Influence of Acoustic Reflection on the Inertial Cavitation Dose in a Franz Diffusion Cell

The exposure of the skin to low-frequency (20–100?kHz) ultrasound is a well-established method for increasing its permeability to drugs. The mechanism underlying this permeability increase has been found to be inertial cavitation within the coupling fluid. This study investigated the influence of acoustic reflections on the inertial cavitation dose during low-frequency (20?kHz) exposure in an in vitro skin sonoporation setup. This investigation was conducted using a passive cavitation detector that monitored the broadband noise emission within a modified Franz diffusion cell. Two versions of this diffusion cell were employed. One version had acoustic conditions that were similar to those of a standard Franz diffusion cell surrounded by air, whereas the second was designed to greatly reduce the acoustic reflection by submerging the diffusion cell in a water bath. The temperature of the coupling fluid in both setups was controlled using a novel thermoelectric cooling system. At an ultrasound intensity of 13.6 W/cm², the median inertial cavitation dose when the acoustic reflections were suppressed, was found to be only about 15% lower than when reflections were not suppressed.     

Non-linear Acoustic Emissions from Therapeutically Driven Contrast Agent Microbubbles

Non-linear emissions from microbubbles introduced to the vasculature for exposure to focused ultrasound are routinely monitored for assessment of therapy and avoidance of irreversible tissue damage. Yet the bubble-based mechanistic source for these emissions, under subresonant driving at typical therapeutic pressure amplitudes, may not be well understood. In the study described here, dual-perspective high-speed imaging at 210,000 frames per second (fps), and shadowgraphically at 10 Mfps, was used to observe cavitation from microbubbles flowing through a 500-µm polycarbonate capillary exposed to focused ultrasound of 692 kHz at therapeutically relevant pressure amplitudes. The acoustic emissions were simultaneously collected via a broadband calibrated needle hydrophone system. The observations indicate that periodic bubble-collapse shock waves can dominate the non-linear acoustic emissions, including subharmonics at higher driving amplitudes. Contributions to broadband emissions through variance in shock wave amplitude and emission timings are also identified. Possible implications for in vivo microbubble cavitation detection, mechanisms of therapy and the conventional classification of cavitation activity as stable or inertial are discussed.     

Effect of Overpressure on Acoustic Emissions and Treated Tissue Histology in ex Vivo Bulk Ultrasound Ablation

Bulk ultrasound ablation is a thermal therapy approach in which tissue is heated by unfocused or weakly focused sonication (average intensities on the order of 100 W/cm²) to achieve coagulative necrosis within a few minutes exposure time. Assessing the role of bubble activity, including acoustic cavitation and tissue vaporization, in bulk ultrasound ablation may help in making bulk ultrasound ablation safer and more effective for clinical applications. Here, two series of ex vivo ablation trials were conducted to investigate the role of bubble activity and tissue vaporization in bulk ultrasound ablation. Fresh bovine liver tissue was ablated with unfocused, continuous-wave ultrasound using ultrasound image-ablate arrays sonicating at 31 W/cm² (0.9 MPa amplitude) for either 20 min at a frequency of 3.1 MHz or 10 min at 4.8 MHz. Tissue specimens were maintained at a static overpressure of either 0.52 or 1.2 MPa to suppress bubble activity and tissue vaporization or at atmospheric pressure for control groups. A passive cavitation detector was used to record subharmonic (1.55 or 2.4 MHz), broadband (1.2–1.5 MHz) and low-frequency (5–20 kHz) acoustic emissions. Treated tissue was stained with 2% triphenyl tetrazolium chloride to evaluate thermal lesion dimensions. Subharmonic emissions were significantly reduced in overpressure groups compared with control groups. Correlations observed between acoustic emissions and lesion dimensions were significant and positive for the 3.1-MHz series, but significant and negative for the 4.8-MHz series. The results indicate that for bulk ultrasound ablation, where both acoustic cavitation and tissue vaporization are possible, bubble activity can enhance ablation in the absence of tissue vaporization, but can reduce thermal lesion dimensions in the presence of vaporization.     

