超音波角頻譜合成孔徑成像原理之研究

Abstract

本論文探討頻譜合成孔徑原理在超音波掃描成像上的應用，改進B式 掃描的橫向解析能力。數值計算採用快速傳立葉轉換之標準演算程序， 分別對於時間與橫向空間座標進行轉換，並且保存原始訊號在各個波 傳方向的性質。本法較傳統使用的時域合成孔徑成像技術(即延遲加 成法)，大幅縮簡計算時間。本研究之實驗用線聚焦浸水式超音波探頭 模擬波束發散之微小探頭，對於內含數個反射體之試體進行B式掃描。 分析結果顯示在探頭近場範圍內的聲場不符合波前為柱面之假設，以 致於反射體影像重建失真，在探頭遠場範圍之反射體影像的橫向解 析度均獲得明顯地改進。本研究的分析演算程序可供未來研發及建構 數位式醫用超音波掃描系統之參考。

The angular-spectrum-based synthetic aperture imaging method is studied in this thesis in an effort to improve the transverse spatial resolution of B-mode ultrasonic inspection. The standard fast Fourier transform (FFT) algorithm is adopted in this numerical scheme and is employed twice to obtain the spectra in time-frequency and spatial-frequency domains. The directions of wave vectors associated with the spatial frequencies are preserved in the process of image reconstruction. Compared to the conventional synthetic aperture imaging technique in time domain (so-called delay-and-sum method), the presented method features faster computing algorithm. In experiment a line focus immersion transducer with short focal length is used to approximate a small transducer transversely scanning at the focal point. It is obviously found that the transverse resolution in the far field was much improved. The near-field images were distorted because the wave fronts are not in the same cylindrical surface. This work is helpful in understanding the feasibility of a digital-typed medical ultrasonic scannign system.