Osteoporosis detection in postmenopausal women using axial transmission multi-frequency bone ultrasonometer: Clinical findings

The objective of this study was to evaluate if the Bone UltraSonic Scanner (BUSS) can detect osteoporosis in postmenopausal women. BUSS is an axial transmission multi-frequency ultrasonometer for acquisition of wave propagation profiles along the proximal anterior tibia. We derived 10 diagnostically significant BUSS parameters that were then compared with the DXA spine T-score, which was used in this study as the “gold standard” for the assessment of osteoporosis (T-score <−2.5). BUSS wave parameters were studied in 331 postmenopausal women examined by 9 trained operators at 3 clinical sites with use of 3 devices. The efficiency of each BUSS parameter in osteoporosis detection was assessed using a receiver operating characteristic curve analysis. Area under the curve (AUC) for each of 10 parameters ranged from 58.1% to 70.2%. Using these parameters a linear classifier was derived which provided at its output 83.0% AUC, 87.7% sensitivity and 63.2% specificity to DXA-identified osteoporosis. The results of this study confirm BUSS’s capability to detect osteoporosis in postmenopausal women.     

Multi-frequency axial transmission bone ultrasonometer

The last decade has seen a surge in the development of axial transmission QUS (Quantitative UltraSound) technologies for the assessment of long bones using various modes of acoustic waves. The condition of cortical bones and the development of osteoporosis are determined by numerous mechanical, micro-structural, and geometrical or macro-structural bone properties like hardness, porosity and cortical thickness. Such complex manifestations of osteoporosis require the evaluation of multiple parameters with different sensitivities to the various properties of bone that are affected by the disease. This objective may be achieved by using a multi-frequency ultrasonic examination The ratio of the acoustic wavelength to the cortical thickness can be changed by varying the frequency of the ultrasonic pulse propagating through the long bone that results in the change in composition of the induced wave comprised of a set of numerous modes of guided, longitudinal, and surface acoustic waves. The multi-frequency axial transmission QUS method developed at Artann Laboratories (Trenton, NJ) is implemented in the Bone Ultrasonic Scanner (BUSS). In the current version of the BUSS, a train of ultrasonic pulses with 60, 100, 400, 800, and 1200 kHz frequencies is used. The developed technology was tested on a variety of bone phantoms simulating normal, osteopenic, and osteoporotic bones. The results of this study confirm the feasibility of the multi-frequency approach for the assessment of the processes leading to osteoporosis.     

The dual-frequency method for ultrasonic assessment of skeletal system

The article is a review of the new dual-frequency method in axial bone quantitative ultrasonometry for assessment of changes in cortical bones in osteoporosis. The method is based on the use of two frequencies for the generation of flexural and longitudinal ultrasonic waves, which opens possibilities for differential diagnostics of changes in various components of the state of the skeletal system, such as cortical layer thickness, porosity, and elastic properties of tissue. The axial scanning and composition of two-dimensional acoustic profiles of bones are carried out with the purpose of using topographic variations in the acoustic properties for diagnostics of the state of a bone. Results of laboratory and clinical tests of Bone UltraSonic Scanner (BUSS) developed in Artann Laboratories on the basis of the stated principles are presented. The sensitivity of measured characteristics to progression of osteoporosis and the detectability of early changes in bones related to this disease are shown.

     

Reflector-based phase calibration of ultrasound transducers

Recently, the measurement of phase transfer functions (PTFs) of piezoelectric transducers has received more attention. These PTFs are useful for e.g. coding and interference based imaging methods, and ultrasound contrast microbubble research. Several optical and acoustic methods to measure a transducer’s PTF have been reported in literature. The optical methods require a setup to which not all ultrasound laboratories have access to. The acoustic methods require accurate distance and acoustic wave speed measurements. A small error in these leads to a large error in phase, e.g. an accuracy of 0.1% on an axial distance of 10 cm leads to an uncertainty in the PTF measurement of ±97° at 4 MHz. In this paper we present an acoustic pulse-echo method to measure the PTF of a transducer, which is based on linear wave propagation and only requires an estimate of the wave travel distance and the acoustic wave speed. In our method the transducer is excited by a monofrequency sine burst with a rectangular envelope. The transducer initially vibrates at resonance (transient regime) prior to the forcing frequency response (steady state regime). The PTF value of the system is the difference between the phases deduced from the transient and the steady state regimes. Good agreement, to within 7°, was obtained between KLM simulations and measurements on two transducers in a 1-8 MHz frequency range. The reproducibility of the method was ±10°, with a systematic error of 2° at 1 MHz increasing to 16° at 8 MHz. This work demonstrates that the PTF of a transducer can be measured in a simple laboratory setting.     

