Generally, a piezoelectric body is used in an ultrasound transducer. This is because a piezoelectric body has a so-called coupling effect between an electric system and a mechanical system for converting mechanical energy into electrical energy and, conversely, electrical energy into mechanical energy. The piezoelectric body has a sheet shape, a plate shape, or a rod shape and is provided with a pair of electrodes, one electrode being fixed to a rear layer and the other electrode being in contact with a medium via an acoustic lens or a matching layer.
Many piezoelectric ultrasound transducers emit sound waves toward a medium or detect sound waves propagating through the medium in a d33 mode or an e33 mode. It is generally considered that the d33 mode is a longitudinal vibration of a column vibrator and that the e33 mode is a thickness vibration of a plate vibrator. Ferroelectrics such as PZT ceramics and PVDF, high dielectrics such as P(VDCN/VAc), and porous polymer electret piezoelectric bodies exhibit the d33 mode or the e33 mode by retaining residual polarization due to an orientation of an electric dipole by polling processing. On the other hand, with piezoelectric crystals without residual polarization, a C axis in the case of piezoelectric crystals such as ZnO, LiNbO3, and KNbO3 and the A axis in the case of quartz are respectively oriented perpendicular to an electrode surface to exhibit the d33 mode or the e33 mode in the case of quartz, a d11 mode or an e11 mode). Piezocomposite materials are dependent on what materials are used.
With a piezoelectric body constituting an ultrasound transducer, a simplest mechanical boundary condition is a case where one end is a fixed end and the other end is a free end. In theory, an acoustic impedance Z (unit: MRayl.) and a boundary condition of an object in contact have a relationship in which Z=0 represents the free end and Z=∞ represents the fixed end. However, the present specification is not as strict, and with the exception of an adhesion layer and an electrode layer, a fixed end is assumed when an impedance Z of a piezoelectric material is smaller or equal to an impedance Z of an object in contact and a free end is assumed when the impedance Z of the piezoelectric material is greater. In addition, a resonance of a longitudinal vibration or a thickness vibration of a piezoelectric body is used by an ultrasound transducer to transmit and receive waves, and a resonance frequency fr thereof is determined primarily by physical properties and dimensions of the piezoelectric body and, to a lesser degree, by a structure of the transducer and how the transducer is pressed against a medium. Therefore, in the present specification, factors which alter the resonance frequency besides than dimensions and properties of a piezoelectric body are eliminated.
First, the resonance frequency fr in the d33 mode or the e33 mode of a piezoelectric body is given using a sound velocity v and a height (thickness) h of the piezoelectric body by Expression (1) below.fr=v/4h  (1)
This is generally referred to as a resonance in a λ/4 resonance mode, λ denotes a wavelength inside the piezoelectric body. There is also a λ/2 resonance mode with both ends free. A resonance frequency thereof is double the resonance frequency of the λ/4 resonance mode.
Meanwhile, the sound velocity v of the piezoelectric body is given by Expression (2) below in the case of a longitudinal vibration of a column vibrator and by Expression (3) below in the case of a thickness vibration of a plate-like thickness vibrator.v=(1/sρ)1/2  (2)v=(c/ρ)1/2  (3)where s denotes elastic compliance, c denotes elastic stiffness, and ρ denotes density.
From Expressions (1) to (3) above, it is understood that respective frequencies of wave transmission and wave reception by a transducer are primarily determined by a height (thickness) h, an elastic modulus s, and a density ρ of a piezoelectric body.
Furthermore, for ultrasound diagnostic devices used in the fields of medicine, architecture, and the like, there are demands for higher frequencies and improved wave transmitting/receiving performances in transducers for the purpose of obtaining images with higher resolutions. In regards to improving a wave transmitting/receiving performance of an ultrasound transducer using a piezoelectric body, electrical impedance matching between the transducer and an electric processing circuit is an important factor in transmitting electrical signals at a high S/N ratio. In addition, in regards to achieving a higher frequency, since wave transmitting/receiving frequencies are determined by the thickness of a piezoelectric body, the piezoelectric body must be thinned. Although thinning of a piezoelectric body causes a decline in electrical impedance and is therefore beneficial in terms of impedance matching with an electrical circuit, a magnitude of decline is, at best, no more than an inverse of a thickness ratio. Furthermore, thinning of a piezoelectric body increases the difficulty of a production process in terms of film thickness control, handling, and the like.
Therefore, in conventional art, a harmonic component of a wave transmission/reception signal of a conventional λ/4 resonant transducer is used in order to obtain a high-frequency signal. However, since a harmonic component has a lower sensitivity than a fundamental wave component and is susceptible to attenuation due to damping by the piezoelectric body or surrounding materials, it is difficult to obtain a signal with a high S/N ratio. In consideration thereof, an e33 thickness-stretch mode will be described with reference to FIG. 1 as an example of ultrasound wave transmission/reception using harmonics. FIG. 1 and the following description are presented in Non-Patent Document 1. Constants of the elements constituting an equivalent circuit shown in FIG. 1 are given in Expressions (4) to (6) below.Cn=pnkt2C0  (4)L=1/ωp12C1  (5)andpn=(1/n2)(8/π2),n=2m−1  (6)where Cn denotes a capacitance of each element, L denotes inductance, kt denotes an electromechanical coupling coefficient in the thickness-stretch mode, and ωp1 denotes resonance frequency.
