利用軟體方法進行乳房攝影散射之矯正

Abstract

乳房攝影術(Mammography)是一種常用的醫學影像照影設備，其主要原理為利用x光拍攝女性的乳房以觀察女性乳房中的構造，藉此進行乳癌等相關疾病的診斷，乳房攝影術的優點為簡單快速且非侵入式，非常適合用來做為初步篩檢的工具。 目前與乳房攝影有關的研究中主要針對以下兩個方向進行研究: 其一為如何有效的降低病患在x光所產生的放射性環境下曝露的時間及強度。另一方面的研究則是解決關於x光經過乳房組織後所產生散射(scatter)的情形，散射會降低影像品質造成影像的模糊，使的影像中許多重要資訊難以辯識。 早期的乳房攝影設備中，會在感測器(detector)與乳房之間加上許多的網格(grid)，藉著網格以阻擋散射過的光子到達感測器來解決散射的情形。由於部分光子被阻隔在網格上而無法到達感測器，光子的散失造成感測器上接收到的光子數降低，若要接收到相同的光子數勢必要提高x光的強度及照射時間。因此本研究將利用影像處理的方式來解決散射的問題來取代網格的功能。 在本研究中，我們使用GATE軟體進行乳房攝影系統的模擬，GATE是利用蒙地卡羅法(Monte Carlo Simulation)進行模擬，蒙地卡羅法可以用來模擬大量且具隨機性的事件，因此可以用來計算出乳房攝影系統中x光光子在穿透過乳房後在感測器上的分佈情形藉此模擬出乳房攝影系統的影像。此外，GATE記錄每個光子的散射與否，因此可以計算出散射的影像，本研究將比較出GATE的散射影像及其他散射數學模型的差異及準確性。

Mammography is a common modality used in medical imaging. The main principle of mammography is the usage of x-ray imagine to observe the internal breast structure. Mammography has the advantages of fast and non-invasive detection for diagnosing breast cancer, and therefore is suitable for early screening of breast cancer. Contemporarily, there are two main topics about mammography that have received much attention. The first area is about the dosage control of x-ray to reduce the exposure time and the intensity of x-ray. The other one is about the reduction of x-ray photon scatter in measured images. x-ray scatter reduces the quality of image and results in the loss of some important information for diagnosis. In the traditional mammography system, the grids are placed between the object and detector. The scatter signal could be blocked by these anti-scatter girds. The signal in x-ray images decays because of the blocking of anti-scatter grids. To compensate for the x-ray decay due to anti-scatter girds, the dose of x-ray should be enhanced or the exposure time should be increased. In this research, software-based scatter correction methods are proposed to replace the hardware-based anti-scatter grid. In this research, simulation software called GATE (Geant4 application for tomographic emission) was used to simulate the mammography system. The simulation method of GATE is based on Monte Carlo simulation. Monte Carlo simulation can simulate large amount of random events. With this characteristic of Monte Carlo simulation, the behavior of x-ray photons can be simulated via Monte Carlo simulation. In this research, the accuracy of each mathematical model and their differences for estimating x-ray scattering are evaluated via Monte Carlo simulation.