the
composite piezoelectric material is further preferable.
[0100] Modifications of the present embodiment will be described
hereinafter with reference to the drawings. In the drawings,
members equivalent to the aforementioned members are denoted with
the same reference numerals, and a detailed description thereof is
omitted to avoid redundancy in the following description.
[0101] FIG. 2 shows a first modification of the ultrasonic
transducer. The ultrasonic transducer of the present modification
has an acoustic lens 132 having a convex opening surface, and a
buffer layer 134 disposed between the acoustic lens 132 and the
receiving piezoelectric resonator 104. The acoustic lens 132 having
the convex opening surface is suitable for a case in which a sound
velocity of a lens material is lower than a sound velocity of 1500
m/sec of the living tissue. The buffer layer 134 improves a bonding
property between the acoustic lens 132 and the receiving
piezoelectric resonator 104.
[0102] In the present modification, the acoustic matching layer 110
in FIG. 1 is not disposed between the transmitting piezoelectric
resonator 102 and the receiving piezoelectric resonator 104, but
the acoustic matching layer is further preferably disposed between
the transmitting piezoelectric resonator 102 and the receiving
piezoelectric resonator 104. Since a silicone resin for use as a
material of the acoustic lens 132 generally has a bad adhesion to
another resin material, the acoustic lens 132 may be bonded to the
receiving piezoelectric resonator 104 after heating and bonding a
polyimide resin film as the buffer layer beforehand.
[0103] FIG. 3 shows a second modification of the ultrasonic
transducer. The ultrasonic transducer of the present modification
does not have the acoustic lens, and an only insulating layer 136
is formed in front of the receiving piezoelectric resonator 104.
The ultrasonic transducer is suitable for a use in which a
transverse resolution does not matter very much, and this
ultrasonic transducer can advantageously be presented
inexpensively. Also in the present modification example, it is
further preferable to dispose the acoustic matching layer between
the transmitting piezoelectric resonator 102 and the receiving
piezoelectric resonator 104.
[0104] FIG. 4 shows a third modification of the ultrasonic
transducer. In the ultrasonic transducer of the present
modification example, a transmitting piezoelectric resonator 138,
receiving piezoelectric resonator 140 and insulating layer 142 are
bent in a concave shape. By this structural characteristic, the
ultrasonic transducer can converge the ultrasound without any
acoustic lens. Also in the present modification example, it is
further preferable to dispose the acoustic matching layer between
the transmitting piezoelectric resonator 138 and the receiving
piezoelectric resonator 140.
[0105] FIG. 7A schematically shows a constitution of a modification
of the control system, and FIG. 7B shows a timing chart of the
control signal V.sub.t.
[0106] In the control system, a controller of the receiving
piezoelectric resonator 104 does not include the selector 160, and
instead includes a transformer 180 connected in parallel, and a
capacitor 182 connected on a secondary side of a transformer 180.
Another constitution is the same as that of the control system
shown in FIG. 6A.
[0107] For the transformer 180, the inductance on the primary side
is small, and during transmission, impedance .omega.L (L is a
primary-side inductance of the transformer 180) for the frequency
f.sub.0 is set to an inductance value by which the short state is
substantially recognized. Moreover, the capacitor 182 has a
capacitance tuned to the frequency nf.sub.0 (n is an integer of 2
or more).
[0108] In the control system, among the received signals from the
receiving piezoelectric resonator 104, a pressure of only a
component of nf.sub.0 (n is an integer of 2 or more), that is, the
harmonic signal is selectively raised by a secondary-side tuning
circuit (transformer 180 and capacitor 182), and an output
V.sub.out is sent to the subsequent signal processor such as the
amplifier.
[0109] In the present embodiment, the ultrasonic transducer has
separate transmitting and receiving piezoelectric resonators which
are superposed and disposed in layers. The control system controls
and holds the state between the electrodes of the receiving
piezoelectric resonator in the short state or the nearly short
state during ultrasonic transmission for a specific time, and holds
the state between the electrodes of the transmitting piezoelectric
resonator in the open state or the nearly open state during
ultrasonic reception for the specific time. Thereby, since the
noise component by the residual vibration is eliminated, the
opening structure is the same, and the ultrasound is
transmitted/received in the whole opening area, reception of the
harmonic signal with a large output is possible.
[0110] Additionally, the constitution of the mechanical sector
scanning integral ultrasonic transducer has been described above as
the present embodiment, but the technique described in the present
embodiment can also be applied to an electronic scanning array
ultrasonic transducer, and is not limited to the mechanical sector
scanning integral ultrasonic transducer. Moreover, it is possible
to variously combine the basic forms and modification structures in
the present embodiment. Various constitutions of the harmonic
imaging ultrasonic transducer are possible in accordance with a
diagnosis object portion and diagnosis precision, and these
constitutions are also included as other modification examples of
the present embodiment.
[0111] [Second Embodiment]
[0112] According to a second embodiment, there is provided an
ultrasonic transducer suitable for harmonic imaging ultrasonic
diagnosis.
[0113] As shown in FIG. 9A and FIG. 9B, the ultrasonic transducer
includes a transmitting piezoelectric resonator 202, receiving
piezoelectric resonator 204, housing 206 for containing these
piezoelectric resonators 202, 204, and acoustic lens 208.
[0114] The transmitting piezoelectric resonator 202 has an annular
band shape, the receiving piezoelectric resonator 204 has a disc
shape, and the receiving piezoelectric resonator 204 is positioned
inside the transmitting piezoelectric resonator 202.
[0115] The transmitting piezoelectric resonator 202 has a
piezoelectric material such as polarized lead zirconium titanate
(PZT) ceramic, and a pair of electrodes disposed opposite each
other via the piezoelectric material. The receiving piezoelectric
resonator 204 has a piezoelectric material such as a composite
piezoelectric material formed of PZT having a composition other
than that of the transmitting piezoelectric resonator 202, lead
titanate (PbTiO.sub.3) ceramic, single-crystal piezoelectric
material (K(Ta.sub.xNb.sub.1-x)O.sub.3), or PZT and a resin, and a
pair of electrodes disposed opposite each other via the
piezoelectric material. The receiving piezoelectric resonator 204
has a voltage output coefficient g.sub.33 larger than that of the
transmitting piezoelectric resonator 202.
[0116] A damping layer 210 is disposed on a back surface of the
transmitting piezoelectric resonator 202, and a damping layer 212
is disposed on a back surface of the receiving piezoelectric
resonator 204. Since the frequency of the received ultrasound is an
integral multiple of the frequency of the transmitted ultrasound, a
damping degree of the damping layer 212 of the back surface of the
receiving piezoelectric resonator 204 is set to be weaker than the
damping degree of the damping layer 210 of the back surface of the
transmitting piezoelectric resonator 202.
[0117] Therefore, the damping layer 212 may be formed of a material
with a relatively small ultrasonic attenuation ratio. This is
because the damping layer 212 includes an insulating damping layer
with alumina dispersed in an epoxy resin, and an influence of
electric cross talk is effectively avoided via the damping
layer.
[0118] The acoustic lens 208, for example, of an epoxy resin is
disposed in front of the piezoelectric resonators 202, 204. The
acoustic lens 208 is imaginarily divided into a peripheral portion
positioned in front of the transmitting piezoelectric resonator 202
and a middle portion positioned in front of the receiving
piezoelectric resonator 204. The peripheral portions of the
transmitting piezoelectric resonator 202, damping layer 210 and
acoustic lens 208 constitute a transmitting ultrasonic vibrator,
and the middle portions of the receiving piezoelectric resonator
204, damping layer 212 and acoustic lens 208 constitute a receiving
ultrasonic vibrator. That is, both the transmitting and receiving
ultrasonic vibrators partially include the acoustic lens 208.
[0119] The peripheral and middle portions of the acoustic lens 208
have a concave opening surface. The opening surface of the
peripheral portion of the acoustic lens 208 has a curvature radius
R.sub.t, and the opening surface of the middle portion of the
acoustic lens 208 has a curvature radius R.sub.r smaller than the
curvature radius R.sub.t. That is, the acoustic lens 208 has an
opening surface having the curvature radius which partially
differs.
