Rausch Ivo, Valladares Alejandra, Sundar Lalith Kumar Shiyam, Beyer Thomas, Hacker Marcus, Meyerspeer Martin, Unger Ewald
QIMP Team, Center for Medical Physics and Biomedical Engineering, Medical University of Vienna, Waehringer Guertel 18-20/4L, 1090, Vienna, Austria.
Division of Nuclear Medicine, Department of Biomedical Imaging and Image-guided Therapy, Medical University of Vienna, Vienna, Austria.
EJNMMI Phys. 2021 Feb 18;8(1):18. doi: 10.1186/s40658-021-00364-9.
PET/MRI phantom studies are challenged by the need of phantom-specific attenuation templates to account for attenuation properties of the phantom material. We present a PET/MRI phantom built from MRI-visible material for which attenuation correction (AC) can be performed using the standard MRI-based AC.
A water-fillable phantom was 3D-printed with a commercially available MRI-visible polymer. The phantom had a cylindrical shape and the fillable compartment consisted of a homogeneous region and a region containing solid rods of different diameters. The phantom was filled with a solution of water and [18F]FDG. A 30 min PET/MRI acquisition including the standard Dixon-based MR-AC method was performed. In addition, a CT scan of the phantom was acquired on a PET/CT system. From the Dixon in-phase, opposed-phase and fat images, a phantom-specific AC map (Phantom MR-AC) was produced by separating the phantom material from the water compartment using a thresholding-based method and assigning fixed attenuation coefficients to the individual compartments. The PET data was reconstructed using the Phantom MR-AC, the original Dixon MR-AC, and an MR-AC just containing the water compartment (NoWall-AC) to estimate the error of ignoring the phantom walls. CT-based AC was employed as the reference standard. Average %-differences in measured activity between the CT corrected PET and the PET corrected with the other AC methods were calculated.
The phantom housing and the liquid compartment were both visible and distinguishable from each other in the Dixon images and allowed the segmentation of a phantom-specific MR-based AC. Compared to the CT-AC PET, average differences in measured activity in the whole water compartment in the phantom of -0.3%, 9.4%, and -24.1% were found for Dixon phantom MR-AC, MR-AC, and NoWall-AC based PET, respectively. Average differences near the phantom wall in the homogeneous region were -0.3%, 6.6%, and -34.3%, respectively. Around the rods, activity differed from the CT-AC PET by 0.7%, 8.9%, and -45.5%, respectively.
The presented phantom material is visible using standard MR sequences, and thus, supports the use of standard, phantom-independent MR measurements for MR-AC in PET/MRI phantom studies.
正电子发射断层显像/磁共振成像（PET/MRI）体模研究面临挑战，因为需要特定体模的衰减模板来考虑体模材料的衰减特性。我们展示了一种由磁共振成像可见材料制成的PET/MRI体模，对于该体模，可以使用基于标准磁共振成像的衰减校正（AC）来进行衰减校正。
使用市售的磁共振成像可见聚合物对一个可注水的体模进行3D打印。该体模为圆柱形，可填充隔室由一个均匀区域和一个包含不同直径实心棒的区域组成。向体模中注入水和[18F]氟代脱氧葡萄糖（FDG）的溶液。进行了一次30分钟的PET/MRI采集，包括基于标准狄克逊法的磁共振成像衰减校正方法。此外，在PET/CT系统上对体模进行了CT扫描。从狄克逊同相、反相和脂肪图像中，通过基于阈值的方法将体模材料与水隔室分离，并为各个隔室指定固定的衰减系数，生成特定体模的AC图（体模磁共振成像衰减校正图）。使用体模磁共振成像衰减校正图、原始狄克逊磁共振成像衰减校正图以及仅包含水隔室的磁共振成像衰减校正图（无壁衰减校正图）对PET数据进行重建，以估计忽略体模壁的误差。将基于CT的衰减校正用作参考标准。计算CT校正后的PET与其他衰减校正方法校正后的PET之间测量活性的平均百分比差异。
在狄克逊图像中，体模外壳和液体隔室均清晰可见且彼此可区分，并且允许分割基于体模的特定磁共振成像衰减校正。与基于CT衰减校正的PET相比，基于狄克逊体模磁共振成像衰减校正图、磁共振成像衰减校正图和无壁衰减校正图的PET在体模整个水隔室中测量活性的平均差异分别为-0.3%、9.4%和-24.1%。在均匀区域靠近体模壁处的平均差异分别为-0.3%、6.6%和-34.3%。在棒周围，活性与基于CT衰减校正的PET相比分别相差0.7%、8.9%和-45.5%。
所展示的体模材料使用标准磁共振序列即可看见，因此，在PET/MRI体模研究中支持使用标准的、与体模无关的磁共振测量进行磁共振成像衰减校正。
