編碼發射波形藉由拉長波長提高傳輸能量以提升超音波影像訊雜比(signal-to-noise ratio, SNR)，並利用脈衝壓縮技術(pulse compression)壓縮回音信號(received echo)恢復軸向解析度(axial resolution)，因此可用於改善都卜勒血流估計中因散射信號微弱導致的靈敏度限制，並可用以補償高頻超音波因組織衰退造成之穿透深度限制。互補式格雷編碼(Golay Complementary Sequence, GCS)有低旁瓣(sidelobe)與低成本之硬體實現等優點，但傳統上因其移動假影(motion artifact)而被視為不適用於都卜勒血流估計。互補式格雷編碼在慢時域(slow-time domain)中具有主瓣(mainlobe)信號與旁瓣信號相差二分之一脈衝重複頻率(pulse repetition frequency, PRF)的特性，故以截止頻率為四分之一脈衝重複頻率之低通濾波(PRF/4 filtering)進行格雷解碼可消除移動假影對都卜勒成像造成的流速誤判，其效果已於前期研究中初步獲得驗證。
本篇研究進一步將前述PRF/4低通濾波之格雷解碼技術實現於商用高頻超音波影像系統，該高頻影像系統廣泛於臨床前研究中用以提供血流相關訊息，我們並於仿體與小鼠活體實驗進行互補式格雷編碼都卜勒影像之成像品質與訊雜比評估。仿體實驗結果顯示不同都卜勒偏移頻率在正負四分之一脈衝重複頻率內血流信號訊雜比均能有一定增加並提高都卜勒偵測的靈敏度，但當主瓣信號都卜勒偏移超出四分之一脈衝重複頻率時將因疊頻失真現象(aliasing)導致錯誤的血流訊息。活體實驗中小鼠的右頸動脈所擷取到的收縮舒張期間可能不盡相同導致偵測的靈敏度差異不大。腎臟血管為動靜脈並行，使用互補式格雷編碼可偵測到較深處靜脈血管的血流，但高流速動脈血管亦會出現疊頻失真現象。腹主動脈為動物體內最深處之血管，傳統發射信號其頻率若高於30MHz即無法偵測到血流，但在35MHz互補式格雷編碼發射時仍可提供一定之血流靈敏度。

Coded excitation can improve the SNR by utilizing elongated waveform to increase the transmit energy. Besides, the axial resolution can be restored by pulse compressing the received echo. Thus, it can be used to enhance the sensitivity of Doppler flow estimation which it is often compromised by the weak blood flow scattering. Also, it could increase the penetration depth of high frequency ultrasound limitation which is resulted from frequency attenuation. The Golay Complementary Sequence (GCS) has the advantages of low range side lobe and low implementation cost, it is conventionally regarded as not applicable to blood flow detection because of potential motion artifact. In the GCS, the spectral difference of the main-lobe signal and the side-lobe signal is half of the pulse-repetition-frequency (PRF). Based on this property, it 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 to eliminate the motion artifact. Also, the performance of this technique has been verified in previous study.
In this research, the PRF/4 filtering has been implemented in a high frequency ultrasound imaging system. The high frequency ultrasound imaging system is widely used in pre-clinical research to provide the flow information. We validate the efficacy of the Doppler flow image quality and SNR using in-vitro and in-vivo experiments. The in-vitro experiment shows that the SNR and the sensitivity could be improved while the different Doppler frequency shifts within ±PRF/4. Furthermore, aliasing occurs when the main-lobe signal is beyond the limit of ±PRF/4. The in-vivo experiment demonstrates that the sensitivity difference of the Right Common Carotid Artery of the mouse is not obvious because the acquisition frames from diastole and systole are not the same. In the kidney vessel, the depth of the flow detection could be increased in the vein of the kidney vessel, but the higher velocity in the artery of the kidney vessel causes aliasing. The abdominal aorta is the deepest vessel in the mouse. Conventionally, the flow estimation is not achievable when the transmit frequency is higher than 30MHz. However, when the transmit signal is GCS, the sensitivity can be as good as the conventional method even with the transmit frequency of 35MHz.

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