正子電腦斷層造影（PET）具有較高的靈敏度與特異性，已被廣泛的應用於功能性檢查與腫瘤偵測上，三維正子電腦斷層造影可以達到更高的系統靈敏度，但是因為其缺乏septa，無可避免的會使散射符合事件隨著真實符合事件的增加而增加，相關的研究發現，散射符合事件的貢獻可能佔所有符合事件的50%以上。本研究針對三維正子電腦斷層造影，提出一種射束阻擋裝置以進行散射矯正。我們將阻擋器置於待掃描物體周圍，在不影響散射光子進入偵檢器的假設條件之下，阻擋器可以衰減一定比例的主要光子。利用此射束阻擋裝置，散射的量或是散射分率可以直接從被阻擋的LOR中求得，並且可以利用cublic spline內差法（beam stopper method, BS）或是雙能窗取樣資訊（beam-stopper scatter fraction method, BS-SF）求得在正旋圖（sinogram）中整個散射的分布。本實驗中，我們利用蒙地卡羅技術模擬數位假體以驗證射束阻擋裝置的散射矯正效果，在腹部（abdomen）假體的實驗中，BS方法正確的評估出散射的分布，並且改善了影像對比；在均勻假體的實驗中，BS-SF方法準確的將散射所造成的貢獻移除，使得活度濃度恢復到理論值；在猶他（Utah）假體的實驗中，BS-SF方法正確的評估出散射分率的分布，並且在沒有增加雜訊的情況下，改善了影像對比與定量結果；最後，在非均勻朱寶（Zubal）假體的實驗中，蒙地卡羅模擬的結果顯示出BS與BS-SF兩種方法都能有效的恢復影像對比，並降低均方根差。總結而言，我們所提出的射束阻擋裝置與散射矯正方法可以有效的移除不同種類的散射事件，包含單次康普敦散射、多重散射，以及從可視範圍（field of view）外來的散射貢獻，並且是一種直接、快速，以及準確的方法。

Fully 3D positron emission tomography (PET) can achieve higher system sensitivity of coincidence events, but the absence of inter-slice septa inevitably leads to increased scattered events. The scattered events can contribute as much as 50% of the total detected events. In this study, we proposed a scatter correction method for 3D PET based on a beam stopper device. The stoppers were placed surrounding the object to attenuate primary beams. The scatter and scatter fraction were directly estimated at those blocked lines of response and then interpolated to other radial bins using cubic splice interpolation (beam stopper method, BS) or the dual-energy window information (beam-stopper scatter fraction method, BS-SF), respectively. The performance was evaluated by using Monte Carlo simulations of several digital phantoms. For the abdomen phantom study conducted by the BS method, the estimated scatter distribution matched well with the true one. For the Utah phantom study, the proposed BS-SF method can accurately estimate the scatter fraction distribution and improve image contrast and quantification without noticeable noise increase. The simulated results also demonstrated a better restoration of image contrast for the non-homogeneous Zubal phantom. We conclude that the proposed scatter correction method could effectively suppress various kinds of scattered events, including single scatter, multiple scatter, and scatter from outside the field-of-view. It is a direct, fast, and accurate technique for scatter correction in 3D PET.

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