Modified multi-element synthetic transmit aperture method for ultrasound imaging: A tissue phantom study

The paper presents the modified multi-element synthetic transmit aperture (MSTA) method for ultrasound imaging. It is based on coherent summation of RF echo signals with apodization weights taking into account the finite size of the transmit subaperture and of the receive element. The work presents extension of the previous study where the modified synthetic transmit aperture (STA) method was considered and verified [1]. In the case of MSTA algorithm the apodization weights were calculated for each imaging point and all combinations of the transmit subaperture and receive element using their angular directivity functions (ADFs). The ADFs were obtained from the exact solution of the corresponding mixed boundary-value problem for periodic baffle system modeling the transducer array. Performance of the developed method was tested using Field II simulated synthetic aperture data of point reflectors for 4MHz 128-element transducer array with 0.3mm pitch and 0.02mm kerf to estimate the visualization depth and lateral resolution. Also experimentally determined data of the tissue-mimicking phantom (Dansk Fantom Service, model 571) obtained using 128 elements, 4MHz, linear transducer array (model L14-5/38) and Ultrasonix SonixTOUCH Research platform were used for qualitative assessment of imaging contrast improvement. Comparison of the results obtained by the modified and conventional MSTA algorithms indicated 15dB improvement of the noise reduction in the vicinity of transducer's surface (1mm depth), and concurrent increase in the visualization depth (86% augment of the scattered amplitude at the depth of 90mm). However, this increase was achieved at the expense of minor degradation of the lateral resolution of approximately 8% at the depth of 50mm and 5% at the depth of 90mm.

Ultrasonically assisted evaluation of the impact of atherosclerotic plaque on the pulse pressure wave propagation: a clinical feasibility study

The purpose of this work was to evaluate ultrasound modality as a non-invasive tool for determination of impact of the degree of the atherosclerotic plaque located in human internal carotid arteries on the values of the parameters of the pulse wave. Specifically, the applicability of the method to such arteries as brachial, common, and internal carotid was examined. The method developed is based on analysis of two characteristic parameters: the value of the mean reflection coefficient modulus |Γ|(a) of the blood pressure wave and time delay Δt between the forward (travelling) and backward (reflected) blood pressure waves. The blood pressure wave was determined from ultrasound measurements of the artery's inner (internal) diameter, using the custom made wall tracking system (WTS) operating at 6.75 MHz. Clinical data were obtained from the carotid arteries measurements of 70 human subjects. These included the control group of 30 healthy individuals along with the patients diagnosed with the stenosis of the internal carotid artery (ICA) ranging from 20% to 99% or with the ICA occlusion. The results indicate that with increasing level of stenosis of the ICA the value of the mean reflection coefficient measured in the common carotid artery, significantly increases from |Γ|(a)=0.45 for healthy individuals to |Γ|(a)=0.61 for patients with stenosis level of 90-99%, or ICA occlusion. Similarly, the time delay Δt decreases from 52 ms to 25 ms for the respective groups. The method described holds promise that it might be clinically useful as a non-invasive tool for localization of distal severe artery narrowing, which can assist in identifying early stages of atherosclerosis especially in regions, which are inaccessible for the ultrasound probe (e.g. carotid sinus or middle cerebral artery).

