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Abstract:
The purpose of this work was to present a new approach that allows the influence of cortical bone on noninvasive measurement of broadband ultrasound attenuation (BUA) to be corrected. The method, mplemented here at 1 MHz makes use of backscattered signal and once refined and clinically confirmed, it would offer an alternative to ionizing radiation based methods, such as DEXA (Dual-nergy X-ray absorptiometry), quantitative computed tomography (QCT), radiographic absorptiometry (RA) or single X-ray absorptiometry (SXA), which are clinically approved for assessment of progress of osteoporosis. In addition, as the method employs reflected waves, it might substantially enhance the applicability of BUA - from being suitable to peripheral bones only it would extend this applicability to include such embedded bones as hip and femoral neck. The proposed approach allows the cortical layer parameters used for correction and the corrected value and parameter of the ancellous bone (BUA) to be determined simultaneously from the single (pulse-echo) bone backscattered wave; to the best of the authors' knowledge such approach was not previously reported. The validity of the method was tested using acoustic data obtained from a custom- esigned bone-mimicking phantom and a calf femur. The relative error of the attenuation coefficient assessment was determined to be 3.9% and 4.7% for the bone phantom and calf bone specimens, respectively. When the cortical shell influence was not taken into account the corresponding errors were considerably higher 8.3% (artificial bone) and 9.2% (calf femur). As indicated above, once clinically proven, the use of this BUA measurement technique in reflection mode would augment diagnostic power of the attending physician by permitting to include bones, which are not accessible for transmission mode evaluation, e.g. hip, spine, humerus and femoral neck.

Keywords:
broadband ultrasound attenuation, correction of influence of cortical bone, trabecular bone

Abstract:
The purpose of this work was to determine the influence of standing waves and possible multiple reflections under the conditions often encountered in examining the effects of ultrasound exposure on the cell cultures in vitro. More specifically, the goal was to quantitatively ascertain the influence of ultrasound exposure under free field (FF) and standing waves (SW) and multiple reflections (MR) conditions (SWMR) on the biological endpoint (50% cell necrosis). Such information would help in designing the experiments, in which the geometry of the container with biological tissue may prevent FF conditions to be established and in which the ultrasound generated temperature elevation is undesirable. This goal was accomplished by performing systematic, side-by-side experiments in vitro with C6 rat glioma cancer cells using 12 well and 96 well plates. It was determined that to obtain 50% of cell viability using the 12 well plates, the spatial average, temporal average (ISATA) intensities of 0.32 W/cm2 and 5.89 W/cm2 were needed under SWMR and FF conditions, respectively. For 96 well plates the results were 0.80 W/cm2 and 2.86 W/cm2 respectively. The corresponding, hydrophone measured pRMS maximum pressure amplitude values, were 0.71 MPa, 0.75 MPa, 0.75 MPa and 0.73 MPa, respectively. These results suggest that pRMS pressure amplitude was independent of the measurement set-up geometry and hence could be used to predict the cells' mortality threshold under any in vitro experimental conditions or even as a starting point for (pre-clinical) in vivo tests. The described procedure of the hydrophone measurements of the pRMS maximum pressure amplitude at the k/2 distance (here 0.75 mm) from the cell's level at the bottom of the dish or plate provides the guideline allowing the difference between the FF and SWMR conditions to be determined in any experimental setup. The outcome of the measurements also indicates that SWMR exposure might be useful at any ultrasound assisted therapy experiments as it permits to reduce thermal effects. Although the results presented are valid for the experimental conditions used in this study they can be generalized. The analysis developed provides methodology facilitating independent laboratories to determine their specific ultrasound exposure parameters for a given biological end-point under standing waves and multiple reflections conditions. The analysis also permits verification of the outcome of the experiments mimicking pre- and clinical environment between different, unaffiliated teams of researchers.

