Negative Pressure Amplitude Research Articles

Correlation of Escherichia coli inactivation with histotripsy bubble cloud size

Bacterial loads can be effectively reduced with cavitation-mediated focused ultrasound, or histotripsy. Our previous in vitro work with Escherichia coli (E. coli) established strong trends of bactericide with increasing peak negative pressure amplitude, pulse length, PRF, and treatment time. The current study correlates bactericide as a function of the histotripsy bubble cloud size produced for these treatment parameters at several frequencies. Histotripsy was applied to E. coli suspensions in 10-ml sample vials at 810 kHz, 1.2 MHz, or 3.25 MHz for 40 min. Separately, cavitation was recorded using a Photron Fastrax high-speed camera for acoustic parameters equivalent to those used in the E. coli studies. The images were used to quantify the maximum size of each bubble cloud, with the assumption that the cloud was axially symmetric around the propagation direction. A strong linear relationship exists between log kill versus cloud size (R2 = 0.96). Remarkably, across all variables studied, the log-reduction in viable bacteria exhibited a direct proportional relationship with the dimensions of the bubble cloud. This strong correlation between bacterial reduction and cavitation bubble cloud size could have significance for clinical applications where bacteria cannot easily be sampled. [Work supported by NIH R01AR080120 and R01EB023910.]

Comparative Study of Histotripsy Pulse Parameters Used to Inactivate Escherichia coli in Suspension.

Bacterial loads can be effectively reduced using cavitation-mediated focused ultrasound, or histotripsy. In this study, gram-negative bacteria (Escherichia coli) in suspension were used as model bacteria to evaluate the effectiveness of two regimens of histotripsy treatments: cavitation histotripsy (CH) and boiling histotripsy (BH). Ten-milliliter volumes of Escherichia coli were treated at different negative focal pressure amplitudes and over time periods up to 40 min. Cavitation activity was characterized with coaxial passive cavitation detection (PCD) and synchronized plane wave B-mode imaging. CH treatments exhibited a threshold behavior that was consistent with PCD metrics of cavitation. Above the threshold, bacterial inactivation followed a monotonically increasing log-linear relationship that indicated an exponential inactivation rate. BH exhibited no threshold, but instead followed a different monotonically increasing inactivation rate. Inactivation rates were larger for BH at or below the CH threshold, and larger for CH substantially above the threshold. CH studies performed at different pulse lengths at the same duty cycle had similar inactivation rates, suggesting that at any given pressure amplitude, the "on time" was the most important variable for inactivating E. coli. The maximum inactivation was produced by CH at the highest pressure amplitudes used, leading to a log reduction >4.2 for a 40 min treatment. The results of this study suggest that both CH and BH can be used to inactivate E. coli in suspension, with the optimal regimen depending on the attainable peak negative focal pressure at the target.

Numerical study on aerodynamic drag and noise of high-speed pantograph by introducing spanwise waviness

It is noteworthy to explore potential measures for further reducing the aerodynamic drag and noise of high-speed pantographs. This paper proposes a method of introducing spanwise waviness into the upper and lower arms. The improved delayed detached eddy simulation (IDDES) method and the Ffowcs Williams and Hawkings (FW-H) equation are used for flow field simulation and sound propagation calculation, respectively. The effects of wavelength, wave amplitude and inclination on the aerodynamic force and radiated noise of the wave cylinder were firstly investigated, and then the aerodynamic characteristics of the pantograph with and without spanwise waviness configurations were compared. Results show that the appropriate spanwise waviness on the cylinder can diffuse the negative pressure core area in the wake, reduce the negative pressure amplitude, suppress the vortex shedding and weaken the fluctuation amplitude of the aerodynamic force, resulting in the reduction of aerodynamic drag and noise. For the high-speed pantograph by introducing spanwise waviness, when the ratio of wavelength and wave amplitude to cylinder diameter is 1.2 and 1/8 respectively, the aerodynamic drag and noise of the pantograph by introducing spanwise waviness are reduced by 1.58% and 1.16 dBA, respectively.

