JPS6053399

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DESCRIPTION JPS6053399
[0001]
The present invention relates to a probe for an ultrasonic diagnostic apparatus using a
piezoelectric ceramic as electroacoustic conversion means. An ultrasonic probe used in an
ultrasonic diagnostic apparatus is required to have a wide band, low loss, and low pull in order to
improve the sensitivity and resolution in the distance direction. In general, an ultrasonic probe
having a vibrator main body formed of a piezoelectric ceramic material as an electroacoustic
conversion means has high electroacoustic conversion efficiency, but has acoustic consistency
with a human body and a medium such as water. It is bad that it is difficult to widen the
frequency band width. In addition, in an ultrasonic probe having a vibrator main body formed of
a polymeric piezoelectric material such as porphy or vinylidene, a good match with the human
body and a medium such as water can be obtained, and the frequency bandwidth is set wide.
However, it is well known that there is a disadvantage that the electro-acoustic conversion
efficiency, particularly the electro-acoustic conversion efficiency at the time of transmission is
low. For this reason, conventionally, an ultrasonic probe having an acoustic impedance matching
layer using a piezoelectric ceramic as an electroacoustic conversion material has been proposed.
In the probe in which one matching layer is formed on a piezoelectric ceramic, a relative
bandwidth of about 40-is obtained, and in the probe in which a double matching layer is formed,
a relative bandwidth of about 70 inches is obtained. It is obtained. However, in order to further
improve the performance of the diagnostic device, a further broad band characteristic is required
of the probe, which is disadvantageous in the performance of the conventional probe. In order to
meet the above requirements, the present invention provides an ultrasonic probe which has low
loss and low ripple characteristics, and realizes a far wider band characteristic than a
conventional probe in which a double matching layer is formed. Intended to be provided. That is,
according to the present invention, in the ultrasonic probe having a triple matching layer and
using the piezoelectric ceramic as the electric sound conversion material, the characteristic
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acoustic impedance density of the first matching layer formed on the piezoelectric ceramic
vibrator is calculated. When the characteristic impedance impedance of the second matching
layer formed on the first matching layer is Ih + and the characteristic acoustic impedance density
'tels of the third matching layer formed on the second matching layer, 12.6 X t O '(Kq / s-m'))) S
18 18. IX10 (Kf/s・ni )3.8X10 CF−r/a・
7F/)≦h≦6.0X10 (Kg/s・nl)1.7X10 (Kr/s−nl)≦、
)3く2.4X10 (Kg/s−r+?
And the sound velocity of the longitudinal waves of the first matching layer, the second matching
layer, and the third matching layer is 91 + 11 respectively. v, the thickness t1 of the first
matching layer, the thickness t of the second matching layer, and the thickness of the third
matching layer, where t0 is the mechanical resonant frequency (electrical antiresonant
frequency) tfo of the piezoelectric ceramic vibrator itself It is an ultrasonic probe characterized in
that t3 is set. Next, the present invention will be described. FIG. 1 is a schematic cross-sectional
view of the ultrasonic probe of the present invention. In FIG. 1, 10 is a piezoelectric ceramic
polarized in the direction of propagation of ultrasonic waves (indicated by an arrow in the
figure), and 14 ° 15 is an electrode. Generally, the piezoelectric ceramic has a characteristic
acoustic impedance density 2 ° of about 30 × 10 ′ to 38 × 10 ′ Kf / 8 · −, which is more
than 20 times that of water. 11.12.13 are the ith, xz, and third acoustic matching layers
respectively, and the acoustic matching layers close to the medium 16 are arranged such that the
characteristic acoustic impedance density is small. Here, the characteristic acoustic impedance
density is represented by the product of the velocity of sound and the density. In FIG. 1, suffix 0
indicates piezoelectric ceramic, 1 indicates matching layer 1, 2 indicates matching layer 2.3
indicates matching layer 3, electromechanical coupling coefficient, ε-3 indicates dielectric
constant, Zo (=) os) + Zt (=, h8) r Zt (-) ts). Z, (=) 3S) is the characteristic blue tide impedance, t6 *
t1 * 12.1s is the layer thickness, vOr 'I, l 7! 1 V3 is the speed of sound, and zL is the load acoustic
impedance. An ultrasonic probe with a triple matching layer (S is a cross-sectional area) has the
possibility of broadband characteristics, but on the other hand, if the optimization of the
alignment no is not completely performed, a large ripple may be generated in the passband. Will
lead to an increase in insertion loss. Therefore, since the triple matching layer is used, the
performance of the overall probe is rather deteriorated, and the possibility of producing a probe
which can not be put to practical use at all is extremely increased. The present invention focuses
on the fact that the ultrasonic probe having a triple matching layer is a quadruple mode
transducer, and applies the synthesis theory of the multimode filter to achieve optimization of
the matching layer, thereby achieving wide band and low ripple. And it is an attempt to supply
high-performance probes such as low loss. Next, the principle of the present invention will be
described. The equivalent circuit of the ultrasonic probe having the triple matching layer shown
in FIG. 1 is represented as shown in FIG. 2 by using a Mason ceramic equivalent circuit in the
piezoelectric ceramic portion. In FIG. 2, co is a damping capacity, φ is an electro-mechanical
conversion ratio, and between each value, φ2-ω n Coz o k ′ ′ / π (ω (ω).