[0120] The curvature radius R.sub.t of the opening surface of the
peripheral portion of the acoustic lens 208 has a value equal to an
acoustic focal length F.sub.2 for the ultrasound having the center
frequency f.sub.0. Moreover, the curvature radius R.sub.r Of the
opening surface of the middle portion of the acoustic lens 208 has
a value equal to an acoustic focal length F.sub.1 for the
ultrasound having the center frequency nf.sub.0 (n is an integer of
2 or more). For the curvature radius R.sub.t of the opening surface
of the peripheral portion of the acoustic lens 208 and the
curvature radius R.sub.r of the opening surface of the middle
portion of the acoustic lens 208, values are preferably selected
such that respective focuses agree with each other.
[0121] Furthermore, the acoustic lens 208 preferably has an
acoustic matching function. That is, the peripheral portion of the
acoustic lens 208 has a thickness which is 1/4 of a wavelength
corresponding to the ultrasound with the center frequency f.sub.0
on average, and the middle portion of the acoustic lens 208 has a
thickness which is 1/40 of a wavelength corresponding to the
ultrasound with the center frequency nf.sub.0 on average. For
example, for the harmonic signal with the frequency 2f.sub.0, an
average thickness T.sub.r of the middle portion of the acoustic
lens 208 is preferably 1/2 of an average thickness T.sub.t of the
peripheral portion of the acoustic lens 208. Here, the average
thickness T.sub.r is a distance from the surface of the receiving
piezoelectric resonator 204 to a middle between a bottom of the
concave surface of the curvature radius R.sub.r and an upper end of
the concave surface of the curvature radius R.sub.r. Moreover, the
average thickness T.sub.t is a distance from the surface of the
transmitting piezoelectric resonator 202 to a middle between an
imaginary bottom of the curvature radius R.sub.t and the upper end
(i.e., a lens effective end with respect to the transmitting
piezoelectric resonator) of the concave surface of the curvature
radius R.sub.t.
[0122] When this condition and the agreement of the focuses are
both satisfied, a difference in thickness sometimes occurs in a
boundary of two lens portions different in curvature radius from
each other. In order to avoid differences in thickness, as shown in
FIG. 9B, the surface position of the receiving piezoelectric
resonator 204 projects upward from the surface position of the
transmitting piezoelectric resonator 202 by a length T.sub.d.
[0123] The structure including the transmitting piezoelectric
resonator 202, receiving piezoelectric resonator 204 and acoustic
lens 208 is fixed inside the housing 206 via an insulating layer
214. A front electrode of the receiving piezoelectric resonator 204
is connected to a front electrode of the transmitting piezoelectric
resonator 202 via a wiring 216, and a front electrode of the
transmitting piezoelectric resonator 202 is connected to the
housing 206 via a wiring 218. A two-core coaxial cable 220 has a
lead wire 222 connected to a back electrode of the transmitting
piezoelectric resonator 202, a lead wire 224 connected to a back
electrode of the receiving piezoelectric resonator 204, and a
shielding wire 226 connected to the housing 206.
[0124] The ultrasound with the center frequency f.sub.0 transmitted
from the transmitting piezoelectric resonator 202 is converged by
the acoustic lens 208, and focused in a position F.sub.2. The echo
signal propagated through the living tissue having a large
nonlinear effect, and including the harmonic signal is incident
upon the receiving piezoelectric resonator 204 having a resonant
frequency nf.sub.0 (n is an integer of 2 or more) via the acoustic
lens 208, and is converted to the electric signal.
[0125] With the receiving piezoelectric resonator 204 of a material
having the same sound velocity as that of the transmitting
piezoelectric resonator, the thickness of the receiving
piezoelectric resonator 204 is set to be substantially {fraction
(1/n)} of the thickness of the transmitting piezoelectric resonator
202. Thereby, the receiving piezoelectric resonator 204 selectively
receives a frequency component of nf.sub.0 (n is an integer of 2 or
more).
[0126] Moreover, since the curvature radii R.sub.t and R.sub.r are
determined in order to allow the focus for the transmitted
frequency with the center frequency f.sub.0 to agree with the focus
for the received frequency with the center frequency nf.sub.0 (n is
an integer of 2 or more), the acoustic lens can also obtain a
satisfactory spatial resolution. Furthermore, since the average
thickness of the acoustic lens is set to 1/4.lambda. for each
frequency, broad-band and high-sensitivity transmission/reception
can be performed.
[0127] In the second embodiment, since the transmitting
piezoelectric resonator 202 and receiving piezoelectric resonator
204 are separately disposed substantially in the same plane, the
ultrasound generated by the transmitting piezoelectric resonator
202 is not transmitted through the receiving piezoelectric
resonator 204 or is not reflected by the receiving piezoelectric
resonator. Therefore, it is unnecessary to control the state
between the electrodes in the open or short state, and control is
remarkably simple.
[0128] In the second embodiment, when a piezoelectric g constant of
the receiving piezoelectric resonator 204 is set to be larger than
the piezoelectric g constant of the transmitting piezoelectric
resonator 202, and the piezoelectric material having a large
mechanical quality factor Qm is used, the selection property of
harmonic signal reception is further enhanced.
[0129] Modification examples of the second embodiment will be
described hereinafter with reference to the drawings. In the
drawings, members equivalent to the aforementioned members are
denoted with the same reference numerals, and a detailed
description thereof is omitted to avoid redundancy in the following
description.
[0130] FIG. 10 shows a first modification of the ultrasonic
transducer. In the ultrasonic transducer of the present
modification example, an acoustic lens 230 has an opening surface
with a constant curvature radius. The acoustic lens 230 is
imaginarily divided into a peripheral portion positioned in front
of the transmitting piezoelectric resonator 202, and a middle
portion positioned in front of the receiving piezoelectric
resonator 204. The peripheral portions of the transmitting
piezoelectric resonator 202, damping layer 210 and acoustic lens
230 constitute a transmitting ultrasonic vibrator, and the middle
portions of the receiving piezoelectric resonator 204, damping
layer 212 and acoustic lens 230 constitute a receiving ultrasonic
vibrator.
[0131] The peripheral portion of the acoustic lens 230 has an
average thickness T.sub.t equal to 1/4 of the wavelength of the
ultrasound with the fundamental frequency f.sub.0, and the middle
portion of the acoustic lens 230 has an average thickness T.sub.r
which is 1/4 of the wavelength of the ultrasound with the
fundamental frequency nf.sub.0 (n is an integer of 2 or more) Here,
the average thickness T.sub.t is a distance from the surface of the
transmitting piezoelectric resonator 202 to the middle between a
bottom and an upper end (i.e., the lens effective end with respect
to the transmitting piezoelectric resonator 202) of the acoustic
lens 230. The average thickness T.sub.r is a distance from the
surface of the receiving piezoelectric resonator 204 to the middle
between a bottom and an upper end (i.e., the lens effective end
with respect to the receiving piezoelectric resonator 204) of the
concave surface of the acoustic lens 230.
[0132] The opening surface of the acoustic lens 230 has a curvature
radius equal to a radius of a spherical surface which circumscribes
a circle obtained by connecting a point of the average thickness
T.sub.t of the peripheral portion of the acoustic lens 230, and a
circle obtained by connecting a point of the average thickness
T.sub.r of the middle portion of the acoustic lens 230.
[0133] Thereby, the spatial resolution is slightly deteriorated,
but acoustic matching conditions are approximately satisfied, and a
high-sensitivity reception is performed with respect to the
harmonic signal. Moreover, since the curvature radius of the
acoustic lens 230 is the same over the whole opening, processing is
facilitated. Therefore, an inexpensive ultrasonic transducer can be
presented for harmonic imaging ultrasonic diagnosis.
[0134] FIG. 11 shows a second modification of the ultrasonic
transducer. The ultrasonic transducer of the present modification
has a disc-shaped transmitting piezoelectric resonator 232, and an
annular band shaped receiving piezoelectric resonator 234, and the
transmitting piezoelectric resonator 232 is positioned inside the
receiving piezoelectric resonator 234.
[0135] An acoustic lens 236 is imaginarily divided into a middle
portion positioned in front of the transmitting piezoelectric
resonator 232, and a peripheral portion positioned in front of the
receiving piezoelectric resonator 234. The middle portions of the
transmitting piezoelectric resonator 232, damping layer 212 and
acoustic lens 236 constitute a transmitting ultrasonic vibrator.
