Study of red cell aggregation in pulsatile flow from ultrasonic Doppler power measurements

Axial shear rate: A hemorheological factor for erythrocyte aggregation under Womersley flow in an elastic vessel based on numerical simulation

Erythrocyte aggregation (EA) is a highly dynamic, vital phenomenon to interpreting human hemorheology, which would be helpful for the diagnosis and prediction of circulatory anomalies. Previous studies of EA on erythrocyte migration and the Fåhraeus Effect are based on the microvasculature. They have not considered the natural pulsatility of the blood flow or large vessels and mainly focused on shear rate along radial direction under steady flow to comprehend the dynamic properties of EA. To our knowledge, the rheological characteristics of non-Newtonian fluids under Womersley flow have not reflected the spatiotemporal behaviors of EA or the distribution of erythrocyte dynamics (ED). Hence, it needs to interpret the ED affected by temporal and spatial flow variation to understand the effect of EA under Womersley flow. Here, we demonstrated the numerically simulated ED to decipher EA's rheological role in axial shear rate under Womersley flow. In the present study, the temporal and spatial variations of the local EA were found to mainly depend on the axial shear rate under Womersley flow in an elastic vessel, while mean EA decreased with radial shear rate. The localized distribution of parabolic or M-shape clustered EA was found in a range of the axial shear rate profile (-15 to 15s^-1) at low radial shear rates during a pulsatile cycle. However, the linear formation of rouleaux was realized without local clusters in a rigid wall where the axial shear rate is zero. In vivo, the axial shear rate is usually considered insignificant, especially in straight arteries, but it has a great impact on the disturbed blood flow due to the geometrical properties, such as bifurcations, stenosis, aneurysm, and the cyclic variation of pressure. Our findings regarding axial shear rate provide new insight into the local dynamic distribution of EA, which is a critical player in blood viscosity. These will provide a basis for the computer-aided diagnosis of hemodynamic-based cardiovascular diseases by decreasing the uncertainty in the pulsatile flow calculation.

Specification and Evaluation of Plasticizer Migration Simulants for Human Blood Products: A Delphi Study

Potentially toxic plasticizers are commonly added to polyvinyl chloride medical devices for transfusion in order to improve their flexibility and workability. As the plasticizers are not chemically bonded to the PVC, they can be released into labile blood products (LBPs) during storage. Ideally, LBPs would be used in laboratory studies of plasticizer migration from the medical device. However, short supply (i.e., limited stocks of human blood in collection centres) has prompted the development of specific simulants for each type of LBP in the evaluation of new transfusion devices. We performed a Delphi study with a multidisciplinary panel of 24 experts. In the first (qualitative) phase, the panel developed consensus definitions of the specification criteria to be met by each migration simulant. Next, we reviewed the literature on techniques for simulating the migration of plasticizers into LBPs. A questionnaire was elaborated and sent out to the experts, and the replies were synthesized in order to obtain a consensus. The qualitative study established specifications for each biological matrix (whole blood, red blood cell concentrate, plasma, and platelet concentrate) and defined the criteria required for a suitable LBP simulant. Ten criteria were suggested: physical and chemical characteristics, opacity, form, stability, composition, ability to mimic a particular clinical situation, ease and safety of use, a simulant–plastic interaction correlated with blood, and compatibility with analytical methods. The questionnaire data revealed a consensus on the use of natural products (such as pig’s blood) to mimic the four LBPs. Opinions diverged with regard to synthetic products. However, an isotonic solution and a rheological property modifier were considered to be of value in the design of synthetic simulants. Consensus reached by the Delphi group could be used as a database for the development of simulants used to assess the migration of plasticizers from PVC bags into LBPs.

Depletion-model-based numerical simulation of the kinetics of red blood cell aggregation under sinusoidal pulsatile flow

BACKGROUND

Previous numerical modeling studies on red blood cell (RBC) aggregation have elucidated the inverse relationship between shear rate and RBC aggregation under steady flow. However, information on the cyclic variation in RBC aggregation under pulsatile flow remains lacking.

OBJECTIVE

RBC aggregation was simulated to investigate the complex interrelationships among the parameters of RBC motion under pulsatile flow.