Passive Cavitation Detection during Pulsed HIFU Exposures of Ex Vivo Tissues and In Vivo Mouse Pancreatic Tumors

Pulsed high-intensity focused ultrasound (pHIFU) has been shown to enhance vascular permeability, disrupt tumor barriers and enhance drug penetration into tumor tissue through acoustic cavitation. Monitoring of cavitation activity during pHIFU treatments and knowing the ultrasound pressure levels sufficient to reliably induce cavitation in a given tissue are therefore very important. Here, three metrics of cavitation activity induced by pHIFU and evaluated by confocal passive cavitation detection were introduced: cavitation probability, cavitation persistence and the level of the broadband acoustic emissions. These metrics were used to characterize cavitation activity in several ex vivo tissue types (bovine tongue and liver and porcine adipose tissue and kidney) and gel phantoms (polyacrylamide and agarose) at varying peak-rare factional focal pressures (1–12 MPa) during the following pHIFU protocol: frequency 1.1 MHz, pulse duration 1 ms and pulse repetition frequency 1 Hz. To evaluate the relevance of the measurements in ex vivo tissue, cavitation metrics were also investigated and compared in the ex vivo and in vivo murine pancreatic tumors that develop spontaneously in transgenic KrasLSL.G12 D/+; p53 R172 H/+; PdxCretg/+ (KPC) mice and closely re-capitulate human disease in their morphology. The cavitation threshold, defined at 50% cavitation probability, was found to vary broadly among the investigated tissues (within 2.5–10 MPa), depending mostly on the water-lipid ratio that characterizes the tissue composition. Cavitation persistence and the intensity of broadband emissions depended both on tissue structure and lipid concentration. Both the cavitation threshold and broadband noise emission level were similar between ex vivo and in vivo pancreatic tumor tissue. The largest difference between in vivo and ex vivo settings was found in the pattern of cavitation occurrence throughout pHIFU exposure: it was sporadic in vivo, but it decreased rapidly and stopped over the first few pulses ex vivo. Cavitation activity depended on the interplay between the destruction and circulation of cavitation nuclei, which are not only used up by HIFU treatment but also replenished or carried away by circulation in vivo. These findings are important for treatment planning and optimization in pHIFU-induced drug delivery, in particular for pancreatic tumors.     

A Comparison of Sonothrombolysis in Aged Clots between Low-Boiling-Point Phase-Change Nanodroplets and Microbubbles of the Same Composition

We present enhanced cavitation erosion of blood clots exposed to low-boiling-point (−2°C) perfluorocarbon phase-change nanodroplets and pulsed ultrasound, as well as microbubbles with the same formulation under the same conditions. Given prior success with microbubbles as a sonothrombolysis agent, we considered that perfluorocarbon phase-change nanodroplets could enhance clot disruption further beyond that achieved with microbubbles. It has been hypothesized that owing to their small size and ability to penetrate into a clot, nanodroplets could enhance cavitation inside a blood clot and increase sonothrombolysis efficacy. The thrombolytic effects of lipid-shell-decafluorobutane nanodroplets were evaluated and compared with those of microbubbles with the same formulation, in an aged bovine blood clot flow model. Seven different pulsing schemes, with an acoustic intensity (I_SPTA) range of 0.021–34.8 W/cm² were applied in three different therapy scenarios: ultrasound only, ultrasound with microbubbles and ultrasound with nanodroplets (n = 5). Data indicated that pulsing schemes with 0.35 W/cm² and 5.22 W/cm² produced a significant difference (p < 0.05) in nanodroplet sonothrombolysis performance compared with compositionally identical microbubbles. With these excitation conditions, nanodroplet-mediated treatment achieved a 140% average thrombolysis rate over the microbubble-mediated case. We observed distinctive internal erosion in the middle of bovine clot samples from nanodroplet-mediated ultrasound, whereas the microbubble-mediated case generated surface erosion. This erosion pattern was supported by ultrasound imaging during sonothrombolysis, which revealed that nanodroplets generated cavitation clouds throughout a clot, whereas microbubble cavitation formed larger cavitation clouds only outside a clot sample.     

