Acoustic Watts Research Articles

In vivo exposure of the bladder using a non-invasive high intensity focused ultrasound toroidal transducer

A toroidal high-intensity focused ultrasound (HIFU) transducer was used to expose normal bladder wall tissues non-invasively in vivo in a porcine model in order to investigate the potential to treat bladder tumors. The transducer was divided into 32 concentric rings with equal surface areas, operating at 2.5 MHz. Eight animals were split into two groups of 4. In the first group, post-mortem evaluation was performed immediately after ultrasound exposure. In the second group, animals survived for up to seven days before post-mortem evaluation. The ultrasound imaging guided HIFU device was hand-held during the procedure using optical tracking to ensure correct targeting. One thermal lesion in each animal was created using a 40 s exposure at 80 acoustic Watts (free-field) in the trigone region of the bladder wall. The average (±Standard Deviation) abdominal wall and bladder wall thicknesses were 10.3 ± 1.4 mm and 1.1 ± 0.4 mm respectively. The longest and shortest axes of the HIFU ablations were 7.7 ± 2.9 mm and 6.0 ± 1.8 mm, respectively, resulting in an ablation of the whole thickness of the bladder wall in most cases. Ablation were performed at an average depth (distance from the skin surface to the centre of the HIFU lesion) of 42.5 ± 3.8 mm and extended throughout the thickness of the bladder. There were two cases of injury to tissues immediately adjacent to the bladder wall but without signs of perforation, as confirmed by histological analysis. Non-invasive HIFU ablation using a hand-held toroidal transducer was successfully performed to destroy regions of the bladder wall in vivo.

Multi-focal HIFU reduces cavitation in mild-hyperthermia

BackgroundMild-hyperthermia therapy (40–45 °C) with high-intensity focused ultrasound (HIFU) is a technique being considered in a number of different treatments such as thermally activated drug delivery, immune-stimulation, and as a chemotherapy adjuvant. Mechanical damage and loss of cell viability associated with HIFU-induced acoustic cavitation may pose a risk during these treatments or may hinder their success. Here we present a method that achieves mild heating and reduces cavitation by using a multi-focused HIFU beam. We quantify cavitation level and temperature rise in multi-focal sonications and compare it to single-focus sonications at the transducer geometric focus.MethodsContinuous wave sonications were performed with the Sonalleve V2 transducer in gel phantoms and pork at 5, 10, 20, 40, 60, 80 acoustic watts for 30 s. Cavitation activity was measured with two ultrasound (US) imaging probes, both by computing the raw channel variance and using passive acoustic mapping (PAM). Temperature rise was measured with MR thermometry at 3 T. Cavitation and heating were compared for single- and multi-focal sonication geometries. Multi-focal sonications used four points equally spaced on a ring of either 4 mm or 8 mm diameter. Single-focus sonications were not steered.ResultsMulti-focal sonication generated distinct foci that were visible in MRI thermal maps in both phantoms and pork, and visible in PAM images in phantoms only. Cavitation activity (measured by channel variance) and mean PAM image value were highly correlated (r > 0.9). In phantoms, cavitation exponentially decreased over the 30-second sonication, consistent with depletion of cavitation nuclei. In pork, sporadic spikes signaling cavitation were observed with single focusing only. In both materials, the widest beam reduced average and peak cavitation level by a factor of two or more at each power tested when compared to a single focus. The widest beam reduced peak temperature by at least 10 °C at powers above 5 W, and created heating that was more spatially diffuse than single focus, resulting in more voxels in the mild heating (3–8 °C) range.ConclusionsMulti-focal HIFU can be used to achieve mild temperature elevation and reduce cavitation activity.

