Technical NoteArtifacts caused by transcranial magnetic stimulation coils and EEG electrodes in T2∗-weighted echo-planar imaging
Introduction
Basic research in magnetic resonance imaging (MRI) of human brain function as well as foreseeable clinical applications attempt to expand the MRI capabilities by a combination with transcranial magnetic stimulation (TMS) and electroencephalographic (EEG) recordings [1], [2], [3], [4]. Potential usages comprise the monitoring of electrophysiologic activity in parallel to dynamic imaging of blood oxygenation level dependent (BOLD) contrast, e.g., in epilepsy patients, a BOLD MRI assessment of TMS-induced neuronal activity, and functional imaging of TMS-related brain plasticity, e.g., during motor learning or rehabilitation.
Technically, the actual use of TMS and EEG may be arranged in an interleaved MRI acquisition mode so that problems due to induced currents are avoided. Nevertheless, pertinent combinations are potentially hampered by impaired image quality originating from static magnetic field inhomogeneities in the vicinity of TMS coils or EEG electrodes. Such phenomena also occur for ‘MR-compatible’ materials [5], [6] because functional MRI recordings are commonly based on strongly T2∗-weighted, i.e., susceptibility-sensitized, gradient-echo sequences such as long-TE FLASH, EPI, or spiral imaging.
The purpose of the present study was to investigate the distortions in T2∗-weighted EPI of phantoms and humans that are introduced by nonferromagnetic TMS coils and commercially available EEG electrodes.
Section snippets
Materials and methods
All studies were conducted at 2.0 T (Siemens Vision, Erlangen, Germany) using the standard imaging headcoil. We tested two differently sized TMS coils of figure-eight shape with a long axis of 166 mm and a diameter of 86 mm (Dantec Medical A/S, Skovlunde, Denmark) and a long axis of 190 mm and a diameter of 100 mm (Magstim, Carmarthenshire, Wales, UK). Both coils were especially designed for MR experiments using nonmagnetic materials and are not yet commercially available. To allow for remote
TMS coils
As demonstrated by the frontal and top view in Figs. 1 a and 1b high -resolution short-TE gradient-echo images of the water phantom revealed no distortions in the presence of the 166 mm long TMS coil. The position of the coil relative to the inner water volume is indicated by the “figure eight”-shaped structure representing the water tube attached to the coil surface. The resulting 3D images are identical to those acquired in the absence of a coil (not shown).
In contrast, significant
Conclusion
Nonferromagnetic TMS coils may cause severe artifacts in T2∗-weighted gradient-echo images such as in long-TE EPI scans commonly employed for functional neuroimaging. This particularly holds true for image orientations crossing the plane of the TMS coil. Also, EEG electrodes and fixation gels may impair T2∗-weighted images. Pertinent signal losses are localized though larger than the size of the electrodes.
The observed effects in phantoms are much less pronounced in applications to humans. This
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