Acoustic characterization of high intensity focused ultrasound (HIFU) fields is important both for the accurate prediction of ultrasound induced bioeffects in tissues and for the development of regulatory standards for clinical HIFU devices. In this paper, a method to determine HIFU field parameters at and around the focus is proposed. Nonlinear pressure waveforms were measured and modeled in water and in a tissue-mimicking gel phantom for a 2 MHz transducer with an aperture and focal length of 4.4 cm. Measurements were performed with a fiber optic probe hydrophone at intensity levels up to 24,000 W/cm(2). The inputs to a Khokhlov-Zabolotskaya-Kuznetsov-type numerical model were determined based on experimental low amplitude beam plots. Strongly asymmetric waveforms with peak positive pressures up to 80 MPa and peak negative pressures up to 15 MPa were obtained both numerically and experimentally. Numerical simulations and experimental measurements agreed well; however, when steep shocks were present in the waveform at focal intensity levels higher than 6000 W/cm(2), lower values of the peak positive pressure were observed in the measured waveforms. This underrepresentation was attributed mainly to the limited hydrophone bandwidth of 100 MHz. It is shown that a combination of measurements and modeling is necessary to enable accurate characterization of HIFU fields.

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Acoustic characterization of high intensity focused ultrasound fields: a combined measurement and modeling approach.

Therapeutic ultrasound (US) has been of increasing interest during the past few years. However, the development of this technique depends on the availability of high-performance transducers. These transducers have to be optimised for focusing and steering high-power ultrasonic energy within the target volume. Recently developed high-power 1-3 piezocomposite materials bring to therapeutic US the exceptional electroacoustical properties of piezocomposite technology: these are high efficiency, large bandwidth, predictable beam pattern, more flexibility in terms of shaping and definition of sampling in annular arrays, linear arrays or matrix arrays. The construction and evaluation of several prototypes illustrates the benefit of this new approach that opens the way to further progress in therapeutic US.

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New piezoelectric transducers for therapeutic ultrasound

A passive cavitation detector (PCD) identifies cavitation events by sensing acoustic emissions generated by the collapse of bubbles. In this work, a dual passive cavitation detector (dual PCD), consisting of a pair of orthogonal confocal receivers, is described for use in shock wave lithotripsy. Cavitation events are detected by both receivers and can be localized to within 5 mm by the nature of the small intersecting volume of the focal areas of the two receivers in association with a coincidence detection algorithm. A calibration technique, based on the impulse response of the transducer, was employed to estimate radiated pressures at collapse near the bubble. Results are presented for the in vitro cavitation fields of both a clinical and a research electrohydraulic lithotripter. The measured lifetime of the primary growth-and-collapse of the cavitation bubbles increased from 180 to 420 microseconds as the power setting was increased from 12 to 24 kV. The measured lifetime compared well with calculations based on the Gilmore-Akulichev formulation for bubble dynamics. The radiated acoustic pressure 10 mm from the collapsing cavitation bubble was measured to vary from 4 to 16 MPa with increasing power setting; although the trends agreed with calculations, the predicted values were four times larger than measured values. The axial length of the cavitation field correlated well with the 6-dB region of the acoustic field. However, the width of the cavitation field (10 mm) was significantly narrower than the acoustic field (25 mm) as bubbles appeared to be drawn to the acoustic axis during the collapse. The dual PCD also detected signals from "rebounds," secondary and tertiary growth-and-collapse cycles. The measured rebound time did not agree with calculations from the single-bubble model. The rebounds could be fitted to a Rayleigh collapse model by considering the entire bubble cloud as an effective single bubble. The results from the dual PCD agreed well with images from high-speed photography. The results indicate that single-bubble theory is sufficient to model lithotripsy cavitation dynamics up to time of the main collapse, but that upon collapse bubble cloud dynamics becomes important.

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A dual passive cavitation detector for localized detection of lithotripsy-induced cavitation in vitro.

Acoustic holography is a powerful technique for characterizing ultrasound sources and the fields they radiate, with the ability to quantify source vibrations and reduce the number of required measurements. These capabilities are increasingly appealing for meeting measurement standards in medical ultrasound; however, associated uncertainties have not been investigated systematically. Here errors associated with holographic representations of a linear, continuous-wave ultrasound field are studied. To facilitate the analysis, error metrics are defined explicitly, and a detailed description of a holography formulation based on the Rayleigh integral is provided. Errors are evaluated both for simulations of a typical therapeutic ultrasound source and for physical experiments with three different ultrasound sources. Simulated experiments explore sampling errors introduced by the use of a finite number of measurements, geometric uncertainties in the actual positions of acquired measurements, and uncertainties in the properties of the propagation medium. Results demonstrate the theoretical feasibility of keeping errors less than about 1%. Typical errors in physical experiments were somewhat larger, on the order of a few percent; comparison with simulations provides specific guidelines for improving the experimental implementation to reduce these errors. Overall, results suggest that holography can be implemented successfully as a metrological tool with small, quantifiable errors.

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Acoustic holography as a metrological tool for characterizing medical ultrasound sources and fields.

For the characterization of high-intensity focused ultrasound (HIFU) fields, hydrophone measurements should be performed in water in the whole range of the radiated power; however, cavitation occurs at high output, leading to the destruction of the hydrophone. To avoid this problem, a new hydrophone with additional protective layers covering the electrodes was developed and tested in the experiments. A single-element 1-MHz focusing ultrasound source was used for detailed measurements of the acoustic field in the axial and lateral planes. Measurements were performed with a new membrane hydrophone up to the focal peak compressional and rarefactional pressures of 55.6 and of 12.8 MPa, respectively, when the shock front had already formed in the waveform. Numerical modeling for the beams of periodic waves with an initially uniform amplitude distribution was performed, based on the Khokhlov-Zabolotskaya-Kuznetsov equation. Numerical solutions were compared with the experimental data and found to be in good agreement (within 10%). The influences of the limited bandwidth and the spatial averaging effect of the hydrophone on the accurate measurements of the acoustic field parameters were also investigated.

Membrane hydrophone measurement and numerical simulation of HIFU fields up to developed shock regimes

One of the well-known methods of ultrasound focusing is the use of a transducer having the form of a concave spherical cap In the theoretical analysis, O’Neil’s formula is widely used, which describes acoustic pressure on the axis of the transducer under the assumption that normal velocity of its surface is uniform Usually, it is believed that modern piezoelectric focused sources are very close to this theoretical model The paper presented describes results of experimental investigation which showed that there exists a significant disagreement between the observed acoustic field and that predicted by the theory of O’Neil Two types of measurements have been made to clarify this problem The acoustic pressure field of a focused piezoceramic transducer (with aperture diameter of 100 mm, radius of curvature of 100 mm, resonance frequency of 1 MHz) has been measured in a water tank using a miniature hydrophone In addition, the radiator surface vibration has been studied in air using an optical interferome

J. Zhang

Papers

Lamb waves in piezoelectric focused radiator as a reason for discrepancy between O’Neil’s formula and experiment