Methods to calibrate the absolute receive sensitivity of single-element, focused transducers

Determining the Responsivity of Air-Coupled Piezoelectric Transducers Using a Comparative Method: Theory and Experiments

The responsivity of an ultrasonic transducer is an important parameter in evaluating its effective frequency band, the electroacoustic conversion efficiency, and the measurement capability of the system. The determination of the responsivity of a traditional immersion or contact piezoelectric transducer has been well investigated. However, due to the high attenuation of waves propagating in air and the large acoustic impedance mismatch between the active piezoceramic material and the load medium, there are few reports of the calibration of an air-coupled piezoelectric transducer. In this work, we present a comparative method of measuring the responsivity of an air-coupled transducer: the air-coupled transducer is used to receive a broadband pulse signal to evaluate its frequency spectrum, and a toneburst signal with known vibration displacement is measured by the air-coupled transducer in order to calibrate the amplitude of the responsivity. The effects of transmitter responsivity, input pulse characteristics, attenuation, and diffraction are taken into account to improve the accuracy of the responsivity determination. In addition, the measurement of the amplitude of the responsivity by comparing the measured displacements avoids the complicated task of characterizing the effects of electrical equipment. The determined responsivity is checked by comparing the measured displacements using different methods at different frequencies in order to evaluate its frequency spectrum and by measuring the nonlinearity parameters of the material to evaluate its amplitude. The agreement between results obtained using different methods demonstrates that the calibrated responsivity of the air-coupled transducer is valid, and the proposed method is effective.

Cavitation Emissions Nucleated by Definity Infused through an EkoSonic Catheter in a Flow Phantom

The EkoSonic endovascular system has been cleared by the U.S. Food and Drug Administration for the controlled and selective infusion of physician specified fluids, including thrombolytics, into the peripheral vasculature and the pulmonary arteries. The objective of this study was to explore whether this catheter technology could sustain cavitation nucleated by infused Definity, to support subsequent studies of ultrasound-mediated drug delivery to diseased arteries. The concentration and attenuation spectroscopy of Definity were assayed before and after infusion at 0.3, 2.0 and 4.0 mL/min through the EkoSonic catheter. PCI was used to map and quantify stable and inertial cavitation as a function of Definity concentration in a flow phantom mimicking the porcine femoral artery. The 2.0 mL/min infusion rate yielded the highest surviving Definity concentration and acoustic attenuation. Cavitation was sustained throughout each 15 ms ultrasound pulse, as well as throughout the 3 min infusion. These results demonstrate a potential pathway to use cavitation nucleation to promote drug delivery with the EkoSonic endovascular system.

Simultaneously Determining Sensitivity and Effective Geometrical Parameters of Ultrasonic Piezoelectric Transducers Using a Self-Reciprocity Method

Determination of the sensitivity of a transducer is essential in evaluating its central frequency and effective bandwidth, its electroacoustic conversion capability, or the measurement ability of an ultrasonic test system. In this work, a calibration method based on self-reciprocity is proposed for the determination of transducer sensitivity, which can be applied to both planar and focused transducers. The two-port electrical network of the experimental setup is analyzed, and a simplified measurement procedure is described in which the "impedance mismatch" problem is solved, and only input and output currents are needed. An acoustic transfer function is introduced, both to reduce the effects of wave energy loss on the determination of transducer sensitivity and to help extract the effective geometrical parameters of the transducer through these measured output current signals. The effects of diffraction, attenuation, and effective geometrical parameters on sensitivity determination are discussed in this work; the experimental results show that when all these factors are taken into account, accurate sensitivities for both planar and focused transducers can be obtained at various experimental distances.