Keywords:
Standing wave, Ultrasound pressure, Ultrasound intensity, C6 glioma, Anticancer therapy, Sonodynamic therapy, Ultrasound bio-effects

Abstract:
In therapeutic applications of High Intensity Focused Ultrasound (HIFU) the guidance of the HIFU beam and especially its focal plane is of crucial importance. This guidance is needed to appropriately target the focal plane and hence the whole focal volume inside the tumor tissue prior to thermo-ablative treatment and beginning of tissue necrosis. This is currently done using Magnetic Resonance Imaging that is relatively expensive. In this study an ultrasound method, which calculates the variations of speed of sound in the locally heated tissue volume by analyzing the phase shifts of echo-signals received by an ultrasound scanner from this very volume is presented. To improve spatial resolution of B-mode imaging and minimize the uncertainty of temperature estimation the acoustic signals were transmitted and received by 8 MHz linear phased array employing Synthetic Transmit Aperture (STA) technique. Initially, the validity of the algorithm developed was verified experimentally in a tissue-mimicking phantom heated from 20.6 to 48.6°C. Subsequently, the method was tested using a pork loin sample heated locally by a 2 MHz pulsed HIFU beam with focal intensity ISATA of 129 W/cm2. The temperature calibration of 2D maps of changes in the sound velocity induced by heating was performed by comparison of the algorithm-determined changes in the sound velocity with the temperatures measured by thermocouples located in the heated tissue volume. The method developed enabled ultrasound temperature imaging of the heated tissue volume from the very inception of heating with the contrast-to-noise ratio of 3.5–12 dB in the temperature range 21–56°C. Concurrently performed, conventional B-mode imaging revealed CNR close to zero dB until the temperature reached 50°C causing necrosis. The data presented suggest that the proposed method could offer an alternative to MRI-guided temperature imaging for prediction of the location and extent of the thermal lesion prior to applying the final HIFU treatment.

Keywords:
Ultrasonic temperature imaging, HIFU, Echo phase shift, Velocity image contrast

Abstract:
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 4 MHz 128-element transducer array with 0.3 mm pitch and 0.02 mm 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, 4 MHz, 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 15 dB improvement of the noise reduction in the vicinity of transducer’s surface (1 mm depth), and concurrent increase in the visualization depth (86% augment of the scattered amplitude at the depth of 90 mm). However, this increase was achieved at the expense of minor degradation of the lateral resolution of approximately 8% at the depth of 50 mm and 5% at the depth of 90 mm.

Keywords:
Synthetic aperture imaging, Ultrasound imaging, Directivity function, Beamforming

Abstract:
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, 4 MHz, 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 (12 dB) 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.

Keywords:
synthetic aperture imaging, ultrasound imaging, directivity function, beamforming

Abstract:
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).

Keywords:
Pulse wave, Ultrasound, Vascular impedance, Stenosis

Abstract:
This work addresses the difficulties in the measurements of the nonlinear medium parameter B/A and presents a modification of the finite amplitude method (FAM), one of the accepted procedures to determine this parameter. The modification is based on iterative, hybrid approach and entails the use of the versatile and comprehensive model to predict distortion of the pressure–time waveform and its subsequent comparison with the one experimentally determined. The measured p–t waveform contained at least 18 harmonics generated by 2.25 MHz, 29 mm effective diameter, single element, focused PZT source (f-number 3.5) and was recorded by Sonora membrane hydrophone calibrated in the frequency range 1– 40 MHz. The hydrophone was positioned coaxially at the distal end of the specially designed, two-section assembly comprising of one, fixed length (60 mm), water-filled cylindrical container and the second, variable length (60–120 mm) container that was filled with unknown medium. The details of the measurement chamber are described and the reasons for this specific design are analyzed. The data were collected with the variable length chamber filled with 1.3-butanediol, which was used as a close approximation of tissue mimicking phantom. The results obtained provide evidence that a novel combination of the FAM with the semi-empirical nonlinear propagation model based on the hyperbolic operator is capable of reducing the overall uncertainty of the B/A measurements as compared to those reported in the literature. The overall uncertainty of the method reported here was determined to be ±2%, which enhances the confidence in the numerical values of B/A measured for different, clinically relevant media. Optimization of the approach is also discussed and it is shown that it involves an iterative procedure that entails a careful selection of the acoustic source and its geometry and the axial distance over which the measurements need to be performed. The optimization also depends critically on the experimental determination of the source surface pressure amplitude.