Progress in developing a thermal method for measuring the output power of medical ultrasound transducers that exploits the pyroelectric effect

Progress in developing a new measurement method for ultrasound output power is described. It is a thermal-based technique with the acoustic power generated by a transducer being absorbed within a specially developed polyurethane rubber material, whose high absorption coefficient ensures energy deposition within a few mm of the ultrasonic wave entering the material. The rate of change of temperature at the absorber surface is monitored using the pyroelectric voltage generated from electrodes disposed either side of a 60 mm diameter, 0.061 mm thick membrane of the piezoelectric polymer polyvinylidene fluoride (pvdf) bonded to the absorber. The change in the pyroelectric output voltage generated by the sensor when the transducer is switched ON and OFF is proportional to the delivered ultrasound power. The sensitivity of the device is defined as the magnitude of these switch voltages to a unit input stimulus of power (watt). Three important aspects of the performance of the pyroelectric sensor have been studied. Firstly, measurements have revealed that the temperature dependent sensitivity increases over the range from approximately 20 °C to 30 °C at a rate of +1.6% °C⁻¹. Studies point to the key role that the properties of both the absorbing backing layer and pvdf membrane play in controlling the sensor response. Secondly, the high sensitivity of the technique has been demonstrated using an NPL Pulsed Checksource, a 3.5 MHz focused transducer delivering a nominal acoustic power level of 4 mW. Finally, proof-of-concept of a new type of acoustic sensor responding to time-averaged intensity has been demonstrated, through fabrication of an absorber-backed hydrophone of nominal active element diameter 0.4 mm. A preliminary study using such a device to resolve the spatial distribution of acoustic intensity within plane-piston and focused 3.5 MHz acoustic fields has been completed. Derived beam profiles are compared to conventional techniques that depend on deriving intensity from acoustic pressure measurements made using the sensor as a calibrated hydrophone.     

Analytical and numerical calculations of optimum design frequency for focused ultrasound therapy and acoustic radiation force

Focused ultrasound therapy relies on acoustic power absorption by tissue. The stronger the absorption the higher the temperature increase is. However, strong acoustic absorption also means faster attenuation and limited penetration depth. Hence, there is a trade-off between heat generation efficacy and penetration depth. In this paper, we formulated the acoustic power absorption as a function of frequency and attenuation coefficient, and defined two figures of merit to measure the power absorption: spatial peak of the acoustic power absorption density, and the acoustic power absorbed within the focal area. Then, we derived “rule of thumb” expressions for the optimum frequencies that maximized these figures of merit given the target depth and homogeneous tissue type. We also formulated a method to calculate the optimum frequency for inhomogeneous tissue given the tissue composition for situations where the tissue structure can be assumed to be made of parallel layers of homogeneous tissue. We checked the validity of the rules using linear acoustic field simulations. For a one-dimensional array of 4 cm acoustic aperture, and for a two-dimensional array of 4 × 4 cm² acoustic aperture, we found that the power absorbed within the focal area is maximized at 0.86 MHz, and 0.79 MHz, respectively, when the target depth is 4 cm in muscle tissue. The rules on the other hand predicted the optimum frequencies for acoustic power absorption as 0.9 MHz and 0.86 MHz, respectively for the 1D and 2D array case, which are within 6% and 9% of the field simulation results. Because radiation force generated by an acoustic wave in a lossy propagation medium is approximately proportional to the acoustic power absorption, these rules can be used to maximize acoustic radiation force generated in tissue as well.     

