三維超音波頻率成像法於肝臟組織辨識之應用

Abstract

本論文以積體成像法(volume rendering)為工具，使用光線投擲(ray casting)原理來將三維超音波資料成像。本研究提出以超音波回波信號的中心頻率作為超音波三維成像的重要參數，探討肝臟組織仿體在酒精影響下，組織的變異特性以及其超音波回波信號中心頻率在空間上的變化。在第一次的實驗，中心頻率的統計結果顯示，有10%酒精影響的肝臟組織，其中心頻率的變異係數會大於無酒精影響的肝臟組織。振幅統計結果來看，超音波回波信號的振幅參數，不是很容易辨別組織差異的參數。組織受酒精影響而變異的情況下，中心頻率這項參數是較振幅參數來得敏感(sensitive)。第二次的實驗以小塊肝葉為實驗仿體，探討肝臟組織在有無酒精影響下，超音波回波信號中心頻率表現的特性。在受酒精影響的肝臟組織，其超音波回波信號之中心頻率的分佈較廣而且其變異係數也來得較大。第三次的實驗則採取大範圍的掃描，更深入探討不同的超音波投影成像方法之特性，如平均值、中間值、最大值、Distance to maximum、重心法、attenuated intensity…也以光學影像來比對肝臟組織受到酒精影響的區域；中心頻率影像在酒精影響肝臟組織變異的區域呈現出圈狀結構，相當迥異於平均值及中間值投影。最後使用Field II軟體模擬不同散射子密度的phantom下，超音波信號中心頻率的變動情況，當散射子的密度增加時，中心頻率的CV值也有增加的趨勢。其相關係數R值為0.7603，顯示散射子密度的改變跟超音波中心頻率的變異係數有高度的正相關。

This research uses volume rendering as a tool for imaging 3D ultrasound data and the method used is ray casting. An important parameter is proposed in this research. It is called the central frequency of ultrasound echo signals which is used for rendering 3D ultrasound data. Then the liver tissue characterization and the change of the central frequency in the space domain are discussed under the condition that the liver tissue is affected by the alcohol. In the first experiment, the statistics of the central frequency show that the variance coefficient of central frequency is relatively larger under the condition that the liver tissue is affected by the 10% alcohol than that the liver tissue have no alcohol to influence. The statistics of ultrasound amplitude show that the amplitude of the ultrasound echo signals is not a well parameter to distinguish the tiny difference of the liver tissue. It reveals that the parameter of central frequency is more sensitive than amplitude. In the second experiment, a small lobe of liver is used as experimental phantom. The distribution of histogram of the central frequency is wider and the variance coefficient is larger when the liver tissue is affected by 10% alcohol. In the third experiment, we take a large scan for liver lobe and discuss various rendering methods such as mean value, median, maximum, Distance to maximum, Center of gravity, and attenuated intensity etc. The optical image of cross sectional view of liver lobe is used to compare the region which is affected by alcohol. We find that the central frequency image in the affected region appears a lot of circle structure and it is very different with respect to mean or median value projection method. Finally, we simulate ultrasound echo signals and see whether the central frequency changes under different simulating phantom of scattering density by using Field II software packages. The result shows that the density of scattering increases and the CV value of the central frequency has a trend of increase too. Their correlation coefficient is 0.7603 and it shows a high relationship.