| Summary: | 碩士 === 國立陽明大學 === 生物醫學影像暨放射科學系暨研究所 === 100 === Objective： SPECT is an important molecular imaging modality in both clinical and preclinical applications. However, its quantitative accuracy is limited by photon attenuation and scatter effect when photons interact with atoms. In this study, we developed a new attenuation correction (AC) method, CT-based mean attenuation correction (CTMAC) method, and compared with various state-of-the-art methods to assess the AC phenomenon as described above. AC and scatter correction (SC) were realized by using the SPECT/CT data that were acquired from various physical phantoms and a rat. Materials and Methods： The physical phantoms and a SD rat, which were injected 99mTc, were scanned by a parallel-hole small animal SPECT scanner for evaluating AC. The parameters were set as follows: 1.2 mm parallel-hole aperture diameter, 96 projections acquired by three γ-cameras, 120 seconds per projection which were recorded within three energy windows (130-137, 137-144, and 144-151 keVs). After finishing the above experiments, they were imaged by the 80 kVp micro-CT scanner and were reconstructed with a modified 3D cone-beam Feldkamp algorithm. Moreover, the SPECT data were reconstructed with FBP except CT-based iterative attenuation compensation during reconstruction (CTIACR) method which was based on OSEM algorithm. Chang’s, CT-based attenuation correction (CTAC), CTIACR and CTMAC methods were performed for AC, and scatter was estimated and corrected by taking advantage of the triple-energy-window method (TEW). After filtering, the data were subtracted from the primary window projections before reconstruction. Absolute quantification was derived from a known activity point source scan. Results： In the physical-phantom studies, we compared the images with original, SC only, and the scatter-corrected images with AC performed by using the Chang’s, CTAC, CTIACR, as well as the CTMAC methods. The mean percentage error (MPE) for evaluating the above five configurations are -10.61, -36.90, -3.02, 0.71, 1.31 and 3.81% in the rat-sized phantom, -12.59, -35.85, -4.16, -4.35, -1.23 and -1.12% in the part of the quartering phantom which contained the higher activity concentration, -6.86, -31.32, 2.77, 2.46, 0.44 and 7.22% in the phantom’s part of the lower activity concentration, -4.22, -28.12, 13.41, -3.39, -1.72 and -0.77% in the inner layer of the concentric circle phantom, -36.32, -53.24, -34.58, -39.85, -27.74 and -36.09% in the outer layer of the concentric circle phantom. For the SD rat examination, we found out that SC in combination with four AC methods can obtain quite similar results. Conclusion： The effect of photon attenuation in rat-sized objects is significant. The CTMAC method needs the shortest correction time and can obtain a very good AC result. Attenuation correction obtained from CT data in combination with scatter correction allows accurate quantification in small animal SPECT imaging but it takes more time.
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