The safety of research participants is the main concern for simultaneous EEG-fMRI acquisitions. We also aim to protect the BrainAmp MR system from damage within the harsh MR environment. This support tip explains the factors in the MR environment that make the BrainAmp MR (EEG/ExG) amplifiers and Powerpacks vulnerable to heating. We also provide recommendations on how to minimize the risk of heating and damage.
Continuing development of MR scanner technology and the application of EEG-fMRI means that we need to continuously revisit safety guidelines and consider how advancements in the MR community influence the EEG system. We previously covered how to perform safe EEG-fMRI measurements using our recommended sequence and setup guidelines. The recommendations in the previous articles are still valid. The purpose of this article is to provide additional information and recommendations on safety risks in the MR environment.
While our current sequence guidelines provide more flexibility to the user by giving B1+rms limits, we need to consider that the B1+rms metric quantifies the amount of (RF) magnetic field. However, the amount of gradient magnetic field acting on the EEG system should also be considered when designing fMRI sequences and combined EEG-fMRI studies. This article focuses on how the direction and strength of the gradient fields affect the EEG system.
The gradient magnetic field
In short, rapidly changing gradients are applied during fMRI to alter the main magnetic field in a spatially localized manner. This helps with spatial encoding of the MR signal received from thousands of voxels. The switching of the gradient fields induces eddy currents in conductive material within the magnetic environment. Keep in mind, the EEG system consists of conductive components. Specifically, the shielding of the amplifiers, which minimizes interference with the MR imaging, is susceptible to heating from eddy currents induced by the changing gradient fields.
The susceptibility of the amplifier to heating increases with stronger gradient fields. Several factors influence the strength of the gradient fields acting on the EEG system: the spatial variation of the gradient magnetic field strength within the scanner, the direction of the strongest gradient field with respect to the amplifiers, and the gradient amplitude and the rate of change of the gradient fields defined by the scanners gradient performance settings. The next sections will describe how the spatial distribution of the gradient fields and certain MR sequence parameters influence the potential heating of the EEG system.
Spatial Distribution of the Gradient Fields
The rapidly switching gradient fields are generated by a set of three orthogonal gradient coils (x- , y- and z-gradient coil). When current is passed through one of these coils, a magnetic field is produced, which alters the local static magnetic field by a small amount (in the mT range) along the given axis. The direction of the current and the local magnetic field is altered over time generating a gradient magnetic field (dB/dt).
Figure 1: Axes in the scanner. The z-axis corresponds to the superior-inferior axis, the x-axis is equivalent to the left-right axis, and the y-axis is the anterior-posterior axis.
During image acquisition, gradients are generated in all three directions simultaneously. This causes the spatial distribution of the strength of the gradient magnetic field to vary with radial distance from the isocenter. The smallest gradient magnetic field is observed at the isocenter and the maximum change in the magnetic field is observed near the edge of the gradient coil (Stafford 2020).
Figure 2: Maximum gradient magnetic field (dB/dt) distribution within the scanner. A) Radial distribution of maximum dB/dt in the xy-plane. The diameter of the circles are 20 cm (blue), 40 cm (orange), and 50 cm (green). B) A representation of the maximum change in gradient magnetic field along the z-axis, which is orthogonal to the xy-plane. The figure was adapted from Stafford (2020).
Figure 2 illustrates how the maximum change in magnetic field increases when moving further away from the isocenter. A larger change in the magnetic field results in a stronger electromotive force that induces eddy currents within the amplifier. Figure 2 shows placing amplifiers side-by-side as well as stacking too many amplifiers in the y-direction is suboptimal as the electromotive force (EMF) increases towards the edge of the scanner bore. Stacking amplifiers in the z-isoline reduces the amount of EMF acting on the sides of the amplifiers. However, the spatial distribution of the gradient field needs to also be considered in the anterior-posterior direction. We recommend stacking up to three BrainAmp MR (EEG/ExG) and/or PowerPacks. A stack of four BrainAmp MR (EEG/ExG) or PowerPacks causes the top amplifier to be too close to the scanner bore wall. Subsequently, we also do not recommend placing PowerPacks or amplifiers directly on the scanner bore floor. Please refer to our operating instructions and contact Technical Support for EEG configurations consisting of four amplifiers. Figure 3 summarizes our recommended setups for the most common channel number and amplifier combinations. We also recommend contacting Technical Support for advice on amplifier setup for high channel number systems.
Figure 3: Recommended EEG system setup for
a) 32-channel system with Carbon Wire Loops (CWLs) or 64-channel system. We recommend this setup, as it is ideal for experimental designs with visual paradigm and allows better dissipation of heat.
b) An alternative setup for 32-channel system with CWLs or 64-channel system.
c) The recommended setup for 64-channel system with CWLs or 96-channel system. A indicates the BrainAmp MR (EEG/ExG) and B indicates the PowerPacks.
d) The BrainAmp MR (EEG/ExG) and PowerPacks are stacked in the z-isoline.
Does the direction of the different types of gradients influence the EEG system?
In the previous section, the three orthogonal gradient magnetic fields (Gx, Gy, Gz) were introduced. During echo planar imaging (EPI), which is typically used for simultaneous EEG-fMRI acquisition, these three gradients serve different purposes for image acquisition. Figure 4 illustrates a multi-echo EPI sequence diagram during the acquisition of one slice of an imaging volume. One gradient is initially applied to select a slice perpendicular to the direction of the slice, which is typically the z-gradient in EPI imaging. There are two additional types of spatial gradients that are then applied during EPI imaging to help localize and read the MR signal: 1) the frequency encoding gradient and 2) the phase encoding gradient. These correspond to the x-gradient and the y-gradient. Fully explaining MRI sequences is beyond the scope of this article, for those of you interested in learning more we recommend Schmitt, 2015.
