Sound Speed Mismatch Research Articles

Reformulation of frequency-difference matched-field processor for high-frequency known-source localization.

Frequency-difference matched-field processing is a high-frequency source localization technique formulated by matching the frequency-difference autoproduct of the measured field and replicas at the difference-frequency. Although it successfully localizes sound sources by sparse vertical array in shallow or deep ocean with an environmental mismatch, there is still some ambiguity in replica modeling and signal processing. Here, the existing conventional processor is modified to match the bandwidth-averaged autoproduct of the measured field with replicas of the bandwidth-averaged autoproduct, or approximately its self-term for the expected source locations. The proposed processor is consistent with the perspective of matched-field processing and can naturally relieve some drawbacks of the existing one, such as low peak or low dynamic range on the ambiguity surface. Numerical tests are carried out in several shallow ocean environments and the source localization using experimental data are performed to confirm the properties of the proposed processor. It is found that the high-frequency diffracted field always leaves traces on its bandwidth-averaged autoproduct field. These high-frequency marks cause a bias in source localization in the presence of a sound speed mismatch even in low difference-frequencies.

Standardised quantitative ultrasound imaging approach for the contact-less three-dimensional analysis of neocartilage formation in hydrogel-based bioscaffolds

In this work we present a standardised quantitative ultrasound imaging (SQUI) approach for the non-destructive three-dimensional imaging and quantification of cartilage formation in hydrogel based bioscaffolds. The standardised concept involves the processing of ultrasound backscatter data with respect to an acellular phantom in combination with the compensation of sound speed mismatch diffraction effects between the bioscaffold and the phantom. As a proof-of-concept, the SQUI approach was tested on a variety of bioscaffolds with varying degree of neocartilage formation. These were composed of Gelatine Methacryloyl (GelMA) hydrogels laden with human adipose-derived stem cells (hADSCs). These were cultured under chondrogenic stimulation following a previously established protocol, where the degree of the neocartilage formation was modulated using different GelMA network densities (6, 8, 10 % w/v) and culture time (0, 14, 28 days). Using the SQUI approach we were able to detect marked acoustic and morphological changes occurring in the bioscaffolds a result of their different chondrogenic outcome. We defined an acoustic neocartilage indicator, the sonomarker, for the selective imaging and quantification of neocartilage formation. The sonomarker, of backscatter intensity logIBC -2.4, was found to correlate with data obtained via standard destructive bioassays. The ultrasonic evaluation of human specimens confirmed the sonomarker as a relevant intensity, although it was found to shift to higher intensity values in proportion to the cartilage condition as inferred from sound speed measurements. This study demonstrates the potential of the SQUI approach for the realization of non-destructive analysis of cartilage regeneration over-time. Statement of significanceAs tissue engineering strategies for neocartilage regeneration evolve towards clinical implementation, alternative characterisation approaches that allow the non-destructive monitoring of extracellular matrix formation in implantable hydrogel based bioscaffolds are needed. In this work we present an innovative standardized quantitative ultrasound imaging (SQUI) approach that allows the non-destructive, volumetric, and quantitative evaluation of neocartilage formation in hydrogel based bioscaffolds. The standardised concept aims to provide a robust approach that accounts for the dynamic changes occurring during the conversion from a cellular bioscaffold towards the formation of a neocartilage construct. We believe that the SQUI approach will be of great benefit for the evaluation of constructs developing neocartilage, not only for in-vitro applications but also potentially applicable to in-vivo applications.

