Acoustic Threshold Research Articles

Fluorocarbon nanodrops as acoustic temperature probes.

This work investigated the use of superheated fluorocarbon nanodrops for ultrasound thermal imaging and the use of mixed fluorocarbons for tuning thermal and acoustic thresholds for vaporization. Droplets were fabricated by condensing phospholipid-coated microbubbles containing C3F8 and C4F10 mixed at various molar ratios. Vaporization temperatures first were measured in a closed system by optical transmission following either isothermal pressure release or isobaric heating. The vaporization temperature was found to depend linearly on the percentage of C4F10 in the droplet core, indicating excellent tunability under these fluorocarbon-saturated conditions. Vaporization temperatures were then measured in an open system using contrast-enhanced ultrasound imaging, where it was found that the mixed droplets behaved like pure C4F10 drops. Additionally, the critical mechanical index for vaporization was measured at the limits of therapeutic hyperthermia (37 and 60 °C), and again the mixed droplets were found to behave like pure C4F10 drops. These results suggested that C3F8 preferentially dissolves out of the droplet core in open systems, as shown by a simple mass transfer model of multicomponent droplet dissolution. Finally, proof-of-concept was shown that pure C4F10 nanodrops can be used as an acoustic temperature probe. Overall, these results not only demonstrate the potential of superheated fluorocarbon emulsions for sonothermetry but also point to the limits of tunability for fluorocarbon mixtures owing to preferential release of the more soluble species to the atmosphere.

The effect of seismic waterguns on the inner ears of round goby

Abstract High-pressure acoustic sources, such as seismic exploration technology or sonar, can temporarily or permanently affect the auditory system of some fish species. The majority of studies done on fish hearing aim to determine safe acoustic operating thresholds for different fishes in order to mitigate possible negative effects of these anthropogenic noise sources. In contrast, the present study was designed to evaluate the effects of a hydraulic watergun on invasive round goby in Lake Michigan as a potential population suppression tool to protect critical spawning habitat for native fishes. The goal of the present study was to evaluate the potential sub-lethal effects of pulse pressure sound technology on the hearing end organs of round goby. Round goby from Little Traverse Bay near Charlevoix, MI were exposed to 6 discharges from a seismic watergun at an average peak sound pressure level of 229 dB re 1 μPa. Fish were monitored for 60 h post-exposure and assessed for damage to otoliths and inner ear sensory hair cells. We found no significant hair cell loss or otolith damage, suggesting that a higher peak pressure level or longer duration of discharges are needed to cause significant damage to round goby inner ear anatomy.

Role of saposin C and D in auditory and vestibular function.

Saposins are small proteins derived from a precursor protein, prosaposin. Each of the four saposins (A-D) is necessary for the activity of lysosomal glycosphingolipid hydrolases. Individual saposin mutations lead to lysosomal storage diseases, some of which are associated with hearing loss. Here we evaluate the effects of the loss of saposins C and D on auditory and vestibular function in transgenic mice. Transgenic mice with either loss of saposin C function or a combined loss of saposin C + D function were studied. Light microscopy and immunofluorescence were used to evaluate histologic and morphologic changes in the auditory and vestibular organs. Acoustic brainstem response thresholds and distortion product otoacoustic emissions were used to study the auditory phenotype. A null mutation of saposin C did not result in any identifiable histologic changes or loss of hearing through postnatal day 55. Combined losses of saposins C and D similarly did not result in any changes in organ of Corti histology or loss of hearing. However, inclusions within the vestibular end organs was noted, consistent with afferent and efferent neuronal sprouting, although to a much milder degree than seen in the previously studied prosaposin knockout mouse. Loss of saposin C and D function, although causing mild phenotypic changes in the vestibular end organs, otherwise results in minimal functional impairment and no changes in the auditory system. It is more likely that the auditory and vestibular effects of the loss of prosaposin are mediated through the actions of saposin A and/or B. NA.