Feedback effects of pesticides flow atomization on the operating characteristics of pulsed engines

Traditional pulse-jet fogger have problems such as poor atomization or unstable operation. A pulse-jet fogger test device with an adjustable liquid flow rate was designed. Four experimental settings were used, no spraying and spraying with three liquid flow rates (11 L/h, 14 L/h, and 20 L/h). Pressure waveform in the combustion chamber and smoke inhalation depth tests showed following results: under different chemical fluid flow conditions, influences of the thermal atomization of the chemical liquid were observed in the pulse engine. Feedback effects were mainly changes in parameters and during each pulsation period, the turning time of the airflow in the pulse engine had different effects. The fuel consumption rate and pressure peak-to-peak value under 20 L/h chemical fluid flow were higher than other working conditions, and the positive and negative pressure amplitude fluctuated the least and had the best pressure stability. The 14 L/h chemical liquid flow demonstrated the worst stability, with the most significant fluctuations in pressure amplitude and large cyclical changes and poor consistency of the maximum depth of smoke inhalation in each cycle in the atomization section, with a high maximum relative difference of 34.6%, causing significant turbulence in the airflow oscillation system.

Moving model test on the aerodynamic pressure of bilateral inverted-L-shaped noise barriers caused by high-speed trains

With the increasing speed of high-speed trains, the aerodynamic impact of trains on high-speed railway noise barriers has gradually become a focus of attention. Compared with traditional vertical noise barriers, the bilateral inverted-L-shaped noise barrier (BILSNB) has better noise insulation performance, but its aerodynamic impact may be more prominent. In this research, a moving train-BILSNB test system with a scale ratio of 1:16.8 is established, and the influence of train speed and the opening width on the aerodynamic pressure is analysed and discussed. The large eddy simulation is applied to investigate the influence mechanism of flow field on the aerodynamic performance of the BILSNB. The main conclusions are as follows. (1) The pressure amplitude caused by the passing of the train head is 36.67% greater than that generated by the passing of the train tail on average. (2) The maximum pressure of the BILSNB appears at the bottom area. When the height of the measuring point increases from 0.5H to 1.6H, the pressure coefficient of peak positive pressure, peak negative pressure and pressure amplitude decrease by 29.0%, 17.0% and 15.5%, respectively. (3) The aerodynamic pressure in the BILSNB has a longitudinal end effect: the pressure coefficient of positive peak pressure and pressure amplitude at the inlet section are 27.6% and 17.5% higher than the corresponding average value of all middle sections, respectively. (4) The pressure amplitude of the BILSNB is approximately proportional to 2.15 power of the train speed. The pressure coefficient of the BILSNB is approximately a power function of the opening width, and the value range of the index is about (−0.26, −0.30). (5) The mechanism of the longitudinal end effect is as follows: When the train arrives at the inlet section, the high-speed airflow in front of the train head cannot flow around in time resulting in the impact effect of the airflow on the inlet section becomes greater than that on the middle section. The influence mechanism of the opening width is as follows: The narrower the opening width is, the more intense the airflow inside the BILSNB is compressed, and the impact effect of the airflow on the noise barrier is more significant.

Miniaturized acoustic focus tunable photoacoustic transmitter probe

Aiming to provide dynamically adjustable acoustic field for in vivo ultrasound application scenarios, a miniaturized optical fiber photoacoustic (PA) transmitter is presented. Different from the existing laser generated focused ultrasound probes with fixed acoustic focal length, its focus can be continuously adjusted from infinity to 2.166 mm by deforming the PA conversion film using pneumatic actuation method. The PA film is designed into a circular suspending elastic membrane structure with clamped boundary, which consists of a PDMS/candle soot nanoparticles (CSNPs) mixture layer sandwiched between two pure polydimethylsiloxane (PDMS) layers. Using soft lithography strategy, the PA film together with a structured PDMS substrate and a multimode fiber (MMF) are assembled together to construct the final PA probe with 2.68 mm aperture. For a proof-of-concept demonstration, three different focusing statuses (with 5.196 mm, 3.246 mm and 2.166 mm focal lengths) are provided and their axial acoustic field distributions are characterized as well. Under the excitation condition with the laser fluence of 13.26 mJ/cm2, acoustic signal with 9 MHz center frequency, 92.91% fractional bandwidth at − 6 dB and peak negative pressure (PNP) amplitude up to 8.91 MPa at the 2.166 mm focal point can be achieved, resulting in mechanical index (MI) of 2.97. Referring to the reported data, the current PA transmitter probe demonstrates excellent application potential.

