Aggregation of red blood cells studied by ultrasound backscattering

Pilot clinical study of quantitative ultrasound spectroscopy measurements of erythrocyte aggregation within superficial veins

BACKGROUND:

An enhanced inflammatory response is a trigger to the production of blood macromolecules involved in abnormally high levels of erythrocyte aggregation.

OBJECTIVE:

This study aimed at demonstrating for the first time the clinical feasibility of a non-invasive ultrasound-based erythrocyte aggregation quantitative measurement method for potential application in critical care medicine.

METHODS:

Erythrocyte aggregation was evaluated using modeling of the backscatter coefficient with the Structure Factor Size and Attenuation Estimator (SFSAE). SFSAE spectral parameters W (packing factor) and D (mean aggregate diameter) were measured within the antebrachial vein of the forearm and tibial vein of the leg in 50 healthy participants at natural flow and reduced flow controlled by a pressurized bracelet. Blood samples were also collected to measure erythrocyte aggregation ex vivo with an erythroaggregometer (parameter S₁₀).

RESULTS:

W and Din vivo measurements were positively correlated with the ex vivoS₁₀ index for both measurement sites and shear rates (correlations between 0.35–0.81, p < 0.05). Measurement at low shear rate was found to increase the sensitivity and reliability of this non-invasive measurement method.

CONCLUSIONS:

We behold that the SFSAE method presents systemic measures of the erythrocyte aggregation level, since results on upper and lower limbs were highly correlated.

Quantitative Measurement of Erythrocyte Aggregation as a Systemic Inflammatory Marker by Ultrasound Imaging: A Systematic Review

This systematic review is aimed at answering two questions: (i) Is erythrocyte aggregation a useful biomarker in assessing systemic inflammation? (ii) Does quantitative ultrasound imaging provide the non-invasive option to measure erythrocyte aggregation in real time? The search was executed through bibliographic electronic databases CINAHL, EMB Review, EMBASE, MEDLINE, PubMed and the grey literature. The majority of studies correlated elevated erythrocyte aggregation with inflammatory blood markers for several pathologic states. Some studies used "erythrocyte aggregation" as an established marker of systemic inflammation. There were limited but promising articles regarding the use of quantitative ultrasound spectroscopy to monitor erythrocyte aggregation. Similarly, there were limited studies that used other ultrasound techniques to measure systemic inflammation. The quantitative measurement of erythrocyte aggregation has the potential to be a routine clinical marker of inflammation as it can reflect the cumulative inflammatory dynamics in vivo, is relatively simple to measure, is cost-effective and has a rapid turnaround time. Technologies like quantitative ultrasound spectroscopy that can measure erythrocyte aggregation non-invasively and in real time may offer the advantage of continuous monitoring of the inflammation state and, thus, may help in rapid decision making in a critical care setup.

Computational Biomechanics of Human Red Blood Cells in Hematological Disorders

We review recent advances in multiscale modeling of the biomechanical characteristics of red blood cells (RBCs) in hematological diseases, and their relevance to the structure and dynamics of defective RBCs. We highlight examples of successful simulations of blood disorders including malaria and other hereditary disorders, such as sickle-cell anemia, spherocytosis, and elliptocytosis.

Interest of the attenuation coefficient in multiparametric high frequency ultrasound investigation of whole blood coagulation process

Previous studies [R. Libgot, F. Ossant, Y. Gruel, P. Lermusiaux, and F. Patat, Proc.-IEEE Utrason. Symp. 4, 2259-2262 (2005); R. Libgot-Calle, F. Ossant, Y. Gruel, P. Lermusiaux, and F. Patat, Ultrasound Med. Biol. 34, 252-264 (2008); F. Ossant, R. Libgot, P. Coupe, P. Lermusiaux, and F. Patat, Proc.-IEEE Ultrason. Symp. 2, 846-849 (2004)] showed the potential of an in vitro high frequency ultrasound (beyond 20 MHz) device to describe the blood clotting process. The parameters were simultaneously estimated in double transmission (DT) with the calculation of the velocity of longitudinal waves and in backscattering (BS) modes with the estimation of the integrated BS coefficient and the effective scatterer size. The aim of the present study was to show how the integrated attenuation coefficient (IAC) assessed in DT mode could provide additional information on this process, especially regarding the fibrin polymerization which is an important part of the coagulation process. A characteristic time t(a) of the variations in IAC that could be linked to fibrin formation was identified.

