Quantitative real-time imaging of myocardium based on ultrasonic integrated backscatter

Forward-Looking Multimodal Endoscopic System Based on Optical Multispectral and High-Frequency Ultrasound Imaging Techniques for Tumor Detection

We developed a forward-looking (FL) multimodal endoscopic system that offers color, spectral classified, high-frequency ultrasound (HFUS) B-mode, and integrated backscattering coefficient (IBC) images for tumor detection in situ. Examination of tumor distributions from the surface of the colon to deeper inside is essential for determining a treatment plan of cancer. For example, the submucosal invasion depth of tumors in addition to the tumor distributions on the colon surface is used as an indicator of whether the endoscopic dissection would be operated. Thus, we devised the FL multimodal endoscopic system to offer information on the tumor distribution from the surface to deep tissue with high accuracy. This system was evaluated with bilayer gelatin phantoms which have different properties at each layer of the phantom in a lateral direction. After evaluating the system with phantoms, it was employed to characterize forty human colon tissues excised from cancer patients. The proposed system could allow us to obtain highly resolved chemical, anatomical, and macro-molecular information on excised colon tissues including tumors, thus enhancing the detection of tumor distributions from the surface to deep tissue. These results suggest that the FL multimodal endoscopic system could be an innovative screening instrument for quantitative tumor characterization.

Analysis of Two Quantitative Ultrasound Approaches

There are two well-known ultrasonic approaches to extract sets of quantitative parameters: Lizzi-Feleppa (LF) parameters: slope, intercept, and midband; and quantitative ultrasound (QUS)-derived parameters: effective scatterer diameter (ESD) and effective acoustic concentration (EAC). In this study, the relation between the LF and QUS-derived parameters is studied theoretically and experimentally on ex vivo mouse livers. As expected from the theory, LF slope is correlated to ESD ([Formula: see text]), and from experimental data, LF midband is correlated to EAC ([Formula: see text]). However, LF intercept is not correlated to ESD ([Formula: see text]) nor EAC ([Formula: see text]). The unexpected correlation observed between LF slope and EAC ([Formula: see text]) results likely from the high correlation between ESD and EAC due to the inversion process. For a liver fat percentage estimation, an important potential medical application, the parameters presenting the better correlation are EAC ([Formula: see text]) and LF midband ([Formula: see text]).

Spatial Angular Compounding Technique for H-Scan Ultrasound Imaging

H-Scan is a new ultrasound imaging technique that relies on matching a model of pulse-echo formation to the mathematics of a class of Gaussian-weighted Hermite polynomials. This technique may be beneficial in the measurement of relative scatterer sizes and in cancer therapy, particularly for early response to drug treatment. Because current H-scan techniques use focused ultrasound data acquisitions, spatial resolution degrades away from the focal region and inherently affects relative scatterer size estimation. Although the resolution of ultrasound plane wave imaging can be inferior to that of traditional focused ultrasound approaches, the former exhibits a homogeneous spatial resolution throughout the image plane. The purpose of this study was to implement H-scan using plane wave imaging and investigate the impact of spatial angular compounding on H-scan image quality. Parallel convolution filters using two different Gaussian-weighted Hermite polynomials that describe ultrasound scattering events are applied to the radiofrequency data. The H-scan processing is done on each radiofrequency image plane before averaging to get the angular compounded image. The relative strength from each convolution is color-coded to represent relative scatterer size. Given results from a series of phantom materials, H-scan imaging with spatial angular compounding more accurately reflects the true scatterer size caused by reductions in the system point spread function and improved signal-to-noise ratio. Preliminary in vivo H-scan imaging of tumor-bearing animals suggests this modality may be useful for monitoring early response to chemotherapeutic treatment. Overall, H-scan imaging using ultrasound plane waves and spatial angular compounding is a promising approach for visualizing the relative size and distribution of acoustic scattering sources.

Active droplet sorting in microfluidics: a review

The ability to manipulate and sort droplets is a fundamental issue in droplet-based microfluidics. Various lab-on-a-chip applications can only be realized if droplets are systematically categorized and sorted. These micron-sized droplets act as ideal reactors which compartmentalize different biological and chemical reagents. Array processing of these droplets hinges on the competence of the sorting and integration into the fluidic system. Recent technological advances only allow droplets to be actively sorted at the rate of kilohertz or less. In this review, we present state-of-the-art technologies which are implemented to efficiently sort droplets. We classify the concepts according to the type of energy implemented into the system. We also discuss various key issues and provide insights into various systems.

Ultrasound Imaging Techniques for Spatiotemporal Characterization of Composition, Microstructure, and Mechanical Properties in Tissue Engineering

Ultrasound techniques are increasingly being used to quantitatively characterize both native and engineered tissues. This review provides an overview and selected examples of the main techniques used in these applications. Grayscale imaging has been used to characterize extracellular matrix deposition, and quantitative ultrasound imaging based on the integrated backscatter coefficient has been applied to estimating cell concentrations and matrix morphology in tissue engineering. Spectral analysis has been employed to characterize the concentration and spatial distribution of mineral particles in a construct, as well as to monitor mineral deposition by cells over time. Ultrasound techniques have also been used to measure the mechanical properties of native and engineered tissues. Conventional ultrasound elasticity imaging and acoustic radiation force imaging have been applied to detect regions of altered stiffness within tissues. Sonorheometry and monitoring of steady-state excitation and recovery have been used to characterize viscoelastic properties of tissue using a single transducer to both deform and image the sample. Dual-mode ultrasound elastography uses separate ultrasound transducers to produce a more potent deformation force to microscale characterization of viscoelasticity of hydrogel constructs. These ultrasound-based techniques have high potential to impact the field of tissue engineering as they are further developed and their range of applications expands.

