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Several in vitro studies have employed ultrasonic techniques to detect varying properties of coagulating blood under static or stirred conditions. Most of those studies mainly addressed on the development and feasibility of modalities and however were not fully considering the effect of blood flow. To better elucidate this issue, ultrasonic backscattering were measured from the coagulating porcine blood circulated in a mock flow loop with various steady laminar flows at mean shear rates from 10 to 100 s-1. A 3 ml of 0.5 M CaCl2 solution for inducing blood coagulation was added to that of 30 ml blood circulated in the conduit. For each measurement carried out with a 10-MHz transducer, backscattered signals digitized at 100-MHz sampling frequency were acquired for a total of 20 min at temporal resolution of 50 A-lines per s. The integrated backscatter (IB) was calculated for assessing backscattering properties of coagulating blood. The results show that blood coagulation tended to be increased corresponding to the addition of CaCl2 solution: the IB was increased approximately 6.1 plusmn 0.6 (mean plusmn standard deviation), 5.4 plusmn 0.9, and 4.5 plusmn 1.2 dB at 310 plusmn 62, 420 plusmn 88, and 610 plusmn 102 s associated with mean shear rates of 10, 40, and 100 s-1, respectively. The rate of increasing IB for evaluating the growth of clot was estimated to be 0.075 plusmn0.017,0.052 plusmn0.027, and 0.038 plusmn 0.012 DeltadB Deltas-1 corresponding to the increase of mean shear rates. These results consistently demonstrate that higher shear rate tends to prolong the duration for the flowing blood to be coagulated and to decrease the rate of IB. Moreover, the laminar flow was changed to turbulent flow during that the blood was clotting discerned by spatial variations of ultrasound backscattering in the conduit. All these results validate that ultrasound backscattering is feasible to be utilized for detect- - ing and assessing blood coagulation under dynamic conditions.