METHODS

A two-dimensional particle model was used to simulate RBC motion driven by hydrodynamic, aggregation, and elastic forces in a sinusoidal pulsatile flow field. The kinetics of RBCs motion was simulated on the basis of the depletion model.

RESULTS

The simulation results corresponded with previously obtained experimental results for the formation and destruction of RBC aggregates with a parabolic radial distribution during a pulsatile cycle. In addition, the results demonstrated that the cyclic variation in the mean aggregate size of RBCs increased as velocity amplitude increased from 1 cm/s to 3 cm/s under a mean steady flow of 2 cm/s, as mean steady flow velocity decreased from 6 cm/s to 2 cm/s under a velocity amplitude of 1.5 cm/s, and as stroke rate decreased from 180 beats per minute (bpm) to 60 bpm.

CONCLUSIONS

The present simulation results verified previous experimental results and improved the current understanding of the complex spatiotemporal changes experienced by RBC aggregates during a sinusoidal pulsatile cycle.

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.

Protocol for Robust In Vivo Measurements of Erythrocyte Aggregation Using Ultrasound Spectroscopy

Erythrocyte aggregation is a non-specific marker of acute and chronic inflammation. Although it is usual to evaluate this phenomenon from blood samples analyzed in laboratory instruments, in vivo real-time assessment of aggregation is possible with spectral ultrasound techniques. However, variable blood flow can affect the interpretation of acoustic measures. Therefore, flow standardization is required. Two techniques of flow standardization were evaluated with porcine and equine blood samples in Couette flow. These techniques consisted in either stopping the flow or reducing it. Then, the sensibility and repeatability of the retained method were evaluated in 11 human volunteers. We observed that stopping the flow compromised interpretation and repeatability. Conversely, maintaining a low flow provided repeatable measures and could distinguish between normal and high extents of erythrocyte aggregation. Agreement was observed between in vivo and ex vivo measures of the phenomenon (R² = 82.7%, p value < 0.0001). These results support the feasibility of assessing in vivo erythrocyte aggregation in humans by quantitative ultrasound means.

Microfluidic-based speckle analysis for sensitive measurement of erythrocyte aggregation: A comparison of four methods for detection of elevated erythrocyte aggregation in diabetic rat blood

Biochemical alterations in the plasma and red blood cell (RBC) membrane of diabetic blood lead to excessive erythrocyte aggregation (EA). EA would significantly impede the blood flow and increase the vascular flow resistance contributing to peripheral vascular diseases. In this study, a simple microfluidic-based method is proposed to achieve sensitive detection of hyperaggregation. When a blood sample is delivered into the device, images of blood flows are obtained with a short exposure time for a relatively long measuring time. A micro-particle image velocimetry technique was employed to monitor variation of the flow rate of blood as a function of time. Given that EA formation in the channel creates clear speckle patterns, the EA extent can be estimated by calculating a speckle area (ASpeckle) through a normalized autocovariance function. The hematocrit effect is assessed by comparing optical images transmitted through blood samples. EA variations caused by dextran treatment are quantitatively evaluated using characteristic time (λSpeckle) obtained by fitting the variations of ASpeckle. Other indices including number of RBCs in an aggregate (NRBC), characteristic time of erythrocyte sedimentation rate (λESR), and aggregation index estimated from ultrasound signals (AIEcho) are determined under different EA conditions using conventional techniques. The four different methods are applied to diabetic blood samples to compare their indices under hyperaggregation conditions. It is found that the proposed method can detect variation of EA reasonably, compared with conventional measurement techniques. These experimental demonstrations support the notion that the proposed method is capable of effectively monitoring the biophysical properties of diabetic blood.