Transcranial thermal ablation with a 230 kHz MRI-guided focused ultrasound system in a large animal mode

We evaluated the feasibility of thermal ablation in using a 230 kHz transcranial MRI-guided Focused Ultrasound (TcMRgFUS) system in three rhesus macaques. Thalamic targets were sonicated at 40–50s at 90–560 acoustic Watts. Focal heating sufficient to create an MRI-evident lesion was achieved in 4/6 targets where thermal dose exceeded 240 CEM43°C. Focal heating increased linearly as a function of the applied energy at a rate of 3.2±0.4°C/kJ (R²:0.81). Lesion sizes were consistent with 240 CEM43°C contours. The findings suggest that the lesions were consistent with thermal mechanisms. No evidence of cavitation-related petechiae were evident after sonication. Similar tests in macaques with a version of this system operating at 670 kHz (Hynynen et al., Eur. J. Radiol. 2006) measured skull-induced heating of 130°C/kJ/cm² of outer skull surface, more than twice of that measured here (63°C per kJ/cm²). While no or minimal focal heating was observed at 670 kHz, we reached ablation-level thermal dose values at 230 kHz. Thus, these preliminary results thus suggest that this low frequency system can expand the “treatment envelope” for TcMRgFUS.

Pre-Clinical Study of In Vivo Magnetic Resonance-Guided Bubble-Enhanced Heating in Pig Liver

Bubble-enhanced heating (BEH) can be exploited to increase heating efficiency in treatment of liver tumors with non-invasive high-intensity focused ultrasound (HIFU). The objectives of this study were: (i) to demonstrate the feasibility of increasing the heating efficiency of sonication exploiting BEH in pig liver in vivo using a clinical platform; (ii) to determine the acoustic threshold for such effects with real-time, motion-compensated magnetic resonance-guided thermometry; and (iii) to compare the heating patterns and thermal lesion characteristics resulting from continuous sonication and sonication including a burst pulse. The threshold acoustic power for generation of BEH in pig liver in vivo was determined using sonication of 0.5-s duration (“burst pulse”) under real-time magnetic resonance thermometry. In a second step, experimental sonication composed of a burst pulse followed by continuous sonication (14.5 s) was compared with conventional sonication (15 s) of identical energy (1.8 kJ). Modification of the heating pattern at the targeted region located at a liver depth between 20 and 25 mm required 600–800 acoustic watts. The experimental group exhibited near-spherical heating with 40% mean enhancement of the maximal temperature rise as compared with the conventional sonication group, a mean shift of 7 ± 3.3 mm toward the transducer and reduction of the post-focal temperature increase. Magnetic resonance thermometry can be exploited to control acoustic BEH in vivo in the liver. By use of experimental sonication, more efficient heating can be achieved while protecting tissues located beyond the focal point.

Robert Beyer’s laboratory in nonlinear acoustics: 1975–1982 and earlier perspectives.

Professor Robert T. Beyer’s laboratory at Brown University transitioned in the mid-1970s to conduct experiments (under Navy funding) on the nonlinear interaction of sound with noise (by T.K.S.) and, shortly after, on the nonlinear interaction of sound with turbulence (by M.S.K.). In this presentation, T.K.S. and M.S.K. recall their experiences in Beyer’s laboratory during their tenure as graduate students. Under Beyer’s guidance and support both graduate students (“unofficially” numbers 20 and 21) built up their experiments—mostly from scratch—but were able to use some remnants left over from an earlier era of graduate students, which included some intricately made ultrasonic transducers and precise mechanical staging. Beyer’s earlier graduate students’ dissertations also proved to be an invaluable source of knowledge. T.K.S. and M.S.K. developed considerable apparatus for their research. Prototype electronic equipment in that era involved mostly analog electronics because of the significant challenges associated with building the digital version at that time. The development of equipment including the complex acoustic watt meter (T.K.S.) and the hot film anemometer (M.S.K.) will be presented along with descriptions of their measurements of nonlinear scattering. Professor Beyer’s mentoring—the teacher, the historian, and storyteller—along with his fatherly guidance are heartfelt today.

