Frequency-dependent Backscatter Research Articles

Hydro-acoustic classification and abundance estimation of mesopelagic fish in deep scattering layers (DSL) of the Indian Ocean

Abstract Deep scattering layers (DSL) in oligotrophic systems are typically comprised of a variety of coexisting organisms, including fish, zooplankton, jellyfish, and squid, and hence there is uncertainty about the proportion of the acoustic backscatter in these layers that can be attributed to mesopelagic fish. Here, acoustic targets were classified using a multi-frequency acoustic classification algorithm based on data collected in the Indian Ocean at 18, 38, 70, and 120 kHz during three acoustic surveys in 2018. Frequency-dependent backscattering strength information (∆Sv) was extracted from acoustic data that coincided with trawl hauls dominated by mesopelagic fish. Five ∆Sv frequency pairs were used to separate the acoustic backscattering into three broad scattering categories, i.e. mesopelagic fish, crustacean-like/tunicates, and squids/others. Results indicated that the DSL is highly diverse and dominated by mesopelagic fish, with average densities at a regional scale ranging from 12.0 (±10.9) to 26.0 (±21.7) g m−2, and proportions of the acoustic backscatter attributed to mesopelagic fish ranging from 0.5 to 0.7. These estimates are generally lower than previously estimated for other regions of the Indian Ocean. The situation may well be similar elsewhere, particularly in oligotrophic systems, with potential ramifications for global mesopelagic fish biomass estimates.

Broadband acoustic characterization of backscattering from a rough stratification interface.

Broadband acoustic analysis of scattering from sharp density gradients in the water column generally treat the interfaces as smooth surfaces. However, these interfaces may exhibit roughness owing to external water column forcing and local convective processes. In this work we extend broadband backscatter analysis methods to consider interface roughness by drawing upon methods developed for sea surface and seabed acoustic backscattering. The one-dimensional acoustic model from Weidner and Weber [J. Acoust. Soc. Am. 150(6), 4353-4361 (2021)], which predicts a decay in the reflected wave amplitude from stratification interfaces with increasing frequency, was expanded for surface applications. The expanded model was used to analyze the scattered pressure field from interfaces over a range of surface roughness magnitudes. Analysis of model results indicate that stratification interface roughness, quantified by the root-mean-squared interface slope angle and root-mean-squared height of the interface, modifies the model-predicted frequency-dependent backscattering. A broadband acoustic inversion procedure to remotely measure the magnitude of the vertical extent of stratification gradients and the corresponding sound speed perturbation was defined. The broadband inversion method was tested on data collected in the Baltic Sea with well-documented, strong salinity-driven stratification.

Broadband acoustic characterization of backscattering from a rough stratification surface

This work aims to elucidate the utility and limitations of broadband scientific echo sounders in characterizing the frequency-dependent scattering from stratification interfaces. A recently published 1D acoustic backscattering model for the oceanic stratification structure was expanded for this work to consider echo sounding applications and the importance of roughness (vertical and spatial variability) along the stratification interface. The expanded model was tested in utilizing data from a recent acoustic experiment in the Kattegat basin, a region with well-documented, strong, salinity-driven stratification. A suite of broadband split-beam echosounders (45–270 kHz) provided broadband backscattering data, which were validated using high resolution in situ observations of water column structure, collected with a CTD (Conductivity, Temperature, Depth) sensor and MVP (Moving Vessel Profiler). Overall, the frequency dependence of backscattering agrees with model predictions of a gradual decay in the reflected wave amplitude from stratification structure with increasing frequency. Stratification interface roughness drives changes in frequency-dependent scattering away from a smooth surface solution as a function of different roughness regimes, primarily defined by the root-mean-squared slope of the interface. Results suggest a path to remote estimations of physical medium properties through broadband acoustic inversion.

Examining the relationship between morphological variation and modeled broadband scattering responses of reef-associated fishes from the Southeast United States