Acoustic characterization of a new trisacryl contrast agent. Part I: In vitro study

The objective of the study was to acoustically characterize trisacryl polymeric microparticles (TMP), which are derived from biocompatible embolic agents.With significant acoustic properties, these polymeric particles could be potentially used as targeted ultrasound contrast agents, directed towards a specific site, with ligands conjugation on the polymeric network surface. In the in vitro study, a pulser/receiver (PRF of 1 kHz), associated to different transducers (5, 10 and 15 MHz), was used to measure the acoustic properties of the TMP inserted in a Couette flow device. Acoustic characterization according to TMP concentration (0.12-15.63 mg/ml), frequency (4.5-17 MHz, defined by each transducer bandwidth), ultrasound pressure (137-378 kPa) and exposure time (0-30 min) was conducted. Particle attenuation was also evaluated according to TMP concentration and emission frequency. Backscattering increased non linearly with concentration and maximum enhancement was of 16.4 dB ± 0.89 dB above 7.8 mg/ml. This parameter was found non-linear with increasing applied pressure and no harmonic oscillation could be noticed. Attenuation reached approximately 1.4 dB/cm at 15 MHz and for the 15.6 mg/ml suspension.The TMP have revealed in vitro ultrasound properties comparable to those observed with known contrast agents, studied in similar in vitro systems. However, such set-ups combined with a rather aqueous suspending medium, have some limitations and further investigations need now to be conducted to approach in vivo conditions in terms of flow and blood environment.     

A study on the control of dye solution temperature on the line-width and wavelength stability of a copper vapour laser pumped single mode dye laser

This paper presents a study on the line-width and wavelength stability of a single mode narrow line-width (≈ 100 MHz) dye laser pumped by a copper vapour laser, with and without precision temperature control of dye solution. The single mode dye laser system was based on a specially designed SS metal dye cell and grazing incidence grating (GIG) dye resonator with intra-cavity double prism beam expander and etalon. A high precision wavelength-meter was used to record the line-width and frequency stability data. With the coarse dye solution temperature control of 23 ± 2 °C, dye laser line-width varied in a periodic fashion (in every 30 s) between 100 and 770 MHz. Frequency stability was ± 215 MHz (1 min). This is attributed to switching from single to double mode due to temperature induced cavity length change. For the precise dye solution control of 23 ± 0.1 °C, the periodic variation of the line-width was removed completely and the line-width was always ≤ 100 MHz. The frequency stability also improved to ± 42 MHz (1 min). It is established that the dye temperature control is very crucial for achieving, highly frequency stable single axial mode operation.     

Noninvasive estimation of the ocular elastic modulus for age-related macular degeneration in the human eye using sequential ultrasound imaging

Introduction

Elastic modulus estimation may be an important clinical criterion, as it seems to affect such eye parameters as intraocular pressure, ocular pulsation, blood flow, effect of topical medications, and post-refractive surgery complications. The purpose of this study was to examine the differences in elasticity in the ocular axial length, posterior wall thickness (posterior pole), and retina-choroid thickness under normal and aged-related macular degeneration (AMD) conditions in the human eye by directly estimating the elastic modulus with sequential and noninvasive ultrasound image processing.

Materials and Methods

In this study, 25 healthy subjects and 20 patients with non-neovascular AMD participated in the experiment. The deformation of the ocular axial length, posterior wall thickness and retina-choroid complex thickness was captured using high-resolution ultrasonography before and after loading. The B-mode (20 MHz) and A-mode (8 MHz) frames were obtained and processed with an echo tracking technique. The elastic modulus was estimated using changes in ocular axial length, posterior wall thickness and retina-choroid complex thickness and with applied stress measurements.

Results

There was a significant difference (p < 0.05) in the ocular axial length elastic modulus between the AMD and healthy subjects (AMD patients: 95.165 ± 26.431 kPa, vs. healthy subjects: 49.539 ± 25.867 kPa). Moreover, there was a statistically significant difference (p < 0.05) in the posterior wall thickness elastic modulus between AMD patients and healthy subjects (AMD patients: 50.519 ± 12.295 kPa, vs. healthy subjects: 20.519 ± 11.827 kPa). However, no statistically significant difference (p-value > 0.05) was found in the retina-choroid complex elastic modulus between the two groups (AMD patients: 20.134 ± 3.898 kPa, vs. healthy subjects: 15.630 ± 4.250 kPa).

