Ultrasonic backscatter from flowing whole blood. I: Dependence on shear rate and hematocrit

Echogenicity variations from porcine blood I: the "bright collapsing ring" under pulsatile flow

The temporal and radial variations of the echogenicity from porcine blood were investigated using a linear M12L transducer with a GE LOGIQ 700 Expert system. The "bright collapsing ring" (BRCR) phenomenon, a bright echogenic ring converging from the periphery to the center of the tube wall and eventually collapsing during a pulsatile cycle in cross-sectional B-mode images, was observed from porcine blood in a mock flow loop within a 0.95-cm diameter tube under certain flow conditions. The BRCR phenomenon from porcine blood was stronger as the peak speed was increased from 10 to 25 cm/s, and the mean echogenicity and the "black hole" (BLH) phenomenon, a central echo-poor zone surrounded by a bright hyperechoic zone, became weaker. As stroke rate was increased from 20 to 60 beats/min (bpm), both the BRCR and the BLH phenomena became weaker. These two phenomena were observed at three transmitting frequencies (9, 11 and 13 MHz). As hematocrit was increased from 12 to 45%, the BRCR phenomenon became more apparent. The nonlinear behavior of backscatter as a function of hematocrit reaching a maximum at hematocrit of 10 approximately 20% was observed near the tube wall, but it changed at the center of the tube, indicating the importance of hemodynamics on the ultrasonic backscatter from flowing blood. The combined effects of shear rate and acceleration on red blood cell aggregation are suggested as a possible mechanism for these phenomena.

Cyclic and radial variation of the Doppler power from porcine whole blood

The Doppler power from porcine blood was observed in a mock flow loop to have cyclic and radial variation during a pulsatile cycle. It was found to decrease with shear rate under steady flow, except near the center of the tube at which other mechanisms such as the effects of radial distribution on the rouleaux might be involved. Under pulsatile flow, the timing of the peak of the Doppler power measured at the center of the tube became closer to the peak systole from 20 to 60 beats/minute (BPM), and the power and velocity peaks coincided at 60 BPM. The overall radial variation of the Doppler power during a whole pulsatile cycle was prominent due to the increase of shear rate from the center to 4.5 mm radial position within a tube of 6.35 mm radius. The cyclic variation of the Doppler power varied with the radial position, being relatively large at the center, reaching a minimum at an intermediate radial position, and increasing again near the wall. The peak of the Doppler power occurred at early systole near the tube wall and lagged the flow closer to the center. The "black hole" phenomenon was observed only over portions of the flow cycle. All these complex variations of the Doppler power across the tube over a cycle are thought to be the result of red cell aggregation, which can be affected by shear rate and acceleration.

Noninvasive in vivo measurements of hematocrit

OBJECTIVE

To develop a clinically applicable method for noninvasive acoustic determination of hematocrit values in vivo.

METHODS

The value of hematocrit was determined initially in vitro from the pulseecho measurements of acoustic attenuation. The testing was carried out in a laboratory setup with an ultrasonic transducer operating at 20 MHz and with the use of human blood samples at 37 degrees C. The attenuation coefficient measurements in blood in vivo were implemented by multigated, 20-MHz pulsed Doppler insonation. The Doppler signal was recorded in the brachial and radial arteries. Both in vitro and in vivo hematocrit data were compared with those obtained by the centrifuge method.

RESULTS

The attenuation coefficient in vitro was determined from the measurements of 168 samples with hematocrit values varying between 23.9% and 51.6%. The attenuation from 20-MHz data was equal to 3.66 + 0.089 hematocrit (decibels per centimeter). The uncertainty of in vivo measurements in the brachial artery was determined to be within +/- 5% hematocrit. However, the measurements in the radial artery resulted in a clinically unacceptable uncertainty of +/- 20% hematocrit.

CONCLUSIONS

The method proposed appears to be promising for in vivo determination of hematocrit, because 5% hematocrit error is adequate for monitoring changes in patients in shock or during dialysis. It was found that the multigate system largely simplified placement of an ultrasonic probing beam in the center of the blood vessel. Current work focuses on enhancing the method's applicability to arbitrarily selected vessels and to reducing the hematocrit measurement error to much less than 5% hematocrit.

High-frequency backscatter and attenuation measurements of porcine erythrocyte suspensions between 30-90 MHz

There are now diagnostic ultrasonic imaging devices that operate at very high frequencies (VHF) of 20 MHz and beyond for clinical applications in ophthalmology, dermatology and vascular surgery. To be able to better interpret these images and to further the development of these devices, knowledge of ultrasonic attenuation and scattering of biologic tissues, such as blood, in the high-frequency range is crucial. VHF attenuation and backscatter experiments were made on porcine red blood cell (RBC) suspensions, for which much data on attenuation and backscatter can be found in the literature in the lower frequency range. Attenuation and backscatter at hematocrits of 6%, 10%, 15%, 20%, 25% and 30% from 30 to 90 MHz were measured using a modified substitution method that allows the utilization of focused transducers. The results show that the attenuation coefficient from all suspensions increased linearly with frequency and the backscatter coefficient for low hematocrit suspensions was found to have a maximum between 10% and 15%. At higher hematocrits, a decrease in the frequency-dependence was observed, possibly indicating that Rayleigh scattering is no longer valid because the wavelength in the VHF range is comparable to the size of a porcine RBC.

A new ultrasound principle for characterizing erythrocyte aggregation: in vitro reproducibility and validation

RATIONALE AND OBJECTIVES

There is no method currently available to quantify erythrocyte aggregation in vivo. In this work, using a Couette system, we defined new ultrasound indexes potentially applicable for non-invasive investigations.

METHODS

Two ultrasound protocols were developed: (1) a protocol in which decreasing shear rates ranging from 200 to 1 s-1 were applied to solutions; and (2) a protocol in which a 200 s-1 shear rate was initially applied followed by stoppage of flow (a kinetics protocol). New ultrasound indexes were defined as: the power PUS at the nominal frequency of each transducer, Rayleigh's slope (tangent of the curve PUS = f(log(F)) through the 3.5 to 15 MHz frequency bandwidth) and kinetic indexes characterizing the aggregation/aggregability of the suspension.

RESULTS

Using washed erythrocytes resuspended in saline, it was shown that the ultrasound intensity is dependent at 3.54 +/- 5.9% (NS) to the power of the frequency (theoretical value = 4). Using 10 total blood samples extracted from a single pig, good reproducibility for all indexes (5%) was demonstrated.

CONCLUSIONS

A suitable and reproducible methodology was developed and validated for studying erythrocyte aggregation in calibrated in vitro conditions.

Simulation of ultrasound backscattering by red cell aggregates: effect of shear rate and anisotropy

Tissue characterization using ultrasound (US) scattering allows extraction of relevant cellular biophysical information noninvasively. Characterization of the level of red blood cell (RBC) aggregation is one of the proposed application. In the current paper, it is hypothesized that the microstructure of the RBCs is a main determinant of the US backscattered power. A simulation model was developed to study the effect of various RBC configurations on the backscattered power. It is an iterative dynamical model that considers the effect of the adhesive and repulsive forces between RBCs, and the effect of the flow. The method is shown to be efficient to model polydispersity in size, shape, and orientation of the aggregates due to the flow, and to relate these variations to the US backscattering properties. Three levels of aggregability at shear rates varying between 0.05 and 10 s(-1) were modeled at 40% hematocrit. The simulated backscattered power increased with a decrease in the shear rate or an increase in the RBC aggregability. Angular dependence of the backscattered power was observed. It is the first attempt to model the US power backscattered by RBC aggregates polydisperse in size and shape due to the shearing of the flow.

