Usual ultrasonic Doppler blood flowmeters have been variously used for the diagnoses of circulatory diseases. The flowmeters, however, cannot give the absolute blood velocity (
Vb) without knowing the beam incident angle. Several non-invasive Doppler methods to detect the absolute
Vb have been already reported, but those methods have so many unsolved problems to apply to clinical uses.
This report describes a new useful method to eliminate the clinical difficulties. Two transducers were used in this method to form angle 8 by two ultrasonic beams. The absolute
Vb can be derived from two output readings (a and b) of transducers.
Vb=a/
h'√1+ (cotθ-b/a/sinθ)
2 (fora>b),
where h' = 2× (carrier frequency) / (sound velocity)
This method has the following merits in clinical use.
(1)
Vb can be measured independent of the incident angle.
(2)
Vb can be measured independent of the distance to blood vessel if θ is an acute angle.
(3) The probe size can be miniaturized so as to facilitate its handling in clinical use.
The proposed method was measured by using conventional Doppler flowmeter which uses a zero crossing counter (ZCC) as a detection circuit of stochastic Doppler signal. Model experiments were performed by utilizing an oscillating flow of human blood induced by a sinusoidal pump.
The experimental data reasonably agreed with the predicted values, which were not affected by the change of beam incident angle. Although ±11% accuracy is unavoidable-in this method, it has been proved by clinical application that this method is very useful.
From these experimental results, it is concluded that this non-invasive method can give a new quantitative information on blood flow (the absolute velociy). This method will provide useful data for diagnosing the circulation system.
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