在高頻超音波都卜勒血流估計中，一般以掃掠式掃描法(swept-scan)來擷取流速資訊，而超音波探頭的橫向移動會造成時變的組織反射信號，造成慢時域(slow time)方向上產生組織訊號頻譜加寬(spectral broadening)的效應，所以當為了將組織訊號濾除而使用雜波高通濾波器(wall filter)的時候，必頇把雜波濾波器的截止頻率提高而造成低流速的血流信號連帶被濾除，導致所偵測出來的血流資訊也會產生錯估之結果。
為了抑制因掃掠式掃描法所產生的組織頻譜加寬效應，本研究採用微氣泡對比劑非線性成像方法來達到組織抑制效果。基頻脈衝反相都卜勒(Fundamental pulse inversion Doppler, FPID)法可將組織線性信號與微氣泡特殊非線性信號在都卜勒頻譜上分離在不同的頻段以減少組織干擾，我們以一低通濾波器將位於頻譜上DC附近之非線性信號取出，然而流速估計的範圍會因低通濾波器被限制在±PRF/4之間，但此時wall filter設計便能以保留較低流速的血流信號為目標做調整，進而提升都卜勒系統偵測血流之能力。結果顯示，在不同馬達移動速度的情況下，其所能估計出之血流資訊量相差不多，當馬達移動速度為4mm/s、8mm/s、16mm/s，血流流速為3mm/s時，其CPD分別為
II
0.63、0.65與0.58，表示其並不受因馬達橫向移動所產生的頻譜加寬效應所影響。而在低血流流速3mm/s時，當馬達移動速度為16mm/s，在有使用FPID情形下比沒使用FPID的情形，其SCR有7.71dB(P < 0.001)的提升，而CPD則有20.2 %(P < 0.001)的提升。

Swept-scan mode is often used in High-frequency ultrasound Doppler estimation for the blood flow information. But it will cause the tissue spectral broadening effect in the Doppler spectrum when using Swept-scan. In order to remove the tissue signal, the cut-off frequency of wall filter must be increased. The increase of cut-off frequency leads to the loss of low-velocity blood signal and an incorrect estimate result of blood flow detection.
To alleviate the effect of wall filtering on low-velocity blood signal, the suppression of tissue Doppler signal with micro-bubble contrast agent and non-linear imaging can be helpful. With the method of fundamental pulse-inversion Doppler (FPID), the non-linear bubble signal can be isolated from the linear tissue signal in the Doppler spectrum to reduce the clutter interference. With the FPID method, we need a low pass filter to get the non-linear signal out from DC of Doppler spectrum. So that, the range of blood velocity estimation would be restricted in ±PRF/4. But the cut-off frequency of wall filter can be lower. The results show that the FPID helps to retain the low-velocity blood flow signal in color Doppler imaging and thus improves the efficacy of blood flow detection. With different lateral velocity of scanning motor, the CPD result of FPID are 0.63(Vmotor = 4mm/s), 0.65(Vmotor = 8mm/s) and 0.58(Vmotor = 16mm/s). It shows that FPID will not be affected by the tissue spectral broadening effect. The results also show that, with FPID method, the SCR increases
IV
by 7.71dB (P < 0.001) and the CPD increases by 20.2 % (P < 0.001) in the case of 3mm/s of blood flow velocity and 16mm/s lateral velocity of the scanning motor.

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