Multi-functional biomedical ultrasound: Imaging, manipulation, neuromodulation and therapy

As a safe, low-cost, portable and fast imaging modality, ultrasound has been widely used in various clinical fields. Ultrasound imaging is used in every branch of medicine and can be used to visualize almost every organ. Recently, numerous new ultrasound technologies, such as microbubble-based nonlinear contrast imaging, ultrasound molecular imaging, ultrasound-assisted targeted drug delivery, ultrasonic neuro-modulation and theranostics-based integrative application, have been developed for early-stage diagnosis and treatment, based on the multi-disciplinary developments in physics, engineering, and biomedicine. The nature of biomedical ultrasound technology is also changing, from being an imaging technology into a multi-functional tool that allows both precise diagnosis and treatment. This review offers a critical analysis of the state-of-the-art biomedical ultrasound technology and its application in clinical diagnosis and drug delivery. In this paper, we first introduce some basic physical characteristics of biomedical ultrasound technology, including its wave and mechanical properties, the history of the development of ultrasound contrast agent microbubbles, and the biomedical effects of ultrasound, such as cavitation and heat deposition. Then, we review the recent progress and our research on the preparation and acoustic characterization of microbubbles, nonlinear ultrasound contrast imaging and ultrasound molecular imaging, and ultrasound-assisted drug delivery and treatment based on the large surface of a drug-loaded microbubble and on the surface modification-enabled elastic shell. In addition, we discuss the integrative applications of ultrasound diagnostics and therapeutics in clinical medicine. Moreover, we review the advances in ultrasound-based deep brain stimulation, which indicates that ultrasound technology might be used as a potential noninvasive tool in neuroscience and in the treatment of brain disorders. Finally, we review the challenges associated with the development of biomedical ultrasound technology, which include acoustic power thresholds in drug delivery and deep brain stimulation for the safety and precision control of acoustic fields in inhomogeneous media. In conclusion, besides assessing the conventional tissue anatomy and hemodynamics, we argue that biomedical ultrasound techniques can provide information on tissue elasticity, metabolic function, distribution of temperature, and even biochemical processes. Owing to its advantages such as acoustic radiation force, the use of ultrasound might open a new era in neuro-modulation in the neurosciences, in addition to its traditional areas of use such as imaging and thermal therapy.

Acoustic strength of water and effect of ultrasound on the liquid-vapor phase diagram

The structure-time approach is used to develop an analytical model that makes it possible to predict the dependences of the acoustic cavitation threshold of water on temperature and background pressure. The calculated dependences are compared with the results of experiments carried out in the leading laboratories. It is demonstrated that the proposed approach allows the estimation of the effect of the acoustic field on the phase state of the substance under study. The calculated liquid-vapor phase curves for water in the presence of acoustic fields are presented.

Peculiarities of hearing impairment depending on interaction with acoustic stimuli.

Aims:The functional state of the auditory analyzer of several operators groups was study. The objective of this study was to determine some characteristics of hearing impairment in relation with features of acoustic stimuli and informative significance of noise.Materials and Methods:236 employees (middle age 35.4 ± 0.74 years) were divided into four groups according to features of noise perception at the workplaces. The levels of permanent shifts of acoustic thresholds were estimated using audiometric method.Statistical Analysis Used:Common statistical methods were used in research. Mean quantity and mean absolute errors were calculated. Statistical significance between operators' groups was calculated with 0.05 confidential intervals.Results:The peculiarities of hearing impairment in observed groups were different. Operators differentiating acoustic signals had peak of hearing impairment in the field of language frequencies, while the employees who work with noise background at the workplaces had maximal hearing threshold on the 4000 Hz frequency (P ≤ 0.05).Conclusions:Hearing impairment depends both on energy and human interaction with acoustic irritant. The distinctions in hearing impairment may be related with the necessity of recognizing of acoustic signals and their frequency characteristics.

The ratio of the subjective audiometry in patients with acoustic trauma and “noisy” production workers.

