Feasibility of 3D harmonic contrast imaging.

Improved endocardial border delineation with the application of contrast agents should allow for less complex and faster tracing algorithms for left ventricular volume analysis. We developed a fast rotating phased array transducer for 3D imaging of the heart with harmonic capabilities making it suitable for contrast imaging. In this study the feasibility of 3D harmonic contrast imaging is evaluated in vitro. A commercially available tissue mimicking flow phantom was used in combination with Sonovue. Backscatter power spectra from a tissue and contrast region of interest were calculated from recorded radio frequency data. The spectra and the extracted contrast to tissue ratio from these spectra were used to optimize the excitation frequency, the pulse length and the receive filter settings of the transducer. Frequencies ranging from 1.66 to 2.35 MHz and pulse lengths of 1.5, 2 and 2.5 cycles were explored. An increase of more than 15 dB in the contrast to tissue ratio was found around the second harmonic compared with the fundamental level at an optimal excitation frequency of 1.74 MHz and a pulse length of 2.5 cycles. Using the optimal settings for 3D harmonic contrast recordings volume measurements of a left ventricular shaped agar phantom were performed. Without contrast the extracted volume data resulted in a volume error of 1.5%, with contrast an accuracy of 3.8% was achieved. The results show the feasibility of accurate volume measurements from 3D harmonic contrast images. Further investigations will include the clinical evaluation of the presented technique for improved assessment of the heart.

A fast rotating scanning unit for real-time three-dimensional echo data acquisition.

Most three-dimensional (3-D) echocardiography (3-DE) systems today are based on off-line methods where a large number of cross-sectional 2-D scans have to be acquired sequentially before a 3-D image can be reconstructed. Because acquisition is done step-by-step based on ECG triggering plus respiratory gating, this introduces motion artefacts and takes significant acquisition time. Another 3-D approach is based on 2-D transducers and parallel beam-forming. Such a system is very complex. In this manuscript, a fast continuously-rotating scanning unit, based on a 64-element phased-array transducer, is described. Typical rotation speed of the 3-D unit is 8 rotations per s. Therefore, 16 3-D volume datasets can be acquired per s in real-time. The first clinical examples as acquired with this probe are presented.

Simulation of circular array ultrasound transducers for intravascular applications.

The beam shape of a circular array transducer that is commonly used in intravascular ultrasound catheters was investigated in linear mode of operation. For this use, a simulation program which can simulate the radio frequency (rf)-response of a number of scatterers has been developed. The program is based on the impulse response method, which is implemented in the frequency domain. Due to the unusual geometry of the transducer, the far field gets peculiarly shaped for large apertures. Instead of having a far field with its maximum intensity in a single lobe on the acoustical axis, the far field splits up into a dual-lobe far field with maximum intensity in two lobes off the acoustical axis. A formula is derived that predicts the occurrence of these beam shapes.

Effect of catheter placement on 3-D velocity profiles in curved tubes resembling the human coronary system.

Novel measurement techniques based on intravenous ultrasound (IVUS) technology ('IVUS-Flowmetry') require the location of a catheter inside the coronary bed. The present study quantifies disturbances in the 3-D velocity profile induced by catheter placement inside a tube, applying computational fluid dynamics. Two curved, circular meshes (radius K = 0.025 m and K = 0.035 m) with and without a catheter inside the lumen were applied. The catheter was located at the inner curve, the outer curve and at the top position. Boundary conditions were: no slip on the wall, zero stress at the outlet, uniform inflow with entrance velocities of 0.1, 0.2 and 0.4 m/s. Curvature-associated centrifugal forces shifted the maximal velocity to the outer curve and introduced two symmetrical vortices. Additional catheter placement redistributed the 3-D axial velocity field away from the catheter, which was accompanied by the appearance of multiple low-strength vortices. In addition, peak axial velocity increased, peak secondary velocities decreased, axial pressure drop increased and shear stress increased. Flow calculations simulated to resemble IVUS-based flowmetry changed by only 1% after considering secondary velocity. In conclusion, placement of a catheter inside a curved tube resembling the human coronary system changes the velocity field and reduces secondary patterns. The present study supports the usefulness of catheter-based flowmetry during resting flow conditions. During hyperemic flow conditions, flow measurements might be accompanied by large axial pressure drops because the catheter, itself, might act as a significant stenosis.

