對於非純正子射源的研究，由於射源本身衰變形式複雜，易造成影像雜訊的提高和影像對比度的降低。因此非純正子射源影像的評估與後續量化修正以及影像重建的改善等研究更有其必要性，最有利的工具便是利用蒙地卡羅模擬來分析。SimSET (Simulation System for Emission Tomography) 非常有效率的模擬系統，但侷限於偵檢器幾何形狀以及無法支援非純正子射源的模擬。GATE (Geant4 application for tomographic emission) 提供非常優異的物理模組，但對於立體像素的假體模擬是非常沒有效率。為了想保持準確的物理模組和有效率的模擬速度，於是我們統合這兩套系統，稱之Sim-GATE 整合系統，符合上列之優點，又可額外支援非純正子射源的模擬。
這套系統中加入時間資訊和衰變能階的概念。Sim-GATE先從SimSET可以在假體內快速模擬出光子資訊(PHG)，利用光子資訊連接到GATE端偵檢器部分完成結合。驗證射源為F-18、Cu-64和I-124之線射源在水假體，主要驗證項目包括能譜分析和靈敏度驗證。此外也測量F-18和Cu-64線射源在空氣中不同位置的空間解析度。
結果都相當吻合所預期。我們期望這套新系統可以提供快速且精確的模擬，以及未來可以修正非純正子射源影像品質。

The utilization of non-pure positron emitting nuclides with complex decay characteristics for animal molecular imaging raises several questions about their ability to perform high quality imaging. Monte Carlo simulation is a powerful tool to accurately quantify the micoPET images with these non-pure positron emitters. SimSET (Simulation System for Emission Tomography) is an efficient Monte Carlo code, but it’s limited in terms of the versatile detector simulation and the support of non-pure positron emitters. GATE (Geant4 application for tomographic emission) based on GEAN4 offers the ability and the flexibility to model novel detection systems but are not efficient at voxel-based phantoms. In order to accurately model the scanner systems and maintain the efficient simulation speed as well, our purpose is to develope an efficient and realistic Monte Carlo simulation for non-pure positron emitters with SimSET-GATE workflow (SimGATE).

The current SimSET software didn’t provide the time information and the simulation of non-pure positron emitters. In order to perform the complex decay scheme of non-pure positron emitter, we modified the SimSET photon history generator (PHG) to support the non-pure positron emitters. The modified SimSET PHG was inserted into the GATE simulation to evade the relatively slow MCS code based on GEANT4 in simulating photon interactions inside voxelized phantoms. For validation, acquisitions of F-18, Cu-64 line source for different positions in a water phantom were performed with MicroPET R4 scanner, together with the corresponding simulations. Energy spectra, sensitivity and images with I-124 line source obtained from SimGATE and GATE were compared.

Preliminary results indicated that the modified SimSET PHG version with SimSET-GATE workflow for these positron emitters are in rather good agreement between experimental and simulated resolution in different radial positions. Excellent agreement in spectra and different count rates were found between GATE–only and the new workflow as well. In summary, our new model allows fast and accurate modeling of microPET acquisition for the non-pure positron emitter. In the future study, we will explore the possibility of fully Monte Carlo simulation to correct all the non-true coincidences for the non-pure positron emitter.

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