| Summary: | 碩士 === 國立臺灣大學 === 電信工程學研究所 === 87 === Besides the B Mode image, the ultrasound instrument has the ability to estimate the blood velocity for diagnosing the human circulatory system ( for example : color flow mapping system ) . The difference between B mode image and color flow mapping image is the former displays the scattering strength of scatterers in the sample volume and the latter displays the velocity information of sactterers in the sample volume .
In this thesis we conclude the factors to make doppler spectrum broaden are
(1) the viscous fluid : because the blood velocity is not equal in the vessel , the sample volume decides the range of doppler spectrum .
(2) the transit time : the sample volume limits the time duration of the scatterer and this makes the doppler spectrum broaden .
We develop a blood model to prove wideband signal has narrower doppler spectrum bandwidth than narrowband signal does in viscous fluid . therefore , the increase on transit time of wideband signal results in the narrowest doppler spectrum bandwidth .
By means of variable-sampling method rather than equal-sampling one, we show that the former can increase the transit time significantly. Besides, we discover that using variable-sampling method to estimate blood velocity is more stable than using equal-sampling one and it also has the ability of anti-noising and anti-partial-aliasing .
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