This study applied high frequency ultrasounds, up to 50 MHz, for better detecting the process of blood coagulation (BC) and clot formation (CF) using only a 5 m1 volume of blood sample. Measurements were carried out using various focused ultrasonic transducers of central frequencies ranged from 5 to 50 MHz Different hematocrits between 25 and 55% of the porcine blood were restituted from the separated plasma and red cells. A 2.5 ml calcium chloride was added into the blood sample for inducing clot formation. Per each measurement, backscattered signals of the blood digitized at a 500 MHz sampling frequency were collected for 30 minutes at a 1 A-line per second temporal resolution. Thus it allowed direct observation of changes of backscattered signals associated with process of BC and CF by both backscattered strengths and M-mode image. Results indicated that the backscattered strength was decreased initially corresponding to the addition of calcium chloride. The backscattered strength was increased gradually as more blood coagulated and was then saturated when the clot was finally formed. The rising time indicating the early stage of blood coagulation was found ranged from 212 to 35. seconds corresponding to the applied ultrasound frequencies from 5 to 50 MHz, respectively, meaning that high frequency ultrasound has a better sensitivity than that of the low frequency ultrasound. This discrepancy might be directly interpreted due to that ultrasonic backscattering is significantly dependent on the ultrasound frequency It moreover suggests that high frequency ultrasound is capable of sensitively examining the variation of the blood clinically.
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