Haematologic determinants of left atrial spontaneous echo contrast in mitral stenosis

OBJECTIVES

To determine if a relationship exists in mitral stenosis, in patients with either sinus rhythm or atrial fibrillation, between left atrial spontaneous echo contrast and the haematologic indices haematocrit, red cell concentration, mean corpuscular volume, platelet count and volume.

METHODS

Left atrial spontaneous echo contrast severity was graded on a scale of 0-4 in 163 patients with symptomatic mitral stenosis (84 patients in sinus rhythm, 79 patients in atrial fibrillation) undergoing transesophageal echocardiography, cardiac catheterization and full blood examination as part of assessment prior to balloon mitral valvuloplasty.

RESULTS

In sinus rhythm, spontaneous echo contrast grade was negatively correlated with cardiac index (r=-0.33), mitral valve area (r=-0.25) and mitral regurgitation grade (r=-0.22) and positively correlated with haematocrit (r=0.24) and red cell concentration (r=0.25). Spontaneous echo contrast grade was not correlated with left atrial diameter or mean corpuscular volume. In atrial fibrillation, spontaneous echo contrast grade was also negatively correlated with mitral valve area (r=-0.25) and mitral regurgitation (r=-0.36) but was positively correlated with left atrial diameter (r=0.34) and was not correlated with cardiac index, haematocrit or red cell concentration. There was no correlation between spontaneous echo contrast grade and platelet variables in either group.

CONCLUSIONS

Natural variation in red cell concentration in patients with symptomatic mitral stenosis was an independent predictor of the severity of left atrial spontaneous echo contrast in sinus rhythm, but no relationship between red cell concentration and spontaneous echo contrast grade was evident in atrial fibrillation.

A point process approach to assess the frequency dependence of ultrasound backscattering by aggregating red blood cells

To study the shear-thinning rheological behavior of blood, an acoustical measurement of the erythrocyte aggregation level can be obtained by analyzing the frequency dependence of ultrasonic backscattering from blood. However, the relation that exists among the variables describing the aggregation level and the backscattering coefficient needs to be better clarified. To achieve this purpose, a three-dimensional random model, the Neyman-Scott point process, is proposed to simulate red cell clustering in aggregative conditions at a low hematocrit (H<5%). The frequency dependence of the backscattering coefficient of blood, in non-Rayleigh conditions, is analytically derived from the model, as a function of the size distribution of the aggregates and of their mass fractal dimension. Quantitative predictions of the backscatter increase due to red cell aggregation are given. The parametric model of backscatter enables two descriptive indices of red cell aggregation to be extracted from experimental data, the packing factor W and the size factor delta. Previously published backscatter measurements from porcine whole blood at 4.5% hematocrit, in the frequency range of 3.5 MHz-12.5 MHz, are used to study the shear-rate dependence of these two indices.

Doppler power variation from porcine blood under steady and pulsatile flow

Although a number of recent studies have demonstrated that the echogenicity of blood varies as a function of time under pulsatile flow, the fundamental mechanisms responsible for it are still uncertain. To better understand this phenomenon, the Doppler power from porcine blood and polystyrene microsphere suspensions was measured at the center of the tube as functions of two crucial parameters, flow velocity and stroke rate (for pulsatile flow), under steady and pulsatile flow in a mock flow loop. In the present study, the experimental results were obtained with a 10-MHz pulsed Doppler system with a frequency response estimated more accurately by electronic injection, and validated by comparing to the radiofrequency (RF) signal acquired from the same Doppler instrument. The results show that the Doppler power from microspheres and porcine red blood cell (RBC) suspensions did not vary appreciably (< 2 dB), with either the speed or stroke rate (for pulsatile flow only) under steady and pulsatile flow. It was found that the Doppler power from porcine whole blood under steady flow decreased with the speed by approximately 13 dB from 3 to 33 cm/s and was only 3 dB higher than that from RBC suspension at 33 cm/s, suggesting minimal RBC aggregation in whole blood at this speed. The apparent cyclic variation from whole blood was observed at 20 and 40 beats/min (BPM). The cyclic variation became more obvious as the speed and stroke rate decreased. The mean Doppler power over a cycle increased as the peak speed decreased. The Doppler power reached a maximum near peak systole and a minimum at late diastole at the center of the tube. This pattern cannot be explained by RBC aggregation due to the shear rate alone, and may be attributed to acceleration and deceleration along with aggregation. The cyclic variation was not observed at 60 BPM, probably because of a lack of time for aggregation to occur.

In vivo measurements of ultrasonic backscattering in blood

Ultrasonic backscattering in blood including its dependence on the hematocrit, plasma proteins, shear rate, and flow disturbance, has been studied extensively theoretically and experimentally in vitro. However, much of the result has never been validated in vivo. To do so, backscattering measurements were made on pigs using a 10-MHz non-focused intravascular transducer in direct contact with blood. The probe was placed in either the abdominal aorta or the inferior vena cava. The backscattering coefficient (BSC) of blood flowing in these vessels as well as downstream from a stenosis was measured using an approach that was originally developed for measurements with focused transducers. With this approach, 6% porcine red cell saline suspensions prepared immediately after each in vivo measurement were used as the reference medium. Result from seven pigs at hematocrits ranging from 29 to 36% (31.9 +/- 2.5%) demonstrated that BSC of blood in the vena cava, (4.62 +/- 2.06) x 10(-5) cm-sr-1, is consistently higher than that in the aorta, (2.65 +/- 1.22) x 10(-5) cm-sr-1. The difference has been attributed to the lower shear rate and the formation of red cell aggregation in venous blood. These in vivo results are in agreement with those obtained in vitro. In response to stenoses created by ligating the aorta, backscattering of the blood measured downstream from the stenosis showed that the closer the site of measurement relative to the stenosis, the higher the backscatter, presumably resulting from the higher degree of flow disturbance. In vitro backscattering results on porcine whole blood were also acquired at 20 MHz with a Diasonics intravascular scanner.

Integrated backscatter assessment of left atrial spontaneous echo contrast in chronic nonvalvular atrial fibrillation: relation with clinical and echocardiographic parameters

Integrated backscatter (IB) provides the quantitative assessment of left atrial spontaneous echo contrast (SEC). The IB intensity of the left atrial cavity relative to the left ventricular cavity is related to atrial thrombus in patients with atrial fibrillation (AF) or sinus rhythm. However, little is known about the relation between the quantitative SEC value of the left atrial cavity and variables implying thromboembolism in nonvalvular AF. To examine this relation, we performed transesophageal echo-cardiography with IB analysis in 65 patients with chronic nonvalvular AF. The quantitative SEC value of the left atrial cavity was defined as the difference between atrial IB intensity and ventricular IB intensity (corrected IB intensity). The corrected IB intensity was correlated with the left atrial dimension (r = 0.25, P =.049), the left atrial appendage velocity (r = -0.41, P <.001), and the duration of AF (r = 0.23, P =. 023). The corrected IB intensity was higher in patients who had a history of hypertension (3.2 +/- 2.2 dB versus 2.0 +/- 1.6 dB, P =. 018), SEC (3.9 +/- 1.9 dB versus 1.4 +/- 1.1 dB, P =.002), and left atrial thrombus (4.5 +/- 2.7 dB versus 2.2 +/- 1.7 dB, P <.001) when compared with those who did not have these abnormalities. The corrected IB intensity was significantly lower in patients with significant mitral regurgitation than in those without it (1.1 +/- 1. 2 dB versus 2.7 +/- 2.0 dB, P =.036). When the cutoff value of the corrected IB intensity was set at >/=2.0 dB, the sensitivity for left atrial thrombus was 78% and the specificity was 55%. In patients with chronic nonvalvular AF, the quantitative SEC value of the left atrial cavity depends on the duration of AF as well as the left atrial dimension and appendage velocity. Although IB may be capable of identifying patients with higher risk of cardiogenic embolism, a large-scale prospective study is needed to actually establish this.