Modified synthetic transmit aperture algorithm for ultrasound imaging

The modified synthetic transmit aperture (STA) algorithm is described. The primary goal of this work was to assess the possibility to improve the image quality achievable using synthetic aperture (SA) approach and to evaluate the performance and the clinical applicability of the modified algorithm using phantoms. The modified algorithm is based on the coherent summation of back-scattered RF echo signals with weights calculated for each point in the image and for all possible combinations of the transmit-receive pairs. The weights are calculated using the angular directivity functions of the transmit-receive elements, which are approximated by a far-field radiation pattern of a narrow strip transducer element vibrating with uniform pressure amplitude over its width. In this way, the algorithm takes into account the finite aperture of each individual element in the imaging transducer array. The performance of the approach developed was tested using FIELD II simulated synthetic aperture data of the point reflectors, which allowed the visualization (penetration) depth and lateral resolution to be estimated. Also, both simulated and measured data of cyst phantom were used for qualitative assessment of the imaging contrast improvement. The experimental data were obtained using 128 elements, 4MHz, linear transducer array of the Ultrasonix research platform. The comparison of the results obtained using the modified and conventional (unweighted) STA algorithms revealed that the modified STA exhibited an increase in the penetration depth accompanied by a minor, yet discernible upon the closer examination, degradation in lateral resolution, mainly in the proximity of the transducer aperture. Overall, however, a considerable (12dB) improvement in the image quality, particularly in the immediate vicinity of the transducer's surface was demonstrated. The modified STA method holds promise to be of clinical importance, especially in the applications where the quality of the "near-field" image, that is the image in the immediate vicinity of the scanhead is of critical importance such as for instance in skin- and breast-examinations.

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.

Influence of the ultrasound transducer bandwidth on selection of the complementary Golay bit code length

In contrast to previously published papers [A. Nowicki, Z. Klimonda, M. Lewandowski, J. Litniewski, P.A. Lewin, I. Trots, Comparison of sound fields generated by different coded excitations - Experimental results, Ultrasonics 44 (1) (2006) 121-129; J. Litniewski, A. Nowicki, Z. Klimonda, M. Lewandowski, Sound fields for coded excitations in water and tissue: experimental approach, Ultrasound Med. Biol. 33 (4) (2007) 601-607], which examined the factors influencing the spatial resolution of coded complementary Golay sequences (CGS), this paper investigates the effect of ultrasound imaging transducer's fractional bandwidth on the gain of the compressed echo signal for different spectral widths of the CGS. Two different bit lengths were considered, specifically one and two cycles. Three transducers having fractional bandwidth of 25%, 58% and 80% and operating at frequencies 6, 4.4 and 6 MHz, respectively were examined (one of the 6 MHz sources was focused and made of composite material). The experimental results have shown that by increasing the code length, i.e. decreasing the bandwidth, the compressed echo amplitude could be enhanced. The smaller the bandwidth was the larger was the gain; the pulse-echo sensitivity of the echo amplitude increased by 1.88, 1.62 and 1.47, for 25%, 58% and 80% bandwidths, respectively. These results indicate that two cycles bit length excitation is more suitable for use with bandwidth limited commercially available imaging transducers. Further, the time resolution is retained for transducers with two cycles excitation providing the fractional bandwidth is lower than approximately 90%. The results of this work also show that adjusting the code length allows signal-to-noise-ratio (SNR) to be enhanced while using limited (less that 80%) bandwidth imaging transducers. Also, for such bandwidth limited transducers two cycles excitation would not decrease the time resolution, obtained with "conventional" spike excitation. Hence, CGS excitation could be successfully implemented with the existing, relatively narrow band imaging transducers without the need to use usually more expensive wideband, composite ones.

Wave envelopes method for description of nonlinear acoustic wave propagation

A novel, free from paraxial approximation and computationally efficient numerical algorithm capable of predicting 4D acoustic fields in lossy and nonlinear media from arbitrary shaped sources (relevant to probes used in medical ultrasonic imaging and therapeutic systems) is described. The new WE (wave envelopes) approach to nonlinear propagation modeling is based on the solution of the second order nonlinear differential wave equation reported in [J. Wójcik, J. Acoust. Soc. Am. 104 (1998) 2654-2663; V.P. Kuznetsov, Akust. Zh. 16 (1970) 548-553]. An incremental stepping scheme allows for forward wave propagation. The operator-splitting method accounts independently for the effects of full diffraction, absorption and nonlinear interactions of harmonics. The WE method represents the propagating pulsed acoustic wave as a superposition of wavelet-like sinusoidal pulses with carrier frequencies being the harmonics of the boundary tone burst disturbance. The model is valid for lossy media, arbitrarily shaped plane and focused sources, accounts for the effects of diffraction and can be applied to continuous as well as to pulsed waves. Depending on the source geometry, level of nonlinearity and frequency bandwidth, in comparison with the conventional approach the Time-Averaged Wave Envelopes (TAWE) method shortens computational time of the full 4D nonlinear field calculation by at least an order of magnitude; thus, predictions of nonlinear beam propagation from complex sources (such as phased arrays) can be available within 30-60 min using only a standard PC. The approximate ratio between the computational time costs obtained by using the TAWE method and the conventional approach in calculations of the nonlinear interactions is proportional to 1/N2, and in memory consumption to 1/N where N is the average bandwidth of the individual wavelets. Numerical computations comparing the spatial field distributions obtained by using both the TAWE method and the conventional approach (based on a Fourier series representation of the propagating wave) are given for circular source geometry, which represents the most challenging case from the computational time point of view. For two cases, short (2 cycle) and long (8 cycle) 2 MHz bursts, the computational times were 10 min and 15 min versus 2 h and 8 h for the TAWE method versus the conventional method, respectively.