The peripheral portions of the receiving piezoelectric resonator
234, damping layer 212 and acoustic lens 236 constitute a receiving
ultrasonic vibrator.
[0136] Both the middle portion and the peripheral portion of the
acoustic lens 236 have a convex opening surface. The opening
surface of the middle portion of the acoustic lens 236 has a
curvature radius R.sub.t, and the opening surface of the peripheral
portion of the acoustic lens 236 has a curvature radius R.sub.r
larger than the curvature radius R.sub.t.
[0137] An original object of the acoustic lens 236 is to converge
the ultrasound, but the acoustic lens is further preferably
provided with a function of an acoustic matching layer. The
acoustic matching layer for the harmonic signal is thinner than the
acoustic matching layer for the fundamental frequency. For the
acoustic lens 236, a lens portion in the vicinity of a center is
thick, and a peripheral lens portion is thin. Therefore, the
transmitting piezoelectric resonator 232 is disposed inside the
receiving piezoelectric resonator 234.
[0138] FIG. 12 shows a main part of a third modification of the
ultrasonic transducer. The main part of the ultrasonic transducer
of the present modification is a part contained in the housing. The
part includes a circular transmitter 248 or a transmitting
ultrasonic vibrator, and an annular band shaped receiver 250 or a
receiving ultrasonic vibrator which surrounds the transmitter. The
transmitter 248 includes a middle portion of the piezoelectric
resonator 242, and the receiver 250 includes the peripheral portion
of the piezoelectric resonator 242, and a damping layer 252
disposed on the back surface of the vibrator.
[0139] The piezoelectric resonator 242 is disc-shaped, and has an
electrode 244 common to the transmitter 248 and receiver 250 on a
front surface thereof, and a circular electrode 246 of the
transmitter 248 and an annular band shaped electrode 256 of the
receiver 250 on a back surface thereof. The receiver 250 of the
piezoelectric resonator 242 has a thickness which is {fraction
(1/n)} of the thickness of the transmitter 248, and can selectively
receive an n-dimensional high order harmonic wave having a
frequency component of nf.sub.0 (n is an integer of 2 or more) with
respect to a fundamental ultrasound f.sub.0 transmitted from the
transmitter 248.
[0140] The damping layer 252 has a concave depression in a middle
portion corresponding to the transmitter 248 of the piezoelectric
resonator 242, and an annular band portion outside the depression
is bonded to the receiver 250 of the piezoelectric resonator 242. A
bottom of the concave depression does not contact the transmitter
248 of the piezoelectric resonator 242, and a gap 254 is formed in
the back surface of the transmitter 248.
[0141] In the structure, the transmitter 248 has a large mechanical
quality factor Qm, and can radiate a broad-band transmission
ultrasound with a large amplitude. In the present modification
example, the damping layer 252 has the concave depression in the
middle portion corresponding to the transmitter 248, but may have a
through hole.
[0142] FIG. 13A and FIG. 13B show a main part of a fourth
modification of the ultrasonic transducer. The main part of the
ultrasonic transducer of the present modification is contained in
the housing, and has four transmitting ultrasonic vibrators 260 and
four receiving ultrasonic vibrators 270. Both the transmitting
ultrasonic vibrator 260 and the receiving ultrasonic vibrator 270
have the same fan shape, and these vibrators are alternately
disposed in a radiant shape. That is, the fan-shaped transmitting
ultrasonic vibrators 260 and receiving ultrasonic vibrators 270 are
alternately arranged along an angular direction.
[0143] The transmitting ultrasonic vibrator 260 has a transmitting
piezoelectric resonator 262 for transmitting a fundamental
ultrasound with a frequency f.sub.0, a transmitting acoustic lens
264 disposed on a front surface of the vibrator, and a damping
layer 266 disposed on a back surface of the transmitting
piezoelectric resonator 262. The transmitting acoustic lens 264 has
a surface curvature radius R.sub.t centering on a point F.sub.2,
and is focused at the point F.sub.2 along an acoustic line 268 for
the ultrasound with the fundamental frequency f.sub.0 transmitted
from the transmitting piezoelectric resonator 262.
[0144] The receiving ultrasonic vibrator 270 has a receiving
piezoelectric resonator 272 for selectively receiving a harmonic
ultrasound with a frequency nf.sub.0 (n is an integer of 2 or
more), a receiving acoustic lens 274 disposed on a front surface of
the vibrator, and a damping layer 276 disposed on a back surface of
the receiving piezoelectric resonator 272. The receiving acoustic
lens 274 has a surface curvature radius R.sub.r centering on a
point F.sub.1, and is focused at the point F.sub.2 along an
acoustic line 278 for the harmonic ultrasound with the frequency
nf.sub.0.
[0145] In the structure, opening areas of the transmitting
ultrasonic vibrator 260 and receiving ultrasonic vibrator 270 are
the same for transmission/reception, and focuses can be matched
only by a lens surface shape.
[0146] In the ultrasonic transducer of the second embodiment, the
transmitting and receiving ultrasonic vibrators are separated from
each other in the plane. Therefore, when the shape of the acoustic
lens disposed on the front surface of the piezoelectric resonator
is optimized, transmission/reception is efficiently performed.
Additionally, while the focuses of the fundamental ultrasound and
harmonic ultrasound agree with each other, the spatial resolution
is satisfactory, and reception of the harmonic signal with a large
output is possible.
[0147] The constitution of the mechanical sector scanning integral
ultrasonic transducer has been described above as the second
embodiment, but the content described in the second embodiment can
also be applied to the electronic scanning array ultrasonic
transducer, except the fourth modification example, and is not
limited to application to the mechanical sector scanning integral
ultrasonic transducer. Moreover, it is possible to variously
combine the present embodiment modes and modification
constitutions. Various constitutions of the harmonic imaging
ultrasonic transducer are possible in accordance with the diagnosis
object portion and diagnosis precision, and these constitutions are
also included as other modification examples of the present
embodiment.
[0148] [Third Embodiment]
[0149] As shown in FIG. 14, an ultrasonic transducer 300 has a
transmitting ultrasonic vibrator for transmitting the fundamental
ultrasound, a receiving ultrasonic vibrator for receiving the
harmonic signal, and a housing 306 in which these vibrators are
contained. The transmitting ultrasonic vibrator has a transmitting
piezoelectric resonator 302, and a backing layer (dumping layer)
310 disposed on the back surface of vibrator. Moreover, the
receiving ultrasonic vibrator has a receiving piezoelectric
resonator 304, and a backing layer (dumping layer) 312 disposed on
the back surface of the vibrator.
[0150] The transmitting piezoelectric resonator 302 has an annular
band plate shape, the receiving piezoelectric resonator 304 has a
disc shape, and the receiving piezoelectric resonator 304 is
positioned inside the transmitting piezoelectric resonator 302. A
concave acoustic lens 308, for example, of an epoxy resin or the
like is disposed on the front surface of the transmitting
piezoelectric resonator 302 and receiving piezoelectric resonator
304.
[0151] Furthermore, the transmitting and receiving ultrasonic
vibrators partially have the acoustic lens 308. That is, the
receiving ultrasonic vibrator has a part, that is, a circular
middle portion of the acoustic lens 308 positioned on the front
surface of the receiving piezoelectric resonator 304. The
transmitting ultrasonic vibrator has a part, that is, an annular
band shaped peripheral portion of the acoustic lens 308 positioned
on the front surface of the transmitting piezoelectric resonator
302.
[0152] The transmitting ultrasonic vibrator transmits a fundamental
ultrasound having the center frequency f.sub.0 in response to input
of the electric signal, and the receiving ultrasonic vibrator
receives the harmonic signal having the center frequency nf.sub.0
(n is an integer of 2 or more) generated in an object by the
fundamental ultrasound.
[0153] The transmitting and receiving ultrasonic vibrators, that
is, the structure including the transmitting piezoelectric
resonator 302, receiving piezoelectric resonator 304, acoustic lens
308 and backing layers 310 and 312 is fixed inside the housing 306
via an insulating layer 314.