Microfluidic droplet sorting with a high frequency ultrasound beam

This paper presents experimental results demonstrating the feasibility of high frequency ultrasonic sensing and sorting for screening single oleic acid (lipid or oil) droplets under continuous flow in a microfluidic channel. In these experiments, hydrodynamically focused lipid droplets of two different diameters (50 μm and 100 μm) are centered along the middle of the channel, which is filled with deionized (DI) water. A 30 MHz lithium niobate (LiNbO(3)) transducer, placed outside the channel, first transmits short sensing pulses to non-invasively determine the acoustic scattering properties of the individual droplets passing through the beam's focus. Integrated backscatter (IB) coefficients, utilized as a sorting criterion, are measured by analyzing the received echo signals from each droplet. When the IB values corresponding to 100 μm droplets are obtained, a custom-built LabVIEW panel commands the transducer to emit sinusoidal burst signals to commence the sorting operation. The number of droplets tested for the sorting is 139 for 50 μm droplets and 95 for 100 μm droplets. The sensing efficiencies are estimated to be 98.6% and 99.0%, respectively. The sorting is carried out by applying acoustic radiation forces to 100 μm droplets to direct them towards the upper sheath flow, thus separating them from the centered droplet flow. The sorting efficiencies are 99.3% for 50 μm droplets and 85.3% for 100 μm droplets. The results suggest that this proposed technique has the potential to be further developed into a cost-effective and efficient cell/microparticle sorting instrument.

Changes in ultrasonic properties of liver tissue in vitro during heating-cooling cycle concomitant with thermal coagulation

The present work considers the ultrasonic properties of porcine liver tissue in vitro measured during heating concomitant with thermal coagulation followed by natural cooling, so as to provide information about changes in the ultrasonic properties of the tissue after thermal coagulation. The excised liver samples were heated in a degassed water bath up to 75°C and naturally cooled down to 30°C. The tissue was observed to begin thermally coagulating at temperatures lower than 75°C. The ultrasonic parameters considered include the speed of sound, the attenuation coefficient, the backscatter coefficient and the nonlinear parameter of B/A. They were more sensitive to temperature when heating than during natural cooling. All of the parameters were shown to rise significantly on completion of the heating-cooling cycle. At 35°C after thermal coagulation, the B/A value was increased by 96%, the attenuation and backscatter coefficients were increased by 50%∼68% and 33%∼37%, respectively, in the typical frequency ranges of 3 MHz∼5 MHz used for ultrasonic imaging and the speed of sound was increased by 1.4%. The results of this study added to the evidence that tissue characterization, in particular, based on the B/A could be valuable for ultrasonically imaging the thermal lesions following high-intensity focused ultrasound (HIFU) surgery.

Backscattering measurement from a single microdroplet

Backscattering measurements for acoustically trapped lipid droplets were undertaken by employing a P[VDF-TrFE] broadband transducer of f-number = 1, with a bandwidth of 112%. The wide bandwidth allowed the transmission of the 45 MHz trapping signal and the 15 MHz sensing signal using the same transducer. Tone bursts at 45 MHz were first transmitted by the transducer to hold a single droplet at the focus (or the center of the trap) and separate it from its neighboring droplets by translating the transducer perpendicularly to the beam axis. Subsequently, 15 MHz probing pulses were sent to the trapped droplet and the backscattered RF echo signal received by the same transducer. The measured beam width at 15 MHz was measured to be 120 μ m. The integrated backscatter (IB) coefficient of an individual droplet was determined within the 6-dB bandwidth of the transmit pulse by normalizing the power spectrum of the RF signal to the reference spectrum obtained from a flat reflector. The mean IB coefficient for droplets with a 64 μ m average diameter (denoted as cluster A) was -107 dB, whereas it was -93 dB for 90-μm droplets (cluster B). The standard deviation was 0.9 dB for each cluster. The experimental values were then compared with those computed with the T-matrix method and a good agreement was found: the difference was as small as 1 dB for both clusters. These results suggest that this approach might be useful as a means for measuring ultrasonic backscattering from a single microparticle, and illustrate the potential of acoustic sensing for cell sorting.

Layer-dependent variation in the anisotropy of apparent integrated backscatter from human coronary arteries

Clinical imaging of the coronary arteries in the cardiac catheterization laboratory using intravascular ultrasound (IVUS) is known to display a three-layered appearance, corresponding to the intima/plaque, media and adventitia. It is not known whether ultrasonic anisotropy arising from these tissues may alter this pattern in future IVUS systems that insonify in the forward direction or obliquely. In anticipation of such devices, the current study was carried out by imaging fresh human coronary arteries in two orthogonal directions in vitro. Twenty-six sites from 12 arteries were imaged with a side-looking IVUS system, and with an acoustic microscope both radially and axially. Side-looking IVUS and radial acoustic microscopy scans demonstrated the typical "bright-dark-bright" pattern of the backscatter, with the media being significantly darker than the other two layers. Images obtained in the axial orientation exhibited a markedly different pattern, with the relative brightness of the media significantly larger than that of the intima/plaque.

A two-dimensional CVIB imaging system with a speckle tracking algorithm

Quantitative ultrasound tissue characterization based on integrated backscatter (IB) has shown great potential in detecting myocardial ischemia. The magnitude of the cyclic variation in IB (CVIB) has been considered one promising parameter in assessing regional myocardial contractile performance. This lab has previously developed a novel ultrasonic fusion imaging method based on CVIB. However, the major problem for clinical applications of this technique is that the myocardial tissue could not be tracked effectively without cardiologist's intervention. This paper introduced a speckle tracking method into the CVIB-weighted imaging system, called speckle tracking algorithm with adaptive window size (STAWAWS), to track myocardial tissue particle automatically. This method provides a way to obtain the particle's positions frame by frame in a series of B-mode images. Then using the RF signals according to the particle's positions the IB curve can be calculated to produce CVIB value. The method was applied on the experimental and clinical data cases's analysis. The results of dog's data processing showed that this method could eliminate the misunderstanding of myocardial ischemia especially near the endocardium. The results of clinical data suggested that this method had clinical significance in detecting ischemic myocardium. Though the CVIB-weighted images obtained by the use of this auto-tracking method can improve the accuracy of detecting myocardial ischemia, it is not real-time analysis and the clinical data cases are not sufficient. Further clinical validation is still needed in the future' work.

2-D CVIB imaging in animal experiments

To demonstrate the feasibility and validity of 2-D CVIB (Cyclic variation of Integrated Backscatter) imaging method, animal experiments were conducted. Among 10 anesthetized open-chest dogs, acute myocardial ischemia was induced by occluding left anterior descending coronary artery. While scanning normal hearts and ischemic hearts with the ultrasonic B scanner, digital radiofrequency data were acquired by a real-time acquisition system in synchronism. The offline analysis to the radio-frequency signal with the 2-D CVIB imaging method was performed to verify the consistency between the imaging results and the design of the experiments. The experimental results showed that the 2-D CVIB imaging method was successful in detecting the ischemic myocardium and might provide a new noninvasive way for the cardiologists to both quantitatively and visually evaluate the contractile performance of the myocardium.

