Peter A. Lewin

Pulse elongation and deconvolution filtering for medical ultrasonic imaging

Range sidelobe artifacts which are associated with pulse compression methods can be reduced with a new method composed of pulse elongation and deconvolution (PED). While pulse compression and PED yield similar signal-to-noise ratio (SNR) improvements, PED inherently minimizes the range sidelobe artifacts. The deconvolution is implemented as a stabilized inverse filter. With proper selection of the excitation waveform an exact inverse filter can be implemented. The excitation waveform is optimized in a minimum mean square error (MMSE) sense. An analytical expression for the power spectrum of the optimal pulse is presented and several techniques to numerically optimize the excitation pulse are shown. The effects of PED are demonstrated in computer simulations as well as ultrasonic images.

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.

Application of time-delay spectrometry for calibration of ultrasonic transducers

Time-delay spectrometry (TDS) can conveniently be used for calibration and performance evaluation of piezoelectric electroacoustic transducers. The main emphasis of the work reported here is an experimental evaluation of the TDS technique. The TDS concept is introduced through a theoretical analysis. The experimental evaluation is carried out using specially designed measurement methods and instrumentation which uses a spectrum analyzer as the central analog signal processing unit. The optimal performance of the TDS measurement systems is analyzed in terms of relevant instrumentation parameters. The advantages and disadvantages of TDS, including practical performance limitations, are discussed, along with the measurement uncertainties of the method. It is shown that TDS in the frequence range covering both underwater acoustics and medical ultrasonics applications offers a viable alternative to other calibration techniques, such as those based on a gated burst measurement system.<<ETX>>

Calibration of ultrasonic hydrophone probes up to 100 MHz using time gating frequency analysis and finite amplitude waves.

A number of ultrasound imaging systems employs harmonic imaging to optimize the trade off between resolution and penetration depth and center frequencies as high as 15 MHz are now used in clinical practice. However, currently available measurement tools are not fully adequate to characterize the acoustic output of such nonlinear systems primarily due to the limited knowledge of the frequency responses beyond 20 MHz of the available piezoelectric hydrophone probes. In addition, ultrasound hydrophone probes need to be calibrated to eight times the center frequency of the imaging transducer. Time delay spectrometry (TDS) is capable of providing transduction factor of the probes beyond 20 MHz, however its use is in practice limited to 40 MHz. This paper describes a novel approach termed time gating frequency analysis (TGFA) that provides the transduction factor of the hydrophone probes in the frequency domain and significantly extends the quasi-continuous calibration of the probes up to 60 MHz. The verification of the TGFA data was performed using TDS calibration technique (up to 40 MHz) and a nonlinear calibration method (up to 100 MHz). The nonlinear technique was based on a novel wave propagation model capable of predicting the true pressure-time waveforms at virtually any point in the field. The spatial averaging effects introduced by the finite aperture hydrophones were also accounted for. TGFA calibration results were obtained for different PVDF probes, including needle and membrane designs with nominal diameters from 50 to 500 micro m. The results were compared with discrete calibration data obtained from an independent national laboratory and the overall uncertainty was determined to be +/-1.5 dB in the frequency range 40-60 MHz and less than +/-1 dB below 40 MHz.