[0154] The receiving piezoelectric resonator 304 has a disc shaped
piezoelectric material, and a pair of electrodes formed entirely on
opposite surfaces of the piezoelectric material. Similarly, the
transmitting piezoelectric resonator 302 has an annular plate
shaped piezoelectric material, and a pair of electrodes formed
entirely on opposite surfaces of the piezoelectric material. The
electrode on the front side, that is, the ultrasonic emission
surface side of the receiving piezoelectric resonator 304 is
electrically connected to the electrode on the front side, that is,
the ultrasonic emission surface side of the transmitting
piezoelectric resonator 302 via a wiring 316. The front electrode
of the transmitting piezoelectric resonator 302 is electrically
connected to the housing 306 via a wiring 318.
[0155] A two-core coaxial cable 320 extending through the housing
306 has a lead wire 322, lead wire 324 and shielding wire 326. The
lead wire 322 is electrically connected to a back electrode of the
transmitting piezoelectric resonator 302 via a wiring 332, the lead
wire 324 is electrically connected to a back electrode of the
receiving piezoelectric resonator 304 via a wiring 334, and the
shielding wire 326 is electrically connected to the conductive
housing 306. Furthermore, an internal space 336 of the housing 306
is filled with a seal material such as an epoxy resin.
[0156] FIG. 15A and FIG. 15B show models of the transmitting and
receiving piezoelectric resonators of the in-plane separated
ultrasonic transducer shown in FIG. 14. As shown in FIG. 15A, the
transmitting piezoelectric resonator 302 has an annular band shape,
the receiving piezoelectric resonator 304 has a disc shape, and the
receiving piezoelectric resonator 304 is positioned inside the
transmitting piezoelectric resonator 302. In FIG. 15B, not only a
second order or third order harmonic signal 344, but also
ultrasounds of all frequency components contained in transmitted
ultrasounds 342 and superimposed onto the harmonic signal reach the
disc shaped receiving piezoelectric resonator 304.
[0157] As seen from FIG. 15A, in the in-plane separated ultrasonic
transducer, an opening area of the transmitting piezoelectric
resonator decreases as compared with the in-plane
transmission/reception integral ultrasonic transducer for
transmission/reception by the whole opening surface. Moreover, as
shown in FIG. 15B, the thickness of the receiving piezoelectric
resonator decreases with an increase of a degree of the received
harmonic signal.
[0158] Sensitivity deterioration with the decrease of the opening
area of the transmitting piezoelectric resonator and the decrease
of the thickness of the receiving piezoelectric resonator will be
considered hereinafter. It is then proved that a satisfactory
sensitivity can be obtained by appropriately selecting the
piezoelectric materials of the transmitting piezoelectric resonator
302 and receiving piezoelectric resonator 304.
[0159] Assuming that an opening area of the transmitting
piezoelectric resonator 302 is St, and opening area of the
receiving piezoelectric resonator 304 is S.sub.r, as compared with
the conventional in-plane transmission/reception integral
ultrasonic transducer in which the opening area is S.sub.t+S.sub.r,
a transmission ultrasonic energy drops substantially to
S.sub.t/(S.sub.t+S.sub.r).
[0160] An output voltage V.sub.out of the receiving piezoelectric
resonator 304 is represented by the following equation (1).
V.sub.out=q.sub.r/C.sub.r=d.sub.33r.multidot.T.multidot.S.sub.r/(.epsilon.-
S.sub.r/t.sub.r)=g.sub.33r.multidot.t.sub.r.multidot.T (1)
[0161] Here, q.sub.r denotes a charge piezoelectrically converted
and generated on the electrode of the receiving piezoelectric
resonator 304, C.sub.r denotes a capacitance of the receiving
piezoelectric resonator 304, e denotes permittivity of the
receiving piezoelectric resonator 304, d.sub.33r denotes a
piezoelectric constant of the receiving piezoelectric resonator
304, g.sub.33r denotes a voltage output coefficient of the
receiving piezoelectric resonator, t.sub.r denotes a thickness of
the receiving piezoelectric resonator, and T denotes an ultrasonic
reception stress.
[0162] Furthermore, assuming that the frequency of the received
ultrasound is f.sub.r, and longitudinal wave sound velocity of the
piezoelectric resonator material is V.sub.r, the following equation
results.
t.sub.r=.lambda./2=V.sub.r/2f.sub.r (2)
[0163] Therefore, the equation (1) results in the following
equation.
V.sub.out=g.sub.33r't.sub.r.multidot.T
=.sub.g33r.multidot.V.sub.r.multido- t.T/2f.sub.r (3)
[0164] Furthermore, assuming that the ultrasonic reception stress T
is proportional to the opening of the transmitting piezoelectric
resonator, the following equation results.
V.sub.out=g.sub.33r.multidot.V.sub.r.multidot.T/2f.sub.r=g.sub.33r.multido-
t.V.sub.r.multidot.S.sub.r.multidot.P.sub.0/2f.sub.r=g.sub.33r.multidot.V.-
sub.r.multidot.S.sub.td.sub.33tV.sub.drive/2f.sub.r (4)
[0165] Moreover, when the received ultrasound is an n-dimensional
harmonic signal, the following equation results.
V.sub.out=g.sub.33r.multidot.V.sub.r.multidot.S.sub.t
d.sub.33t.multidot.V.sub.drive/2nf.sub.r (5)
[0166] Here, S.sub.t denotes an opening of the transmitting
piezoelectric resonator, P.sub.0 denotes an ultrasonic sound
pressure per unit area generated by the transmitting piezoelectric
resonator, d.sub.33t denotes a piezoelectric constant of the
transmitting piezoelectric resonator, and V.sub.drive denotes a
drive voltage applied to the transmitting piezoelectric
resonator.
[0167] From the equations (4) and (5), when the n-dimensional
harmonic signal is received in the in-plane separated ultrasonic
transducer, the received frequency increases n times, and the
output voltage V.sub.out drops to {fraction (1/n)}.
[0168] Moreover, as compared with the in-plane
transmission/reception integral ultrasonic transducer, a
transmission opening area is S.sub.t/(S.sub.t+S.sub.r) times that
of the in-plane transmission/reception integral ultrasonic
transducer, and the output voltage V.sub.out accordingly drops
further.
[0169] Usually, the piezoelectric resonator of the same
piezoelectric material is used in the transmitting and receiving
piezoelectric resonators. Based on this assumption, for example,
when S.sub.t=S.sub.r, the frequency f.sub.r of a detected
ultrasound is 2f.sub.0, and the output voltage V.sub.out drops to
1/4 (=-12 dB).
[0170] Here, the in-plane separated ultrasonic transducer shown in
FIG. 15A and FIG. 15B is used as a comparison object, and the
in-plane separated ultrasonic transducer for detecting only the
fundamental ultrasound as shown in FIG. 16A and FIG. 16B is
considered. This ultrasonic transducer corresponds to a usual pulse
echoing transducer of a whole surface integral type.
[0171] In the ultrasonic transducer, a transmitting piezoelectric
resonator 352 and receiving piezoelectric resonator 354 have the
same thickness and use the same piezoelectric material. For
example, the piezoelectric resonator of a PZT based piezoelectric
material having a longitudinal wave sound velocity V.sub.t=4260
[m/s], and a voltage output coefficient
g.sub.33t=30.times.10.sup.-3 [Vm/N] is disposed.
[0172] For the ultrasonic transducer of FIG. 16A and FIG. 16B,
since the receiving piezoelectric resonator 354 is also used in
transmission, the same transmission/reception as that for the
fundamental pulse echo diagnosis by the conventional integral
ultrasonic transducer is performed, and the transmission opening
area of the ultrasonic transducer of FIG. 16A and FIG. 16B is
substantially S.sub.t+S.sub.r.
[0173] The reception voltage V.sub.r1 in the fundamental wave
reception is represented by replacing suffix r in the equation (4)
with suffix t for convenience as follows.
V.sub.r1=g.sub.33t.multidot.V.sub.t.multidot.(S.sub.t+S.sub.r).multidot.Q.-
sub.t.multidot.d.sub.t.multidot.V.sub.drive /2f.sub.t (6)
[0174] Here, Q.sub.t denotes a resonance sharpness of general
mechanical vibration of the ultrasonic vibrator including the
backing layer and acoustic matching layer.
[0175] On the other hand, when the reception voltage in the
n-dimensional harmonic reception is V.sub.rn, similar to the
equation (6), the following equation results.
