Comparisons of image quality factors for phase aberration correction with diffuse and point targets: theory and experiments

Acoustoelectric Time-Reversal for Ultrasound Phase-Aberration Correction

Acoustoelectric imaging (AEI) is a technique that combines ultrasound (US) with radio frequency recording to detect and map local current source densities. This study demonstrates a new method called acoustoelectric time reversal (AETR), which uses AEI of a small current source to correct for phase aberrations through a skull or other US-aberrating layers with applications to brain imaging and therapy. Simulations conducted at three different US frequencies (0.5, 1.5, and 2.5 MHz) were performed through media layered with different sound speeds and geometries to induce aberrations of the US beam. Time delays of the acoustoelectric (AE) signal from a monopole within the medium were calculated for each element to enable corrections using AETR. Uncorrected aberrated beam profiles were compared with those after applying AETR corrections, which demonstrated a strong recovery (29%-100%) of lateral resolution and increases in focal pressure up to 283%. To further demonstrate the practical feasibility of AETR, we further conducted bench-top experiments using a 2.5 MHz linear US array to perform AETR through 3-D-printed aberrating objects. These experiments restored lost lateral restoration up to 100% for the different aberrators and increased focal pressure up to 230% after applying AETR corrections. Cumulatively, these results highlight AETR as a powerful tool for correcting focal aberrations in the presence of a local current source with applications to AEI, US imaging, neuromodulation, and therapy.

Basic concept and clinical applications of quantitative ultrasound (QUS) technologies

In the field of clinical ultrasound, the full digitalization of diagnostic equipment in the 2000s enabled the technological development of quantitative ultrasound (QUS), followed by multiple diagnostic technologies that have been put into practical use in recent years. In QUS, tissue characteristics are quantified and parameters are calculated by analyzing the radiofrequency (RF) echo signals returning to the transducer. However, the physical properties (and pathological level structure) of the biological tissues responsible for the imaging features and QUS parameters have not been sufficiently verified as there are various conditions for observing living tissue with ultrasound and inevitable discrepancies between theoretical and actual measurements. A major issue of QUS in clinical application is that the evaluation results depend on the acquisition conditions of the RF echo signal as the source of the image information, and also vary according to the model of the diagnostic device. In this paper, typical examples of QUS techniques for evaluating attenuation, speed of sound, amplitude envelope characteristics, and backscatter coefficient in living tissues are introduced. Exemplary basic research and clinical applications related to these technologies, and initiatives currently being undertaken to establish the QUS method as a true tissue characterization technology, are also discussed.

Soft-Tissue Aberration Correction for Histotripsy

Acoustic aberrations caused by natural heterogeneities of biological soft tissue are a substantial problem for histotripsy, a therapeutic ultrasound technique that uses acoustic cavitation to mechanically fractionate and destroy unwanted target tissue without damaging surrounding tissue. These aberrations, primarily caused by sound speed variations, result in severe defocusing of histotripsy pulses, thereby decreasing treatment efficacy. The gold standard for aberration correction (AC) is to place a hydrophone at the desired focal location to directly measure phase aberrations, which is a method that is infeasible in vivo. We hypothesized that the acoustic cavitation emission (ACE) shockwaves from the initial expansion of inertially cavitating microbubbles generated by histotripsy can be used as a point source for AC. In this study, a 500-kHz, 112-element histotripsy phased array capable of transmitting and receiving ultrasound on all channels was used to acquire ACE shockwaves. These shockwaves were first characterized optically and acoustically. It was found that the shockwave pressure increases significantly as the source changes from a single bubble to a dense cavitation cloud. The first arrival of the shockwave received by the histotripsy array was from the outer-most cavitation bubbles located closest to the histotripsy array. Hydrophone and ACE AC methods were then tested on ex vivo porcine abdominal tissue samples. Without AC, the focal pressure is reduced by 49.7% through the abdominal tissue. The hydrophone AC approach recovered 55.5% of the lost pressure. Using the ACE AC method, over 20% of the lost pressure was recovered, and the array power required to induce cavitation was reduced by approximately 31.5% compared to without AC. These results supported our hypothesis that the ACE shockwaves coupled with a histotripsy array with transmit and receive capability can be used for AC for histotripsy through soft tissue.

