Mark E. Schafer

Transducer characterization using the angular spectrum method

A measurement technique for analyzing the surface velocity patterns of ultrasonic transmitters is presented. The technique is based on the angular spectrum method of wave field analysis. In this approach, acoustic propagation between parallel planar surfaces is modeled using the two‐dimensional (2‐D) Fourier transform of the wave field, with each element in the spatial frequency domain multiplied by the appropriate phase factor. The technique was extended from the basic monochromatic model to the wideband pulsed case. An experimental system was built to measure the acoustic fields from various transducers, including single‐element and multielement phased arrays. Backpropagation results are shown for circular planar, circular focused, and rectangular phase steered transducers. The results demonstrate the ability of the extended angular spectrum method to reconstruct the surface velocity distribution of complex acoustic radiators.

Prediction of ultrasonic field propagation through layered media using the extended angular spectrum method

The angular spectrum method is a powerful technique for modeling the propagation of acoustic fields. The technique can predict an acoustic pressure field distribution over a plane, based upon knowledge of the pressure field distribution at a parallel plane. Predictions in both the forward and backward propagation directions are possible. In addition to predicting the effects of diffraction, the model also includes the effects of attenuation, refraction, dispersion, phase distortion, and the effects of finite amplitude acoustic propagation. No other model currently exists which can predict the propagation of wideband acoustic fields produced by sources of arbitrary geometry including all of the above propagation effects. Prior investigations have focused on using backward propagation predictions to analyze the surface vibration patterns of acoustic radiators. In contrast, the current effort has placed particular emphasis on verifying the model in the forward propagation case. In this paper, both forward and backward predictions are presented which demonstrate the ability of the model to characterize a three-dimensional acoustic field based upon measurements at a single plane. Results are also presented which examine the ability of the extended model to predict acoustic propagation through media composed of stacked homogeneous layers. The model has immediate applications in the study of acoustic phenomena and in the field of acoustic transducer design. Additionally, significant progress has been made toward the ultimate goal of predicting the degradation of acoustic transducer performance due to propagation through inhomogeneous, nonlinear, tissue-like media.

Factors affecting the choice of preamplification for ultrasonic hydrophone probes

This paper gives a systematic analysis of the effects of including an integrated (built-in) preamplifier into the ultrasonic piezoelectric probes (hydrophones) that are finding increasing use in biomedical applications. The design parameters considered include the end-of-cable sensitivity, gain, dynamic range, power supply requirements, construction intricacy, and cost. The rationale behind the inclusion of a preamplifier is given, and it is shown that the additional complexity introduced with the preamplifier into the measurement chain may not be warranted in all applications. Both the drawbacks and advantages of hydrophone preamplification are demonstrated, especially for the case of high pressure amplitude ultrasonic field measurements. Guidelines are developed for the potential user to identify the need for preamplification and the factors that influence the selection of the appropriate circuitry.

Development of a cost-effective shock wave hydrophone

The design of a new wideband, quantitative shock wave hydrophone is presented. The sensor not only has a wideband (>50 MHz) and linear (up to 100 MPa) response, but it also operates in a measurement environment in which the hydrophone elements sensitivity changes as a function of shock wave exposure. Thin films of polyvinylidene difluoride (PVDF) were used in a disposable hydrophone configuration. A self-monitoring feature, based on the change in hydrophone electrode resistance as the electrode materials are removed by shock wave action, indicates that the hydrophone element needs to be replaced. Development efforts include: 1) reducing the manufacturing costs; 2) determining the relationship between electrode resistance and hydrophone sensitivity; 3) developing a resistance monitoring approach; and 4) rapidly making reliable electrical connection to the disposable hydrophone elements. Acoustical characterization data indicate that the hydrophones fulfill the requirements for accurate, wideband measurement of lithotripter fields; the disposable feature makes the system cost effective for research, quality assurance and regulatory submissions

Exploratory Analysis of Estimated Acoustic Peak Rarefaction Pressure, Recanalization, and Outcome in the Transcranial Ultrasound in Clinical Sonothrombolysis Trial

Acoustic peak rarefaction pressure (APRP) is the main factor that influences ultrasound‐enhanced thrombolysis. We sought to determine whether recanalization rate and functional outcomes in the Transcranial Ultrasound in Clinical SONothrombolysis (TUCSON) trial could be predicted by estimated in vivo APRP.