Non-lethal methods are sought to provide indices or measure of absolute abundance to fit fishery stock assessment models, including those computed for reef fishes. Calibrated scientific echosounders provide the foundation for stock assessments in low-diversity ecosystems but are hampered by complicated target strength responses in high diversity systems like coral and rocky reefs. Newly available broadband echosounders present advantages over narrowband predecessors including increased spatial resolution, increased signal to noise and spectral resolution; however, a better understanding of the sound scattering properties is necessary for operational use in surveys of diverse reef fish assemblages. To gain insight into the feasibility of implementing broadband techniques, we first model the frequency dependent backscatter of ecologically and commercially important species; results will aid our understanding of the backscattering as well as provide a supporting dataset to develop algorithms for classification of species/functional groups. Computed-tomography (CT) scans were performed on 149 individuals across reef fish species from the northeastern Gulf of Mexico to generate three-dimensional swim bladder models for estimation of morphometrics to input into numerical scattering models. Principal component analysis of the swim bladder morphology indicated significant variation among species; however, closely related species (confamilials) were not different. We employed boundary element method modeling to examine the frequency-dependent backscatter responses across a range of fish orientations (+/- 45°). Comparisons of frequency-dependent backscatter revealed strongest similarities between closely related species and greatest differences between more distantly related species or species groups. Patterns in the spectra have the potential to be used to classify targets toward species (or species group) for estimates of density from marine ecosystem acoustic surveys. Given that broadband echosounders are not yet commonplace, and many surveys in the region utilize narrowband echosounders, we also developed narrowband target strength – length relationships for six of the reef fish species with model fits ranging widely (r2 = 0.05 to 0.93). The potential utility of broadband echosounders to enhance ecosystem studies, or to develop fisheries-independent indices of abundance for use in stock assessment is discussed.

Low frequency sound scattering from a submerged bubble cloud: The Seneca Lake experiment

Acoustic backscatter from the sea surface is governed by the roughness of the surface and subsurface microbubble distributions. At low frequencies, scattering results primarily from coherent and/or collective scatter from bubbles entrained by the subsurface vorticity or carried to depth by Langmuir circulation and thermal convection. In 1947, Carstensen and Foldy published seminal work on sound scattering and attenuation from bubble screens (JASA 19, 481-501, 1947) in which they employed an effective medium approximation to model the problem. Drawing insights from this work, W. Carey postulated that resonance scattering from submerged bubble clouds can be described by a Minnaert formula modified to account for the enhanced compressibility of the mixture. In 1990, experiments to test this concept were conducted at the US Navy Sonar Test Facility at Seneca Lake, New York (JASA 92, 2993-2996, 1992). Measurements of frequency-dependent backscatter from a submerged bubble cloud proved consistent with model pre...

Hydrodynamic influences on acoustical and optical backscatter in a fringing reef environment

AbstractObservations of hydrodynamics along with optical and acoustical water characteristics in a tropical fringing reef environment reveal a distinct signature associated with flow characteristics and tidal conditions. Flow conditions are dominated by tidal forcing with an offshore component from the reef flat during ebb. Measurements span variable wave conditions enabling identification of wave effects on optical and acoustical water properties. High‐frequency acoustic backscatter (6 MHz) is strongly correlated with tidal forcing increasing with offshore directed flow and modulated by wave height, indicating dominant hydrodynamic influence. Backscatter at 300 and 1200 kHz is predominantly diurnal suggesting a biological component. Optical backscatter is closely correlated with high‐frequency acoustic backscatter across the range of study conditions. Acoustic backscatter frequency dependence is used along with changes in optical properties to interpret particle‐size variations. Changes across wave heights suggest shifts in particle‐size distributions with increases in relative concentrations of smaller particles for larger wave conditions. Establishing a connection between the physical processes of a fringing tropical reef and the resulting acoustical and optical signals allows for interpretation and forecasting of the remote sensing response of these phenomena over larger scales.

Vertical Air Motions and Raindrop Size Distributions Estimated Using Mean Doppler Velocity Difference From 3- and 35-GHz Vertically Pointing Radars

Vertical profiles of vertical air motion and raindrop size distributions (DSDs) within stratiform rain are estimated using two collocated vertically pointing radars (VPRs) operating at 3 and 35 GHz. Different raindrop backscattering cross sections occur at 3 and 35 GHz with Rayleigh scattering occurring for all raindrops at 3 GHz and Mie scattering occurring for larger raindrops at 35 GHz. This frequency-dependent backscattering cross section causes differently shaped reflectivity-weighted Doppler velocity spectra leading to radar transmit frequency-dependent radar moments of intrinsic reflectivity factor, mean Doppler velocity, and spectrum variance. The retrieval method described herein uses four radar moments as inputs to retrieve four outputs at each height within a precipitation column. The inputs include 3-GHz VPR mean Doppler velocity and unattenuated reflectivity factor and 35-GHz VPR mean Doppler velocity and spectrum variance. The outputs include vertical air motion and three parameters of a gamma-shaped DSD. To account for different VPR sample volumes, radar observations were accumulated over 45 s and over several range gates to represent time-space scales larger than either VPR sample volumes. Observed variability over this common time-space scale is used to estimate retrieval uncertainties. The retrieved air motions and DSD parameters compare well against retrievals from a collocated 449-MHz VPR that estimated air motions from Bragg scattering signals and DSD parameters from Rayleigh scattering signals.