Spontaneous echo contrast: where there's smoke there's fire

Left atrial (LA) spontaneous echo contrast (SEC), or "smoke," is a frequent finding on transesophageal echocardiography (TEE), but it is rarely detected with transthoracic echocardiography. LA SEC is characterized by dynamic smoke-like echoes within the LA cavity or appendage. Most patients with LA SEC have atrial arrhythmias, mitral stenosis, or a mitral valve prosthesis, and they have an enlarged LA, conditions that are associated with LA stasis. Conversely, mitral regurgitation is protective against LA SEC. LA SEC is present in almost all patients with LA thrombus and is associated with previous embolic events in many patient populations. In patients with nonvalvular atrial fibrillation, LA SEC predicts future embolism and death. LA SEC may therefore assist in selecting patients with atrial fibrillation or with mitral stenosis and sinus rhythm who benefit the most from anticoagulation. Hematological studies have shown that LA SEC is a marker of an hypercoagulable state. LA SEC is a manifestation of red cell aggregation, arising from an interaction between red cells and plasma proteins such as fibrinogen, at low shear rates. LA SEC does not require platelets. The detection of LA SEC on ultrasound arises from the increased amplitude of backscatter from red cell aggregates rather than single cells. Patients with LA SEC should be considered for anticoagulant therapy and may require correction of underlying cardiovascular abnormalities. Future directions in LA SEC include further assessment of integrated backscatter for quantification, assessment of its prognostic role in clinically low-risk patients with nonvalvular AF, and novel pharmacological treatment.

Rheo-acoustical study of the shear disruption of reversible aggregates. Ultrasound scattering from concentrated suspensions of red cell aggregates

Shear-induced disruption of reversible aggregates or clusters in a concentrated suspension is investigated by ultrasound backscattering in the low shear regime. Fractal aggregates are considered as non-Brownian scatterers much smaller than the wavelength with acoustic properties close to those of the surrounding liquid, so that the attenuation of the coherent field is weak and multiple scattering can be neglected. The concept of variance in local particle volume fraction is used to deduce a first-order expression of the ultrasound scattering cross section per unit volume for Rayleigh scatterers in a dense suspension. On the basis of a scaling law for the shear-induced disruption of aggregates, the shear stress dependence of the ultrasonic scattered intensity from a dense suspension of clusters is derived. In a second part, the shear breakup of hardened red blood cell aggregates is investigated in plane-plane flow geometry by ultrasound scattering. Rheo-acoustical experiments are analyzed within the framework of the self-consistent field approximation and the scaling laws currently used in microrheological models. Finally, the ability of ultrasonic, light reflectometry and viscometry methods to provide quantitative information about red blood cell aggregation and membrane adhesiveness is discussed.

Modeling and analysis of ultrasound backscattering by spherical aggregates and rouleaux of red blood cells

The present study concerns the modeling and analysis of ultrasound backscattering by red blood cell (RBC) aggregates, which under pathological conditions play a significant role in the rheology of blood within human vessels. A theoretical model based on the convolution between a tissue matrix and a point spread function, representing, respectively, the RBC aggregates and the characteristics of the ultrasound system, was used to examine the influence of the scatterer shape and size on the backscattered power. Both scatterers in the form of clumps of RBC aggregates and rouleaux were modeled. For all simulations, the hematocrit was kept constant at 10%, the ultrasound frequency was 10 MHz, the insonification angle was varied from 0 to 90 degrees , and the scatterer size (diameter for clumps and length for rouleaux) ranged from 4 mum to 120 mum. Under Rayleigh scattering by assuming a Poisson distribution of scatterers in space, the ultrasound backscattered power increased linearly with the particle volume. For non-Rayleigh scatterers, the intensity of the echoes diminished as the scatterer volume increased, with the exception of rouleaux at an angle of 90 degrees . As expected, the backscattered power was angularly dependent for anisotropic particles (rouleaux). The ultrasound backscattered power did not always increase with the size of the aggregates, especially when they were no longer Rayleigh scatterers. In the case of rouleaux, the anisotropy of the backscattered power is emphasized in the non-Rayleigh region.

Pathophysiologic correlates of thromboembolism in nonvalvular atrial fibrillation: II. Dense spontaneous echocardiographic contrast (The Stroke Prevention in Atrial Fibrillation [SPAF-III] study)

We analyzed transesophageal echocardiograms from 772 participants in the Stroke Prevention in Atrial Fibrillation (SPAF-III) study, characterizing spontaneous echocardiographic contrast (SEC) in the left atrium or appendage as faint or dense. The association of dense SEC with stroke risk factors and anatomic, hemodynamic, and hemostatic parameters related to specific thromboembolic mechanisms was evaluated by multivariate analysis. Spontaneous echocardiographic contrast was present in 55% of patients and was dense in 13%. Age (odds ratio [OR] 2.4/decade, P <.001), constant atrial fibrillation (OR 6.9, P <.001), history of hypertension (OR 3. 2, P <.001), and current tobacco smoking (OR 2.6, P =.04) were independent clinical predictors of dense SEC. Multivariate analysis of clinical, echocardiographic, and hemostatic parameters yielded age as the sole independent clinical predictor of dense SEC (OR 2. 4/decade, P <.001). Other independent predictors were measures of left atrial/appendage flow dynamics, left atrial size (OR 2.4/cm diameter, M-mode, P <.001), atherosclerotic aortic plaque (OR 2.8, P =.002), and plasma fibrinogen >350 mg/dL (P <.001). Results were similar when SEC of any density was analyzed. In conclusion, SEC occurred in more than half of these patients with prospectively defined nonvalvular atrial fibrillation but was usually faint. Dense SEC was strongly associated with previously reported clinical predictors of stroke, linking them to thromboembolism through atrial stasis. Diverse pathophysiologic factors including atrial stasis, fibrinogen level, and aortic plaque influence SEC.

A system-based approach to modeling the ultrasound signal backscattered by red blood cells

A system-based model is proposed to describe and simulate the ultrasound signal backscattered by red blood cells (RBCs). The model is that of a space-invariant linear system that takes into consideration important biological tissue stochastic scattering properties as well as the characteristics of the ultrasound system. The formation of the ultrasound signal is described by a convolution integral involving a transducer transfer function, a scatterer prototype function, and a function representing the spatial arrangement of the scatterers. The RBCs are modeled as nonaggregating spherical scatterers, and the spatial distribution of the RBCs is determined using the Percus-Yevick packing factor. Computer simulations of the model are used to study the power backscattered by RBCs as a function of the hematocrit, the volume of the scatterers, and the frequency of the incident wave (2-500 MHz). Good agreement is obtained between the simulations and theoretical and experimental data for both Rayleigh and non-Rayleigh scattering conditions. In addition to these results, the renewal process theory is proposed to model the spatial arrangement of the scatterers. The study demonstrates that the system-based model is capable of accurately predicting important characteristics of the ultrasound signal backscattered by blood. The model is simple and flexible, and it appears to be superior to previous one- and two-dimensional simulation studies.