Aggregation of red blood cells in suspension: study by light-scattering technique at small angles

Red blood cells (RBCs) in the presence of plasma proteins or other macromolecules have a tendency to form aggregates. Light-scattering technique was used to investigate the RBC aggregation process. A highly diluted suspension of RBCs was illuminated with a 632.8-nm HeNe laser. Angular-resolved measurements of light intensity scattered by an RBC suspension from a 200-microm thick optical glass cuvette during 10 min of their aggregation process were performed at 1 to 4 off-axis deg with a very high angular resolution, at hematocrits in the range of 3.5 x 10(-2) to 10(-1). The angular spreading of forward-scattered light at small angles during the RBC aggregation process was described in terms of a new, effective phase function model that has been used for fitting the experimental data. The aggregated RBCs' optical properties, such as effective scattering anisotropy and scattering cross section, were determined. The results were compared with prediction of Mie theory for equivolumetric spherical particles. The time dependence of the aggregates mean radius and of the mean number of cells per aggregate was also calculated. Last, the potential of the proposed technique (forward-scattering light technique) as a new quantitative investigation of cellular aggregation process was estimated.

High frequency ultrasound device to investigate the acoustic properties of whole blood during coagulation

This study was designed to investigate the changes in acoustic properties of whole blood during the coagulation process. High frequency (from 20 to 40 MHz) ultrasound parameters were measured both in double transmission (DT) and backscattering (BS) mode to assess sound velocity and backscatter coefficient, respectively. The integrated backscatter coefficient (IBC) and the effective scatterer size (ESS) were deducted from the backscatter coefficient. Measurements were performed on whole blood samples collected from 12 healthy volunteers. During the blood clotting process (2 h observation), acoustic parameters were measured with 15 s time resolution for the transmission parameter and 5 s (for the 5 first min) and 30 s (for the end of the observation time) for the backscattering parameters. The results obtained clearly showed that simultaneous measurements of parameters in DT and BS modes are able to identify several stages during the in vitro blood clotting process. In particular, red blood cell (RBC) aggregation can be described from the backscattering parameters and liquid-gel transition phase of blood from the sound velocity. Intra- and inter-individual dispersion of these parameters were also measured and discussed.

Effects of contrast media on erythrocyte aggregation during sedimentation

To evaluate the effects of contrast media (CMs) on erythrocyte aggregation, we measured the erythrocyte sedimentation with Westergren method at 25 degrees C. CMs were diatrizoate (Urografin 76%) for ionic CM and iopamidol (Iopamiron 370) for nonionic CM. Swine red blood cells (RBCs) were suspended in autologous plasma containing diatrizoate (URO), iopamidol (IOP), and saline (SAL) at 6.7% w/w, as well as in plasma alone (PLA), at 40% of the hematocrit. Sigmoid sedimentation curves were fitted to the Puccini et al. (1977) equation, and the average number of RBCs per aggregate m was calculated by Stokes' law against the time t. According to the Murata-Secomb (1988) theory we estimated the collision rate K between two aggregates from dm/dt in the stationary phase during sedimentation. Corresponding to the maximal ESR, the dm/dt (in cells/s) was 0.52 in PLA, 0.09 in SAL, 0.06 in URO and 0.03 in IOP, so that K also decreased in proportion to dm/dt from 145 fL/s in PLA to 8 fL/s in IOP. Both the ionic and nonionic CMs tend to inhibit the RBC aggregation more than that in SAL; the latter iopamidol appears to be inhibitory more than the former diatrizoate in autologous plasma.