In vitro assessment of stiffness-dependent histotripsy bubble cloud activity in gel phantoms and blood clots

As a bubble-based ablative therapy, the efficacy of histotripsy has been demonstrated in healthy or acutely diseased models. Chronic conditions associated with stiff tissues may require additional bubble activity prior to histotripsy liquefaction. In this study, histotripsy pulses were generated in agarose phantoms of Young's moduli ranging from 12.3 to 142 kPa, and in vitro clot models with mild and strong platelet-activated retraction. Bubble cloud emissions were tracked with passive cavitation imaging, and the threshold acoustic power associated with phantom liquefaction was extracted with receiver operator characteristic analysis. The power of histotripsy-generated emissions and the degree of liquefaction were tabulated for both clot models. For the agarose phantoms, the acoustic power associated with liquefaction increased with Young's modulus. When grouped based on agarose concentration, only two arms displayed a significant difference in the liquefaction threshold acoustic power (22.1 kPa versus 142 kPa Young's modulus). The bubble cloud dynamics tracked with passive cavitation imaging indicated no strong changes in the bubble dynamics based on the phantom stiffness. For identical histotripsy exposure, the power of acoustic emissions and degree of clot lysis did not vary based on the clot model. Overall, these results indicate that a fixed threshold acoustic power mapped with passive cavitation imaging can be utilized for predicting histotripsy liquefaction over a wide range of tissue stiffness.

For Whom the Bubble Grows: Physical Principles of Bubble Nucleation and Dynamics in Histotripsy Ultrasound Therapy

Histotripsy is a focused ultrasound therapy for non-invasive tissue ablation. Unlike thermally ablative forms of therapeutic ultrasound, histotripsy relies on the mechanical action of bubble clouds for tissue destruction. Although acoustic bubble activity is often characterized as chaotic, the short-duration histotripsy pulses produce a unique and consistent type of cavitation for tissue destruction. In this review, the action of histotripsy-induced bubbles is discussed. Sources of bubble nuclei are reviewed, and bubble activity over the course of single and multiple pulses is outlined. Recent innovations in terms of novel acoustic excitations, exogenous nuclei for targeted ablation and histotripsy-enhanced drug delivery and image guidance metrics are discussed. Finally, gaps in knowledge of the histotripsy process are highlighted, along with suggested means to expedite widespread clinical utilization of histotripsy.

Characterization of cavitation-radiated acoustic power using diffraction correction

A method is developed for compensating absolute pressure measurements made by a calibrated passive cavitation detector (PCD) to estimate the average acoustic power radiated from a region of interest (ROI) defined to encompass all cavitating bubbles. A diffraction correction factor for conversion of PCD-measured pressures to cavitation-radiated acoustic power per unit area or volume is derived as a simple analytic expression, accounting for position- and frequency-dependent PCD sensitivity. This approach can be applied to measurements made by any PCD without precise knowledge of the number, spatial, or temporal distribution of cavitating bubbles. The diffraction correction factor is validated in simulation for a wide range of ROI dimensions and frequencies. The correction factor is also applied to emission measurements obtained during in vitro ultrasound-enhanced sonophoresis experiments, allowing comparison of stable cavitation levels between therapeutic configurations with different source center frequencies. Results incorporating sonication at both 0.41 and 2.0 MHz indicate that increases in skin permeability correlate strongly with the acoustic power of subharmonic emissions radiated per unit skin area.

Post Hoc Analysis of Passive Cavitation Imaging for Classification of Histotripsy-Induced Liquefaction in Vitro