Influence of warfarin therapy on left atrial spontaneous echo contrast in nonvalvular atrial fibrillation

To examine whether warfarin therapy had any influence on left atrial spontaneous echo contrast, we performed serial transesophageal echocardiography with integrated backscatter analysis in 12 patients with non-valvular atrial fibrillation. We found that the integrated backscatter intensity of the left atrial cavity did not change after 1 to 2 months of warfarin therapy, and concluded that this therapy does not influence spontaneous echo contrast.

Ultrasonic backscattering from porcine whole blood of varying hematocrit and shear rate under pulsatile flow

It was shown previously that ultrasonic scattering from whole blood varies during a flow cycle under pulsatile flow both in vitro and in vivo. It has been postulated that this cyclic variation may be associated with the dynamics of red cell aggregation because the shearing force acting on the red cell aggregates across the lumen is a function of time during a flow cycle. In all studies, the local shear rate variation as a function of time is unknown. The effect of shear rate on the red cell aggregation and, thus, on ultrasonic scattering from blood can only be merely speculated. One solution to this problem is to estimate the shear rate in a flow conduit by finite element analysis (FEA). An FEA computational fluid dynamics (CFD) tool was used to calculate local shear rate in a series of experiments in which ultrasonic backscattering from porcine whole blood under pulsatile flow was measured as a function of hematocrit and shear rate intravascularly with a 10-MHz catheter-mounted transducer in a mock flow loop. The results show that, at 20 beats per min (BPM), the magnitudes of the cyclic variation for hematocrits at 30, 40, and 50% were approximately 4 dB. However, at 60 BPM, the magnitude of cyclic variation was found to be minimal. The results also confirm previous findings that the amplitude and the timing of the peak of ultrasonic backscattering from porcine whole blood under pulsatile flow during a flow cycle are dependent upon the shear rate and hematocrit in a complicated way.

Embolic risk based on aortic atherosclerotic morphologic features and aortic spontaneous echocardiographic contrast

BACKGROUND

Different aortic atherosclerotic plaque morphologic features may have varying embolic potentials. Spontaneous echocardiographic contrast (SEC) in the aorta, as in the left atrium, has been associated with an increased risk of embolic events and often occurs with complex aortic atherosclerosis. Thus an evaluation of their isolated and combined association with embolic events was undertaken.

METHODS

We retrospectively studied all patients who underwent biplane or multiplane transesophageal echocardiography meeting the following inclusion and exclusion criteria: age >/=55 years and no other cardiac pathologic condition known to be associated with embolic events other than aortic atherosclerosis or aortic SEC. The 105 patients meeting the criteria were divided into those with aortic atherosclerosis and/or aortic SEC (the study group) and those without these aortic pathologic conditions (the comparison group). Complex aortic atherosclerosis was defined as mobile, ulcerated, or protuberant (> 4 mm). SEC was defined as a pulsatile, swirling echo pattern within the aortic lumen. Embolic events included strokes, transient ischemic attacks, or peripheral emboli.

RESULTS

The 61 study patients and 44 comparison patients did not significantly differ with respect to the reason for referral, age, or sex. Embolic events occurred in 35 patients. Those with ulcerated or mobile plaques had a greater prevalence of embolic events (odds ratio 4.50; 95% confidence interval, 1.30-15.5; P <.05). The highest embolic event rate was seen in patients with any complex atherosclerosis and concomitant SEC (odds ratio 9.00; 95% confidence interval, 2.06-39.3; P <.01). Patients with SEC alone or protuberant plaques alone did not have a higher event rate (odds ratio 1.71 and 0.60; 95% confidence interval, 0.57-5.17 and 0.15-2. 47, respectively).

CONCLUSIONS

Embolic events were associated with the presence of ulcerated or mobile aortic plaques. In addition, the combination of aortic SEC and any complex atherosclerosis had the highest embolic association.

The effect of hemodynamics, vessel wall compliance and hematocrit on ultrasonic Doppler power: an in vitro study

Previous in vitro studies in rigid tubes under pulsatile flow conditions have reported a lack of a cyclic variation in blood echogenicity that contradicts in vivo results. To investigate whether or not these variations can be attributed to the compliance of the vessel wall, a series of in vitro experiments with compliant tubes, under pulsatile flow conditions, was performed. Two important factors that may affect the Doppler power were investigated: 1. the dependence on hematocrit and 2. the effect of the vessel wall elasticity. In the present study, it is shown that, at the low beat rates, the peak of the mean Doppler power within the flow cycle depends on the vessel wall compliance. When the vessel becomes more compliant, the peak is shifted from the early to the late systole. Additionally, there is a correlation between the power peak and hematocrit that is more evident in compliant vessels. At a higher pulsation rate of 37 beats/min, a different variation is observed. A drop in the power occurs near peak systole in compliant tube experiments and is more pronounced as the vessel becomes more constricted. The observed power drop agrees with previously reported in vivo results, but is not seen in rigid tube experiments. The results of this study suggest that proper interpretation of cyclic variations in Doppler power requires a knowledge of hemodynamic parameters, such as the modulus of elasticity of the vessel wall, propagation velocity or, possibly, the phase angle of input impedance.

An in vitro study of the effects of Doppler angle, fibrinogen, and hematocrit on ultrasonic Doppler power

For a better understanding of the relationship between the Doppler power and erythrocyte aggregation of whole blood under steady flow in a conduit, the effects of Doppler angle, fibrinogen concentration, and hematocrit were investigated in a mock flow loop. The results show that, at a mean shear rate of 102 s(-1), there was minimal angular dependence; but at a mean shear rate of 52 s(-1), there was a weak angular dependence as the Doppler angle was varied from 40 degrees to 70 degrees . These results suggest that there was, perhaps, no or little alignment of the red cell aggregates at high shear rates. The Doppler power was found to increase nonlinearly as the fibrinogen concentration was increased; and the effect of other plasma proteins on red cell aggregation may not be negligible, although fibrinogen is the dominant factor. The results show that the variation of the Doppler power over the lumen is hematocrit dependent for hematocrits below 26%

Expression of cell adhesion molecules and the appearance of adherent leukocytes on the left atrial endothelium with atrial fibrillation: rabbit experimental model

To assess the expression of cell adhesion molecules and the appearance of leukocytes adhering to the left atrial endothelium with atrial fibrillation (AF), 10 Japanese white rabbits were anesthetized and 3 pacing leads were placed in the right atrium. For the AF model, the right atrium was stimulated by electrical pacing (the stimulation frequency of each lead being adjusted to different intervals) for 8h while the control model was subjected to a sham operation without atrial stimulation. The left atrial appendage was excised from the heart and examined immunohistochemically. P-selectin staining of the endothelium in both models was linear and regional, and intracellular adhesion molecule-1 (ICAM-1) in the AF model was confined to leukocytes and endothelial cells with adherent leukocytes. The expression of P-selectin (p<0.05) and the appearance of positively ICAM-1 stained adherent leukocytes (p<0.05) were significantly greater in the AF model than in the control model. In conclusion, AF could regulate the expression of at least 2 critical adhesion molecules, P-selectin and ICAM-1, and the appearance of adherent leukocytes; suggesting that these molecules may play an important role in left atrial thrombus formation with AF. Although anticoagulant therapy has generally been carried out with warfarin in AF patients, neutralizing antibodies to cell adhesion molecules should be tried to prevent thromboembolic complications.