Timing-error-difference calibration of a two-dimensional array imaging system using the overlapping-subaperture algorithm

Timing errors in the transmitting and receiving electronic channels of an imaging system can generate different transmission and reception phase-aberration profiles. To decide if these two profiles need to be measured separately, an overlapping-subaperture algorithm has been proposed in a previous paper to measure the difference between timing errors in transmitting and receiving channels connected to each element in a two-dimensional array. This algorithm has been used to calibrate a custom built imaging system with a curved linear two-dimensional array, and the results are presented in this paper. The experimental results have demonstrated that the overlapping-subaperture algorithm is capable of calibrating the timing-error-difference profile of this imaging system with a standard deviation of only a few nanoseconds. Experimental results have also shown that the time-error-difference profile of this imaging system is smaller than one tenth of a wavelength and there is no need to measure the transmission and reception phase-aberration profiles separately. The derived average phase-aberration profile using the near-field signal-redundancy algorithm can be used to correct phase aberrations for both transmission and reception.

Correction of tissue-motion effects on common-midpoint signals using reciprocal signals

The near field signal redundancy algorithm for phase-aberration correction is sensitive to tissue motion because several separated transmissions are usually needed to acquire a set of common-midpoint signals. If tissues are moving significantly due to, for example, heart beats, the effects of tissue motion on common-midpoint signals need to be corrected before the phase-aberration profile can be successfully measured. Theoretical analyses in this paper show that the arrival-time difference between a pair of common-midpoint signals due to tissue motion is usually very similar to that between the pair of reciprocal signals acquired using the same two transmissions. Based on this conclusion, an algorithm for correcting tissue-motion effects on the peak position of cross-correlation functions between common-midpoint signals is proposed and initial experimental results are also presented.

Transcranial ultrasonic therapy based on time reversal of acoustically induced cavitation bubble signature

Brain treatment through the skull with high-intensity focused ultrasound can be achieved with multichannel arrays and adaptive focusing techniques such as time reversal. This method requires a reference signal to be either emitted by a real source embedded in brain tissues or computed from a virtual source, using the acoustic properties of the skull derived from computed tomography images. This noninvasive computational method focuses with precision, but suffers from modeling and repositioning errors that reduce the accessible acoustic pressure at the focus in comparison with fully experimental time reversal using an implanted hydrophone. In this paper, this simulation-based targeting has been used experimentally as a first step for focusing through an ex vivo human skull at a single location. It has enabled the creation of a cavitation bubble at focus that spontaneously emitted an ultrasonic wave received by the array. This active source signal has allowed 97 +/- 1.1% of the reference pressure (hydrophone-based) to be restored at the geometrical focus. To target points around the focus with an optimal pressure level, conventional electronic steering from the initial focus has been combined with bubble generation. Thanks to step-by-step bubble generation, the electronic steering capabilities of the array through the skull were improved.

Evaluating the robustness of dual apodization with cross-correlation

We have recently presented a new method to suppress side lobes and clutter in ultrasound imaging called dual apodization with cross-correlation (DAX). However, due to the random nature of speckle, artifactual black spots may arise with DAX-processed images. In this paper, we present one possible solution, called dynamic DAX, to reduce these black spots. We also evaluate the robustness of dynamic DAX in the presence of phase aberration and noise. Simulation results using a 5 MHz, 128-element linear array are presented using dynamic DAX with aberrator strengths ranging from 25 ns root-mean-square (RMS) to 45 ns RMS and correlation lengths of 3 mm and 5 mm. When simulating a 3 mm diameter anechoic cyst, at least 100% improvement in the contrast-to-noise ratio (CNR) compared with standard beamforming is seen using dynamic DAX, except in the most severe case. Layers of pig skin, fat, and muscle were used as experimental aberrators. Simulation and experimental results are also presented using dynamic DAX in the presence of noise. With a system signal-to-noise ratio (SNR) of at least 15 dB, we have a CNR improvement of more than 100% compared with standard beamforming. This work shows that dynamic DAX is able to improve the contrast-to-noise ratio reliably in the presence of phase aberration and noise.