V.sub.rn=g.sub.33r.multidot.V.sub.r.multidot.S.sub.t.multidot.Q.sub.r.mult-
idot.d.sub.t.multidot.V.sub.drive/2nf.sub.t (7)
[0176] For the ultrasonic transducer of FIG. 15A and FIG. 15B, to
compensate for the sensitivity deterioration, the n-dimensional
harmonic signal can preferably be received with a signal level
which is not less than the fundamental wave reception voltage in
the ultrasonic transducer of FIG. 16A and FIG. 16B. For this, the
ultrasonic transducer of FIG. 15A and FIG. 15B may satisfy
V.sub.rn/V.sub.r1.gtoreq.1.
[0177] When the equations (6) and (7) are assigned to this
relation, the following equation results.
V.sub.rn/V.sub.r1=(g.sub.33r.multidot.V.sub.r.multidot.(S.sub.t+S.sub.r).m-
ultidot.Q.sub.r/n)/(g.sub.33t.multidot.V.sub.t.multidot.S.sub.t.multidot.Q-
.sub.t) .gtoreq.1 (8)
[0178] Finally, the following equation results.
(g.sub.33r.multidot.V.sub.r.multidot.Q.sub.r)/(g.sub.33t.multidot.V.sub.t.-
multidot.Q.sub.t).gtoreq.n.multidot.(1+R) (9)
[0179] Here, R=S.sub.t/S.sub.r
[0180] When the mechanical resonance sharpness Q.sub.t of the
transmitting piezoelectric resonator 302 is 5 or more, trailing of
a transmitted ultrasonic pulse lengthens, and a depth-direction
resolution is deteriorated. Moreover, when the sharpness is 1 or
less, a fundamental ultrasonic band is excessively broadened, a
fundamental wave component mixed amount in 2f.sub.0 increases, and
S/N is deteriorated. Therefore, a value of Q.sub.t is preferably
between 1 and 5.
[0181] Furthermore, when the mechanical resonance sharpness Q.sub.r
of the receiving piezoelectric resonator 304 is 5 or more, the
trailing of the reception voltage V.sub.rn lengthens, and the
depth-direction resolution is deteriorated. Additionally, when the
sharpness is 1 or less, a band of the reception voltage V.sub.rn is
excessively broadened, a fundamental wave component ratio
increases, and S/N is deteriorated. Therefore, a value Of Q.sub.r
is preferably between 1 and 5.
[0182] The material of the transmitting piezoelectric resonator may
have a piezoelectric constant d.sub.33 and mechanical quality
factor Qm which satisfy d.sub.33>200.times.10.sup.-12 [m/V],
70<Qm<1000.
[0183] When the value of Qm of the piezoelectric resonator is
large, a damping effect of the backing layer is relaxed, and
thereby the value of Q.sub.t may be adjusted. In order to set the
damping effect to be lower than usual and enhance the resonance
sharpness, the backing layer 310 may have a high ultrasonic
attenuation ratio, and a low acoustic impedance Zd, for example, an
acoustic impedance Zd which is 1/3 or less of the acoustic
impedance Zp of the transmitting piezoelectric resonator 302. A
material preferable for the backing layer 310 contains, for
example, a composite resin formed by mixing an appropriate amount
of a tungsten powder in a highly flexible epoxy resin. The tungsten
powder is preferably mixed to such an extent that the acoustic
impedance of the backing layer 310 is about 1/3 of the acoustic
impedance of the piezoelectric resonator and the attenuation ratio
is of the order of 8 dB/cm/MHz. Here, with the attenuation ratio of
5 dB/cm/MHz or less, Q excessively increases, time axis pulse width
lengthens, and distance-direction resolution is deteriorated. The
material of the backing layer 310 is not limited to the
aforementioned composite material, and a composite material mixed,
for example, with an alumina or zirconia powder may be used.
[0184] Since the disc-shaped receiving piezoelectric resonator 304
has a narrow band filter characteristic at the center frequency
2f.sub.0 or 3f.sub.0, only a 2f.sub.0 or 3f.sub.0 component is
selectively converted to a voltage signal. With a relative relation
between the acoustic impedance Zp of the piezoelectric resonator
and the acoustic impedance Zd of the backing layer, a back surface
side ultrasonic vibration of the piezoelectric resonator is divided
into an ultrasonic vibration amplitude transmitted toward the
backing layer and an ultrasonic vibration amplitude reflected
toward the piezoelectric resonator. A division ratio is one of
measures for determining the mechanical resonance sharpness Q of
the transmitting ultrasonic vibrator. When Zd is 1/3 or less of Zp,
a reflected ultrasonic component increases, and it is possible to
control the mechanical resonance sharpness Q of the transmitting
ultrasonic vibrator to an optimum value of 2 to 5.
[0185] The piezoelectric material of the receiving piezoelectric
resonator 304 may be a material which has a large Qm, large voltage
output coefficient g.sub.33, and high longitudinal wave sound
velocity. The voltage output coefficient g.sub.33 may satisfy
g.sub.33>30.times.10.s- up.-3 [V/Nm]. Examples of the material
preferably include a piezoelectric single crystal represented, for
example, by a chemical formula K(Nb.sub.1-xTa.sub.x)O.sub.3,
0.ltoreq.x .ltoreq.0.2. Another preferable material is a lead
titanate based piezoelectric ceramic material. A further preferable
material is a bismuth layer structure ferroelectric material (BLSF)
represented by chemical formula Bi.sub.4Ti.sub.3O.sub.12 or
Ma.sub.1-xMb.sub.xBi.sub.2McO.sub.8, 0.ltoreq.x .ltoreq.0.2. Here,
Ma and Mb are alkaline earth metal elements such as Sr and Ba, and
Mc is a +5 valence metal element such as Ta and Nb.
[0186] When the value of Qm of the piezoelectric resonator is
large, the damping effect of the backing layer may be relaxed to
adjust the value of Q.sub.r. In order to set the damping effect to
be lower than usual and enhance the resonance sharpness, the
backing layer 312 may have a high ultrasonic attenuation ratio, and
a low acoustic impedance Zd, for example, an acoustic impedance Zd
which is 1/3 or less of the acoustic impedance Zp of the receiving
piezoelectric resonator 304. A material preferable for the backing
layer 312 contains, for example, a composite resin formed by mixing
an appropriate amount of a tungsten oxide powder or a barium
ferrite powder in a highly flexible epoxy resin. The material of
the backing layer 312 is not limited to the aforementioned
composite material, and the composite material mixed, for example,
with an alumina or zirconia powder may be used.
[0187] For example, the piezoelectric material of the transmitting
piezoelectric resonator 302 is a PZT ceramic material, and the
piezoelectric material of the receiving piezoelectric resonator 304
is a potassium niobate (KNbO.sub.3) piezoelectric single crystal
with a longitudinal wave sound velocity V.sub.t=5900 [m/s], and
voltage output coefficient g.sub.33t=55.times.10.sup.-3 [Vm/N].
[0188] Here, when Q.sub.t=1, Q.sub.r=5, and these values and
material constants are assigned to a left side of the equation (9),
then the left side=12.7. When R=1, that is, transmission opening
area=reception opening area, n=6 is possible. Therefore, even a
six-dimensional harmonic signal can be satisfactorily received. In
actuality, when the value Of Q.sub.r is too large, the time axis
pulse width increases, and the depth-direction resolution is
deteriorated. Therefore, the value Of Q.sub.r is preferably small.
With a decrease to Q.sub.r=2.5, the left side of the equation (9)
indicates 6.4, and n=3 is possible. Therefore, even a third order
harmonic signal can be satisfactorily received.
[0189] Moreover, for the transmitting ultrasonic vibrator, assuming
that Q.sub.t=2, R=1, then n=2 is possible at Q.sub.r=3.2.
Therefore, even the third order harmonic signal can satisfactorily
be received. Furthermore, when R=0.5, that is, the reception
opening area S.sub.r is 1/20 of the transmission opening area, n=2
is possible at Q.sub.t=2, Q.sub.r2.4. Therefore, even a second
order harmonic signal can satisfactorily be received.
[0190] These transducer properties do not need to be manufactured
using an undeveloped technique, and can be sufficiently realized by
a conventional transducer manufacturing technique.