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= 2πf, (2) θ, -ωt1 / vl (i-0, 1, 2, 3) (3) Here, the piezoelectric ceramic portion operates as a halfwave oscillator, and is set in relation to the characteristic impedance density) o> h> 2t>; js. At this
time, it can be understood from the mechanical input impedance characteristic when the probe is
viewed from the load side that the present probe has all four resonance modes close to each
other. FIG. 3 shows these four resonance modes normalized by the vibration velocity v6 of one
end face of the piezoelectric ceramic. It can be seen from FIG. 3 that the odd-order (first and
third) resonance modes and the even-order (second and fourth) resonance modes are 180 ° out
of phase with each other. (In FIG. 3, v1 is the vibration velocity in the probe. That is, a probe
having a triple matching layer can be regarded as a quadruple mode band pass filter, and the
design theory of the multimode filter can be applied. Considering that the damping capacity C6
minimizes the influence by externally adding an inductance in series as a design principle, the
right side portion of 1: φ in the equivalent circuit of FIG. 2 is examined. In the right part of 1: of
the equivalent circuit of FIG. 2, the impedance of the to-hand arm is za, and the impedance of the
lattice arm is zb. It can be equivalently converted to a 1: n transformer with Janmar + rB type
circuit and frequency characteristics, the even resonance modes are owned by z8 and the odd
resonance modes are owned by zb. At this time, if z a and z b have different signs, it will be a pass
band, and by designing z a and z b, it is possible to design a probe. The image impedance 2 °
seen from the load side is given by z, 2 = 5K (4) and * stop is the stopband when ZOf is an
imaginary number, and is the passband when it is a real number, and the probe has a triple
matching layer In case generally 91 ', 92' as shown in FIG. Five stop zones indicated by 93 ', 94'
and 95 'and six pass zones indicated by 41.42 DEG 43.44.45.46 appear. Of these, the stop zone
93 'appears near the center frequency fc of the probe, but the frequency bandwidth of this stop
zone is much narrower than the other stop zones. The stop zones which have a very large
influence on the ripples of the probe passband mainly are '2', 44 ', and the stop zones 91', 95 'at
both ends do not give much shadow qIi. In FIG. 4, the solid line indicates real value 2 and the
dotted line indicates an imaginary value. Therefore, the matching layers may be designed such
that the stop regions 42 ', 43', 44 'are minimized and that 2.degree. 2 coincides with the load
impedance z2 near the center frequency fc.
