High-Field and Hybrid MR Imaging, University Hospital Essen, University Duisburg-Essen, Essen, Germany.
Erwin L. Hahn Institute for Magnetic Resonance Imaging, University Duisburg-Essen, Essen, Germany.
Med Phys. 2022 Feb;49(2):865-877. doi: 10.1002/mp.15446. Epub 2022 Jan 25.
Truncation artifacts in the periphery of the magnetic resonance (MR) field-of-view (FOV) and thus, in the MR-based attenuation correction (AC) map, may hamper accurate positron emission tomography (PET) quantification in whole-body PET/MR, which is especially problematic in patients with obesity with overall large body dimensions. Therefore, an advanced truncation correction (TC) method to extend the conventional MR FOV is needed.
The extent of MR-based AC-map truncations in obese patients was determined in a dataset including n = 10 patients that underwent whole-body PET/MR exams. Patient inclusion criteria were defined as BMI > 30 kg/m and body weight > 100 kg. Truncations in PET/MR patients with obesity were quantified comparing the MR-based AC-map volume to segmented non-AC PET data, serving as the reference body volume without truncations to demonstrate the need of improved TC. The new method implemented in this study, termed "advanced HUGE," was modified and extended from the original HUGE method by Blumhagen et al. in order to provide improved TC across the entire axial MR FOV and to unlock new clinical applications of PET/MR. Advanced HUGE was then systematically tested in PET/MR NEMA phantom measurements. Relative differences between computed tomography (CT) AC PET data of the phantom setup (reference) and MR-based Dixon AC, respectively Dixon + advanced HUGE AC, were calculated. The applicability of the method for advanced TC was then demonstrated in first MR-based measurements in healthy volunteers.
It was found that the MR-based AC maps of obese patients often reveal truncations in the anterior-posterior direction. Especially, the abdominal region could benefit from improved TC, where maximal relative differences in the AC-map volume up to -17% were calculated. Applying advanced HUGE to improve the MR-based AC in PET/MR, PET quantification errors in the large-volume phantom setup could be considerably reduced from average -18.6% (Dixon AC) to 4.6% compared to the CT AC reference. Volunteer measurements demonstrate that formerly missing AC-map volume in the Dixon-VIBE AC-map could be added due to advanced HUGE in the anterior-posterior direction and thus, potentially improves AC in PET/MR.
The advanced HUGE method for truncation correction considerably reduces truncations in the anterior-posterior direction demonstrated in phantom measurements and healthy volunteers and thus, further improves MR-based AC in PET/MR imaging.
磁共振（MR）视野（FOV）边缘的截断伪影，以及由此导致的基于 MR 的衰减校正（AC）图中出现的截断伪影，可能会妨碍全身 PET/MR 中 PET 定量的准确性，这在肥胖患者中尤其成问题，因为他们的整体身体尺寸较大。因此，需要一种先进的截断校正（TC）方法来扩展传统的 MR FOV。
在包括 n = 10 名患者的数据集，这些患者接受了全身 PET/MR 检查，确定了肥胖患者基于 MR 的 AC 图截断的程度。患者纳入标准定义为 BMI>30 kg/m2 和体重>100 kg。通过比较基于 MR 的 AC 图体积与分段的非 AC PET 数据来量化 PET/MR 肥胖患者中的截断，分段的非 AC PET 数据作为没有截断的参考体体积，以证明需要改进 TC。本研究中实施的新方法称为“高级 HUGE”，是对 Blumhagen 等人的原始 HUGE 方法进行修改和扩展得到的，旨在提供整个轴向 MR FOV 的改进 TC，并解锁 PET/MR 的新临床应用。高级 HUGE 随后在 PET/MR NEMA 体模测量中进行了系统测试。计算体模设置（参考）的 CT AC PET 数据与基于 MR 的 Dixon AC 之间的相对差异，以及 Dixon + 高级 HUGE AC 之间的相对差异。然后，在第一组健康志愿者的基于 MR 的测量中证明了该方法在高级 TC 中的适用性。
研究发现，肥胖患者的基于 MR 的 AC 图通常在前-后方向上显示截断。特别是腹部区域可以从改进的 TC 中受益，在那里计算出的 AC 图体积的最大相对差异高达-17%。在 PET/MR 中应用高级 HUGE 来改善基于 MR 的 AC，可大大降低大容量体模设置中 PET 量化误差，从平均-18.6%（Dixon AC）降低到与 CT AC 参考相比的 4.6%。志愿者测量结果表明，由于在前后方向上的高级 HUGE，以前在 Dixon-VIBE AC 图中丢失的 AC 图体积现在可以添加，从而可能提高 PET/MR 中的 AC。
在体模测量和健康志愿者中，用于截断校正的高级 HUGE 方法显著减少了在前-后方向上的截断，从而进一步提高了 PET/MR 成像中基于 MR 的 AC。