MRI-guided gas bubble enhanced ultrasound heating in in vivo rabbit thigh

In this study, we propose a focused ultrasound surgery protocol that induces and then uses gas bubbles at the focus to enhance the ultrasound absorption and ultimately create larger lesions in vivo. MRI and ultrasound visualization and monitoring methods for this heating method are also investigated. Larger lesions created with a carefully monitored single ultrasound exposure could greatly improve the speed of tumour coagulation with focused ultrasound. All experiments were performed under MRI (clinical, 1.5 T) guidance with one of two eight-sector, spherically curved piezoelectric transducers. The transducer, either a 1.1 or 1.7 MHz array, was driven by a multi-channel RF driving system. The transducer was mounted in an MRI-compatible manual positioning system and the rabbit was situated on top of the system. An ultrasound detector ring was fixed with the therapy transducer to monitor gas bubble activity during treatment. Focused ultrasound surgery exposures were delivered to the thighs of seven New Zealand white rabbits. The experimental, gas-bubble-enhanced heating exposures consisted of a high amplitude 300 acoustic watt, half second pulse followed by a 7 W, 14 W or 21 W continuous wave exposure for 19.5 s. The respective control sonications were 20 s exposures of 14 W, 21 W and 28 W. During the exposures, MR thermometry was obtained from the temperature dependency of the proton resonance frequency shift. MR T2-enhanced imaging was used to evaluate the resulting lesions. Specific metrics were used to evaluate the differences between the gas-bubble-enhanced exposures and their respective control sonications: temperatures with respect to time and space, lesion size and shape, and their agreement with thermal dose predictions. The bubble-enhanced exposures showed a faster temperature rise within the first 4 s and higher overall temperatures than the sonications without bubble formation. The spatial temperature maps and the thermal dose maps derived from the MRI thermometry closely correlated with the resulting lesion as examined by T2-weighted imaging. The lesions created with the gas-bubble-enhanced heating exposures were 2–3 times larger by volume, consistently more spherical in shape and closer to the transducer than the control exposures. The study demonstrates that gas bubbles can reliably be used to create significantly larger lesions in vivo. MRI thermometry techniques were successfully used to monitor the thermal effects mediated by the bubble-enhanced exposures.

Initial experience with a novel focused ultrasound ablation system for ring ablation outside the pulmonary vein.

Atrial fibrillation has been shown to initiate from triggers within pulmonary veins. Several studies have documented that electrical isolation of those triggers can lead to maintenance of sinus rhythm. The complication of pulmonary vein stenosis has limited the utility of delivering ablation energy within the pulmonary vein. We utilize a focused ultrasound catheter ablation system for delivery of transmural ablation lines proximal to the pulmonary vein ostium. Nine dogs (weight 30-39 kg) were anesthetized and ventilated. Through a transseptal approach, pulmonary veins were engaged with the focused balloon ultrasound catheter. Ultrasound power was delivered at 40 acoustic watts outside the pulmonary vein ostium, focused 2 mm off the balloon surface, with a depth of approximately 6 mm, for 30-120 seconds. Following ablation, lesions were histopathologically analyzed. Of nine animals studied, fourteen pulmonary veins were ablated. We found successful delivery of near circumferential and transmural ablation lines in 6/14 pulmonary veins. In each of the six circumferential ablations, successful alignment of the ultrasound transducer along the longitudinal axis of the parabolic balloon occurred. The final four ablations were conducted with an enhanced catheter design that assured axial alignment. Of these ablations, all four were circumferential. The remaining 8 pulmonary veins had incomplete delivery of lesions. In each of these veins the ultrasound transducer was misaligned with the balloon axis when therapy was delivered. Focused ultrasound ablation is a new means of performing pulmonary vein isolation. This method provides delivery of lesions outside the vein, limiting the risk of pulmonary vein stenosis for the treatment of atrial fibrillation.