HIFU-induced cavitation and heating in ex vivo porcine subcutaneous fat

The present study is motivated by the fact that there are no published studies quantifying cavitation activity and heating induced by ultrasound in adipose tissue and that there are currently no reliable techniques for monitoring successful deposition of ultrasound energy in fat in real time. High-intensity focused ultrasound (HIFU) exposures were performed in excised porcine fat at four different frequencies (0.5, 1.1, 1.6 and 3.4 MHz) over a range of pressure amplitudes and exposure durations. The transmission losses arising from reflection at the skin interface and attenuation through skin and fat were quantified at all frequencies using an embedded needle hydrophone. A 15 MHz passive cavitation detector (PCD) coaxial to the HIFU transducer was used to capture acoustic emissions emanating from the focus during HIFU exposures, while the focal temperature rise was measured using minimally invasive needle thermocouples. Repeatable temperature rises in excess of 10°C could be readily instigated across all four frequencies for acoustic intensities (Ispta) in excess of 50 W/cm² within the first 2 s of exposure. Even though cavitation could not be initiated at 1.1, 1.6 and 3.4 MHz over the in situ peak rarefactional (p_-) pressure range 0-3 MPa explored in the present study, inertial cavitation activity was always initiated at 0.5 MHz for pressures greater than 1.6 MPa (p_-) and was found to enhance focal heat deposition. A good correlation was identified between the energy of broadband emissions detected by the PCD and the focal temperature rise at 0.5 MHz, particularly for short 2 s exposures, which could be exploited as a tool for noninvasive monitoring of successful treatment delivery. (E-mail: zoe.kyriakou@eng.ox.ac.uk)     

The speed of sound and attenuation of an IEC agar-based tissue-mimicking material for high frequency ultrasound applications

This study characterized the acoustic properties of an International Electromechanical Commission (IEC) agar-based tissue mimicking material (TMM) at ultrasound frequencies in the range 10–47 MHz. A broadband reflection substitution technique was employed using two independent systems at 21°C ± 1°C. Using a commercially available preclinical ultrasound scanner and a scanning acoustic macroscope, the measured speeds of sound were 1547.4 ± 1.4 m?s⁻¹ and 1548.0 ± 6.1 m?s⁻¹, respectively, and were approximately constant over the frequency range. The measured attenuation (dB?cm⁻¹) was found to vary with frequency f (MHz) as 0.40f + 0.0076f². Using this polynomial equation and extrapolating to lower frequencies give values comparable to those published at lower frequencies and can estimate the attenuation of this TMM in the frequency range up to 47 MHz. This characterisation enhances understanding in the use of this TMM as a tissue equivalent material for high frequency ultrasound applications.     

Ultrasound-enhanced rt-PA thrombolysis in an ex vivo porcine carotid artery model

Ultrasound is known to enhance recombinant tissue plasminogen activator (rt-PA) thrombolysis. In this study, occlusive porcine whole blood clots were placed in flowing plasma within living porcine carotid arteries. Ultrasonically induced stable cavitation was investigated as an adjuvant to rt-PA thrombolysis. Aged, retracted clots were exposed to plasma alone, plasma containing rt-PA (7.1 ± 3.8 μg/mL) or plasma with rt-PA and Definity^® ultrasound contrast agent (0.79 ± 0.47 μL/mL) with and without 120-kHz continuous wave ultrasound at a peak-to-peak pressure amplitude of 0.44 MPa. An insonation scheme was formulated to promote and maximize stable cavitation activity by incorporating ultrasound quiescent periods that allowed for the inflow of Definity^®-rich plasma. Cavitation was measured with a passive acoustic detector throughout thrombolytic treatment. Thrombolytic efficacy was measured by comparing clot mass before and after treatment. Average mass loss for clots exposed to rt-PA and Definity^® without ultrasound (n = 7) was 34%, and with ultrasound (n = 6) was 83%, which constituted a significant difference (p < 0.0001). Without Definity^® there was no thrombolytic enhancement by ultrasound exposure alone at this pressure amplitude (n = 5, p < 0.0001). In the low-oxygen environment of the ischemic artery, significant loss of endothelium occurred but no correlation was observed between arterial tissue damage and treatment type. Acoustic stable cavitation nucleated by an infusion of Definity^® enhances rt-PA thrombolysis without apparent treatment-related damage in this ex vivo porcine carotid artery model.     

Control of Acoustic Cavitation for Efficient Sonoporation with Phase-Shift Nanoemulsions