Control of the dynamics of a boiling vapour bubble using pressure-modulated high intensity focused ultrasound without the shock scattering effect: A first proof-of-concept study

Boiling histotripsy is a promising High-Intensity Focused Ultrasound (HIFU) technique that can be used to induce mechanical tissue fractionation at the HIFU focus via cavitation. Two different types of cavitation produced during boiling histotripsy exposure can contribute towards mechanical tissue destruction: (1) a boiling vapour bubble at the HIFU focus and (2) cavitation clouds in between the boiling bubble and the HIFU source. Control of the extent and degree of mechanical damage produced by boiling histotripsy is necessary when treating a solid tumour adjacent to normal tissue or major blood vessels. This is, however, difficult to achieve with boiling histotripsy due to the stochastic formation of the shock scattering-induced inertial cavitation clouds. In the present study, a new histotripsy method termed pressure-modulated shockwave histotripsy is proposed as an alternative to or in addition to boiling histotripsy without inducing the shock scattering effect. The proposed concept is (a) to generate a boiling vapour bubble via localised shockwave heating and (b) subsequently control its extent and lifetime through manipulating peak pressure magnitudes and a HIFU pulse length. To demonstrate the feasibility of the proposed method, bubble dynamics induced at the HIFU focus in an optically transparent liver tissue phantom were investigated using a high speed camera and a passive cavitation detection systems under a single 10, 50 or 100 ms-long 2, 3.5 or 5 MHz pressure-modulated HIFU pulse with varying peak positive and negative pressure amplitudes from 5 to 89 MPa and −3.7 to −14.6 MPa at the focus. Furthermore, a numerical simulation of 2D nonlinear wave propagation with the presence of a boiling bubble at the focus of a HIFU field was conducted by numerically solving the generalised Westervelt equation. The high speed camera experimental results showed that, with the proposed pressure-modulated shockwave histotripsy, boiling bubbles generated by shockwave heating merged together, forming a larger bubble (of the order of a few hundred micron) at the HIFU focus. This coalesced boiling bubble then persisted and maintained within the HIFU focal zone until the end of the exposure (10, 50, or 100 ms). Furthermore, and most importantly, no violent cavitation clouds which typically appear in boiling histotripsy occurred during the proposed histotripsy excitation (i.e. no shock scattering effect). This was likely because that the peak negative pressure magnitude of the backscattered acoustic field by the boiling bubble was below the cavitation cloud intrinsic threshold. The size of the coalesced boiling bubble gradually increased with the peak pressure magnitudes. In addition, with the proposed method, an oval shaped lesion with a length of 0.6 mm and a width of 0.1 mm appeared at the HIFU focus in the tissue phantom, whereas a larger lesion in the form of a tadpole (length: 2.7 mm, width: 0.3 mm) was produced by boiling histotripsy. Taken together, these results suggest that the proposed pressure-modulated shockwave histotripsy could potentially be used to induce a more spatially localised tissue destruction with a desired degree of mechanical damage through controlling the size and lifetime of a boiling bubble without the shock scattering effect.

Expansion of cylindrical tubular workpieces on high-voltage magnetic-pulse installation with controlled vacuum discharger

Purpose. An experimental verification of the existence of a range of values for the parameters of the capacitive energy storage of the magnetic-pulse installations with controlled vacuum discharger, in which, with a high probability, there is a «cut» of the discharge current pulses and the expansion of cylindrical thin-walled tubular workpieces using an external coil. Methodology. High voltage magnetic-pulse installation of NTU «KhPI» with controlled vacuum discharger, multiturn coil with inside dielectrical die and inside aluminum alloy workpiece are used. The capacitance and charge voltage of capacitive energy storage are changed. Discharge current pulses are measured by Rogowski coil and the oscillograph. Results. Parts of complicated shape are made by expansion of cylindrical tubular workpieces with help of external coil. Pressed metallic tubular part is removable from inner dielectric rod. Originality. The frequency of «cut» pulse is defined by negative magnetic field pressure amplitude. It is shown that we must coordinate this frequency and charge voltage with capacitive storage parameters by high probability of pulse «cut». Practical value. It is shown how to use installations with controlled vacuum dischargers in magnetic forming technology based on «cut» pulses.

A Study on the Acoustic Response of Pickering Perfluoropentane Droplets in Different Media.