In vivo ultrasound-induced occlusion of arteries and veins

In this study rabbit ear vessels were sonicated in vivo with focused spherically curved ultrasound transducer (diameter 100 mm, radius of curvature 80 mm, frequency 2 MHz). The sonication duration was varied from 0.2 to 5 s. The acoustic power was between about 50 and 200 acoustic watts. The vessels were visualized under microscope during and after the sonication to follow the blood flow and the vessel diameter. A video camera on the microscope was used to record the observations. The sonication caused blood vessels to contract temporarily at low powers and at high powers the vessels occluded and remained closed over the 2-h follow-up. However, the x-ray angiograms performed by injecting x-ray contrast agent in the ear vessels after the animal was sacrificed showed often that complete occlusion was not achieved. A further increase in the power caused the skin to coagulate and these effects had less effect on the blood flow. The vessels were completely sealed by performing a follow-up sonication at about 34 W for 10 s. This coagulated the tissue around the blood vessels and stopped the flow.

Design and performance of a high-power, low-frequency sound source

A well-controlled low-frequency, high-intensity sound source that is self-contained on a telescoping semi-trailer is described. The three major components of the sound source are: the air-supply system, the Wyle Laboratory air-stream modulator, and the acoustic horn. The modulator has been available for some 20 years and is capable of producing 30 000 acoustic watts of output power from 10 to 630 Hz. It has been modified to allow for CO2 cooling of the voice coil. The air-supply system consists of a rotary lobe blower driven by a 150 horsepower diesel engine. The acoustic horn is a true 10-Hz exponential horn, 17.1 m (56 ft) long. The mouth and throat diameters are 2.34 m (92 in.) and 0.01 m (4 in.). Measurements of the acoustic output and performance will be presented.

T. J. Schultz' contributions to sound‐intensity measurement and theory

In the mid 1950s, during and following his doctoral research at Harvard University, Ted Schultz contributed significantly to the early development of sound‐intensity measurement techniques. By using back‐to‐back pressure transducers, he was able to develop a sound‐intensity probe that satisfactorily measured sound intensity. However, in explaining his observed results, he was impeded by the paucity of theoretical understanding of complex sound‐intensity fields. Ted and other researchers in the late 1950s and early 1960s eventually dropped further development of the early “acoustic watt meter” on which Ted held a U.S. patent, after frustrating and disappointing research results. Later, Ted and others came to understand that great care must be taken to correct the errors between transducer channels or to use phase‐matched transducers. In any event, Ted's pioneering experiments in sound intensity contributed immensely to later developments and to the current more advanced state‐of‐the‐art.

Experimental and theoretical results of acoustic agglomeration at high pressures and high temperatures

Acoustic agglomeration (AA) of the fly-ash particles from coal combustion in pressurized, fluidized bed combustors for power plants is an intermediate treatment of the aerosol to improve the fractional removal efficiency of conventional particle removal devices such as cyclones so that recently proposed clean air requirements can be met. Tests were performed on Penn State's high-pressure (10 atm), high-temperature (891 °C) acoustic agglomerator (6-in. i.d., 14-ft length) at 158 dB and about 850 Hz, with particle loading of 10–18 g/m3 and residence times of about 4 s. The sound source was a high-efficiency siren of Penn State's design providing about 1500 acoustic watts. The mass fraction in the critical 0.5- to 5-μm particle size range was reduced by a factor of 5 and the 10- to 20-μm size range increased by a factor of 3. An acoustic agglomeration simulation model (AASM) computer code that predicts AA quite reliably at both atmospheric and HTHP conditions was developed at Penn State. The simulation results in terms of particle size distribution agree quite well with the test results from both test conditions. The simulation also predicts that better AA could have been obtained at higher frequencies (2000 Hz) and higher acoustic intensities (160 dB). [Work supported by DOE.]

