Effect on High-Intensity Fields of a Tough Hydrophone With Hydrothermal PZT Thick-Film Vibrator and Titanium Front Layer.

A novel tough hydrophone was fabricated by depositing hydrothermally synthesized lead zirconate titanate polycrystalline film on the back-side surface of a titanium plate. Our developed tough hydrophone resisted damage in a high-pressure field (15 MPa) at a focal point of a sinusoidal continuous wave driven by a concave high-intensity focused ultrasound (HIFU) transducer with up to 50 W of power input to the sound source. The hydrophone was suitable for the HIFU field, even though the hydrophone has a flat-shape tip of 3.5 mm diameter, which is slightly larger than the wavelength of a few megahertz. In this paper, experiments are performed to assess the effect on the HIFU field of changing the shape of the tough hydrophone, with the aim of developing a tough hydrophone. The spatial distribution of the acoustic bubbles around the focal point was visualized by using ultrasonic diagnostic equipment with the tough hydrophone located at the focal point of the HIFU transducer. From the visualization, the trapped acoustic bubbles were seen to arise from the standing wave, which implies that the acoustic pressure is reduced by this cloud of acoustic bubbles that appeared during hydrophone measurement. Although cavitation and acoustic bubbles may be unavoidable when using high-intensity ultrasound, the estimated result of evaluating acoustic fields without misunderstanding by acoustic bubbles can be obtained by the aid of visualizing bubbles around the tough hydrophone.

Measurement of incident position of hypervelocity particles on piezoelectric lead zirconate titanate detector.

A cosmic dust detector for use onboard a satellite is currently being developed by using piezoelectric lead zirconate titanate (PZT). The characteristics of the PZT detector have been studied by bombarding it with hypervelocity iron (Fe) particles supplied by a Van de Graaff accelerator. One central electrode and four peripheral electrodes were placed on the front surface of the PZT detector to measure the impact positions of the incident Fe particles. It was demonstrated that the point of impact on the PZT detector could be identified by using information on the time at which the first peak of the output signal obtained from each electrode appeared.

High frequency ultrasound imaging of surface and subsurface structures of fingerprints.

High frequency ultrasound is suitable for non-invasive evaluation of skin because it can obtain both morphological and biomechanical information. A specially developed acoustic microscope system with the central frequency of 100 MHz was developed. The system was capable of (1) conventional C-mode acoustic microscope imaging of thinly sliced tissue, (2) ultrasound impedance imaging of the surface of in vivo thick tissue and (3) 3D ultrasound imaging of inside of the in vivo tissue. In the present study, ultrasound impedance imaging and 3D ultrasound imaging of in vivo fingerprints were obtained. The impedance image showed pores of the sweat glands in the surface of fingerprint and 3D ultrasound imaging showed glands of the rear surface of fingerprint. Both findings were not visualized by normal optical imaging, thus the system can be applied to pathological diagnosis of skin lesions and assessment of aging of the skin in cosmetic point of view.

High frequency ultrasound characterization of artificial skin.

Regenerated skin with 3-dimensional structure is desired for the treatment of large burn and for the plastic surgery. High frequency ultrasound is suitable for non-destructive testing of the skin model because it provides information on morphology and mechanical properties. In this paper, spectral parameters of ultrasound radio-frequency signal from a specially developed high-frequency ultrasound imaging system were evaluated for tissue characterization of artificial skin. Results suggest that spectral parameters are useful for classification of epidermis and dermis in the artificial skin model. The system is also a useful tool for the noninvasive and nondestructive evaluation of skin.

Ultrasound speed and impedance microscopy for in vivo imaging.

Ultrasound speed and impedance microscopy was developed in order to develop in vivo imaging system. The sound speed mode realized non-contact high resolution imaging of cultured cells. This mode can be applied for assessment of biomechanics of the cells and thinly sliced tissues. The impedance mode visualized fine structures of the surface of the rat's brain. This mode can be applied for intra-operative pathological examination because it does not require slicing or staining.

Ultrasonic tissue characterization of atherosclerosis by a speed-of-sound microscanning system.

We have been developing a scanning acoustic microscope (SAM) system for medicine and biology featuring quantitative measurement of ultrasonic parameters of soft tissues. In the present study, we propose a new concept sound speed microscopy that can measure the thickness and speed of sound in the tissue using fast Fourier transform of a single pulsed wave instead of burst waves used in conventional SAM systems. Two coronary arteries were frozen and sectioned approximately 10 microm in thickness. They were mounted on glass slides without cover slips. The scanning time of a frame with 300 x 300 pixels was 90 s and two-dimensional distribution of speed of sound was obtained. The speed of sound was 1680 +/- 30 m/s in the thickened intima with collagen fiber, 1520 +/- 8 m/s in the lipid deposition underlying the fibrous cap, and 1810 +/- 25 m/s in a calcified lesion in the intima. These basic measurements will help in the understanding of echo intensity and pattern in intravascular ultrasound images.

Oscillation mode conversion and energy confinement of acoustically agitated bubbles.

The acoustically agitated bubble oscillation in liquids that is considered to be related to the half-subharmonic acoustic bubble oscillations is discussed in terms of parametric decay instability. At the frequency of about 40 kHz, the half-subharmonic bubble oscillation mode should be a surface bubble oscillation that does not easily emit acoustic waves into water and confines acoustic energy from longitudinal waves. The half-subharmonic bubble oscillation is the dominant mode that leads to parametric decay instability of bubble oscillations.