Acoustic droplet vaporization (ADV) is the physical process of liquid-to-gas phase transition mediated by pressure variations in an ultrasound field. In this study, the acoustic response of novel particle-stabilized perfluoropentane droplets was studied in bulk and confined media. The oil/water interface was stabilized by cellulose nanofibers. First, their acoustic responses under idealized conditions were examined to assess their susceptibility to undergo ADV. Second, the droplets were studied in a more realistic setting and placed in a confined medium. Lastly, an imaging setup was developed and tested on the droplets. The acoustic response could be seen when the amplitude of the peak negative pressure (PNP) was above 200 kPa, suggesting that this is the vaporization pressure threshold for these droplets. Increasing the PNP resulted in a decrease in signal intensity over time, suggesting a more destructive behavior. The imaging setup was able to differentiate between the droplets and the surrounding tissue. Results obtained within this study suggest that these droplets have potential in terms of ultrasound-mediated diagnostics and therapy.

Study on Electroosmosis Consolidation of Punctiform Electrode Unit

In the electroosmosis method, when the distance between the opposite electrode and the same electrode is equal, the two‐dimensional effect of electroosmotic consolidation is significant, and the use of one‐dimensional model will overestimate the potential gradient, making the calculated pore pressure value too large. Aiming at this problem, according to the electrode arrangement rule and the minimum composition, a punctiform electrode unit model is proposed, and electroosmotic experiments are carried out on the symmetric and asymmetric unit models. The two‐dimensional electroosmotic consolidation governing equation of the punctiform electrode unit is established. The electric potential field of the electrode unit and the finite element form of the electroosmotic consolidation equation are given by the Galerkin method. The PyEcFem finite‐element numerical library is developed using Python programming to calculate. The research results show the following: (1) The two‐dimensional effect of the potential field distribution of the punctiform electrode unit is significant. The reduction of spacing of the same nature electrode in the symmetrical unit can make the potential distribution close to a uniform electric field. The asymmetry prevents the electric potential field distribution from being reduced to a one‐dimensional model. (2) The number of anodes will affect the electroosmosis effect of the soil. The more the anodes, the better the electroosmosis reinforcement effect of the soil, and the distribution of negative excess pore water pressure will be more uniform. (3) In the early stage of electroosmosis, the more the drainage boundaries, the faster the generation of negative pore pressure, but in the middle and late stages of electroosmosis, the potential value becomes the decisive factor, and the amplitude of negative pore water pressure in asymmetric units is higher than that in symmetric units. The potential distribution will not affect the degree of consolidation but will affect the extreme pore water pressure.

Aerodynamic Effects as a Maglev Train Passes Through a Noise Barrier

The aerodynamic performance of a maglev (magnetic levitation) train (MT) passing through a two-sided noise barrier was simulated by the improved delayed detached eddy simulations. The influence of the presence of noise barrier on both sides on the flow field around the MT and the pressure field in the noise barrier area are analyzed. The effect of changes of the noise barrier height and the centre of track (COT) distance parameter on its flow field were also studied. Numerical simulation results were verified by the moving model test. The results show that the noise barrier increases the pressure in the area around a train. The maximum pressure changes most as the tip of the head of the MT passes through the measuring point. The maximum positive and negative pressure amplitudes of the head wave on the inner wall surface of the noise barrier are increased by 83.5% and 58.4% compared to those the same height position measurement point in open air. On the inner wall of the barrier, the fluctuation of the pressure decreases with the increased height of the probe, which is opposite to the trend on the outer wall. Inside the noise barrier, the wind velocity in the x-direction is greater than that in open air when the tip of the nose and tail of the car passes. But the velocity in the y-direction when the tip of the head and tail cars passes is limited, and two speed peaks appear in the horizontal direction train induced wind. The higher the noise barrier, the higher pressure of the same height measurement point in the noise barrier area. Due to the passage of the train, the top of the noise barrier has a certain vorticity at the position of the head car and the tail car. Changing the height of the noise barrier changes how the vortex structure at the tail of the train develops. Moving the noise barrier further from the COT causes the fluctuations at the pressure measurement point inside the noise barrier to diminish meaning the pressure enhancement effect caused by the height of the noise barrier also diminishes.

