Frequency characteristics of an ultrasonic varifocal liquid crystal lens.

Compound lens systems with mechanical actuators are used to focus objects at near to far distances. The focal length of ultrasound varifocal liquid crystal (LC) lenses can be controlled by modulating the refractive index spatial distribution of the medium through the acoustic radiation force, resulting in thin and fast-response varifocal lenses. The frequency characteristics of such a lens are evaluated in this paper, and several axisymmetric resonant vibration modes over 20 kHz are observed. The effective lens aperture decreased with the wavelength of the resonant flexural vibration generated on the lens, meaning that this parameter can be controlled with the driving frequency.

Simulation of ultrasonically induced electrical potentials in bone.

Low-intensity pulsed ultrasound treatment is known to shorten the healing period of bone fractures by 30%-40%, but the initial mechanism of the healing process remains unknown. One possible mechanism is related to the piezoelectricity of bone. However, the complex geometry of bones results in inherent challenges to evaluating electric fields induced therein. Therefore, in this study, we investigate the piezoelectric responses of bones by using simulations to study the wave propagation and induced potentials in bone, according to the piezoelectric finite-difference time-domain (PE-FDTD) method. First, we verify the suitability of the PE-FDTD method by comparing the simulated electric field results with the experimental data obtained by an ultrasound receiver using bone as the piezoelectric element. Next, ultrasound irradiation into a real bone model (the radius of a 66-year-old woman) is simulated at different incident angles. At normal incidence and off-axis incidence (45°), the maximum electric field strength was 4.3 and 5.6 mV/cm, respectively. We also present evidence of significant shear wave contribution to the induced potential. The results of this study confirm the existence of ultrasonically induced potentials in heterogenous bones with complex shapes, equal in magnitude to potentials generated in electrically stimulated bone healing.

Precise Observation of Ultrasonic Pulses Using an SPR Sensor.

Surface plasmon resonance (SPR) type ultrasonic sensors enable local measurements and have a flat frequency response in a wide frequency range. They are expected to be used in photoacoustic microscopy (PAM) and other applications that require broadband ultrasonic detection. In this study, we focus on the precise measurement of ultrasound pressure waveforms via a Kretschmann-type SPR sensor. The noise equivalent pressure was estimated to be 52 Pa [Formula: see text], and the maximum amplitude of the wave measured via the SPR sensor linearly responded to the pressure until 427 kPa [Formula: see text]. Furthermore, the observed waveform for each applied pressure agreed well with the waveforms measured via the calibrated ultrasonic transducer (UT) in the MHz range. Moreover, we focused on the effect of the sensing diameter on the frequency response of the SPR sensor. The results show that the beam diameter reduction improved the frequency response at high frequencies. Evidently, we found that the sensing diameter of the SPR sensor must be carefully selected in consideration of the measurement frequency.

Development of evaluation system for cerebral artery occlusion in emergency medical services: noninvasive measurement and utilization of pulse waves.

Rapid reperfusion therapy can reduce disability and death in patients with large vessel occlusion strokes (LVOS). It is crucial for emergency medical services to identify LVOS and transport patients directly to a comprehensive stroke center. Our ultimate goal is to develop a non-invasive, accurate, portable, inexpensive, and legally employable in vivo screening system for cerebral artery occlusion. As a first step towards this goal, we propose a method for detecting carotid artery occlusion using pulse wave measurements at the left and right carotid arteries, feature extraction from the pulse waves, and occlusion inference using these features. To meet all of these requirements, we use a piezoelectric sensor. We hypothesize that the difference in the left and right pulse waves caused by reflection is informative, as LVOS is typically caused by unilateral artery occlusion. Therefore, we extracted three features that only represented the physical effects of occlusion based on the difference. For inference, we considered that the logistic regression, a machine learning technique with no complex feature conversion, is a reasonable method for clarifying the contribution of each feature. We tested our hypothesis and conducted an experiment to evaluate the effectiveness and performance of the proposed method. The method achieved a diagnostic accuracy of 0.65, which is higher than the chance level of 0.43. The results indicate that the proposed method has potential for identifying carotid artery occlusions.

Control of the surface profile of powder using the flexural vibration of a V-shaped plate.