Keywords:
pulsed finite-amplitude acoustic waves, nonlinear propagation, nonlinearity parameter B/A

Abstract:
The primary goal of this work was to verify experimentally the applicability of the recently introduced time-averaged wave envelope (TAWE) method as a tool for fast prediction of four dimensional (4D) pulsed nonlinear pressure fields from arbitrarily shaped acoustic sources in attenuating media. The experiments were performed in water at the fundamental frequency of 2.8 MHz for spherically focused (focal length F = 80 mm) square (20mm x 20 mm) and rectangular (10mm x 25 mm) sources similar to those used in the design of 1D linear arrays operating with ultrasonic imaging systems. The experimental results obtained with 10-cycle tone bursts at three different excitation levels corresponding to linear, moderately nonlinear and highly nonlinear propagation conditions (0.045, 0.225 and 0.45 MPa on-source pressure amplitude, respectively) were compared with those yielded using the TAWE approach. The comparison of the experimental results and numerical simulations has shown that the TAWE approach is well suited to predict (to within ± 1 dB) both, the spatial–temporal and spatial–spectral pressure variations in the pulsed nonlinear acoustic beams. The obtained results indicated that implementation of the TAWE approach enabled shortening of computation time in comparison with the time needed for prediction of the full 4D pulsed nonlinear acoustic fields using a conventional (Fourier-series) approach. The reduction in computation time depends on several parameters, including the source geometry, dimensions, fundamental resonance frequency, excitation level as well as the strength of the medium nonlinearity. For the non-axisymmetric focused transducers mentioned above and excited by a tone burst corresponding to moderately nonlinear and highly nonlinear conditions the execution time of computations was 3 and 12h, respectively, when using a 1.5 GHz clock frequency, 32-bit processor PC laptop with 2 GB RAM memory, only. Such prediction of the full 4D pulsed field is not possible when using conventional, Fourier-series scheme as it would require increasing the RAM memory by at least 2 orders of magnitude.

Keywords:
rectangular focused apertures, pulsed acoustic fields, nonlinear distortion, numerical modelling and experiments

Abstract:
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.

Keywords:
ultrasound imaging, transducer bandwidth, complementary Golay sequences

Keywords:
ultrasound exposure, therapcutic ultrasound, membraue pcrmeability, giant vesicles

Abstract:
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. Wojcik, 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 approximateratio 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/N)**2, 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.

Keywords:
Nonliear propagation, Envelope waves, Fast calculations

Abstract:
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.

Keywords:
coded excitation, sound fields

Abstract:
Objective:
The goal of this work was to develop a clinically applicable method for non-invasive acoustic determination of hematocrit in vivo.

Methods:
The value of hematocrit (HCT) was determined initially in vitro from the pulse-echo measurements of acoustic attenuation. The testing was carried out using a laboratory setup with ultrasound transducer operating at 20 MHz and employing human blood samples at the temperature of 37C. The attenuation coefficient measurements in blood in vitro and in vivo were implemented using multi-gated (128-gates), 20 MHz pulse Doppler flow meter. The Doppler signal was recorded in the brachial artery. Both in vitro and in vivo HCT data were compared with those obtained using widely accepted, conventional centrifuge method.