Measurement of the Local Pressure near a Cavitation Bubble Cloud Formed by the Focused Ultrasound

The growth of a cavitation bubble cloud and the corresponding pressure near the tip of the cavitation bubble cloud formed by the backscattering of the focused ultrasound from a laser-induced bubble have been analyzed experimentally. The laser-induced bubble which is generated near the geometrical focus of the focused ultrasound is used as a reflector of the incident wave to yield strong negative pressure which leads to the cavitation inception and following cavitation bubble cloud formation. In the present study, optical observation by a high-speed video camera with frame rate up to 1.25 Mfps and the pressure measurement by the fiber optic prove hydrophone (FOPH) are carried out simultaneously to estimate the cavitation inception pressure. It is shown that the cavitation bubble cloud grows conically along with the propagation axis of the incident wave. The distance between the top interface of the cavitation bubble cloud and the geometrical focus of the focused ultrasound becomes a limited value even though the focused ultrasound keeps on irradiating, which indicates that the cavitation bubble cloud grows within the focal region with sufficient negative pressure due to backscattering. The pressure measurement reveals that the negative pressure amplitude backscattered from a free surface is larger than that of the focused ultrasound itself. Negative peaks in the pressure waveform have a long term fluctuation whose increments correspond the sudden growth of the cavitation bubble cloud. Cavitation inception pressure was measured defining it as the negative pressure at the top interface of the cavitation bubble cloud just before the FOPH was covered with cavitation bubbles. The measured cavitation inception pressure was -24 MPa in the present study.

The induction of inertial cavitation from polymeric nanocups—From theory to observation

The inability for therapeutics to distribute throughout the entirety of the tumor is a major challenge in cancer therapy. Acoustic cavitation from microbubbles promotes drug distribution and improves therapeutic efficacy. Yet microbubbles are too large to navigate the microvasculature of the tumor, and are destroyed by the ultrasound wave. Thus, there is a need for submicron cavitation nucleation agents. Recently, we have developed a submicron polymeric nanocup capable of trapping a nanobubble within the surface crevice. Using a modified Rayleigh-Plesset model that accounts for the size and shape of the crevice on the surface of the nanocup, we predicted that the cavity trapped bubble expands and contracts yet remains in the cavity. With a sufficient peak negative pressure amplitude, the model indicated that the surface bubble extends beyond and detaches from the crevice before inertial collapse. To verify our predictions, we exposed nanocups to high intensity focused ultrasound at different driving frequencies and observed bubble nucleation from nanocups using the Brandaris ultra high-speed camera. Acoustic emissions were recorded using a passive cavitation detector. These direct observations of the induction of inertial cavitation from nanocups verified the predictions made by the modified Rayleigh-Plesset crevice model.

Measurement of middle ear pressure changes during balloon eustachian tuboplasty: a pilot study

Conclusion: The middle ear pressure changes detected during BET can be directly attributed to the balloon inflation and may represent a second, immediate, mechanism of action of BET. BET seems to be safe with respect to the risk of a barotrauma. Further human studies are now necessary to confirm the results and gain more insight into the mechanism of action of BET.Objective: Since the introduction of Balloon Eustachian Tuboplasty (BET) as a treatment of chronic Eustachian tube dysfunction, the precise mechanism of action is unknown. Long-term effects of BET may be related to observed microfractures of the Eustachian tube cartilage. However, clinical observations indicate a second, immediate mode of action. Therefore, this study investigated and characterized middle ear pressure changes occurring directly during BET procedure.Methods: Using a micro-optical pressure sensor, pressure changes during BET were monitored transtympanically in a cadaveric animal study using heathland sheep.Results: Middle ear pressure amplitudes during BET are dependent on the speed of balloon inflation as well as the maximum inflation pressure. A 10-bar inflation pressure yielded a mean middle ear pressure of 5.34 mmHg (71.0 daPA). Negative pressure amplitudes occurring on withdrawal of the balloon catheter are influenced by the speed of withdrawal. No pressure amplitudes capable of causing barotrauma to membranous ear structures could be detected.

Treatment envelope of transcranial histoptripsy applied without aberration correction

Due to the characteristically high absorption, attenuation, and aberrating effects of the skull on ultrasound, transcranial HIFU therapies have typically been restricted to targeting brain regions at depths ≥2cm from the interior surface of the skull. Previous studies using histotripsy, a cavitation based ultrasound therapy relying on short duration (<2 acoustic cycles), high negative pressure amplitude (P ≤ -30MPa) ultrasound pulses to generate cavitation clouds to fractionate tissue, have shown that targeted bubble clouds can be generated through the skull without aberration correction. Here, we present results from experiments probing the minimum depths with respect to the skull surface at which histotripsy applied without aberration correction can generate lesions. Using a 500 kHz, 256-element transducer, histotripsy was applied through an ex vivo human skull to generate lesions in a tissue mimicking phantom embedded within. Lesions were generated at multiple locations within the skull through skull sections with characteristically different geometries. MRI was used to assess lesion generation following treatment. Using histotripsy without aberration correction, we were able to generate lesions at typical depths of between 6 and 12 mm from the interior skull surface. These results demonstrate the potential of histotripsy to target shallower brain regions than current HIFU therapies.