In the pharmaceutical field, a technique to level the surface of powdered medicines is important in the dispensing process because the flatness of the powder surface affects the packaging accuracy directly. This paper investigates a method for leveling of the surface profile of powder using ultrasound vibration on a plate. The system used comprises a V-shaped plate and two ultrasound transducers, and the plate configuration required was determined from the results of simulations performed by finite element analysis. The resonant longitudinal vibration of the transducers generated the resonant flexural vibration mode of the plate at 27.4 kHz, which resulted in transportation and leveling of the powder. The powder was aggregated at the nodal positions of the flexural standing wave, and a correlation was observed between the sound pressure distribution over the plate and the surface profile of the powder. The powder leveling accuracy was investigated by varying the driving phase difference between the transducers, and it was found that a smaller standing wave ratio for the flexural vibration produced higher leveling accuracy. When the input voltage to the transducers was increased, the leveling time decreased and the leveling accuracy improved; the highest leveling accuracy obtained was 2.2 mm at 130 Vpp.

Concentration-Dependent Viscoelasticity of Poloxamer-Shelled Microbubbles.

The oscillation of shelled microbubbles during exposure to ultrasound is influenced by the mechanical properties of the shell components. The oscillation behavior of bubbles coated with various phospholipids and other amphiphiles has been studied. However, there have been few investigations of how the adsorption conditions of the shell molecules relate to the viscoelastic properties of the shell and influence the oscillation behavior of the bubbles. In the present study, we investigated the oscillation characteristics of microbubbles coated with a poloxamer surfactant, that is, Pluronic F-68, at several concentrations after the adsorption kinetics of the surfactant at the gas-water interface had reached equilibrium. The dilatational viscoelasticity of the shell during exposure to ultrasound was analyzed in the frequency domain from the attenuation characteristics of the acoustic pulses propagated in the bubble suspension. At Pluronic F-68 concentrations lower than 2.0 × 10-2 mol L-1, the attenuation characteristics typically exhibited a sharp peak. At concentrations higher than 2.0 × 10-2 mol L-1, the peak flattened. The dilatational elasticity and viscosity of the shell were estimated by fitting the theoretical model to the experimental values, which revealed that both the elasticity and viscosity increased markedly at approximately 2.0 × 10-2 mol L-1. This suggests that the adsorption properties of Pluronic F-68 strongly affect the oscillation characteristics of microbubbles of a size suitable for medical ultrasound diagnostics.

Study on ultrasonic wave propagation in equine leg bone for screening bucked shin.

For simple, safe, portable, and inexpensive evaluation suitable for leg bone diseases of racehorses in the field, an ultrasonic measurement technique was applied to evaluate wave velocities. A digital model of the third metacarpal bone with the bucked shin was fabricated using high-resolution peripheral quantitative computerized tomography data of a racehorse. This model was anisotropic and heterogeneous, and was constructed using the measured ultrasonic wave velocities in the bone. With this model, ultrasonic wave propagation along the bone axis was simulated using the elastic finite-difference time-domain method. We found two main waves with different propagation velocities. The fast-waves showed a wave velocity close to the longitudinal wave in the axial direction. However, the apparent velocities changed dramatically owing to bone surface irregularities (changes of the shape) in the area of bucked shin. The slow-waves showed a wave velocity close to the shear wave, which was unaffected by the bone surface irregularities. The simple comparison of different wave behaviors may be a suitable parameter for the initial in vivo screening of bucked shin in the legs of racehorses, which can be performed in the field.

Varifocal Concave-Convex Lens Using Viscoelastic Gel and Ultrasound Vibration.

A varifocal concave-convex lens using ultrasound and transparent viscoelastic gel is reported. The configuration of the lens is simple and thin, consisting of four pieces of a piezoelectric ultrasound transducer, a glass disk, and a transparent silicone gel film. It uses a combination of the ultrasound resonant flexural standing- and traveling-wave modes excited by in-phase and four-phase drives so that the lens can change its shape to both concave and convex by switching the resonance mode with the same structure. The acoustic radiation force (ARF) originated from the resonant flexural vibration modes changed the surface profile of the gel. Convex and concave deformation were generated at the center of the lens at the resonance frequencies of 38 and 60 kHz, respectively, indicating that a varifocal concave-convex lens could be fabricated by controlling the driving frequency, voltage amplitude, and phase differences among the ultrasound transducers. The deformational displacement on the lens surface and the change in the focal length increased with the input voltage amplitude. The optical microscopic images observed through the lens were enlarged 1.28× (reduced 0.92× ) in the convex (concave) mode with 20 [Formula: see text]. The response time for focusing and the temperature stability under operation were evaluated. By switching the resonance vibration modes of the lens through the input signals to multiple ultrasound transducers, the variable-focus function with both concave and convex lenses was achieved in the same configuration.

