by Dr. Tracy Warbrick
Application Specialist EEG-fMRI (Brain Products)
In this article we provide an overview of our EEG-fMRI safety recommendations especially with respect to our recent developments. We will consider our sequence guidelines and what limits apply to our various caps. We will also provide an overview of recommendations for setting up the system in the scanner. Recommendations for sources of further information will also be made throughout.
We take safety very seriously for EEG-fMRI because the EEG system is vulnerable to the effects of the MR environment. The EEG system is a recording circuit (EEG electrode versus reference electrode) and consists of electrodes, lead wires, and an amplifier. We place this system inside a static magnetic field, and during scanning we use strong switching gradient fields and radio frequency (RF) pulses. The EEG-system is susceptible to heating due to RF-coupling as well as eddy currents. This has consequences for the function of equipment and the wellbeing of the participant. As such, we provide recommendations for how to use the BrainAmp MR systems safely in the MR environment at 3 T:
We strive to keep our products and guidelines up to date and when we revise any of our documentation, we make it available for free on our website; please be sure to check for current versions so you always have the most recent information.
Let’s start with our sequence guidelines
We introduced the B1+rms thresholds in 2020. We’ve always had limitations for acceptable MRI sequences to restrict the amount RF exposure of the EEG system, but previously they were a little more restrictive and not so easy to adapt to advanced fMRI sequences. We wanted to make the guidelines more flexible and allow researchers to use advanced fMRI sequences, e.g. MB fMRI, so we introduce the B1+rms threshold. Further information on B1+rms can be found in this article.
Keep in mind that the system is intended for simultaneous EEG-fMRI measurements, as such we have fMRI BOLD imaging in mind when testing and setting the limits. The B1+rms limit might also allow other, non-fMRI sequences, to be used but there will also be sequences that are outside of these guidelines. We appreciate that researchers might want to use sequences with a higher RF load, e.g. turbo spin echo sequences, arterial spin labelling, or even fMRI sequences with a high RF load, but this is not possible if the B1+rms is above the threshold. The potential consequences for using sequences that are outside the guidelines are damage to the EEG equipment or injury to the participant due to heating of components of the EEG system. We would like to avoid these consequences; therefore, we provide guidelines on how to use the EEG system safely.
B1+rms in practice
Since introducing the B1+rms limits we’ve had some questions from our users on how to determine B1+rms and where to find it. Specific sequence parameters are outside of our expertise so we are unable to comment on which values you should use. Also, the exact parameters to change to manipulate B1+rms for a given sequence will vary across scanners. However, we can offer a few tips on how to use B1+rms.
The parameters that influence the B1+rms of a sequence are similar to those we consider in relation to reducing specific absorption rate (SAR), for example flip angle, RF pulse duration, number of slices in a given repetition time (TR). Your scanner operator should be able to help you adjust the relevant parameters in your MRI sequence.
The location of the B1+rms display on the scanner console is different for each manufacturer, however, it is likely to be close to where the SAR parameters are displayed.
Note that on Siemens’ systems there is a ‘predicted’ and a ‘current’ B1+rms. The ‘predicted’ value should be used to determine the B1+rms of the sequence you would like to use.
What B1+rms threshold applies to your setup?
In 2020 we also made some modifications to the standard BrainCap MR: shorter cable tree, increased resistance on drop-down leads (e.g. ECG lead), and the introduction of a shorter (10 cm) bundled cable to connect the EEG cap to the amplifier. The updated features of the new design allowed the BrainCap rev. 3 to be tested with stronger MRI sequences. The R-Net MR has same safety features as the BrainCap MR rev. 3.
Our current standard setup includes the BrainCap MR rev. 3 or the R-Net MR used with a 10 cm bundled connection cable. The B1+rms threshold for this standard setup is 1.5 µT. However, we know that some labs have older caps or prefer to use the longer (30 cm) connection cable due to their local setup. Don’t worry, you can continue to use your existing BrainCap MRs but note that the B1+rms threshold is more restrictive (1.0 µT) and is in line with our previous recommendations for the BrainCap MR. Figure 1 provides an overview of what B1+rms threshold applies to different combinations of caps and connection cable lengths. All connection cables longer than 30 cm (e.g. 100 cm) are intended only for the cap preparation outside the magnet room. These cables should never be part of a recording setup inside the scanner room.
Note that adding carbon wire loops doesn’t change the B1+rms threshold for the BrainCap MR or R-Net MR.
Figure 1. Overview of the B1+rms threshold that should be used for different combinations of EEG caps and connection cables. Note that by connection cable, we mean the cable that connects the BrainCap MR or R-Net MR to the BrainAmp MR (plus). Note that connection cables longer than 30 cm are intended for cap preparation outside of the scanner room and are not intended to be part of the set up inside the scanner.
Tip: For older caps, we recommend replacing the 30 cm ribbon cable that connects the cap to the amplifier with a 10 cm bundled cable. This doesn’t reduce the overall length as much as for the new BrainCap MR, or change the B1+rms threshold, but it will help with finding a better position for the BrainAmp MR (plus) inside the scanner bore.
How do I know what threshold applies to my cap?
If the cable tree of your BrainCap MR is longer than 31 cm it is an older (or customized) BrainCap MR.
To be sure whether your BrainCap MR is a rev. 3 you should check the document delivered with your cap which provides the layout and specification.
Setting up the system in the bore
In addition to staying within the sequence guidelines you should also follow our placement recommendations for setting up the BrainAmp MR (plus) system inside the scanner. The exact setup can be dependent on local factors such as the type of scanner you have, the head coil that you plan to use, and whether you have any other equipment inside the scanner bore. But there are some basic principles that should always be followed. Figure 2 illustrates a recommended setup and lists the key points to consider when setting up your BrainAmp MR (plus) system.
Figure 2. How to position the BrainAmp MR (plus) system inside the scanner bore. Part A shows a BrainCap MR with CWLs inside a head coil, the top part of the head coil is removed to show the dedicated space for the EEG cable tree. 10 cm bundled cables are used to connect the EEG cap and CWLs to the amplifiers. Part B shows the full head coil and the BrainAmp MR plus and PowerPack positioned behind the head coil on a platform inside the scanner bore. A 30 cm ribbon cable is used to connect the EEG cap to the amplifier.
We have provided a refresher on the main safety considerations for setting up your EEG-fMRI study and how these recommendations apply to our new solutions. If you would like to learn more about how to setup your BrainAmp MR (plus) system safely you can watch the recording of our recent webinar “Getting ready for simultaneous EEG-fMRI: safety and setup basics” that is available on our Brain Products Academy webinar channel.
In this article we have covered setting up EEG measurements, for information on using the BrainAmp ExG MR for peripheral physiology measurements please refer to our series of support tips on ExG-fMRI measurements, the first of which covered EMG-fMRI and can be found here.