Left ventricular wall motion analysis using real-time three-dimensional ultrasound.

This study tested the ability of real-time 3-D (RT 3-D) echocardiography to detect and delineate regions of abnormal contraction (akinesia or dyskinesia) in a canine model of regional myocardial injury and to develop methods to simplify injury assessments. Closed chest RT 3-D scans were obtained and regional left ventricular (LV) contractile function was assessed in nine animals at baseline and after myocardial cryoinjury with a 1-cm cryoprobe. Evaluation of contractile function was based on radial shortening of LV chamber cross-sections at multiple levels. Radial length changes were analyzed using color-coded circumferential maps of the LV. Seven sets of motion maps demonstrated new areas of poorly contracting myocardium in the cryoinjured region relative to baseline. Two sets of data were excluded due to insufficient LV visualization. Motion maps derived from RT 3-D echo have the ability to detect and localize regions of abnormal LV wall motion.

Quantification of aortic regurgitation by real-time 3-dimensional echocardiography in a chronic animal model: computation of aortic regurgitant volume as the difference between left and right ventricular stroke volumes.

BACKGROUND: The accuracy of conventional 2-dimensional echocardiographic and Doppler techniques for the quantification of valvular regurgitation remains controversial. In this study, we examined the ability of real-time 3-dimensional (RT3D) echocardiography to quantify aortic regurgitation by computing aortic regurgitant volume as the difference between 3D echocardiographic-determined left and right ventricular stroke volumes in a chronic animal model. METHODS: Three to 6 months before the study, 6 sheep underwent surgical incision of one aortic valve cusp to create aortic regurgitation. During the subsequent open chest study session, a total of 25 different steady-state hemodynamic conditions were examined. Electromagnetic (EM) flow probes were placed around the main pulmonary artery and ascending aorta and balanced against each other to provide reference right and left ventricular stroke volume (RVSV and LVSV) data. RT3D imaging was performed by epicardial placement of a matrix array transducer on the volumetric ultrasound system, originally developed at the Duke University Center for Emerging Cardiovascular Technology. During each hemodynamic steady state, the left and right ventricles were scanned in rapid succession and digitized image loops stored for subsequent measurement of end-diastolic and end-systolic volumes. Left and right ventricular stroke volumes and aortic regurgitant volumes were then calculated and compared with reference EM-derived values. RESULTS: There was good correlation between RT3D left and right ventricular stroke volumes and reference data (r = 0.83, y = 0.94x + 2.6, SEE = 9.86 mL and r = 0.63, y = 0.8x - 1.0, SEE = 5.37 mL, respectively). The resulting correlation between 3D- and EM-derived aortic regurgitant volumes was at an intermediate level between that for LVSV and that for RVSV (r = 0.80, y = 0.88x + 7.9, SEE = 10.48 mL). RT3D tended to underestimate RVSV (mean difference -4.7 +/- 5.4 mL per beat, compared with -0.03 +/- 9.7 mL per beat for the left ventricle). There was therefore a small overestimation of aortic regurgitant volume (4.7 +/- 10.4 mL per beat). CONCLUSION: Quantification of aortic regurgitation through the computation of ventricular stroke volumes by RT3D is feasible and shows good correlation with reference flow data. This method should also be applicable to the quantification of other valvular lesions or single site intracardiac shunts where a difference between right and left ventricular cavity stroke volumes is produced.

Real-time, volumetric echocardiography: usefulness of volumetric scanning for the assessment of cardiac volume and function.