[0191] As seen from the above description, in the harmonic imaging
in-plane separated ultrasonic transducer, when the transmitting and
receiving piezoelectric resonators satisfying the equation (9) are
used, the harmonic signal can be received at the reception voltage
having substantially the same degree as that of the fundamental
wave transmission/reception of the whole surface opening.
[0192] A first modification of the ultrasonic transducer of the
third embodiment will be described with reference to FIG. 17A and
FIG. 17B.
[0193] As shown in FIG. 17A and FIG. 17B, in the in-plane separated
ultrasonic transducer of the modification example, a transmitting
piezoelectric resonator 362 of the transmitting ultrasonic vibrator
is a PZT ceramic disc vibrator, a receiving piezoelectric resonator
364 of the receiving ultrasonic vibrator for receiving the harmonic
signal is an annular piezoelectric resonator of KNbO.sub.3.
Contrary to the aforementioned embodiment, the transmitting
piezoelectric resonator 362 is positioned inside the receiving
piezoelectric resonator 364.
[0194] When an opening area ratio R (=S.sub.r/S.sub.t) is 0.5, in
accordance with relative setting of Q.sub.t and Q.sub.r, the
harmonic signal can be received at the signal level substantially
of the same degree as that of the whole surface opening and
fundamental wave transmission/reception as described above.
[0195] When the general PZT ceramic vibrator has an annular shape,
dividing vibration occurs in the vicinity of the resonant
frequency, and a satisfactory resonant characteristic cannot be
obtained in many cases. However, when KNbO.sub.3 is used in the
annular piezoelectric resonator as in the present modification
example, the resonant characteristic is largely improved. This fact
has been experimentally confirmed by the present inventor, et
al.
[0196] A second modification of the transmitting piezoelectric
resonator of the third embodiment will next be described with
reference to FIG. 18 to FIG. 20.
[0197] In the third embodiment, the annular transmitting
piezoelectric resonator has the whole surface electrode formed on
the whole surface of the vibrator, but in the modification of the
embodiment, as shown in FIG. 18, an annular transmitting
piezoelectric resonator 370 has an energy trapped electrode
structure 372.
[0198] Here, the term "energy enclosed electrode structure"
indicates an electrode which partially covers a piezoelectric
material face, and satisfies equation (10) described later.
[0199] The energy trapped electrode structure 372 has a pair of
electrode plates disposed opposite to each other via an annular
piezoelectric material 374. As shown in FIG. 19, one electrode
plate of the pair, for example, the front electrode plate has a
plurality of circular electrodes 382, and a thin wiring 384 via
which the adjacent circular electrodes 382 are connected to each
other. Moreover, as shown in FIG. 20, the other electrode plate of
the pair, for example, the back electrode plate has the same number
of circular electrodes 392 as that of circular electrodes 382, and
a thin wiring 394 via which the adjacent circular electrodes 392
are connected to each other.
[0200] As seen from FIG. 19 and FIG. 20, the circular electrode 382
is disposed opposite the circular electrode 392 via the
piezoelectric material 374. The wiring 384 crosses over the wiring
394 only in one place, and there is no other place in which the
wirings are disposed opposite each other via the piezoelectric
material 374.
[0201] Furthermore, assuming that an electrode diameter is a, and a
frequency drop ratio by electrode formation is D, a thickness h of
the piezoelectric material 374 satisfies the following equation. 1
a 2 h < 2 2
[0202] The piezoelectric resonator is formed, for example, by
forming the whole surface electrodes on opposite surfaces of the
circular PZT piezoelectric material with the thickness satisfying
the above formula, polarizing the electrode, and selectively
etching the front and back whole surface electrodes by a process
such as photolithography.
[0203] For the diameter of the circular electrode 382, a value of
about 1/3 to 2/3 of a width w of the annular piezoelectric material
374 is selected. For the diameter of the circular electrode 392, a
value smaller than the diameter of the circular electrode 382 by 5
to 10% is selected by considering a positional deviation during
etching so that one of the pair of circular electrodes is prevented
from projecting from the other electrode.
[0204] The electrode 382 on the ultrasonic emission side is
connected to the front electrode of the disc-shaped piezoelectric
resonator 304 disposed inside the annular piezoelectric resonator
via a wiring 416, and connected to the housing 306 via a wiring 418
in FIG. 14.
[0205] For the annular piezoelectric resonator having the usual
whole surface electrode, the dividing vibration sometimes occurs at
the resonant frequency. The occurrence of the dividing vibration
not only reduces a transmission ultrasonic sound pressure, but also
displaces a ratio of the resonant frequency of the transmitting
ultrasonic vibrator to the resonant frequency of the receiving
ultrasonic vibrator from 1:2. This hinders transmission of the
correct fundamental wave and reception of the harmonic signal.
[0206] Since the annular piezoelectric resonator of the present
modification has the energy trapped electrode structure, the
dividing vibration hardly occurs in the vicinity of the resonant
frequency. This can accurately maintain the ratio of the resonant
frequency of the transmitting ultrasonic vibrator to the resonant
frequency of the receiving ultrasonic vibrator at 1:2, and this
preferably realizes effective harmonic imaging.
[0207] When the energy trapped electrode is used in the
piezoelectric resonator electrode in this manner, in the annular
piezoelectric resonator, no unnecessary vibration component is
superimposed, and the fundamental ultrasound formed only of the
longitudinal ultrasonic component can efficiently be generated.
[0208] In the present modification example, the transmitting
piezoelectric resonator having the energy trapped electrode has
been illustrated as an improvement of the transmitting
piezoelectric resonator, but the receiving piezoelectric resonator
may have the energy trapped electrode as an improvement example of
the receiving piezoelectric resonator. A large Q.sub.r is obtained
by the receiving piezoelectric resonator having the energy trapped
electrode, and a large reception voltage V.sub.rn is accordingly
obtained.
[0209] Additionally, the respective constitutions of the
embodiments of the present invention can of course be modified and
altered in various ways.
[0210] For example, in the third embodiment, the ultrasonic
transducer has a circular opening, but the opening shape is not
limited to a circle. The opening of the ultrasonic transducer may
be, for example, rectangular, elliptical, or strip-shaped.
Moreover, the ultrasonic transducer may be an electronic scanning
array transducer, and each element constituting an array may have a
fundamental wave transmitting vibrator and high order harmonic wave
receiving vibrator in the same plane as in the third
embodiment.
[0211] [Fourth Embodiment]
[0212] According to a fourth embodiment, there is provided an
ultrasonic transducer system suitable for harmonic imaging
ultrasonic diagnosis, which includes the ultrasonic transducer
described in the third embodiment.
[0213] As shown in FIG. 21, the ultrasonic transducer system
includes the ultrasonic transducer 300, and a pulser circuit 402
for supplying a drive pulse signal to the transmitting
piezoelectric resonator 302 of the ultrasonic transducer 300. In
one example, the pulser circuit 402 generates a high-voltage spike
wave if necessary, and can adjust a pulse width and fall time. In
another example, the pulser circuit 402 generates a high-voltage
trapezoidal wave if necessary, and can adjust the pulse width and
fall time. In a further example, the pulser circuit 402 generates a
high-voltage burst wave if necessary, and can adjust a burst length
and window function.
[0214] The ultrasonic transducer system further has a receiver
circuit 404 for receiving the output signal of the receiving
piezoelectric resonator 304 of the ultrasonic transducer 300, a
signal processor circuit 406 for processing a signal from the
receiver circuit 404, and an image processor circuit 408 for
forming a signal from the signal processor circuit 406 into an
image. The image obtained by the image processor circuit 408 is
finally displayed on a monitor screen (not shown).
[0215] The present invention is applied to ultrasonic diagnosis in
which the second order or third order harmonic signal is utilized.
In ultrasonic diagnosis, the fundamental ultrasound is required not
to contain frequency components other than the component of the
center frequency f.sub.0, particularly not to contain the 2f.sub.0
or 3f.sub.0 frequency component. Particularly, when the fundamental
ultrasound contains the 2f.sub.0 or 3f.sub.0 frequency component,
during reception, it is impossible to distinguish the harmonic
signal as a detection object from the 2f.sub.0 or 3f.sub.0
frequency component mixed beforehand in the fundamental
ultrasound.