Lower extremity volumetric arterial blood flow in normal subjects

The objective of this clinical study was to establish normal values for volumetric blood flow in the leg at rest using Doppler ultrasound, and to determine what biophysical factors influence resting volumetric flow. Arterial blood flow was measured at four sites in the legs of 40 healthy subjects using an ATL Ultramark 9 HDI system. All subjects were nonhypertensive nonsmokers with ankle brachial index values greater than 1 and no history of vascular disease. The subjects, 20 of each gender, in age ranging from 20 to 64 y were examined. Blood flow was calculated from the time-averaged, intensity-weighted mean velocity Doppler waveforms and vessel cross-sectional area at the same site. Thigh and calf circumference measurements were used to estimate muscle masses. The mean flow and standard error measured in four arteries in the leg were: 284+/-21 mL/min in the common femoral (CFA); 152+/-10 mL/min in the superficial femoral (SFA); 72+/-5 mL/min in the popliteal; and 3+/-1 mL/min in the dorsalis pedis. Although women tended to have higher time-averaged mean velocities in the CFA and SFA than men (t-test, p < 0.008), their arterial cross-sectional areas tended to be smaller (t-test, p < 0.004) and no statistically significant difference was found between men and women in volumetric flow at any site. No correlation was found between age, weight, height, muscle mass and volumetric flow at all four sites. These estimates of lower extremity volumetric flow in healthy subjects provide a baseline for future studies of flow rates in patients with vascular disease.

In situ measurements of Doppler power vs. flow turbulence intensity in red cell suspensions

Whereas previous studies have shown that ultrasonic backscatter and Doppler power from blood are affected by flow turbulence, turbulence level has only been inferred from the flow Reynolds number and not directly measured. In this study, both ultrasonic Doppler power and flow turbulence intensity were measured in situ to quantify the relationship between Doppler power and flow turbulence. Three grid meshes of different geometries were used in a steady-flow mock loop to generate controlled levels of flow turbulence in porcine red blood cell saline suspensions. Doppler power was measured by a 10-MHz PW Doppler flowmeter, and the turbulence intensity by using constant-temperature hot film anemometry. We showed that Doppler power is affected by turbulence and hematocrit in a complex way. At a fixed hematocrit, Doppler power increases nonlinearly with turbulence intensity and, at fixed turbulence intensity, Doppler power peaks at an optimal hematocrit level that increases with turbulence level. The shape factor, introduced by Lucas and Twersky (1987) to take into account effects of shape and orientation of the scatterers in a dense distribution of small and tenuous scatterers, was estimated by fitting the experimental data to the theoretical model. The results indicate that shape factor decreases with increasing turbulence intensity.

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.

Inhibition of red cell aggregation prevents spontaneous echocardiographic contrast formation in human blood

BACKGROUND

Spontaneous echocardiographic contrast (SEC) is a pattern of blood echogenicity that has been attributed to ultrasonic backscatter from blood cell aggregates that form under low shear conditions. Patients with left atrial SEC have an increased thromboembolic risk. This study examined the role of red cell and platelet aggregates in the pathogenesis of SEC in human blood and the effects on SEC of antithrombotic therapy and red cell disaggregatory agents.

METHODS AND RESULTS

Blood echogenicity was examined with the use of quantitative videodensitometry over a controlled range of flow velocities in an in vitro model characterized by nonlaminar flow conditions. One hundred ninety study samples were prepared from single fresh blood donations (40 to 120 mL) from 24 healthy volunteers and 11 patients. Whole blood echogenicity was unaltered by depletion of platelets, stimulation of platelet aggregation with adenosine diphosphate, or inhibition of platelet aggregation with aspirin. Low flow-related echogenicity increased with increasing hematocrit (P<.001) but was abolished when red cells were lysed selectively with saponin (P<.001). In the presence of red cells, low flow-related echogenicity increased with increasing fibrinogen concentration (P<.001) and with plasma paraproteins. Low flow-related echogenicity in whole blood was unaltered by heparin and warfarin but was reduced in a dose-dependent manner by dextran 40 (40 mg/mL, 70% reduction, P<.001) and poloxamer 188 (8 mg/mL, 47% reduction, P<.001), which inhibited red cell aggregation.

CONCLUSIONS

These results support protein-mediated red cell aggregation as the mechanism of SEC in human blood. Inhibition of red cell aggregation, indexed by resolution of SEC, may provide an alternative to anticoagulant and antiplatelet therapy to reduce cardiac thromboembolic risk.

An approach for measuring ultrasonic backscattering from biological tissues with focused transducers

When the standard substitution method is used with a focused transducer to measure the backscattering coefficient from biological tissues including blood, it yields erroneous results. Extending the backscattering measurements to frequencies beyond 15 MHz necessitates the use of focused transducers because of the worsened signal-to-noise ratio--caused by the increased attenuation and the smaller transducer aperture size--in order to make the measurements close to the transducer. An approach which allows the use of focused transducers in backscattering measurements has been developed. It has been used to measure the backscattering coefficient of red cell suspensions of hematocrit ranging from a few percent to 30% in the frequency range from 5 MHz to 30 MHz. The results at hematocrits below 20% agree well with those obtained with the standard substitution method, although they differ as the hematocrit is increased beyond 20%. The experimental results also show that the fourth-power dependence of backscatter on frequency is in general approximately valid for suspended erythrocytes of hematocrit between 6% and 30%.

Direct effects of DC cardioversion on blood echogenicity: an in vitro study

Exacerbation of left atrial spontaneous echo contrast (SEC) after cardioversion of atrial fibrillation has been attributed to left atrial mechanical dysfunction induced by the procedure ("atrial stunning"). An in vitro model was devised to determine whether electrically induced changes in blood properties might contribute to SEC formation after cardioversion. Human blood echogenicity was examined quantitatively by videodensitometry before and after shocks of 1, 2, 5, and 20 J. Changes in blood cell numbers, cell morphology, and erythrocyte sedimentation rate were determined by haematological analysis. Immediately following electrical discharges, transient and dose-related, highly echogenic microbubbles were noted, but shocks of increasing intensity did not induce SEC at high blood velocity or alter the severity of SEC at low blood velocity. No quantitative or qualitative changes in haematological parameters were observed. These results suggest that direct effects of electrical shock on blood do not contribute to SEC after cardioversion. Systemic haematological responses to electric shock that might indirectly promote red cell aggregation in vivo cannot be excluded by this in vitro study.

In vivo and in vitro studies on the mechanism and clinical significance of spontaneous echocardiographic contrast in patients with atrial dysrhythmias

The pathogenesis of spontaneous echocardiographic contrast (SEC) is complex and multifactorial. Although originally described in low-flow state situations such as in the left atrium of patients with mitral stenosis or in the false lumen of patients with aortic dissection, its detection is highly dependent on technical factors such as the frequency of the transducer used. Multiple blood components have been implicated in SEC formation and erythrocyte aggregation currently appears to be the most likely mechanism. SEC is related to atrial fibrillation and is commonly found in patients with thrombus or prior history of thromboembolism. In addition, it may represent a prognostic marker for patients with atrial fibrillation because patients with SEC have a higher incidence of subsequent thromboembolic events. Therapeutic options include anticoagulation and, perhaps, antiplatelet therapy. Further prospective studies are necessary to better define SEC pathogenesis and treatment.

Integrated backscatter for quantification of left atrial spontaneous echo contrast

OBJECTIVES

This study was designed to develop a quantitative method of spontaneous echo contrast (SEC) assessment using integrated backscatter and to compare integrated backscatter SEC measurement with independent qualitative grades of SEC and clinical and echocardiographic predictors of thromboembolism.

BACKGROUND

Left atrial SEC refers to dynamic swirling smokelike echoes that are associated with low flow states and embolic events and have been graded qualitatively as mild or severe.