Propagation Through Inhomogeneous Media Using the Angular Spectrum Method

This paper presents a technique for analyzing the acoustic fields generated by ultrasonic transmitters when radiating into layered, inhomogeneous media. The technique is based on the angular spectrum method of wavefield analysis, which is used to forward- or backpropagate acoustic fields between two parallel planar surfaces. Wave propagation is modelled as shift invariant filtering in the spatial frequency domain, with each spatial frequency component multiplied by the appropriate phase propagation factor. By modifying the phase propagation factors, the effects of attenuation, dispersion, refraction, and phase distortion may be modelled. Simulation results demonstrate several features of this approach, and experimental results show the ability of the technique to determine the pressure and velocity fields from different transducers. Wideband propagation is considered as an extension to the basic monochromatic model. 1 .O INTRODUCTION The design of ultrasonic imaging transducers has evolved over the last thirty-five years [l] to the point that there are standard design approaches employed to produce a transducer with specified imaging properties [2]. These imaging properties are typically stated in terms of resolution in a homogeneous medium such as water. Unfortunately, tissue is not homogeneous, and the resolving power of the transducer is affected by the attenuation, refraction, and phase distortion present in tissue. The goal of our work has been to develop a prediction technique which could simultaneously account for several of these tissue propagation properties. In addition, the technique was to be flexible, computationally efficient, and suitable for the analysis of practical transducer geometries, including phased linear arrays. This latter requirement stemmed from recent results [3,4], which note that the inhomogeneous nature of tissue is the limiting factor in the development of higher resolution diagnostic ultrasound equipment. These requirements formed the basis for the approach which was taken. The technique was based on the angular spectrum, or Fourier decomposition method [5], which is one of the more powerful techniques for predicting acoustic field distributions in homogeneous media. The angular spectrum method decomposes the pressure distribution over a plane surface into a two-dimensional spectrum of plane waves. Propagation is then modelled by multiplying each Fourier spectral component by the appropriate phase factor, effectively performing a linear filtering operation. The advantages of this technique include: high computational efficiency using the FFT algorithm, high spatial resolution even in the nearfield of the acoustic source, and the ability to predict acoustic fields over an entire plane in a single two-dimensional FFT operation. The method has been used to analyze transducer performance [6], to predict transducer beam patterns [7], and to image scattering objects [a], all in homogeneous media. For completeness, the basic derivation of the angular spectrum method is presented in Section 2. One of the underlying assumptions in the development of the angular spectrum method is that the medium is

Challenges and regulatory considerations in the acoustic measurement of high-frequency (>20 mhz) ultrasound

This article examines the challenges associated with making acoustic output measurements at high ultrasound frequencies (>20 MHz) in the context of regulatory considerations contained in the US Food and Drug Administration industry guidance document for diagnostic ultrasound devices. Error sources in the acoustic measurement, including hydrophone calibration and spatial averaging, nonlinear distortion, and mechanical alignment, are evaluated, and the limitations of currently available acoustic measurement instruments are discussed. An uncertainty analysis of acoustic intensity and power measurements is presented, and an example uncertainty calculation is done on a hypothetical 30‐MHz high‐frequency ultrasound system. This analysis concludes that the estimated measurement uncertainty of the acoustic intensity is +73%/−86%, and the uncertainty in the mechanical index is +37%/−43%. These values exceed the respective levels in the Food and Drug Administration guidance document of 30% and 15%, respectively, which are more representative of the measurement uncertainty associated with characterizing lower‐frequency ultrasound systems. Recommendations made for minimizing the measurement uncertainty include implementing a mechanical positioning system that has sufficient repeatability and precision, reconstructing the time‐pressure waveform via deconvolution using the hydrophone frequency response, and correcting for hydrophone spatial averaging.