[0216] For this reason, when a second order harmonic signal is
utilized in the ultrasonic diagnosis, the ultrasound transmitted
from the transmitting ultrasonic vibrator preferably contains no
2f.sub.0 frequency component. Similarly, when a third order
harmonic signal is utilized in the ultrasonic diagnosis, the
ultrasound transmitted from the transmitting ultrasonic vibrator
preferably contains no 3f.sub.0 frequency component.
[0217] FIG. 22A shows a drive voltage waveform 412 of the spike
wave as one example of the drive signal supplied to the
transmitting piezoelectric resonator. FIG. 22B shows a frequency
characteristic of the drive voltage waveform 412 of the spike wave,
and shows a negative inclination 416 in the vicinity of a first dip
frequency, first dip frequency 422, second dip frequency 424, and
first peak frequency 426. Such trapezoidal wave cannot strictly
realize a 5 function or a rectangular wave as an ideal drive
waveform, and is a waveform generally utilized as an actual drive
signal waveform in a pulse echo diagnosis method.
[0218] As seen from FIG. 22B, the frequency characteristic of the
drive voltage waveform 412 of the spike wave indicates a peak/dip
characteristic and whole drooping characteristic. It is well known
that the frequency characteristic of the d function indicates
neither the peak/dip characteristic nor the whole drooping
characteristic. When the peak/dip characteristic has a pulse width
on a time axis, the characteristic appears. It is further found
that the whole drooping characteristic and deterioration of
steepness of a level change in the peak/dip frequency appear by
disposing a fall inclination.
[0219] A spectrum T(j.omega.) of the transmitted ultrasonic signal
is represented by a product of a response signal spectrum
H(j.omega.) and drive waveform spectrum D(j.omega.) during d
function driving as represented in the following equation (11).
T(j.omega.)=H(j.omega.).multidot.D(j.omega.) (11)
[0220] It is understood from this equation that when D(j.omega.)
has a dip, that is, a drop of the level in the frequency
characteristic in 2f.sub.0, transmission waveform T(j.omega.) also
has a drop in the frequency, and as a result the 2f.sub.0 component
of the fundamental ultrasound to be transmitted is inhibited.
[0221] As shown in FIG. 22B, the drive signal waveform having the
spectrum such that the frequency of the first dip 422 is 2f.sub.0
is used, and the transmission spectrum T(j.omega.) with the
inhibited 2f.sub.0 component can therefore be obtained.
[0222] FIG. 24 shows the characteristic of the first dip frequency
with respect to a fall time tf in the frequency characteristic of
the drive signal of the spike wave. It is seen from a
characteristic curve of the first dip frequency shown in FIG. 24
that the fundamental ultrasound with the inhibited 2f.sub.0
component can be obtained by setting the fall time tf to 99 ns in
detection of the second order harmonic signal with a frequency of
10 MHz. As a result, a level down of -2.5 [dB/MHz].times.(10 MHz -5
MHz)=-12.5 dB can be realized. On the other hand, the level down in
the dip decreases, but apparently the level down of -12.5 dB or
more can be realized due to both effects.
[0223] When the spike wave fall time is appropriately selected in
this manner, the fundamental ultrasound with the 2f.sub.0 or
3f.sub.0 component inhibited therein can be generated.
[0224] Moreover, FIG. 24A shows a drive voltage waveform 432 of a
trapezoidal wave as another example of the drive signal supplied to
the transmitting piezoelectric resonator. FIG. 24B shows the
frequency characteristic of the drive voltage waveform 432 of the
trapezoidal wave, and shows a negative inclination 436 in the
vicinity of the first dip frequency, first dip frequency 442,
second dip frequency 444, and first peak frequency 446.
[0225] Also in the trapezoidal wave, similar to the aforementioned
spike wave, a specific relation is established between the first
dip frequency and the fall time. Therefore, the fundamental
ultrasound with the 2f.sub.0 or 3f.sub.0 component inhibited
therein can be generated by appropriately selecting the fall
time.
[0226] FIG. 25A shows the drive voltage waveform of a burst wave as
another example of the drive signal supplied to the transmitting
piezoelectric resonator, and FIG. 25B shows the frequency
characteristic. In this case, since a core of a burst wave 452 is a
sine wave, a side lobe is remarkably small, a main lobe band width
is also small by about -30 dB, and an ideal drive signal waveform
is obtained.
[0227] Also in the burst wave, a burst length tp and first dip
frequency have a relation shown in FIG. 26. Therefore, the first
dip frequency can be controlled by adjusting the burst length tp.
Therefore, when the drive signal having the spectrum D(j.omega.) is
used, a transmitted wave with either 2f.sub.0 or 3f.sub.0 frequency
component completely inhibited therein can be obtained.
[0228] In the ultrasonic transducer system of the fourth
embodiment, the center frequency of the pulser circuit 402 is
f.sub.0, and the circuit supplies the drive pulse signal having the
frequency characteristic with the first dip frequency of 2f.sub.0
to the transmitting ultrasonic vibrator. Thereby, the ultrasound
with the 2f.sub.0 component inhibited therein is generated from the
transmitting ultrasonic vibrator.
[0229] Alternatively, the pulser circuit 402 may supply the drive
pulse signal having the frequency characteristic with the center
frequency being in f.sub.0 and the first dip frequency being in
3f.sub.0 to the transmitting ultrasonic vibrator. Thereby, the
ultrasound with the 3f.sub.0 component inhibited therein is
generated from the transmitting ultrasonic vibrator.
[0230] According to the fourth embodiment, the ultrasound with the
inhibited 2f.sub.0 or 3f.sub.0 component can be transmitted by
controlling the drive signal waveform. As a result, the second
order or third order harmonic signal generated in the object by the
fundamental ultrasound can be received at a high S/N.
[0231] Additionally, the respective constitutions of the embodiment
of the present invention can of course be modified or changed in
various ways.
[0232] For example, in the fourth embodiment, the ultrasonic
transducer has a circular opening, but the opening shape is not
limited to circle. The opening of the ultrasonic transducer may be,
for example, rectangular, elliptical, or strip-shaped. Moreover,
the ultrasonic transducer may be an electronic scanning array
transducer, and each element constituting the array may have a
fundamental wave transmitting vibrator and high order harmonic wave
receiving vibrator in the same plane as in the fourth
embodiment.
[0233] [Fifth Embodiment]
[0234] The ultrasonic transducer of a fifth embodiment will be
described with reference to FIG. 27.
[0235] The ultrasonic transducer of the fifth embodiment is similar
to the ultrasonic transducer described in detail in the third
embodiment with reference to FIG. 14, and in FIG. 27, members
denoted with the same reference numerals as those of FIG. 14 are
equivalent members.
[0236] The ultrasonic transducer of the fifth embodiment has a
thin-piece spiral heater 502 inside the acoustic lens 308. One end
of the heater 502 is electrically connected to the surface
electrode of the transmitting piezoelectric resonator 302 via a
fine conductor 504, and the other end thereof is connected to the
housing 306 via fine conductor 506. The spiral thin-piece heater
502 is preferably disposed as close as possible to the transmitting
piezoelectric resonator 302 to such an extent that the heater does
not contact the electrode on the ultrasonic emission surface side
of the transmitting piezoelectric resonator 302.
[0237] The heater 502 gives a temperature gradient to the
transmitting piezoelectric resonator 302 along a thickness
direction, and gives a functionally gradient characteristic to at
least one of the piezoelectric constant and permittivity.
[0238] It is known that the vibrator characteristic can be changed
by imparting the functionally gradient characteristic to the
piezoelectric constant or the permittivity of the piezoelectric
resonator (Akira Yamada: "Piezoelectric Function Inclination type
Broad Band Ultrasonic Transducer" in commemoration of 2000,
Advanced Technique Symposium "Piezoelectric Material and Elastic
Wave Device" Text (February, 2000) pp. 31 to 38).
[0239] In FIG. 28, a broken line 512 shows an impedance
characteristic of the piezoelectric resonator having no
functionally gradient, and a solid line 514 shows the impedance
characteristic of the piezoelectric resonator having the
functionally gradient characteristic with the piezoelectric
constant e.sub.33.