Timing-error-difference calibration using reciprocal signals

Timing errors in transmission and reception electronic channels of medical ultrasound imaging systems are generally smaller than one-tenth of a wavelength and do not influence the focusing quality of the system. However, these errors influence the performance of the near-field-signal-redundancy algorithm for correcting phase-aberrations generated by speed heterogeneity in the medium due to its high sensitivity to errors. The effect of timing errors is to make the transmission and reception phase-aberration profiles different. When the difference is much smaller than the period of the signal, an algorithm has been proposed in a previous work to measure the average of the transmission and reception phase-aberration profiles, and it can be used as an approximation to correct phase-aberrations on both transmission and reception. However, when the difference is large, the transmission and reception phase-aberration profiles need to be measured separately. In this paper, several algorithms that use reciprocal signals are proposed to measure the difference profile of the transmission and reception phase-aberration profiles. Their performances are theoretically analyzed, simulated, and experimentally tested. From the measured average and difference profiles, the transmission and reception phase-aberration profiles can be derived separately and used to correct phase-aberrations on transmission and reception, respectively.

The cross algorithm for phase-aberration correction in medical ultrasound images formed with two-dimensional arrays

Common-midpoint signals in the near-field signal-redundancy (NFSR) algorithm for one-dimensional arrays are acquired using three consecutive transducer elements. An all-row-plus-two-column algorithm has been proposed to implement the one-dimensional NFSR algorithm on two dimensional arrays. The disadvantage of this method is that its ambiguity profile is not linear and a timeconsuming iterative method has to be used to linearize the ambiguity profile. An all-row-plus-two-column-and-a-diagonal algorithm has also been proposed. Its ambiguity profile is linear, but it is very sensitive to noise and cannot be used. In this paper, a novel cross algorithm is proposed to implement the NFSR algorithm on two-dimensional arrays. In this algorithm, common-midpoint signals are acquired using four adjacent transducer elements, which is not available in one-dimensional arrays. Its advantage includes a linear ambiguity profile and a higher measurement signal-to-noise ratio. The performance of the cross algorithm is evaluated theoretically. The region of redundancy is analyzed. The procedure for deriving the phaseaberration profile from peak positions of cross-correlation functions between common-midpoint signals is discussed. This algorithm is tested with a simulated data set acquired with a two-dimensional array, and the result shows that the cross algorithm performs better than the all-row plus-twocolumn NFSR algorithm.

Towards aberration correction of transcranial ultrasound using acoustic droplet vaporization

We report on the first experiments demonstrating the transcranial acoustic formation of stable gas bubbles that can be used for transcranial ultrasound aberration correction. It is demonstrated that the gas bubbles can be formed transcranially by phase-transitioning single, superheated, micron-size, liquid dodecafluoropentane droplets with ultrasound, a process known as acoustic droplet vaporization (ADV). ADV was performed at 550 kHz, where the skull is less attenuating and aberrating, allowing for higher-amplitudes to be reached at the focus. Additionally, it is demonstrated that time-reversal focusing at 1 MHz can be used to correct for transcranial aberrations with a single gas bubble acting as a point beacon. Aberration correction was performed using a synthetic aperture approach and verified by the realignment of the scattered waveforms. Under the conditions described below, time-reversal aberration correction using gas bubbles resulted in a gain of 1.9 +/- 0.3 in an introduced focusing factor. This is a small fraction of the gain anticipated from complete transmit-receive of a fully-populated two-dimensional array with sub-wavelength elements. (E-mail: khaworth@umich.edu).