Acoustic cavitation can be used to temporarily disrupt cell membranes for intracellular delivery of large biomolecules. Termed sonoporation, the ability of this technique for efficient intracellular delivery (i.e., >50% of initial cell population showing uptake) while maintaining cell viability (i.e., >50% of initial cell population viable) has proven to be very difficult. Here, we report that phase-shift nanoemulsions (PSNEs) function as inertial cavitation nuclei for improvement of sonoporation efficiency. The interplay between ultrasound frequency, resultant microbubble dynamics and sonoporation efficiency was investigated experimentally. Acoustic emissions from individual microbubbles nucleated from PSNEs were captured using a broadband passive cavitation detector during and after acoustic droplet vaporization with short pulses of ultrasound at 1, 2.5 and 5 MHz. Time domain features of the passive cavitation detector signals were analyzed to estimate the maximum size (R_max) of the microbubbles using the Rayleigh collapse model. These results were then applied to sonoporation experiments to test if uptake efficiency is dependent on maximum microbubble size before inertial collapse. Results indicated that at the acoustic droplet vaporization threshold, R_max was approximately 61.7 ± 5.2, 24.9 ± 2.8, and 12.4 ± 2.1 μm at 1, 2.5 and 5 MHz, respectively. Sonoporation efficiency increased at higher frequencies, with efficiencies of 39.5 ± 13.7%, 46.6 ± 3.28% and 66.8 ± 5.5% at 1, 2.5 and 5 MHz, respectively. Excessive cellular damage was seen at lower frequencies because of the erosive effects of highly energetic inertial cavitation. These results highlight the importance of acoustic cavitation control in determining the outcome of sonoporation experiments. In addition, PSNEs may serve as tailorable inertial cavitation nuclei for other therapeutic ultrasound applications.     

Unmyelinated Peripheral Nerves Can Be Stimulated in Vitro Using Pulsed Ultrasound

Appreciation for the medical and research potential of ultrasound neuromodulation is growing rapidly, with potential applications in non-invasive treatment of neurodegenerative disease and functional brain mapping spurring recent progress. However, little progress has been made in our understanding of the ultrasound–tissue interaction. The current study tackles this issue by measuring compound action potentials (CAPs) from an ex vivo crab walking leg nerve bundle and analysing the acoustic nature of successful stimuli using a passive cavitation detector (PCD). An unimpeded ultrasound path, new acoustic analysis techniques and simple biological targets are used to detect different modes of cavitation and narrow down the candidate biological effectors with high sensitivity. In the present case, the constituents of unmyelinated axonal tissue alone are found to be sufficient to generate de novo action potentials under ultrasound, the stimulation of which is significantly correlated to the presence of inertial cavitation and is never observed in its absence.     

A Study of Bubble Activity Generated in Ex Vivo Tissue by High Intensity Focused Ultrasound

Cancer treatment by extracorporeal high-intensity focused ultrasound (HIFU) is constrained by the time required to ablate clinically relevant tumour volumes. Although cavitation may be used to optimize HIFU treatments, its role during lesion formation is ambiguous. Clear differentiation is required between acoustic cavitation (noninertial and inertial) effects and bubble formation arising from two thermally-driven effects (the vapourization of liquid into vapour, and the exsolution of formerly dissolved permanent gas out of the liquid and into gas spaces). This study uses clinically relevant HIFU exposures in degassed water and ex vivo bovine liver to test a suite of cavitation detection techniques that exploit passive and active acoustics, audible emissions and the electrical drive power fluctuations. Exposure regimes for different cavitation activities (none, acoustic cavitation and, for ex vivo tissue only, acoustic cavitation plus thermally-driven gas space formation) were identified both in degassed water and in ex vivo liver using the detectable characteristic acoustic emissions. The detection system proved effective in both degassed water and tissue, but requires optimization for future clinical application. (E-mail: jmclaughlan7@gmail.com)     

Gold nanoparticle targeted photoacoustic cavitation for potential deep tissue imaging and therapy

The laser generation of vapor bubbles around plasmonic nanoparticles can be enhanced through the application of an ultrasound field; a technique referred to as photoacoustic cavitation. The combination of light and ultrasound allows for bubble formation at lower laser fluence and peak negative ultrasound pressure than can be achieved using either modality alone. The growth and collapse of these bubbles leads to local mechanical disruption and acoustic emission, and can potentially be used to induce and monitor tissue therapy. Photoacoustic cavitation is investigated for a broad range of ultrasound pressures and nanoparticle concentrations for gold nanorods and nanospheres. The cavitation threshold fluences for both nanoparticle types are found to drastically reduce in the presence of an ultrasound field. The results indicate that photoacoustic cavitation can potentially be produced at depth in biological tissue without exceeding the safety limits for ultrasound or laser radiation at the tissue surface.OCIS codes: (170.0170) Medical optics and biotechnology, (170.5120) Photoacoustic imaging, (170.5180) Photodynamic therapy     

Ultrasonic effects on alveolar macrophages in suspension

Rabbit alveolar macrophages in suspension were exposed to 5 or 10 min of continuous 2-MHz ultrasound with 5, 10, and 15 W/cm² spatial average intensities. Viability as determined by dye exclusion decreased with increasing intensity. Pressure experiments indicated that this was a result of acoustic cavitation. Ultrasound induced clumping of cells and often reduced membrane ruffling. Some cells were disintegrated. Cells that appeared to be otherwise intact had swollen mitochondria with ruptured cristae.     