Unifying Concepts of Statistical and Spectral Quantitative Ultrasound Techniques.

Quantitative ultrasound (QUS) techniques using radiofrequency (RF) backscattered signals have been used for tissue characterization of numerous organ systems. One approach is to use the magnitude and frequency dependence of backscatter echoes to quantify tissue structures. Another approach is to use first-order statistical properties of the echo envelope as a signature of the tissue microstructure. We propose a unification of these QUS concepts. For this purpose, a mixture of homodyned K-distributions is introduced to model the echo envelope, together with an estimation method and a physical interpretation of its parameters based on the echo signal spectrum. In particular, the total, coherent and diffuse signal powers related to the proposed mixture model are expressed explicitly in terms of the structure factor previously studied to describe the backscatter coefficient (BSC). Then, this approach is illustrated in the context of red blood cell (RBC) aggregation. It is experimentally shown that the total, coherent and diffuse signal powers are determined by a structural parameter of the spectral Structure Factor Size and Attenuation Estimator. A two-way repeated measures ANOVA test showed that attenuation (p-value of 0.077) and attenuation compensation (p-value of 0.527) had no significant effect on the diffuse to total power ratio. These results constitute a further step in understanding the physical meaning of first-order statistics of ultrasound images and their relations to QUS techniques. The proposed unifying concepts should be applicable to other biological tissues than blood considering that the structure factor can theoretically model any spatial distribution of scatterers.

The Best of Quantitative Ultrasound

The objective of these studies was to demonstrate that quantitative ultrasound (QUS) techniques could be used to improve diagnostic ultrasound imaging. Specifically, QUS techniques were applied to improving detection of breast and thyroid cancer, early detection of preterm birth, quantification of fatty liver and thermometry. QUS techniques were employed that utilized parameterization of the frequency-dependent backscatter coefficient, the attenuation coefficient and/or envelope statistics. The backscatter coefficient was parameterized through the effective scatterer diameter (ESD) and the effective acoustic concentration (EAC). Backscattered ultrasound data were acquired from tissue samples using clinical ultrasound scanners providing the RF signal and from laboratory systems utilizing single-element focused transducers. The data spanned frequency ranges from 2 to 50 MHz. Combinations of the ESD, EAC and envelope statistics were successful in differentiating malignant from benign tumors in rodent models of breast cancer. Similarly, the ESD combined with envelope statistics provided differentiation of malignant from benign nodules in mouse models of thyroid cancer. Cervical attenuation was an early predictor of preterm birth in rat models and in humans. Attenuation and the slope and magnitude of the backscatter coefficient were observed to predict the degree of fatty liver in mouse models and in humans. The EAC was observed to track temperature when using thermal therapies in different tissues. QUS techniques have continued to mature and have demonstrated new sources of contrast leading to improved diagnostic capabilities and the ability to monitor and assess therapy.

Dependences of the backscatter coefficient on the frequency, the bone volume fraction, and the trabecular thickness in bovine femoral trabecular bone in Vitro

The present study aims to provide insight into the dependences of the backscatter coefficient on the frequency, the bone volume fraction (BV/TV), and the trabecular thickness (Tb.Th) in bovine femoral trabecular bone in vitro. The frequency-dependent backscatter coefficient was measured in 20 bovine femoral trabecular bone samples over a frequency range from 0.2 to 0.7 MHz. The backscatter coefficient measured at 0.5 MHz exhibited significant positive correlations with the BV/TV (r = 0.76) and the Tb.Th (r = 0.77). The power-law fits to the measurements showed that the backscatter coefficient was empirically proportional to the frequency, the BV/TV, and the Tb.Th with exponents of n = 2.9, 1.3, and 1.8, respectively. The present study also used a scattering model, in which the scattering coefficient was proportional to the product of the mean velocity fluctuation and the correlation function integrated over the volume, to predict the measurements. The measurements were found to largely fall within the range of the predictions obtained from the scattering model.

