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Purpose: Radiofrequency (RF) exposure during MR examination is limited by IEC 60601-2-33 to prevent thermal hazards to patients. These limits are also the basis to derive the maximum induced field for the demonstration of MR safety of implants per ISO/TS 10974 (2018). One limit is the head-averaged specific absorption rate (SAR), for which the head extent is defined differently by MR and implant vendors. The purpose of this technical note is to inform MR safety stakeholders on the sensitivity of safety evaluations due to different head extent definitions., Methods: RF distributions from the validated MRIxViP exposure libraries of 12 high-resolution human anatomical models were scaled to the normative SAR limits for different definitions of the head extent to compare the corresponding induced SAR and electric (E-)field levels., Results: The definitions of the head extent used by major implant vendors and defined in ISO/TS 10974 (2018) are larger than those introduced in IEC 60601-2-33 (2022), resulting in lower RF head exposure by up to 2.4 dB (factor 1.7). Other proposed definitions of the head result in intermediate values., Conclusion: The different head extents result in different maximum RF exposures affecting the risk assessment by up to a factor of 1.7. The results of this study can be used to estimate the additional uncertainty in safety assessments. Future revisions of MR standards should eliminate this inconsistency., (© 2023 The Authors. Magnetic Resonance in Medicine published by Wiley Periodicals LLC on behalf of International Society for Magnetic Resonance in Medicine.)