Probability of Cavitation for Single Ultrasound Pulses Applied to Tissues and Tissue-Mimicking Materials

In this study, the negative pressure values at which inertial cavitation consistently occurs in response to a single, two-cycle, focused ultrasound pulse were measured in several media relevant to cavitation-based ultrasound therapy. The pulse was focused into a chamber containing one of the media, which included liquids, tissue-mimicking materials, and ex vivo canine tissue. Focal waveforms were measured by two separate techniques using a fiber-optic hydrophone. Inertial cavitation was identified by high-speed photography in optically transparent media and an acoustic passive cavitation detector. The probability of cavitation (P_cav) for a single pulse as a function of peak negative pressure (p₋) followed a sigmoid curve, with the probability approaching one when the pressure amplitude was sufficient. The statistical threshold (defined as P_cav = 0.5) was between p₋ = 26 and 30 MPa in all samples with high water content but varied between p₋ = 13.7 and >36 MPa in other media. A model for radial cavitation bubble dynamics was employed to evaluate the behavior of cavitation nuclei at these pressure levels. A single bubble nucleus with an inertial cavitation threshold of p₋ = 28.2 megapascals was estimated to have a 2.5 nm radius in distilled water. These data may be valuable for cavitation-based ultrasound therapy to predict the likelihood of cavitation at various pressure levels and dimensions of cavitation-induced lesions in tissue.     

Cavitation threshold of microbubbles in gel tunnels by focused ultrasound

The investigation of inertial cavitation in micro-tunnels has significant implications for the development of therapeutic applications of ultrasound such as ultrasound-mediated drug and gene delivery. The threshold for inertial cavitation was investigated using a passive cavitation detector with a center frequency of 1 MHz. Micro-tunnels of various diameters (90 to 800 microm) embedded in gel were fabricated and injected with a solution of Optison(trade mark) contrast agent of concentrations 1.2% and 0.2% diluted in water. An ultrasound pulse of duration 500 ms and center frequency 1.736 MHz was used to insonate the microbubbles. The acoustic pressure was increased at 1-s intervals until broadband noise emission was detected. The pressure threshold at which broadband noise emission was observed was found to be dependent on the diameter of the micro-tunnels, with an average increase of 1.2 to 1.5 between the smallest and the largest tunnels, depending on the microbubble concentration. The evaluation of inertial cavitation in gel tunnels rather than tubes provides a novel opportunity to investigate microbubble collapse in a situation that simulates in vivo blood vessels better than tubes with solid walls do.     

Measurement and correlation of acoustic cavitation with cellular bioeffects

Using broadband noise as a measure of cavitation activity, this study determined the kinetics of cavitation during sonication of Optison contrast agent and tested whether cellular bioeffects can be predicted by cavitation dose. Cell suspensions were exposed to ultrasound at varying acoustic frequency, pressure, exposure time, Optison concentration and cell type to obtain a broad range of bioeffects, i.e., intracellular uptake and loss of viability, as quantified by flow cytometry. We found that cavitation activity measured by broadband noise increased and peaked within 20 ms and then decayed with a half-life of tens to hundreds of milliseconds. Intracellular uptake and loss of viability correlated well with the cavitation dose determined by the time integral of broadband noise magnitude. These results demonstrate that broadband noise correlates with bioeffects over a broad range of experimental conditions, which suggests a noninvasive feedback method to control ultrasound's bioeffects in real time.     

Ultrasound safety with midfrequency transcranial sonothrombolysis: preliminary study on normal macaca monkey brain