Evaluation of the impact of backscatter intensity variations on ultrasound attenuation estimation

Quantitative ultrasound based approaches such as attenuation slope estimation can be used to determine underlying tissue properties and eventually used as a supplemental diagnostic technique to B-mode imaging. The authors investigate the impact of backscatter intensity and frequency dependence variations on the attenuation slope estimation accuracy. The authors compare three frequency domain based attenuation slope estimation algorithms, namely, a spectral difference method, the reference phantom method, and two spectral shift methods: a hybrid method and centroid downshift method. Both the reference phantom and hybrid method use a tissue-mimicking phantom with well-defined acoustic properties to reduce system dependencies and diffraction effects. The normalized power spectral ratio obtained is then filtered by a Gaussian filter centered at the transmit center frequency in the hybrid method. A spectral shift method is then used to estimate the attenuation coefficient from the normalized and filtered spectrum. The centroid downshift method utilizes the shift in power spectrum toward lower frequencies with depth. Numerical phantoms that incorporate variations in the backscatter intensity from -3 to 3 dB, by varying the scatterer number density and variations in the scatterer diameters ranging from 10 to 100 μm are simulated. Experimental tissue mimicking phantoms with three different scatterer diameter ranges (5-40, 75-90, and 125-150 μm) are also used to evaluate the accuracy of the estimation methods. The reference phantom method provided accurate results when the acoustical properties of the reference and the sample are well matched. Underestimation occurs when the reference phantom possessed a higher sound speed than the sample, and overestimation occurs when the reference phantom had a lower sound speed than the sample. The centroid downshift method depends significantly on the bandwidth of the power spectrum, which in turn depends on the frequency dependence of the backscattering. The hybrid method was the least susceptible to changes in the sample's acoustic properties and provided the lowest standard deviation in the numerical simulations and experimental evaluations. No significant variations in the estimation accuracy of the attenuation coefficient were observed with an increase in the scatterer number density in the simulated numerical phantoms for the three methods. Changes in the scatterer diameters, which result in different frequency dependence of backscatter, do not significantly affect attenuation slope estimation with the reference phantom and hybrid approaches. The centroid method is sensitive to variations in the scatterer diameter due to the frequency shift introduced in the power spectrum.

Dependences of the attenuation and the backscatter coefficients on the frequency and the porosity in bovine trabecular bone: Application of the binary mixture model

The present study aims to investigate the dependences of the attenuation and the backscatter coefficients on the frequency and the porosity in bovine trabecular bone in vitro. The frequencydependent attenuation and backscatter coefficients were measured in 22 bovine femoral trabecular bone samples over a frequency range from 1.4 to 3.0 MHz by using a pair of transducers with a diameter of 12.7 mm and a center frequency of 2.25 MHz. The binary mixture model for ultrasonic scattering in trabecular bone, in which trabecular bone is assumed to be an isotropic binary mixture composed of a bone matrix and marrow, was applied to predict the measurements. The experimental results showed that the attenuation and the backscatter coefficients increased with increasing frequency from 1.4 to 3.0 MHz and decreased with increasing porosity from 66.9 to 91.5%. The predictions of the binary mixture model showed good agreements with the measurements, suggesting that scattering may be the dominant attenuation mechanism in dense bovine trabecular bone at frequencies from 1.4 to 3.0 MHz.

Ultrasound characterization of red blood cells distribution: a wave scattering simulation study

Ultrasonic backscattered signals from blood contain frequency-dependent information that can be used to obtain quantitative parameters describing the aggregation state of red blood cells (RBCs). However the relation between the parameters describing the aggregation level and the backscatterer coefficient needs to be better clarified. For that purpose, numerical wave simulations were performed to generate backscattered signals that mimic the response of two-dimensional (2D) RBC distributions to an ultrasound excitation. The simulated signals were computed with a time-domain method that has the advantages of requiring no physical approximations (within the framework of linear acoustics) and of limiting the numerical artefacts induced by the discretization of object interfaces. In the simple case of disaggregated RBCs, the relationship between the backscatter amplitude and scatterer concentration was studied. Backscatter coefficients (BSC) in the frequency range 10 to 20 MHz were calculated for weak scattering infinite cylinders (radius 2.8 μm) at concentrations ranging from 6 to 36%. At low concentration, the BSC increased with scatterer concentrations; at higher concentrations, the BSC reached a maximum and then decreased with increasing concentration, as it was noted by previous authors in in vitro blood experiments. In the case of aggregated RBCs, the relationship between the backscatter frequency dependence and level of aggregation at a concentration of 24% was studied for a larger frequency band (10 – 40 MHz). All these results were compared with a weak scattering model based on the analytical computing of the structure factor.