METHODS

We performed transesophageal echocardiography in 43 patients and acquired digital integrated backscatter image sequences of the interatrial septum to internally calibrate the left ventricular cavity and left atrial cavity under different gain settings. Patients were independently assessed as having no, mild or severe SEC. We compared intensity of integrated backscatter in the left atrial cavity relative to that in the left ventricular as well as to the independently assessed qualitative grades of SEC. Fourier analysis characterized the temporal variability of SEC. The integrated backscatter was compared with clinical and echocardiographic predictors of thromboembolism.

RESULTS

The left atrial cavity integrated backscatter intensity of the mild SEC subgroup was 4.7 dB higher than that from the left ventricular cavity, and the left atrial intensity of the severe SEC subgroup was 12.5 dB higher than that from the left ventricular cavity. The left atrial cavity integrated backscatter intensity correlated well with the qualitative grade. Fourier transforms of SEC integrated backscatter sequences revealed a characteristic dominant low frequency/high amplitude spectrum, distinctive from no SEC. There was a close relationship between integrated backscatter values and atrial fibrillation, left atrial size, left atrial appendage flow velocities and thrombus.

CONCLUSIONS

Integrated backscatter provides an objective quantitative measure of SEC that correlates well with qualitative grade and is closely associated with clinical and echocardiographic predictors of thromboembolism. The relationship between integrated backscatter measures and cardioembolic risk will be defined in future multicenter studies.

Stratification of thromboembolic risk of atrial fibrillation by transthoracic echocardiography and transesophageal echocardiography: the relative role of left atrial appendage function, mitral valve disease, and spontaneous echocardiographic contrast

The role of transesophageal echocardiography (TEE) in thromboembolic risk stratification in atrial fibrillation (AF) has not been established. Left atrial appendage contractile dysfunction in patients with AF predisposes to thrombus formation. The extent of blood stasis and propensity for thrombus can be assessed during TEE by measurement of the peak Doppler velocity of blood outflow from the appendage. Spontaneous echocardiographic contrast (SEC) is a swirling pattern of blood echogenicity that may be detected by TEE in the left atrium in patients with AF. The presence of SEC reflects left atrial blood stasis and a prothrombotic state. SEC is associated with an increased risk of systemic thromboembolic events. Parameters derived from TEE may provide additional prognostic data to clinical history and transthoracic echocardiography in thromboembolic risk stratification in AF.

Power Doppler ultrasound evaluation of the shear rate and shear stress dependences of red blood cell aggregation

The use of power Doppler ultrasound at 10 MHz is evaluated as a method to study the shear rate and the shear stress dependences of red blood cell aggregation. This evaluation was based on six in vitro experiments conducted in a 1.27-cm diameter tube under steady flow conditions. Porcine whole blood was circulated in the flow model at flow rates ranging between 125 to 1500 ml/min (mean shear rate across the tube ranging between 6 and 74 s-1). For each flow condition, the variation of the Doppler power across the tube and the velocity profile were measured by moving the Doppler sample volume across the tube diameter. For each radial position, the shear rate within the Doppler sample volume was also determined by considering the radial power pattern of the ultrasound beam. To estimate the shear stress within the Doppler sample volume, the apparent viscosity of blood samples withdrawn from the flow model was measured for each experiment. The variation of the Doppler power as a function of the shear rate within the sample volume showed a rapid reduction of the power between 1 and 5 s-1, a transition region between 5 and 10 s-1, and a very slow reduction beyond 10 s-1. Little variation of the Doppler power was measured for shear stress higher than 2 dyn/cm2. The maximum Doppler power for all flow rates was usually found near the center of the tube. Based on the ultrasonic scattering models, which predict that the Doppler power is related to the volume square of the scatterers, the method described in the present study showed a very high sensitivity to the presence of red blood cell aggregation for shear rates below 10 s-1.

Cyclic variation of Doppler power from whole blood under pulsatile flow

The echogenicity and Doppler power from whole blood under pulsatile flow have been found to vary during the flow cycle in previous studies both in vitro and in vivo. The present study was undertaken to better understand this phenomenon. Doppler power from whole blood under pulsatile flow was measured with a pulsed Doppler flowmeter as a function of the flow cycle, radial position and compliance of the vessel in a mock flow loop. It was found that the cyclic variation is more pronounced if the stroke rate is less than 56 beats/min and that the peak of the Doppler power from whole blood flowing near the center stream coincided with the peak of the flow velocity. However, it began to lead the velocity peak as the measurement site was moved away from the center stream. The lead increased as the radial distance was increased. The results also show that the compliance of the vessel can affect, to a certain extent, the magnitude of the cyclic variation. Results from intravascular Doppler measurements rule out the possibility that the cyclic variation is primarily due to the variation in attenuation caused by vessel wall during a flow cycle.

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.

Spontaneous echocardiographic contrast in the thoracic aorta: factors associated with its occurrence and its association with embolic events

Spontaneous echocardiographic contrast is associated with embolic events when it occurs in the left atrium. Because little is known about spontaneous echocardiographic contrast in the aorta, we investigated this association retrospectively in 343 patients without aortic dissection or aneurysm who had undergone transesophageal echocardiography. Two independent readers concurred on the presence of spontaneous echocardiographic contrast in the aorta in 93% of the study patients, with the remainder agreed on by consensus. Spontaneous echocardiographic contrast was found in 65 patients (19%) and was associated with older age (p < 0.0001), male sex (p < 0.0001), slightly larger aortas (p < 0.0001), and complex aortic atherosclerosis (p = 0.0001). Thirty-four (28.6%) of 119 patients with clinical embolic events had spontaneous echocardiographic contrast in the aorta in contrast to 31 (13.8%) of 224 patients referred for other reasons (p = 0.0001). This finding remained significant when spontaneous echocardiographic contrast in the aorta was the only abnormality allowed (n = 207, p = 0.0065) or when other echocardiographic variables known to be related to embolic events were included in a multivariate analysis. Thus, spontaneous echocardiographic contrast in the aorta can often be detected by transesophageal echocardiography and is associated with a higher prevalence of embolic events.

Transesophageal echocardiographic determinants of embolism in nonrheumatic atrial fibrillation

The purpose of the study was to determine the relation of transesophageal echocardiographic findings to symptoms of systemic embolism in patients with nonrheumatic atrial fibrillation. Transthoracic and transesophageal echocardiography were used to study 107 patients with atrial fibrillation including 49 patients without embolic complications and 58 patients who had suffered from previous cerebral or peripheral embolism. A multiple logistic regression analysis revealed that left atrial thrombi (odds ratio 9.0, 95% CI 2.4-33.6, p < 0.005) and the presence of dense left atrial spontaneous contrast (odds ratio 8.4, 95% CI 1.3-53.1, p < 0.05) were independently related to embolic symptoms. Intensive left atrial spontaneous contrast was associated with an increased left atrial diameter (odds ratio 2.0, 95% CI 1.1-3.6, p < 0.05), the presence of chronic atrial fibrillation (odds ratio 6.9, 95% CI 1.6-29.8, p < 0.01) and aortic atherosclerosis (odds ratio 2.6, 95% CI 1.2-5.5, p < 0.05). It was further negatively correlated to mitral regurgitation (odds ratio 0.4, 95% CI 0.2-0.9, p < 0.05). In conclusion, dense spontaneous echo contrast and left atrial thrombi are associated to thromboembolic complications in patients with nonrheumatic atrial fibrillation. Classifying of spontaneous contrast seems to be useful when estimating the thromboembolic risk in atrial fibrillation.