BACKGROUND: A novel imaging system has been introduced which uses a dedicated two-dimensional echo probe for rapid beam forming to scan a pyramidal volume in real time. Real-time volumetric echocardiography has the potential to determine accurate cardiovascular anatomy, volume and function in the beating heart without reconstructions. The results of animal and human studies using volumetric echocardiography are evaluated for the potential for clinical applications. IMAGING METHODOLOGY: A new type of ultrasound imaging, high-speed volumetric scanning based on phased array principles permits real-time three-dimensional, volumetric echocardiography (real-time 3-DE). The system requires no off-line reconstruction techniques, thus enabling dynamic three-dimensional visualization and quantification of the heart in real time using a transthoracic approach. Real-time 3-DE uses a 2-D matrix phased array transducer. Image formation employs 16:1 parallel processing to scan a pyramidal volume composed of multiple steering directions in the azimuth dimension and in the elevation dimension. The finished transducer is mounted in a hand-held case with a circular aperture of 16 mm diameter. The array consists of approximately 1,600 elements, operating at 2.5 MHz. Real-time 3-DE permits simultaneous, multiple plane display of two sector arcs (B-scans) and C-scan (parallel to the transducer face or inclined) on a single monitor, conveying the three-dimensional nature of the ultrasound data. This system also allows these planes to be angled for extra diagnostic flexibility. The motion of all the structures during the cardiac cycle can be evaluated in dynamic mode. METHODS: Real-time 3-DE was assessed for accuracy of volume measurement by measuring the volume of balloons of different size and shape, and the hearts of 15 closed chest dogs with myocardial contrast enhancement, and compared to the volumes measured by left ventricular angiography in the dogs. Real-time 3-DE was used to evaluate the endocardial border determination of the entire left ventricle by injecting contrast agent in 12 patients. The endocardial border determination of each segment was scored, and the endocardial border score index calculated. Both real-time 3-D images and cine magnetic resonance imaging (MRI) were performed in 16 patients to assess the accuracy of volume measurement of the left ventricle in humans. The endocardial border of the left ventricle was manually traced, and the volumes calculated by Simpson's rule. RESULTS: The volumes measured by real-time 3-DE correlated well with the true volumes for different sizes of balloon and for asymmetric balloons. The end-diastolic volume and end-systolic volume linear correlation of real-time 3-DE versus angiography measurements using manual tracing in vivo also gave a good correlation (r = 0.97, p < 0.001; r = 0.92, p < 0.01). Fifty-eight of 192 segments were rated as good at baseline and 143 rated as good after Levovist injection. Endocardial border determination was improved by Levovist injection in 100 of 137 segments (74.6%). The endocardial border score index was significantly higher after Levovist administration than at baseline (p < 0.003). The end-diastolic volume and end-systolic volume of the left ventricle measured by real-time 3-DE in humans correlated well with those measured by MRI (end-diastolic volume: r = 0.97, p < 0.001; end-systolic volume: r = 0.96, p < 0.001). CONCLUSIONS: Transthoracic real-time, volumetric echocardiography opens a new and exciting field of echocardiography. The results of these studies demonstrate that this system can accurately measure the ventricular volume and function without use of geometric assumptions. This volumetric mode or V-mode scanning is a new imaging modality that provides a practical methodology to investigate important clinical and research questions.

Anatomic validation of a novel method for left ventricular volume and mass measurements with use of real-time 3-dimensional echocardiography.

Assessment of left ventricular (LV) volumes and mass is a critical element in the evaluation of patients with cardiovascular disease. However, most non-invasive methods used for the quantitative measurements of LV volume and mass have important intrinsic limitations. Real-time 3-dimensional echocardiography (RT3D echo) is a new technique capable of acquiring volumetric images without cardiac or respiratory gating. The purpose of this study was to develop and validate a system for rapid LV volume and mass measurements with the use of RT3D echo images. To this end, in 11 explanted sheep hearts, the left ventricle was instrumented with a latex balloon and filled with known volumes of saline solution. Two independent observers made volume calculations from images acquired with RT3D echo. In addition, 21 open-chest sheep were imaged with RT3D echo for LV mass calculation. Anatomic LV mass was determined after removing the heart. A strong correlation was observed between the actual LV volumes and those calculated from the RT3D echo images (r = 0.99; y = 1.31 + 0.98x; standard error of the estimate = 2.2 mL). An analysis of intraobserver and interobserver variabilities revealed high indexes of agreement. A strong correlation was observed between actual LV mass and that calculated from RT3D echo images (r = 0.94; y = 14.4 + 0.89x; standard error of the estimate = 8.5 gm). Thus RT3D echo images allow rapid and accurate measurements of LV volume and mass. This technique may expand the use of cardiac ultrasonography for the quantitative assessment of heart disease.

Angular scatter ultrasound imaging of wavelength scale targets.

A bistatic ultrasound imaging system is demonstrated that uses two 32-element linear phased array transducers oriented at an angle of 40 degrees to one another. The system simultaneously acquires and displays in real time one conventional backscatter image and one "angular scatter" image formed using side-scattered echoes from the same B-mode sector region. Experiments are presented that show differences in the magnitudes of backscatter and angular scatter signals acquired from three nylon monofilaments with diameters less than one wavelength and from soft tissue structures in vivo. The relative magnitudes of angular scatter signals from the monofilaments are qualitatively consistent with a theoretical analysis of acoustic scattering from elastic cylinders. Larger tissue features are more clearly defined in angular scatter images. This result is attributed to the orientation of specularly reflecting surfaces and the expected influence of scattering angle on the system's sensitivity to different scatterer spacings.