Implementation of the near-field signal redundancy phase-aberration correction algorithm on two-dimensional arrays

Near-field signal-redundancy (NFSR) algorithms for phase-aberration correction have been proposed and experimentally tested for linear and phased one-dimensional arrays. In this paper the performance of an all-row-plus-two-column, two-dimensional algorithm has been analyzed and tested with simulated data sets. This algorithm applies the NFSR algorithm for one-dimensional arrays to all the rows as well as the first and last columns of the array. The results from the two column measurements are used to derive a linear term for each row measurement result. These linear terms then are incorporated into the row results to obtain a two-dimensional phase aberration profile. The ambiguity phase aberration profile, which is the difference between the true and the derived phase aberration profiles, of this algorithm is not linear. Two methods, a trial-and-error method and a diagonal-measurement method, are proposed to linearize the ambiguity profile. The performance of these algorithms is analyzed and tested with simulated data sets.

Overdetermined least-squares aberration estimates using common-midpoint signals

As medical ultrasound imaging moves to larger apertures and higher frequencies, tissue sound-speed variations continue to limit resolution. In geophysical imaging, a standard approach for estimating near-surface aberrating delays is to analyze the time shifts between common-midpoint signals. This requires complete data-echoes from every source/receiver pair in the array. Unfocused common-midpoint signals remain highly correlated in the presence of delay aberrations; there is also tremendous redundancy in the data. In medical ultrasound, this technique has been impaired by the wide-angle, random-scattering nature of tissue. This has made it difficult to estimate azimuth-dependent aberration profiles or to harness the full redundancy in the complete data. Prefiltering the data with two-dimensional fan filters mitigates these problems, permitting highly overdetermined, least-squares solutions for the aberration profiles at many steering angles. In experiments with a tissue-mimicking phantom target and silicone rubber aberrators at nonzero stand-off distances from a one-dimensional phased array, this overdetermined, fan-filtering algorithm significantly outperformed other phase-screen algorithms based on nearest-neighbor cross-correlation, speckle brightness maximization, and common-midpoint signal analysis. Our results imply that there is still progress to be made in imaging with single-valued focusing operators. It also appears that the signal-to-noise penalty for using complete data sets is partially compensated by the overdetermined nature of the problem.

Analysis and correction of ultrasonic wavefront distortion based on a multilayer phase-screen model

A model is introduced that incorporates the cumulative wavefront distortion effects caused by spatial heterogeneities along the path of propagation, and a corresponding model-based wavefront distortion-correction method is presented. In the proposed model, a distributed heterogeneous medium is lumped into a series of parallel phase screens. The distortion effects can be compensated--without a priori knowledge of the distorting structure--by backpropagation of received wavefronts through hypothetical multiple phase screens located between the imaging system and targets, while each pointwise time shift is adjusted iteratively to maximize a specified image quality factor at the final layer. Theoretical analyses indicate that the mean speckle brightness decreases monotonically with the root-mean-square value of distributed phase distortions; therefore, the speckle brightness can be used as an image quality factor. Experimental one-dimensional (1-D) array data with simulated distortion effects based on a real 2-D abdominal-tissue map were used to evaluate the performance of the proposed method and existing aberration-correction techniques. The simulated characteristics of wavefront distortion and relative performance of existing correction techniques were similar to reports based on abdominal-wall data and breast data. This investigation shows that the proposed method provides better compensation for wavefront distortion.

