Naohiro Hozumi

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 mum 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 plusmn 30 m/s in the thickened intima with collagen fiber, 1520 plusmn 8 m/s in the lipid deposition underlying the fibrous cap, and 1810 plusmn 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.

Acoustic impedance micro-imaging for biological tissue using a focused acoustic pulse with a frequency range up to 100 MHz

We have proposed a new method for two-dimensional acoustic impedance imaging for biological tissue that can per- form micro-scale observation without slicing the specimen. A tissue was placed on a plastic plate of 0.5 mm in thickness. An acoustic pulse with a frequency range up to 100 MHz was trans- mitted from the rear side of the plate, the acoustic beam being focused at the boundary between the tissue and plate. The reflec- tion intensity was interpreted into local acoustic impedance of the target tissue. An acoustic impedance microscopy with 200 x 200 pixels, its field of view being 2 x 2 mm, was obtained by mechani- cally scanning the transducer. Quantification of acoustic imped- ance was performed using water or an appropriate material as a reference. The accuracy was evaluated using saline with various NaCl content. A rat cerebellum was employed as the specimen. The development of parallel fiber in cerebella cultures was clearly observed as the contrast in acoustic impedance. The pro- posed technique is believed to be a powerful tool for biological tissue characterization, as neither staining nor slicing is required.

Acoustic impedance microscopy for biological tissue characterization

A new method for two-dimensional acoustic impedance imaging for biological tissue characterization with micro-scale resolution was proposed. A biological tissue was placed on a plastic substrate with a thickness of 0.5mm. A focused acoustic pulse with a wide frequency band was irradiated from the rear side of the substrate. In order to generate the acoustic wave, an electric pulse with two nanoseconds in width was applied to a PVDF-TrFE type transducer. The component of echo intensity at an appropriate frequency was extracted from the signal received at the same transducer, by performing a time-frequency domain analysis. The spectrum intensity was interpreted into local acoustic impedance of the target tissue. The acoustic impedance of the substrate was carefully assessed prior to the measurement, since it strongly affects the echo intensity. In addition, a calibration was performed using a reference material of which acoustic impedance was known. The reference material was attached on the same substrate at different position in the field of view. An acoustic impedance microscopy with 200×200 pixels, its typical field of view being 2×2 mm, was obtained by scanning the transducer. The development of parallel fiber in cerebella cultures was clearly observed as the contrast in acoustic impedance, without staining the specimen. The technique is believed to be a powerful tool for biological tissue characterization, as no staining nor slicing is required.

Investigation of filler effect on treeing phenomenon in epoxy resin under ac voltage

Epoxy resin is widely used as an insulation material in many electrical apparatuses because of its excellent electrical and manufacture characteristics. It is usually mixed with filler to improve mechanical and thermal characteristics. In order to qualitatively clarify the effect of fillers on treeing phenomena, treeing tests were performed with epoxy specimens mixed with different kinds of fillers. The property of the interface is also clarified by the treatment of silica filler using silane coupling. Although tree initiation voltage decreased with introducing silica fillers, the fillers prevented the growth of the tree. The effect of filler shape was more significant on round-shape filler than on square-shape filler. Silane coupling treatment to the fillers did not bring a significant change in tree initiation voltage, however, brought a reduction in tree propagation. The change in tree propagation rate was explained by considering the field relaxation and energy dispersion due to branching at the filler-resin interface. Tree propagation along the interface between resin and a quartz plate was observed and analyzed in order to ensure the above explanation.

Numerical analysis of ultrasound propagation and reflection intensity for biological acoustic impedance microscope

This paper proposes a new method for microscopic acoustic imaging that utilizes the cross sectional acoustic impedance of biological soft tissues. In the system, a focused acoustic beam with a wide band frequency of 30-100 MHz is transmitted across a plastic substrate on the rear side of which a soft tissue object is placed. By scanning the focal point along the surface, a 2-D reflection intensity profile is obtained. In the paper, interpretation of the signal intensity into a characteristic acoustic impedance is discussed. Because the acoustic beam is strongly focused, interpretation assuming vertical incidence may lead to significant error. To determine an accurate calibration curve, a numerical sound field analysis was performed. In these calculations, the reflection intensity from a target with an assumed acoustic impedance was compared with that from water, which was used as a reference material. The calibration curve was determined by changing the assumed acoustic impedance of the target material. The calibration curve was verified experimentally using saline solution, of which the acoustic impedance was known, as the target material. Finally, the cerebellar tissue of a rat was observed to create an acoustic impedance micro profile. In the paper, details of the numerical analysis and verification of the observation results will be described.

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.

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 rats brain. This mode can be applied for intra-operative pathological examination because it does not require slicing or staining.

9D-1 Precise Calibration for Biological Acoustic Impedance Microscope

This report deals with the scanning acoustic microscope for imaging cross sectional acoustic impedance of biological soft tissues. A focused acoustic beam with a wide frequency range up to about 100 MHz was transmitted to the tissue object in contact with the rear surface of plastic substrate. The reflected signals from the target and reference are interpreted into local acoustic impedance. Two-dimensional profile is obtained by scanning the transducer. As the incidence is not vertical, not only longitudinal wave but also transversal wave is generated in the substrate. The error in estimated acoustic impedance assuming vertical incidence was discussed. The error is not negligible if the angle of focusing is large, or the acoustic impedance of the reference material is far different from the target. However it can be compensated, if the beam pattern and acoustic parameters of coupling medium and substrate were known. The improvement of precision brought by the compensation was ensured by using a droplet of saline solution of which acoustic impedance was known. Finally, a cerebellum tissue of rat was observed with a good precision.

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.

Measurement of acoustic property for signal recovery in PEA method

Recently, the acoustic-impedance microscope has been developed by the application of pulsed electro-acoustic (PEA) method for the biomedical sample observation. The acoustic-impedance microscope can measure the difference of acoustic impedance in thickness direction in materials. When measuring the space charge distribution of a polymer insulating material by using the normal PEA method, the acoustic waves to obtain the space charge profile are detected by piezoelectric device in the system.