[0240] As seen from FIG. 28, in the piezoelectric resonator having
the functionally gradient characteristic, a third order
piezoelectric vibration 516 largely generated in the piezoelectric
resonator having no functionally gradient characteristic is
inhibited. Therefore, the third order piezoelectric vibration 516
can be inhibited by imparting the functionally gradient
characteristic to the transmitting piezoelectric resonator.
[0241] In the ultrasonic transducer of the fifth embodiment, the
functionally gradient characteristic is imparted to the
transmitting piezoelectric resonator 302 by heating the vibrator by
the heater 502, with the intention of inhibiting third order
piezoelectric vibration.
[0242] Since the transmission/reception of the ultrasound in the
ultrasonic transducer of the fifth embodiment is the same as that
of the ultrasonic transducer shown in FIG. 14, a description
thereof is omitted to avoid redundancy. Only a part associated with
the heater 502 will be described hereinafter.
[0243] When the drive signal, for example, a burst wave signal is
applied between the housing 306 and the wiring 332 in the
transmitting piezoelectric resonator 302, a current flows through
the wiring 332, transmitting piezoelectric resonator 302, conductor
504, spiral thin-piece heater 502, conductor 506, and housing 306
in order. When current flows through the spiral thin-piece heater
502, the current is converted into Joule heat. Since the spiral
thin-piece heater 502 is disposed in the vicinity of the
transmitting piezoelectric resonator 302, the heat generated in the
spiral thin-piece heater 502 is efficiently transmitted to the
transmitting piezoelectric resonator 302.
[0244] On the other hand, the backing layer 310 formed, for
example, of a resin with a tungsten powder densely dispersed in a
satisfactorily thermally conductive silicone resin, is bonded to
the back surface of the transmitting piezoelectric resonator 302.
Furthermore, the space 336 inside the housing 306 is also filled
with a material having a satisfactory thermal conductivity such as
the silicone resin. Therefore, the heat transmitted through the
transmitting piezoelectric resonator 302 is satisfactorily radiated
from the back surface side of the vibrator.
[0245] As a result, a temperature gradient is generated in the
thickness direction of the transmitting piezoelectric resonator
302, and the functionally gradient characteristic concerning the
permittivity and/or the piezoelectric constant is imparted into the
transmitting piezoelectric resonator 302. Therefore, as described
with reference to FIG. 28, the third order piezoelectric vibration
of the transmitting piezoelectric resonator 302 is inhibited.
Thereby, the ultrasound in which the component of 3f.sub.0 is
inhibited is generated from the transmitting ultrasonic
vibrator.
[0246] In the fifth embodiment, the third order piezoelectric
vibration of the transmitting piezoelectric resonator 302 is
inhibited. On the other hand, a second order piezoelectric
vibration 518 of the transmitting piezoelectric resonator 302 is
excited. Therefore, the ultrasonic transducer of the fifth
embodiment may be combined with the drive control for inhibiting
the component of 2f.sub.0 described in the fourth embodiment.
[0247] The temperature gradient to be applied to the transmitting
piezoelectric resonator 302 strongly depends on the temperature
characteristic of the permittivity or the piezoelectric constant of
the transmitting piezoelectric resonator 302. Generally, it is
known that with a lower Curie point of the piezoelectric resonator,
dependence of the permittivity or the piezoelectric constant on
temperature increases, and a temperature difference to be applied
to the front/back surface of the piezoelectric resonator may be
small.
[0248] For example, to impart the functionally gradient
characteristic such that the permittivity is 3200 on the front
surface and 2200 on the back surface of the vibrator to the
transmitting piezoelectric resonator 302, when the temperature
characteristic of the permittivity of the transmitting
piezoelectric resonator 302 changes by 1% per 1.degree. C., the
temperature difference to be applied to the front/back surface may
be around 26.degree. C.
[0249] According to the fifth embodiment, when the functionally
gradient characteristic regarding the permittivity and/or the
piezoelectric constant is imparted to the transmitting
piezoelectric resonator, the third order piezoelectric vibration is
inhibited.
[0250] Furthermore, when the drive control described in the fourth
embodiment is combined for use, the ultrasonic pulse close to the
ideal waveform only of the fundamental wave component is
transmitted from the transmitting ultrasonic vibrator.
[0251] Additionally, the respective constitutions of the embodiment
of the present invention can of course be modified or changed in
various ways.
[0252] For example, in the fifth embodiment, the ultrasonic
transducer has a circular opening, but the opening shape is not
limited to a circle. The opening of the ultrasonic transducer may
be, for example, rectangular, elliptical, or strip-shaped.
Moreover, the ultrasonic transducer may be an electronic scanning
array transducer, and each element constituting the array may have
a fundamental wave transmitting vibrator and high order harmonic
wave receiving vibrator disposed in the same plane as in the fifth
embodiment.
[0253] In the fifth embodiment, the example in which means for
imparting the temperature gradient comprises the spiral thin-piece
heater has been described, but the means for imparting the
temperature gradient may be other means, such as a Peltier element.
Particularly, since the Peltier element has a cooling end on one
end thereof and a heating end on the other end thereof, a heat
utilization efficiency is high, and controllability is also
satisfactory. Therefore, the element can be said to be preferable
means when there is an allowance in an outer dimension of the
transducer.
[0254] As another modification of the ultrasonic transducer, an
ultrasonic transducer including the transmitting piezoelectric
resonator having the functionally gradient characteristic in at
least one of the piezoelectric constant and the permittivity will
be described.
[0255] In the fifth embodiment, for the object of inhibiting the
third order piezoelectric vibration of the transmitting
piezoelectric resonator 302, the ultrasonic transducer including
the means for imparting the functionally gradient characteristic to
the transmitting piezoelectric resonator 302 has been described.
However, in order to achieve the same object, instead of disposing
the means on the ultrasonic transducer, the transmitting
piezoelectric resonator itself may include the functionally
gradient characteristic on at least one of the piezoelectric
constant and the permittivity.
[0256] The ultrasonic transducer of the present modification is
structured by replacing the transmitting piezoelectric resonator
302 with the piezoelectric resonator having the functionally
gradient characteristic in the ultrasonic transducer shown in FIG.
14. FIG. 29 shows a partial section of a functionally gradient
piezoelectric resonator 520 having the functionally gradient
characteristic, with which the transmitting piezoelectric resonator
302 of the ultrasonic transducer of FIG. 14 is replaced.
[0257] The functionally gradient piezoelectric resonator 520 has a
pair of electrodes 522a, 522b, and a piezoelectric layer 524 held
between the electrodes. The piezoelectric layer 524 has a plurality
of piezoelectric thin films 526a, 526b, . . . , 526z laminated on
one another. Each of the piezoelectric thin films slightly differs
in the permittivity and/or the piezoelectric constant along a
lamination direction with respect to the adjacent piezoelectric
thin film.
[0258] For example, for the piezoelectric thin films 526a, 526b, .
. . , 526z, there is almost no difference in other constants
excluding the permittivity and including the Curie point. The
uppermost piezoelectric thin film, that is, the ultrasonic emission
surface side piezoelectric thin film 526a has a permittivity of
3200, and the lowermost piezoelectric thin film 526z has a
permittivity of 2200. As a whole, the films have the functionally
gradient characteristic shown in FIG. 30.
[0259] The piezoelectric resonator 520 itself having the
functionally gradient characteristic has a large inclination as
compared with the functionally gradient characteristic created by
imparting the temperature gradient to the piezoelectric resonator
having no functionally gradient characteristic. Therefore, the
third order piezoelectric vibration 516 is more dramatically
inhibited.
[0260] The piezoelectric layer of the functionally gradient
piezoelectric resonator is not limited to the lamination of the
piezoelectric thin films different in the permittivity and/or the
piezoelectric constant from one another. At least one of the
piezoelectric constant and the permittivity may only have the
functionally gradient characteristic. For example, the layer may be
formed by diffusing an impurity ion from one surface of a
plate-shaped piezoelectric material.
[0261] Additional advantages and modifications will readily occur
to those skilled in the art. Therefore, the invention in its
broader aspects is not limited to the specific details and
representative embodiments shown and described herein. Accordingly,
various modifications may be made without departing from the spirit
or scope of the general inventive concept as defined by the
appended claims and their equivalents.
* * * * *