Effects of red blood cell aggregates dissociation on the estimation of ultrasound speckle image velocimetry

Ultrasound speckle image of blood is mainly attributed by red blood cells (RBCs) which tend to form RBC aggregates. RBC aggregates are separated into individual cells when the shear force is over a certain value. The dissociation of RBC aggregates has an influence on the performance of ultrasound speckle image velocimetry (SIV) technique in which a cross-correlation algorithm is applied to the speckle images to get the velocity field information. The present study aims to investigate the effect of the dissociation of RBC aggregates on the estimation quality of SIV technique. Ultrasound B-mode images were captured from the porcine blood circulating in a mock-up flow loop with varying flow rate. To verify the measurement performance of SIV technique, the centerline velocity measured by the SIV technique was compared with that measured by Doppler spectrograms. The dissociation of RBC aggregates was estimated by using decorrelation of speckle patterns in which the subsequent window was shifted as much as the speckle displacement to compensate decorrelation caused by in-plane loss of speckle patterns. The decorrelation of speckles is considerably increased according to shear rate. Its variations are different along the radial direction. Because the dissociation of RBC aggregates changes ultrasound speckles, the estimation quality of SIV technique is significantly correlated with the decorrelation of speckles. This degradation of measurement quality may be improved by increasing the data acquisition rate. This study would be useful for simultaneous measurement of hemodynamic and hemorheological information of blood flows using only speckle images.

In vivo observation of the hypo-echoic "black hole" phenomenon in rat arterial bloodstream: a preliminary Study

The "black hole," a hypo-echoic hole at the center of the bloodstream surrounded by a hyper-echoic zone in cross-sectional views, has been observed in ultrasound backscattering measurements of blood with red blood cell aggregation in in vitro studies. We investigated whether the phenomenon occurs in the in vivo arterial bloodstream of rats using a high-frequency ultrasound imaging system. Longitudinal and cross-sectional ultrasound images of the rat common carotid artery (CCA) and abdominal aorta were obtained using a 40-MHz ultrasound system. A high-frame-rate retrospective imaging mode was employed to precisely examine the dynamic changes in blood echogenicity in the arteries. When the imaging was performed with non-invasive scanning, blood echogenicity was very low in the CCA as compared with the surrounding tissues, exhibiting no hypo-echoic zone at the center of the vessel. Invasive imaging of the CCA by incising the skin and subcutaneous tissues at the imaging area provided clearer and brighter blood echo images, showing the "black hole" phenomenon near the center of the vessel in longitudinal view. The "black hole" was also observed in the abdominal aorta under direct imaging after laparotomy. The aortic "black hole" was clearly observed in both longitudinal and cross-sectional views. Although the "black hole" was always observed near the center of the arteries during the diastolic phase, it dissipated or was off-center along with the asymmetric arterial wall dilation at systole. In conclusion, we report the first in vivo observation of the hypo-echoic "black hole" caused by the radial variation of red blood cell aggregation in arterial bloodstream.

High spatial and temporal resolution observations of pulsatile changes in blood echogenicity in the common carotid artery of rats

Previous studies have found that ultrasound backscatter from blood in vascular flow systems varies under pulsatile flow, with the maximum values occurring during the systolic period. This phenomenon is of particular interest in hemorheology because it is contrary to the well-known fact that red blood cell (RBC) aggregation, which determines the intensity of ultrasound backscatter from blood, decreases at a high systolic shear rate. In the present study, a rat model was used to provide basic information on the characteristics of blood echogenicity in arterial blood flow to investigate the phenomenon of RBC aggregation under pulsatile flow. Blood echogenicity in the common carotid arteries of rats was measured using a high-frequency ultrasound imaging system with a 40-MHz probe. The electrocardiography-based kilohertz visualization reconstruction technique was employed to obtain high-temporal-resolution and high-spatial-resolution time-course B-mode cross-sectional and longitudinal images of the vessel. The experimental results indicate that blood echogenicity in rat carotid arteries varies during a cardiac cycle. Blood echogenicity tends to decrease during early systole and reaches its peak during late systole, followed by a slow decline thereafter. The time delay of the echogenicity peak from peak systole in the present results is the main difference from previous in vitro and in vivo observations of backscattering peaks during early systole, which may be caused by the very rapid heart rates and low RBC aggregation tendency of rats compared with humans and other mammalian species. The present study may provide useful information elucidating the characteristics of RBC aggregation in arterial blood flow.