Introduction: The problem of diagnosis and treatment of sensorineural hearing loss (SHL), including forms developed under the influence of noise, takes one of the leading places in ot­olaryngology. However, there are not many studies on acoustic trauma, although this problem has recently become more and more important. Objective: A comparison of subjective audiometry in patients with sensorineural hearing loss after acute acoustic trauma and chronic noise exposure. Materials and methods. In the work the results of exa­mination of 84 patients with acoustic trauma, 15 healthy as the control group and 15 workers employed on 'noise' occupations as a comparison group are given. Subjective audiometry was fully carried out by clinical audiometer AC-40 «Interacoustics» (Denmark). Hearing indices were investigated in the conventional (0,125-8) kHz and extended (9-16) kHz frequency bands. Results: Subjective audiometry showed a reduction in sound perception in all patients. Ac­cor­ding to the threshold tone audiometry in patients with acoustic trauma hearing thresholds were authentically (P <0,05) increased by 4, 6 and 8 kHz tones of conventional (0,125-8) kHz frequency band and by 14-16 kHz tones of the extended (9-16 kHz) in comparison with the control group, as with workers employed on noise occupations. All the examined patients had deterioration of speech-test audiometry and above-threshold audiometry. Conclusions: According to su­b­jective audiometry, the type similar disorders of auditory function are in patients with acoustic trauma as in patients with long-term noise exposure, but they are more pronounced and develop much faster. The most informative features which show the origin and progression of hearing loss in patients with acoustic trauma are: increasing hearing thresholds by 14 and 16 kHz tones of the extended (9-16 kHz) frequency band and by 4, 6 and 8 kHz tones of con­ven­tional (0,125-8) kHz frequency band plus the reduction of differential thresholds (DT) according to the Lusher’s method at 4 kHz.

Assessing the underwater acoustics of the world's largest vibration hammer (OCTA-KONG) and its potential effects on the Indo-Pacific humpbacked dolphin (Sousa chinensis).

Anthropogenic noise in aquatic environments is a worldwide concern due to its potential adverse effects on the environment and aquatic life. The Hongkong-Zhuhai-Macao Bridge is currently under construction in the Pearl River Estuary, a hot spot for the Indo-Pacific humpbacked dolphin (Sousa chinensis) in China. The OCTA-KONG, the world's largest vibration hammer, is being used during this construction project to drive or extract steel shell piles 22 m in diameter. This activity poses a substantial threat to marine mammals, and an environmental assessment is critically needed. The underwater acoustic properties of the OCTA-KONG were analyzed, and the potential impacts of the underwater acoustic energy on Sousa, including auditory masking and physiological impacts, were assessed. The fundamental frequency of the OCTA-KONG vibration ranged from 15 Hz to 16 Hz, and the noise increments were below 20 kHz, with a dominant frequency and energy below 10 kHz. The resulting sounds are most likely detectable by Sousa over distances of up to 3.5 km from the source. Although Sousa clicks do not appear to be adversely affected, Sousa whistles are susceptible to auditory masking, which may negatively impact this species' social life. Therefore, a safety zone with a radius of 500 m is proposed. Although the zero-to-peak source level (SL) of the OCTA-KONG was lower than the physiological damage level, the maximum root-mean-square SL exceeded the cetacean safety exposure level on several occasions. Moreover, the majority of the unweighted cumulative source sound exposure levels (SSELs) and the cetacean auditory weighted cumulative SSELs exceeded the acoustic threshold levels for the onset of temporary threshold shift, a type of potentially recoverable auditory damage resulting from prolonged sound exposure. These findings may aid in the identification and design of appropriate mitigation methods, such as the use of air bubble curtains, “soft start” and “power down” techniques.

Enhancing acoustic cavitation using artificial crevice bubbles

We study the response of pre-defined cavitation nuclei driven continuously in the kHz regime (80, 100 and 200kHz). The nuclei consist of stabilized gaspockets in cylindrical pits of 30μm diameter etched in silicon or glass substrates. It is found that above an acoustic pressure threshold the dynamics of the liquid–gas meniscus switches from a stable drum-like vibration to expansion and deformation, frequently resulting in detachment of microbubbles. Just above this threshold small bubbles are continuously and intermittently ejected. At elevated input powers bubble detachment becomes more frequent and cavitation bubble clouds are formed and remain in the vicinity of the pit bubble. Surprisingly, the resulting loss of gas does not lead to deactivation of the pit which can be explained by a rectified gas diffusion process.

