Magnetic Resonance, Siemens AG Healthcare Sector, Erlangen 91052, Germany.
Institute of Medical Physics, Friedrich-Alexander-University Erlangen-Nürnberg, Erlangen 91052, Germany.
Med Phys. 2014 Feb;41(2):022303. doi: 10.1118/1.4861097.
In quantitative PET imaging, it is critical to accurately measure and compensate for the attenuation of the photons absorbed in the tissue. While in PET/CT the linear attenuation coefficients can be easily determined from a low-dose CT-based transmission scan, in whole-body MR/PET the computation of the linear attenuation coefficients is based on the MR data. However, a constraint of the MR-based attenuation correction (AC) is the MR-inherent field-of-view (FoV) limitation due to static magnetic field (B0) inhomogeneities and gradient nonlinearities. Therefore, the MR-based human AC map may be truncated or geometrically distorted toward the edges of the FoV and, consequently, the PET reconstruction with MR-based AC may be biased. This is especially of impact laterally where the patient arms rest beside the body and are not fully considered.
A method is proposed to extend the MR FoV by determining an optimal readout gradient field which locally compensates B0 inhomogeneities and gradient nonlinearities. This technique was used to reduce truncation in AC maps of 12 patients, and the impact on the PET quantification was analyzed and compared to truncated data without applying the FoV extension and additionally to an established approach of PET-based FoV extension.
The truncation artifacts in the MR-based AC maps were successfully reduced in all patients, and the mean body volume was thereby increased by 5.4%. In some cases large patient-dependent changes in SUV of up to 30% were observed in individual lesions when compared to the standard truncated attenuation map.
The proposed technique successfully extends the MR FoV in MR-based attenuation correction and shows an improvement of PET quantification in whole-body MR/PET hybrid imaging. In comparison to the PET-based completion of the truncated body contour, the proposed method is also applicable to specialized PET tracers with little uptake in the arms and might reduce the computation time by obviating the need for iterative calculations of the PET emission data beyond those required for reconstructing images.
在定量 PET 成像中，准确测量和补偿组织中吸收的光子的衰减至关重要。在 PET/CT 中，可以从低剂量 CT 基础的透射扫描中轻松确定线性衰减系数，而在全身 MR/PET 中，线性衰减系数的计算基于 MR 数据。然而，基于 MR 的衰减校正 (AC) 的一个限制是由于静态磁场 (B0) 不均匀性和梯度非线性，MR 固有视野 (FoV) 限制。因此，基于 MR 的人体 AC 图可能会在视野 (FoV) 的边缘被截断或几何变形，并且，基于 MR 的 AC 的 PET 重建可能会产生偏差。这在横向方向上尤其具有影响，因为患者的手臂在身体旁边休息，并未被完全考虑在内。
提出了一种通过确定局部补偿 B0 不均匀性和梯度非线性的最佳读出梯度场来扩展 MR FoV 的方法。该技术用于减少 12 名患者的 AC 图中的截断，分析并比较了对 PET 量化的影响，与未应用 FoV 扩展的截断数据以及已建立的基于 PET 的 FoV 扩展方法相比。
在所有患者中，基于 MR 的 AC 图中的截断伪影都得到了成功的减少，并且平均身体体积增加了 5.4%。在某些情况下，与标准截断衰减图相比，在个体病变中观察到高达 30%的与患者相关的 SUV 大的变化。
该技术成功地扩展了基于 MR 的衰减校正中的 MR FoV，并显示出在全身 MR/PET 混合成像中对 PET 量化的改善。与基于 PET 的截断体轮廓的完成相比，该方法还适用于在手臂中摄取量很少的特殊 PET 示踪剂，并且通过避免对重建图像所需的重建图像之外的 PET 发射数据进行迭代计算，可能会减少计算时间。