Piezoelectric and Opto-Acoustic Material Properties of Bone.

In this chapter, recent piezoelectric and opto-acoustic studies on bone are introduced. The former are certainly related to ultrasound since piezoelectricity is one of the electro-mechanical properties. The latter are divided into two parts: Photo Acoustics (PA) and Brillouin Scattering (BS). PA is the energy conversion from light to ultrasound while Brillouin scattering is the interaction between phonons and photons. These studies seem very different; however, they are all studies on the ultrasonic material characterization of bone. Another common aspect of these studies is that they are generally targeting the material characterization of bone extracellular matrix. These studies have started later than the conventional ultrasonic bone studies and are expected to provide different characteristics of bone in the micrometer scale area.

Signal Amplification of the Transient Response Measured by the Subnanosecond Pump-Probe Method Based on Surface Plasmon Resonance.

A pump-probe system with a subnanosecond pulsed laser is expected to be a compact and inexpensive transient spectroscopic system that enables nondestructive and noncontact evaluations of the physical properties. However, an improvement in the sensitivity and a theoretical model to complement the measurement signal are necessary to obtain the transient signal precisely because of the low sensitivity and large time resolution. We have developed a highly sensitive pump-probe system with a subnanosecond pulsed laser that combines signal amplification based on surface plasmon resonance (SPR) in this study. An integrated theoretical model of the transient response obtained by a subnanosecond pump-probe under the SPR condition was proposed. Our model consisted of the profile descriptions of the used pulse source, temperature change, generated thermoelastic stress, estimated permittivity change in the metal film, and estimated reflectivity change. The theoretical estimations in the time domain and the incident angle dependence were compared with those of the experimental results to verify our theory. As a result, the estimations were well in agreement with the experimental results. Moreover, the signal-amplification mechanism based on SPR was discussed using our theory. The amplification was caused by the broadening of the resonant curve of SPR and the shift of the resonant angle, which seemed to come from the increase in the electron-phonon scattering rate and the thermal expansion of the metal film, respectively. A clear mechanism of SPR-based signal amplification of the subnanosecond pump-probe was identified through experimental and theoretical approaches.

Ultrasound liquid crystal lens with a variable focus in the radial direction for image stabilization.

New technologies for adaptive optics are becoming increasingly important for miniature devices such as cell-phone cameras. In particular, motion-free autofocusing and optical image stabilization require sophisticated approaches for alternative lens architectures, materials, and processing to replace multiple solid elements. We discuss a new method, to the best of our knowledge, that provides image stabilization via an annular piezoelectric ceramic that uses ultrasound to drive a liquid crystal layer sandwiched between two circular glass substrates. The piezoelectric ceramic is divided into four quadrants that are independently driven with sinusoidal voltages at the resonant frequency of the lens. The technique is based on ultrasound vibrations with a suitable driving scheme. The lens configuration was modeled via finite-element analysis. Various combinations of the four-channel ultrasound transducer can be used to define the focal point of the liquid crystal lens. Clear optical images could be obtained with the lens. By using two-dimensional fast Fourier transforms, the focal point position was defined and shifted in the radial direction.

Simulation study on the effects of cancellous bone structure in the skull on ultrasonic wave propagation.

The transcranial Doppler method (TCD) enables the measurement of cerebral blood flow velocity and detection of emboli by applying an ultrasound probe to the temporal bone window, or the orbital or greater occipital foramina. TCD is widely used for evaluation of cerebral vasospasm after subarachnoid hemorrhage, early detection of patients with arterial stenosis, and the assessment of brain death. However, measurements often become difficult in older women. Among various factors contributing to this problem, we focused on the effect of the diploe in the skull bone on the penetration of ultrasound into the brain. In particular, the effect of the cancellous bone structure in the diploe was investigated. Using a 2D digital bone model, wave propagation through the skull bone was investigated using the finite-difference time-domain (FDTD) method. We fabricated digital bone models with similar structure but different BV/TV (bone volume/total volume) values in the diploe. At a BV/TV of approximately 50-60% (similar to that of older women), the minimum ultrasound amplitude was observed as a result of scattering and multiple reflections in the cancellous diploe. These results suggest that structural changes such as osteoporosis may be one factor hampering TCD measurements.