Numerical models of perfused hyperthermia phantoms

Finite difference numerical models based on bioheat transfer [H. H. Pennes, J. Appl. Physiol. 1, 93–122 (1948)] and incomplete countercurrent heat exchange [S. Weinbaum and L. M. Jiji, J. Biomech. Eng. 107, 121–139 (1985)] have been developed and tested. Predicted performance of hyperthermia phantoms heated by ultrasound applicators will be compared with actual performance to validate the models. The models will then be used as aids in designing ultrasound applicators to produce heat deposition patterns believed advantageous for hyperthermia treatment of experimental animals and patients. The 3D models admit adiabatic or isothermal boundaries and arrays of paired, straight tubules for counter current heat exchange. Results of parametric variations will be shown. Preliminary results obtained by using as inputs the design parameters for a proposed kidney phantom perfused at 110 ml/100 g (of tissue-equivalent gel) per minute have yielded satisfactory time constants on the order of 150 s. When the simulated ultrasound applicator emitted 56 acoustic watts at 1 MHz, applied as an 8-cm-diam, unfocused Gaussian beam, the predicted steady-state, spatial maximum temperature rise was 6 °C. [Work supported by PHS Grant No. R01 CA33749 awarded by the National Cancer Institute, DHHS.]

Analysis and design of a 600 acoustic watt siren for an acoustic agglomeration of submicron particles in gas streams

The 600 acoustic watt siren to provide either pure tone or warble tone sound in the frequency range of 1000 to 4000 Hz was designed on the basis of a lumped parameter analytical model using electrical analogies of the acoustical elements. The pressure source, rotor and stator ports, the exponential horn, and the agglomeration chamber are represented. The time varying nature of the porting arrangement is recognized. The power output of the siren as a function of frequency (siren speed), supply pressure and air flow agrees well with the predicted values. Acoustic efficiencies ranging from 25% to 33% were attained. Since axial clearances between rotor and stator were the limiting factors larger sirens contemplated for industrial use are expected to provide higher efficiencies. Thus, overall efficiencies including the compressed air supply system in the vicinity of 27% appear feasible. The siren has operated without breakdown for about 1000 hours.

High‐powered acoustic transmitter for atmospheric sounders

A high‐power narrow‐beam acoustic transmitter for use in an atmospheric sounder has been designed and evaluated. The transmitter uses thirteen electroacoustic horn drivers manifolded to a 2‐m aperture horn‐reflector antenna. The evaluation of the transmitter included measurements of beam pattern, total efficiency, efficiency of individual drivers, total power output, and finite amplitude distortion. In particular at a frequency of 1250 Hz the 3‐dB beamwidth was 9°, the total efficiency was 10%, individual driver efficiency was approximately 17%, and the total power output was approximately 50 acoustic watts. At this power power finite amplitude distortion was quite perceptible at the mouth of the horn.

High-power acoustic transmitter arrays

A high-power acoustic transmitter with design goals of producing a narrow bean (≈5° half angle to 3-dB points), minimum side lobes (40 to 50 dB down) and high acoustic power (≈500 acoustic watts) was producted by NOAA's Wave Propagation Laboratories for use in the FAA sponsored wind shear measuring system. The antenna was required to operate under all weather conditions and to produce a 1250-Hz pure tone for a duration of 0.1 to 0.5 sec with repetition rates of 6 to 8 sec. The transmitter design consists of an amplitude-shaded array made up of 144 omnidirectional elements mounted in a 12×12 configuration on a fiat reflecting plane. Results of acoustical impedance measurements performed on a set of apertures which simulated the element openings and measurements aimed at compensating for element-to-element interaction are presented along with beam patterns from a full-scale antenna.