Comparison of sound fields generated by different coded excitations--experimental results

This work reports the results of measurements of spatial distributions of ultrasound fields obtained from five energizing schemes. Three different codes, namely, chirp signal and two sinusoidal sequences were investigated. The sequences were phase modulated with 13 bits Barker code and 16 bits Golay complementary codes. Moreover, two reference signals generated as two and sixteen cycle sine tone bursts were examined. Planar, 50% (fractional) bandwidth, 15 mm diameter source transducer operating at 2 MHz center frequency was used in all measurements. The experimental data were collected using computerized scanning system and recorded using wideband, PVDF membrane hydrophone (Sonora 804). The measured echoes were compressed, so the complete pressure field in the investigated location before and after compression could be compared. In addition to a priori anticipated increase in the signal to noise ratio (SNR) for the decoded pressure fields, the results indicated differences in the pressure amplitude levels, directivity patterns, and the axial distance at which the maximum pressure amplitude was recorded. It was found that the directivity patterns of non-compressed fields exhibited shapes similar to the patterns characteristic for sinusoidal excitation having relatively long time duration. In contrast, the patterns corresponding to compressed fields resembled those produced by brief, wideband pulses. This was particularly visible in the case of binary sequences. The location of the maximum pressure amplitude measured in the 2 MHz field shifted towards the source by 15 mm and 25 mm for Barker code and Golay code, respectively. The results of this work may be applicable in the development of new coded excitation schemes. They could also be helpful in optimizing the design of imaging transducers employed in ultrasound systems designed for coded excitation. Finally, they could shed additional light on the relationship between the spatial field distribution and achievable image quality and in this way facilitate optimization of the images obtained using coded systems.

Acousto-optic, point receiver hydrophone probe for operation up to 100 MHz

This work describes the results of initial evaluation of a wideband acousto-optic hydrophone probe designed to operate as point receiver in the frequency range up to 100 MHz. The hydrophone was implemented as a tapered fiber optic (FO) probe sensor with a tip diameter of approximately 7 microm. Such small physical dimensions of the sensor eliminate the need for spatial averaging corrections so that true pressure-time (p-t) waveforms can be faithfully recorded. The theoretical considerations that predicted the FO probe sensitivity to be equal to 4.3 mV/MPa are presented along with a brief description of the manufacturing process. The calibration results that verified the theoretically predicted sensitivity are also presented along with a brief description of the improvements being currently implemented to increase this sensitivity level by approximately 20 dB. The results of preliminary measurements indicate that the fiber optic probes will exhibit a uniform frequency response and a zero phase shift in the frequency range considered. These features might be very useful in rapid complex calibration i.e. determining both magnitude and phase response of other hydrophones by the substitution method. Also, because of their robust design and linearity, these fiber optic hydrophones could also meet the challenges posed by high intensity focused ultrasound (HIFU) and other therapeutic applications. Overall, the outcome of this work shows that when fully developed, the FO probes will be well suited for high frequency measurements of ultrasound fields and will be able to complement the data collected by the current finite aperture piezoelectric PVDF hydrophones.