The effect of flow acceleration on the cyclic variation of blood echogenicity under pulsatile flow

It has been shown that the echogenicity of blood varies during a flow cycle under pulsatile flow both in vitro and in vivo. In general, the echogenicity of flowing whole blood increases during the early systole phase and then reduces to a minimum at late diastole. While it has been postulated that this cyclic variation is associated with the dynamics of erythrocyte aggregation, the mechanisms underlying this increasing echogenicity with flow velocity remain uncertain. The effect of flow acceleration has also been proposed as an explanation for this phenomenon, but no specific experiments have been conducted to test this hypothesis. In addition, the influence of ultrasonic attenuation on the cyclic variation of echogenicity requires clarification. In the present study, a Couette flow system was designed to simulate blood flowing with different acceleration patterns, and the flow velocity, attenuation, and backscattering coefficient were measured synchronously from 20%- and 40%-hematocrit porcine whole blood and erythrocyte suspensions using 35-MHz ultrasound transducers. The results showed ultrasonic attenuation exerted only minor effects on the echogenicity of blood under pulsatile flow conditions. Cyclic variations of echogenicity were clearly observed for whole blood with a hematocrit of 40%, but no variations were apparent for erythrocyte suspensions. The echogenicity did not appear to be enhanced when instantaneous acceleration was applied to flowing blood in any case. These findings show that flow acceleration does not promote erythrocyte aggregation, even when a higher peak velocity is applied to the blood. Comparison of the results obtained with different accelerations revealed that the cyclic variation in echogenicity observed during pulsatile blood flow may be jointly attributable to the effect of shear rate and the distribution of erythrocyte on aggregation.

Evaluation of distal turbulence intensity for the detection of both plaque ulceration and stenosis grade in the carotid bifurcation using clinical Doppler ultrasound

OBJECTIVES

To determine the interrelationship of stenosis grade and ulceration with distal turbulence intensity (TI) in the carotid bifurcation measured using conventional clinical Doppler ultrasound (DUS) in vitro, in order to establish the feasibility of TI as a diagnostic parameter for plaque ulceration.

METHODS

DUS TI was evaluated in a matched set of ulcerated and smooth-walled carotid bifurcation phantoms with various stenosis severities (30, 50, 60 and 70 %), where the ulcerated models incorporated a type 3 ulceration.

RESULTS

Post-stenotic TI was significantly elevated owing to ulceration in the mild and moderate stenoses (P < 0.001). TI increased with stenosis severity in both the ulcerated and non-ulcerated series, with a statistically significant effect of increasing stenosis severity (P < 0.001). Whereas TI in the mild and non-ulcerated moderate stenoses was less than 20.4 ± 1.3 cm s(-1), TI in the ulcerated moderate and severe models was higher than 25.6 ± 1.3 cm s(-1), indicating a potential diagnostic threshold.

CONCLUSION

We report a two-curve relationship of stenosis grade and ulceration to distal TI measured using clinical DUS in vitro. Clinical DUS measurement of distal TI may be a diagnostic approach to detecting ulceration in the mild and moderately stenosed carotid artery.

KEY POINTS

• Patients with carotid artery plaque ulcerations are at higher risk of stroke. • Clinical Doppler ultrasound is routinely used to detect carotid artery stenosis. • Doppler ultrasound turbulence intensity can detect ulceration in realistic flow models. • Turbulence intensity also increases with stenosis severity independent of ulceration. • Doppler ultrasound should help in assessing both stenosis severity and ulceration.

The acute effects of smoking on the cyclic variations in blood echogenicity of carotid artery

The objective of this research is to study the cyclic variations in echogenicity (CVE) as an acute response to smoking. CVEs, caused by the aggregation of red blood cells (RBC) were measured from the cross-sectional images of the common carotid artery using coded harmonic imaging of a commercial ultrasound system. The amplitude of the CVE (A(cve)) was analyzed among 28 smokers before and after smoking. A(cve) was increased in 22 smokers and decreased in six smokers after 1-2 cigarettes were smoked. Heart rate (HR) was also estimated from the ultrasonic images before and after smoking. The smokers were optimally divided into two clusters with respect to the change in A(cve) and the intrinsic characteristics of smokers (i.e., daily consumed cigarettes and smoking years) through a two-step cluster analysis (TSCA). The increase in A(cve) after smoking was significantly higher in the heavy smoker cluster compared with the light smoker cluster. The results suggest that the acute changes in A(cve) in response to smoking are different between heavy smokers and light smokers. This preliminary study demonstrates the potential application of coded harmonic ultrasound imaging to detect or characterize RBC aggregation. In addition, the results may be useful for understanding the acute physiologic changes caused by smoking.

