基於超音波對比劑諧波訊號統計特性之適應性脈波壓縮濾波器效能分析

Abstract

超音波對比劑是粒徑小於7微米受殼層包覆的氣泡，一旦對比劑在被製造出來，群集分佈於一空間中，其濃度、粒徑分佈會隨著時間持續變化，回波訊號的特性也會持續改變。 編碼發射-脈波壓縮系統是能夠提升CTR的技術，發射端使用線性唧聲訊號，增強訊號能量；接收端，考慮到對比劑的非線性響應，需要特定的脈波壓縮濾波器來匹配。前人基於諧波成像的概念設計SQL脈波壓縮濾波器，然而其設計上無法滿足實際狀況，無法隨對比劑粒徑分佈變化而調整。 本論文中提出一個基於諧波成像方法及回波訊號統計特性設計的適應性脈波壓縮濾波器，視對比劑回波訊號為隨機程序，透過對回波訊號頻譜估測以取得對比劑分佈特性，並從估測的頻譜中決定、調整濾波器參數，使此濾波器能夠隨回波頻譜特性而達到適應性的能力。 從效能分析的結果看來對比劑粒徑分佈越集中，APCF有較佳的效能，然而若對比劑粒徑呈瑞雷分佈，APCF的效能提升不太顯著，與SQLPCF效能十分接近。此外，發射訊號頻率、頻寬也是影響APCF效能的主要因素，因此僅有在特定情況下，APCF才會有顯著的效能增強表現。

Ultrasound contrast agents are shelled air bubbles whose diameter is less than 7 micrometer. After being reconstituted, ultrasound contrast agents form a population suspending in the medium, and concentration as well as size distribution of the population are always changing so that echoes from the population changing, too. Coded excitation-pulse compression system is a technique to increase CTR. In this study, linear chirp is chosen to be the signal of coded excitation for transmitting, and, considering the nonlinear response of ultrasound contrast agents, a specific pulse compression filter should be designed at receiving end. In previous research, based on the concept of harmonic imaging, a kind of pulse compression filter, called SQLPCF, had been published. SQLPCF, however, failed to adapt to the fact that size distribution is changing all the time. In this study, based on the concept of harmonic imaging and statistical properties of echo signal, an adaptive pulse compression filter, APCF, has been proposed. The parameters of APCF are determined from the estimated Power Spectral Density of echoes, which are modeled as a random process. Due to estimated Power Spectral Density, parameters of APCF have the information of size distribution of UCAs so that APCF is able to adapt to all circumstances. The results of performance analysis show that APCF works better when the size distribution is narrow, but, however, when the size distribution is Rayleigh-like, performance of APCF, comparing with that of SQLPCF, improves slightly. Besides, frequency and bandwidth of transmitting signal also dominate the performance of APCF. Thus, performance of APCF will improve significantly only under some specific circumstances.