Mechanical properties of bovine aortic endothelial cells in suspension studied by ultrasonic interferometry

OBJECTIVE

Cell adhesion phenomenon has been extensively studied in the last decade and was shown to be mediated by specialized molecules and driven by physical forces. Cohesion of the vessel wall cells is also dependent on adhesion molecules but less is known about the physical forces involved. To investigate endothelial cell/endothelial cell interaction from a mechanical point of view, we have used an ultrasonic interferometry device, named EchoCell, which has been previously designed to study red blood cell-red bood cell (RBC-RBC) interaction.

METHODS

Bovine aortic endothelial (BAE) cells were cultured, detached, then suspended in buffer and their mechanical and geometrical properties studied with the EchoCell system. The ultrasonic apparatus measures both the accumulation rate of cells in suspension on a solid plate and the acoustical impedances of the suspension and the sediment.

RESULTS

In suspension, BAE exhibited, in our experimental conditions (3x10(6) cells per ml), a spherical size evaluated by calculation at a mean radius of 7+/-2 microm. Moreover, no BAE aggregation occurred at the concentrations used. The acoustical impedance of the BAE suspensions calculated from all the samples studied, in the cell concentration range from 1.5x10(6) to 6x10(6) cells per ml, was 1.52x10(6) Rayl (kg m(-2) s(-1)). Furthermore, the acoustical impedance of the cell sediment was found to be independent on the initial cell suspension concentration and equal to 1.63x10(6) Rayl (kg m(-2) s(-1)). Estimation of the volume fraction of BAE inside the sediment allows to evaluate the ultrasonic velocity and the elastic bulk modulus of cells.

CONCLUSION

The ultrasonic interferometry method appears particularly interesting to study geometrical and mechanical (acoustical impedance, sound velocity, elastic bulk modulus) properties of BAE cells.

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.

The progression of thrombus in an ex-vivo shunt model evaluated by intravascular ultrasound radiofrequency analysis

We tested the ability of ultrasound radiofrequency (RF) signal analysis to characterize thrombus accumulation in a Dacron graft incorporated into the exteriorized arteriovenous shunt in 3 baboons with constant blood flow for 60 min. Thrombus formation was quantified by sequential measurements of 111Indium-labeled platelet deposition. RF signals were acquired every 15 min at 2 sites in the graft, using a 2.9 Fr intravascular ultrasound catheter-based transducer (30 MHz) and digitized at 250 MHz in 8-bit resolution. Regions of interest were placed within a 0.5-mm perimeter adjacent to the graft wall. Integrated backscatter increased significantly (p < 0.001) with increasing platelet deposition. However, mean-to-standard deviation ratio of the RF envelope showed no significant change and the distribution pattern of the RF probability function remained constant and consistent with a Rayleigh scattering process. These results provide a basis for using RF analysis to monitor the time-course of thrombus formation.

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%

Effects of a sudden flow reduction on red blood cell rouleau formation and orientation using RF backscattered power

In most studies that were aimed at evaluating the kinetics of red blood cell (RBC) aggregation, human blood was initially circulated at a high shear rate to disrupt the aggregates, and measurements were performed following a complete flow stoppage, during the process of rouleau formation. However, it is known that a very low shear rate can enhance the formation of aggregates, as demonstrated by the modal relationship of the shear-rate dependence of RBC aggregation. The objective of the present study was, thus, to evaluate the influence of sudden flow reductions compared to a complete flow stoppage on the kinetics of rouleau formation, using ultrasound backscattering. Horse blood models, characterized by different aggregation levels, were obtained by diluting the plasma with a saline solution in different proportions. Blood was circulated in a 12.7-mm vertical tube at a flow rate of 1250 mL min-1 (prereduction flow rate) to disrupt the aggregates. The ultrasound radiofrequency (RF) signal was recorded from the center of the tube following different levels of sudden flow reduction or complete stoppage (postreduction flow rate). All measurements were performed over 2 min, using a 10-MHz transducer. No power increase was observed after complete flow stoppage. For postreduction flow rates varying between 20 and 160 mL min-1, the backscattered power increased proportionally with the kinetics of RBC aggregation. The echo buildup was also faster and stronger when the postreduction flow rate was increased. An unexpected pattern of variation of the backscattered power was found for horse RBCs characterized by high kinetics of rouleau formation. The power increased rapidly to a plateau, followed by another rapid increase and another plateau. Rouleau formation, random disorientation and reorientation were postulated to explain the phasic power increases observed.