Intravascular imaging.

Based on three-dimensional (3D) information, quantitative data such as plaque volume can be calculated. The procedure includes automatic contour detection based in image segmentation methods and greatly speeds up clinical evaluation. With the use of additional X-ray information, the true tortuous vessel geometry can be reconstructed in 3D. This allows, by numerical modelling techniques, to calculate endothelial shear stress values which in turn may indicate sites prone to stenosis. With a decorrelation technique for radio frequency (RF) echo information from sequential data in the same beam direction and integration method over the entire cross section, blood velocity can be shown colour-coded during the cardiac cycle, while even blood flow quantification seems to be possible. In vitro as well as animal experiments have shown the feasibility of the method. Intravascular imaging can be used to study the biomechanical properties of atheroma components. Local radial strain as a measure of local tissue hardness can be estimated in principle. Hard or soft plaques can be identified from the strain images independently of the echogenic contrast between plaque and vessel wall.

New developments in intravascular ultrasound imaging.

IntraVascular Ultrasound Imaging (IVUS) has already been proposed in the early days of diagnostic ultrasound. Today, it has come under further full attention as a result of minimal invasive techniques. Not only excellent intravascular two-dimensional (2D) images are presently obtained, also three-dimensional (3D) reconstructed images show their diagnostic value. Based on 3D information, quantitative data such as plaque volume can be calculated. The procedure includes automatic contour detection based on image segmentation methods and greatly speeds up clinical evaluation. With the use of additional X-ray information, the true tortuous vessel geometry can be reconstructed in 3D. This allows, by numerical modelling techniques, to calculate endothelial shear stress values, which in turn may indicate sites prone to stenosis. With a decorrelation technique for radiofrequency (RF) echo information from sequential data in the same beam direction and integration method over the entire cross section, blood velocity can be shown colour-coded during the cardiac cycle, while even blood flow quantification seems to be possible. In vitro as well as in vivo experiments have shown the feasibility of the method. Intravascular imaging can be used to study the biomechanical properties of atheroma components. Local radial strain, used as a measure of local tissue hardness, can be estimated to identify hard or soft plaques independently of the echogenicity contrast between plaque and vessel wall.

Blood flow imaging and volume flow quantitation with intravascular ultrasound.

Current intravascular ultrasound techniques produce real-time imaging of a vessel cross-section with a scan plane approximately normal to blood flow. When a cluster of randomly distributed blood particles moves across the ultrasound beam, the received echo signals decorrelate as a function of time. This phenomenon may be used to estimate blood velocities by measuring the decorrelation rate from a sequence of blood scattering signals. A decorrelation-based method for measuring local blood velocity and quantifying volume flow from cross-sectional radio frequency intravascular echo signals was developed. Serial in vitro measurements were performed with a flow phantom to test the principle of the proposed velocity estimation method. An in vivo pig experiment was carried out to study the feasibility of applying this method in clinical settings. Preliminary results of this study indicate that the proposed decorrelation method is able to extract cross-sectional velocity data and volumetric flow both in vitro and in vivo.

Performance of time delay estimation methods for small time shifts in ultrasonic signals.

In this study several time delay estimation (TDE) methods were investigated for estimation of time shifts of less than 10 ns at a frequency of 30 MHz. Using simulated and experimental echosignals we investigated the performance of five methods: two phase related methods (phase shift and phase difference method); two correlation methods (cross-correlation and correlation interpolation method); and a demodulation method. The results showed that the correlation interpolation method is by far the most accurate for all time delays. With this method, estimation errors of about 200 ps are achievable with an signal-to-noise ratio (SNR) of 40 dB (f0 = 30 MHz, bandwidth = 20 MHz) for time shifts of up to 10 ns.

Potentials of volumetric blood-flow measurement.