Conclusion

Although the results were obtained examining a relatively low number of patients, it would appear that noninvasive ultrasound estimation of the local elastic moduli of ocular axial length and posterior wall thickness is suited to aid in detection of the non-exudative AMD thus manifesting its potential as a screening tool in symptom-free individuals.     

Analysis of 2-D motion tracking in ultrasound with dual transducers

We study displacement and strain measurement error of dual transducers (two linear arrays, aligned orthogonally and coplanar). Displacements along the beam of each transducer are used to obtain measurements in two-dimensions. Simulations (5 MHz) and experiments (10 MHz) are compared to measurements with a single linear array, with and without angular compounding. Translation simulations demonstrate factors of 1.07 larger and 8.0 smaller biases in the axial and lateral directions respectively, for dual transducers compared to angular compounding. As the angle between dual transducers decreases from 90° to 40°, for 1% compression simulations, the lateral RMS error ranges from 2.1 to 3.9 μm compared to 9 μm with angular compounding. Simulation of dual transducer misalignment of 1 mm and 2° result in errors of less than 9 μm. Experiments demonstrate factors of 3.0 and 5.2 lower biases for dual transducers in the axial and lateral directions respectively compared to angular compounding.     

High-overtone self-focusing acoustic transducers for high-frequency ultrasonic Doppler

This work reports the potential use of high-overtone self-focusing acoustic transducers for high-frequency ultrasonic Doppler. By using harmonic frequencies of a thick bulk Lead Zirconate Titanate (PZT) transducer with a novel air-reflector Fresnel lens, we obtained strong ultrasound signals at 60 MHz (3rd harmonic) and 100 MHz (5th harmonic). Both experimental and theoretical analysis has demonstrated that the transducers can be applied to Doppler systems with high frequencies up to 100 MHz.     

Development of calibration techniques for ultrasonic hydrophone probes in the frequency range from 1 to 100 MHz

The primary objective of this work was to develop and optimize the calibration techniques for ultrasonic hydrophone probes used in acoustic field measurements up to 100 MHz. A dependable, 100 MHz calibration method was necessary to examine the behavior of a sub-millimeter spatial resolution fiber optic (FO) sensor and assess the need for such a sensor as an alternative tool for high frequency characterization of ultrasound fields. Also, it was of interest to investigate the feasibility of using FO probes in high intensity fields such as those employed in HIFU (high intensity focused ultrasound) applications. In addition to the development and validation of a novel, 100 MHz calibration technique the innovative elements of this research include implementation and testing of a prototype FO sensor with an active diameter of about 10 μm that exhibits uniform sensitivity over the considered frequency range and does not require any spatial averaging corrections up to about 75 MHz. The results of the calibration measurements are presented and it is shown that the optimized calibration technique allows the sensitivity of the hydrophone probes to be determined as a virtually continuous function of frequency and is also well suited to verify the uniformity of the FO sensor frequency response. As anticipated, the overall uncertainty of the calibration was dependent on frequency and determined to be about ±12% (±1 dB) up to 40 MHz, ±20% (±1.5 dB) from 40 to 60 MHz and ±25% (±2 dB) from 60 to 100 MHz. The outcome of this research indicates that once fully developed and calibrated, the combined acousto-optic system will constitute a universal reference tool in the wide, 100 MHz bandwidth.     

Nonlinear change of on-axis pressure and intensity maxima positions and its relation with the linear focal shift effect