Development of a high intensity focused ultrasound (HIFU) hydrophone system

In the past few years, High Intensity Focused Ultrasound (HIFU) has developed from a scientific curiosity to an accepted therapeutic modality. Concomitant with HIFUs growing clinical use, there has been a need for reliable, economical and reproducible measurements of HIFU acoustic fields. A number of approaches have been proposed and investigated, most notably by Kaczkowski et al (Proc. 2003 IEEE Ultrasonics Symposium, 982-985). We are developing a similar reflective scatterer approach, incorporating several novel features which improve the hydrophones bandwidth, reliability, and reproducibility. For the scattering element, we have used a fused silica optical fiber with a polyamide protective coating. The fused silica core is 73 microns in diameter with a 5 micron thick polyamide coating for a total diameter of 83 microns. The fiber was prepared by cleaving to yield a perpendicular/flat cut. The fiber is maintained in position using a capillary tube arrangement which provides structural rigidity with minimal acoustic interference. The receiver is designed as a segmented, truncated spherical structure with a 10cm radius; the scattering element is positioned at the center of the sphere. Each segment is approximately 6.3 cm square. The receiver is made from 25 micron thick, biaxially stretched PVDF, with a Pt-Au electrode on the front surface. Each segment has its own high impedance, wideband preamplifier, and the signals from multiple segments are summed coherently. As an additional feature, the system is designed to pulse the PVDF elements so that the pulse-echo response can be used to align the fiber at the center. This is important when the need arises to change the fiber due to, for instance, cavitation damage. The hydrophone can also be designed with a membrane structure to allow the region around the scatterer to be filled with a fluid which suppresses cavitation. Initial tests of the system have demonstrated a receiver array sensitivity of -279 dB re 1 microVolt/Pa (before preamplification), with a scattering loss at the fiber of approximately 39dB, producing an effective sensitivity of -318 dB re 1 microVolt/Pa. The addition of the closely coupled wideband preamplifiers boosts the signal to a range which is sufficient for the measurement of HIFU transducers. The effective bandwidth of the system exceeds 15MHz, through careful design and the use of PVDF as a sensor material. In order to test the system, a HIFU transducer in the 4.0MHz frequency range was tested at low output settings using a conventional PVDF membrane hydrophone. The prototype system was then used to characterize the same HIFU transducer at full power. The results showed good correlation between waveforms and cross-axis beam measurements, taking into account the additional shock losses at higher output settings.

Ultrasound for defect detection and grading in wood and lumber

Ultrasonic inspection of solid wood and lumber has been investigated for over 20 years, with limited commercial impact. Potential applications run from logs to finished boards, with the goal of either characterizing internal defects (knots, splits, rot), or assessing overall strength. This paper reviews recent efforts to apply ultrasound technology to the Forest Products industry.

Self-monitoring shock wave hydrophone

The present invention relates to a hydrophone specifically designed for use in high pressure shock wave fields, comprising a thin piezoelectric polymer film secured to a rigid hoop structure, having a centrally located active element and two conducting leads extending from the active element on each side of the film. Under the action of high pressure shock waves, the conductive material which makes up the active area electrode and the conductive leads is slowly removed, altering the hydrophones sensitivity and eventually rendering it unusable. The present invention provides an improved design for a hydrophone which monitors the loss of electrode and lead integrity due to shock wave action so that the hydrophone may be replaced before it produces invalid readings. The leads on each side of the centrally located active element are electrically switched to measure the resistance between the leads and the central portion. In another embodiment, the film may be a disposable item allowing for rapid replacement once damaged.