Bursting Bubbles and Bilayers

This paper discusses various interactions between ultrasound, phospholipid monolayer-coated gas bubbles, phospholipid bilayer vesicles, and cells. The paper begins with a review of microbubble physics models, developed to describe microbubble dynamic behavior in the presence of ultrasound, and follows this with a discussion of how such models can be used to predict inertial cavitation profiles. Predicted sensitivities of inertial cavitation to changes in the values of membrane properties, including surface tension, surface dilatational viscosity, and area expansion modulus, indicate that area expansion modulus exerts the greatest relative influence on inertial cavitation. Accordingly, the theoretical dependence of area expansion modulus on chemical composition - in particular, poly (ethylene glyclol) (PEG) - is reviewed, and predictions of inertial cavitation for different PEG molecular weights and compositions are compared with experiment. Noteworthy is the predicted dependence, or lack thereof, of inertial cavitation on PEG molecular weight and mole fraction. Specifically, inertial cavitation is predicted to be independent of PEG molecular weight and mole fraction in the so-called mushroom regime. In the “brush” regime, however, inertial cavitation is predicted to increase with PEG mole fraction but to decrease (to the inverse 3/5 power) with PEG molecular weight. While excellent agreement between experiment and theory can be achieved, it is shown that the calculated inertial cavitation profiles depend strongly on the criterion used to predict inertial cavitation. This is followed by a discussion of nesting microbubbles inside the aqueous core of microcapsules and how this significantly increases the inertial cavitation threshold. Nesting thus offers a means for avoiding unwanted inertial cavitation and cell death during imaging and other applications such as sonoporation. A review of putative sonoporation mechanisms is then presented, including those involving microbubbles to deliver cargo into a cell, and those - not necessarily involving microubbles - to release cargo from a phospholipid vesicle (or reverse sonoporation). It is shown that the rate of (reverse) sonoporation from liposomes correlates with phospholipid bilayer phase behavior, liquid-disordered phases giving appreciably faster release than liquid-ordered phases. Moreover, liquid-disordered phases exhibit evidence of two release mechanisms, which are described well mathematically by enhanced diffusion (possibly via dilation of membrane phospholipids) and irreversible membrane disruption, whereas liquid-ordered phases are described by a single mechanism, which has yet to be positively identified. The ability to tune release kinetics with bilayer composition makes reverse sonoporation of phospholipid vesicles a promising methodology for controlled drug delivery. Moreover, nesting of microbubbles inside vesicles constitutes a truly “theranostic” vehicle, one that can be used for both long-lasting, safe imaging and for controlled drug delivery.

Hydrophone spatial averaging corrections from 1 to 40 MHz

The purpose of this study was to develop and experimentally verify a practical spatial averaging model for frequencies up to 40 MHz. The model is applicable to focused sources of circular geometry, accounts for the effects of hydrophone probe finite aperture, and allows calibration by substitution to be performed when the active elements of reference and tested hydrophone probes differ significantly. Several broadband sources with focal numbers between 3 and 20 were used to produce ultrasound fields with frequencies up to 40 MHz. The effective diameters of the ultrasonic hydrophone probes calibrated in the focal plane of the sources ranged from 150 to 500 /spl mu/m. Prior to application of the spatial averaging corrections, the hydrophones with diameters smaller than that of the reference hydrophone exhibited experimentally determined absolute sensitivities higher than the true ones. This discrepancy increased with decreasing focal numbers and increasing frequency. It was determined that the error was governed by the cross-section of the beam in the focal plane and the ratio of the effective diameters of the reference and tested hydrophone probes. In addition, the error was found to be reliant on the frequency-dependent effective hydrophone radius. After applying the spatial averaging correction, the overall uncertainty in the hydrophone calibration was on the order of /spl plusmn/1 dB. The model developed is being extended to be applicable to frequencies beyond 40 MHz, which are becoming increasingly important in diagnostic ultrasound imaging applications.

Development of calibration techniques for ultrasonic hydrophone probes in the frequency range from 1 to 100 MHz

