Fast Statistical Image Reconstruction for Emission Tomography: Application to SPECT

核子醫學電腦斷層掃瞄(nuclear medicine emission computer tomography-ECT)，包括PET（正子斷層掃描）和SPECT（單光子斷層掃描），已經成為現代醫療診斷的利器，它與具人體解剖結構資訊的X-ray CT和MRI最大的不同，在於它能提供人體中生理與代謝功能之訊息。核子醫學電腦斷層掃瞄的主要目標是將標記藥物的(radionuclide)注射人體，透過影像重建法(image reconstruction method)去重建它在人體內的密度分佈，然後提供診斷。因此為增加診斷的精確度，一個好的影像重建法是不可或缺的。統計性影像重建法(statistical image reconstruction methods)，特別是貝斯(Bayesian)影像重建法已經證明比傳統的濾波反投影(FBP)要來的準確。貝斯影像重建法的優點除了考慮雜訊本質，有正確系統模型(system model)之外，還可以藉由已知物體(object)的特質，適時加入事前資訊prior(or penalty)，以便調整影像重建的結果，增加其精確度。目前，統計性影像重建法已經漸漸成為核醫影像重建的重要方法，但是在廣為使用之前，仍然有些問題須要解決，其中最重要的問題即是影像重建速度太慢。OSEM(ordered subset expectation-maximization)演算法，因為具有倍增於原來EM-ML(EM Maximum Likelihood)演算法的速度，以及類似EM-ML的簡單操作，備受矚目。但是OSEM因為缺乏推導理論基礎，無法證明收斂性質，不易加入事前資訊以應用於貝斯影像重建。針對這個問題，許多人提出了具收斂性的快速重建法。但是，這些方法都需要使用者設定一個鬆散調整參數(relaxation parameter)，使用上比較不方便。過去，我們曾提出一個新的演算法，叫做COSEM-MAP不但具有OSEM增加重建速度的性質，也可以證明整個演算的收斂，並加入事前資訊，而且不需要設定任何參數。在此，我們先介紹這個COSEM-MAP的方法，然後利用實際的SPECT假體資料，用COSEM-MAP來做影像重建，與FBP以及OSEM的結果做比較，主要目的是去證明快速影像重建法應用到SPECT的可行性。

關鍵字

單光子斷層掃描；統計性影像重建法；正子斷層掃描

並列摘要

Nuclear medicine imaging, including PET and SPECT, has become a powerful diagnostic tool, in that nuclear medicine imaging is able to visualize physiological functions or functional metabolism. In nuclear medicine, an estimate of the three-dimensional spatial distribution of injected radionuclide is needed for diagnosis. This estimate is obtained through a reconstruction algorithm. Statistical image reconstructions have been proven to outperform the traditional FBP reconstruction in many aspects. However, the main problem with statistical reconstruction is its computation load and slow convergence. The OSEM (ordered subset expectation maximization) algorithm, which has an order-of-magnitude speed enhancement over the original MLEM (maximum likelihood EM) algorithm, is the most popular statistical reconstruction method used in many clinical hospitals worldwide. Nevertheless, the OSEM algorithm does not converge for real data so that its noise and resolution are difficult to predict. Furthermore, the OSEM is not easy to include a prior term. Many fast OS-type algorithms were proposed to solve for the speed and the convergence problems of the OSEM. They all need a user-specified relaxation schedule to ensure the speed and convergence property. But there is no easy way to decide the relaxation schedule. Previously, we had proposed a fast, convergent OS-type algorithm, called COSEM-MAP, to retain the speed while at the same time without any relaxation schedule. Here, we briefly review the COSEM algorithm, and then apply the method to a set of real SPECT phantom data acquired from the Chang Gung Memorial Hospital. A segmented attenuation correction method is applied to the SPECT data. The COSEM-MAP result is compared to those of the popular FBP and OSEM methods often used in clinical reconstruction. Our COSEM-MAP method shows potential effectiveness as compared to the FBP and OSEM.

並列關鍵字

SPECT ； Statistical Reconstruction Method ； Iterative Reconstruction ； PET

國際替代計量

Fast Statistical Image Reconstruction for Emission Tomography: Application to SPECT

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