Quantification of blood echogenicity: evaluation of a semiquantitative method of grading spontaneous echo contrast

Spontaneous echo contrast (SEC) is an echogenic, swirling pattern of blood flow which may be observed by transesophageal echocardiography (TEE) in the left atrium in low flow states, such as atrial fibrillation (AF). The presence of SEC has been proposed as a marker of increased thromboembolic risk. Evaluation of the severity of SEC might be useful in stratification of thromboembolic risk. The aim of this study was to validate a semiquantitative method of grading SEC against quantitative videodensitometric analysis. TEE studies were performed in 50 patients with AF. The severity of left atrial SEC was graded by three independent observers and by videodensitometry. There was a strong, positive correlation between the semiquantitative grades of SEC and quantitative videodensitometric scores (r = 0.85, P < 0.0001). Inter- and intraobserver correlations in the grading of SEC were very high (observer 1 vs. 2: r = 0.98, P = 0.0001; observer 1 vs. 3: r = 0.93, P = 0.0001; observer 1 vs. 1: r = 0.97, P = 0.0001). Semiquantitative grading of SEC can be performed rapidly and reliably by experienced observers. These results support the use of semiquantitative grading in studies of the pathogenesis and prognostic implications of SEC.

Ultrasound backscatter at 30 MHz from human blood: influence of rouleau size affected by blood modification and shear rate

High frequency intravascular ultrasound may show a high intensity backscatter from blood which hampers the discrimination between lumen and arterial wall. In this study, the acoustic behaviour of blood at 30 MHz in relation to rouleau size was analyzed. In a Couette viscometer, high frequency (20-40 MHz) backscatter data from normal and modified blood samples from eight volunteers were obtained at shear rates from 0 to 1000 s-1. The acoustic behaviour of blood was quantified by the integrated backscatter power and the spectral slope of the backscatter coefficient. Backscatter from blood depended on rouleau size. At a shear rate of zero, both whole blood and rouleau-enhanced blood showed a 11-dB-higher integrated backscatter power than rouleau-suppressed blood, which itself was 10 dB higher than that of hemolysed blood, the latter showing a 6-dB-higher backscatter than saline. Platelets did not contribute to the backscatter power. Plasma and saline produced no detectable integrated backscatter power other than noise. The spectral slope of whole and rouleau-enhanced blood was small (1 and 0.5, respectively), whereas rouleau-suppressed blood and hemolysed blood (both with a slope of 3.3) behaved almost like a Rayleigh scattering medium (slope = 4). The backscatter from rouleau-suppressed blood showed no shear rate dependence. At low shear rates ( < 0.8 s-1 for integrated backscatter power and < 0.2 s-1 for the spectral slope), whole blood and rouleau-enhanced blood tended to the results from the static situation (no shear). At high shear rates ( > 80 s-1 for integrated backscatter power and >11 s-1 for spectral slope), these samples tended to the results of rouleau-suppressed blood. Ultrasound backscatter at 30 MHz from human blood is only caused by red blood cells. With increasing aggregate (rouleau) size, the integrated backscatter power increased by 11 dB, and the spectral slope decreased from 3.3 to 1.

Time-domain ultrasonic contrast blood flowmetry

Time-domain ultrasonic blood flow estimation methods have recently received considerable attention because of their advantages over conventional Doppler methods. Among them are that they may yield better spatial resolution and that methods based on frame-to-frame speckle tracking do not require a knowledge of the angle between directions of blood flow and the ultrasound beam. These methods, however, suffer from an intrinsic problem of poor signal-to-noise ratio, since the echoes scattered back by blood are much weaker than those of the surrounding structures. In addition, the maximal velocity that can be estimated by frame-to-frame tracking via conventional ultrasonic scanners is limited by the frame rate of the scanner. In this article, we will present experimental results to show that these problems may be alleviated by using a high frame rate scanner in conjunction with the injection of an ultrasonic contrast agent. In this study the contrast agent used was Albunex.

Spontaneous echo contrast and hemorheologic abnormalities in cerebrovascular disease

BACKGROUND AND PURPOSE

Spontaneous echo contrast (SEC) is thought to represent a risk factor for cardioembolic stroke. In vitro studies suggest that SEC results from interaction between red cells and fibrinogen. To better understand the relation between SEC and stroke and to investigate the in vivo genesis of SEC, we examined the relation between SEC, the constituents of the blood, and plasma and serum viscosity in patients with acute stroke or chronic cerebrovascular disease.

METHODS

Fifty patients with acute stroke or chronic cerebrovascular disease referred for transesophageal echocardiogram (TEE) were studied by transthoracic echocardiography and TEE. Complete blood count, fibrinogen, albumin, gamma-globulin, and plasma and serum viscosity determinations were made. Left atrial SEC was graded as absent, mild, or marked by means of TEE.

RESULTS

SEC was absent in 31 patients, mild in 10 patients, and marked in 9 patients. Higher grade of SEC was associated with a significantly greater percentage of patients with atrial fibrillation and larger left atrial dimension. Atrial fibrillation was present in 23% of the patients in the SEC absent group, 50% of the patients in the mild SEC group, and 78% of the patients in the marked SEC group (P < .01). Left atrial diameter averaged 3.8 +/- 0.6 cm in the SEC absent group, 4.3 +/- 1.1 in the mild SEC group, and 4.9 +/- 0.7 in the marked SEC group (P < .001). Hematocrit, white blood cell count, and platelet count did not differ among the three groups. Fibrinogen, gamma-globulin, plasma viscosity, and serum viscosity values were all significantly higher in the presence of SEC (P < .05). Fibrinogen values were 361 +/- 97 mg/dL in the SEC absent group and 427 +/- 135 mg/dL in the marked SEC group. gamma-Globulin levels were 0.75 +/- 0.23 g/dL in the SEC absent group and 1.06 +/- 0.48 g/dL in the marked SEC group. Both plasma viscosity (1.97 cp) and serum viscosity (1.64 cp) were higher in the marked SEC group than in the SEC absent group (1.77 and 1.50 cp, respectively).

CONCLUSIONS

In patients with acute stroke or chronic cerebrovascular disease, the severity of SEC was not related to albumin, hematocrit, white cell count, or platelet count but rather to elevated fibrinogen levels and concomitant increases in both plasma and serum viscosity. Moreover, increasing grade of SEC was associated with significantly increased left atrial diameter and a higher percentage of patients in atrial fibrillation.

Hematologic correlates of spontaneous echo contrast in patients with atrial fibrillation and implications for thromboembolic risk

Spontaneous echo contrast has been observed in conditions of low blood flow velocity, such as rheumatic mitral stenosis and atrial fibrillation (AF). The phenomenon has been attributed to increased echogenicity due to aggregation of blood cells at low shear rates. The aim of this study was to determine whether abnormalities of blood composition also might contribute to spontaneous echo contrast formation by promoting cellular aggregation. Transesophageal echocardiography was performed in 185 patients with AF (31 with valvular and 154 with nonvalvular AF). The left atrium was examined for thrombus and spontaneous echo contrast, which was graded from 0 (nil) to 4+ (severe) by 2 independent observers. Forty milliliters of venous blood was obtained from each patient for hematologic analysis. Spontaneous echo contrast was observed in 46% of patients (74% with valvular and 41% with nonvalvular AF). In linear regression analysis, positive correlations were found between grade of spontaneous echo contrast and erythrocyte sedimentation rate (p < 0.001), low-shear blood viscosity (p < 0.001) and anticardiolipin antibody (p = 0.02) in the total study population, and in patients with nonvalvular AF. Spontaneous echo contrast correlated with mitral valve area (p < 0.01) and gradient (p = 0.03), but not with hematologic parameters in patients with valvular AF. Left atrial thrombus was present in 6 patients, all of whom had spontaneous echo contrast. Age (< 0.01), spontaneous echo contrast (p = 0.03) and the fibrinogen concentration (p = 0.03) correlated with previous embolic events.(ABSTRACT TRUNCATED AT 250 WORDS)

High frequency ultrasonic backscatter from erythrocyte suspension

Previous studies have shown that ultrasonic backscattering from red blood cells suspended in saline is proportional to the fourth power of frequency for frequencies below 15 MHz, as predicted by Rayleigh scattering theory. Recently, we have extended the measurements up to 30 MHz, because scattering of ultrasound by red blood cells may no longer be negligible at these frequencies and can affect, to a great degree, the operation of intravascular imaging devices. The experimental results show that the fourth power dependence on frequency of the backscattering coefficient for porcine erythrocytes suspended in saline solution appears to be valid up to 30 MHz. To confirm this, backscattering cross-section of porcine red cells was computed as a function of frequency using the T-matrix method. Since at higher frequencies the shape of the scatterers may also play a significant role, its effect was investigated by treating the red cell as a sphere, a disc, and a biconcave disc of the same volume. Good agreement was obtained between the experimental and theoretical results.