Cyclic and radial variation of the echogenicity of blood in human carotid arteries observed by harmonic imaging

To better understand the characteristics of erythrocyte aggregation in flowing blood, echogenicity variation in blood was observed both in vitro and in vivo. However, few noninvasive observations of blood echogenicity variation during the cardiac cycle in human arteries have been reported. In the present study, to reduce the dynamic range between the blood vessel lumen and the surrounding tissue, coded harmonic images were acquired from human carotid arteries using a GE LOGIQ 700 Expert system (GE, Milwaukee, WI, USA) with an M12L probe, which enabled the noninvasive detection of the cyclic and radial variation of echogenicity in arterial vessels. It was found that blood echogenicity increased during systole, reaching a maximum at peak systole and then decreased to a weak level during diastole. The echogenicity profiles of blood along the vessel diameter were found to be approximately parabolic in the cardiac cycle, except for the hypoechoic zone near the center of the vessel at peak systole. The present results for human carotid arteries corroborate previous in vitro observations that showed a cyclic and radial variation of blood echogenicity, which was thought to be caused by the enhancement of erythrocyte aggregation due to the combined effects of flow acceleration and shear rate during systole.

Cyclic variations of high-frequency ultrasonic backscattering from blood under pulsatile flow

It was shown previously that ultrasonic scattering from whole blood varies during the flow cycle under pulsatile flow both in vitro and in vivo. It has been postulated that the cyclic variations of the backscattering signal are associated with red blood cell (RBC) aggregation in flowing whole blood. To obtain a better understanding of the relationship between blood backscattering and RBC aggregation behavior for pulsatile flowing blood, the present study used high-frequency ultrasound to characterize blood properties. The backscattering signals from both whole blood and an RBC suspension at different peak flow velocities (from 10 to 30 cm/s) and hematocrits (20% and 40%) under pulsatile flow (stroke rate of 20 beats/min) were measured with 3 single-element transducers at frequencies of 10, 35, and 50 MHz in a mock flow loop. To avoid the frequency response problem of a Doppler flowmeter, the integrated backscatter (IB) and flow velocity as functions of time were calculated directly using RF signals from flowing blood. The experimental results showed that cyclic variations of the IB curve were clearly observed at a low flow velocity and a hematocrit of 40% when using 50 MHz ultrasound, and that these variations became weaker as the peak flow velocity increased. However, these cyclic variations were detected only at 10 cm/s when using 10 MHz ultrasound. These results demonstrate that a high flow velocity can stop the formation of rouleaux and that a high hematocrit can promote RBC aggregation to produce cyclic variations of the backscattering signal under pulsatile flow. In addition, slight cyclic variations of the IB curve for an RBC suspension were observed at 35 and 50 MHz. Furthermore, the peak of the IB curve from whole blood led the peak of the velocity waveform when using high-frequency ultrasound, which could be explained by the assumption that a rapid flow can promote RBC aggregation under pulsatile flow. Together, the experimental results showed that the sensitivity and resolution of detecting blood properties are higher for 50 MHz ultrasound than for 10 MHz ultrasound.

Cyclic changes in blood echogenicity under pulsatile flow are frequency dependent

Previous in vivo and in vitro studies have demonstrated that blood echogenicity varies under pulsatile flow, but such changes could not always be measured at physiological stroke rates. The apparent contradiction between these studies could be a result of the use of different ultrasound frequencies. Backscattered signals from porcine blood were measured in a pulsatile Couette flow apparatus. Cyclic changes in shear rate for stroke rates of 20 to 70 beats per minute (BPM) were applied to the Couette system, and different blood samples were analyzed (normal blood and blood with hyperaggregating erythrocytes promoted with dextran). To confirm that cyclic echogenicity variations were observable, spectral analysis was performed to verify if changes in echo-amplitude corresponded to the stroke rate applied to the flow. Echogenicity was measured with two single-element transducers at 10 and 35 MHz. At 35 MHz, cyclic variations in backscatter were observed from 20 to 70 BPM. However at 10 MHz, they were detected only at 20 BPM. For all cases except for hyperaggregating red blood cells (RBCs) at 20 BPM, the magnitude of the cyclic variations were higher at 35 MHz. We conclude that cyclic variations in RBC aggregation exist at physiological stroke rates, unlike what has been demonstrated in previous in-vitro studies at frequencies of 10 MHz. The increased sensitivity at 35 MHz to small changes in aggregate size might be the explanation for the better characterization of RBC aggregation at high stroke rates. Our results corroborate in-vivo observations of cyclic blood echogenicity variations in patients using a 30-MHz intravascular ultrasound catheter.

