B1+rms for Implants: A Review of Intended Use

The B1+rms metric, requested by Active Implantable Medical Device (AIMD) manufacturers, characterizes MR equipment radiofrequency (RF) electromagnetic field outputs. Specific Absorption Rate (SAR) is the physiologically appropriate power deposition metric for patient heating considerations, but B1+rms is better suited for implant purposes because it more directly assesses AIMD/RF exposure interactions in conductive implants. The formal SAR definitions only specify limits, not how to compute SAR, thus each MR equipment make, model and software version may compute SAR differently via proprietary algorithms. Since a wide range of SAR occurs for a given B1+ exposure condition, SAR estimates are larger than the actual exposure. Consequently, the SAR safety margin size that fulfills limit requirements is unknown to AIMD manufacturers and MR equipment users. By design, the B1+rms metric has no limit or safety margin, but is precisely defined to enable transparency to all B1+rms users. Because B1+rms enables a more precise assessment of AIMD/RF exposure interactions, more RF performance becomes available for MR Conditional AIMD purposes. The AIMD manufacturer is responsible to assess all relevant uncertainties, set appropriate safety margins, and provide device-specific limits. These tasks are simplified with B1+rms. Additionally, since MR image quality requires control of B1+ outputs, the clinical workflow of AIMD scanning can be improved with B1+rms labelling whereas the SAR variability between patients undergoing the same B1+ exposure may require unique sequence adjustments to comply with MR Conditional SAR constraints, impacting image quality. However, confusion remains regarding B1+rms computation, application, and why it is an independently sufficient and superior metric for MR Conditional labeling purposes. This review compares the SAR and B1+rms metrics, their suitability for AIMD labeling, describes how AIMD manufacturers use B1+rms, and clarifies various B1+rms concerns. Reasons why B1+rms is a superior RF measure for MR Conditional labeling purposes are identified. EVIDENCE LEVEL: 5. TECHNICAL EFFICACY: Stage 1.