Three-dimensional reconstruction of the "bright ring" echogenicity from porcine blood upstream in a stenosed tube

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.

Numerical simulation of spatiotemporal red blood cell aggregation under sinusoidal pulsatile flow

Previous studies on red blood cell (RBC) aggregation have elucidated the inverse relationship between shear rate and RBC aggregation under Poiseuille flow. However, the local parabolic rouleaux pattern in the arterial flow observed in ultrasonic imaging cannot be explained by shear rate alone. A quantitative approach is required to analyze the spatiotemporal variation in arterial pulsatile flow and the resulting RBC aggregation. In this work, a 2D RBC model was used to simulate RBC motion driven by interactional and hydrodynamic forces based on the depletion theory of the RBC mechanism. We focused on the interaction between the spatial distribution of shear rate and the dynamic motion of RBC aggregation under sinusoidal pulsatile flow. We introduced two components of shear rate, namely, the radial and axial shear rates, to understand the effect of sinusoidal pulsatile flow on RBC aggregation. The simulation results demonstrated that specific ranges of the axial shear rate and its ratio with radial shear rate strongly affected local RBC aggregation and parabolic rouleaux formation. These findings are important, as they indicate that the spatiotemporal variation in shear rate has a crucial role in the aggregate formation and local parabolic rouleaux under pulsatile flow.

Evaluation method of the degree of red blood cell aggregation considering ultrasonic propagation attenuation by analyzing ultrasonic backscattering properties

PURPOSE

Red blood cell (RBC) aggregation is one of the main factors that determines blood viscosity and an important indicator for evaluating blood properties. As a noninvasive and quantitative method for diagnosing blood properties, our research group estimated the size of RBC aggregates by fitting the scattered power spectrum from the blood vessel lumen with the theoretical scattering characteristics to evaluate the degree of RBC aggregation. However, it was assumed that the propagation attenuation of ultrasound in the vascular lumen was the same regardless of whether RBCs were aggregated or not, which caused systematic errors in the estimated size.

METHODS

To improve the size estimation accuracy, we calculated and corrected the attenuation of the blood vessel lumen during RBC aggregation and non-aggregation. The attenuation in the blood vessel lumen was calculated with the spectra acquired from two different depths.

RESULTS

In the basic experiments using microparticles, the estimation accuracy decreased as the concentration increased in the case of the conventional method, but the estimated size tended to approach the true size irrespective of the concentration, removing the propagation attenuation component with the proposed method. In the in vivo experiment on the human hand dorsal vein, the size was estimated to be larger during RBC aggregation and smaller during non-aggregation using the proposed method.

CONCLUSION

These results suggest that the proposed method can provide precise size estimation by considering the propagation attenuation component regardless of differences in blood conditions such as RBC concentration and degree of aggregation.

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.

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.

Improvement of ultrasound speckle image velocimetry using image enhancement techniques

Ultrasound-based techniques have been developed and widely used in noninvasive measurement of blood velocity. Speckle image velocimetry (SIV), which applies a cross-correlation algorithm to consecutive B-mode images of blood flow has often been employed owing to its better spatial resolution compared with conventional Doppler-based measurement techniques. The SIV technique utilizes speckles backscattered from red blood cell (RBC) aggregates as flow tracers. Hence, the intensity and size of such speckles are highly dependent on hemodynamic conditions. The grayscale intensity of speckle images varies along the radial direction of blood vessels because of the shear rate dependence of RBC aggregation. This inhomogeneous distribution of echo speckles decreases the signal-to-noise ratio (SNR) of a cross-correlation analysis and produces spurious results. In the present study, image-enhancement techniques such as contrast-limited adaptive histogram equalization (CLAHE), min/max technique, and subtraction of background image (SB) method were applied to speckle images to achieve a more accurate SIV measurement. A mechanical sector ultrasound scanner was used to obtain ultrasound speckle images from rat blood under steady and pulsatile flows. The effects of the image-enhancement techniques on SIV analysis were evaluated by comparing image intensities, velocities, and cross-correlation maps. The velocity profiles and wall shear rate (WSR) obtained from RBC suspension images were compared with the analytical solution for validation. In addition, the image-enhancement techniques were applied to in vivo measurement of blood flow in human vein. The experimental results of both in vitro and in vivo SIV measurements show that the intensity gradient in heterogeneous speckles has substantial influence on the cross-correlation analysis. The image-enhancement techniques used in this study can minimize errors encountered in ultrasound SIV measurement in which RBCs are used as flow tracers instead of exogenous contrast agents.