Pulse Doppler ultrasound detection, characterization and size estimation of emboli in flowing blood

A theory describing pulse Doppler ultrasound signals due to backscattering due to emboli in flowing blood is presented. From this theory, the minimum detectable size of a formed-element embolus can be established as a function of carrier frequency and vessel size. Emboli can be sized and characterized, based on the ratio of the amplitude of the Doppler signal during embolus passage through the sample volume to background bloodflow Doppler signal when no embolus is present. This ratio is defined as the "embolus to blood ratio" (EBR). Size estimation of emboli can be done by insonating an embolus with a single frequency and measuring the EBR, only if the embolus does not exceed a certain size, and if the vessel diameter and per cent hematocrit are known. Using two different frequencies, the vessel geometry (diameter and sample volume length) and per cent hematocrit can be eliminated from calculation of embolus size. Sources of uncertainty in the EBR and their effect on embolus size estimation are discussed. Discrimination between gas and formed-element emboli is described, given a detector with sufficient dynamic range, and use of three carrier frequencies. The theory presented here is in agreement with experimental findings of other investigators.

Analysis of ultrasonic scattering in blood via a continuum approach

Since the pioneering work by Reid et al. on measuring ultrasonic scattering in blood, this phenomenon has been extensively studied both theoretically and experimentally. The knowledge on ultrasonic scattering properties of blood is needed for the design of ultrasonic methods for measuring blood flow, and a better interpretation of ultrasonic images. The development of high frequency intravascular or intracardiac imaging devices raises the possibility of measuring blood properties, e.g., erythrocyte aggregation and fibrinogen concentration, in situ. A number of theoretical approaches have been developed to analyze this phenomenon where in general ultrasound wavelength is much greater than the erythrocytes. These results show that the backscattering coefficient of blood, defined as power backscattered by a unit volume of blood per steradian per unit incident intensity, is proportional to variance of the erythrocyte number fluctuation and backscattering cross-section of a single erythrocyte. In this paper, we will show that similar results can also be obtained by taking a continuum approach.

The role of echocardiography in the evaluation of cardiac source of embolism: left atrial spontaneous echo contrast

Left atrial (LA) spontaneous echo contrast, also known as "smoke," is a frequent transesophageal echocardiographic finding characterized by swirling, smokelike echoes in the LA cavity or appendage. LA smoke is associated with conditions favoring stasis of LA blood, including atrial fibrillation, mitral stenosis, the absence of mitral regurgitation, and LA enlargement. LA spontaneous echo contrast is a marker of previous embolic events in patients with atrial fibrillation, mitral stenosis, or mitral valve replacement. Most LA thrombi are accompanied by smoke. Recent studies show that LA spontaneous echo contrast is also associated with increased fibrinogen, hematocrit, and blood viscosity, indicating a relatively hypercoagulable state in addition to stasis. These findings suggest that LA spontaneous echo contrast is a manifestation of erythrocyte aggregation, and that hematologic factors may contribute to the association between spontaneous echo contrast and thromboembolism.

Maximal blood flow velocity in severe coronary stenoses measured with a Doppler guidewire. Limitations for the application of the continuity equation in the assessment of stenosis severity

In vitro and animal experiments have shown that the severity of coronary stenoses can be assessed using the continuity equation if the maximal blood flow velocity of the stenotic jet is measured. The large diameter and the low range of velocities measurable without frequency aliasing with the conventional intracoronary Doppler catheters precluded the clinical application of this method for hemodynamically significant coronary stenoses in humans. This article reports the results obtained using a 12 MHz steerable angioplasty guidewire in a consecutive series of 52 patients undergoing percutaneous coronary angioplasty (61 coronary stenoses). The ratio between coronary flow velocity in a reference segment and in the stenosis was used to estimate the percent cross-sectional area stenosis. A Doppler recording suitable for quantitation was obtained in the stenotic segment in only 10 of 61 arteries (16%). The time-averaged peak velocity increased from 15 +/- 5 to 115 +/- 26 cm/sec from the reference normal segment to the stenosis. Volumetric coronary flow calculated from the product of mean flow velocity and cross-sectional area was similar in the stenosis and in the reference segment (33.2 +/- 14.9 vs 33.5 +/- 17.0 mL/min, respectively, difference not significant). The percent cross-sectional area stenosis and minimal luminal cross-sectional area derived from the Doppler velocity measurements using the continuity equation and calculated with quantitative angiography were also similar (Doppler, 86.7 +/- 5.1% and 1.00 +/- 0.48 mm2; quantitative angiography, 85.9 +/- 7.9% and 1.02 +/- 0.50 mm2).(ABSTRACT TRUNCATED AT 250 WORDS)

Hemodynamic and hemorheologic determinants of left atrial spontaneous echo contrast and thrombus formation in patients with idiopathic dilated cardiomyopathy

The purpose of the present study was to evaluate the specific role of hemorheologic and hemodynamic parameters for spontaneous echo contrast and thrombus formation in vivo. We therefore investigated the association between the presence of left atrial spontaneous echo contrast and thrombus formation by transesophageal echocardiography and multiple clinical, hemodynamic, and hemorheologic parameters in 70 patients with idiopathic dilated cardiomyopathy. Transesophageal echocardiography showed left atrial spontaneous echo contrast and left atrial thrombi in 33% and 19% of patients, respectively. Patients with left atrial spontaneous echo contrast had a lower cardiac index (2.1 +/- 0.9 versus 2.6 +/- 0.9 L/min/m2; p < 0.02), a lower left atrial (21 +/- 8 versus 38 +/- 10 cm/sec; p < 0.001) and left atrial appendage flow velocity (17 +/- 14 versus 39 +/- 13 cm/sec; p < 0.001), a larger left atrial diameter (53 +/- 6 versus 46 +/- 10 mm; p < 0.002), and more often presented with atrial fibrillation (62% versus 32%; p < 0.02). Plasma fibrinogen concentration (4.0 +/- 1.1 versus 3.5 +/- 0.7 gm/L; p < 0.02) and plasma viscosity (1.83 +/- 0.10 versus 1.76 +/- 0.15 mPa.sec; p < 0.05) were higher in patients with spontaneous echo contrast. Multivariate analysis revealed an association between the presence of spontaneous echo contrast and left atrial flow velocity p < 0.0001) and plasma viscosity (p < 0.01). In patients with left atrial (appendage) thrombus or a history of embolism, left atrial appendage flow velocity was lower (15.0 +/- 8.2 versus 29.6 +/- 14.5 cm/sec; p < 0.005) and spontaneous echo contrast was more frequently observed (52% versus 23%; p < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)