Effect of the sound speed mismatch between fluid and channel on the particle alignment in a standing surface acoustic wave device

Standing surface acoustic wave (SSAW) combined with a microfluidic channel has been emerging as a device to manipulate biological and chemical particles for separation, mixing, and enrichment. Particles are trapped at pressure minima in the interference pattern determined by the acoustic properties of the fluid and the channel wall. To predict particle motion, it is essential to analyze the pressure distribution formed in the fluid. Here, we investigated the effect of sound speeds of fluid and channel wall on the pattern of aligned particles. We developed the numerical full model capable of analyzing wave propagation in each component of a SSAW device and tracing particles. The computational simulation with varied sound speeds of fluid and channel wall demonstrated how the particle alignment pattern is altered by the mismatch between their sound speeds. The computational results were compared with the experimental ones obtained for fluids with varied sound speeds. We revealed the difference in the propagation path of acoustic wave between fluid and channel wall plays a critical role in determining the particle movement. Our study provides an intuition on how particles subjected to SSAW migrate, which is beneficial in designing an acoustofluidic device to manipulate particles in a desired manner.

Reference Phantom Method for Ultrasonic Imaging of Thin Dynamic Constructs

Quantitative ultrasound has a great potential for the non-destructive evaluation of tissue engineered constructs, where the local attenuation and the integrated backscatter coefficient (IBC) can be used for monitoring the development of biological processes. The local determination of both parameters can be achieved using the reference phantom method (RPM). However, its accuracy can be affected when evaluating constructs of evolving sound speed, attenuation and thickness, for example, when evaluating biodegradable hydrogels developing neocartilage. To assess the feasibility of using the RPM under such dynamic conditions while employing a 50-MHz transducer, we conducted a series of experiments on 3-mm-thick acellular hydrogels laden with microspheres. The ultrasonic evaluation procedure used was validated by detecting and compensating for large attenuation variations occurring in the construct, up to 20-fold with respect to the reference phantom, with estimations errors below 1%. We found that sound speed mismatch does not affect the local attenuation estimation, but causes a strong diffraction effect by reducing the backscatter intensity. Such intensity reduction was compensated by determining the IBC percentage change (IBCΔ) as function of sound speed mismatch with respect to the reference phantom (ΔSS), with the equation IBCΔ = (0.63 ± 0.07) ΔSS + (8.54 ± 0.76) 10–3 ΔSS2. The investigated ΔSS interval was up to 120 m/s and using two different concentrations of microspheres, with estimation errors below 7% relative to the construct's actual IBC. Finally, we found that the spectral difference method is sufficient to measure within a few millimetres in depth mismatch, and when combining with sound speed mismatch, we found negligible additional effects. These results pave the way for the use of a generic reference phantom for the evaluation of thin dynamic constructs, thus simplifying the need for using different phantoms depending on the construct's properties.

Multiple time-reversal focusing with a virtual source array

Time reversal (TR) is the process of generating a spatio-temporal focus at a probe source (PS) location by backpropagating a time-reversed version of a received signal. While TR focusing requires the PS for a coherent acoustic focus at its origin, the requirement of the PS has been partially relaxed by the concept of a virtual source array (VSA) [J. Acoust. Soc. Am. 125, 3828–3834 (2009)]. A VSA can serve as a remote platform or lens and redirect a focused field to a selected location beyond the VSA for which the field is assumed as a homogeneous medium with constant sound speed. The objective of this study is to extend VSA-based single TR focusing to simultaneous multiple focusing. This is achieved using the optimization theory by employing the multiple constraints method derived from a constraint matrix, which consists of appropriately synchronized transfer functions. It is found that simultaneous multiple focusing can be achieved with distortionless response at selected multiple locations, and its performance degrades in the presence of sound speed mismatch. Possible applications for the VSA-based TR focusing are discussed, and numerical simulation results are presentedTime reversal (TR) is the process of generating a spatio-temporal focus at a probe source (PS) location by backpropagating a time-reversed version of a received signal. While TR focusing requires the PS for a coherent acoustic focus at its origin, the requirement of the PS has been partially relaxed by the concept of a virtual source array (VSA) [J. Acoust. Soc. Am. 125, 3828–3834 (2009)]. A VSA can serve as a remote platform or lens and redirect a focused field to a selected location beyond the VSA for which the field is assumed as a homogeneous medium with constant sound speed. The objective of this study is to extend VSA-based single TR focusing to simultaneous multiple focusing. This is achieved using the optimization theory by employing the multiple constraints method derived from a constraint matrix, which consists of appropriately synchronized transfer functions. It is found that simultaneous multiple focusing can be achieved with distortionless response at selected multiple locations, and its perf...