Design and characterization of a real-time angular scatter ultrasound imaging system.

A novel ultrasound imaging system has been implemented using two 32-element linear phased array transducers oriented at an angle of 40 degrees to one another. The system simultaneously acquires and displays, in real time, a conventional backscatter image and an angular scatter image formed using side-scattered echoes from the same region. The design of the system is shaped by the influence of the scatter angle on the spatial resolution and receive signal processing requirements of the instrument. The subtended scatter angles are restricted to values >90 degrees to ensure that the angular scatter receiver effectively tracks the transmitted pulse and that the spatial resolution in the two images is comparable. The system is sufficiently tolerant of small variations in the average acoustic velocity of the medium to guarantee reliable pulse tracking in biomedical applications. The angular scatter signal magnitude is significantly weighted by the directivity of the receive array. The imaging system will most effectively demonstrate angular variations in scattering at scatter angles between 125 and 145 degrees , where the angular response of the receiver is near its peak.

Real-time three-dimensional echocardiography for measurement of left ventricular volumes.

Left ventricular (LV) volumes are important prognostic indexes in patients with heart disease. Although several methods can evaluate LV volumes, most have important intrinsic limitations. Real-time 3-dimensional echocardiography (RT3D echo) is a novel technique capable of instantaneous acquisition of volumetric images. The purpose of this study was to validate LV volume calculations with RT3D echo and to determine their usefulness in cardiac patients. To this end, 4 normal subjects and 21 cardiac patients underwent magnetic resonance imaging (MRI) and RT3D echo on the same day. A strong correlation was found between LV volumes calculated with MRI and with RT3D echo (r = 0.91; y = 20.1 + 0.71x; SEE 28 ml). LV volumes obtained with MRI were greater than those obtained with RT3D echo (126 +/- 83 vs 110 +/- 65 ml; p = 0.002), probably due to the fact that heart rate during MRI acquisition was lower than that during RT3D echo examination (62 +/- 11 vs 79 +/- 16 beats/min; p = 0.0001). Analysis of intra- and interobserver variability showed strong indexes of agreement in the measurement of LV volumes with RT3D echo. Thus, LV volume measurements with RT3D echo are accurate and reproducible. This technique expands the use of ultrasound for the noninvasive evaluation of cardiac patients and provides a new tool for the investigational study of cardiovascular disease.

Real-time, three-dimensional echocardiography: feasibility of dynamic right ventricular volume measurement with saline contrast.

BACKGROUND: The asymmetry and complex shape of the right ventricle have made it difficult to determine right ventricular (RV) volume with 2-dimensional echocardiography. Three-dimensional cardiac imaging improves visualization of cardiac anatomy but is also complex and time consuming. A newly developed volumetric scanning system holds promise of obviating past limitations. METHODS: Real-time, transthoracic 3-dimensional echocardiographic images of the right ventricle were obtained with a high-speed volumetric ultrasound system that uses a 16:1 parallel processing schema from a 2.5 MHz matrix phased-array scanner to interrogate an entire pyramidal volume in real time. The instrumentation was used to measure RV volume in 8 excised canine hearts; dynamic real-time 3-dimensional images were also obtained from 14 normal subjects. RESULTS: Three-dimensional images were obtained in vitro and in vivo during intravenous hand-agitated saline injection to determine RV volumes. The RV volumes by real-time 3-dimensional echocardiography are well correlated with those of drained in vitro (y = 1.26x - 9.92, r = 0.97, P <.0001, standard error of the estimate = 3.26 mL). For human subjects, the end-diastolic and end-systolic RV volumes were calculated by tracing serial cross-sectional, inclined C scans; functional data were validated by comparing the scans with conventional 2-dimensional echocardiographic indexes of left ventricular stroke volume. CONCLUSIONS: These data indicate that RV volume measurements of excised heart by real-time 3-dimensional echocardiography are accurate and that beat-to-beat RV quantitative measurement applying this imaging method is possible. The new application of real-time 3-dimensional echocardiography presents the opportunity to develop new descriptors of cardiac performance.

FLASH correlation: a new method for 3-D ultrasound tissue motion tracking and blood velocity estimation.

