音波都卜勒影像系統(ultrasonic Doppler imaging system)為利用都卜勒效應計算出血流相對速度，並以影像呈現。為提升其影像信噪比(signal-to-noise ratio)，常使用發射時間較長、平均聲壓低的編碼激發(coded excitation)技術，並在接收後藉由解碼壓縮處理一回復空間解析度(spatial resolution)。
在編碼激發波形中，壓縮結果可分為主瓣(mainlobe)與旁瓣(sidelobe)兩部分，旁瓣會造成成像假影而造成流速誤判，其中格雷編碼(Golay code)由於需要多次發射進行壓縮解碼，難以抑制移動觀測物產生的旁瓣假影，因此格雷編碼傳統上被認為不適用於都卜勒影像系統。
在本篇研究中，我們藉由格雷編碼旁瓣在慢時域(slow time)與主瓣相差二分之一脈衝重複頻率(PRF)的都卜勒偏移頻此一特性，設計一PRF/4慢時域濾波器(slow-time filter)，以消除格雷編碼產生的旁瓣，達到提升格雷編碼超音波都卜勒影像的成像精確度的效果。
我們將針對中心頻率20 M Hz的格雷編碼波形比較傳統解碼、改良解碼與PRF/4濾波器三種解碼方式並進行分析，結果顯示當血流頻譜的都卜勒偏移頻率在正負四分之一脈衝重複頻率內，在物體移動速度對應至0.115 PRF時，PRF/4濾波器能比傳統解碼多抑制20 dB的主瓣旁瓣比(range sidelobe level)，血流仿體都卜勒影像也顯示可減少假影的產生並提升血流估計精確度。

Ultrasonic Doppler imaging system uses Doppler effects to estimate the direction and speed of the blood flow. Golay code can be utilized to improve the SNR in Ultrasonic Doppler imaging system by transmitting long waveforms repeatedly without excess sound pressure. The received echo is decoded for pulse compression to restore its axial resolution. However, sidelobes in the decoded signal will produce the image artifacts and degrade the image quality. Golay code is conventionally assumed to be not applicable in Doppler system because Golay code is hard to suppress sidelobes when the object moves.
In this study, our analyses show that the main lobe component of Golay can produce the correct Doppler frequency but the range side lobe counterpart erroneously shifts by half of the pulse-repetition-frequency (PRF). Based on the spectral difference, an optimized separation between the main lobe and side lobe components can be performed by decoding the received echoes with a low-pass filter whose cut-off frequency is PRF/4 in the slow-time domain. The PRF/4 filter can retain the main lobe component with Doppler frequency within ±PRF/4 while eliminating the side lobe component outside ±PRF/4.
Using a 20 MHz ultrasonic Doppler imaging system, we prove that sidelobes is effectively suppressed by the PRF/4 filter compared to the conventional and the modified methods. In the case of high flow velocity, color-flow imaging with the PRF/4 filter also exhibits accurate estimation of the flow region while the conventional and the modified methods suffer from noticeable side lobe artifacts. In the case of 0.115 Doppler frequency , The PRF/4 filter's range sidelobe level is up to 20 dB lower than the conventional method's. Therefore, the robustness of Golay excitation improves in Doppler detection by using the PRF/4 filter for decoding.

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