In vivo droplet vaporization for occlusion therapy and phase aberration correction

The objective was to determine whether a transpulmonary droplet emulsion (90%, <6 microm diameter) could be used to form large gas bubbles (>30 microm) temporarily in vivo. Such bubbles could occlude a targeted capillary bed when used in a large number density. Alternatively, for a very sparse population of droplets, the resulting gas bubbles could serve as point beacons for phase aberration corrections in ultrasonic imaging. Gas bubbles can be made in vivo by acoustic droplet vaporization (ADV) of injected, superheated, dodecafluoropentane droplets. Droplets vaporize in an acoustic field whose peak rarefactional pressure exceeds a well-defined threshold. In this new work, it has been found that intraarterial and intravenous injections can be used to introduce the emulsion into the blood stream for subsequent ADV (B- and M-mode on a clinical scanner) in situ. Intravenous administration results in a lower gas bubble yield, possibly because of filtering in the lung, dilution in the blood volume, or other circulatory effects. Results show that for occlusion purposes, a reduction in regional blood flow of 34% can be achieved. Individual point beacons with a +24 dB backscatter amplitude relative to white matter were created by intravenous injection and ADV.

The influences of ambiguity phase aberration profiles on focusing quality in the very near field--part I: single range focusing on transmission

Most phase aberration measurement algorithms have an ambiguity for constant and tilted phase aberration profiles. Based on the Fresnel (near field) approximation with single range focusing and the Fraunhofer (far field) approximation, constant and tilted phase aberration profiles change the position of the focal point only and do not influence the image focusing quality. Therefore, ambiguity phase aberration profiles are generally considered to be harmless and ignored in those algorithms and related theoretical analyses. However, Fresnel and Fraunhofer approximations may become invalid under many medical ultrasound imaging situations, e.g., when the imaging field is in the very near field (f-number approximately 1). In the very near field, although it is known that constant and tilted phase aberration profiles may degrade the focusing quality, it seems that there is a lack of quantitative analysis results in the literature about their influences, and this is the purpose of the current paper. In this paper, a quantitative analysis with a very near field approximation is performed for single range focusing on transmission, which is a commonly used transmission focusing method in medical ultrasound imaging. The tolerable levels of constant and tilted phase aberration profiles are derived as a function of the imaging system's f-number and wavelength. Because some phase aberration measurement algorithms may also have an ambiguity for quadratic phase aberration profiles, they are also included in the analysis. The theoretical results are compared with numerical and simulation results. These results have shown that the influences of tilted and quadratic phase-aberration profiles can be ignored only under certain conditions in the very near field.

Effects of phase aberration on high frame rate imaging

A high frame-rate (HFR) imaging method (about 3750 frames/s for imaging of biological soft tissues at a depth of 200 mm) has been developed recently with limited diffraction beams. This method uses the fast Fourier transform (FFT) and inverse fast Fourier transform (IFFT) to construct images, and can be implemented with simple and inexpensive hardware, compared to the conventional delay-and-sum method where a digital beam former is usually used. In this paper, phase aberration effects are studied for both the high frame rate and the conventional methods by adding random phase shifts to echo signals obtained from an experiment. In the study, two broadband linear arrays were used to construct images of an ATS 539 tissue-equivalent phantom that has a frequency-dependent attenuation of about 0.5 dB/MHz/cm. The first array has 48 elements, a central frequency of 2.25 MHz, an aperture of 18.288 mm, and a width of 12.192 mm in elevation. The second has 64 elements, a central frequency of 2.5 MHz, and a dimension of 38.4 mm x 10 mm. The-6dB pulse-echo bandwidth of both arrays is about 40% of their center frequencies. Radiofrequency (RF) signals were digitized at 20 mega samples/s at a 12-bit resolution to construct images. Results show that phase aberration has about the same effect on both methods in terms of image resolution and contrast, although the high frame-rate method can be implemented with a simpler system.

A Fourier transform-based sidelobe reduction method in ultrasound imaging

Focusing is widely used to increase the resolution in medical ultrasound imaging systems. Focusing increases signal levels returning from the mainlobe direction and decreases those from sidelobe directions. The sidelobes, when not completely cancelled, deteriorate the resulting image resolution. This paper proposes a method of improving the resolution by scaling the received signal according to the ratio between the mainlobe and the sidelobe levels computed in the frequency domain by the use of Fourier transform. The proposed method is verified by computer simulation and experiment and is shown to be highly effective in narrowing the mainlobe width and decreasing the sidelobe levels at the same time.

