Shigemi Saito

Broadband ultrasonic transducers using a LiNbO/sub 3/ plate with a ferroelectric inversion layer

Some heat treatment of a lithium niobate (LiNbO/sub 3/) plate induces domain inversion, thereby yielding a ferroelectric inversion layer. In such a piezoelectric plate with an inversion layer, even-order thickness-extensional modes, as well as odd-order modes, can be excited piezoelectrically. Therefore, the ultrasonic transducer using such a piezoelectric plate is expected to operate over a wide frequency range. In this paper, it is shown that broadband ultrasonic transducers can be obtained at a certain thickness ratio of inversion layer to plate, and that the transducer characteristics differ depending on whether the inversion layer is on the front side or on the backside. The broadband characteristics are experimentally demonstrated by fabricating transducers with 9 MHz or 75 MHz center frequency using 36/spl deg/ rotated Y-cut LiNbO/sub 3/ plates with a ferroelectric inversion layer.

Speckle Noise Reduction by Superposing Many Higher Harmonic Images

A novel ultrasonic imaging method offering high resolution and high-quality images for clinical diagnosis has been developed. This method produces an image using many higher harmonic components contained in the echoes from the inside of a human body, generated due to ultrasonic nonlinear propagation through biological tissues. A new ultrasonic probe has been designed to detect higher harmonic components efficiently over a broad band produced by nonlinear propagation through biological tissues. This probe has a bilayer structure consisting of a lead zirconate titanate (PZT) transmitter and a polyvinylidene fluoride (PVDF) receiver. Experiments employing the new probe show that the receiving transducer easily detects higher harmonics from the fundamental through the tenth in the spectrum of reflected sound from an agar-gel phantom submerged in water. By scanning the probe, the harmonic images of the fundamental through the ninth harmonic component are successfully obtained. In addition, it is demonstrated that speckle noise can be reduced by averaging many harmonic images.

Measurement of the acoustic nonlinearity parameter in liquid media using focused ultrasound

The principle and the experimental results for a finite amplitude method to measure the acoustic nonlinearity parameter B/A of liquid media using focused ultrasound are presented. The present method takes both absorption and diffraction into account. A liquidlike sample is inserted in the focal region of the acoustic field formed by a spherically focusing Gaussian source. The response of the amplitudes and phases of the fundamental and second harmonic components of the transmitted wave is observed at the post focal region to obtain the experimental value of a quantity named RF, which incorporates the nonlinearity parameter of the sample. The measurement of linear acoustic properties is also carried out using the same focusing source. The value of B/A is determined through comparison between experimental and theoretical values of RF. The experimental results for 4.5‐ml volume samples at the fundamental frequency of 1.9 MHz validate the present method. This suggests the possiblity of obtaining the B/A imagi...

Second harmonic component of a nonlinearly distorted wave in a focused sound field

The second harmonic in a focused nonlinear field is examined with a numerical computation and with experiments done in water. The computation is accomplished by rewriting an expression derived by Lucas and Muir [J. Acoust. Soc. Am. 74, 1522–1528 (1983)] into a form appropriate for numerical computation at any point. The primary velocity distribution at the source is experimentally defined and utilized in the computation. The experiment is conducted with a 2‐MHz focusing sound source consisting of a circular transducer coupled to an acoustic lens. The experimental values for amplitude and phase of the second harmonic component are in reasonable agreement with the theory. The second harmonic component focuses more sharply than the fundamental component, and the amplitude ratio of the second harmonic component to the fundamental component attains the maximum value at the focus. The results lead to an interpretation that the component generated in the focal region is the dominant contributor to the propagatin...

Ultrasonic Focusing Gaussian Source to Receive Nonlinearly Generated Second Harmonic Sound by Itself.

A focusing source to concurrently detect the nonlinearly generated second harmonic sound contained in the reflected wave has been produced. A 36° rotated Y-cut LiNbO3 plate of 20 MHz fundamental resonance frequency with a reversed polarization layer formed by heat treatment is bonded to a plane surface of a solid acoustic lens whose thickness is set at a small value so as to avoid the influences of nonlinear distortion and multiple reflection within the lens. One of the electrodes on the piezoelectric plate is fabricated to a 20-point-star shape. The formation of a 20 MHz focused Gaussian beam and the capability of receiving the 40 MHz second harmonic sound are confirmed through experiments.

