Gregersen Fróði, Eroğlu Hasan H, Göksu Cihan, Puonti Oula, Zuo Zhentao, Thielscher Axel, Hanson Lars G
Section for Magnetic Resonance, DTU Health Tech, Technical University of Denmark, Kgs Lyngby, Denmark.
Danish Research Centre for Magnetic Resonance, Centre for Functional and Diagnostic Imaging and Research, Copenhagen University Hospital - Amager and Hvidovre, Copenhagen, Denmark.
Imaging Neurosci (Camb). 2024 May 20;2. doi: 10.1162/imag_a_00176. eCollection 2024.
Volume conductor models of the human head are routinely used to estimate the induced electric fields in transcranial brain stimulation (TBS) and for source localization in electro- and magnetoencephalography (EEG and MEG). Magnetic resonance current density imaging (MRCDI) has the potential to act as a non-invasive method for dose control and model validation but requires very sensitive MRI acquisition approaches. A double-echo echo-planar imaging (EPI) method is here introduced. It combines fast and sensitive imaging of the magnetic fields generated by the current flow of transcranial electric stimulation with increased robustness to physiological noise. For validation, noise floor measurements without injected currents were obtained in five subjects for an established multi-echo gradient-echo (MGRE) sequence and the new EPI method. In addition, data with current injection were acquired in each subject with a right-left (RL) and anterior-posterior (AP) electrode montage with both sequences to assess the accuracy of subject-specific detailed head models. In line with previous findings, the noise floor measurements showed that the MGRE results suffered from spatial low-frequency noise patterns, which were mostly absent in the EPI data. A recently published approach optimizes the ohmic conductivities of subject-specific head models by minimizing the difference between simulated and measured current-induced magnetic fields. Here, simulations demonstrated that the MGRE noise patterns have a larger negative impact on the optimization results than the EPI noise. For the current injection measurements, a larger discrepancy was found for the RL electrode montage compared with the AP electrode montage consistently for all subjects. This discrepancy that remained in part also after optimization of the ohmic conductivities, was similar for the data of the two sequences and larger than the measurement noise, and thus demonstrates systematic biases in the volume conductor models. We have shown that EPI-based MRCDI is superior to established techniques by mitigating the effects of previously reported spatial low-frequency noise in MRCDI if limited spatial resolution is acceptable. Additionally, the consistent inter-subject results indicate that MRCDI is capable of picking up inaccuracies in computational head models and will be useful to guide systematic improvements.
人体头部的容积导体模型通常用于估计经颅磁刺激（TBS）中感应电场以及用于脑电图（EEG）和脑磁图（MEG）的源定位。磁共振电流密度成像（MRCDI）有潜力作为一种非侵入性方法用于剂量控制和模型验证，但需要非常灵敏的MRI采集方法。本文介绍了一种双回波平面回波成像（EPI）方法。它将经颅电刺激电流流动产生的磁场的快速灵敏成像与对生理噪声增强的稳健性相结合。为了进行验证，在五名受试者中使用既定的多回波梯度回波（MGRE）序列和新的EPI方法获取了未注入电流时的本底噪声测量值。此外，在每个受试者中使用左右（RL）和前后（AP）电极组合通过两种序列采集注入电流的数据，以评估特定受试者详细头部模型的准确性。与先前的研究结果一致，本底噪声测量表明MGRE结果受到空间低频噪声模式的影响，而这些模式在EPI数据中大多不存在。最近发表的一种方法通过最小化模拟和测量的电流感应磁场之间的差异来优化特定受试者头部模型的欧姆电导率。在此，模拟表明MGRE噪声模式对优化结果的负面影响比对EPI噪声的影响更大。对于注入电流测量，在所有受试者中，与AP电极组合相比，RL电极组合发现了更大的差异。在欧姆电导率优化后，这种差异部分仍然存在，对于两种序列的数据相似且大于测量噪声，因此表明容积导体模型中存在系统偏差。我们已经表明，如果有限的空间分辨率是可接受的，基于EPI的MRCDI通过减轻先前报道的MRCDI中空间低频噪声的影响优于既定技术。此外，受试者间一致的结果表明MRCDI能够检测到计算头部模型中的不准确之处，并将有助于指导系统性改进。