Next, based on this principle, subtraction and experiment are performed to verify the
effectiveness of the present invention. Acoustic impedance density,). Is 36.40 × 10 ′ (Kg /
B−rr?), And the coupling coefficient k is 0.536. Specific electricity ratio ε ;, /. The matching
layer 'r3 layer was formed on this vibrator using a 230 piezoceramic ceramic vibrator. The probe
of the structure shown in FIG. 1 was designed as the characteristic impedance density 211 rat, is
and the total thickness parameter of the matching layer. Here, for simplicity, the thickness of the
matching layer is calculated with the same wavelength for all three layers. As a representative
example of a probe according to this design) + = 16.2 × 10 '(K9 / 8 "m'))! Calculation examples
will be shown for the cases of 4.8 × 10 ′ (Kp / s−m ′) and −2.0 × 10 ′ (Kf / s − + n ′)
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and the thickness of the matching layer. The frequency characteristic of 2 ° in the passband is
shown in FIG. Here, the frequency is the electrical antiresonance frequency f of the piezoelectric
ceramic vibrator. Is standardized. The solid line in the figure indicates that 2 ° 2 is a real
number 1 and the dotted line indicates that an imaginary value is taken. As apparent from the
figure, the stop zone 43 'shown in FIG. 4 completely disappears, and at the same time, the stop
zones 42' and 44 'become smaller, and further 2.degree. It can be seen that the real value shows
a value close to the impedance of water over a wide range. Therefore, it is suggested that the
probe according to the present design can realize extremely flat loss characteristics in the pass
band, and can realize a probe excellent in bonding with the human body. Next, the details of the
present invention will be described according to an embodiment. A disk-like piezoelectric ceramic
vibrator having a diameter of 1.76 cr ++ and a thickness of 1.0 H, which is the same as the
material constant of 12i described above, was fabricated, and polarized in the thickness direction
to mirror-polish the main surface. A Cr-Au electrode film having a thickness of 3000 A was
formed on the main surface of this vibrator. A first composite layer made of an optical glass
made of an optical glass having the same value as the values of the above-mentioned design
example and the thickness of the front tube on the entire surface on one side of this disc shaped
vibrator, a second composite made of epoxy resin The ultrasonic probe was manufactured by
bonding a layer and an m3 matching layer made of urethane resin. A tuning coil was added in
series using this probe to emit ultrasonic waves into water, and the insertion loss of the round
trip was measured by receiving the reflected wave from the aluminum block reflector at a water
depth of 5 crn. The reciprocation insertion loss characteristic of the prototyped probe is shown
by a solid line in FIG.
It can be seen that a low ripple characteristic with an insertion loss of 2.6 dB, a 6 dB relative
bandwidth of 91 bits, and 0, 7 dB or less is obtained. The characteristic shown by the dotted line
in FIG. 6 is a calculated value, and the deviation between the measured value and the calculated
value is considered to be mainly due to the spread of the ultrasonic beam and to the elastic loss
of the probe. Very good agreement is observed. It is understood that such characteristics of
broadband low ripple and low loss are characteristics that can not be realized at all by the
conventional ultrasonic probe, and the ultrasonic probe of the present invention has extremely
excellent performance. Ru. In addition, the thickness and characteristic acoustic impedance
density of each matching layer become uneven due to manufacturing variations and material
constants, and the thickness and characteristic acoustic impedance density of each matching
layer can be used in designing a probe. It is important to find out the optimum range where
sufficient performance can be obtained. Therefore, the relationship between these parameters
and the passband characteristics is calculated from both the experiment and the theory, using the
thickness of each matching layer and the characteristic impedance density as parameters
centering on the values of the above-mentioned embodiment, and the band extremely difficult to
realize in the conventional structure. We have found an optimum range in which a practically
sufficient performance of 3 dB or less of internal ripple can be obtained. As a result, as a
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practically sufficient range, 12.68610 'Kv / s-m "<2m <18.1 X 10' Kq / s n?
3.8X10 K9/s −≦1. <6.0 × 10 ′ Kp / s ··· 1, 7 × 10 ′ Kf / s−m ′ <23 <2.4
× 10 ′ Kg / s ·········· Further, the electric antiresonance frequency of the piezoelectric sessian
vibrator is f. The thickness of each matching layer (when λ + = v + / fo (1 = 1.2, 3)) is obtained
when the binding is completed. In addition, as a material having a characteristic impedance
density falling within the above range, each stock glass for the matching layer 1, epoxy thread
resin for the matching layer 2, and nylon resin. There is a phenolic resin, etc., and there is a
polyimide resin, a urethane resin, a polysulfone resin, a nylon resin, etc. in relation to the
matching layer 1, and these are all readily available materials. In order to realize a probe having
such high performance, it is necessary to limit the thickness of each matching layer and the
characteristic acoustic impedance density to the above-mentioned range. If it deviates from this
range, insertion loss and loss fluctuation become large, which is not preferable in practical use.