Histotripsy utilizes focused ultrasound to generate bubble clouds for transcutaneous tissue liquefaction. Bubble activity maps are under development to provide image guidance and monitor treatment progress. The aim of this paper was to investigate the feasibility of using plane wave B-mode and passive cavitation images to be used as binary classifiers of histotripsy-induced liquefaction. Prostate tissue phantoms were exposed to histotripsy pulses over a range of pulse durations (5- ) and peak negative pressures (12-23 MPa). Acoustic emissions were recorded during the insonation and beamformed to form passive cavitation images. Plane wave B-mode images were acquired following the insonation to detect the hyperechoic bubble cloud. Phantom samples were sectioned and stained to delineate the liquefaction zone. Correlation between passive cavitation and plane wave B-mode images and the liquefaction zone was assessed using receiver operating characteristic (ROC) curve analysis. Liquefaction of the phantom was observed for all the insonation conditions. The area under the ROC (0.94 versus 0.82), accuracy (0.90 versus 0.83), and sensitivity (0.81 versus 0.49) was greater for passive cavitation images relative to B-mode images ( ) along the azimuth of the liquefaction zone. The specificity was greater than 0.9 for both imaging modalities. These results demonstrate a stronger correlation between histotripsy-induced liquefaction and passive cavitation imaging compared with the plane wave B-mode imaging, albeit with limited passive cavitation image range resolution.

Calibration of focused ultrasonic transducers and absolute measurements of fluid nonlinearity with diffraction and attenuation corrections

This paper presents analytical and experimental techniques for absolute determination of the acoustic nonlinearity parameter (β) in fluids using focused transducers. When focused transducers are used for β measurements, the geometrical and mechanical calibrations are generally required for accurate determination of the receiver transfer function from which the absolute pressure can be calculated. The fundamental and second harmonic wave amplitudes in harmonic generation measurements should be modified to account for beam diffraction and material absorption. All these issues are resolved in this study and the proposed technique is validated through the β measurement in water. An experimental method is developed to determine the effective radius and focal length of focused transducers. A simplified self-reciprocity calibration procedure for a broadband focused receiver is described. The diffraction and attenuation corrections for the fundamental and second harmonic waves are explicitly derived using the multi-Gaussian beam model, and the effects on the β determination are discussed. When the diffraction and attenuation corrections are all properly made, the measurement of β over a large range of propagation distances is possible with errors less than 8%.

Quantitative Frequency-Domain Passive Cavitation Imaging

Passive cavitation detection has been an instrumental technique for measuring cavitation dynamics, elucidating concomitant bioeffects, and guiding ultrasound therapies. Recently, techniques have been developed to create images of cavitation activity to provide investigators with a more complete set of information. These techniques use arrays to record and subsequently beamform received cavitation emissions, rather than processing emissions received on a single-element transducer. In this paper, the methods for performing frequency-domain delay, sum, and integrate passive imaging are outlined. The method can be applied to any passively acquired acoustic scattering or emissions, including cavitation emissions. To compare data across different systems, techniques for normalizing Fourier transformed data and converting the data to the acoustic energy received by the array are described. A discussion of hardware requirements and alternative imaging approaches is additionally outlined. Examples are provided in MATLAB.

A self-reciprocity calibration method for broadband focused transducers

A procedure is developed for self-calibration of broadband, spherically focused ultrasonic transducers based on reciprocity. The input and received signals are measured in a pulse-echo configuration. These signals are used in conjunction with a multi-Gaussian beam model to obtain the electromechanical transfer function of the transducer. This calibration procedure is advantageous because it reduces the experimental configuration to a single transducer and a reflector. Experimental results indicate that the transfer function is insensitive to on-axis reflector placement. This result supports the feasibility of integrating the calibration procedure into actual testing in some situations.

Accuracy of a bistatic scattering substitution technique for calibration of focused receivers

A recent method for calibrating single-element, focused passive cavitation detectors (PCD) compares bistatic scattering measurements by the PCD and a reference hydrophone. Here, effects of scatterer properties and PCD size on frequency-dependent receive calibration accuracy are investigated. Simulated scattering from silica and polystyrene spheres was compared for small hydrophone and spherically focused PCD receivers to assess the achievable calibration accuracy as a function of frequency, scatterer size, and PCD size. Good agreement between measurements was found when the scatterer diameter was sufficiently smaller than the focal beamwidth of the PCD; this relationship was dependent on the scatterer material. For conditions that result in significant disagreement between measurements, the numerical methods described here can be used to correct experimental calibrations.