B1+RMS, The root-mean-square value of the MRI Effective Component of the RF Magnetic (B1) Field
When performing magnetic resonance imaging (MRI) examinations on patients with implants and devices, it is important to be aware of, and to control, the amount of radiofrequency (RF) power that is utilized in order to ensure patient safety. The metric typically presented in MR Conditional labeling that addresses this matter is the whole body averaged specific absorption rate (SAR).
SAR is the rate at which energy is absorbed by the body when exposed to the RF electromagnetic field (B1) and is measured in units of Watts per kilogram (W/kg) of body weight. SAR is patient dependent and varies depending on the patient’s size and mass (weight). Of note is that there is no direct way to measure SAR prior to or during an MRI procedure. As a result, MR system manufacturers rely on numerical models to conservatively estimate the SAR for a particular scan sequence and each manufacturer uses conservative assumptions for their SAR models to ensure that no patient is exposed to RF energy that exceeds the specified SAR limits.
When active implants and devices are present in patients, the use of SAR appears to be an unreliable means of characterizing RF-induced heating. Therefore, it is potentially dangerous to use SAR in these cases, as reported by Baker, et al. (2004). A number of years ago, the Joint Working Group comprised of scientists and scientists affiliated with MR system and device manufacturers, along with the FDA, recommended that B1+RMS be used as a metric for implant heating as opposed to using SAR.
B1+RMS is the root-mean-square value of the MRI effective component of the RF magnetic (B1) field or, in other words, the time-averaged RF magnetic field component relevant for creating an MR image that is generated by the MR system during a scan. B1+RMS is measured in units of micro-Tesla (μT). The MR scanner measures the B1+ field (the positively rotating RF magnetic field produced by the scanner) needed for an imaging sequence and uses the time averaged B1+ field, or B1+RMS, that will occur due to a particular imaging sequence. Thus, the B1+RMS value is calibrated by the MR system’s software during the “prep” or “pre-scan” phase or measurements of an MRI exam. An important characteristic of B1+RMS is that it is not an estimated value but it is a known quantity based on the pulse sequence and the associated parameters. Furthermore, B1+RMS is not patient-dependent nor is it calculated differently based on a given MR system manufacturer.
Understanding the importance of B1+RMS as it pertains to MR Conditional labeling of active implants and devices necessitates an appreciation of basic MRI physics. When a patient enters the MR scanner, protons in the body align in the direction of the static magnetic field (B0). similar to a compass aligning with the Earth’s magnetic field. An MR imaging sequence is composed of a series of RF pulses that produce a magnetic field that interacts with these magnetically-aligned protons and rotates them through a specific angle commonly referred to as the “flip angle”. The RF magnetic field produced by the scanner is called the “B1” field of which only one part, known as the positively rotating or “+” component, is useful for “flipping” the magnetically-aligned protons and allows MR images to be created. The maximum 10-second, time-averaged B1+ field strength of the RF pulses in the imaging sequence is the root-mean-square or “RMS” B1+ value of the imaging sequence.
In 2013, the (International Electrotechnical Commission (IEC) mandated that all MR systems manufactured going forward must display the B1+RMS.Therefore, it is unlikely to see this B1+RMS information on older scanners or those with software that has not been updated.
B1+RMS in MR Conditional Labeling for Active Implants
Considering that B1+RMS is a more precise RF exposure metric than SAR, device manufacturers have begun to use values for B1+RMS that must not be exceeded when scanning patients with active implants. When following B1+RMS as a condition of use, the SAR value is irrelevant, unless, of course, a particular operating mode is specified. UsingB1+RMS tends to provide better “performance” for the MRI exam because it has fewer limitations vs. using SAR values for patients with implants and devices.
The parameters that an MR system operator uses to modify B1+RMS will vary with the particular MR system. By way of examples, parameters and options that can be adjusted to reduce B1+RMS include, the following:
- Increase the RF pulse duration
- Utilize a “Low SAR” mode of other similar option
- Increase the repetition time (TR) without reducing the number of slices
- Reduce the number of slices for a given TR
- Reduce the Echo Train Length (ETL)
- Reduce the refocusing angle (FSE sequences)
- Reduce the flip angle (e.g., for gradient echo pulse sequences)
- Use a GRE sequence instead of a spin echo or fast spin echo pulse sequence
Besides being a presumably more precise RF exposure metric than SAR, another advantage to using B1+RMS as opposed to SAR is that, once you adjust a particular sequence to a desired B1+RMS value, that information can be saved in your protocol library for future use. The B1+RMS will then remain at that value for the next patient unless the scan parameters are changed.
In summary, B1+RMS is being used by device manufactures in their MR Conditional labeling for active implants. Therefore, it is vital to understand B1+RMS and, if needed, how to adjust MRI protocols to achieve an acceptable B1+RMS value in order to ensure patient safety.
[Excerpted with permission from William Faulkner, B.S., R.T. (R)(MR)(CT), FSMRT, MRSO (MRSC), William Faulkner and Associates, LLC, www.t2star.com and Medtronic, Inc.]
Baker KB, et al. Evaluation of specific absorption rate as a dosimeter of MRI-related implant heating. J Magn Reson Imaging. 2004;20:315-20.
International Electrotechnical Commission. IEC 60601-2-33:2010+A11:2011 and Corrigendum 1:2012. Medical electrical equipment. Part 2-33: Particular requirements for the basic safety and essential performance of magnetic resonance equipment for medical diagnosis. 3rd. Geneva: International Electrotechnical Commission, 2010.