Factors influencing electrical place pitch perception in bimodal listeners.

Factors that might affect perceptual pitch match between acoustic and electric stimulation were examined in 25 bimodal listeners using magnitude estimation. Pre-operative acoustic thresholds in both ears, and duration of severe-profound loss, were first examined as correlates with degree of match between the measured pitch and that predicted by the spiral ganglion frequency-position model. The degree of match was examined with respect to (1) the ratio between the measured and predicted pitch percept on the most apical electrode and (2) the ratio between the slope of the measured and predicted pitch function. Second, effect of listening experience was examined to assess whether adaptation occurred over time to match the frequency assignment to electrodes. Pre-experience pitch estimates on the apical electrode were within the predicted range in only 28% of subjects, and the slope of the electrical pitch function was lower than predicted in all except one subject. Subjects with poorer hearing tended to have a lower pitch and a shallower electrical pitch function than predicted by the model. Pre-operative hearing thresholds in the contralateral ear and hearing loss duration were not correlated with the degree of pitch match, and there was no significant group effect of listening experience.

An acoustic threshold for third formant in American English /r/

It is well known that a low F3 is the most salient acoustic feature of American English /r/, and that the degree of F3 lowering is correlated with the degree to which /r/ is perceptually acceptable to native listeners as a “good” vs. “misarticulated” /r/. Identifying the point at which F3 lowering produces a “good” /r/ would be helpful in remediation of /r/-production difficulties in children and second language learners. Such a measure would require normalization across speakers. Hagiwara (1995) observed that F3 for /r/ in competent adult speakers was at or below 80% of the average vowel frequencies for a given speaker. In this study, we investigate whether children’s productions start to sound “good” when they lower F3 to the 80% demarcation level or below. Words with /r/ and vowel targets from 20 children with a history of /r/ misarticulation were extracted from acoustic records of speech therapy sessions. Three experienced clinicians judged correctness of /r/ productions. Measured F3’s at the midpoint of /r/ and a range of vowels were compared for these productions. Preliminary findings suggest that the 80% level is a viable demarcation point for good vs. misarticulated articulation of /r/.

Cavitation thresholds of contrast agents in an in vitro human clot model exposed to 120-kHz ultrasound.

Ultrasound contrast agents (UCAs) can be employed to nucleate cavitation to achieve desired bioeffects, such as thrombolysis, in therapeutic ultrasound applications. Effective methods of enhancing thrombolysis with ultrasound have been examined at low frequencies (<1 MHz) and low amplitudes (<0.5 MPa). The objective of this study was to determine cavitation thresholds for two UCAs exposed to 120-kHz ultrasound. A commercial ultrasound contrast agent (Definity(®)) and echogenic liposomes were investigated to determine the acoustic pressure threshold for ultraharmonic (UH) and broadband (BB) generation using an in vitro flow model perfused with human plasma. Cavitation emissions were detected using two passive receivers over a narrow frequency bandwidth (540-900 kHz) and a broad frequency bandwidth (0.54-1.74 MHz). UH and BB cavitation thresholds occurred at the same acoustic pressure (0.3 ± 0.1 MPa, peak to peak) and were found to depend on the sensitivity of the cavitation detector but not on the nucleating contrast agent or ultrasound duty cycle.