Virtual Source Array-Based Multiple Time-Reversal Focusing

Time reversal (TR) is the process of generating a spatio-temporal focus at a probe source (PS) location by transmitting a time-reversed version of a received signal. While TR focusing requires the PS for a coherent acoustic focus at its origin, the requirement of the PS has been partially relaxed by the introduction of the concept of a virtual source array (VSA) (J. Acoust. Soc. Am. 2009, 125, 3828–3834). A VSA can serve as a remote platform or lens and redirect a focused field to a selected location beyond the VSA for which the field is assumed as a homogeneous medium with constant sound speed. The objective of this study is to extend VSA-based single TR focusing to simultaneous multiple focusing. This is achieved using the optimization theory by employing the multiple constraints method derived from a constraint matrix, which consists of appropriately synchronized transfer functions. Through numerical simulations, it is found that simultaneous multiple focusing can be achieved with distortionless response at selected multiple locations, and its performance degrades in the presence of sound speed mismatch. For achieving robust multiple focusing in the mismatch environment, singular value decomposition is applied to obtain the weight vector (i.e., backpropagation vector) that best approximates the column vectors of the constraint matrix. Numerical simulation results show that VSA-based multiple TR focusing using SVD is not a method to simultaneously focus on multiple locations, but a method of constructing a field which robustly passes through multiple locations in sound speed mismatch environment.

On System-Dependent Sources of Uncertainty and Bias in Ultrasonic Quantitative Shear-Wave Imaging.

Ultrasonic quantitative shear-wave imaging methods have been developed over the last decade to estimate tissue elasticity by measuring the speed of propagating shear waves following acoustic radiation force excitation. This work discusses eight sources of uncertainty and bias arising from ultrasound system-dependent parameters in ultrasound shear-wave speed (SWS) measurements. Each of the eight sources of error is discussed in the context of a linear, isotropic, elastic, homogeneous medium, combining previously reported analyses with Field II simulations, full-wave 2-D acoustic propagation simulations, and experimental studies. Errors arising from both spatial and temporal sources lead to errors in SWS measurements. Arrival time estimation noise, speckle bias, hardware fluctuations, and phase aberration cause uncertainties (variance) in SWS measurements, while pulse repetition frequency (PRF) and beamforming errors, as well as coupling medium sound speed mismatch, cause biases in SWS measurements (accuracy errors). Calibration of the sources of bias is an important step in the development of shear-wave imaging systems. In a well-calibrated system, where the sources of bias are minimized, and averaging over a region of interest (ROI) is employed to reduce the sources of uncertainty, an SWS error can be expected.

Automated acoustic localization and call association for vocalizing humpback whales on the Navy's Pacific Missile Range Facility.

Time difference of arrival (TDOA) methods for acoustically localizing multiple marine mammals have been applied to recorded data from the Navy's Pacific Missile Range Facility in order to localize and track humpback whales. Modifications to established methods were necessary in order to simultaneously track multiple animals on the range faster than real-time and in a fully automated way, while minimizing the number of incorrect localizations. The resulting algorithms were run with no human intervention at computational speeds faster than the data recording speed on over forty days of acoustic recordings from the range, spanning multiple years. Spatial localizations based on correlating sequences of units originating from within the range produce estimates having a standard deviation typically 10 m or less (due primarily to TDOA measurement errors), and a bias of 20 m or less (due primarily to sound speed mismatch). An automated method for associating units to individual whales is presented, enabling automated humpback song analyses to be performed.