Current intravascular ultrasound techniques produce real-time imaging of a vessel cross-section with a scan plane normal to blood flow. When randomly distributed blood particles travel through this ultrasound imaging plane, the received echo signals decorrelate as a function of time. The speed of such a decorrelation procedure is proportional to the flow velocity. This phenomenon provides a potential to estimate blood velocities by means of decorrelation analysis. In this paper, we present a method for measuring local blood velocity and quantifying volume flow directly from cross-sectional intravascular ultrasound data. This method is based on multiple decorrelation assessments with a sequence of radio frequency echo signals. The velocity measurement is obtained by comparing the measured decorrelation value with the prior knowledge of the beam characteristics of an intravascular ultrasound transducer. Volume flow is derived by integrating the cross-sectional area and its corresponding velocity vector over the vessel lumen. The decorrelation-based method was tested in vitro with a flow phantom. Measurements were also carried out in vivo in pig experiments to determine the usefulness of this method in clinical settings. Preliminary results of these experiments indicate that the proposed decorrelation method is able to extract cross-sectional velocity profiles and volumetric flow both in vitro and in vivo.

Intravascular elastography: principles and potentials.

Many intravascular therapeutic techniques for the treatment of significant atherosclerotic lesions are mechanical in nature: angioplasty, stenting and atherectomy. The selection of the most adequate treatment would be advantageously aided by knowledge of the mechanical properties of the lesion. Based on the success of conventional intravascular ultrasound (IVUS) imaging in accurately depicting the morphology of atheromatous lesions, ultrasonic tissue characterization has been proposed to determine the composition of atherosclerotic plaques. Elastography is an ultrasound-based imaging technique capable of producing cross-sectional elasticity images called elastograms. The technique involves analysis of echo signals obtained at two states of incremental intravascular pressure. High resolution, local tissue displacement estimation by cross-correlation is followed by computation of local strain. Strain is utilized as an indicator of the local compliance of tissue under the assumption of constant stress within the scan plane. Using vessel-mimicking phantoms, we demonstrate the feasibility of intravascular elastography experimentally. The elastograms are able to depict lesions of different elasticity independently of the echogenicity contrast, since the information provided by the elastograms is generally independent of that obtained from the conventional IVUS image. Thus, the elastogram can complement the characterization of lesions from the conventional IVUS image. Progress to in vitro and in vivo testing is expected in conjunction with ongoing improvements in the current instrumentation and processing.

Intravascular elasticity imaging using ultrasound: feasibility studies in phantoms.

A technique is described for measuring the local hardness of the vessel wall and atheroma using intravascular ultrasound. Strain images were constructed using the relative local displacements, which are estimated from the time shifts between gated echo signals acquired at two levels of intravascular pressure. Time shifts were estimated using one-dimensional correlation with bandlimited interpolation around the peak. Tissue-mimicking phantoms with typical morphology and hardness topology of some atherosclerotic vessels were constructed. Hard and soft regions could be distinguished on the strain image, independently of their contrast in echogenicity. Thus, the potential of ultrasonic hardness imaging to provide information that may be unavailable from the echogram alone was demonstrated. The strain images of the homogeneous and layered phantoms showed some artifacts that need to be corrected for, to obtain images of the modulus of elasticity. For in vitro and in vivo experiments, the spatial resolution of the technique needs to be improved. Furthermore, two-dimensional correlation techniques may be necessary in case of nonradial expansion and an off-centre catheter position.

Influence of data processing on cyclic variation of integrated backscatter and wall thickness in stunned porcine myocardium.

This study was performed to investigate the relationship between the cyclic variation of integrated backscatter and myocardial wall thickening in stunned myocardium. Different definitions of cyclic variation were evaluated to be able to compare with other studies. Ultrasound data were acquired from 10 open-chested Yorkshire pigs (25-33 kg) at baseline, during regional ischemia and during 30 min of stunning, using a broadband ultrasound transducer (3-7 MHz) sutured directly upon the left ventricular myocardial wall. Cyclic variation of integrated backscatter and myocardial wall thickening were calculated using three definitions obtained from the literature. Independent of the definition, cyclic variation of wall thickness and integrated backscatter were blunted during acute ischemia and returned transiently to or above baseline during the first minute of reperfusion, followed by a gradual decrease to a level under baseline during stunning. An early return of the cyclic variation of the integrated backscatter was not observed in pigs, independent of the data processing used. The relationship between integrated backscatter and wall thickness was maintained.

