Kanglyeol Ha

Application of Acoustic Microscopy to Dental Material Characterization

Acoustic microscopy, which has been established for the evaluation of the elastic properties of solid materials in imaging and in quantitative measurements, is introduced into the dental field as a research tool to investigate directly the change of visco-elastic properties in dental h ard tissues (enamel and dentine) associated with the progress of caries. A point-focus-beam (PFB) acoustic microscope is employed to image primary caries lesions in human enamel with a spatial resolution of 3 pm at 400 MHz in both amplitude and phase. Destruction of structure and decrease of acoustic impedance, mainly due to the demineralization , are observed. A method of quantitative material characterization , u sing the V(z) curve analysis of 1 i ne-f ocus-beam (LFB) acoustic microscopy, is applied to study changes in the visco-elastic propert i es assoc i a ted with the demi neral i zation. Preliminary acoustic measurements are reported for the hard tissues of human enamel and dentine, and also for a hydroxyapatite ceramic. It is found that in order to analyze the visco-elastic properties the propagation characteristics of leaky surface acoustic waves (LSAWs) for enamel and of leaky s urface skimming compressional waves (LSSCWs) for dentine, as a function of frequency, should be made.

Focal Length Controllable Ultrasonic Transducer Using Bimorph-Type Bending Actuator

Using a bimorph-type bending actuator, we propose a new method for controlling the focal length of a transducer by electric DC voltage. We designed two focal length controllable ultrasonic transducers with actuators, a line-focus and a point-focus transducer. The polyvinylidene fluoride (PVDF) piezoelectric type polymer film is used for transmitting and receiving of ultrasonic signals. Using the new method, it is confirmed by investigation of the underwater acoustic field that the focal length can be controlled to within 10% of the radius of the transducer curvature.

A Basic Study on Nondestructive Evaluation of Potatoes Using Ultrasound

This paper presents the acoustic properties of potatoes measured by the pulse transmission method with longitudinal ultrasonic waves in a frequency range of 50 kHz to 1 MHz. The average ultrasonic velocity has been measured to be 824 m/s at 100 kHz and the attenuation coefficient is approximately proportional to f1.4, where f is frequency. The attenuation for defective potatoes is measured to be much higher than that for normal ones. Therefore, it is suggested that defective potatoes can be detected by attenuation measurements of longitudinal ultrasonic waves.

Aspect Ratio Dependence of Electromechanical Coupling Coefficient k31 of Lateral-Excitation Piezoelectric Vibrator

Using the theory of coupled vibrations, the lateral-excitation mode is theoreticaliy analyszed. As a result, a formula for the electromechanical coupling coefficient k31 was derived as a function of the aspect ratio between the width and length of a rectangular thin-plate piezoelectric vibrator. This formula can be used in the design for device fabrication using a lateral-excitation-mode piezoelectric vibrator with an arbitrary aspect ratio.

Arrayed Ultrasonic Transducers on Arc Surface for Plane Wave Synthesis

In ultrasonic computed tomography (UCT), it is necessary to synthesize a plane wave using waves emitted from sound sources arranged in the interior surface of a cylinder. In order to transmit a plane wave into a cylindrical surface, an ultrasonic transducer which has many vibrating elements with piezoelectric transverse effect arrayed on an arc surface is proposed. To achieve a wide beam width, the elements should have a small radiation area with a much narrow width. The measured electroacoustic efficiency for the elements was approximately 40% and the beam width defined by -3 dB level from the maximum was as wide as 120 deg. It was confirmed that plane wave synthesis is possible using the proposed transducer array.