We report a new method for tracking tissue motion and blood flow in three dimensions (3-D) using successive volumetric ultrasound scans. The method is based on combining the concepts of feature tracking and 3-D correlation search to achieve a compromise between accuracy and computational efficiency. This paper introduces the new method and the experimental setup used for its evaluation in a tissue-mimicking material. Results are presented demonstrating that the new method has both satisfactory tracking performance and the potential for practical real-time implementation.

Real-time three-dimensional echocardiography for determining right ventricular stroke volume in an animal model of chronic right ventricular volume overload.

BACKGROUND: The lack of a suitable noninvasive method for assessing right ventricular (RV) volume and function has been a major deficiency of two-dimensional (2D) echocardiography. The aim of our animal study was to test a new real-time three-dimensional (3D) echo imaging system for evaluating RV stroke volumes. METHODS AND RESULTS: Three to 6 months before hemodynamic and 3D ultrasonic study, the pulmonary valve was excised from 6 sheep (31 to 59 kg) to induce RV volume overload. At the subsequent session, a total of 14 different steady-state hemodynamic conditions were studied. Electromagnetic (EM) flow probes were used for obtaining aortic and pulmonic flows. A unique phased-array volumetric 3D imaging system developed at the Duke University Center for Emerging Cardiovascular Technology was used for ultrasonic imaging. Real-time volumetric images of the RV were digitally stored, and RV stroke volumes were determined by use of parallel slices of the 3D RV data set and subtraction of end-systolic cavity volumes from end-diastolic cavity volumes. Multiple regression analyses showed a good correlation and agreement between the EM-obtained RV stroke volumes (range, 16 to 42 mL/beat) and those obtained by the new real-time 3D method (r=0.80; mean difference, -2.7+/-6.4 mL/beat). CONCLUSIONS: The real-time 3D system provided good estimation of strictly quantified reference RV stroke volumes, suggesting an important application of this new 3D method.

3D ultrasound tissue motion tracking using correlation search.

Several methods for ultrasound tissue motion tracking and blood velocity estimation have been proposed and clinically applied. While providing valuable information to the clinician that was not obtainable from B-mode anatomical imaging, these methods still suffer from various fundamental and practical limitations that compromise their performance in certain clinical situations. A significant limitation is the inability of most of these methods to estimate the complete 3D motion or velocity vectors. With the introduction of ultrasound volumetric imaging, the need for a method that is capable of obtaining the complete motion vector is even more pressing. In this paper, we investigate the implementation of a correlation search scheme to estimate the 3D motion vectors using successive volumetric ultrasound scans. We present tracking results for motion along different axes and discuss the advantages and limitations of performing the correlation search in 3D.

Speckle structure in three dimensions.

Ultrasound speckle has long been recognized as a noise source in diagnostic imaging. The advent of three-dimensional imaging and flow detection requires the characterization of the three-dimensional acoustical speckle pattern. Ultrasound data were acquired by using an automated three-dimensional translation stage to measure the radio-frequency (rf) backscatter signals from a volume scattering phantom. The data samples were processed off-line to locate and measure envelope-detected speckle peaks, i.e., local maxima. Results indicate that speckle has a distinctive structure in which three-dimensional peaks can be located and measured. These peaks are brighter on average than the mean speckle brightness level and are uniformly distributed throughout the volume. The lateral breadth of the speckle peaks, defined as the breadth of the -6 dB contour in the lateral-elevational plane, is over twice the width predicted by previous investigators. This is the first attempt to physically measure the breadth of bright spots in the speckle pattern. A rational for the discrepancy between previous theory and the measurement in this paper is given.

Feasibility study for a two-dimensional diagnostic ultrasound velocity mapping system.

A receive beam-tracking technique is introduced for simultaneously measuring more than one dimension of an existing velocity vector in medical ultrasound. By using flexible beamforming and a conventional linear-array transducer, vectors can be measured at real-time frame rates over a significant field of view. Both simulations and in vitro experiments show that this technique should provide more accurate measurements of blood flow velocities in the human body than is available with current color flow mapping systems.

Optical transducer for reception of ultrasonic waves.

A new optical transducer for the detection of acoustic pressure in the diagnostic ultrasound frequency range is described. This transducer is based on the modulation of an evanescent light field by the incident acoustic energy. Theoretical design considerations are presented for the purpose of developing the most sensitive transducer. Based on these considerations an experimental transducer was constructed. Although less sensitive than predicted this device was capable of transducing ultrasonic pulses with a 1.0-MHz center frequency at diagnostic ultrasound amplitude levels. The techniques developed here are applicable for two-dimensional transduction and may prove a viable alternative to piezoelectric array transducers.