Monitoring imaging of lesions induced by high intensity focused ultrasound based on differential ultrasonic attenuation and integrated backscatter estimation

We investigated the feasibility of two monitoring imaging methods to visualize and evaluate the high intensity focused ultrasound (HIFU) induced lesions in vitro during and after their formation, which were based on differential ultrasonic parameter estimation. Firstly, ultrasonic attenuation slope of tissue sample was estimated based on the spectral analysis of ultrasound RF backscattered signals. The differential attenuation slope maps were acquired, which were interpreted as the differences between the pretreatment image and those obtained in different stages during HIFU therapy. Secondly, ultrasonic integrated backscatter (IBS), defined as the frequency average of the backscatter transfer function over the useful bandwidth, was proposed quantitatively to evaluate the extent of lesions with the same RF signals as the first method. Differential IBS maps were also acquired to visualize temporal evolution of lesion formation. It was found in pig liver in vitro that more precise definition of the treated area was obtained from the differential IBS images than from differential attenuation slope images. Dramatic increase in both attenuation and IBS value was observed during the therapy, which may be related to dramatic enhancement of cavitation due to boiling and accompanying tissue damage. Two methods to obtain one differential image were compared and the cumulative differential image was found to be able to eliminate noises and artifacts to some extent, which was the cumulation of a series of differential images acquired from the differences between the temporally adjacent RF data frames. Moreover, we presented a bidirectional color code for identification of the artifacts due to tissue movements caused by HIFU radiation force. We conclude that cumulative differential IBS images have the potential to monitor the formation of HIFU-induced lesions.

Differential ultrasonic imaging for the characterization of lesions induced by high intensity focused ultrasound

High intensity focused ultrasound (HIFU) is an effective technique for noninvasive local creating coagulative necrotic lesions in deep target volumes without damage to the overlaying or surrounding tissues. It is very important to detect and evaluate lesions generated by HIFU during treatment procedures. This study describes the development of several differential ultrasonic imaging techniques to characterize lesions based on estimation of relative changes in tissue properties derived from backscattered RF data. A single, spherical HIFU transducer was used to produce lesions in soft tissues. The RF signals were recorded as outputs from a modified diagnostic ultrasound system. After some preprocessing, the integrated backscatter values, which can be used as an indicator of the microstructure and backscattering property of tissues, were calculated before and after HIFU treatment. The differential integrated backscatter values were subsequently used to form images revealing the lesion areas. The differential attenuation imaging with the same RF data was also performed, which has been proposed by a few researchers. The results of the differential integrated backscatter imaging were compared with that of the differential attenuation imaging and the former method offers some advantages over the latter method. The two methods above are both based on spectrum analysis and would spend much computational time. Therefore, some simple digital differential imaging methods, including absolute difference (AD), sum absolute differences (SAD), and sum squared differences (SSD) algorithms, were also proposed to detect HIFU-induced lesions. However, these methods cannot provide the information of the degree of tissue damage. Experiments in vitro bovine muscle and liver validated the method of differential integrated backscatter imaging for the characterization of HIFU-induced lesions. And the AD, SAD, and SSD algorithms can be implemented in real-time during HIFU therapy to visualize the lesions.

Computer simulation model on ultrasonic myocardial backscatter

Myocardial ultrasonic tissue characterization with integrated backscatter (IB) has been studied in recent years. To evaluate the dynamic characters of the myocardium, dynamic tracing of each unit volume of myocardium is a key point. Thus, a 2D CVIB imaging algorithm based on an auto-tracing method was developed in our previous work. Where, an auto-tracking method was used in CVIB-weighted images so that the POI in each frame throughout the cardiac cycle is automatically traced to reflect the real myocardial tissue movement. It is necessary to validate the auto-tracing method. However, at present there are no appropriate models to support the process. In order to validate the auto-tracing method, two myocardium models have been established to simulate the short and long axis view of echocardiography. The motions of partially ischemic myocardium have been simulated. The models have been used to validate the 2D CVIB imaging methods in the detection of myocardial ischemia. The simulation results show that the auto-tracing algorithm is effective. The model developed in this work provides a tool for studying and developing technologies in myocardial behavior.

On the statistics of ultrasonic spectral parameters

Several factors affect the accuracy and precision of ultrasonic spectrum analysis, which is used for characterization of normal and diseased tissue in a variety of organs. For example, averaging procedures and the sequence of operations affect the accuracy and precision of spectrum analysis. Averaging procedures and logarithmic conversion (i.e., conversion to dB) introduce a constant bias that affects spectral amplitudes and the values of intercept and midband fit; the bias depends on the sequencing of the log conversion and averaging as well as the number of independent spectra or spectral parameters that are averaged. We derive expressions that permit correction of such biases. Furthermore, we show that standard deviations for slope and midband-fit estimation can be minimized by averaging spectra before dB conversion and before computing spectral parameters by linear regression. Experimental results using phantoms agree remarkably with theoretical predictions for the data window functions studied in this article, Hamming and rectangular.

Ultrasonic tissue characterization of the mouse myocardium: Successful in vivo cyclic variation measurements

BACKGROUND

Measurements of the systematic variation of backscattered ultrasonic energy from myocardium during the heart cycle (cyclic variation) have been successfully used to characterize a wide spectrum of cardiac pathologies in large animal models and human subjects. The purpose of this study was to evaluate the feasibility of extending cyclic variation measurements to the study of genetically manipulated mouse models of cardiac diseases as a method for developing further insights into the disease-altered properties of the myocardium and its characterization with ultrasound.

METHODS

Parasternal long-axis images of the heart were obtained in 9 wild-type mice under light anesthesia using a commercial imaging system with a 15-MHz nominal center frequency linear array. Images of a tissue-mimicking phantom and the mouse hearts were obtained for a series of specific receiver gains for each of a series of specific dynamic range settings. Analyses of these data formed the basis for gray-scale image calibration. Cyclic variation measurements were obtained by determining the average gray-scale value for a region of interest placed in the midmyocardium of the posterior wall for each frame acquired during 4 cardiac cycles and converting these mean gray-scale values to backscatter values expressed in decibels using the determined calibration. Results are expressed in terms of the magnitude and time delay of cyclic variation. To evaluate repeatability of these measurements the same group of mice underwent the identical imaging protocol 2 weeks after the first study.

