| Summary: | 碩士 === 國立清華大學 === 生醫工程與環境科學系 === 103 === 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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