Two-dimensional arrays for medical ultrasound.

The design, fabrication and evaluation of two-dimensional transducer arrays are described for medical ultrasound imaging. A 4 x 32, 2.8 MHz array was developed to use new signal processing techniques for improved B-scan imaging including elevation focusing, phase correction and synthetic aperture imaging. Laboratory measurements from typical array elements showed 50 omega insertion loss of -56 dB, -6 dB fractional bandwidth of 43%, interelement crosstalk of -19 dB, and -6 dB pulse-echo angular response of 62 degrees. Simulations of pulse-echo beam plots have shown grating lobes 20 dB below the main lobe at +/- 7 degrees in the elevation direction. The complete 2-D array has been used for measurements of phase aberrations in breast, and the individual 32 element linear arrays have been used to obtain conventional B-scans. Several 16 x 16 arrays have also been developed for high speed volumetric imaging. These include 96 transmit elements and 32 receive channels. With a lambda/4 matching layer, laboratory measurements show 50 omega insertion loss of -72 dB, -6 dB fractional bandwidth of 63%, interelement crosstalk of -29 dB and -6 dB angular response of 25 degrees. Pulse-echo sensitivity was improved by 21 dB through the use of integrated circuit preamplifiers of high impedance mounted in the transducer handle. In vivo cardiac, abdominal, and obstetric B-scans with elevation focusing, as well as high speed C-scans, have been obtained with these 2-D arrays.

High-speed ultrasound volumetric imaging system. I. Transducer design and beam steering.

Transducer design and phased array beam steering are developed for a volumetric ultrasound scanner that enables the 3-D visualization of dynamic structures in real time. The authors describe the design considerations and preliminary evaluation of a high-speed, online volumetric ultrasound imaging system that uses the principles of pulse-echo, phased array scanning with a 2-D array transducer. Several 2-D array designs are analyzed for resolution and main lobe-side lobe ratio by simulation using 2-D fast Fourier transform methods. Fabrication techniques are described for 2-D array transducer. Experimental measurements of pulse-echo point spread responses for 2-D arrays agree with the simulations. Measurements of pulse-echo sensitivity, bandwidth, and crosstalk are included.

High-speed ultrasound volumetric imaging system. II. Parallel processing and image display.

For pt.I see ibid., vol.38, no.2, p.100-8 (1991). The authors describe the design, application, and evaluation of parallel processing to the high-speed volumetric ultrasound imaging system. The scanner produces images analogous to an optical camera or the human eye and supplies more information than conventional sonograms. Potential medical applications include improved anatomic visualization, tumor localization, and better assessment of cardiac function. The system uses pulse-echo phased array principles to steer a 2-D array transducer of 289 elements in a pyramidal scan format. Parallel processing in the receive mode produces 4992 scan lines at a rate of approximately 8 frames/s. Echo data for the scanned volume is presented online as projection images with depth perspective, stereoscopic pairs, or multiple tomographic images. The authors also describe the techniques developed for the online display of volumetric images on a conventional CRT oscilloscope and show preliminary volumetric images for each display mode.

Real time volumetric ultrasound imaging system.

A real time volumetric ultrasound imaging system has been developed for medical diagnosis. The scanner produces images analogous to an optical camera and supplies more information than conventional sonograms. Potential medical applications include improved anatomic visualization, tumor localization, and better assessment of cardiac function. The system uses pulse-echo phased array principles to steer a two-dimensional array transducer of 289 elements in a pyramidal scan format. Parallel processing in the receive mode produces 4992 scan lines at a rate of approximately 8 frames/second. Echo data for the scanned volume is presented as projection images with depth perspective, stereoscopic pairs, multiple tomographic images, or C-mode scans.

Angle independent ultrasonic blood flow detection by frame-to-frame correlation of B-mode images.

We have previously reported initial clinical results of a novel blood velocity imaging technique utilizing a two-dimensional correlation search applied to consecutively acquired echoes. In this paper, we describe both the physical principles underlying this technique and test tank experiments which define its performance under a variety of conditions. The results indicate that, unlike Doppler flow imaging systems, this technique defines the flow velocity vector in two dimensions and is not subject to aliasing.