Varifocal optical lens using ultrasonic vibration and thixotropic gel.

A variable focus optical lens using a thixotropic gel and ultrasonic vibration is discussed. The surface profile of the gel could be deformed via acoustic radiation force generated by ultrasound. A thixotropic gel in which the viscosity was changed by shear stress was employed as a transparent lens material. The thixotropic gel allowed the lens to maintain shape deformation in the absence of continuous ultrasound excitation. The lens had a simple structure with no mechanical moving parts and included an annular piezoelectric transducer, a glass disk, and the thixotropic gel film. The axisymmetric concentric flexural vibration mode was generated on the lens at 71 kHz, which resulted in static surface deformation of the gel via the acoustic radiation force. The preservation rate was investigated after switching off the ultrasonic excitation. There was a trade-off between the preservation rate of the lens deformation and the response time for focusing. The focal length could be controlled via the input voltage to the lens, and a variable-focus convex lens could be realized; the change in the focal length with 4.0 Vpp was 0.54 mm. The optical transmittance of the lens was measured and the transmittance ranged 70%-80% in the visible spectral region.

Decrease in Longitudinal Wave Velocity in Glycated Collagen.

Diabetic patients have a higher risk of bone fracture than those without diabetes, despite a normal bone mineral density. This higher riskmay result fromthe deterioration of collagen because of glycation. The objective of this study was to investigate the elastic properties of glycated collagen using the micro-Brillouin scattering technique. Using single-layer uniaxial collagen films with a thickness of approximately [Formula: see text], the longitudinal wave velocities, propagating in the parallel andperpendiculardirectionswith respect to the collagen fiber orientation, were measured in dry and wet film specimens. The wave velocities in the glycated collagen specimens decreased as a function of glycation time. This decrease depended on the direction of collagen fiber alignment and wave propagation. The lowest velocity due to glycation in thewet filmswas foundwhen the ultrasound propagated perpendicular to the fiber direction. These results indicate that the glycation of collagen in the bone may also reduce bone elasticity and suggest that the effects of glycation on collagen films may be anisotropic.

Ultrasound liquid crystal lens with enlarged aperture using traveling waves.

A new type of ultrasonically controlled concave liquid crystal lens based on traveling waves (TWs) with a divided electrode structure and an appropriate driving scheme is proposed in this Letter. The lens uses an annular piezoelectric ceramic divided into four parts for four-phase driving and consists of a liquid crystal layer in a sandwich structure between two circular glass substrates. The lens configuration was simulated by finite element analysis using the Ansys software. Here we discuss the use of TWs to expand the lens aperture and clarify the lens' optical characteristics using a Shack-Hartmann wavefront sensor. The effective lens aperture using TWs was 4.4 mm, and the focal length was 3.8 m.

Site dependence of ultrasonically induced electrical potentials in bone.

The success rate of low-intensity pulsed ultrasound (LIPUS) therapy depends on the bone site. However, the initial mechanism of physical stimulation by ultrasound and bone cellular response remains unclear. One possible physical stimulation is the induced electrical potentials due to the piezoelectricity. In this study, the output electrical potentials of ultrasound transducers made from bovine bones were investigated. Transducers made from the radius bone showed the largest electric potentials, followed by tibia, femur, and humerus. There was clear site dependence of the induced electric potentials of bone, in good accordance with the success rate of LIPUS therapy.

Anisotropic Longitudinal Wave Propagation in Swine Skull.

To understand the in-plane elastic character of ultrasonic waves in the skull, longitudinal wave velocities were studied in the MHz range using a conventional pulse technique. Taking advantage of the thickness of swine skulls, anisotropic in-plane wave velocity changes in the outer and diploe layers were experimentally investigated using structural information measured by X-ray computer tomography (CT). The velocities in the thin inner layer were difficult to measure. The main trabecular alignment (MTA) in the thick swine diploe layer was almost perpendicular to the thickness direction and changed with position inside the skull. The degree of anisotropy of in-plane longitudinal wave velocity ranged 1.07-1.33 in both outer and diploe layers, depending on position and swine sample. The angle of the fastest velocity in the outer layer was different from that in most parts of the diploe layer. Anisotropic character in the diploe layer gradually changed with position in the thickness direction.

