編碼波形於脈衝反相基頻影像之應用

The applications of coded waveforms in pulse inversion fundamental imaging

鄭雲謙(Yun-Chien Cheng)

指導教授 ： 李百祺

國立臺灣大學/電機資訊學院/電機工程學研究所/碩士(2006年)

https://doi.org/10.6342/NTU.2006.02510

摘要

醫用超音波影像中，微氣泡對比劑常用於增加血液灌流區和組織的對比度。微氣泡在特定聲場作用下，會因共振而產生比生物組織更強的非線性訊號，脈衝反相技術利用這樣的特性，分別激發兩次相位相差180度的反相訊號，將二回波相加以消除線性成分而保留非線性成分。若取出相加後殘餘訊號之基頻部分成像，則稱為脈衝反相基頻成像技術(pulse inversion fundamental imaging)。先前之研究已證實此方法比傳統的基頻影像或是二次諧波影像有更好的血液灌流區對組織對比解析度。本研究將進一步應用編碼波形中的高斯啾聲脈衝在脈衝反相基頻影像上，以拉長脈衝的方式取代提升脈衝峰值來增加脈衝能量，可提升影像之訊雜比且避免破壞氣泡與組織，然後利用壓縮濾波器壓縮回波訊號，恢復因脈衝拉長而下降之軸向解析度。利用模擬與仿體實驗，本論文討論壓縮濾波器、聲壓、微氣泡粒徑分佈和載波等因素對於壓縮氣泡脈衝反相基頻訊號之影響。本研究發現，根據壓縮濾波器設計原理，濾波器無法壓縮含有過多非線性成分之波形，所以本研究嘗試的四種濾波器中，軸向解析度的回復程度最多也只有傳統基頻影像壓縮的一半不到。此外，設計濾波器時要求越寬，壓縮後產生之高頻雜訊與近場旁瓣越小，反而能夠得到較好的對比解析度。拉長脈衝則可使訊雜比提高，啾聲載波中初始頻率變化大的頻率漸增、寬頻的載波則能產生較大的脈衝反相基頻訊號，這些都能提升對比解析度。微氣泡粒徑方面，半徑和脈衝頻率相對應的氣泡有很強的共振訊號，所以接近特定半徑的氣泡越多，得到的對比度越好。本研究顯示，使用接近共振半徑之微氣泡對比劑、高聲壓頻率漸減之寬頻高斯啾聲長脈衝、並以限制寬鬆的濾波器壓縮脈衝反相基頻訊號，可以得到最好的對比解析度與軸向解析度。未來希望以單氣泡實驗取得更具體的微氣泡散射訊號和氣泡特性參數，希望能設計出成功壓縮效能更好的非線性壓縮濾波器。

關鍵字

微氣泡對比劑 ； 脈衝反相基頻技術 ； 壓縮濾波器 ； 對比解析度 ； 軸向解析度

並列摘要

In medical ultrasound imaging, micro bubble contrast agents are used to improve the contrast between blood perfusion region and soft tissue. Micro bubble oscillation produces stronger nonlinear response than tissue. Pulse inversion (PI) imaging excites two phase inverted pulses and sums the echoes to cancel the linearly propagated signal and keep the nonlinear components. The imaging keeping fundamental part of residue by filtering is called PI fundamental imaging. Generally, PI fundamental imaging has better contrast to tissue ration than traditional fundamental imaging and second harmonic imaging. In this research, we use chirp excitation, which is one of coded waveforms as an attempt to improve CTR by increasing pulse length and maintain the axial resolution by received pulse compression. In this study, chirp excitation is applied to pulse inversion fundamental imaging and the effects of pulse, acoustic pressure, bubble radius, and compression filters on imaging compression are discussed. However, because the compression filter is designed assuming linear propagation, the range side lobe becomes significant in PI fundamental imaging because the signal is from the nonlinear response. The less strict filter constraint produces less high frequency noise and lower range side lobe in compression. By linear compression, the axial resolution recovery of PI fundamental image is 50 % less than fundamental image. Chirps with big initial frequency change such as frequency increasing or broad bandwidth chirp produce strong PI fundamental signal. Contrast agents having more bubbles with oscillation radius scatter stronger nonlinear signal. All of these increase image CTR. To sum up, using contrast agents with oscillation radius, Gaussian chirps, having high amplitude, decreasing frequency, and broad bandwidth, and less strict constraint filter can get image with best CTR and axial resolution. Our future work will focus on single bubble experiments and alternative pulse compression filter design.

並列關鍵字

Microbubble contrast agents ； Pulse inversion fundamental imaging ； Compression filter ； Contrast to tissue ratio ； Axial resolution

參考文獻

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