Results:
The attenuation coefficient in vitro was determined from the measurements of 168 samples with hematocrit varying between 23.9 and 51.6%. Those experiments indicated that the coefficient increased linearly with hematocrit. The HCT value was obtained from the 20 MHz data using regression analysis. The attenuation (() was determined as a 42.14 + 1.02*HCT (Np/m). The corresponding standard deviation (SD), and the correlation coefficient were calculated as SD = 2.4 Np/m, and R = 0.9, (p<0.001), respectively The absolute accuracy of in vivo measurements in the brachial artery was determined to be within 5% HCT.

Conclusions:
The method proposed appears to be promising for in vivo determination of hematocrit as 5% error is adequate to monitor changes in patients in shock or during dialysis. It was found that the multigate system largely simplified the placement of an ultrasonic probing beam in the center of the blood vessel. Current work focuses on enhancing the method’s applicability to arbitrary selected vessels and reducing the HCT measurement error to well below 5%.

Keywords:
hematocrit, blood, Doppler, power Doppler, multigate Doppler

Abstract:
Coded ultrasonography is intensively studied in many laboratories due to its remarkable properties: increased depth penetration, signal-to-noise ratio (SNR) gain and improved axial resolution. However, no data concerning the spatial behavior of the pressure field generated by coded bursts transmissions were reported so far. Five different excitation schemes were investigated. Flat, circular transducer with 15 mm diameter, 2 MHz center frequency and 50% bandwidth was used. The experimental data was recorded using the PVDF membrane hydrophone and collected with computerized scanning system developed in our laboratory. The results of measured pressure fields before and after compression were then compared to those recorded using standard ultrasonographic short-pulse excitation. The increase in the SNR of the decoded pressure fields is observed. The modification of the spatial pressure field distribution, especially in the intensity and shape of the sidelobes is apparent. Coded sequences are relatively long and, intuitively, the beam shape could be expected to be very similar to the sound field of long-period sine burst. This is true for non-compressed distributions of examined signals. However, as will be shown, the compressed sound fields, especially for the measured binary sequences, are similar rather to field distributions of short, wideband bursts.

Keywords:
coded excitation, ultrasonic field distribution, pulse compression, matched filtration, medical imaging

Abstract:
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 μm 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.

Keywords:
Ultrasound imaging, Nonlinear propagation, Spatial averaging, Safety indices

Abstract:
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.

Keywords:
Nonlinear propagation modeling, Nonlinear propagation, JW model

Abstract:
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 μm. 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.

Keywords:
Time gating frequency analysis (TGFA), Time delay spectrometry (TDS), High frequency hydrophone calibration, Nonlinear hydrophone calibration, High frequency ultrasound, Ultrasonic metrology

Abstract:
Harmonic imaging at frequencies up to 15 MHz is now routinely used in clinical practice and frequencies well beyond 20 MHz are considered for diagnostic ultrasound imaging applications. However, currently available measurement tools are not fully adequate to characterize such high frequency systems, primarily due to the combined effects of limited frequency responses and spatial averaging effects. To alleviate this problems, a comprehensive wave propagation model has been developed and tested. The model can predict the linear and nonlinear acoustic wave propagation generated by differently shaped acoustic radiators at virtually any point in the field and takes into account spatial averaging effects introduced by hydrophone probes and their associated frequency responses. The applicability of the model in hydrophone probe calibration up to 100 MHz is demonstrated. Also, a novel calibration technique termed Time-Gating Frequency Analysis (TGFA) is briefly described and calibration results in the frequency range up to 60 MHz for hydrophones having effective diameters between 150 and 500 /spl mu/m are presented. Also presented are the results of the investigation that determined the effect of using hydrophone probes of different diameters and bandwidth on Spatial-Peak Pulse-Average Intensity (I/sub SPPA/). It was found that the values of I/sub SPPA/ increased with decreasing effective aperture of the hydrophone probe and its bandwidth.

Keywords:
Ultrasonic imaging, Frequency, Sonar equipment, Probes, Calibration, Acoustic propagation, Nonlinear acoustics, Bandwidth, Acoustic measurements, Current measurement