An ultrasonic transducer for second imaging using a LiNbO/sub 3/ plate with a local ferroelectric inversion layer

A new lithium niobate (LiNbO/sub 3/) transducer of separated transmitter-receiver configuration is proposed for application to second harmonic imaging in a high-frequency range. In this transducer, a domain-inverted layer of half the plate thickness is formed selectively by Ti-diffusion and heat treatment in the central part of a LiNbO/sub 3/ plate. The uniformly polarized surrounding region is used as the transmitter that will generate the fundamental wave component. A transducer with the transmission frequency of 50 MHz and the reception frequency of 100 MHz is fabricated using a 36/spl deg/ rotated Y-cut LiNbO/sub 3/ plate, and its performance is demonstrated.

Multiple-frequency ultrasonic imaging by transmitting pulsed waves of two frequencies.

PurposeThe aim of this study was realization of a broadband measurement system that is capable of effectively carrying out a frequency compound method. In the present method, the secondary wave components of difference and sum frequencies are generated along with the higher harmonic components through the nonlinear interaction of two-frequency ultrasound. A multiple-frequency beam is generated together with the initially radiated frequency components.MethodsFor the structure of a transducer capable of simultaneously radiating two sound waves with different frequencies, a coaxial arrangement of a circular-disc piezoelectric transducer and a ring piezoelectric transducer was designed. The radiating frequencies chosen were 2 and 8 MHz. In addition to the 4-MHz second harmonic sound of the 2-MHz primary sound, sounds of the 6-MHz difference frequency and the 10-MHz sum frequency can be generated.ResultsBy measuring the acoustic pressure distribution, the formation of a multiple-frequency beam was confirmed. The signal-to-noise ratio in an agar-gel phantom image was increased by 5–6 dB with application of the frequency compound method. The validity of the proposed method was demonstrated through the generation of a human finger image. Further, it was found that the influence of the Doppler effect was small enough that almost all the secondary waves were attributable to the nonlinear propagation of sounds.ConclusionsA multiple-frequency sound beam was realized by radiating a two-frequency sound. The effectiveness of the presented method was demonstrated through actual imaging.

B/A measurement for liquid media using an LN transducer with inverted-domain layer

A system to measure the acoustic nonlinearity parameter B/A for a liquid medium utilizing a high-frequency focusing Gaussian source has been developed. The 19 MHz sound source employing a LiNbO3 (LN) plate with a ferroelectric inversion layer can emit and receive double frequency sound of 38 MHz. An LN plate with a twisted star-shape electrode is attached to a synthetic silica lens whose radius is much larger than the aperture size and thickness to generate a focused Gaussian beam. Linear properties such as the density, sound speed and attenuation coefficient are determined through a comparison of the pressure amplitude reflected from the solid surface located at the focal plane and the reflector distance to maximize the signal amplitude with those for the case in which the medium is replaced by distilled water. Furthermore, the ratio of the second harmonic amplitude in a sound returned from the reflector set at the focal plane and that in the case of distilled water yields the B/A value through a calculation containing the linear property obtained in advance. The measured result for the linear property and the B/A value of various alcohol samples shows a reasonable agreement with the literature values.

A property of the nonlinearly geznnerated second-harmonic component of focused ultrasound detected by a concave receiver

The property of the fundamental and second-harmonic components of the acoustic pressure in a beam emitted from a uniformly excited focusing source has been investigated for the cases of the free field and the field with an inserted sample. Calculation of the detection by a concave receiver placed beyond the focal region, which is based on the successive approximation solution for the Khokhlov-Zabolotskaya-Kuznetsov equation, is validated through experiments. The range dependence of the fundamental component amplitude differs from that of the second harmonic, unlike the case of a Gaussian source. The cancellation of the second harmonic components generated in front of, and beyond, the focal region is degraded compared with the case of a Gaussian source. The result of the experimental determination of the nonlinearity parameter B/A for the inserted sample, utilizing the signal received beyond the sample, shows slightly poorer agreement with the literature values, owing to the less sensitive property of the second-harmonic component to the inserted sample.

Development of Multiple-Frequency Ultrasonic Imaging System Using Multiple Resonance Piezoelectric Transducer

The authors have developed a multiple frequency imaging system using a multiple resonance transducer (MRT) consisting of 1–3 composite materials with a low mechanical quality factor Q bonded together. The MRT has a structure consisting of thin and thick piezoelectric plates, two matching layers, and a backing layer. This makes it possible to obtain B-mode images of satisfactory resolution using ultrasonic pulses owing to their short duration. In this paper, the vibration property of the MRT derived through equivalent-circuit analysis is first shown. By utilizing the result, an MRT capable of transmitting ultrasonic pulses for generation of the images of biological tissues with satisfactory resolution is designed and prototyped. Setting the prototype transducer in the mechanical sector probe of commercial ultrasonic diagnosis equipment, the speckle reduction effect is demonstrated using images of various phantoms to mimic biological tissues and a human thyroid.