環形探頭之景深改善

Abstract

本研究提出以頻率編碼孔徑假說來改善單一探頭因為聚焦點固定而使景深範圍受限的缺點。頻率編碼孔徑指的是當單一探頭的壓電材料具有不同厚度的單元時，可以藉由編輯發射訊號的頻率範圍來控制探頭中對應頻率響應的單元，使單一探頭也可以達到發射聚焦的效果，克服景深範圍因為聚焦點固定的限制。為了驗證頻率編碼孔徑假說，本研究以單一探頭架構為基礎，設計一個非均勻厚度的環形探頭。非均勻厚度的環形探頭是由六個等表面積但不同厚度的同心圓所構成，孔徑大小為6 mm，厚度依序由內向外遞增10 μm，最內圈的頻率為61.17 MHz(60μm)，最外圈則為33.36 MHz(110μm)。當發射波形是由低頻向高頻變化的升頻編碼訊號時，環形探頭會依序由外向內被激發而使探頭聚焦延遲曲線的曲率增加，使探頭聚焦在較淺的位置。反之，當發射波形是由高頻往低頻變化的降頻編碼訊號時，環形探頭會由內向外被激發而使探頭聚焦延遲曲線的曲率減少，使探頭聚焦在較深的位置。如此，發射聚焦的效果不需使用陣列就可以實現。 根據PzFlex模擬的結果顯示，當環形探頭的頻寬為80%時，以降頻編碼訊號作為發射訊號時，環形探頭的景深範圍可以增加約133%，但橫向解析度同樣維持在未編碼之前的100μm。而當環形探頭的頻寬降低至10%時，雖然同樣利用頻率編碼波形可以讓景深範圍增加至280%且聚焦點偏移約0.6mm，但橫向解析度也會降低至127μm。我們實際製作的環形探頭頻寬約有80%，同樣使用降頻編碼波形後，環形探頭約可以增加64%的景深範圍，而且橫向解析度保持與未編碼之前相同，約在100μm左右，與模擬的結果相符。本研究成功利用頻率編碼的方式使環形探頭的景深範圍得到改善，並且證明當環形探頭頻寬降低時，可以利用頻率編碼孔徑的方式達到近似陣列探頭發射聚焦的效果。

The hypothesis of this research is that frequency encoded aperture can be used to improve depth of focus (DOF). Frequency encoded aperture refers to the transducer design where different positions on the aperture have different frequency response. To test the hypothesis, a non-uniform thickness single element annular transducer (SEAT) is proposed. The sub-elements have different frequency response and thus can be controlled by different excitation signals. Because it is spatially separable, dynamic focusing can be achieved to some extent and thus DOF can possibly be improved. In this study, we design two types of coding signals which is up-frequency coding signal (frequency increased with time) and down-frequency coding signal (frequency decreased with time). When we choose the up-frequency coding signal, the curvature of delay profile increase and the focal depth becomes shallower. Also, when we choose the down-frequency coding signal, the delay profile decrease and the focal depth becomes deeper. With this method, SEAT can achieve dynamic focusing. According to the simulation results, when the bandwidth of SEAT is around 80%, the DOF of SEAT can be improved by 83% with the up-frequency coding signal and 133% with the down-frequency coding signal. But, the focus point still remains un-change. The lateral resolution maintains at 100μm while using up and down-frequency coding signal. When the bandwidth of SEAT is reduced to 10%, the DOF can be improved around 280% and the focal point can be shifted by 0.6mm. The tradeoff is that the lateral beam width of SEAT is increased. Based on the experiment results, the DOF of an 80% bandwidth SEAT can be improved around 55% with the up-frequency coding signal and 64% with the down-frequency coding signal. The lateral resolution of SEAT still maintains at 100μm. In this study, we demonstrate the frequency encoded aperture improves the DOF of SEAT. However, dynamic focusing on SEAT can be only achieved when the bandwidth of SEAT is sufficiently low.