正子斷層掃描為分子影像上一項重要的工具。使用三維的取像模式可增加造影的靈敏度，但散射光子被偵測的機率也隨之升高。散射事件會提高背景雜訊，降低重建影像的對比度及量化分析的準確性。目前臨床上被廣泛應用於正子斷層掃描系統的散射事件修正為單次散射光子模擬法，是針對散射一次的光子做模擬，未考慮來自照野外及多次散射的散射事件貢獻使其準確度受到限制。而本篇研究所提出的射束擋塊散射修正法，是利用高原子序物質製成的射束擋塊，置於待測物周圍，用來部分阻擋真實事件，藉由放置及未放置射束擋塊裝置的兩組掃描資訊可計算出散射事件的分布。此方法藉由直接量測來估算散射事件分布，能同時修正不同類型的散射事件，包含多次散射以及來自照野外的散射事件。在先前發表的研究中已使用蒙地卡羅模擬來驗證射束擋塊散射修正法的可行性。而此篇研究主要是將此散射修正法應用在臨床正子電腦斷層掃描系統上。藉由實際假體實驗以及蒙地卡羅模擬與單次散射光子模擬法比較，來驗證射束擋塊散射修正法的修正效果。結果顯示射束擋塊散射修正法能有效提高影像對比度及量化準確性，因此本研究所提出的散射修正法可有效且準確的應用於臨床。

Positron emission tomography (PET) is an important tool for molecular imaging. Fully three-dimensional (3D) acquisition mode can achieve, high sensitivity of coincidence events, but the absence of inter-slice septa, inevitably leads to increased scattered events. Scatter coincidence add a background to the true coincidence distribution, decreasing contrast by misplacing events during reconstruction and degrades the accuracy of quantitative analysis. Most commercial PET systems employ the single scatter simulation (SSS) algorithm for scatter correction. However, the SSS method achieves limited accuracy because it estimates scatter from single scatter events and does not consider the scatter from out of field of view (OFOV). We have developed a reliable scatter correction scheme for PET/CT imaging with beam stopper device (BS), with the aim of accurately estimating scatter distribution from the projection image. In this research, we proposed an accurate scatter correction method employing the beam stopper device. By directly estimating the scatter component at the LOR corresponding to the stopper, various kinds of scattered events including the most challenging multiple scatter and scatter from outside the FOV can be effectively compensated. Furthermore, no cumbersome iterative process and preliminary 2D estimates of the emitter and attenuator distributions are needed, which could reduce the computation cost and the implementation complexity of the BS method. We conclude that the BS method appears to be a feasible way to correct scattered events in PET/CT imaging.

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