Physical properties of tissues relevant to arterial ultrasound imaging and blood velocity measurement

A review was undertaken of physical phenomena and the values of associated physical quantities relevant to arterial ultrasound imaging and measurement. Arteries are multilayered anisotropic structures. However, the requirement to obtain elasticity measurements from the data available using ultrasound imaging necessitates the use of highly simplified constitutive models involving Young's modulus, E. Values of E are reported for healthy arteries and for the constituents of diseased arteries. It is widely assumed that arterial blood flow is Newtonian. However, recent studies suggest that non-Newtonian behavior has a strong influence on arterial flow, and the balance of published evidence suggests that non-Newtonian behavior is associated primarily with red cell deformation rather than with aggregation. Hence, modeling studies should account for red cell deformation and the shear thinning effect that this produces. Published literature in healthy adults gives an average hematocrit and high-shear viscosity of 0.44 +/- 0.03 and 3.9 +/- 0.6 mPa.s, respectively. Published data on the acoustic properties of arteries and blood is sufficiently consistent between papers to allow compilation and derivation of best-fit equations summarizing the behavior across a wide frequency range, which then may be used in future modeling studies. Best-fit equations were derived for the attenuation coefficient vs. frequency in whole arteries (R(2) = 0.995), plasma (R(2) = 0.963) and blood with hematocrit near 45% (R(2) = 0.999), and for the backscatter coefficient vs. frequency from blood with hematocrit near 45% (R(2) = 0.958).

Assessment by transient elastography of the viscoelastic properties of blood during clotting

Blood clotting is a natural process that can be both beneficial and life-threatening for the human body. It allows the maintenance of hemostasis after vascular injury, but it can also cause deep vein thrombosis and heart stroke. This study aimed better to understand the clotting process from a biomechanical point of view by using an acoustic method. The long-term objective is the staging of the age of clots in deep veins for therapy planning. The transient elastography method using a shear elasticity probe served to evaluate the shear wave velocity (V(S)) and shear wave attenuation (alpha(S)) of porcine whole blood during in vitro clot formation. By solving an inverse problem, it was then possible to provide images of the elasticity (mu(B)) and of the viscosity (eta(B)) from clotting blood. The time-varying elasticity and viscosity were very similar to what has been observed for the sol-gel transition of polymers. The mechanical properties of blood clot, which were modified by varying the hematocrit and by adding heparin or fibrinogen, were clearly assessed by the transient elastography technique. It is concluded that the shear elasticity probe is an appropriate tool to quantify and follow the sol-gel transition of blood during clotting.

Aggregate formation of erythrocytes in postcapillary venules

The purpose of the present study was to obtain information on erythrocyte aggregate formation in vivo. The movements of erythrocytes in postcapillary venules of the rat spinotrapezius muscle at various flow rates were recorded with a high-speed video camera before and after infusion of dextran 500. To distinguish aggregates, the following criteria were used: 1) a fixed distance (4 microm) between the center points of two adjacent cells, 2) lack of visible separation between the adjacent cells, and 3) movement of the adjacent cells in the same direction. Without dextran 500 infusion, 11 and 5% of erythrocytes formed aggregates in low (33.2 +/- 28.3 s) and high pseudoshear (144.2 +/- 58.3 s) conditions, respectively, based on the above criteria. After dextran 500 infusion, 53% of erythrocytes satisfied the criteria in the low pseudoshear condition (26.5 +/- 17.0 s) and 13% of erythrocytes met the criteria in the high pseudoshear condition (240.0 +/- 85.9 s), indicating erythrocyte aggregation is strongly associated with shear rate. Approximately 90% of aggregate formation occurred in a short time period (0.15-0.30 s after entering the venule) in a region 15 to 30 microm from the entrance. The time delay may reflect rheological entrance conditions in the venule.