Decorrelation of intravascular echo signals: potentials for blood velocity estimation.

When blood particles travel through an intravascular ultrasound imaging plane, the received echo signals decorrelate at a rate that is related to the flow velocity. In this paper, the feasibility of extracting blood velocity from the decorrelation function of radio frequency signals was investigated through theoretical analysis and computer simulation. A computer model based on the impulse response method was developed to generate the ultrasound field of a 30-MHz intravascular transducer. The decorrelation due to the scatterer displacement as well as other nonmotion related decorrelation sources were studied. The computer simulations show that the decorrelation function is linearly related to the lateral displacement. The monotonic relationship between correlation and displacement provides possibilities to estimate flow velocity with decorrelation measurements. Because of the complexity of the beam profile in the near field, assessment of local velocities requires detailed knowledge of the decorrelation at each axial beam position. Sources of signal decorrelation other than the lateral displacement may cause a bias in the decorrelation based velocity measurements. For localized decorrelation estimation, measurement variations in small range windows present a major challenge. An approach based on multiple decorrelation measurements should be adopted in order to reduce the variations. In conclusion, results of this study suggest that it is feasible to measure flow velocity by quantifying the decorrelation of intravascular ultrasound signals from blood.

Temporal correlation of blood scattering signals in vivo from radiofrequency intravascular ultrasound.

One limitation encountered using high frequency intravascular ultrasound (IVUS) is the echogenicity of blood, which increases dramatically at frequencies of 20-40 MHz. Because of the higher velocity of moving blood particles, the echo pattern of flowing blood shows more variations in time than that of the wall. To investigate the time-varying characteristics of the blood scattering measurements were performed on the radiofrequency (RF) data collected in vivo from five pig experiments. After positioning the echo catheter inside the iliac artery, an M-mode sequence of 30 RF traces was acquired at a high pulse repetition rate (5 kHz). The RF correlation time was measured on the regions of blood and the arterial wall. Two processing techniques, temporal averaging and correlation, were tested for suppression of the blood echo intensity. The correlation time Tc measured in the blood region was approximately 1 ms, which was shorter than that measured in the wall region (Tc >> 6 ms). The correlation values calculated in a small window showed a large variation in the blood region while the wall region produced a constant high output. After processing eight consecutive RF traces (delta T = 200 microseconds), the temporal averaging method results in a 50% intensity reduction in the blood region. Using the correlation output as a weighting function, the blood echo intensity can be further reduced to only 10% of its original value. Application of the RF correlation processing to a cross-sectional image data demonstrates the feasibility of this technique to remove most of the blood echoes and enhance the image contrast of the luminal interface.

Two decades of transesophageal phased array probes.

After its introduction about two decades ago, transesophageal echocardiography (TEE) has rapidly evolved into an important diagnostic feature for the cardiologist, since it offers anatomic and hemodynamic information which cannot be obtained precordially. Part of this success was due to the developments in transducer technology which resulted in smaller probes with progressively better imaging qualities. A short review of past, recent and future developments of TEE phased array probes, in particular those at the Erasmus University in Rotterdam, will be given. Furthermore, this article discusses basic parameters of the transducer dictating image quality such as centre frequency, array aperture and focusing illustrated with several simulations. The simulations show that a poor design of the transducer will limit the resolution and will give artefacts in the two-dimensional image.

The relationship between myocardial integrated backscatter, perfusion pressure and wall thickness during isovolumic contraction: an isolated pig heart study.