RESULTS

The mean magnitude of cyclic variation was found to be 4.6 +/- 0.2 dB with a corresponding normalized time delay of 1.02 +/- 0.03 for data averaged over all dynamic range settings. There was no significant difference among results obtained with each of the dynamic range settings. A comparison of these results with those from data acquired 2 weeks after the initial study showed no significant difference.

CONCLUSION

This study represents the first reported measurement of cyclic variation in mice and demonstrates that reliable cyclic variation measurements can be obtained among individual animals and over different time points and, hence, forms the basis for subsequent investigations addressing specific cardiac pathologies and effects arising from myocardial anisotropy.

Hardware and software platform for real-time processing and visualization of echographic radiofrequency signals

In this paper the architecture of a hardware and software platform, for ultrasonic investigation is presented. The platform, used in conjunction with an analog front-end hardware for driving the ultrasonic transducers of any commercial echograph, having the radiofrequency echo signal access, make it possible to dispose of a powerful echographic system for experimenting any processing technique, also in a clinical environment in which real-time operation mode is an essential prerequisite. The platform transforms any echograph into a test-system for evaluating the diagnostic effectiveness of new investigation techniques. A particular user interface was designed in order to allow a real-time and simultaneous visualization of the results produced in the different stages of the chosen processing procedure. This is aimed at obtaining a better optimization of the processing algorithm. The most important platform aspect, which also constitutes the basic differentiation with respect to similar systems, is the direct processing of the radiofrequency echo signal, which is essential for a complete analysis of the particular ultrasound-media interaction phenomenon. The platform completely integrates the architecture of a personal computer (PC) giving rise to several benefits, such as the quick technological evolution in the PC field and an extreme degree of programmability for different applications. The PC also constitutes the user interface, as a flexible and intuitive visualization support, and performs some software signal processing, by custom algorithms and commercial libraries. The realized close synergy between hardware and software allows the acquisition and real-time processing of the echographic radiofrequency (RF) signal with fast data representation.

Novel ultrasonic fusion imaging method based on cyclic variation in myocardial backscatter

Quantitative ultrasonic tissue characterisation of the myocardium based on integrated backscatter (IB) has the potential of becoming an effective method for detecting and evaluating myocardial ischaemia. To facilitate IB-based clinical applications, a new imaging method has been developed that combines the anatomical information of a B-mode image with the contractile performance of a selected myocardial region. To produce such a fusion image, a region of interest (ROI) in a B-mode cardiac image was first selected by the user. Algorithms for detection of the endocardium and epicardium were developed, and the resulting mean distance between the computer-detected curve and the manually traced curve was 0.83mm for the endocardium and 0.58mm for the epicardium. The cyclic variation of IB (CVIB) of each myocardial tissue element within the ROI was then calculated over one cardiac cycle. Finally, a grey-scale B-mode image at the end of diastole was displayed as a still image, and the pixels representing the myocardial tissue in the ROI colour-coded according to the corresponding CVIB over the past heart cycle. Both the B-mode image and the colour-coded region were refreshed (up-dated) at the next end-of-diastole. Preliminary results from normal (CVIB= 10-12dB) and ischaemic (CVIB = 5-7 dB) canine hearts are presented that demonstrate the utility of this new imaging method.

Calibrated parametric medical ultrasound imaging

The goal of this study was to develop a calibrated on-line technique to extract as much diagnostically-relevant information as possible from conventional video-format echograms. The final aim is to improve the diagnostic potentials of medical ultrasound. Video-output images were acquired by a frame grabber board incorporated in a multiprocessor workstation. Calibration images were obtained from a stable tissue-mimicking phantom with known acoustic characteristics. Using these images as reference, depth dependence of the gray level could fairly be corrected for the transducer performance characteristics, for the observer-dependent equipment settings and for attenuation in the examined tissues. Second-order statistical parameters still displayed some nonconsistent depth dependencies. The results obtained with two echoscanners for the same phantom were different; hence, an a posteriori normalization of clinical data with the phantom data is indicated. Prior to processing of clinical echograms,. the anatomical reflections and echoless voids were removed automatically. The final step in the preprocessing concerned the compensation of the overall attenuation in the tissue. A 'sliding window' processing was then applied to a region of interest (ROI) in the 'back-scan converted' images. A number of first and second order statistical texture parameters and acoustical parameters were estimated in each window and assigned to the central pixel. This procedure results in a set of new 'parametric' images of the ROI, which can be inserted in the original echogram (gray value, color) or presented as a color overlay. A clinical example is presented for illustrating the potentials of the developed technique. Depending on the choice of the parameters, four full resolution calibrated parametric images can be calculated and simultaneously displayed within 5 to 20 seconds. In conclusion, an on-line technique has been developed to estimate acoustic and texture parameters with a reduced equipment dependence and to display acoustical and textural information that is present in conventional echograms.

Critical appraisal of left ventricular function assessment by the automated border detection method on echocardiography. Is it good enough?

Many studies have attempted to validate the echocardiographic automated border detection (ABD) method for assessing left ventricular ejection fraction (LVEF) by comparing it with various echocardiographic and non-echocardiographic standards. The main basis of assessing its accuracy has been the coefficient of correlation. The fallacy of using coefficient of correlation for assessing agreement between two methods of measurement has been well emphasized in the literature. In the present study we used the Bland and Altman test for testing the accuracy of the ABD method. We compared the ABD method for LVEF assessment with the manual edge detection technique on echocardiography and with radionuclide ventriculography in 34 patients. The majority of patients (76%) had regional wall motion abnormality. The ABD method could be adequately performed in 25 (74%) patients. LVEF was significantly underestimated by the ABD method with very wide limits of agreement when compared with radionuclide ventriculography and the manual edge detection technique (-9.2+/-21.7 and -2.7+/-18.4 respectively, mean error+/-2 standard deviations). Stated simply, the ABD method could overestimate LVEF by 12.5 and 15.7 or underestimate by 30.9 and 21.1 when compared with radionuclide ventriculography and manual edge detection technique, respectively. This large error is by no means acceptable for clinical purposes. It is concluded that at the present stage, the ABD method cannot replace radionuclide ventriculography and manual edge detection technique for assessing LVEF.

