Abstract
Blood flow measurement based on the ultrasound Doppler effect has not yet advanced beyond the qualitative stage, and many theoretical and technical problems with regard to haemodynamics, ultrasound acoustics, measuring technique, equipment design and construction, signal processing, etc. must be investigated and overcome for quantitative measurement. This paper focuses especially attention on the following three problems concerning the zero-crossing counter widely used in the ultrasonic Doppler flowmeter.
(1) The ultrasound Doppler signal due to the blood flow is always accompanied with various kinds of noise such as internal noise generated in the equipment. The output value of the zero-crossing counter for this Doppler signal under the influence of the noise is necessarily different from that without the noise. Therefore, the output value of the zero-crossing counter for the Doppler signal with noise was analysed. Further, a noise-free counting method with the zero-crossing counter was newly developed, and the relationship between the output value of the zero-crossing counter based on this counting method and the actual zero-crossing number was investigated.
(2) Undesired signals caused by the vessel wall motion and artificial transducer movement must be rejected and a high-pass filter is usually employed for this purpose because the frequency bands of the undesired signals are lower than that of the Doppler signal. The relationship between the zero-crossing counter output for the Doppler signal fed through the high-pass filter and the actual mean flow velocity was theoretically analysed and experimentally confirmed.
(3) The zero-crossing counter output contains a signal due to the blood flow velocity change and a fluctuation due to, the random motion of blood cells. In order to determine the ratio of the random fluctuation to the output, an experiment was carried out, and an experimental equation was obtained from the result representing the dependence of the random fluctuation on the cut-off frequency of the low-pass filter in the zero-crossing counter and on the zero-crossing counter output.
