Khalifé Maya, Fernandez Brice, Jaubert Olivier, Soussan Michael, Brulon Vincent, Buvat Irène, Comtat Claude
Institut du Cerveau et de la Moelle épinière (ICM), CNRS UMR 7225-Inserm U1127-Université Paris 6 UPMC UMR S1127, Paris, France. Laboratoire Imagerie Moléculaire In Vivo (IMIV), UMR 1023 Inserm/CEA/Université Paris Sud-ERL 9218 CNRS, CEA/I2BM/SHFJ, Orsay, France.
Phys Med Biol. 2017 Sep 21;62(19):7814-7832. doi: 10.1088/1361-6560/aa8851.
In brain PET/MR applications, accurate attenuation maps are required for accurate PET image quantification. An implemented attenuation correction (AC) method for brain imaging is the single-atlas approach that estimates an AC map from an averaged CT template. As an alternative, we propose to use a zero echo time (ZTE) pulse sequence to segment bone, air and soft tissue. A linear relationship between histogram normalized ZTE intensity and measured CT density in Hounsfield units ([Formula: see text]) in bone has been established thanks to a CT-MR database of 16 patients. Continuous AC maps were computed based on the segmented ZTE by setting a fixed linear attenuation coefficient (LAC) to air and soft tissue and by using the linear relationship to generate continuous μ values for the bone. Additionally, for the purpose of comparison, four other AC maps were generated: a ZTE derived AC map with a fixed LAC for the bone, an AC map based on the single-atlas approach as provided by the PET/MR manufacturer, a soft-tissue only AC map and, finally, the CT derived attenuation map used as the gold standard (CTAC). All these AC maps were used with different levels of smoothing for PET image reconstruction with and without time-of-flight (TOF). The subject-specific AC map generated by combining ZTE-based segmentation and linear scaling of the normalized ZTE signal into [Formula: see text] was found to be a good substitute for the measured CTAC map in brain PET/MR when used with a Gaussian smoothing kernel of [Formula: see text] corresponding to the PET scanner intrinsic resolution. As expected TOF reduces AC error regardless of the AC method. The continuous ZTE-AC performed better than the other alternative MR derived AC methods, reducing the quantification error between the MRAC corrected PET image and the reference CTAC corrected PET image.
在脑部PET/MR应用中，准确的PET图像定量需要准确的衰减图。一种已实现的脑部成像衰减校正（AC）方法是单图谱法，即从平均CT模板估计AC图。作为替代方案，我们建议使用零回波时间（ZTE）脉冲序列来分割骨骼、空气和软组织。借助16名患者的CT-MR数据库，已建立了骨骼中直方图归一化ZTE强度与以亨氏单位（[公式：见正文]）测量的CT密度之间的线性关系。通过为空气和软组织设置固定的线性衰减系数（LAC），并利用该线性关系为骨骼生成连续的μ值，基于分割后的ZTE计算连续的AC图。此外，为了进行比较，还生成了其他四种AC图：一种为骨骼设置固定LAC的ZTE衍生AC图、PET/MR制造商提供的基于单图谱法的AC图、仅软组织的AC图，最后是用作金标准的CT衍生衰减图（CTAC）。所有这些AC图都在有和没有飞行时间（TOF）的情况下，以不同的平滑程度用于PET图像重建。当与对应于PET扫描仪固有分辨率的[公式：见正文]高斯平滑核一起使用时，通过将基于ZTE的分割与归一化ZTE信号的线性缩放相结合生成的特定受试者AC图被发现是脑部PET/MR中测量的CTAC图的良好替代品。正如预期的那样，无论采用何种AC方法，TOF都能减少AC误差。连续的ZTE-AC比其他替代的MR衍生AC方法表现更好，减少了MRAC校正的PET图像与参考CTAC校正的PET图像之间的定量误差。
