10TH INTERNATIONAL SYMPOSIUM ON PARTICLE IMAGE VELOCIMETRY – PIV13 Delft, The Netherlands, July 1-3, 2013
Simultaneous measurement of velocity field and RBC aggregation of
pulsatile blood flow using ultrasound speckle image velocimetry
Eunseop Yeom1 and Sang Joon Lee1
1
Department of Mechanical Engineering, Pohang University of Science and Technology, Pohang, South Korea
yesgood@postech.ac.kr
ITRODUCTION
Variations of hemodynamic parameters have been known as an important factor for various cardiovascular diseases. Accurate measurement of wall shear stress in blood vessels is essential for early detection and therapeutic treatments of cardiovascular diseases [1]. For that, many velocity field measurement techniques have been proposed and developed. However, the accurate in vivo measurement of the wall shear stress remains a problematic task due to the shear-thinning feature of blood flows. Red blood cells (RBCs) aggregation is a phenomenon that RBCs aggregate and form rouleaux and rouleaux networks. RBC aggregation is totally responsible for the non-Newtonian flow feature. In addition, RBC aggregation is closely related to microcirculatory disorders such as atherosclerosis, diabetes and other cardiovascular diseases [2, 3]. An ultrasound speckle image velocimetry (SIV) was employed for simultaneous measurement of velocity fields and RBC aggregation under pulsatile blood flows.
METHOD
In the SIV method, a cross-correlation algorithm is applied to a pair of ultrasound images scattered by RBC aggregates for the measurement of instantaneous velocity field. In addition, RBC aggregation in a blood flow could be estimated by analyzing ultrasound signals, because backscattering signal is increased by RBC aggregation [4, 5]. In this study, a rat extracorporeal loop model was used to generate a pulsatile velocity waveform of real blood flow without changing rheological properties [6]. The extracorporeal loop is filled with heparin (10 IU/ml) solution to prevent blood clotting. The model was constructed by connecting the loop between the vein and the artery of a rat (Fig. 1). To acquire ultrasound signals of blood flows in the loop, a 35-MHz mechanical sector-scan probe sweeps the loop with 20 degree and A-line data are saved at a frame rate of 50 Hz. The 128 A-lines are converted into B-mode images. The interrogation window used 128 × 8 pixels with 50% overlapping for SIV measurement. To minimize erroneous vectors, the velocity fields are filtered with a 3 × 3 median kernel and smoothened with a 3 × 3 Gaussian kernel.
Figure 2 (a) Instantaneous velocity field in the loop at a systolic instant and (b) cyclic variations of centerline velocity and echogenicity in the near-wall region along a cardiac cycle.
RESULT AND DISCUSSION
Figure 2a shows a typical instantaneous velocity field at a systolic instant. The echogenicity is calculated in dB using the ratio of speckle amplitude of a blood flow to water signal. The centerline velocity and the echogenicity in the near-wall region exhibit cyclic variation. The centerline velocity and echogenicity were phase-averaged over ten cycles. As shown in Fig. 2b, the phasic variation of centerline velocity shows inverse relationship with that of echogenicity. High shear rate during acceleration phase disaggregates the rouleaux and rouleaux networks. Then the decrease of flow velocity induces re-aggregation of RBCs. Although the shape of pulsatile waveform and flow velocity are different from previous studies, the variation trend of RBC aggregation is nearly similar [7, 8]. The SIV technique can be used to simultaneously measure the variations of hemodynamic and hemorheological properties under in vivo and in situ conditions. This kind of research would be useful for investigating the between hemodynamic and hemorheological features of blood flows related with circulatory vascular diseases.
ACKNOWLEDGMENTS
This work was supported by the Creative Research Initiatives (Diagnosis of Biofluid Flow Phenomena and Biomimic Research) of MEST/NRF of Korea.
REFERENCES
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[8] Paeng D G, et al. "Cyclic and radial variation of the echogenicity of blood in human carotid arteries observed by harmonic imaging" Ultrasound in Medicine and Biology 36 (2010) pp. 1118-1124