Next, specific examples illustrating the above range will be described. The first matching layer is
a high density, high modulus optical glass (2, 17.6 × 10 ′ to 18.1 × 10 ° Kf / 8-W?), The
second matching layer ヲ honifene phenylene) 'resin (> t = s, 4x106-6.
OX 10 'Kg / s-door), the third matching layer is a polycarbonate resin (Js = 2.2 x 10' to 2.4 x 10
'K 9 / 5-i)' ii = range of thickness of each matching layer Was set in the above-mentioned range
to make a probe. In yet another embodiment, the first matching layer is a low density optical
glass (J □ = 12.6 × 10 ′ to 13.2 × 10 ′ Kg / s · ff /), and the second matching layer is an
epoxy resin ( 22 = 3.8 × 106-4. I X 10 'Kg / s-m'), The third matching layer is a polysulfone resin
() s = 1.7 x l □ 'to 1.9 x 10' to / 8 嘩-) t- The thickness of the matching layer was set to the above
range, the probe was damaged, and the reciprocation insertion loss characteristics were
measured. An example of those results is shown in FIG. The alternate long and short dash line is
the characteristic of the former probe in which the matching layer is a high density high modulus
optical glass / polyphenylene sulfide / polycarbonate, and the double dotted line is the latter in
which the matching layer is a low density optical glass / epoxy / polysal 7 on These are examples
of the characteristics of the probe, and both of them have sufficiently low practicality and pull
characteristics. still. It goes without saying that as the characteristic acoustic impedance of the
matching layer is closer to that of the matching layer used in the probe shown in FIG. 5, a high
performance probe can be obtained. In addition, although a figure example is not shown, in a
probe using a matching layer having a characteristic acoustic impedance density and thickness
exceeding the scope of the present invention, the loss variation in the passband is large and none
can be practically used. understood. Although the disk-shaped ultrasonic probe has been
described above by way of example, it is needless to say that the present invention can be applied
as it is to other ultrasonic probes. As described above, according to the present invention, a highperformance ultrasound probe can be realized using easily available interest, and the industrial
value is also enormous.
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[0002]
Brief description of the drawings
[0003]
FIG. 1 shows a schematic cross-sectional view of an ultrasonic probe having three matching
layers, 10 being a piezoelectric ceramic, 11 ° 12. 13 is a matching layer, 14.15 is an electrode,
16 is a load medium.
Arrows indicate polarization direction, electromechanical coupling coefficient, ε :, dielectric
constant, Zn, Z (* Zt, ZB characteristic acoustic impedance, zL load impedance, jn + tl + t, t3i'J.
Thickness, '16 + V @ + v2 + 13 is the speed of sound. FIG. 2 shows an equivalent circuit diagram
of an ultrasonic probe having three matching layers, where co is a damping capacity and φ is an
electromechanical metamorphic ratio. FIG. 3 shows the vibration mode gold at each resonance of
the ultrasonic probe having three matching layers, v6 is the piezoelectric ceramic, the vibration
speed of one end face of the beam, and vx is the vibration speed of the probe notice portion. FIG.
4 is a general image impedance characteristic diagram of an ultrasonic probe having three
matching layers, the image impedance when the probe is viewed from the Zozn load side, fc is
the center frequency of the probe, 41.42. 43.44.45.46 is the passband, 41 ', 42', 43 ', 44', 45 'is
the stopband. FIG. 5 is an image impedance characteristic diagram of the probe according to the
present invention. FIG. 6 is a representative reciprocating insertion loss characteristic diagram of
the probe according to the present invention. FIG. 7 is a reciprocating insertion loss
characteristic of the probe according to the present invention as well. E 1 Figure 2 c. Fig. 3 + 0 1
+ 12 13 Fig. 4 frequency Fig. 5 frequency (f / f 0) Fig. 6 0.4 0.6 0.8 1.0 +, 2 1.4 frequency (f / f)
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