單光子射出斷層攝影在核子醫學領域是一個重要的技術，它是藉由放射性核種所釋放出的光子進行採集，並經由電腦計算重建成功能性影像，協助醫師的診斷工作。然而放射性核種所釋放的光子行徑過程中會因組織的吸收或發生散射作用，影響重建影像正確活度分佈的呈現，因此需要將衰減效應的部分給補償回去，以達到定量的準確性。目前臨床上單光子射出斷層掃描常用的衰減修正為張氏法。本研究所提出的衰減修正法為Improved Attenuation Correction (IAC)，所提出的方法初始假設物體內為均勻的活度分佈，藉由衰減分佈圖計算光子朝不同投影角方向射出時，光子所經過的路徑長度以及路徑上不同物質的直線衰減係數來求得衰減修正因子，並直接對原始正弦圖之投影值作衰減校正，校正後之重建影像則作為下次疊代之活度分佈，重複上述步驟直到兩次疊代後的影像差異小於所給定之閾值。我們利用蒙地卡羅模擬腦部、胸腔假體來驗證該法的可行性，並且與傳統的衰減修正法張氏法進行比較。從結果顯示IAC衰減修正法修正後的影像品質較張氏法高，定量的結果也較張氏法準確。因此該法相當具有臨床實際應用的價值。

Single photon emission computed tomography (SPECT), an important technique in nuclear medicine, can collect the emitted photons of the radionuclide and reconstruct the images of radionuclide distribution inside body through computer calculation. However, attenuation and scatter effect occurs when photon passing through the body and will reduce quantitative accuracy of measured radioactivity concentration. In order to achieve the quantitative accuracy, it is necessary to compensate the photon attenuation in the object. The most used attenuation correction method in SPECT is Chang’s method. In this study, we proposed a method called Improved Attenuation Correction (IAC) method. The IAC method estimates the ACF map by calculating the equivalent linear attenuation coefficient based on the attenuation map and the activity map. The attenuation map can be calculated from CT images, while the activity map can be assumed uniform in the initial step. After attenuation correction using the calculated ACF map of IAC, the image can be reconstructed by filter back-projection algorithm and the activity map for next iteration. The iteration stops when the difference between the reconstructed images of two successive iterations is smaller than a predefined criterion. The performance was evaluated by using Monte Carlo simulations of two digital phantoms (brain phantom and thorax phantom). The simulated results demonstrated that the proposed method achieved better image quality and quantitative accuracy than Chang’s method. We concluded that the proposed attenuation correction method could effectively restore the activity concentration and is potential for clinical uses.

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