JPS6258466B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a probe having an impedance conversion circuit that eliminates the adverse effects of a shielded wire connecting an ultrasonic probe and an imaging device main body.

Until now, the piezoelectric material used in the probes of mechanical scanning medical ultrasound diagnostic equipment or flaw detection equipment has mainly been lead zirconate titanate (PZT).
Since its dielectric constant is several hundred or more and its area is several cm ² , its capacitance is sufficiently larger than that of the shielded wire, and the influence of the shielded wire can be almost ignored. However, as the frequency increases, piezoelectric materials other than PZT, such as lithium niobate (LiNb)
As piezoelectric materials with low dielectric constants and small areas such as zinc oxide (ZnO) are used, the influence of shielded wires can no longer be ignored.

The case where LiNb is used as the piezoelectric material will be explained. The dielectric constant of this piezoelectric material is about 30, and when a probe is constructed from a single element (vibrator) with a diameter of 10 mm and a thickness of about 0.1 mm, the capacitance is
It will be about 200PF. On the other hand, the shield wire that connects the probe and the imaging device itself is usually 100~
A wire with a capacity of about 200 PF/m is used, and lowering the capacity further than this increases the wire diameter, which is undesirable. The shielded wire will be about 5-10m, so
The capacitance of the entire shielded wire reaches 500 to 2000 PF, which is larger than the capacitance of the piezoelectric material itself.
When such a capacitor is inserted in parallel with a piezoelectric body,
Significant deterioration in sensitivity occurs when receiving ultrasonic waves. Figure 1 shows an equivalent circuit when a piezoelectric body receives waves, and consists of a constant current source 1 proportional to the magnitude of the incident ultrasonic wave, a damping capacity 2 of electrostatic capacitance C _d , and a motional admittance (size Ym) 3. Become. Since the piezoelectric body is normally used in a sufficiently damped state, the admittance 3 is considered to be sufficiently larger than ωC _d and will be ignored hereafter. However, ω is the resonance angular frequency of the piezoelectric material. This piezoelectric body has a capacitance C _s
Consider a case where shielded wires 4 are inserted in parallel. This equivalent circuit is shown in FIG. When an ultrasonic wave is incident as in FIG. 1, a constant current I _s is generated, and the voltage V ₂ between the terminals of the piezoelectric body becomes V ₂ =I _s /ω(C _d +C _s ). This voltage is reduced to C _d /(C _d +C _s ) compared to the case without the shield wire 4. for example,
Assuming that C _d = 200PF and C _s = 700PF, it can be seen that when the shield wire 4 is inserted, it decreases to 22% of the value without it. By the way, conventionally, in order to reduce the influence of the shield wire 4, a coil 5 having an inductance L has been inserted as shown in FIG. In this case, if the Q value of the coil 5 is _Ql , the output voltage _{V3 becomes QlV2 ,} which increases by a factor of _Ql compared to the case where the coil 5 is not provided. However, since the magnitude of Q _l is determined by the structure of the coil and the frequency band used, it cannot be increased indefinitely. Furthermore, as Q _l increases, the band becomes narrower, leading to waveform deterioration, so it is usually about 2 to 3. Therefore, if there is a shielded wire,
Coils inserted in parallel only cancel out the decrease due to the capacitance of the shield wire, and it has been difficult to obtain an output voltage higher than that.

The present invention was made in view of this point, and its purpose is to provide an impedance converter between the shield wire and the probe, not only to eliminate the adverse effects of the shield wire, but also to create a probe with even higher sensitivity. The point is that we have achieved this. The present invention will be explained below using the drawings.