Effects of tissue anisotropy on the spectral characteristics of ultrasonic backscatter measured with a clinical imaging system

In this paper, we report the effects of inherent tissue anisotropy on the spectral properties of backscattered ultrasound when measured with a commercially-available imaging system. We insonified five specimens of bovine tendon immersed in a water tank and rotated in 10 degrees increments while being imaged with a Hewlett-Packard Sonos 1500 system. The backscattered RF signals corresponding to each angle of insonification were digitized and the spectral characteristics of the backscattered ultrasound were determined. The mean anisotropy, defined as the average difference between values at perpendicular and parallel insonification, for band-limited estimates of backscattered power, centroid frequency, upper-band to lower-band power ratio, and upper-band to total-band power ratio were found to be 24.6 +/- 1.1 dB, 142 +/- 27 kHz, 32 +/- 13%, and 22 +/- 5%, respectively (mean +/- SE). The magnitude of each of these backscatter spectral parameters was larger at perpendicular insonification compared with the corresponding values at parallel insonification, consistent with previous measurements of the inherent anisotropy of ultrasonic attenuation and backscatter in tissue.

Imaging and spectrum analysis of contrast agents in the in vivo rabbit eye using very-high-frequency ultrasound

We have conducted initial studies that demonstrated the feasibility of employing ultrasonic contrast agents with very-high-frequency ultrasound (VHFU), using wideband transducers with center frequencies near 40 MHz. These studies were undertaken with an ultimate objective of quantifying perfusion in vessels in the eye and other organs. We expanded the model developed by Lizzi et al. (1983) to incorporate the scattering characteristics from encapsulated bubbles, such as contrast agents. Our analysis shows how the spectral slopes and intercepts measured from contrast agents are related to factors that include the radii and concentration of contrast-agent particles. We conducted in vitro experiments to validate the theoretical predictions and obtained excellent agreement. We obtained in vivo VHFU data from the eyes of anesthetized rabbits before and after injection of Albunex and Aerosomes. Digitally computed B-mode images demonstrated echo enhancement within the ciliary body and its processes. The magnitudes of these enhancements were quantified using calibrated spectrum-analysis techniques.

Statistical framework for ultrasonic spectral parameter imaging

This study examines the statistics of ultrasonic spectral parameter images that are being used to evaluate tissue microstructure in several organs. The parameters are derived from sliding-window spectrum analysis of radiofrequency echo signals. Calibrated spectra are expressed in dB and analyzed with linear regression procedures to compute spectral slope, intercept and midband fit, which is directly related to integrated backscatter. Local values of each parameter are quantitatively depicted in gray-scale cross-sectional images to determine tissue type, response to therapy and physical scatterer properties. In this report, we treat the statistics of each type of parameter image for statistically homogeneous scatterers. Probability density functions are derived for each parameter, and theoretical results are compared with corresponding histograms clinically measured in homogeneous tissue segments in the liver and prostate. Excellent agreement was found between theoretical density functions and data histograms for homogeneous tissue segments. Departures from theory are observed in heterogeneous tissue segments. The results demonstrate how the statistics of each spectral parameter and integrated backscatter are related to system and analysis parameters. These results are now being used to guide the design of system and analysis parameters, to improve assays of tissue heterogeneity and to evaluate the precision of estimating features associated with effective scatterer sizes and concentrations.

Ultrasonic Tissue Characterization: Review of a Noninvasive Technique for Assessing Myocardial Viability

The determination of myocardial perfusion and myocardial viability has prognostic and therapeutic implications, particularly in the current era of percutaneous transluminal coronary angioplasty and thrombolytic therapy. Several modes of investigation, including positron emission tomography, thallium-201 scintigraphy, and nuclear magnetic resonance imaging are used to differentiate viable from nonviable myocardium. Though these noninvasive tests are useful diagnostic modalities, they are expensive, time consuming, and too cumbersome to be used in the acute setting. Expeditious distinction between viable and nonviable myocardium, during acute coronary syndromes, is of great importance since reperfusion can minimize the extent of ischemic injury and infarction. An expanding body of evidence confirms that ultrasonic tissue characterization has great potential to become a practical bedside diagnostic tool in the search for salvageable myocardium. Further clinical investigative studies would help accomplish a better understanding of the complex interaction between ultrasound and myocardium. (ECHOCARDIOGRAPHY, Volume 13, July 1996)

Quantitative three-dimensional reconstruction of left ventricular volume with complete borders detected by acoustic quantification underestimates volume

Recently a new acoustic-quantification (AQ) technique has been developed to provide on-line automated border detection with an integrated backscatter analysis. Prior studies have largely correlated AQ areas with volumes without direct comparison of volumes for agreement. By using complete AQ-detected borders as the input to a validated method for three-dimensional echocardiographic (3DE) reconstruction, we can compare an entire cavity volume measured with the aid of AQ against a directly measured volume. This would also explore the possibility of applying AQ to 3DE reconstruction to reduce tracing time and enhance routine applicability. To compare reconstructed volumes with actual values in a stable standard allowing direct volume measurement, the left ventricles of 13 excised animal hearts were studied with a 3DE system that automatically combines two-dimensional (2D) images and their locations. Intersecting 2D views were obtained with conventional scanning and AQ imaging, with gains optimized to permit 3D reconstruction by detecting the most continuous AQ borders for each view, with maximal cavity size. Reconstruction was performed with manually traced central endocardial reflections and AQ-detected borders visually reproduced the left ventricular shapes; the AQ reconstructions, however, were consistently smaller. The reconstructed left ventricular (LV) volumes correlated well with actual values by both manual and AQ techniques (r = 0.93 and 0.88, with standard errors of 2.3 cc and 2.0 cc, p = not significant [NS]). Agreement with actual values was relatively close for the manually traced borders (y = 0.93x + 0.68, mean difference = -0.8 +/-2.2 cc). AQ-derived reconstructions consistently underestimated LV volume by 39 +/- 10% (y = 0.62x-0.09, mean difference = -7.8 +/- 3.0 cc, different from manually traced and actual volumes by analysis of variance [ANOVA], F = 69, p<0.00001). The AQ-detected threshold signal was displaced into the cavity, and volume between walls and false tendons was excluded, leading to underestimation, which increased with increasing cavity volume (r = 0.76). The AQ technique can therefore be applied to 3DE reconstruction, providing volumes that correlate well with directly measured values in a stable in vitro standard, minimizing observer decisions regarding manual border placement after image acquisition. However, when the complete borders needed for 3D reconstruction are used, absolute volumes are underestimated with current algorithms that integrate backscatter and displace the detected threshold into the ventricular cavity.