A comprehensive experimental, analytical and numerical study of the true focal region drift relative to the geometrical focus (focal shift effect) in acoustic focused beams and its nonlinear evolution is presented. For this aim, the concept of Fresnel number, proportional to the linear gain, is introduced as a convenient parameter for characterizing focused sources. It is shown that the magnitude of the shift is strongly dependent on the Fresnel number of the source, being larger for weakly focused systems where a large initial shift occurs. Analytical expressions for axial pressure distributions in linear regime are presented for the general case of truncated Gaussian beams. The main new contribution of this work is the examination of the connection between the linear and nonlinear stages of the focal shift effect, and its use for the estimation of the more complicated nonlinear stage. Experiments were carried out using a continuous-wave ultrasonic beam in water, radiated by a focused source with nominal frequency f = 1 MHz, aperture radius a = 1.5 cm and geometrical focal distance R = 11.7 cm, corresponding to a Fresnel number N_F = 1.28. The maximum measured shifts for peak pressure and intensity were 4.4 and 1.1 cm, respectively. The evolution of the different maxima with the source amplitude, and the disparity in their axial positions, is interpreted in terms of the dynamics of the nonlinear distortion process. Analytical results for the particular case of a sound beam with initial Gaussian distribution are also presented, demonstrating that the motion of peak pressure and peak intensity may occur in opposite directions.     

Large GMI effect in Co-rich amorphous wire by tensile stress

Quality Co₆₈Fe_4.5Si₁₅B_12.5 amorphous wires are fabricated and their giant magneto impedance (GMI) effect are investigated at frequencies ranging from 0.1 to 20 MHz with or without tensile stress applied. Experimental results indicate that the GMI effect of these wires can be effectively improved by applying a small axial tensile stress. There is a pronounced increase from 1.3% to 47.3% in impedance ratio at 0.6 MHz when a stress of 103 MPa is applied. The GMI response is further increased to 261% by doing so at 15 MHz. The field sensitivity of these wires is optimized and it increases from 0.55 to 2.73%/(A/m) at 0.6 MHz and it is proved that the most sensitive field response is obtained by applying a tensile stress of 84.5 MPa below 1 MHz. With reference to the stress evolution of sensitivity at varying frequencies, the maximum field sensitivity shifts to a lower stress with increasing frequency. Quantitatively, a stress as small as 18.4 MPa is enough to yield a field sensitivity as high as 6.7%/(A/m) at 15 MHz. This demonstrates the possibility of achieving small stress (<100 MPa) induced large enhancement of GMI effect and field sensitivity at frequencies of several tens of MHz that are of much technical interest in sensor applications.     

Osteocytes exposed to far field of therapeutic ultrasound promotes osteogenic cellular activities in pre-osteoblasts through soluble factors

Low intensity pulsed ultrasound (LIPUS) was reported to accelerate the rate of fracture healing. When LIPUS is applied to fractures transcutaneously, bone tissues at different depths are exposed to different ultrasound fields. Measurement of LIPUS shows pressure variations in near field (nearby transducer); uniform profile was found beyond it (far field). Moreover, we have reported that the therapeutic effect of LIPUS is dependent on the axial distance of ultrasound beam in rat fracture model. However, the mechanisms of how different axial distances of LIPUS influence the mechanotransduction of bone cells are not understood. To understand the cellular mechanisms underlying far field LIPUS on enhanced fracture healing in rat model, the present study investigated the effect of ultrasound axial distances on (1) osteocyte, the mechanosensor, and (2) mechanotransduction between osteocyte and pre-osteoblast (bone-forming cell) through paracrine signaling. We hypothesized that far field LIPUS could enhance the osteogenic activities of osteoblasts via paracrine factors secreted from osteocytes. The objective of this study was to investigate the effect of axial distances of LIPUS on osteocytes and osteocyte–osteoblast mechanotransduction. In this study, LIPUS (plane; 2.2 cm in diameter, 1.5 MHz sine wave, I_SATA = 30 mW/cm²) was applied to osteocytes (mechanosensor) at three axial distances: 0 mm (near field), 60 mm (mid-near field) and 130 mm (far field). The conditioned medium of osteocytes (OCM) collected from these three groups were used to culture pre-osteoblasts (effector cell). In this study, (1) the direct effect of ultrasound fields on the mechanosensitivity of osteocytes; and (2) the osteogenic effect of different OCM treatments on pre-osteoblasts were assessed. The immunostaining results indicated the ultrasound beam at far field resulted in more β-catenin nuclear translocation in osteocytes than all other groups. This indicated that osteocytes could detect the acoustic differences of LIPUS at various axial distances. Furthermore, we found that the soluble factors secreted by far field LIPUS exposed osteocytes could further promote pre-osteoblasts cell migration, maturation (transition of cell proliferation into osteogenic differentiation), and matrix calcification. In summary, our results of this present study indicated that axial distance beyond near field could transmit ultrasound energy to osteocyte more efficiently. The LIPUS exposed osteocytes conveyed mechanical signals to pre-osteoblasts and regulated their osteogenic cellular activities via paracrine factors secretion. The soluble factors secreted by far field exposed osteocytes led to promotion in migration and maturation in pre-osteoblasts. This finding demonstrated the positive effects of far field LIPUS on stimulating osteocytes and promoting mechanotransduction between osteocytes and osteoblasts.     