Magnetic aspect sensitivity of high‐latitude E region irregularities measured by the RAX‐2 CubeSat

The second Radio Aurora Explorer (RAX‐2) satellite has completed more than 30 conjunction experiments with the Advanced Modular Incoherent Scatter Radar chain of incoherent scatter radars in Alaska and Resolute Bay, Canada. Coherent radar echoing occurred during four of the passes: three when E region electron drifts exceeded the ion acoustic speed threshold and one during HF heating of the ionosphere by the High Frequency Active Auroral Research Program heater. In this paper, we present the results for the first three passes associated with backscatter from natural irregularities. We analyze, in detail, the largest drift case because the plasma turbulence was the most intense and because the corresponding ground‐to‐space bistatic scattering geometry was the most favorable for magnetic aspect sensitivity analysis. A set of data analysis procedures including interference removal, autocorrelation analysis, and the application of a radar beam deconvolution algorithm mapped the distribution of E region backscatter with 3 km resolution in altitude and ∼0.1° in magnetic aspect angle. To our knowledge, these are the highest resolution altitude‐resolved magnetic aspect sensitivity measurements made at UHF frequencies in the auroral region. In this paper, we show that despite the large electron drift speed of ∼1500 m/s, the magnetic aspect sensitivity of submeter scale irregularities is much higher than previously reported. The root‐mean‐square of the aspect angle distribution varied monotonically between 0.5° and 0.1° for the altitude range 100–110 km. Findings from this single but compelling event suggest that submeter scale waves propagating at larger angles from the main E×B flow direction (secondary waves) have parallel electric fields that are too small to contribute to E region electron heating. It is possible that anomalous electron heating in the auroral electrojet can be explained by (a) the dynamics of those submeter scale waves propagating in the E×B direction (primary waves) or (b) the dynamics of longer wavelengths.

Magnetic Resonance Imaging for the Exploitation of Bubble-Enhanced Heating by High-Intensity Focused Ultrasound: A Feasibility Study in ex Vivo Liver

Bubble-enhanced heating (BEH) may be exploited to improve the heating efficiency of high-intensity focused ultrasound in liver and to protect tissues located beyond the focal point. The objectives of this study, performed in ex vivo pig liver, were (i) to develop a method to determine the acoustic power threshold for induction of BEH from displacement images measured by magnetic resonance acoustic radiation force imaging (MR-ARFI), and (ii) to compare temperature distribution with MR thermometry for HIFU protocols with and without BEH. The acoustic threshold for generation of BEH was determined in ex vivo pig liver from MR-ARFI calibration curves of local tissue displacement resulting from sonication at different powers. Temperature distributions (MR thermometry) resulting from “conventional” sonications (20 W, 30 s) were compared with those from “composite” sonications performed at identical parameters, but after a HIFU burst pulse (0.5 s, acoustic power over the threshold for induction of BEH). Displacement images (MR-ARFI) were acquired between sonications to measure potential modifications of local tissue displacement associated with modifications of tissue acoustic characteristics induced by the burst HIFU pulse. The acoustic threshold for induction of BEH corresponded to a displacement amplitude of approximately 50 μm in ex vivo liver. The displacement and temperature images of the composite group exhibited a nearly spherical pattern, shifted approximately 4 mm toward the transducer, in contrast to elliptical shapes centered on the natural focal position for the conventional group. The gains in maximum temperature and displacement values were 1.5 and 2, and the full widths at half-maximum of the displacement data were 1.7 and 2.2 times larger than in the conventional group in directions perpendicular to ultrasound propagation axes. Combination of MR-ARFI and MR thermometry for calibration and exploitation of BEH appears to increase the efficiency and safety of HIFU treatment.

Acoustic parametric pumping of spin waves

Recent experiments demonstrated generation of spin currents by ultrasound. We can understand this acoustically induced spin pumping in terms of the coupling between magnetization and lattice waves. Here we study the parametric excitation of magnetization by longitudinal acoustic waves and calculate the acoustic threshold power. The induced magnetization dynamics can be detected by the spin pumping into an adjacent normal metal that displays the inverse spin Hall effect.