As shown in FIG. 4, the impedance converter consists of a transformer 6 and a coil 7 having an inductance L _s inserted on the secondary side. Let the step-up ratio of the transformer be n, the primary and secondary inductances be L ₁ and L ₂ , respectively, and L ₂ >L _s . Since the transformer multiplies the voltage applied to the piezoelectric body by n when transmitting waves, and multiplies the voltage generated in the piezoelectric body by 1/n when receiving waves, there is no boosting effect when considering wave transmission and reception. However, when the impedance conversion effect is taken into account, sensitivity is improved. In other words, the impedance Z ₂ when viewing the transformer 6 from the secondary side during wave reception is:
Since the load on the primary side is multiplied by 1/n ² , Z ₂ =
1/n ²・1/ωC _s . However, ω is the target angular frequency, and C _s is the capacitance of the shield wire. Therefore, the equivalent circuit when receiving waves is as shown in Figure 5,
The shield wire has a capacitance 8 of 1/n ² C _s and is inserted in parallel with the piezoelectric body. Since n can be easily achieved on the order of 2 to 3, C _s is reduced to about 1/10 to 1/10. Therefore, since C _s can be apparently made sufficiently smaller than C _d , its influence becomes extremely small. It has been found that the influence of the shielded wire can be reduced for the above reasons, but by inserting the coil 7 on the secondary side, the generated voltage is multiplied by Q, so the sensitivity is improved compared to when there is no shielded wire. Furthermore, by setting L ₂ ≫L _s , 2
Since the parallel resonance condition on the next side can be achieved by selecting only L _s , the degree of freedom for the transformer increases. That is, if L ₁ and L 2 are selected so that L ₂ and the braking capacity 2 are arranged in parallel, and L ₁ and the shield wire ₄ are arranged in parallel, the step-up ratio n becomes √ _{2 1} , and n cannot be freely selected. This may make the impedance converter ineffective in some cases, and by setting L ₂ >L _s , the degree of freedom in designing the transformer is dramatically increased.

Examples of the present invention will be described in detail below.

Figure 6 shows the relationship between the voltage that drives the probe and the reflected wave voltage, in the conventional case 9 where an inductance is simply inserted in parallel, and in the case 10 of the present invention shown in Figure 4. 11 is shown for case 11. Thickness 100 for piezoelectric material
LiNb with a diameter of 10 mm and a capacitance of 200 PF is used. Also, as a transformer, the thickness is 2
The core is annular ferrite with an inner diameter of 5 mm and an outer diameter of 10 mm, and is wound with 11 turns on the primary side and 40 turns on the secondary side. Its inductance is 2.8μ each
H, 37 μH. On the secondary side, a coil of several μH is inserted in parallel with the piezoelectric material to cause resonance.

7m of 110PF/m shielded wire is used, and its good capacitance is 770PF.

As is clear from FIG. 6, even though the capacitance of the shield wire is nearly four times the braking capacitance of the piezoelectric material, the sensitivity of the probe with the impedance converter of the present invention depends only on the inductance or It can be seen that there is an improvement of more than 10 dB compared to the case without insertion.

FIG. 7 is a diagram showing the sensitivity when the primary inductance of the transformer is changed in the present invention. It can be seen that the maximum sensitivity is reached at the point where the primary inductance and the shield wire almost resonate.

Note that the capacitance caused by the lead wire on the secondary side of the transformer also causes sensitivity deterioration, so the transformer needs to be built into the probe or installed near the probe.

In particular, it is preferable to incorporate the transformer into the probe because it facilitates shielding the transformer from external noise.

Figure 8 shows a cross section of a probe with a built-in transformer, and the connector 11 following the shield wire is connected to the transformer 1.
2, and its secondary side is connected to the piezoelectric body 13. An inductance 14 is connected in parallel to the secondary side. 15 is a shield case 16 is a backing material.

As explained above, even in the case where the capacitance of the shield wire cannot be ignored with respect to the capacitance of the piezoelectric body, the present invention can achieve an improvement in sensitivity of about 10 dB. It should be noted that the present invention is not limited to the piezoelectric body or the imaging method shown here, but can of course be applied to cases where the capacitance of the shield wire cannot be ignored compared to the capacitance of the piezoelectric body.

Furthermore, in the present invention, a case will be described in which a transformer having a ferrite core is used as the impedance converter.

Probes that transmit and receive ultrasonic waves are increasingly using higher frequencies due to demands for higher resolution. Along with this, piezoelectric materials used in probes are also becoming smaller and their capacitances are also becoming smaller. Therefore, the capacitance of the shielded wire that connects the device body and the probe becomes a problem, and as a solution to this problem, the use of an impedance converter using a transformer can be considered.
Even in this case, in order to shorten the shielded wire from the transducer to the probe, it is desirable to install the transducer as close to the piezoelectric body as possible, and in some cases it is necessary to incorporate it into the probe. Therefore, a smaller transformer is also required.

Currently, the operating frequency of the probe is mainly 2 to 10 MHz, and a transformer having a ferrite core, especially an annular core, is preferable from the viewpoint of miniaturization and low loss.