Ultrasonic backscatter system for automated on-line endocardial boundary detection: evaluation by ultrafast computed tomography

OBJECTIVES

The purpose of this study was to evaluate the accuracy of the recently developed echocardiographic on-line endocardial border detection system using ultrafast computed tomography, an independent and proved tomographic imaging modality.

BACKGROUND

The automated system for on-line endocardial border detection identifies the blood-tissue interface by acoustic quantification of the ultrasonic backscatter signal.

METHODS

Eighteen subjects were screened by conventional echocardiography and acoustic quantification. Ten of these, with high quality echocardiographic images, were also examined by ultrafast computed tomography. Comparable image planes at the midpapillary level were analyzed. Measurements of left ventricular cavity area were compared at end-diastole and end-systole and time course analyses of cavity area during the cardiac cycle were performed.

RESULTS

There was good correlation between values for left ventricular end-diastolic area (r = 0.99), end-systolic area (r = 0.93) and fractional area change (r = 0.91) using the two methods. The on-line backscatter system underestimated end-diastolic area (p < 0.001), but the negative bias was small (-1.6 cm2) and the 95% confidence intervals were narrow (-3.6 cm2 to +0.4 cm2). In contrast, the backscatter system overestimated end-systolic area (p < 0.02); the positive bias for this variable was also small (+2.6 cm2) but the confidence intervals were relatively wide (+7.9 to -2.8 cm2). The negative bias of backscatter values for cavity area was fairly constant during diastole and early systole (range -5% to -10%), but during the second half of systole, backscatter values increased progressively relative to computed tomographic values. Real time values for fractional area change measured by the backscatter system were 13% smaller than those determined by ultrafast computed tomography (p < 0.001), with wide confidence intervals (+3% to -30%). Absolute peak rates of area change during systole and diastole were lower by 39% (p < 0.001) and 41% (p < 0.01), respectively, using the on-line ultrasonic backscatter system. Time course analyses revealed the errors to be consistent with cardiac cycle-dependent alterations in gain sensitivity of the ultrasonic backscatter system.

CONCLUSIONS

The ultrasonic backscatter system is associated with cyclic cavity area measurement errors that need to be addressed if its early promise for on-line assessment of ventricular function is to be fulfilled. Incorporation of an electrocardiographically triggered time-varying gain control may improve accuracy for on-line analysis of ventricular performance.

On-line assessment of left atrial area and function by echocardiographic automatic boundary detection

BACKGROUND

Direct assessment of left atrial (LA) function has not been previously performed by noninvasive techniques; rather, LA function has been evaluated only indirectly via the analysis of transmitral flow velocity by Doppler. The recent development of real-time two-dimensional echocardiographic automatic boundary detection suggests that LA dimensions can be measured instantaneously to provide on-line assessment of its systolic and diastolic functions.

METHODS AND RESULTS

We performed echocardiographic assessment of LA dimensions and function with automatic boundary detection in 45 patients by using the apical four-chamber view. Thirty-seven patients had structural or functional cardiac abnormalities, 35 patients were in sinus rhythm, and 10 patients had atrial fibrillation. Moderate to severe mitral regurgitation (MR) was noted in 16 patients. We also studied 10 control subjects to assess normal values of LA cavity area and indexes of function. From the instantaneously derived LA area, we derived indexes of systolic atrial expansion and diastolic atrial emptying. There were excellent correlations between the on-line-derived LA areas and those measured off line from videotaped images of conventional echocardiography (r = .91 for end-diastolic and .93 for end-systolic areas; SEE, 4.0 and 3.8 cm2, respectively). Patients in atrial fibrillation had depressed diastolic emptying index (0.17 +/- 0.05) compared with those in sinus rhythm (0.28 +/- 0.12; P < .02). Furthermore, patients with chronic MR exhibited larger LA cavity areas and depressed systolic and diastolic LA function as compared with those without MR. In addition, the Doppler-determined mitral E/A ratio was related to the ratio of early diastolic-to-late diastolic change in LA cavity area (r = .79; SEE 0.6; n = 35).

CONCLUSIONS

Instantaneous LA cavity area measurement by echocardiographic automatic boundary detection is accurate and feasible in patients with diverse cardiac disorders. Patients with atrial fibrillation had a depressed diastolic emptying index and those with significant mitral regurgitation had depressed systolic expansion index as well. LA functional indexes in both systole and diastole can be derived providing an approach for quantitative evaluations of left atrial-left ventricular interactions based on geometric assessment noninvasively.

Quantification of ultrasonic anisotropy in normal myocardium with lateral gain compensation of two-dimensional integrated backscatter images

Anisotropy of ultrasonic scattering and attenuation in heart tissue depends on the specific orientation of myofibers with respect to angle of insonification. We used lateral gain compensation (LGC) to correct two-dimensional cardiac images for physiologic anisotropy. Normal hearts excised from three dogs and five pigs were insonified in a water tank with both 2.5 and 5.0 MHz phased-array transducers. Integrated backscatter was measured from a short-axis approach in the anterior wall perpendicular to the principal fiber axis, and in the septum parallel to the fiber axis. The gain in a vertical sector encompassing the septum was adjusted to compensate the image for anisotropy by matching the intensity of scattering from septal and anterior regions. The average gain required to compensate the septum for anisotropy was 16 dB at 2.5 MHz, and 20 dB at 5.0 MHz. Five healthy volunteers underwent imaging with a 2.5 MHz transducer from a parasternal short-axis view. The LGC required in vivo was approximately 16 dB at 2.5 MHz and was equivalent to that required for correction of septal anisotropy in excised hearts. Thus, normal myocardium exhibits substantial ultrasonic anisotropy that can be quantified and compensated for with clinically applicable tissue characterization techniques.