The Relationship of Cavitation to the Negative Acoustic Pressure Amplitude in Ultrasonic Therapy**Supported by the National Natural Science Foundation of China under Grant Nos 81127901, 81420108018, 81527803, 81227004 and 11374155, the Natural Science Foundation of Jiangsu Province under Grant No BK20131017, the Main Scientific and Technological Project of Zhejiang Province under Grant No 2013C03044-1, the Chinese Postdoctoral Science Foundation under

The relationship between the cavitation and acoustic peak negative pressure in the high-intensity focused ultrasound (HIFU) field is analyzed in water and tissue phantom. The peak negative pressure at the focus is determined by a hybrid approach combining the measurement with the simulation. The spheroidal beam equation is utilized to describe the nonlinear acoustic propagation. The waveform at the focus is measured by a fiber optic probe hydrophone in water. The relationship between the source pressure amplitude and the excitation voltage is determined by fitting the measured ratio of the second harmonic to the fundamental component at the focus, based on the model simulation. Then the focal negative pressure is calculated for arbitrary voltage excitation in water and tissue phantom. A portable B-mode ultrasound scanner is applied to monitor HIFU-induced cavitation in real time, and a passive cavitation detection (PCD) system is used to acquire the bubble scattering signals in the HIFU focal volume for the cavitation quantification. The results show that: (1) unstable cavitation starts to appear in degassed water when the peak negative pressure of HIFU signals reaches 13.5 MPa; and (2) the cavitation activity can be detected in tissue phantom by B-mode images and in the PCD system with HIFU peak negative pressures of 9.0 MPa and 7.8 MPa, respectively, which suggests that real-time B-mode images could be used to monitor the cavitation activity in two dimensions, while PCD systems are more sensitive to detect scattering and emission signals from cavitation bubbles.

Effects of Temperature on the Histotripsy Intrinsic Threshold for Cavitation.

Histotripsy is an ultrasound ablation method that depends on the initiation of a dense cavitation bubble cloud to fractionate soft tissue. Previous work has demonstrated that a cavitation cloud can be formed by a single acoustic pulse with one high amplitude negative cycle, when the negative pressure amplitude exceeds a threshold intrinsic to the medium. The intrinsic thresholds in soft tissues and tissue phantoms that are water-based are similar to the intrinsic threshold of water over an experimentally verified frequency range of 0.3-3 MHz. Previous work studying the histotripsy intrinsic threshold has been limited to experiments performed at room temperature (~20°C). In this study, we investigate the effects of temperature on the histotripsy intrinsic threshold in water, which is essential to accurately predict the intrinsic thresholds expected over the full range of in vivo therapeutic temperatures. Based on previous work studying the histotripsy intrinsic threshold and classical nucleation theory, we hypothesize that the intrinsic threshold will decrease with increasing temperature. To test this hypothesis, the intrinsic threshold in water was investigated both experimentally and theoretically. The probability of generating cavitation bubbles was measured by applying a single pulse with one high amplitude negative cycle at 1 MHz to distilled, degassed water at temperatures ranging from 10°C-90°C. Cavitation was detected and characterized by passive cavitation detection and high-speed photography, from which the probability of cavitation was measured vs. pressure amplitude. The results indicate that the intrinsic threshold (the negative pressure at which the cavitation probability=0.5) significantly decreases with increasing temperature, showing a nearly linear decreasing trend from 29.8±0.4 MPa at 10˚C to 14.9±1.4 MPa at 90˚C. Overall, the results of this study support our hypothesis that the intrinsic threshold is highly dependent upon the temperature of the medium, which may allow for better predictions of cavitation generation at body temperature in vivo and at the elevated temperatures commonly seen in high intensity focused ultrasound (HIFU) regimes.