In vitro measurement of attenuation and nonlinear scattering from echogenic liposomes

Echogenic liposomes (ELIP) are an excellent candidate for concurrent imaging and drug delivery applications. They combine the advantages of liposomes-biocompatibility and ability to encapsulate both hydrophobic and hydrophilic drugs-with strong reflections of ultrasound. The objective of this study is to perform a detailed in vitro acoustic characterization - including nonlinear scattering that has not been studied before - along with an investigation of the primary mechanism of echogenicity. Both components are critical for developing viable clinical applications of ELIP. Mannitol, a cryoprotectant, added during the preparation of ELIP is commonly believed to be critical in making them echogenic. Accordingly, here ELIP prepared with varying amount of mannitol concentration are investigated for their pressure dependent linear and non-linear scattered responses. The average diameter of these liposomes is measured to be 125-185 nm. But they have a broad size distribution including liposomes with diameters over a micro-meter as observed by TEM and AFM. These larger liposomes are critical for the overall echogenicity. Attenuation through liposomal solution is measured with four different transducers (central frequencies 2.25, 3.5, 5, 10 MHz). Measured attenuation increases linearly with liposome concentration indicating absence of acoustic interactions between liposomes. Due to the broad size distribution, the attenuation shows a flat response without a distinct peak in the range of frequencies (1-12 MHz) investigated. A 15-20 dB enhancement with 1.67 μg/ml of lipids is observed both for the scattered fundamental and the second harmonic responses at 3.5 MHz excitation frequency and 50-800 kPa amplitude. It demonstrates the efficacy of ELIP for fundamental as well as harmonic ultrasound imaging. The scattered response however does not show any distinct subharmonic peak for the acoustic excitation parameters studied. Small amount of mannitol proves critical for echogenicity. However, mannitol concentration above 100 mM shows no effect.     

Dual-high-frequency ultrasound excitation on microbubble destruction volume

Objective and motivation

The goal of this work was to test experimentally that exposing air bubbles or ultrasound contrast agents in water to amplitude modulated wave allows control of inertial cavitation affected volume and hence could limit the undesirable bioeffects.

Methods

Focused transducer operating at the center frequency of 10 MHz and having about 65% fractional bandwidth was excited by 3 μs 8.5 and 11.5 MHz tone-bursts to produce 3 MHz envelope signal. The 3 MHz frequency was selected because it corresponds to the resonance frequency of the microbubbles used in the experiment. Another 5 MHz transducer was used as a receiver to produce B-mode image. Peak negative acoustic pressure was adjusted in the range from 0.5 to 3.5 MPa. The spectrum amplitudes obtained from the imaging of SonoVue^TM contrast agent when using the envelope and a separate 3 MHz transducer were compared to determine their cross-section at the - 6 dB level.

Results

The conventional 3 MHz tone-burst excitation resulted in the region of interest (ROI) cross-section of 2.47 mm while amplitude modulated, dual-frequency excitation with difference frequency of 3 MHz produced cross-section equal to 1.2 mm.

Conclusion

These results corroborate our hypothesis that, in addition to the considerably higher penetration depth of dual-frequency excitation due to the lower attenuation at 3 MHz than that at 8.5 and 11.5 MHz, the sample volume of dual-frequency excitation is also smaller than that of linear 3-MHz method for more spatially confined destruction of microbubbles.     