Automated, on-line quantification of left ventricular dimensions and function by echocardiography with backscatter imaging and lateral gain compensation

To provide on-line quantification of left ventricular cavity dimensions and function by echocardiography 60 control subjects and 10 patients with cardiac dysfunction were studied. A novel, ultrasound imaging system was used which was developed to detect and track, in real time, ventricular endocardial blood boundaries based on quantitative assessment of acoustic properties of tissue. In addition, lateral gain compensation, a robust and novel image enhancement procedure, was used to provide instantaneous measurement and display of cavity areas and functional indexes on a beat-by-beat basis within regions of interest drawn around the blood pool cavity. In control subjects, short-axis end-diastolic area averaged 13.1 +/- 3.7 cm2 (SD), end-systolic area 5.9 +/- 2.7 cm2, and fractional area change 55.6 +/- 11.2%. Apical views yielded corresponding values of 23.8 +/- 4.5 cm2, 15.5 +/- 3.4 cm2 and 34.7 +/- 7.8%. Instantaneous peak rate of cavity area change approximated 50 cm2/s in systole and 60 cm2/s in diastole in each view. Serial measurements of area and functional index were reproducible over intervals of 2 to 3 weeks. Patients with dilated ventricles exhibited average apical view area values of 49.1 +/- 6.1 cm2 and 43.1 +/- 4.9 cm2 in diastole and systole with a fractional area change of 12.2 +/- 3.0%. Thus, results with on-line echocardiographic backscatter imaging-assisted automated edge detection are reproducible and capable of delineating cardiac dysfunction conveniently, promptly and serially at the bedside.

Detection of unique transmural architecture of human idiopathic cardiomyopathy by ultrasonic tissue characterization

BACKGROUND

Noninvasive approaches to the evaluation of idiopathic cardiomyopathy are limited. Recent work from our laboratory has used quantitative ultrasound to define the three-dimensional structure of normal human myocardium and the myocardial remodeling associated with infarction. Our goal was to define the role of ultrasonic tissue characterization for detection of specific alterations in the three-dimensional transmural architecture of idiopathic dilated cardiomyopathy.

METHODS AND RESULTS

We measured frequency-dependent backscatter from 22 cylindrical biopsy specimens from nine explanted fixed hearts of patients who underwent heart transplantation for idiopathic cardiomyopathy, seven specimens from normal portions, and 12 specimens of infarcted tissue from six explanted fixed human hearts. Consecutive transmural levels from each specimen were insonified with a 5-MHz broadband transducer. The dependence of apparent (uncompensated for attenuation) backscatter, B(f), on frequency (f) was computed from radiofrequency (rf) data as: magnitude of B(f)2 = afn, where n is an index that reflects in part the size of the dominant scatterers in myocardial tissue. Myofiber diameter and percentage fibrosis were determined at each transmural level for each specimen. For cardiomyopathic tissue, the frequency dependence of backscatter (n) increased progressively from epicardial to endocardial (0.02 +/- 0.37 to 1.01 +/- 0.12, p less than 0.05) levels in conjunction with a progressive decrease in myofiber diameter (29.5 +/- 0.9 to 21.4 +/- 0.6 microns, p less than 0.0001). In contrast, in tissue from areas of infarction, the frequency dependence decreased progressively from epicardium to endocardium (0.91 +/- 0.20 to 0.23 +/- 0.21, p less than 0.05) in conjunction with a progressive increase in the percentage of fibrosis (23.5 +/- 9.4% to 54.5 +/- 4.9%, p less than 0.005). Normal tissue exhibited no significant transmural trend for frequency dependence, myofiber diameter, or percentage fibrosis.

CONCLUSIONS

These data indicate the presence of a heterogenous transmural distribution of scattering structures associated with human idiopathic cardiomyopathy and myocardial infarction that may be detected by ultrasonic tissue characterization. The divergence of these transmural trends for frequency dependence of backscatter reflects distinct mechanisms of structural heterogeneity for different pathological processes that comprise a transmural gradation of cell size and fibrosis for idiopathic cardiomyopathy and infarction, respectively.

Quantitative ultrasonic imaging: tissue characterization and instantaneous quantification of cardiac function

Quantitative myocardial tissue characterization is being developed to complement and expand conventional echocardiography by delineating the physical state of myocardium under diverse pathophysiologic conditions. Real-time quantitative integrated backscatter imaging has already been applied to patients with ischemic heart disease, hypertrophic cardiomyopathy, and cardiac allograft rejection in clinical investigations performed in the United States, Europe, and Japan. A recently introduced modification of imaging processing algorithms employed for characterization of tissue facilitates automatic detection of endocardial-blood interfaces and on-line quantification of ventricular size and function. Further progress and anticipated developments in quantitative ultrasonic imaging will undoubtedly augment the clinical applications of tissue characterizations based on myocardial integrated backscatter for improved diagnosis, elucidation of pathophysiology, and assessment of cardiac function.

On-line assessment of ventricular function by automatic boundary detection and ultrasonic backscatter imaging

To provide an approach suitable for on-line analysis of ventricular function, a conventional two-dimensional ultrasound imaging system was modified to detect and track blood-tissue interfaces in real time based on their quantitative acoustic properties. This modification permitted on-line display of the left ventricular cavity area, fractional area change, volumes and ejection fraction on a beat by beat basis. Images were obtained from 54 patients and 12 normal subjects with broad ranges of ventricular dimensions and systolic function. On-line measurements of cavity areas were compared with off-line measurements of cavity areas (analysis of videotaped conventional images). Left ventricular cavity areas measured on-line from short-axis views correlated closely with off-line views as did areas from apical views. On-line fractional area change correlated well with ejection fraction calculated off-line. More than 70% of patients could be studied adequately with the approach developed. Thus, automatic boundary detection based on quantitative assessment of tissue acoustic properties permits on-line quantitation of ventricular cavity areas and indexes of function.

Structural remodeling of human myocardial tissue after infarction. Quantification with ultrasonic backscatter

BACKGROUND

Remodeling of myocardial tissue after infarction may culminate in the development of either a well-healed scar or a thin, expanded heart wall segment that predisposes to ventricular aneurysm formation, congestive heart failure, or ventricular tachycardia. The three-dimensional architecture of mature human infarct tissue and the mechanisms that determine it have not been elucidated. We have previously shown that quantitative ultrasonic backscatter can be used to define the transmural organization of human myofibers in the normal ventricular wall by measuring the dependence of backscatter on the angle of insonification, or ultrasonic anisotropy. We propose that measurement of ultrasonic anisotropy of backscatter may permit quantitative characterization of the transmural architecture of tissue from areas of myocardial infarction and facilitate identification of fundamental mechanisms of remodeling of the ventricular wall.