By incorporating an approximation of pn≅1/n2 into Expression (6) above, Expression (4) becomes Expression (7) below.Cn/C0=k12/n2  (7)
Expression (7) indicates that an effective value of the electromechanical coupling coefficient in an n-th order harmonic decreases to 1/n, in the case of a first order mode, since n=1, Expression (7) becomes Expression (8) below.Cn=1/C0=kt2  (8)
Expression (8) is consistent with an expression when incorporating substitutions of ∈T=C0+Cn and ∈S=C0 in a relational expression (9) between kt and permittivity in the first order mode.∈T/∈S=1+kt2  (9)where ∈S denotes permittivity under a clamped condition, ∈T denotes permittivity under a free condition, and C0 and Cn denote capacitance. For the d33 mode, a similar result is obtained by replacing Expression (4) with Expression (10) below.Cn=pn(k332/1−k332)C0  (10)
In addition, an effective value of an electromechanical coupling coefficient when transmitting and receiving third order harmonics is given by Expression (7) in the case of n=3, and by letting an apparent coupling coefficient be kt′, kt′=kt/n=kt/3 is obtained. This result signifies that the apparent coupling coefficient attenuates to ⅓ when transmitting and receiving third order harmonics.
FIG. 2 is a graph showing frequency characteristics (calculated vales) of a complex permittivity of a piezoelectric body in which a first order mode of resonance in a thickness direction is represented by a frequency of 1 MHz. In FIG. 2, an abscissa represents frequency in units of MHz and an ordinate represents complex permittivity. However, kt=0.3, h/2v=2.485×10−7(s), and tan δm=0.04.
A local maximum and a local minimum of a real part (denoted by reference character α1) and a local maximum of an imaginary part (denoted by reference character α2) at 1 MHz are of the first order mode of resonance in the thickness direction. Subsequently, a third order harmonic component is observed at 3 MHz and a fifth order harmonic component is observed at 5 MHz. Meanwhile, as shown in FIG. 3, applying the third order harmonic component (dashed line) shown in FIG. 2 to a piezoelectric body model (solid line) with 3 MHz as a first order mode revealed that, by reducing a coupling coefficient and a piezoelectric body thickness of the piezoelectric body model to ⅓, a waveform of the piezoelectric body model becomes consistent with that of a piezoelectric body with 3 MHz as a third order mode. This result is consistent with the interpretation presented above. FIG. 3 is a graph showing frequency characteristics (calculated values) of a complex permittivity of a piezoelectric body exhibiting thickness vibration. In FIG. 3, an abscissa represents frequency in units of MHz and an ordinate represents complex permittivity. However, as described earlier, the dashed line represents kt=0.3, h/2v=2.485×10−7(s), and tan δm=0.04. On the other hand, the solid line represents k1=0.1, h/2v=3.300×10−7(s), and tan δm=0.04.
As described above, with conventional art, an apparent electromechanical coupling coefficient decreases to 1/n while a harmonic is being detected and electrical impedance is primarily determined by dimensions of a piezoelectric body.
Meanwhile, with medical ultrasound diagnostic devices, harmonic imaging (THI) diagnostics of body tissue using a harmonic signal is becoming a standard diagnostic modality due to its ability to produce diagnostic images with clarity not possible with conventional B-mode diagnostics. Harmonic imaging technology has many advantages due to the use of higher frequencies including a reduced sidelobe level, increased SiN, enhanced contrast resolution, a thinner beam width, improved lateral resolution, lower sound pressure at short distances, and a lower likelihood of an occurrence of multiple reflections due to less fluctuation in sound pressure.
In consideration thereof, in Patent Document 1, signals received by respective piezoelectric elements of an ultrasound transducer are added in a phasing adder circuit and then commonly inputted to a fundamental wave band filter and a harmonic band filter. In addition, outputs thereof are weighted by gains respectively corresponding to a depth of a diagnosis domain of a subject and then combined. Patent Document 1 proposes an ultrasound diagnostic device which accordingly interpolates an attenuation of a harmonic component in a deep diagnosis domain with a fundamental wave. In other words, when receiving harmonics, the ultrasound diagnostic device disclosed in Patent Document 1 compensates for a reduction in the electromechanical coupling coefficient using a filter and an amplifier.
Similarly, in Patent Document 2, a harmonic piezoelectric element is laminated on a fundamental wave piezoelectric element, and a transmission ultrasound wave is emitted from the fundamental wave piezoelectric element. In addition, a fundamental wave signal component received by the fundamental wave piezoelectric element and a plurality of harmonic components received by the harmonic piezoelectric element are respectively passed through band separation filters to extract desired components, separately subjected to gain adjustment, and finally added together, Patent Document 2 proposes an ultrasound diagnostic device that accordingly obtains a signal in accordance with a depth of a diagnosis domain.
However, with the conventional art described above, filters and amplifiers must be inserted to signal paths from a large number of piezoelectric elements.
Furthermore, an organic material such as PVDF is more favorably used than an inorganic material such as PZT for receiving signals with a high frequency. However, while an inorganic material has high permittivity and therefore high capacitance and low electrical impedance and, as a result, matching with a subsequent stage circuit is relatively easy, an organic material has low permittivity and therefore low capacitance and high electrical impedance and, as a result, matching with a subsequent stage circuit is difficult.    Patent Document 1: Japanese Patent Application Publication No. 2002-11004    Patent Document 2: Japanese Patent Publication No. 4192598    Non-Patent Document 1: “Fundamentals of Piezoelectric Materials Science”, Takuro Ishida, Ohmsha, Ltd.