PURPOSE: To measure the absolute value of a blood flow velocity without being affected by the incident angle of ultrasonic waves even when a catheter 1 is not arranged in parallel with a blood flow direction. ;CONSTITUTION: At least two pieces of ultrasonic vibrator transducers 10, 20 are provided in the catheter to be inserted into a blood vessel. The respective vibrator transducers have the incident angles respectively different in the blood flow direction. The ultrasonic waves are transmitted and received by these vibrator transducers. The absolute value of the blood flow velocity which does not depend on the incident angles of the ultrasonic waves is determined by an absolute value computing element 31 in accordance with the Doppler shift frequencies calculated by an Doppler shift frequency computing element 31.;COPYRIGHT: (C)1993,JPO&Japio
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