Department of Clinical Physiology, Nuclear Medicine and PET, Rigshospitalet, Copenhagen, Denmark.
Eur J Nucl Med Mol Imaging. 2013 Apr;40(4):594-601. doi: 10.1007/s00259-012-2316-4. Epub 2013 Jan 8.
In combined whole-body PET/MR, attenuation correction (AC) is performed indirectly using the available MR image information and subsequent segmentation. Implant-induced susceptibility artifacts and subsequent signal voids may challenge MR-based AC (MR-AC). We evaluated the accuracy of MR-AC in PET/MR in patients with metallic endoprostheses, and propose a clinically feasible correction method.
We selected patients with uni- or bilateral endoprostheses from 61 consecutive referrals for whole-body PET/MR imaging (mMR; Siemens Healthcare). Simultaneous whole-body PET/MR imaging was performed at 120 min after injection of about 300 MBq [(18)F]FDG. MR-AC was performed using (1) original MR images and subsequent Dixon water-fat segmentation, (2) as method 1 with implant-induced signal voids filled with soft tissue, (3) as method 2 with superimposed coregistered endoprostheses from the CT scan, and (4) as method 1 with implant-induced signal voids filled with metal. Following MR-AC (methods 1-4) PET emission images were reconstructed on 344 × 344 matrices using attenuation-weighted OSEM (three iterations, 21 subsets, 4 mm gaussian). Maximum body-weight normalized standardized uptake values (SUVmax) were obtained for both hips. Mean SUV (SUVmean) in homogeneous reference regions in the gluteal muscle and bladder following MR-AC (methods 1-4) are also reported.
In total, four patients presented with endoprostheses, unilateral in two and bilateral in two. The fraction of voxels in MR images affected by the implant was at least twice that of the voxels representing the actual implants. MR-AC using methods 2 and 3 recovered the FDG distribution pattern compared to uncorrected PET images and method 1, while method 4 resulted in severe overestimation of FDG uptake (>460 % SUVmax). When compared to method 1, relative changes in SUVmean in the reference regions from method 2 and 3 were generally small albeit not correlated with the fraction of the attenuation image affected by implant-induced artifacts.
Endoprostheses cause PET/MR artifacts that exceed the volume occupied by the implants, and bias PET quantification. Artifacts and bias can be corrected by semiautomated inpainting with soft tissue with a single composition prior to MR-AC, thus restoring quantitative activity distribution.
在组合式全身 PET/MR 中，使用现有的 MR 图像信息和随后的分割来间接进行衰减校正（AC）。植入物引起的磁化率伪影和随后的信号缺失可能会对基于 MR 的 AC（MR-AC）构成挑战。我们评估了在有金属内假体的患者中，MR-AC 在 PET/MR 中的准确性，并提出了一种临床上可行的校正方法。
我们从 61 例连续转诊的全身 PET/MR 成像（mMR；西门子医疗）患者中选择了单侧或双侧假体的患者。在注射约 300MBq[(18)F]FDG 后 120 分钟进行同步全身 PET/MR 成像。使用（1）原始 MR 图像和随后的 Dixon 水脂分割，（2）用软组织填充植入物引起的信号缺失的方法 1，（3）用从 CT 扫描中配准的核心内假体的方法 2，以及（4）用金属填充植入物引起的信号缺失的方法 1 进行 MR-AC。在进行 MR-AC（方法 1-4）后，使用衰减加权 OSEM（3 次迭代，21 个子集，4mm 高斯）在 344×344 矩阵上重建 PET 发射图像。获得双侧髋关节的最大体重归一化标准摄取值（SUVmax）。还报告了 MR-AC（方法 1-4）后在臀肌和膀胱中的同质参考区域内的平均 SUV（SUVmean）。
共有 4 例患者存在假体，2 例为单侧，2 例为双侧。受植入物影响的 MR 图像中的体素数量至少是代表实际植入物的体素数量的两倍。与未经校正的 PET 图像和方法 1 相比，使用方法 2 和 3 的 MR-AC 恢复了 FDG 分布模式，而方法 4 导致 FDG 摄取的严重高估（>460%SUVmax）。与方法 1 相比，来自方法 2 和 3 的参考区域中的 SUVmean 的相对变化通常较小，尽管与受植入物引起的伪影影响的衰减图像的体积没有相关性。
内假体引起的 PET/MR 伪影超过了植入物占据的体积，并导致 PET 定量测量值出现偏差。通过在进行 MR-AC 之前用单一成分的软组织对伪影进行半自动的内插，可以校正伪影和偏差，从而恢复定量的活性分布。
