PET成像中核素及组织差异对正电子分布影响的Geant4模拟

以肝脏和乳腺组织为例,使用蒙特卡罗软件Geant4模拟了在均匀核素体源内外部组织不同的情况下,其正电子分布(对应于PET成像中的病变尺寸)与核素分布(代表病理学病变尺寸)的差异.模拟结果表明,对于高能正电子核素探测到的小病灶,病理学上的病灶尺寸可能要比之小得多.此外,相比于乳腺组织,体源为高密度组织(钙化乳腺组织)时,正电子扩散程度得到了明显减弱.因此,在图像校正中应适当考虑肿瘤病变组织生理性质对正电子分布的影响.体源模型的建立为还原病理学病灶尺寸提供了一种新的校正方法.

Quantum Chemical Calculations of the Structure,Property and Stability of Penta-coordinated Carbonium Ions Derived from Normal Butane

The structure and energy of the carbonium ions formed upon protonation of butane were studied by the DFT methods. Four stable structures are identified for the protonated form of n-butane, the energy increases in the following order： C2HC3〈C1HC2〈C2HH〈C1HH, and the stability decreases in the following order C2HC3〉C1HC2〉C2HH〉C1HH. The stability of the penta-coordinated carbonium ions may be explained by the electron distribution in the three-center-two-electron bonds. The delocalization of the penta-coordinated carbonium ion CHC with three-center-two-electron bonds on positive charges was stronger than that of the penta-coordinated earbonium ion CHH with three-center-two-electron bonds and its stability was higher than that of the penta-coordinated carbonium ion CHH with three-center-two-electron bonds.