Effects of Ultrasound Frequency and Tissue Stiffness on the Histotripsy Intrinsic Threshold for Cavitation

Histotripsy is an ultrasound ablation method that depends on the initiation of a cavitation bubble cloud to fractionate soft tissue. Previous work has indicated that a cavitation cloud can be formed by a single pulse with one high-amplitude negative cycle, when the negative pressure amplitude directly exceeds a pressure threshold intrinsic to the medium. We hypothesize that the intrinsic threshold in water-based tissues is determined by the properties of the water inside the tissue, and changes in tissue stiffness or ultrasound frequency will have a minimal impact on the histotripsy intrinsic threshold. To test this hypothesis, the histotripsy intrinsic threshold was investigated both experimentally and theoretically. The probability of cavitation was measured by subjecting tissue phantoms with adjustable mechanical properties and ex vivo tissues to a histotripsy pulse of 1–2 cycles produced by 345-kHz, 500-kHz, 1.5-MHz and 3-MHz histotripsy transducers. Cavitation was detected and characterized by passive cavitation detection and high-speed photography, from which the probability of cavitation was measured versus pressure amplitude. The results revealed that the intrinsic threshold (the negative pressure at which probability = 0.5) is independent of stiffness for Young's moduli (E) <1 MPa, with only a small increase (∼2–3 MPa) in the intrinsic threshold for tendon (E = 380 MPa). Additionally, results for all samples revealed only a small increase of ∼2–3 MPa when the frequency was increased from 345 kHz to 3 MHz. The intrinsic threshold was measured to be between 24.7 and 30.6 MPa for all samples and frequencies tested in this study. Overall, the results of this study indicate that the intrinsic threshold to initiate a histotripsy bubble cloud is not significantly affected by tissue stiffness or ultrasound frequency in the hundreds of kilohertz to megahertz range.

Conservation of intraoral pressure changes during swallowing

Background/purposeA relationship between malocclusion and intraoral pressure is suspected but ill understood. This study explores intra-oral pressure changes during normal swallowing to assess differences in patterns of command saliva and water swallows. Materials and methodsWe used a custom-made appliance fitted with miniature pressure transducers located along the midline at the anterior-, mid-, and hind palate to measure absolute pressures during command water and saliva swallows. Data were obtained from eight healthy volunteers (six males, two females; 24–35 years) with full permanent dentitions. Following accommodation, volunteers performed 10 mL water and saliva swallows on command. Each volunteer performed the swallows three times on 5 separate days, yielding data from 15 swallows for each individual. ResultsEach participant had a distinct pressure response during swallowing. At all three locations, negative pressure amplitude was greater than positive pressure amplitude, with no significant difference between swallowing saliva and water. There was a larger rate of negative pressure development (P < 0.01) and a larger value for the pressure-time integral (P < 0.05) at the front of the palate during water swallowing. During water swallowing, the root mean square pressure integral (index of work) at the front of the mouth was larger (P < 0.05), and there was a significant front to mid palate pressure gradient (P < 0.05) that was not evident during saliva swallowing. ConclusionWe tested the hypothesis that a liquid with a low viscosity (water) requires less lingual propulsive deformation than a higher viscosity liquid such as saliva. Although saliva is a more complex fluid than water and varies between individuals, participants produced personally distinct and consistent signature pressure patterns and retained these for both liquids.

Cavitation thresholds and why to be wary!

Many biological studies involving ultrasound exposure report acoustic cavitation “thresholds”. The purpose of these is often to convince the reader either that cavitation activity was avoided, or that it definitely occurred as the exposure was “above the threshold”. Formally, the cavitation threshold is the minimum negative pressure amplitude at which pre-existing bubbles begin to oscillate (non-inertial cavitation) or collapse (inertial cavitation). The cavitation nucleation threshold is that for which bubbles can be drawn out of solution and driven to oscillate. Since the true physical cavitation threshold of a medium can only be measured if it is possible to detect single bubble activity, quoted thresholds are more likely to represent a threshold for detection than for cavitation activity. Furthermore, they may not be relevant beyond the specific experimental conditions tested. In interpreting a negative pressure threshold for cavitation in tissue it is important to know, amongst other things: the tissue status and sample geometry (in order that the in situ pressure may be calculated); details of the ultrasound exposure; method of cavitation detection and detector geometry; data gathering, processing and interpretation methods; threshold definition; sample statistics. Without these details, results may be misleading.