Growth of cortical bone thickness and trabecular bone density in Japanese children.

BACKGROUND: The acquisition of a high bone density at a young age is a strategy to prevent fractures/falls later in life. We therefore decided to investigate the increases in cortical thickness (CoTh) and trabecular bone density (TBD) of children. METHODS: Subjects comprised 1314 students (678 boys and 636 girls) aged between 12 and 18 years. Lifestyle factors were examined with a self-administered questionnaire (sleep times, exercise habits, and calcium intake). Bone growth was assessed based on CoTh and TBD using an ultrasonic bone densitometer. Height, weight, and body fat percentage were also measured. RESULTS: Increases in CoTh and TBD occurred earlier in girls than in boys. Calcium intake was not sufficient at any of the ages examined, and sleep times were shorter than those recommended by the National Sleep Foundation. Increases in CoTh and TBD occurred subsequent to increases in height. Although increases in CoTh were observed with age in both sexes, TBD increased in boys until the age of 17 years and in girls until the age of 15 years. At 18 years of age, the young adult mean value was greater than 100% for CoTh but lower than 100% for TBD. A multivariate analysis identified age, body mass index (BMI), and exercise as independent positive factors for CoTh, while body fat percentage was an independent negative factor. Age and BMI were independent positive factors for TBD in both sexes, whereas body fat percentage was a positive factor in boys only. CONCLUSIONS: The study found that CoTH and TBD varied with age and differed in increase in boys and girls; related factors of bone increase could also be found. The results of this study may contribute to the acquisition of high bone density in children and adolescents.

Multimodal Evaluation of the Spatiotemporal Variations of Periprosthetic Bone Properties.

Titanium implants are widely used in dental and orthopedic surgeries. However, implant failures still occur because of a lack of implant stability. The biomechanical properties of bone tissue located around the implant need to be assessed to better understand the osseointegration phenomena and anticipate implant failure. The aim of this study was to explore the spatiotemporal variation of the microscopic elastic properties of newly formed bone tissue close to an implant. Eight coin-shaped Ti6Al4V implants were inserted into rabbit tibiae for 7 and 13 weeks using an in vivo model allowing the distinction between mature and newly formed bone in a standardized configuration. Nanoindentation and micro-Brillouin scattering measurements were carried out in similar locations to measure the indentation modulus and the wave velocity, from which relative variations of bone mass density were extracted. The indentation modulus, the wave velocity and mass density were found to be higher (1) in newly formed bone tissue located close to the implant surface, compared to mature cortical bone tissue, and (2) after longer healing time, consistently with an increased mineralization. Within the bone chamber, the spatial distribution of elastic properties was more heterogeneous for shorter healing durations. After 7 weeks of healing, bone tissue in the bone chamber close to the implant surface was 12.3% denser than bone tissue further away. Bone tissue close to the chamber edge was 16.8% denser than in its center. These results suggest a bone spreading pathway along tissue maturation, which is confirmed by histology and consistent with contact osteogenesis phenomena.

Prolonged Hyperglycemia Reduces Elasticity of Type II Diabetic Rat Bone.

An increase in bone fracture risk has been reported in patients with diabetes. To evaluate an early effect of glucose intolerance on bone homeostasis, we have characterized bones from spontaneously diabetic torii (SDT) rats, an animal model of type 2 diabetes in comparison with Sprague Dawley (SD) rats as healthy control. Focusing on early effects of diabetes on bone elasticity, longitudinal wave velocities of animal bones were first determined by a micro-Brillouin scattering technique in a non-destructive way. Wave velocities in the cortical and cancellous bones in the tibias of the SDT and SD rats were compared. In a pre-diabetic stage at approximately 10 weeks of age, there seems no significant difference in wave velocities in bones from age-matched SDT and SD rats. By contrast, after the onset of diabetes at approximately 20 weeks of age, the mean velocities of bones from SDT rats were lower than those of SD rat. In addition, the X-ray CT showed that the bone amounts of SDT rats were smaller than those of SD rats in an early diabetic stage at 20 weeks of age. The current study demonstrated that the wave velocity decreased in bones of SDT rats in the early stages of diabetes. While a decrease of bone strength in an early stage of diabetes can be partially explained from decreases in bone amount as well as bone elasticity, further studies will be needed in understanding a detailed mechanism of bone deterioration due to diabetes.