METHODS AND RESULTS

We measured integrated backscatter in 33 transmural sections from 12 cylindrical biopsy specimens (1.4-cm diameter) sampled from central regions of mature infarction in six explanted fixed human hearts. Tissue samples were insonified in two-degree steps around their entire circumference at successive transmural levels with a 5-MHz broad-band piezoelectric transducer. Backscatter radio frequency data were gated from the center of each specimen, and spectral analysis was performed on the gated radio frequency for the computation of integrated backscatter. Histological morphometric analysis was performed on each specimen for determination of the predominant fiber orientation and the percentage of tissue infarcted at consecutive transmural levels. The average percentage of tissue infarcted for all transmural levels was 49 +/- 3% (range, 13-80%). Histological attributes varied from patchy fibrosis to extensive confluent zones of scar tissue. The angle-averaged integrated backscatter for all transmural levels in infarct tissue was approximately 5 dB greater than that previously measured in normal tissue in our laboratory (-48.3 +/- 0.5 versus -53.4 +/- 0.4 dB, infarct versus normal). Marked anisotropy of backscatter was observed in tissue from areas of infarction and was characterized by a sinusoid-like dependence on the angle of insonification at each transmural level. Insonification perpendicular to infarct fibers yielded values for integrated backscatter 14.8 +/- 0.5 dB greater than those for insonification parallel to these fibers. Juxtaposition of the sinusoid-like anisotropy functions from all consecutive transmural levels demonstrated a progressive shift in the orientation of scar tissue elements from epicardial to endocardial levels of 14.6 +/- 1.5 degrees/mm of tissue. The transmural shift in fiber orientation per millimeter of tissue from the area of infarction exceeded that previously measured for normal tissue (9.2 +/- 0.7 degrees/mm) by 59%. This marked augmentation in angular shift per millimeter of tissue results from a generalized structural rearrangement (or reorientation) of fibers across the entire ventricular wall in the infarct zone that we hypothesize is determined in part by dynamic mechanical forces, imposed by the surrounding functional normal tissue, that tether the "infarcted" tissue.

CONCLUSIONS

Myocardial tissue from areas of myocardial infarction manifests substantial anisotropy of ultrasonic scattering that may be useful for quantitative characterization of the alignment and overall three-dimensional anatomic organization of mature infarct scars.

Ultrasonic myocardial tissue characterization in the operating room: initial results using transesophageal echocardiography

Ultrasonic tissue characterization provides quantitative assessment of myocardial function and viability. We have previously reported that normal myocardium is characterized by a diastolic-to-systolic cyclic variation of integrated backscatter (IB), whereas ischemic myocardium exhibits blunting of this pattern. To define the applicability of this measurement to characterize the left ventricular myocardium in the operating room, we studied 26 consecutive patients undergoing open heart surgery (12 coronary artery bypass graft, 13 valvular, 1 other) with 5 MHz transesophageal echocardiography. Images of the left ventricle were obtained in the short-axis plane (papillary muscle level) before cardiopulmonary bypass. M-mode acquisition of myocardial IB was attempted from the anterior and inferior segments in each patient. The cyclic variation of IB was measured in at least two consecutive cycles in addition to a qualitative assessment of wall motion. Quantitative measurement of IB was possible in 39/52 (75%) myocardial segments. Cyclic variation of IB averaged 5.7 +/- 1.4 dB (SD) in segments with normal wall motion (n = 25); no difference was noted in the cyclic variation of IB among anterior or inferior walls. Hypokinetic segments demonstrated significant reduction of the cyclic variation (3.8 +/- 1.8 dB; p less than 0.001). Difficulty with obtaining myocardial IB was related to near-field artifact or lateral displacement of the left ventricular wall during systole. Transesophageal echocardiography holds promise for the evaluation of myocardial function and its preservation during cardiac surgery.

Early identification with ultrasonic integrated backscatter of viable but stunned myocardium in dogs

It has been shown that canine and human hearts exhibit a cardiac cycle-dependent variation of integrated backscatter (cyclic variation) that reflects intrinsic regional contractile performance. To determine whether ultrasound tissue characterization can identify viable though stunned myocardium before recovery of regional wall thickening, transient ischemic injury was produced in eight open chest dogs for 15 min followed by reperfusion for 2 h. Cyclic variation and wall thickening were measured before ischemia, at 15 min after the onset of ischemia and at selected intervals after the onset of reperfusion from multiple sites within the ischemic zone with a novel combined two-dimensional and M-mode acquisition system. Cyclic variation and wall thickening were computed from digitized M-mode integrated backscatter images with an algorithm developed and validated for this purpose. Magnitude and "delay" of cyclic variation and wall thickening were compared. Delay represents the degree of synchrony of regional cyclic variation or wall thickening with global ventricular mechanical systole. Baseline cyclic variation and wall thickening magnitudes were 3.8 +/- 0.2 dB and 37 +/- 1.4%, respectively. With ischemia, cyclic variation and wall thickening decreased to 1.7 +/- 0.2 dB and 17 +/- 2%, respectively (p less than 0.05, compared with baseline). Cyclic variation recovered to baseline levels within 20 min after reperfusion (3.3 +/- 0.4 dB, p = NS). Wall thickening remained depressed for 2 h after the onset of reperfusion (23 +/- 2%, p less than 0.05 compared with baseline). Delay of cyclic variation in a unitless ratio expressed as delay (in milliseconds) divided by the QT interval (in milliseconds) increased from 0.87 +/- 0.03 at baseline to 1.10 +/- 0.12 with ischemia, a change consistent with mild asynchrony, and returned to baseline (0.95 +/- 0.07, p = NS compared with baseline) within 20 min after reperfusion. Delay of wall thickening was 0.88 +/- 0.02 at baseline, increased to 1.23 +/- 0.09 with ischemia and remained significantly increased 2 h after reperfusion (1.07 +/- 0.05, p less than 0.05 compared with baseline). Recovery time constants for cyclic variation and wall thickening with reperfusion reflected earlier restoration of cyclic variation (8.1 min) than of wall thickening (420.5 min). Thus, cyclic variation recovers before wall thickening with reperfusion. Its analysis appears to provide a useful index of the presence of viable and potentially salvageable tissue in regions of stunned myocardium that is independent of wall thickening.