Characterization of red blood cell aggregate formation using an analytical model of the ultrasonic backscattering coefficient

Ultrasound backscattering is well adapted to study the red blood cell (RBC) aggregation phenomenon and growth of RBC aggregates since the backscattered ultrasonic intensity depends on the sixth power of the mean radius of the scattering centers when considered as spherical. Thus, small variations of aggregate size induce large variations of the backscattered intensity. From measurements of the ultrasonic backscattering coefficient (ultrasonic backscattering cross section per unit volume of suspension), an analytical model describing its variation versus time, for human aggregated red blood cells in sedimentation, is proposed. Results given by the model allow to define three phases in the phenomenon: 1) a starting phase characterized by a duration ts; 2) a stationary final phase beginning at time tf; 3) a growing intermediate phase characterized by its duration tf - ts. The analytical model has been applied to describe RBC aggregation in dextran 70,000 dalton of different concentrations, and at various hematocrits. Knowledge of the durations ts, tf and the maximum slope s of the curve during the intermediate phase, determined with the model, allows a means to study RBC aggregate growth.

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.

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.

Roles of hematocrit and fibrinogen in red cell aggregation determined by ultrasonic scattering properties

Parameters of the power spectrum of backscattered echoes were applied to quantitatively evaluate red cell aggregation in vitro. Human red cell suspensions were circulated in a closed loop of tubing, and ultrasonic, radiofrequency, echo-signal data were obtained using a 10-MHz transducer. Data acquisition was performed at 30-s to 1-min intervals for 5 min after flow stoppage. Two parameters of the normalized power spectrum of the echo signals, spectral slope and Y-intercept, were computed, and estimates of two scattering properties, the scatterer size and acoustic concentration were calculated from these parameters using equations based on scattering theory. Size and acoustic concentration were observed as they changed over time after the stoppage of flow. The key findings were that hematocrit affected the rate of cell aggregation while fibrinogen controlled aggregate size and acoustic concentration.

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.

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.

Intravascular ultrasound imaging of blood: the effect of hematocrit and flow on backscatter

This study evaluates two key parameters influencing the ultrasonic backscatter from blood--hematocrit and flow rate--at 30 MHz in an in vitro flow system. A range of hematocrits from 0 to 50% was studied at a constant flow rate; various flow rates between stagnation and physiologic levels were studied at a constant hematocrit. The relation between backscatter intensity and hematocrit was a convex function with a maximum between a hematocrit of 10% and 20%. In the flow rate studies, the blood backscatter intensity was a maximum at a flow rate of 0 and rapidly decreased at higher flow rates. These in vitro results suggest that blood backscatter intensity is minimally dependent on hematocrit in the physiologic range. However, a dramatic increase in backscatter intensity occurs with stagnant flow, presumably the result of red blood cell aggregation. Clinically, blood backscatter intensity may provide an index for risk of thrombus formation.

Ultrasonic interferometry: study of particle sedimentation in liquid

An ultrasonic interferometry method was designed to study sedimentation of particles in liquid. The method, based on A mode echography, measures the amplitude of ultrasonic waves reflected (echo E1) by a fixed interface I1 called "solid plate-sediment" interface formed when particles are sedimenting on a solid plate. The amplitude of the echo depends both on mechanical properties of the three media (solid plate, sediment and suspension) on the thickness of the sediment and on the presence of a second mobile interface I2 called "sediment-suspension" interface. In the first phase of sedimentation when the second interface is very close to the first, two reflected waves interfere. Then, in the second phase of sedimentation when the sediment is thick enough, the amplitude of the echo E1 depends only on the sediment and solid plate properties. The first phase will give information on the sedimentation rate of particles (SR). We have compared SR of particles determined by this method with SR measured in a cylindrical tube of the same geometry as the ultrasonic measurement cell and with theoretical values of the sedimentation rate given by theoretical models.