It is important to note that the frequency encoding gradient and the phase encoding gradient have different effects on the EEG system. Figure 4 shows an example of the gradient field strength and direction of the frequency encoding and phase encoding gradient during slice selection in an EPI sequence. Notice the gradient field strength is higher and switches directions more often for the frequency encoding gradient as compared to the phase encoding gradient. This means more electromotive force or current is induced in a conductive system perpendicular to the frequency encoding direction. By Faraday’s law, the induction of an electromotive force is also proportional to surface area of the conductive loop.
Figure 4: Direction and strength of the gradient field across time during the acquisition of a slice. The amplitude of the gradient in the frequency encoding direction is larger and changes more often than for the phase encoding direction. Courtesy of Gianelli et al. 2010.
In many MR scanners the phase encoding direction can be selected. The frequency encoding direction is always perpendicular to the phase encoding direction. Therefore, selecting the phase encoding direction also sets the frequency encoding direction. We need to consider the frequency encoding direction of MR sequences to minimize the effect of the frequency encoding gradient on the amplifier. Figure 5 shows the cross-sectional surface area of the BrainAmp MR (EEG/ExG) amplifier is smaller in the left-right frequency encoding direction than the anterior-posterior frequency encoding direction. Subsequently, only anterior-posterior phase encoding direction is allowed during simultaneous EEG-fMRI to minimize the effect of the frequency encoding gradient on the amplifiers. Typically, anterior-posterior phase encoding direction will also result in better quality, less distorted fMRI images.
Figure 5: The effect of phase encoding and frequency encoding direction on amplifiers placed into the bore. (Left) When anterior-posterior phase encoding direction is defined, the stronger left-right frequency encoding gradient acts on the sides of the amplifiers. The surface area, indicated by the blue rectangle, is much smaller than when (right) left-right phase encoding direction and the respective anterior-posterior frequency encoding direction is defined. The surface area perpendicular the frequency encoding direction should always be minimized. We recommend using anterior-posterior phase encoding.
How Gradient Performance Mode can Influence the EEG System
A push for faster image acquisition and high-resolution imaging has driven the development of powerful gradients in modern MR scanners. Historically, the strength of the gradients was limited by the capabilities of the gradient amplifiers. However, recent developments in gradient technology have improved the gradient performance considerably. Gradient performance is characterized by the maximum gradient strength (often stated in units of mT/m) that can be achieved along one axis and the fastest possible slew rate, which is the peak gradient amplitude divided by the maximum rate of change in the gradient field over time (often stated in units of T/m/s).
Some scanners allow the user to choose between normal or high gradient performance modes. High gradient performance modes tend to use maximum gradient strength and slew rate. Running sequences in high gradient performance can induce peripheral nerve stimulation, so the gradient performance may already be limited at your local scanning institution. However, if your sequence is utilizing high gradient performance mode recall that stronger and faster gradients will induce more electromotive force and eddy currents in conductive materials, which increases the risk of heating in the BrainAmp MR (EEG/ExG) amplifiers and PowerPacks. We recommend reducing the risk of heating by using normal gradient mode. Gradient mode settings vary across scanner vendors and scanner models. Your local scanner operator can advise you on your scanner’s gradient performance mode settings.
Conditions of use for protecting the BrainAmp MR (EEG/ExG) amplifier series and PowerPacks during simultaneous EEG-fMRI
To protect the amplifiers from heating during acquisition of simultaneous EEG-fMRI, we recommend minimizing the strength of the gradient fields acting upon the surface of the amplifiers by following these three steps:
Stack and center amplifiers along the z-isoline. The smallest gradient field is observed near the isocenter of the scanner. Since the strength of the gradient field increases radially with distance from the isocenter, we recommend stacking up to three amplifiers in the center. Setups that consist of four amplifiers require an alternative, suboptimal setup with regards to electromotive forces and heating. Please refer to the operating instructions for placement guidelines of four amplifiers or contact Technical Support for specific guidelines.
Only run functional MRI sequences with phase encoding in the anterior-posterior direction.
We recommend using normal gradient performance mode for simultaneous EEG-fMRI acquisition. Note, not all scanners allow users to define the gradient mode. The scanner operator at your local institution can advise you on the gradient performance specifications of your scanner system.
In this article, we have described how one component of the MR environment, the magnetic gradient field, influences the EEG system. In particular, the BrainAmp MR family and PowerPacks are susceptible to heating from eddy currents induced by the strength of the gradient field. Proper placement of the BrainAmp MR family and PowerPacks as well as following recommended sequence guidelines ensures safe use of our EEG system.
The gradient field is only one aspect of the MR environment. Safety guidelines concerning the static magnetic field and radio frequency field must also be considered. Further safety and sequence guidelines related to B1+rms and radio frequency field can be found here.
This article covered amplifier placement for EEG measurements. For information on using the BrainAmp ExG MR for peripheral physiology measurements, please refer to our support tip series on ExG-fMRI measurements, which include EMG-fMRI and ECG-fMRI.
If you have any questions about setting up your BrainAmp MR system, please contact our Technical Support team.