Phase aberration correction using near-field signal redundancy. I. Principles [Ultrasound medical imaging]

The signal redundancy principle in the near field is analyzed quantitatively. It is found that common midpoint signals are not identical (or redundant) for echoes coming from arbitrary target distributions in the near field. A dynamic near-field correction is proposed to reduce the difference between common midpoint signals for echoes coming from the region of interest. When phase aberrations are present, it is shown that the dynamic correction can generally be done assuming no phase aberration, and the relative time-shift between common midpoint signals can be used to measure phase-aberration profiles. A phase-aberration correction algorithm based on that principle is proposed. In this algorithm, after common midpoint signals are collected they are dynamically corrected for near-field effects and cross-correlated with one another. In a related way, the phase errors are measured from peak positions of these cross-correlation functions. The phase-aberration profile across the array is derived from these measurements. The relationship between the errors in the derived phase aberration profile and the errors in the measured relative time-shift between common midpoint signals is derived. A method for treating the situation of different transmission and reception phase-aberration profiles is also proposed. This algorithm works for general target distributions, iteration is not required, and it can be used in other near-field, pulse-echo, imaging systems.

Simulation of two-dimensional ultrasonic imaging of biological tissues in the presence of phase aberrations

A simulator of phase aberrations for mathematical modeling of two-dimensional (2-D) ultrasonic medical imaging is developed. Principal characteristics of expected phase aberrations were put into the model to investigate the distorting influence of intervening tissues on the quality of conventional medical B-scan images. Information necessary for numerical simulations, including the form of the phase correlation function, correlation length and distortion magnitude, was obtained from analysis of known experimental data on abdominal and breast imaging in vivo. Examples of simulated acoustical images of some simple phantoms are presented. Improvement of image quality due to one simple phase adaptation algorithm is also presented.

Biomedical ultrasound beam forming

The principles of biomedical ultrasound beam forming control the quality of diagnostic imaging. Beam parameters associated with imaging quality are: (1) lateral and axial resolutions; (2) depth of field; (3) contrast and (4) frame rate. In this paper, we review some of the current beam forming techniques and their principles. We focus on trade-offs among the above four aspects of beam forming and relate them to system parameters such as aperture size, f-number (the ratio between focal length and aperture diameter), central frequency (wavelength), system bandwidth and sidelobes. Methods for steering conventional and limited diffraction beams with array transducers are also reviewed.

VLSI circuits for adaptive digital beamforming in ultrasound imaging

For phased-array ultrasound imaging, alternative beamforming techniques and their VLSI circuits are studied to form a fully digital receive front-end hardware. In order to increase the timing accuracy in beamforming, a computationally efficient interpolation scheme to increase the sampling rate is examined. For adaptive beamforming, a phase aberration correction method with very low computational complexity is described. Image quality performance of the method is examined by processing the non-aberrated and aberrated phased-array experimental data sets of an ultrasound resolution phantom. A digital beamforming scheme based on receive focusing at the raster focal points is examined. The sector images of the resolution phantom, reconstructed from the phased-array experimental data by beamforming at the radial and raster focal points, are presented for comparison of the image resolution performances of the two beamforming schemes. VLSI circuits and their implementations for the proposed techniques are presented.