Vestibular receptors contribute to cortical auditory evoked potentials

Acoustic sensitivity of the vestibular apparatus is well-established, but the contribution of vestibular receptors to the late auditory evoked potentials of cortical origin is unknown. Evoked potentials from 500 Hz tone pips were recorded using 70 channel EEG at several intensities below and above the vestibular acoustic threshold, as determined by vestibular evoked myogenic potentials (VEMPs). In healthy subjects both auditory mid- and long-latency auditory evoked potentials (AEPs), consisting of Na, Pa, N1 and P2 waves, were observed in the sub-threshold conditions. However, in passing through the vestibular threshold, systematic changes were observed in the morphology of the potentials and in the intensity dependence of their amplitude and latency. These changes were absent in a patient without functioning vestibular receptors. In particular, for the healthy subjects there was a fronto-central negativity, which appeared at about 42 ms, referred to as an N42, prior to the AEP N1. Source analysis of both the N42 and N1 indicated involvement of cingulate cortex, as well as bilateral superior temporal cortex. Our findings are best explained by vestibular receptors contributing to what were hitherto considered as purely auditory evoked potentials and in addition tentatively identify a new component that appears to be primarily of vestibular origin.

A Comparison of Acoustic Cavitation Detection Thresholds Measured with Piezo-electric and Fiber-optic Hydrophone Sensors

A Fabry-Perot interferometer fiber-optic hydrophone (FOH) was investigated for use as an acoustic cavitation detector and compared with a piezo-ceramic passive cavitation detector (PCD). Both detectors were used to measure negative pressure thresholds for broadband emissions in 3% agar and exvivo bovine liver simultaneously. FOH-detected half- and fourth-harmonic emissions were also studied. Three thresholds were defined and investigated: (i) onset of cavitation; (ii) 100% probability of cavitation; and (iii) a time-integrated threshold where broadband signals integrated over a 3-s exposure duration, averaged over 5-10 repeat exposures, become statistically significantly greater than noise. The statistical sensitiviy of FOH broadband detection was low compared with that of the PCD (0.43/0.31 in agar/liver). FOH-detected fourth-harmonic data agreed best with PCD broadband (sensitivity: 0.95/0.94, specificity: 0.89/0.76 in agar/liver). The FOH has potential as a cavitation detector, particularly in applications where space is limited or during magnetic resonance-guided studies.

Influence of the pressure field distribution in transcranial ultrasonic neurostimulation

Low-intensity focused ultrasound has been shown to stimulate the brain noninvasively and without noticeable tissue damage. Such a noninvasive and localized neurostimulation is expected to have a major impact in neuroscience in the coming years. This emerging field will require many animal experiments to fully understand the link between ultrasound and stimulation. The primary goal of this paper is to investigate transcranial ultrasonic neurostimulation at low frequency (320 kHz) on anesthetized rats for different acoustic pressures and estimate the in situ pressure field distribution and the corresponding motor threshold, if any. The corresponding acoustic pressure distribution inside the brain, which cannot be measured in vivo, is investigated based on numerical simulations of the ultrasound propagation inside the head cavity, reproducing at best the experiments conducted in the first part, both in terms of transducer and head geometry and in terms of acoustic parameters. In this study, 37 ultrasonic neurostimulation sessions were achieved in rats (N=8) using a 320 kHz transducer. The corresponding beam profile in the entire head was simulated in order to investigate the in situ pressure and intensity level as well as the spatial pressure distribution, thanks to a rat microcomputed tomography scan (CT)-based 3D finite differences time domain solver. Ultrasound pulse evoked a motor response in more than 60% of the experimental sessions. In those sessions, the stimulation was always present, repeatable with a pressure threshold under which no motor response occurred. This average acoustic pressure threshold was found to be 0.68±0.1 MPa (corresponding mechanical index, MI=1.2 and spatial peak, pulse averaged intensity, Isppa=7.5 W cm(-2)), as calibrated in free water. A slight variation was observed between deep anesthesia stage (0.77±0.04 MPa) and light anesthesia stage (0.61±0.03 MPa), assessed from the pedal reflex. Several kinds of motor responses were observed: movements of the tail, the hind legs, the forelimbs, the eye, and even a single whisker were induced separately. Numerical simulations of an equivalent experiment with identical acoustic parameters showed that the acoustic field was spread over the whole rat brain with the presence of several secondary pressure peaks. Due to reverberations, a 1.8-fold increase of the spatial peak, temporal peak acoustic pressure (Psptp) (±0.4 standard deviation), a 3.6-fold increase (±1.8) for the spatial peak, temporal peak acoustic intensity (Isptp), and 2.3 for the spatial peak, pulse averaged acoustic intensity (Isppa), were found compared to simulations of the beam in free water. Applying such corrections due to reverberations on the experimental results would yield a higher estimation for the average acoustic pressure threshold for motor neurostimulation at 320 KHz at 1.2±0.3 MPa (MI=2.2±0.5 and Isppa=17.5±7.5 W cm(-2)). Transcranial ultrasonic stimulation is pressure- and anesthesia-dependent in the rat model. Numerical simulations have shown that the acoustic pattern can be complex inside the rat head and that special care must be taken for small animal studies relating acoustic parameters to neurostimulation effects, especially at a low frequency.