However, since the ferrite core has magnetostriction, the magnetostrictive frequency also increases as the size is reduced, which has an adverse effect on the operation of the probe.

For example, considering an annular ferrite core with an inner diameter d _i , an outer diameter d _e , and a sound velocity v, its fundamental frequency f ₀ is expressed by the following equation.

f ₀ =2v/π(d _i +d _e ) When a pulse magnetic field is applied in the circumferential direction of this annular ferrite, it expands and contracts in the circumferential direction due to magnetostriction, and the inductance of the coil wound around the ferrite core changes. Since an ultrasonic probe is normally driven with a very narrow pulse voltage, its frequency components are distributed over a wide frequency range, and the ferrite core is naturally excited as well. As a result, immediately after a pulse is applied to the probe, the inductance of the coil changes in response to the vibration of the ferrite core, which generates a false signal.

In the case of a ferrite core with d _i = 5 mm, d _e = 9 mm, and v = 5000 m/s, the fundamental frequency f ₀ is 230 KHz, but in addition to this vibration, harmonic components are generated,
Oscillations were observed over a range of up to several MHz. That is, large changes in inductance were observed near these frequencies. The band of the receiver that amplifies the signal from the probe is generally wide because it handles pulse signals, and is usually about 30 to 40% of the center frequency. For this reason, it is difficult to distinguish and remove pseudo-false signals based on vibrations of the ferrite core from true signals, resulting in the problem that the probe cannot be operated normally.

In order to eliminate this problem, the present invention is characterized in that magnetostrictive vibrations of the ferrite core are suppressed.

FIG. 9 shows a voltage waveform 17 that drives the probe, and a subsequent pseudo-false signal waveform 18 based on magnetostrictive vibration.
The true signal waveform 19 is shown. If the magnetostrictive vibration of the ferrite core is not suppressed, a false signal larger than the true signal will be generated as shown in FIG. 9 in a situation where the true signal is attenuated. FIG. 10 shows the case where the ferrite core, including the windings, is hardened with an epoxy resin adhesive (ALTECO 30), and it can be seen that magnetostrictive vibrations are sufficiently suppressed and can be almost ignored.

In order to suppress magnetostrictive vibrations, it is possible to surround the ferrite core with a hard frame, but this would increase the size of the transformer and insert a material other than the ferrite core into the winding, which would reduce the Q value. This is not the preferred method because it invites On the other hand, when a resin adhesive is used, not only does the electrical insulation property increase, but it also sufficiently permeates between the wires, making it possible to easily suppress unnecessary vibrations of the ferrite.

As described above, according to the present invention, a transformer for impedance conversion can be downsized and can be easily built into a probe.

[Brief explanation of the drawing]

Figure 1 shows the equivalent circuit of the piezoelectric body during wave reception;
Figure 3 shows the conventional configuration method, Figure 4 shows the configuration method of the present invention, Figure 5 shows the equivalent circuit thereof, Figure 6 shows the reflected wave voltage characteristics, and Figure 7 shows the configuration method of the present invention. FIG. 8 is a cross-sectional view showing the configuration of the probe; FIG. 9 is a diagram showing how pseudo-false signals are generated when the magnetostriction of the Ferrite core is not suppressed; and FIG. 10 is a diagram showing the transmitting and receiving sensitivity characteristics. FIG. 3 is a diagram showing how the S/N ratio is improved by suppressing magnetostriction.

Claims (1)

[Scope of Claims] 1. A transformer with a larger number of secondary windings than the number of primary windings, and a transformer with a larger number of secondary windings than the number of primary windings, at the connection part between the shielded wire leading from the ultrasonic imaging device to the probe and the probe. An ultrasonic probe comprising an impedance conversion circuit inserted into the secondary side of a transformer and comprising an inductance that resonates substantially in parallel with the capacitance of the piezoelectric body of the probe. 2. Claim 1, characterized in that the primary inductance of the transformer is selected to resonate substantially in parallel with the capacitance of a shielded wire connected to the primary side of the transformer. ultrasonic probe. 3. The ultrasonic probe according to claim 1, wherein a ferrite core that suppresses magnetostrictive vibration is used as the transformer. 4. The ultrasonic probe according to claim 3, wherein an annular core is used as the ferrite core. 5. The ultrasonic probe according to claim 1, wherein a core hardened with a resin adhesive is used as the ferrite core.