Non-Gaussian statistics and temporal variations of the ultrasound signal backscattered by blood at frequencies between 10 and 58 MHz

Very little is known about the blood backscattering behavior and signal statistics following flow stoppage at frequencies higher than 10 MHz. Measurements of the radio frequency (rf) signals backscattered by normal human blood (hematocrit = 40%, temperature = 37 degrees C) were performed in a tube flow model at mean frequencies varying between 10 and 58 MHz. The range of increase of the backscattered power during red blood cell (RBC) rouleau formation was close to 15 dB at 10 and 36 MHz, and dropped, for the same blood samples, below 8 dB at 58 MHz. Increasing the frequency from 10 to 58 MHz raised the slope of the power changes at the beginning of the kinetics of aggregation, and could emphasize the non-Gaussian behavior of the rf signals interpreted in terms of the K and Nakagami statistical models. At 36 and 58 MHz, significant increases of the kurtosis coefficient, and significant reductions of the Nakagami parameter were noted during the first 30 s of flow stoppage. In conclusion, increasing the transducer frequency reduced the magnitude of the backscattered power changes attributed to the phenomenon of RBC aggregation, but improved the detection of rapid growth in aggregate sizes and non-Gaussian statistical behavior.

Effects of aggregation of red cells and linear velocity gradients on the correlation-based method for quantitative IVUS blood flow at 20 MHz

Recent computer simulations suggest that the presence of aggregates of red blood cells (RBCs), at random angles and lengths, does not affect the measurements of blood flow transverse to the ultrasound (US) beam direction using a correlation-based method and an intravascular (IV) US array catheter. However, in case of aggregates of RBCs aligned with the flow, measurements of simulated blood velocity are affected. Blood velocity gradients were also shown not to influence the correlation-based method for blood velocity estimation. The objective of this study was to quantify the influence of aggregates of RBCs and blood velocity gradients on the correlation-based method during in vitro experiments. For this purpose, measurements were performed on washed RBCs (no aggregation), normal human blood, and two types of diseased blood in which a lower or a higher level of aggregation was present. The decorrelation pattern of a circular US transducer as a function of transverse blood flow was studied using a Couette system. Changing the shear rate of the Couette system modified the aggregation level of RBCs and the velocity gradient. With the exception of the results at low shear rates and abnormally high aggregation levels, agreements were found between the autoconvolution of the acoustical beam (reference curve) and the radiofrequency (RF) decorrelation patterns. For the high shear rate present in coronary arteries, the correlation-based method for blood flow estimation should not be influenced by these phenomena.

Decorrelation-based blood flow velocity estimation: effect of spread of flow velocity, linear flow velocity gradients, and parabolic flow

In recent years, a new method to measure transverse blood flow, based on the decorrelation of the radio frequency (RF) signals has been developed. In this paper, we investigated the influence of nonuniform flow on the velocity estimation. The decorrelation characteristics of transverse blood flow using an intravascular ultrasound (IVUS) array catheter are studied by means of computer modeling. Blood was simulated as a collection of randomly located point scatterers; moving this scattering medium transversally across the acoustical beam represented flow. First-order statistics were evaluated, and the signal-to-noise ratio from the signals were measured. The correlation coefficient method was used to present the results. Three velocity profiles were simulated: random spread of blood-flow velocity, linear blood-flow velocity gradient, and parabolic blood-flow. Radio frequency and envelope signals were used to calculate the decorrelation pattern. The results were compared to the mean decorrelation pattern for plug blood-flow. The RF signals decorrelation patterns were in good agreement with those obtained for plug blood flow. Envelope decorrelation patterns show a close agreement with the one for plug blood flow. For axial blood flow, there is a discrepancy between decorrelation patterns. The results presented here suggest that the decorrelation properties of an IVUS array catheter for measuring quantitative transverse blood flow probably will not be affected by different transverse blood-flow conditions.

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.