Ultrasonic characterization of cancellous bone using three models of trabecular structure.

The semi-empirical scattering models of trabecular bone were developed and examined for their abilities to mimic the frequency dependent backscattering coefficient measured in the cancellous bone. In the simulation of the bone, rf echoes the real properties of the bone and experimental conditions were taken into account. Tree types of trabeculae mimicking scatterers were considered. First, the bone consisted of cylinders with varying thickness (Gamma distributed) within the population was assumed. The next two cases accounted for the contribution of thick and thin trabeculae to the total backscattered signal. The second model assumed existence of two populations of the cylindrical scatterers significantly differing in the average value of Gamma distributed diameters. Finally, the mixed model composed of thick and thin trabeculae modeled, respectively, by cylindrical and spherical scatterers was examined. The last selection resulted from the similarity found between scattering on small sphere and finite cylinder. Calculated echoes demonstrated the usefulness of the mixed model. Frequency dependence of backscattering coefficient agreed well with the experimentally determined dependences. The study also showed that the amplitude histograms calculated using demodulated rf echoes deviate from the Rayleigh distribution when the variation in scatterers’ diameters increase.

Ultrasound characterization of red blood cell aggregation with intervening attenuating tissue-mimicking phantoms

The analysis of the ultrasonic frequency-dependent backscatter coefficient of aggregating red blood cells reveals information about blood structural properties. The difficulty in applying this technique in vivo is due to the frequency-dependent attenuation caused by intervening tissue layers that distorts the spectral content of signals backscattered by blood. An optimization method is proposed to simultaneously estimate tissue attenuation and blood structure properties, and was termed the structure factor size and attenuation estimator (SFSAE). An ultrasound scanner equipped with a wide-band 25 MHz probe was used to insonify porcine blood sheared in both Couette and tubular flow devices. Since skin is one of the most attenuating tissue layers during in vivo scanning, four skin-mimicking phantoms with different attenuation coefficients were introduced between the transducer and the blood flow. The SFSAE gave estimates with relative errors below 25% for attenuations between 0.115 and 0.411 dBMHz and kR<2.08 (k being the wave number and R the aggregate radius). The SFSAE can be useful to examine in vivo and in situ abnormal blood conditions suspected to promote pathophysiological cardiovascular consequences.

Semi-empirical bone model for determination of trabecular structure properties from backscattered ultrasound

A novel semi-empirical scattering model of trabecular bone facilitating its characterization and allowing optimization of the interrogating pulse-echo transducer performance was developed. The model accounts for spatial density distribution of the trabeculae and includes measurement conditions such as pressure–time waveform of the probing ultrasound wave, the emitted field structure, and the transfer function and limited bandwidth of the acoustic source operating in pulse-echo mode. These measurement conditions are of importance as they modify the scattered echoes, which in turn are linked to the micro-architecture of the bone. The bone was modeled by a random distribution of long and thin cylindrical scatterers having randomly varying diameters and mechanical properties, and oriented perpendicularly to the ultrasound beam axis. To mimic clinically encountered conditions the relevant empirical data obtained at 1 MHz were input to the model. The data included pulse-echo source pressure field distribution in the focal zone and the above mentioned transfer function. With these data the model allowed frequency dependent backscattering coefficient of the simulated bone structure and its statistical properties to be determined. The results obtained indicated that the computer simulation is of particular relevance in studying scattering properties of the cancellous bone and holds promise as a tool to determine the relationship between the physical dimensions and shape of the scatterers and for monitoring of osteoporosis. The results of simulations also indicated that the new bone model proposed is well suited to mimic clinically relevant conditions. In contrast to the existing bone models, which usually assume scatterers to be randomly distributed as infinitely long identical cylinders with a cross-section much smaller than the probing ultrasound wave, the new model includes two populations of scatterers having different physical dimensions and also allows the mechanical properties of the scatterers to be varied.