Faster than real-time automated acoustic localization and call association for humpback whales on the Navy's Pacific Missile Range Facility

Optimal time difference of arrival (TDOA) methods for acoustically localizing multiple marine mammals have been applied to the data from the Navy's Pacific Missile Range Facility in order to localize and track humpback whales. Modifications to established methods were necessary in order to simultaneously track multiple animals on the range without the need for post-processing and in a fully automated way, while minimizing the number of incorrect localizations. The resulting algorithms were run with no human intervention at computational speeds faster than the data recording speed on over 40 days of acoustic recordings from the range, spanning several years and multiple seasons. Spatial localizations based on correlating sequences of units originating from within the range produce estimates having a standard deviation typically 10 m or less (due primarily to TDOA measurement errors), and a bias of 20 m or less (due to sound speed mismatch). Acoustic modeling and Monte Carlo simulations play a crucial role in minimizing both the variance and bias of TDOA localization methods. These modeling and simulation techniques will be discussed for optimizing array design, and for maximizing the quality of localizations from existing data sets.

Effect of environmental mismatch on range estimation using waveguide invariant: theoretical analysis, simulations and experimental results

The range of the source can be estimated using the concept of waveguide invariant that describes the interference pattern of sound fields in water. Previous works have been shown that the interference pattern suffers from temporal and spatial variations in uncertain environment. This could cause the performance degradation of the processor. Theoretical analyses are presented for water-depth mismatch and sound-speed profile (SSP) mismatch. The theoretical results are then demonstrated by examples using both simulations and ocean acoustic data. The results suggest that the water-depth mismatch just shifts the peak around the correct one and the sound-speed mismatch can be neglected for the very low frequency even without constant water sound speed. But its robustness to SSP mismatch doesnot exist for higher frequency. For sediment-type mismatch, although a large modelled mistake is introduced, the results are acceptable. This implies that the effect of sediment-type mismatch is minor compared to that of mismatches in water column. On the whole, the processor is robust to environmental mismatch and can be applied to lower frequency with minimal environmental information.

Relationship between material parameters and target strength of fluid-filled spherical shells in water: calculations and observations

Thin fluid-filled spherical shells have been used as passive sonar targets for many years. They possess a large target strength which is highly dependent on the sound-speed mismatch between the fluid contained within the shell and the exterior fluid surrounding the shell. In the past, to obtain the appropriate mismatch, the interior fluid mixture contained chlorofluorocarbons (CFCs). Due to a recent production ban on CFCs, it is necessary to choose alternative fluids. The present research analyzes the backscattering target strength of a fluid-filled spherical shell as a function of several material parameters as a guide to choosing alternative fluids and shell materials. Calculations over a broad range of material values display the target strength dependence on the interior fluid parameters as well as the parameters defining the metallic shell. The range of material values presented here is far larger than any previous study addressing the focusing effects of fluid-filled spherical shells. The results should aid in determining liquid fillers and shell materials which yield the maximum possible backscattered returns. Also, several experiments were conducted with stainless steel shells containing a hydrochlorofluorocarbon (HCFC), dichlorofluoroethane. The results are compared with results found from calculations as well as from other experiments involving shells containing a previously used CFC mixture.

SECTOR FOCUSING FOR ROBUST HIGH RESOLUTION ANALYSIS OF THE MATCHED FIELD PROCESSING WORKSHOP DATA SET

Replica correlator, minimum variance distortionless response, and sector-focused processing are applied to the test data cases devised for the Matched Field Processing Workshop. The methods are compared on this data set and their performance is clearly delineated. In all the cases examined from the workshop data set which were appropriate for the application of sector-focused procedures (i.e. the presence of mismatch together with modal noise), sector-focused processing performed successfully. It located the source with robustness similar to the replica correlator processor, but with the higher resolution characteristic of minimum variance distortionless response processing. In all cases where replica correlator localizes correctly, sector focusing also localizes correctly and achieves higher resolution. Sector focusing always reduced signal gain degradation compared to minimum variance distortionless response processing. It also reduced signal gain degradation compared to replica correlator processing for the lowest signal-to-noise ratio case. However, significant range bias error was observed in sector-focused as well as replica correlator processing in the case with general mismatch and colored noise. Sector focusing was not applied to the case with sound speed mismatch but no colored noise.