Decorrelation characteristics of transverse blood flow along an intravascular array catheter: effects of aggregation of red blood cells

A method to measure transverse blood flow, based on the correlation between consecutive radiofrequency (RF) signals, has been introduced. This method was validated for an intravascular (IVUS) rotating single element catheter. Currently, we are implementing the method for an IVUS array transducer catheter. The decorrelation characteristics during transverse blood flow using the IVUS array catheter were investigated using computer modeling. Before this, blood was simulated as a collection of randomly located point scatterers and, by moving this scattering medium transversely across the acoustical beam, blood flow was simulated. This paper presents a more realistic scattering media by simulating aggregates of red blood cells (RBCs) as strings of point scatterers. Three configurations of aggregates of RBCs were simulated. First, aggregates of RBCs were strings with different lengths and parallel to the catheter axis. Second, the strings were with a fixed length and angles of plus or minus 45 degrees with respect to the catheter axis. Third, the strings were with different lengths and random angles ranging from -45 degrees to + 45 degrees. The decorrelation characteristics for these configurations of aggregates of RBCs were investigated and compared with point scatterers. For the aggregates of RBCs parallel to the catheter axis, the decorrelation rate became slower when the aggregate length was increased. RBC aggregations with fixed and random lengths and angles resulted in a decorrelation rate that approaches the decorrelation pattern from point scatterers. Results suggests that the presence of aggregates of RBCs will probably not affect the measurements of transverse blood flow using a decorrelation-based method and an IVUS array catheter.

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.

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.

High-grade stenosis of the internal carotid artery assessed by color and power Doppler imaging

PURPOSE

Visualization of carotid artery stenosis is important to improve our understanding of the etiology of ischemic stroke. Two sonographic methods are available for visualizing carotid stenosis: power Doppler imaging (PDI) and color Doppler imaging (CDI). The purpose of this study was to compare the advantages and limitations of these methods in the evaluation of internal carotid artery (ICA) stenosis.

METHODS

Thirty-two patients with at least 70% extracranial ICA stenosis were included in this prospective study. All subjects were examined with CDI and PDI. Four criteria were used to evaluate diagnostic accuracy and overall efficacy: identifying the target vessel, determining the causal pathology, evaluating blood flow, and the examiner's confidence in the diagnosis.

RESULTS

All patients had high-grade stenosis (17 patients had 70% stenosis, 11 had 80%, 2 had 90% and 2 had pseudo-occlusions of the ICA). PDI was significantly (p < 0.05) superior to CDI in identifying the target vessel and highly significantly (p < 0.01) superior to CDI in evaluating blood flow. There were no significant differences between CDI and PDI in determining the causal pathology or in the examiner's confidence in the diagnosis.

CONCLUSIONS

This study demonstrates that PDI is a good additional method to CDI and should be used with it to evaluate ICA stenosis.

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.

Changes in ultrasonic Doppler backscattered power downstream of concentric and eccentric stenoses under pulsatile flow

The main objective of the present work was to investigate, under pulsatile flow, the patterns of variation of the Doppler power backscattered by blood and Sephadex particles upstream and downstream of concentric and eccentric stenoses ranging from 47% to 91% area reduction. Doppler measurements were performed at 5 diameters upstream and 5, 10, 15 and 20 diameters downstream of the constriction. For the concentric 75% and 85%, and the eccentric 79% and 91% area reduction stenoses, a progressive increase of the power backscattered by red cell suspensions at 40% hematocrit was measured downstream of the narrowing. The maximal power usually occurred around 10 diameters after the stenosis and dropped further downstream. In addition to the increase in the power, a cyclic variation of the backscattered intensity was observed within the flow cycle. For the concentric 52% and eccentric 47% area reduction stenoses, no variation of the Doppler power was measured during flow acceleration and deceleration for all recording sites. A coefficient of correlation of 0.82 was measured between the percentage of area reduction and the ratio of the Doppler mean power at 10 diameters downstream to that at 5 diameters upstream of the stenoses. Using Sephadex particles at low concentration, no increase of the Doppler power was found downstream of the 85% and 91% area reduction stenoses. The possible link between the intensity of turbulence and the power backscattered by blood is discussed along with the influence of the correlation between the scattering particles, under turbulent flow.