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
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.
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.