The influence of finite aperture and frequency response of ultrasonic hydrophone probes on the determination of acoustic output

The influence of finite aperture and frequency response of piezoelectric ultrasonic hydrophone probes on the Thermal and Mechanical Indices was investigated using a comprehensive acoustic wave propagation model. The experimental verification of the model was obtained using a commercially available, 8 MHz, dynamically focused linear array and a single element, 5 MHz, focused rectangular source. The pressure-time waveforms were recorded using piezoelectric polymer hydrophone probes of different active element diameters and bandwidths. The nominal diameters of the probes ranged from 50 to 500 microm and their usable bandwidths varied between 55 and 100 MHz. The Pulse Intensity Integral (PII), used to calculate the Thermal Index (TI), was found to increase with increasing bandwidth and decreasing effective aperture of the probes. The Mechanical Index (MI), another safety indicator, was also affected, but to a lesser extent. The corrections needed were predicted using the model and successfully reduced the discrepancy as large as 30% in the determination of PII. The results of this work indicate that by accounting for hydrophones' finite aperture and correcting the value of PII, all intensities derived from the PII can be corrected for spatial averaging error. The results also point out that a caution should be exercised when comparing acoustic output data. In particular, hydrophone's frequency characteristics of the effective diameter and sensitivity are needed to correctly determine the MI, TI, and the total acoustic output power produced by an imaging transducer.

Hydrophones' effective diameter measurements as a quasi-continuous function of frequency

The spatial averaging effect is strongly dependent on the active aperture of the hydrophone probes used to measure ultrasound fields. An experimental method was developed to determine the effective diameter of the probes as a quasi-continuous function of frequency. The implementation of the method utilizes the time delay spectrometry (TDS) technique and a set of focused acoustic sources. The use of focused sources ensured plane wave conditions for the whole frequency range and TDS eliminated all the reflections from the water tank boundaries. This approach allows effective diameter of circular aperture hydrophones to be determined as a quasi-continuous function of frequency up to 40 MHz. The measurements were performed for both needles and membrane designs having nominal diameters ranging from 50 to 500 microm. The results were successfully employed in the development of spatial averaging correction algorithms. Current efforts are being focused on extension of the frequency range up to 60 MHz by using a novel measurement technique termed time gating frequency analysis.

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.

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.

Nonlinear propagation model for ultrasound hydrophones calibration in the frequency range up to 100 MHz

To facilitate the implementation and verification of the new ultrasound hydrophone calibration techniques described in the companion paper (somewhere in this issue) a nonlinear propagation model was developed. A brief outline of the theoretical considerations is presented and the model's advantages and disadvantages are discussed. The results of simulations yielding spatial and temporal acoustic pressure amplitude are also presented and compared with those obtained using KZK and Field II models. Excellent agreement between all models is evidenced. The applicability of the model in discrete wideband calibration of hydrophones is documented in the companion paper somewhere in this volume.

1-60 MHz measurements in focused acoustic fields using spatial averaging corrections

The purpose of this research was to develop, implement and verify a measurement technique enabling rapid and dependable characterization of ultrasound hydrophone probes beyond 20 MHz. The technique employs focused acoustic sources to optimize signal-to-noise ratio and spatial averaging correction model to account for the finite aperture of the hydrophone probes. To minimize calibration time, substitution technique was chosen and its applicability was tested up to 60 MHz. The overall uncertainty of the measurements was on the order of +/- 1 dB. The results are presented for both needle and membrane type PVDF hydrophones having effective diameters ranging from 130-1200 microns. The fundamental limitations of the technique were determined and it is shown that the spatial averaging error is 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. The technique developed is being extended to frequencies beyond 60 MHz.

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 microm. 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 +/-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.