To investigate the independent effect of myocardial wall thickness and myocardial perfusion pressure on integrated backscatter, experiments were designed in which integrated backscatter of normally perfused myocardial tissue was measured while changes in wall thickness during the cardiac cycle were reduced to a minimum. In nine blood-perfused isolated pig hearts, perfusion pressure was uncoupled from left ventricular pressure generation (Langendorff method) and isovolumic contraction and relaxation were realized by inserting a noncompressible water-filled balloon into the left ventricle. In a first experiment, at constant perfusion pressure (85 mmHg), the integrated backscatter (3-7 MHz), the myocardial wall thickness and the left ventricular pressure were determined simultaneously at various balloon volumes (5-25 mL). A quasistatic increase of balloon volume by 50% resulted in an average decrease of wall thickness of 6.5% (p < 0.01) and a mean increase in the integrated backscatter level of 1.1 dB (p < 0.01). Integrated backscatter levels increased statistically significant by 0.14 +/- 0.014 dB per percent decrease of wall thickness. Measurements of percentage end-systolic myocardial wall thickening ranged from -10% to +10%, mean 0.15 +/- 4.5% (NS from zero); whereas cyclic variation of integrated backscatter ranged from -3.9 to +3.9 dB, mean 0.19 +/- 1.5 dB (NS from zero). In a second experiment, at a constant midrange balloon volume, the same parameters were determined simultaneously at various perfusion pressures (20-120 mmHg). An increase in perfusion pressure by 50% resulted in a small but statistically significant increase of 1.5% in myocardial wall thickness, which could be explained by an increase of intravascular volume. The integrated backscatter levels did not change statistically significantly. Measurements of percentage end-systolic myocardial wall thickening ranged from -8.9 to +7.8%, mean 0.13 +/- 4.0% (NS from zero); whereas cyclic variation of integrated backscatter ranged from -1.8 to +4.2 dB, mean 0.37 +/- 1.3 dB (NS from zero). The magnitude of cyclic variation of integrated backscatter of myocardial tissue in a contractile state is reduced if myocardial muscle is prevented from normal thickening. In addition, changes in intravascular volume during the cardiac cycle have a negligible influence on the absolute backscatter level or its cyclic variation. We conclude, if only wall thickness and perfusion pressure are involved, that integrated backscatter is mainly determined by myocardial wall thickness.

An in vitro evaluation of the line pattern of the near and far walls of carotid arteries using B-mode ultrasound.

This in vitro study was executed to evaluate the double line pattern generated at both near and far walls of human carotid arteries using B-mode ultrasound. Therefore, extravascular (7.5 MHz) and intravascular (30 MHz) ultrasound imaging were performed at the same locations of the carotid artery. The thickness of the double line pattern of the extravascular image (7.5 MHz) was compared to the thickness of the intima-media complex seen on the corresponding intravascular image (30 MHz) and on the histologic section. At the far wall of the extravascular image, the measurements were executed at the leading edge of the echo. The data showed high correlation and agreement with the intravascular (r = 0.91, p < 0.001; mean(diff) = -0.01 and SDdiff = 0.12) and the histologic measurements (r = 0.87, p < 0.001; mean(diff) = -0.12 and SDdiff = 0.13). In addition, the results of the measurements of the intravascular image showed high correlations and agreement with the histologic data (r(near) = 0.86, p < 0.001; mean(diff) = -0.08 and SDdiff = 0.15, respectively, r(far) = 0.92, p < 0.001; mean(diff) = -0.12 and SDdiff = 0.12). For comparison with other studies, near wall measurements were also included. These had to be performed at the trailing edge of the echoes to be compatible with these studies. The results of the measurements of the extravascular image showed poor correlations and lack of agreement with those of the intravascular (r = 0.49, p = 0.03; mean(diff) = 0.09 and SDdiff = 0.25) and of the histologic (r = 0.37, p = 0.03; mean(diff) = 0.04 and SDdiff = 0.23) measurements. These results can easily be explained from the physical limitations of measuring at the trailing edges. We conclude that the double line pattern seen at the far wall of the extravascular image is representative of the intima-media complex.

Transesophageal transducer technology: an overview.

Early developments and basic principles in the field of Transesophageal Echocardiography (TEE) probe technology are summarized. Mechanical and electronical sector scanners are compared, and several probe characteristics and image parameters are discussed. A short review of recent developments in TEE is given.