超音波對比劑成像之最佳發射頻率選擇

Abstract

這個研究主要的目標是能夠透過完整的理論和模擬，藉由一個可適性的控制系統改變其發射訊號頻率，而這個最佳頻率能夠得到最佳的對比劑－組織對比度。 在選擇最佳頻率的過程中，微氣泡的大小分佈會是影響最佳頻率的關鍵因素，先前的研究多半使用單氣泡進行模擬，但是在實際情況中微氣泡必定為群聚的，且是相當大的數量級，不能使用單一氣泡的共振頻來直接求得最佳頻率。 因此本篇從多氣泡的背散射係數的理想最佳頻率驗證模擬的最佳頻率估測，針對固定殼層特性的微氣泡，從線性震盪理論分析最佳頻率範圍，說明探頭、組織、氣泡流動會對功率造成的影響，並模擬非線性震盪，利用掃頻方式求出最佳頻率。 探頭的中心頻率和頻寬必須配合組織衰減和氣泡大小分佈做適當的選擇，否則會因為功率衰減過高，無法求出最佳頻率。而氣泡流動速度快時，必須使用多個訊號平均才能達到收斂的效果，平均數量則取決於對比劑汰換率。利用掃頻方式預估最佳頻率範圍時，仍需氣泡半徑大小分佈的資訊，進行最佳頻率的估測，發射訊號則需利用窄頻訊號的結果會較接近理想值。 在氣泡汰換率不高(小於20%)的情況下，能夠以梯度下降演算法求出功率極值，並得到此時的發射頻率，即定義的最佳頻率，求出之背散射功率可高於探頭中心頻7dB的效果。

The main goal of this study is that through a complete theory, simulate with adaptive control of transmission signal frequency which can approach to best CTR(Contrast-to-Tissue Ratio).In the progressing of optimal frequency, the microbubble size distribution will be one key factor affecting the optimal frequency; most of previous study using one- bubble simulation, which is not practical. By adding microbubble distribution model into consideration, we can acquire the optimal frequency more precisely. Using fixed shell properties microbubbles analysis optimal frequency range from linear oscillation theory, illustrate the probe, tissue, and bubbles stochastic effects on the received PSD, and use the nonlinear oscillation simulation, with different emission signal frequency to find the optimal frequency. Transducer’s center frequency and bandwidth should be choose to matched with tissue attenuation along with bubble size distribution, otherwise the optimal frequency will be meaningless because of high power loss. Considering bubble flow velocity, solution need to be derived from average reference traces, and the numbers depends on the bubble changing rate. Using the signal frequency sweep estimate the optimal frequency range, still need to radius of bubble size distribution of information, and the result will be more close to the ideal value if emission signal using the narrowband signal. In the situation of low bubble replace rate (lower than 20%), the maximum power and optimal frequency can be obtained with gradient decent algorithm, and the result backscatter power is 7dB higher than obtained power in transducer central frequency.