Exvivo ultrasound attenuation coefficient for human cervical and uterine tissue from 5 to 10 MHz

Attenuation estimation and imaging in the cervix has been utilized to evaluate the onset of cervical ripening during pregnancy. This feature has also been utilized for the acoustic characterization of leiomyomas and myometrial tissue. In this paper, we present direct narrowband substitution measurement values of the variation in the ultrasonic attenuation coefficient in ex vivo human uterine and cervical tissue, in the 5-10 MHz frequency range. At 5 MHz, the attenuation coefficient values are similar for the different orientations of uterine tissue with values of 4.1-4.2 dB/cm, 5.1 dB/cm for the leiomyoma, and 6.3 dB/cm for the cervix. As the frequency increases, the attenuation coefficient values increase and are also spread out, with a value of approximately 12.6 dB/cm for the uterus (both parallel and perpendicular), 16.0 for the leiomyoma, and 26.8 dB/cm for the cervix at 10 MHz. The attenuation coefficient measured increases monotonically over the frequency range measured following a power law.     

Experimental study of underwater transmission characteristics of high-frequency 30 MHz polyurea ultrasonic transducer

In this paper, we present the transmission characteristics of a polyurea ultrasonic transducer operating in water. In this study, we used a polyurea transducer with fundamental resonance at approximately 30 MHz. Firstly, acoustic pressure radiated from the transducer was measured using a hydrophone, which has a diameter of 0.2 mm. The transmission characteristics such as relative bandwidth, pulse width, and acoustic sensitivity were calculated from the experimental results. The results of the experiment showed a relative bandwidth of 50% and a pulse width of 0.061 μs. The acoustic sensitivity was 0.60 kPa/V with good linearity, where the correlation coefficient R in the fitting calculation was 0.996. A maximum pressure of 13.1 kPa was observed when the transducer was excited at a zero-to-peak voltage of 21 V. Moreover, we experimentally verified the results. The results of the pulse/echo experiment showed that the estimated diameters of the copper wires were 458 and 726 μm, where the differences between the actual and measured values were 15% and 4%, respectively. Acoustic streaming was also observed so that a particle velocity map was estimated by particle image velocimetry (PIV). The sound pressure calculated from the particle velocity obtained by PIV showed good agreement with the acoustic pressure measured using the hydrophone, where the differences between the calculated and measured values were 12–19%.     

Study of ultrasound stiffness imaging methods using tissue mimicking phantoms

A pilot study was carried out to investigate the performance of ultrasound stiffness imaging methods namely Ultrasound Elastography Imaging (UEI) and Acoustic Radiation Force Impulse (ARFI) Imaging. Specifically their potential for characterizing different classes of solid mass lesions was analyzed using agar based tissue mimicking phantoms. Composite tissue mimicking phantom was prepared with embedded inclusions of varying stiffness from 50 kPa to 450 kPa to represent different stages of cancer. Acoustic properties such as sound speed, attenuation coefficient and acoustic impedance were characterized by pulse echo ultrasound test at 5 MHz frequency and they are ranged from (1564 ± 88 to 1671 ± 124 m/s), (0.6915 ± 0.123 to 0.8268 ± 0.755 db cm^-1 MHz^-1) and (1.61×10⁶ ± 0.127 to 1.76 × 10⁶ ± 0.045 kg m^-2 s^-1) respectively. The elastic property Young’s Modulus of the prepared samples was measured by conducting quasi static uni axial compression test under a strain rate of 0.5 mm/min upto 10 % strain, and the values are from 50 kPa to 450 kPa for a variation of agar concentration from 1.7% to 6.6% by weight. The composite phantoms were imaged by Siemens Acuson S2000 (Siemens, Erlangen, Germany) machine using linear array transducer 9L4 at 8 MHz frequency; strain and displacement images were collected by UEI and ARFI. Shear wave velocity 4.43 ± 0.35 m/s was also measured for high modulus contrast (18 dB) inclusion and X.XX m/s was found for all other inclusions. The images were pre processed and parameters such as Contrast Transfer Efficiency and lateral image profile were computed and reported. The results indicate that both ARFI and UEI represent the abnormalities better than conventional US B mode imaging whereas UEI enhances the underlying modulus contrast into improved strain contrast. The results are corroborated with literature and also with clinical patient images.