正子斷層掃描(PET)及單光子斷層掃描(SPECT)為核子醫學中兩種成像方式，此兩種造影方式各有其功能及特性，而結合PET/SPECT進行雙模式同時造影可同時獲取兩種不同的功能性影像，其原理是利用PET及SPECT兩種核種同位素同時進行掃描，稱為雙同位素同時掃描(dual isotope simultaneous acquisition, DISA)，此技術使臨床及臨床前應用更為廣泛。但雙同位素同時掃描會產生散射污染的問題，較高能量光子會與物質發生作用產生散射(down-scatter)，進而影響使用低能窗成像的影像品質，所以必須進行散射校正。
本研究設計特定厚度的準直儀置入PET偵檢器中，利用不同能量間穿透性不同，高能光子幾乎能直接穿透準直儀，經由偵測偶合事件成像，而低能光子只能從我們所設計的針孔穿出，經由旋轉準直儀即可收取不同角度資訊重建出SPECT影像，即可達成雙同位素同時掃描的目標。另外本研究提出新型的散射修正法，此方法與傳統的散射修正法相較，無需額外收取能窗來評估散射，而是直接使用PET能窗推估散射至低能窗所造成的影響，修正後影像更加接近無散射汙染時的標準影像，且雜訊較小。本方法是利用預先掃描均勻假體，建立PET主能窗與欲散射修正能窗間的轉換矩陣(transform matrix)及縮放因子(scaling factor)。利用此兩種因子，在往後DISA掃描中，即可由PET主能窗資訊直接推估散射，使低能窗影像能減去散射所造成的影響。此外本研究分別以Three-rod假體、非均質假體、MOBY假體探討了散射修正法對於雙同位素間活度比、活度分布、衰減材質及實際臨床應用等問題；由結果顯示本方法並不會因這些因素改變而影響散射修正的效果且在修正影像上也較傳統三能窗散射修正法來的準確。

The capability of SPECT imaging developed on an preclinical PET scanner can provide a combined PET/SPECT dual modality imaging environment, potentially opening the opportunity for many new clinical and preclinical applications, However, an obstacle to the implementation of the protocol is the interference of signal between two isotope, particularly in the down-scatter from high energy gamma-ray into low energy window. In this work, we developed a new down-scatter correction method for simultaneous dual isotopes PET/SPECT imaging. A Siemens Inveon preclinical PET with a slit-slat collimator insert was modeled using the GATE/MPHG Monte Carlo simulation software developed by our laboratory. For dual imaging capability, dual energy window settings at 120-160 keV and 350-650keV were used to acquired SPECT and PET imaging, respectively. The procedure of the proposed method includes two steps: (1) A 18F uniform phantom needs to be acquired for establishing a transform function and a scaling factor between the two energy windows beforehand. (2) During dual isotopes simultaneous acquisition (DISA), the down-scatter distribution can be estimated by multiplying the acquired projections at 350-650keV with the transform function, and then the absolute scatter amount can be finally obtained by scaling the resulting scatter distribution using the scaling factor. Various phantoms were conducted to compare the image quality using the proposed method and triple energy window method, which is common used for scatter correction in DISA. The results indicated that image generated by our method is close to pure 99mTc isotope imaging, and the proposed method always outperforms the conventional triple energy window (TEW) method in terms of coefficient of variation (15.08% vs. 21.04%) and contrast recovery coefficient (0.848 vs. 0.58). In conclusion, we have developed a novel down-scatter correction method for DISA imaging. It is expected that the method can also be applied to dual isotopes SPECT imaging with high energy peaks that down-scattered to low photopeak data, such as Tc-99m/Tl-201

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