Improvement of Calculation for Radiation Impedance of the Vibrators with Cylindrical Baffle

In order to determine the size of the vibrating surface of an individual element and the spaces among elements in a cylindrically arrayed ultrasonic transducer, enormous calculations for mutual- and self-radiation impedances should be accomplished within a limited time. However, the direct application of the equation of Greenspon and Sherman to such numerical calculations requires a very long time to obtain high accuracy. In this study, an effective calculation algorithm was investigated by introducing subintervals into the integration of the Greenspon and Shermans equation. As results, limits of subintervals improving computation time and accuracy were fitted as functions of the size of the vibrating surface of the element and the size of the baffle.

Visualization of Temperature Elevation Due to Focused Ultrasound in Dissipative Acoustic Medium

The temperature change due to focused ultrasound in dissipative acoustic medium is very important because it provides us much information. To measure the temperature change inside of the dissipative acoustic medium non-invasively, we adopt a temperature sensitive film which has thermochromic particles with critical temperature of . As a dissipative acoustic medium, agar layer is chosen in the study. The temperature change due to the ultrasound was measured depending on the concentration of the sugar in the agar layer. The color change on the film due to the ultrasound was investigated when the concentration of sugar was from 25% to 40%. As the result, there were rapid increases of discolored area on the film within 2~5 second after the ultrasound driving and the increasing rates decreased after the period. To compare the simulation results were also shown. However in the simulated result, the discolored areas linearly increased from start to 10 seconds. The reason of the differences between the experimental results and simulated ones is that the change of thermal conductivity and heat capacity of the medium were not considered in the simulation program.

Fabrication and Evaluation of a VHF Focusing Ultrasonic Transducer Made of PVDF Piezoelectric Film

In order to obtain high resolution images, a focusing ultrasonic transducer operated in very high frequency (VHF) range was fabricated and its characteristics were evaluated. A 9- thick PVDF film with only one metalized surface for electric ground was adhered to a CCP (Copper-clad polyimide) film by using epoxy. It was pressed by a metal ball to form a concave surface and its rear side was filled with the epoxy. The radius of curvature and the f-number of the fabricated transducer are 7.5 mm and 1.7, respectively. The pulse-echo measurement results from a target located at the focal point showed that the frequency bandwidth was 35.0 MHz and the insertion loss near the peak frequency of approximately 40 MHz was about 60 dB. Those values agreed well with the simulation results by the KLM equivalent circuit analysis including the effect of the epoxy bonding layer. When the image of thin copper lines by the 35 MHz transducer of the UBM (Ultrasonic Backscattering Microscope) system was compared with the image by the transducer fabricated in this study, the fabricated transducer was observed that the axial resolution was improved although the lateral resolution was degraded.

Experimental Study on Ultrasonic Computed Tomography Using Transducers Arrayed on the Internal Surface of a Cylinder

In this study, ultrasonic array transducers with 32 vibrators arranged on the internal surface of a part of a cylinder were fabricated. The vibrators were operated by the piezoelectric transverse effect. By controlling the phase of the input signal for every vibrator, a quasi plane wave was synthesized. Using the fabricated array, inverse scattering ultrasonic computed tomography (UCT) was carried out with a phantom specimen after checking the plane wave generation. It was confirmed that the plane wave was synthesized successfully and a sound velocity image of the phantom was obtained by the plane wave. Consequently, it was noted that the array could be employed as a transmitter and receiver for data acquisition in UCT.

Frequency Controllable Wide-Beam Ultrasonic Transducer with Transverse Mode

In order to obtain wide-beam characteristics and variable resonant frequency of a ultrasonic transducer for the array source, an electrode of transverse mode piezoelectric vibrator is divided, and an electronic inductance is connected to the divided electrodes. The electronic inductance is made by GIC (General Impedance Converter) circuit. Because the GIC circuit is made of OP-Amps and other passive elements, the value of the inductance can be selected easily. As the results, the electronic inductance is variable in the range from 0.2 mH to 1.2 mH. Using the inductance, the resonance frequency of the transducer can be changed in the range from 73 kHz to 86 kHz. In the directivity of the transducer, it is confirmed that the beam width of the transducer is wider than at -3 dB in water.