The primary objective of this work was to develop and optimize the calibration techniques for ultrasonic hydrophone probes used in acoustic field measurements up to 100 MHz. A dependable, 100 MHz calibration method was necessary to examine the behavior of a sub-millimeter spatial resolution fiber optic (FO) sensor and assess the need for such a sensor as an alternative tool for high frequency characterization of ultrasound fields. Also, it was of interest to investigate the feasibility of using FO probes in high intensity fields such as those employed in HIFU (high intensity focused ultrasound) applications. In addition to the development and validation of a novel, 100 MHz calibration technique the innovative elements of this research include implementation and testing of a prototype FO sensor with an active diameter of about 10 microm that exhibits uniform sensitivity over the considered frequency range and does not require any spatial averaging corrections up to about 75 MHz. The results of the calibration measurements are presented and it is shown that the optimized calibration technique allows the sensitivity of the hydrophone probes to be determined as a virtually continuous function of frequency and is also well suited to verify the uniformity of the FO sensor frequency response. As anticipated, the overall uncertainty of the calibration was dependent on frequency and determined to be about +/-12% (+/-1 dB) up to 40 MHz, +/-20% (+/-1.5 dB) from 40 to 60 MHz and +/-25% (+/-2dB) from 60 to 100 MHz. The outcome of this research indicates that once fully developed and calibrated, the combined acousto-optic system will constitute a universal reference tool in the wide, 100 MHz bandwidth.

Frequency response of PVDF needle-type hydrophones

This paper examines the factors governing the frequency response of ultrasonic polyvinylidene fluoride (PVDF) polymer needle-type hydrophones, in particular the sensitivity variations in the lower frequency range of 1-6 MHz. A theoretical model was used to analyze the influence of the hydrophones diameter, the metal electrodes, thickness, PVDF material properties, the adhesive layer acoustical characteristics and the backing material, on the frequency response of the hydrophone. The results of the theoretical modelling differ by less than +/- 0.5 dB from those obtained experimentally from the reciprocity calibration in the frequency range 1-20 MHz. It is shown that the needle hydrophones diameter and backing material are the main reasons for the sensitivity variations observed in the frequency range below 6 MHz.

Estimation of ultrasonic attenuation in a bone using coded excitation

This paper describes a novel approach to estimate broadband ultrasound attenuation (BUA) in a bone structure in human in vivo using coded excitation. BUA is an accepted indicator for assessment of osteoporosis. In the tested approach a coded acoustic signal is emitted and then the received echoes are compressed into brief, high amplitude pulses making use of matched filters and correlation receivers. In this way the acoustic peak pressure amplitude probing the tissue can be markedly decreased whereas the average transmitted intensity increases proportionally to the length of the code. This paper examines the properties of three different transmission schemes, based on Barker code, chirp and Golay code. The system designed is capable of generating 16 bits complementary Golay code (CGC), linear frequency modulated (LFM) chirp and 13-bit Barker code (BC) at 0.5 and 1 MHz center frequencies. Both in vivo data acquired from healthy heel bones and in vitro data obtained from human calcaneus were examined and the comparison between the results using coded excitation and two cycles sine burst is presented. It is shown that CGC system allows the effective range of frequencies employed in the measurement of broadband acoustic energy attenuation in the trabecular bone to be doubled in comparison to the standard 0.5 MHz pulse transmission. The algorithm used to calculate the pairs of Golay sequences of the different length, which provide the temporal side-lobe cancellation is also presented. Current efforts are focused on adapting the system developed for operation in pulse-echo mode; this would allow examination and diagnosis of bones with limited access such as hip bone.

Wide-band piezoelectric polymer acoustic sources

The design of a wideband acoustic source made of the piezoelectric polymer polyvinylidine fluoride (PVDF) is described. The source was developed for the characterization and absolute calibration of ultrasonic hydrophone probes. Construction details are described and performance characteristics of the wideband PVDF transmitter, including its transmitting voltage response and directivity patterns, are compared with theoretical predictions in the frequency range up to 40 MHz. The Krimholtz-Leedom-Mattaei (KLM) model was used to examine the influence of the PVDF polymer film thickness, the backing acoustic impedance, the cable length, and the electrical source resistance on overall transmit transfer characteristics. A comparison is made with traditional piezoelectric ceramic acoustic sources, and it is shown that piezopolymer transmitters exhibit some improved properties and are well suited for certain ultrasound dosimetry applications. In particular, the polymer sources have been found useful in measurements based on swept-frequency excitation. Those measurements allow characterization of transmitters and receivers to be performed as a virtually continuous function of frequency.<<ETX>>