Thermal Factors in Ultrasonic Tissue Destruction

A purely thermal mechanism of tissue destruction by focused high-intensity ultrasound has previously been postulated in the following two ways. One is that there is a fixed threshold temperature which describes the threshold of damage. The second is that the threshold is time dependent and that the threshold is described by a temperature-time history curve. In experiments on cats, the threshold temperatures for damage in brain tissue during irradiation were measured. Data were taken with miniature thermocouples at 0.1-mm intervals. The ultrasound was delivered at powers of 6–20 acoustic watts, at several frequencies from 1.8 to 4.5 MHz, with single and multiple pulses and with variation of base conditions of the animal (e.g., base temperature, intracranial tissue perfusion). Lesion diameters were correlated with temperatures at various radial distances from the irradiation axis to determine threshold temperatures. Results for irradiations under varied values of irradiating parameters were analyzed. The threshold temperatures observed at different frequencies and under different irradiation conditions will be reported with comparisons made of data on time-temperature histories of lesion thresholds. Studies to test the extrapolability of a previously reported theoretical model using a purely thermal analysis to predict lesion dimensions will be discussed.

Kinetics of protein release from yeast sonicated in batch and flow systems at 20 kHz

AbstractThe release constant, k, of brewers yeast sonicated at powers up to 200 W at 20 kHz has been shown to be independent of cell concentration up to values of 60 g made up to 100 ml. It is inversely proportional to the volume of the treatment vessel in the range 75 to 450 ml, and almost proportional to the input acoustic power from 60 to 195 acoustic watts. A flow system is described and a relationship linking protein release, flow rate, and the protein release constant, determined from batch experiments, is derived. Good agreement between the theoretical prediction of protein release and experimental results with the flow system was obtained.

Large-Scale Acoustic Testing of Gas Turbine Silencer Components and Reciprocating Engine Silencers Using a Dynamic Insertion Loss and Attenuation Facility

Since late 1970, the Maxim Silencer Product Center has been testing full-scale gas turbine silencer components and large reciprocating engine silencers under simulated field conditions of flow and intense soundpressure level. A 73-in. centrifugal fan and Ling Electronics EPT-94B electro-pneumatic transducer are assembled in a duct system to provide more than 100 000 SCFM flow and more than 2000 acoustic watts of shaped acoustic noise as input to test silencers. Full-scale gas turbine splitter panels are tested in a 4-ft by 4-ft duct at input acoustic levels corresponding to the unsilenced exhaust noise of a more than 30-MW gas turbine and at flows varying from zero to more than 150 FPS. A shaped random noise electrical input to the EPT-94B noise source is used. A circular duct transition section is used to modify the basic system for tests of large reciprocating engine silencers. Sine sweep and shaped random noise electrical inputs to the noise source are used.

Acoustic Antennas for Atmospheric Echo Sounding

The successful utilization of acoustic waves for atmospheric echo sounding requires an efficient antenna, with high on-axis gain and suppressed sidelobes. Existing microwave antennas and optical searchlights have been adapted for acoustic use, and their performance measured. A conical horn reflector antenna made of fiberglass has given excellent performance at 1–5 kHz after being coated with a viscous damping tar to keep reverberation time short for the reception of weak echoes with minimum delay following the transmission of 20 acoustic watts from the antenna. Septum dampers have been used successfully on parabolic dish antennas. Antenna beam patterns have been measured on an outdoor range, across a canyon, and the results agree closely with predictions from diffraction theory. Absorbing, cylindrical antenna enclosures have been found effective in reducing sidelobe reception by 10 to 20 dB, also in agreement with predictions. Attention to antenna design should provide unimpaired echo sounder operation in an urban environment, and keep noise pollution from the sounder pulses below the annoyance threshold.

Comparison of Ultrasonic Power Measurement Techniques

Ultrasonic devices are now used in a variety of medical and industrial applications where precise knowledge of the acoustic power output is required. Mechanical watt meters are available which measure radiation pressure and convert the reading to acoustic power. Acoustic power measurements can also be made by measuring the energy absorbed by a liquid medium which is irradiated by sound. In addition, the power output of ultrasonic devices can be estimated by measuring RF power out of the ultrasonic generator multiplied by the quoted acoustic efficiency of the transducer. These methods are reviewed and compared for a variety of transducers and acoustic watt meters.