Sensitivity of ultrasonic hydrophone probes below 1 MHz

Frequency responses of different PVDF polymer hydrophone probes, including membrane and needle designs, were measured and are presented in terms of end-of-cable voltage sensitivity versus frequency over a wide, 4.5 octave bandwidth ranging from 0.25 to 2.5 MHz. The probes are seldom, if at all, characterized in this frequency range due to the difficulties associated with a lack of adequate and readily implementable calibration techniques. To this end, a technique, which uses a combination of swept frequency chirp and reciprocity, so that both the relative and absolute plots of sensitivity versus frequency can be obtained, was developed and tested. Salient features of the technique including the design of a 6 octave auxiliary acoustic source are described. The experimental data indicate that a majority of the PVDF membrane hydrophones exhibit a relatively uniform (to within +/- 2 dB) response. While, in general, this is not the case for commercially available needle hydrophone probes, it is evidenced that a careful attention to the PVDF probe design results in frequency characteristics fairly close to those achievable with a membrane design. The overall uncertainty of the calibration technique was estimated to be better than +/- 1.5 dB in the considered frequency range. The results of this work are important to implement procedures for adequate determination of the Mechanical Index (MI) of ultrasound imaging devices. MI is widely accepted as a predictor of potential bioeffects associated with cavitation phenomena. Current efforts are focused on extending the applicability of the technique to frequencies below 100 kHz.

A novel method for determining calibration and behavior of PVDF ultrasonic hydrophone probes in the frequency range up to 100 MHz

A new calibration technique for PVDF ultrasonic hydrophone probes is described. Current implementation of the technique allows determination of hydrophone frequency response between 2 and 100 MHz and is based on the comparison of theoretically predicted and experimentally determined pressure-time waveforms produced by a focused, circular source. The simulation model was derived from the time domain algorithm that solves the non linear KZK (Khokhlov-Zabolotskaya-Kuznetsov) equation describing acoustic wave propagation. The calibration technique data were experimentally verified using independent calibration procedures in the frequency range from 2 to 40 MHz using a combined time delay spectrometry and reciprocity approach or calibration data provided by the National Physical Laboratory (NPL), UK. The results of verification indicated good agreement between the results obtained using KZK and the above-mentioned independent calibration techniques from 2 to 40 MHz, with the maximum discrepancy of 18% at 30 MHz. The frequency responses obtained using different hydrophone designs, including several membrane and needle probes, are presented, and it is shown that the technique developed provides a desirable tool for independent verification of primary calibration techniques such as those based on optical interferometry. Fundamental limitations of the presented calibration method are also examined.

Experimental study of the acoustical properties of polymers utilized to construct PVDF ultrasonic transducers and the acousto-electric properties of PVDF and P(VDF/TrFE) films

Several acoustic transmission and reflection technique measurements were carried out to determine mechanical properties (acoustic attenuation and velocity) versus frequency of polyvinylidene-fluoride (PVDF) and six other polymers. Acoustic measurements (0.5 to 12 MHz) included time-delay spectrometry (TDS; in which separate transmitting and receiving transducers utilize a swept frequency signal) and two pulse-echo methods (short tone burst echoes utilizing transducers with different center frequencies and Fourier analysis of echoes sent and received by damped transducers operating in the broadband pulse mode). Electrical impedance measurements of piezoelectric thin films of PVDF and P(VDF/TrFE) yielded comparable high frequency mechanical parameters. Of the seven acoustically examined polymers, PVDF had the greatest acoustic impedance, lowest acoustic velocity, and greatest mechanical loss (13.4 dB/cm per MHz). Polymethyl-methacrylate (PMMA; lucite) and polydimethyl-pentane (TPX) had the lowest loss. PMMA had the highest acoustic velocity, and TPX had the lowest acoustic impedance and a velocity almost identical to that of PVDF. These data are useful in the design of backing, matching, and lens materials to be used in association with PVDF transducers.

Wideband spherically focused PVDF acoustic sources for calibration of ultrasound hydrophone probes

Several broadband sources have been developed for the purpose of calibrating hydrophones. The specific configuration described is intended for the calibration of hydrophones In a frequency range of 1 to 40 MHz. All devices used 25 /spl mu/m film of PVDF bonded to a matched backing. Two had radii of curvatures (ROC) of 25.4 and 127 mm with f numbers of 3.8 and 19, respectively. Their active element diameter was 0.28 in (6.60 mm). The active diameter of the third source used was 25 mm, and it had an ROC of 254 mm and an f number of 10. The use of a focused element minimized frequency-dependent diffraction effects, resulting in a smooth variation of acoustic pressure at the focus from 1 to 40 MHz. Also, using a focused PVDF source permitted calibrations above 20 MHz without resorting to harmonic generation via nonlinear propagation.

