Exciting news for our EEG-fMRI customers: Update of MRI sequence conditions

by Dr. Tracy Warbrick
Application Specialist EEG-fMRI (Brain Products
)

We strive to keep our products up to date and we are excited to announce some news that will bring our conditions for use in line with current trends in fMRI. We have updated the MRI sequence conditions for our BrainAmp MR systems1 and BrainCap MR.

Abstract

Safety is always our primary concern for simultaneous EEG-fMRI recordings; however, we are aware that fMRI sequences have progressed considerably since our guidelines were implemented. So, we have updated the MRI sequence conditions for our BrainAmp MR systems1. You will now have more flexibility in terms of which MRI sequences you can use for simultaneous EEG-fMRI. This article introduces the new guidelines and explains why we make these new recommendations.

New Conditions for MRI sequences

Yes, you can use multiband EPI sequences!

Because safety is always important to us for simultaneous EEG-fMRI, we still have some restrictions, but we have made considerable progress with our sequence recommendations. To catch up with advances in the fMRI community, we have tested the BrainAmp MR systems1, which includes the BrainCap MR, under current fMRI conditions.

To reflect developments in the MRI community, we are also changing the way we define sequence related MR-conditions for the BrainAmp MR systems1. Instead of specifying individual sequences or sequence parameters we will recommend a maximum B1+rms for MRI sequences used for simultaneous EEG-fMRI. You will find a summary of the new guidelines in Table 1 and below you will find further information on what B1+rms is and why we use it.

Maximum allowed B1+rms values for 3T and different caps
Current standard BrainCap MR Max. B1+rms = 1 µT
New Standard BrainCap MR*
(available from April 2020)
Max. B1+rms = 1.5 µT
R-Net MR*
(coming soon)
Max. B1+rms = 1.5 µT
*Note that for the new standard BrainCap MR and the R-Net MR a 10 cm cable must be used to attach the cap to the BrainAmp MR amplifier. If you use a longer cable the B1+rms limit of 1 µT applies.
Table 1. Maximum B1+rms values of radio frequency (RF) magnetic field allowed for simultaneous EEG-fMRI using the BrainAmp MR systems1 with different EEG caps by Brain Products. All conditions specified in the user manual must also be met.

What is B1+rms?

In short, B1+rms is a metric used to quantify the amount of radio frequency (RF) magnetic field that is utilized during scanning. This is useful for assessing heating risk and is important for the safety of volunteers and equipment used inside the scanner.

For those who would like a little more detail, B1+rms is the average effective RF magnetic field generated by the RF transmit coil for a specific pulse sequence and is expressed in units of µT. More specifically, it can be defined as a statistical measure of the magnitude of the positive rotating component of the B1-Field. We’re not all MR physicists, so to make things easier we have explained the meaning of each component in this expression in Figure 1. If you are interested in learning more, ISMRM.org has some very useful resources. See below (“Literature and Links” section) for a link to a B1+rms video podcast.

Figure 1. Explanation of B1+rms

Figure 1. Explanation of B1+rms

Why have we started using B1+rms?

Historically we have recommended using low Specific Absorption Rate (SAR) sequences, however, SAR is dependent not only on the imaging parameters but also on the volunteer’s weight. B1+rms on the other hand is volunteer independent and is determined by basic MRI parameters. It is a known value based on the pulse sequence parameters, unlike SAR which is an estimated value. Once a sequence has been adjusted to the required B1+rms, it is saved to the scanning protocol and will remain that way as long as relevant parameters are not changed. SAR, on the other hand, will vary from volunteer to volunteer. As such, B1+rms has been recommended as an alternative metric to SAR for limiting the amount of RF power during scanning (for an overview see Faulkner, 2016).

The B1+rms limit that we specify protects the volunteer and the amplifier from excessive RF power while allowing the user more flexibility with their sequence parameters. By specifying a maximum B1+rms for the BrainAmp MR systems1, the user is free to determine sequence parameters as they see fit, as long as the B1+rms remains below the specified threshold.

You might be asking why we have only just started specifying a maximum B1+rms. Using B1+rms as an alternative to SAR for determining MR conditions is a relatively new development and scanner manufacturers have only recently been required to display B1+rms at the MR scanner console. Our previous sequence guidelines were created before these changes were made, so we specified only that low SAR sequences should be used and gave examples of acceptable parameters.

A little more about the new sequence limits

Sequence parameters are just one aspect of our safety recommendations. The B1+rms limits stated in Table 1 are for 3T scanners; however, all other specifications described in the user manual still apply e.g. position of the amplifier, cable routing, and a head coil with an appropriate option for routing the EEG cables. If your setup does not allow for the guidelines in the manual to be followed (e.g. no appropriate head coil available) please contact us to discuss your setup.

For any BrainCap MR already in the market the maximum allowed B1+rms is 1 µT; whereas, for the new standard BrainCap MR, the maximum allowed B1+rms is 1.5 µT. To give those numbers some context, a B1+rms of 1 µT is approximately the same amount of RF power as for the sequence guidelines previously specified. But now, instead of being constrained by specific sequence parameters you can modify your sequence to suit your needs, as long as it stays below the B1+rms threshold. For example, it would be possible to have a multiband EPI sequence (e.g. with an acceleration factor of 2) and still be within the limit.

The B1+rms of 1.5 µT specified for the new standard BrainCap MR allows more RF power in a given time than the previous guidelines. This is because we have made some modifications to the standard BrainCap MR and have tested the new cap with stronger sequences. So, using multiband EPI as an example, this means that a higher acceleration factor, more slices, or a shorter TR could be used with the new standard BrainCap MR compared to the previous version.

The maximum B1+rms of 1.5 µT will also apply to the R-Net MR, our sponge-based electrode system that will be available in an MR-Conditional version soon.

What should I do for my own measurements?

It is now mandatory for all scanner manufacturers to display B1+rms at the console (IEC 60601-2-33:2010), so this value should be easily accessible in all new scanners. Consult the documentation from your scanner vendor to find out where this value can be found.

Local experts at your scanning facility will be able to advise you on how to select the most appropriate sequence parameters for your study. The exact parameters to change in order to manipulate B1+rms will vary across scanners, but they are similar to those we consider in relation to reducing SAR. We cannot advise you on how to modify your sequence or provide specific recommendations for any given parameter; however, the technician in charge of your scanner can advise you on how to determine an appropriate scanning protocol for your study.

Test conditions

Testing for the new B1+rms limits was done under the conditions recommended in the user manual. Measurements were done in a Siemens Prisma (3T) scanner using a Siemens Head/Neck 64 head coil. Temperature measurements were made using a Neoptix Reflex fiber optic temperature thermometer the positions of the probes were determined by previous measurements using an infrared camera.

Abbreviations

Abbreviation Definition
fMRI Functional magnetic resonance imaging
B1+rms The average effective RF magnetic field generated by the RF transmit coil for a specific pulse sequence
RF Radio Frequency
SAR Specific Absorption rate. A measure of the rate at which RF energy is absorbed per unit of tissue mass (W/kg)
EPI Echo Planar Imaging (used for blood oxygen dependent (BOLD) fMRI)

Literature and Links

If you have any questions on the new MRI sequence guidelines please contact our EEG-fMRI Application Specialist.


1refers to all BrainAmp MR amplifiers, i.e. BrainAmp MR, BrainAmp MR plus and BrainAmp ExG MR


©Brain Products GmbH 2020