CFD simulation of acoustic cavitation in a crude oil upgrading sonoreactor and prediction of collapse temperature and pressure of a cavitation bubble

In acoustic cavitation, high pressure and temperature are generated due to cavitation bubble collapse in the liquid bulk around the bubble which causes physical and chemical changes in the liquid. In this study, pressure distribution in water caused by ultrasonic wave propagation in a sonoreactor was investigated. Active cavitation zones were determined by calculating acoustic pressure threshold for cavitation inception and compared with experimental results. Collapse pressure and temperature were predicted 3000atm and 3200K, respectively, for crude oil at temperature of 25°C by evaluating cavitation bubble dynamics in the exerted acoustic field. As a consequence, the huge amounts of energy generated by this phenomenon can be applied for changes in oil properties and crude oil upgrading.

A Self-Focusing Acoustic Transducer That Exploits Cytoskeletal Differences for Selective Cytolysis of Cancer Cells

Biophysical effects of ultrasonic energy in tissue include changes induced by heat, cavitation, and body force (radiation energy). Conventional acoustic devices generate low-frequency (1-4 MHz) high-intensity acoustic waves (> 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> W/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ), which cause tissue destruction primarily through thermal or cavitation effects. However, these effects may be difficult to precisely control and not specific for cancerous cells over normal tissue. Here, we describe the design, fabrication, and therapeutic potential of high-frequency (18-MHz) acoustic irradiation with a self-focusing acoustic transducer (SFAT). A surface micromachining technique was used on a piezoelectric substrate to produce a SFAT device capable of focusing acoustic energy within an area of 100 μm in diameter at the 800-μm focal length. As we sought to minimize potentially nonspecific heat or cavitation effects by acoustic irradiation, operational parameters were chosen to study bioeffects of the device in the absence of tissue heating or biological effects due to cavitation. By varying the acoustic energy, we identified an acoustic intensity threshold (AIT) of 0.15 W/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 17.3 MHz, sufficient to cause this cytolysis effect in human prostate cancer cells 22RV1 without heat or cavitation. Next, we compared the AIT in various cell lines representative of benign and malignant prostate, breast, and skin cells and observed lower AITs in cancer cells over nonmalignant variants. As decreased stiffness (increased compliance) is a biomechanical characteristic, which differs between malignant and nonmalignant cell lines, we hypothesized that a less organized actin cytoskeletal pattern, which is known to be associated with decreased cell stiffness, would correlate with changes in the AIT. Actin staining of cytoskeletal structures confirmed an association between a pattern of diffuse and less organized actin filaments with decreased AIT. Moreover, the same trend of decreased actin organization and decreased AIT was observed following treatments that changed actin patterns in the MCF-10A breast epithelial cell line. These results suggest that biomechanical properties make malignant cells specifically sensitive to cytolysis caused by this form of acoustic energy. In summary, we describe a miniaturized acoustic transducer capable of producing a heatless and cavitation-free, cancer-specific focused cytolysis by direct body force (radiation pressure) effects alone. Ultimately, this device may lead to a miniaturized cancer-treatment system that can be used to focally and specifically ablate cancerous tissue with microscopic precision.