Voltage sensitivity response of ultrasonic hydrophones in the frequency range 0.25-2.5 MHz

Frequency responses of different PVDF polymer hydrophones, including membrane and needle designs, were measured and are presented in terms of end-of-cable voltage sensitivity vs. frequency over a wide, 4.5-octave bandwidth ranging from 0.25-2.5 MHz. The experimental data indicate that the membrane PVDF hydrophones can exhibit uniform, to within +/- 0.75 dB, responses. However, a widely used bilaminar membrane hydrophone-preamplifier combination may display sensitivity variations of +/- 2 dB. Also, even well-designed needle-type hydrophones show a more distinct sensitivity variation below 1 MHz that is on the order of 3-4 dB. The overall uncertainty of the calibration technique was estimated to be better than +/- 2 dB in the frequency range considered. The technique, which uses a combination of swept frequency chirp and reciprocity so that both the relative and absolute plots of sensitivity vs. frequency can be obtained, is also briefly described. The results of this work are important to implement procedures for adequate determination of the mechanical index of ultrasound (US) imaging devices. Mechanical index is widely accepted as a predictor of potential bioeffects associated with cavitation phenomena. Also, absolute calibration data are essential in development of therapeutic procedures based on the use of high-intensity focused ultrasound (HIFU), and in characterization of conventional therapeutic US applicators operating at frequencies below 1 MHz.

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.

Effects of prenatal ultrasound exposure on adult offspring behavior in the Wistar rat

An ultrasound exposure tank was specifically designed for experimental bioeffects studies. Thirty-six pregnant rats were anesthetized, immersed to the axilla in a water tank, and exposed on Day 15, 17, and 19 of gestation. Twelve rats were exposed to 5.0 MHz pulsed ultrasound of effective pulse duration equal to approximately 0.170 microseconds, pulse repetition rate (PRF) 1 kHz, and a spatial peak, temporal peak intensity (lsptp) of 500 W/cm2, representing a clinically appropriate exposure level. The spatial peak pulse average (lsppa), spatial peak temporal average (lspta), and instantaneous maximum (lm) intensities were determined to be 100 W/cm2, 24 mW/cm2, and 230 W/cm2, respectively. The maximum rarefraction pressure, pr, was measured as 12.5 x 10(5) Pa, and the total power was 2.5 mW. Twelve other rats were exposed to 1500 W/cm2, lsptp, and 12 were sham insonified. Since the focal area was about 0.05 cm2, computer controlled stepper motors moved the rats through the ultrasound field to ensure uniform exposure of the abdominal/pelvic region. Total exposure time was 35 min. A miniature thermocouple was implanted in a few rats to verify that no significant temperature increase took place due to exposure. A total of 278 offspring were maintained until postnatal Day 60 when they were subjected to two of four behavioral tests in random order within sexes. The results indicate no consistently observed dose-related alterations in adult behavior due to prenatal fetal exposure to 5.0 MHz ultrasound below an intensity (lsptp) of 1500 W/cm2.

The effects of prenatal ultrasound exposure on postnatal growth and acquisition of reflexes