We investigated the safety of transcranial-targeting midfrequency (0.1 to 1 MHz) ultrasonic thrombolysis for acute ischemic stroke. We applied a new therapeutic and imaging transducer to healthy Macaca monkey brains via sonication of the ipsilateral middle cerebral artery through an acoustic temporal window. Young adult cynomolgus monkeys (Macaca fascicularis) were assigned to a group without sonication (control), a group maintained for 1 d after sonication (C1) and a group maintained for 7 d after sonication (C7; n = 3 for each). Two elder rhesus monkeys (Macaca mulatta) were ultrasonicated under transvenous injection of the recombinant tissue plasminogen activator alteplase (0.9 mg/kg), and maintained for 7 d (R). An automatic switching circuit alternately operated a therapeutic ultrasound beam (T-beam) generator for thrombolysis (frequency = 490 kHz; intensity = 0.72 W/cm²) and a diagnostic color-flow imaging ultrasound beam (D-beam; frequency = 2.5 MHz; intensity = 0.20 W/cm²). A 15-min protocol, comprising four repeats of a sequence of 120-s T-beam activation followed by 30-s D-beam activation and then 5-min T-beam deactivation together with D-beam activation, was repeated four times over 60 min. After confirmation of neurologic deficits, the brains were removed and investigated histologically and immunohistochemically. Three skull samples were subjected to 494-kHz continuous waveform ultrasound, the transcranial intensity was measured and the mechanical index was calculated. None of the monkeys showed neurologic deficits after ultrasonication. The transskull ultrasound intensity rate was 48 ± 12%. The intracranial mechanical index value was 0.15. The novel system did not cause tissue damage in the primate brain and no cavitation effect was detected intracranially.     

Histotripsy Bubble Dynamics in Elastic,Anisotropic Tissue-Mimicking Phantoms

Elastic, anisotropic tissue such as tendon has proven resistant to mechanical fractionation by histotripsy, a subset of focused ultrasound that uses the creation, oscillation and collapse of cavitation bubbles to fractionate tissue. Our objective was to fabricate an optically transparent hydrogel that mimics tendon for evaluation of histotripsy bubble dynamics. Ex vivo bovine deep digital flexor tendons were obtained (n = 4), and varying formulations of polyacrylamide (PA), collagen and fibrin hydrogels (n = 3 each) were fabricated. Axial sound speeds were measured at 1 MHz for calculation of anisotropy. All samples were treated with a 1.5-MHz focused ultrasound transducer with 10-ms pulses repeated at 1 Hz (p₊ = 127 MPa, p_- = 35 MPa); treatments were monitored with passive cavitation imaging and high-speed photography. Dehydrated fibrin gels were found to be the most similar to tendon in cavitation emission energy (fibrin = 0.69 ± 0.24, tendon = 0.64 ± 0.19 [× 10¹⁰ V²]) and anisotropy (fibrin = 3.16 ± 1.12, tendon = 19.4). Bubble cloud area in dehydrated fibrin (0.79 ± 0.14 mm²) was significantly smaller than most other tested hydrogels. Finally, anisotropy was found to have moderately strong linear relationships with cavitation energy and bubble cloud size (r = –0.65 and –0.80, respectively). Dehydrated fibrin shows potential as a repeatable, transparent, tissue-mimicking hydrogel for evaluation of histotripsy bubble dynamics in elastic, anisotropic tissues.     

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 ex vivo 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.     

The threshold for thermally significant cavitation in dog's thigh muscle in vivo

In this study the threshold of thermally significant transient cavitation in vivo in dog's thigh muscle was investigated as a function of frequency from 0.246 MHz to 1.68 MHz. Cavitation, evidenced by strong emission of wide band noise monitored by a hydrophone, appeared to increase the energy absorption in tissue at the focal zone of a focused ultrasound beam as measured with an embedded thermocouple. This was indicated by a significant increase in the temperature, a loss of smooth temperature rise during the 1 s sound pulse and a significant reduction in the acoustic power transmitted through the thigh. This thermal phenomenon was associated with a strong emission of wide band noise which was monitored by a hydrophone. In addition, strong echoes appeared in ultrasound images during the pulses that caused the noise emission and the thermal effect. These echoes appeared preferentially at locations where there was acoustic heterogeneity. The measured cavitation pressure amplitude threshold was found to depend almost linearly on frequency with a slope of about 5.3 MPa MHz-1. (The extrapolated static pressure threshold was 0.6 MPa). When these measured levels are compared to those typical of clinical application, it appears that the transient cavitation can be avoided when perfusion independent high temperature hyperthermia is induced with focused and pulsed ultrasound fields. However, intensities required during scanned focused ultrasound hyperthermia, where sharply focused transducers are used to heat large tumors at low frequencies (1 MHz or below), could rise above the threshold. Thus, care should be taken when focused ultrasound systems are designed so that the maximum peak pressure is below the threshold in order to avoid unpredictable biological effects induced by transient cavitation. Finally it is unlikely that the present diagnostic ultrasound units which operate at higher frequencies and in pulsed mode could cause transient cavitation in vivo.