非純正子發射射源 (Non-pure positron emitters)如I-124相較於傳統正子射源如F-18具有半衰期長、對特定器官結合特異性高等優點，但其在進行正子衰變時除了放出一對互為180度、能量為511 keV之互毀光子外，具有部分機率會同時伴隨第三個加馬射線，此加馬射線會增加隨機事件 (Random Coincidence)發生的機率而影響影像品質。而本研究概念主要應用其非純正子射源放出第三加馬射線的物理特性，發展一套新型非純正子發射斷層系統 (Non-pure Positron Emission Tomography，NPET)，此系統包含一對互相垂直的PET端以及SPECT端並以同符線路連接，利用同符時間窗內收到的三重事件 (triple events)計算交點得到影像，將NPET之PET端偵測之互毀光子對連線以及SPECT端收到受準直儀限制方向的加馬射線去計算衰變位置，以達到及時偵測及時成像的目標。
本實驗是以蒙地卡羅模擬驗證NPET系統針對非純正子發射射源的物理特性而發展的重建概念之可行性，其結果顯示系統解析度平均可到達3.6 mm左右，系統靈敏度約為69 cps/MBq，但由於本身重建方法受限於散射及隨機事件的影響易導致位置錯估而降低影像品質，現階段尚處於系統發展初期，未來將從多方面著手以求改善與發展。

Non-pure positron emitters (e.g. I-124，Br-76，Y-86) have advantages with longer half-lives which can provide prolong study and target specific properties. However, these isotopes also emit high energy gamma rays (called associated gamma) that can be detected and cause additional random coincidences with the annihilation photons. In this research, we propose a novel imaging system called non-pure positron emission tomography (NPET). The system is based on the idea that the source position can be determined through detection of the three photons (2 annihilation photons and 1 associated gamma) using a triple coincidence circuit. The system comprises of two parts. NPET comprises of mutual perpendicular PET and SPECT connected by a coincidence circuit. The collimator on the SPECT can be used to detect the direction of the associated gamma. As long as these three photons are detected in the same time, the source position is located on the intersection of the incoming direction of the associated gamma and the line of annihilated photons which can provide real time image.
The simulation results demonstrated the feasibility of the novel PET imaging for non-pure positron emitters. The average system resolution for NPET is 3.6 mm and the sensitivity is about 69 cps/MBq. Although the scatter and random effect dominate the located source positions which degrade the image quality, NPET system is in initial stage so that it could be improved and developed variously in the future.

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