The examination of pregnant women using diagnostic ultrasound has increased greatly over past decades in the United States. As sonography techniques have been altered and refined, there has been renewed interest concerning possible effects on the developing fetus, since exposures in mid-gestation occur during the sensitive period of brain development. The present study is concerned with possible neonatal functional deficits due to exposure of the fetus to ultrasound. An ultrasound exposure tank was designed specifically for controlled studies of bioeffects. Thirty-six pregnant rats were anesthetized, immersed to the axilla in a water tank and exposed on the 15th, 17th and 19th days of gestation. Twelve rats were exposed to 5.0 MHz pulsed ultrasound of effective pulse duration equal to approximately 0.170 microseconds, pulse repetition rate 1 kHz, and a spatial peak, temporal peak intensity (ISPTP) of 500 W/cm2, representing a clinically relevant exposure level. The spatial peak, pulse average intensity (ISPPA), spatial peak temporal average intensity (ISPTA) and maximum intensity (Im) were determined to be 100 W/cm2, 24 mW/cm2 and 230 W/cm2, respectively. The maximum rarefaction pressure, pr, was measured as 12.5 x 10(5) Pa, and the total power was 2.5 mW. Twelve other rats were exposed to 1500 W/cm2, ISPTP (ISPPA, 350 W/cm2; ISPTA, 58 mW/cm2; Im, 600 W/cm2). Twelve additional rats were sham-exposed. Since the focal area was about 0.5 cm2, computer-controlled stepper motors moved the rats through the ultrasound field to assure uniform exposure of the abdominal/pelvic region. Total exposure time was 35 min. Additionally, a miniature thermocouple was implanted in a few rats to verify that no significant increase in body temperature took place during exposure. All neonates were subjected to five reflex tests and observed for four physiological parameters. Postnatal growth also was monitored. Analyses of the data indicate there were no significant alterations in neonatal development or postnatal growth due to exposure to 5.0 MHz ultrasound below an intensity (ISPTP) of 1500 W/cm2. Studies continue to be completed at higher exposure levels to determine the margin of safety, and the animals will continue to be monitored and evaluated through young adulthood to determine if there are long-term behavioral effects due to fetal exposure to ultrasound.

Characterization of ultrasonic transducers using a fiberoptic sensor

The PVDF (polyvinylidene fluoride) hydrophones, commonly used to measure the characteristics of ultrasonic transducers, suffer from a number of drawbacks. They disturb the field distribution to be measured and cause spatial averaging effects because of their finite aperture. In addition, they are very delicate and susceptible to damage. To overcome some of these problems, the authors previously proposed the use of an optical fiber-based probe to measure the ultrasonic fields. In this paper, this fiberoptic ultrasonic sensor is used to measure the characteristics of six transducers, focused as well as unfocused, covering a frequency range of 2.25 MHz to 20 MHz. Results obtained using the fiberoptic sensor are compared with those obtained using a calibrated PVDF needle hydrophone with an effective diameter of 0.5 mm. The temporal responses as well as the beam profiles of the transducers measured using the fiberoptic sensor show excellent agreement with the results obtained using the PVDF needle hydrophone.

Thermoacoustic sensor for ultrasound power measurements and ultrasonic equipment calibration

This paper describes a thermoacoustic sensor developed for measurements of the acoustic power and calibration of ultrasonic transducers in the medical imaging and nondestructive testing frequency range. It is shown that the equilibrium temperature produced by ultrasound absorption in an absorbing material and detected by a copper-constantan thermocouple is proportional to the square of the current applied to the acoustic source. It is also demonstrated that the simultaneous measurement of this current and the corresponding equilibrium temperature at a given frequency allow the transmitting current sensitivity of the acoustic source to be calculated. The sensor thus provides a useful and low-cost alternative to the expensive calibration methods such as those based on the reciprocity technique, the planar scanning technique and the radiation force balance. The principles of the sensor's operation are outlined and its construction and characteristics are described. Experimental data in the frequency range of 1-8 MHz are presented and the advantages and disadvantages of the sensor are discussed.

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.

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 hydrophone's 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 hydrophone's diameter and backing material are the main reasons for the sensitivity variations observed in the frequency range below 6 MHz.