Correction of phase aberrations for sectored annular array ultrasound transducers

Two methods for correction of unknown phase aberrations induced by inhomogeneous acoustic velocities in tissues are explored for the two dimensional geometry of a sectored annular array system. The methods employed are adaptations of a cross correlation technique and a speckle brightness maximization technique. The methods correct phase distortions via the introduction of phase shifts in the timing sequence at the beamformer stage of a sectored annular array transducer. The techniques are investigated employing software models and a computer controlled automated scanning system. A 65-element sectored annular array is modelled via a rotating 5 element transducer. Tissue equivalent materials were moulded into a double layer aberrating medium to simulate phase distortions encountered in the rectus abdominis muscle in vivo. A comparison of the effectiveness of the two correction methods is presented. Contrast of an anechoic region is increased from 0.34 +/- 0.08 to 0.48 +/- 0.06 for the cross correlation technique, and up to 0.62 +/- 0.05 for the speckle brightness maximization method. The performance of these correction techniques on target phantoms suggests that considerable improvements in image quality should be possible for clinical systems.

Mathematical simulation of pressure pulse propagation in biological tissues

The propagation of pressure pulses of arbitrary time form in inhomogeneous attenuating media with characteristics similar to real biological tissues is considered. A mathematical model using the quasi-optical approximation is developed to solve the wave equation. Such a model proves to be convenient for computer realization. A spectral analysis of known experimental data on frequency dependent backscatter is realized to separate coherent and incoherent components of backscatter from tissues and determine their parameters. The characteristics of the scattering are utilized for digital simulations of ultrasound propagation from an interrogated tissue volume. The results presented describe the expected space distribution of the pressure amplitude and phase near the transducer focus in the presence of an aberrating layer with parameters characteristic of abdominal imaging.

A phase aberration correction method for ultrasound imaging

A computationally efficient method for phase aberration correction in ultrasound imaging is presented. The method is based on time delay estimation via minimization of the sum of absolute differences between radio frequency samples of adjacent array elements. Effects of averaging estimated aberration patterns over scan angle and truncation to a single bit wordlength are examined. Phase distortions due to near-field inhomogeneities are simulated using silicone rubber aberrators. Performance of the method is tested using experimental data. Simulation studies addressing different factors affecting efficiency of the method, such as the number of iterations, window length, and the number of scan angles used for averaging, are presented. Images of a standard resolution phantom are reconstructed and used for qualitative testing.

Simulation studies of ultrasonic backscattering and B-mode images of liver using acoustic microscopy data

Data obtained from a scanning laser acoustic microscope (SLAM) were used to examine several aspects of ultrasonic backscattering from the liver. Phase interferograms from normal and abnormal human-liver specimens were digitized, and a series of algorithms was used to compute images of propagation velocity within the specimens. The propagation velocity images were then employed to simulate A- and B-mode results. These initial simulations were used to investigate how ultrasonic echo signals are related to tissue microstructure. Among the topics examined were B-mode speckling, frequency and beamwidth effects, and angulation dependencies.

A statistical analysis of phase aberration correction using image quality factors in coherent imaging systems

A fundamental analysis of phase aberration correction techniques which use speckle brightness-based image equality factors (QFs) to iteratively reduce phase errors is presented. Phase aberration arises from the spatial inhomogeneity of acoustic velocity in human tissue and degrades the performance of diagnostic ultrasonic imaging systems. A theoretical analysis is presented indicating that the mean speckle brightness decreases with root-mean-square (RMS) phase error. A general definition of QFs is given using the probability of error as a criterion of performance. The QF is optimized through minimization of the probability of error under different conditions. The analysis provides a theoretical framework for the current correction technique using QFs under a variety of conditions, and is a useful tool to evaluate new QFs and correction techniques.

Synthetic receive aperture imaging with phase correction for motion and for tissue inhomogeneities. II. Effects of and correction for motion

For Pt.I see ibid., vol.39, p.489 (1992). The effects of tissue/transducer motion and artifacts from adaptive focusing on synthetic receive aperture (SRA) imaging are explored using experiment, simulation, and theory. The impact of these issues on the selection of SRA subaperture geometry is discussed, and a technique to address this problem is demonstrated. The results indicate that SRA with phase correction holds promise in improving ultrasonic image quality.