Monte Carlo study on ultrasound backscattering by three-dimensional distributions of red blood cells

A Monte Carlo study on ultrasound backscattering by red blood cells (RBCs) is presented for three-dimensional (3D) distributions of particles. The cells were treated as classical spherical particles and accordingly, the Boltzmann distribution was considered to describe probability distribution of energy states of a system composed of such particles. The well-known Metropolis algorithm can generate configurations according to that probability distribution and therefore, was employed in this study to simulate some realizations of both nonaggregating and aggregating RBCs. The study of nonaggregating particles was motivated to compare simulations with existing experimental results and consequently, to validate the model. In the case of aggregating RBCs, the interaction potential between cells was modeled with the Morse potential and the frequency-dependent backscattering coefficient (BSC) was investigated at different hematocrits (H, particle volume fractions). The impact of aggregation potential on the spectral slope (SS) was also evaluated. It is shown that BSC increased as the magnitude of aggregating potential was raised and the effect was more pronounced at higher hematocrits. Moreover, spectral slopes at nonaggregating and low aggregating conditions were found to be around 4, which is consistent with the Rayleigh scattering theory. However, it had diminished significantly, particularly at higher hematocrits as the magnitude of the attractive potential energy was raised. For instance, at H=40% SS dropped from 4.04 for nonaggregating particles to 3.62 at the highest aggregating potential considered in this study. Our results suggest that this 3D model is capable of reflecting the effects of RBC aggregation on BSC and SS.

Frequency dependence of backscatter from thin, oblique, finite-length cylinders measured with a focused transducer-with applications in cancellous bone

A model is presented for the echo from a thin, oblique, finite-length cylinder. The echo is calculated from the line integral of the transducer directivity pattern along the cylinder axis. The model was validated with broadband pulse-echo measurements from (1) a perpendicular (to the ultrasound beam) nylon wire as a function of lateral displacement from the beam center, (2) a tilted nylon wire as a function of the angle of inclination relative to the ultrasound beam, and (3) a quasi-parallel-nylon-wire phantom, which mimicked the scattering properties of cancellous bone. The transducer directivity pattern (as a function of position and frequency) was measured with a membrane hydrophone. The model predicts an approximately cubic frequency dependence of backscatter coefficient from the phantom, as has been observed experimentally in cancellous bone. The model also predicts the relationship between the cylinder length and the exponent of a power law fit to backscatter coefficient versus frequency, which is 4 for very short (compared to a wavelength) cylinders and asymptotically approaches 3 for very long cylinders.

Comparison of the Faran Cylinder Model and the Weak Scattering Model for predicting the frequency dependence of backscatter from human cancellous femur in vitro

This letter presents the first side-by-side comparison of the Faran Cylinder Model and the Weak Scattering Model for predicting backscatter from human femur. Both models are applied to the same dataset of frequency-dependent backscatter coefficients from 26 human femur cancellous bone samples in vitro. The Faran Cylinder Model predicts a slightly slower rate of increase of backscatter with frequency than the Weak Scattering Model, but both models are in reasonable agreement with the data and with each other, given the uncertainty in the measurements.

Acoustic backscatter and effective scatterer size estimates using a 2D CMUT transducer

Compared to conventional piezoelectric transducers, new capacitive microfabricated ultrasonic transducer (CMUT) technology is expected to offer a broader bandwidth, higher resolution and advanced 3D/4D imaging inherent in a 2D array. For ultrasound scatterer size imaging, a broader frequency range provides more information on frequency-dependent backscatter, and therefore, generally more accurate size estimates. Elevational compounding, which can significantly reduce the large statistical fluctuations associated with parametric imaging, becomes readily available with a 2D array. In this work, we show phantom and in vivo breast tumor scatterer size image results using a prototype 2D CMUT transducer (9 MHz center frequency) attached to a clinical scanner. A uniform phantom with two 1 cm diameter spherical inclusions of slightly smaller scatterer size was submerged in oil and scanned by both the 2D CMUT and a conventional piezoelectric linear array transducer. The attenuation and scatterer sizes of the sample were estimated using a reference phantom method. RF correlation analysis was performed using the data acquired by both transducers. The 2D CMUT results indicate that at a 2 cm depth (near the transmit focus for both transducers) the correlation coefficient reduced to less than 1/e for 0.2 mm lateral or 0.25 mm elevational separation between acoustic scanlines. For the conventional array this level of decorrelation requires a 0.3 mm lateral or 0.75 mm elevational translation. Angular and/or elevational compounding is used to reduce the variance of scatterer size estimates. The 2D array transducer acquired RF signals from 140 planes over a 2.8 cm elevational direction. If no elevational compounding is used, the fractional standard deviation of the size estimates is about 12% of the mean size estimate for both the spherical inclusion and the background. Elevational compounding of 11 adjacent planes reduces it to 7% for both media. Using an experimentally estimated attenuation of 0.6 dB cm−1 MHz−1, scatterer size estimates for an in vivo breast tumor also demonstrate improvements using elevational compounding with data from the 2D CMUT transducer.