Shock wave sensors: I. Requirements and design

In the last 9 years, extracorporeal shock wave lithotripsy has become one of the preferred procedures for the treatment of urinary and gallbladder calculi. While there is still uncertainty as to the mechanisms of stone fragmentation, current hypotheses suggest that acoustical shock wave parameters such as rise time, peak compressional and rarefactional pressure, and frequency content may all influence the treatment's efficiency. Thus, optimization of lithotripsy treatment needs pressure sensors that can adequately characterize the shock wave field. This article presents and discusses the design of reliable, wideband, quantitative shock wave sensors made of piezoelectric material. The development, design, and performance characteristics of the sensors are presented. Sensor construction details are described, as are the methods used to characterize the sensor's acoustical performance. The key acoustical parameters of the sensor, its frequency response, and directivity pattern are presented; theory indicates that the probes feature uniform sensitivity over the frequency range up to 100 MHz. Preliminary experimental results indicate that piezoelectric polymer sensors made of polyvinylidene fluoride (PVDF) with a low acoustical impedance backing are suitable for lithotripter field measurements. The applicability of sensors based on fiber optics to shock wave measurements is also briefly discussed. In a companion article, shock wave measurement techniques are outlined and selected lithotripter test data are presented.

A computerized system for measuring the acoustic output from diagnostic ultrasound equipment

The measurement arrangement, which complies with the requirements of the US Food and Drug Administration, consists of a positioning system with a full range of degrees-of-freedom and a digital oscilloscope, both under complete computer control. The acoustic pressure-time waveform is recorded using membrane-type and needle-type polyvinylidene fluoride (PVDF) hydrophone probes. The overall bandwidth of the system depends on the hydrophone probe used and can range up to 100 MHz. A complete description of the system and the measurement procedures is given, along with a brief discussion of the various factors which affect measurement uncertainty. The largest overall uncertainty of the same associated with acoustic intensity measurements was determined to be no greater than 20% for I(sppa) and 25% for I(spta) (spatial peak pulse average intensity and spatial-peak temporal-average intensity, respectively). Other applications of the system include transducer characterization and research work in ultrasound dosimetry and bioeffects.

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.

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.

Interlaboratory comparison of hydrophone calibrations

The results of an interlaboratory comparison of hydrophone calibration techniques in the frequency range 1-10 MHz are reported. Two membrane hydrophones were calculated to six laboratories, and each laboratory determined the end-of-cable loaded sensitivities using their normal calibration methods; these included optical interferometry, planar scanning, reciprocity combined with time-delay spectrometry, and suspended-sphere radiometry. After converting the results to end-of-cable open-circuit sensitivities, in most cases agreement between the various values was within +/-10% at all frequencies.

A comparison of two calibration methods for ultrasonic hydrophones

Although miniature ultrasonic hydrophones are frequently used to measure the acoustic pressure distributions from diagnostic ultrasound sources, relatively little attention has been devoted to the methods for absolute calibration of these hydrophones. In this study a polyvinylidene (PVDF) hydrophone was used to compare two calibration methods currently in use. One is based on a reciprocity technique and the second involves the planar scanning of a source transducer having a known radiated ultrasonic power. The reciprocity method revealed that the hydrophone response did not vary by more than +/- 1.6 dB from -262.8 dB re IV/microPa over the frequency range of 1-10 MHz. For the planar scanning technique seven ultrasound sources between 1-10 MHz were used, and all calibration points were within +/- 0.5 dB of the corresponding points found by the method of reciprocity.

Acoustic pressure amplitude thresholds for rectified diffusion in gaseous microbubbles in biological tissue

One of the mechanisms often suggested for the biological action of ultrasonic beams irradiating human tissues is concerned with the presence in the tissues of minute gaseous bubbles which may, under the influence of the ultrasonic field be stimulated to grow to a size a which resonance or collapse occurs with severe associated shear stresses. The evidence for the existence of microbubbles in tissues is reviewed. The results of calculations, using two existing theoretical models, of the peak pressure threshold as a function of frequency are presented. The frequency is normalized with the resonant frequency of the bubble, and results are presented for three bubble radii (1, 2, and 3.5 micrometer) and for different values of the gas concentration in the tissue between 0.1 and 1. The results from two models differ suggesting that an improved model and better experimental data for the threshold calculations would be appropriate for further calculations. The thresholds calculated range below the peak pressure amplitudes used in continuous wave diagnostic instruments, indicating the need for a more careful investigation both of this damage mechanism and of the exposures used in routine diagnosis. The results of calculations for typical (transient) exposure conditions from pulse-echo equipment are presented, indicating that rectified diffusion and stable cavitation are improbable phenomena in these circumstances.