Implant Heating: Measurement 

How does one measure or estimate MRI-related implant heating?  
Several international safety organizations and government agencies have published relevant standards and methods for estimating and measuring implant-related heating in the MR environment. The three most important are the American Society of Testing and Materials International (ASTM), the International Electrotechnical Commission (IEC), and the International Standards Organization (ISO). ​
American Society of Testing and Materials International (F2182)
​ASTM standard F2182 is restricted to RF-heating of wholly-implanted passive devices in cylindrical magnets at 1.5T and 3.0T. A procedure is described using a saline-based gel-filled phantom of approximately the same dimensions of the body part where the implant is located. The gel is doped to have electrical and thermal properties similar to human tissue. Temperature change in and around the implant is recorded following 15 minutes of continuous RF exposure to an SAR of at least 2 W/kg averaged over the volume of the phantom. 
pacemaker heating test
A) Gel-filled torso phantom for evaluating heating of a pacemaker and wire (arrows). A black temperature probe is attached to the tip of the pacemaker lead. B) Heat map assessed by infrared camera after RF excitation showing temperature increase at the lead tip. (From Winter et al under CC-BY.)
International Electrotechnical Commission (IEC 60601-2-33)
IEC 60601-2-33 is a 100+ page document dealing with multiple aspects of MRI safety and performance, of which implant heating is only a small part. One of the important advances of this standard with regard to implants is the recommended use of B1+rms over SAR. The term B1+rms refers to the "root mean square" value of B1+, the RF-field rotating in the same direction as magnetic precession and responsible for tipping the nuclear magnetization. The advantage of using B1+rms over SAR is that B1+rms can be directly calculated by measuring transmitter voltages producing the incident RF irradiation. More details about B1+rms ​can be found in this associated Q&A.
Another component of the IEC standard is the provision for a Fixed Parameter Option (FBO) by which manufacturers may certify their devices as MR conditional under a restricted set of RF and gradient field outputs. 
International Standards Organization (ISO/TS 10974)
This is the most comprehensive approach to implant heating issues, although the current standard covers only active devices in cylindrical 1.5T scanners (an updated standard is expected by 2021-2). A four-tiered approach is taken, each level progressively resulting in less restrictive conditions for imaging for the implant but requiring a more complex analysis.  In brief, the 4 tiers are
  1. RF heating measurements made in phantoms for worst case scenarios. No modeling required.
  2. Basic human body electromagnetic modeling to predict locations of the highest electric fields to which the implant might be exposed.
  3. Modeling of both the human body and the implant, with calculation of electrical fields tangential to the device.
  4. Concurrent human and implant modeling.
In general, Tier 1 and 2 assessments are usually done for small compact devices, such as cochlear implants, pressure sensors, implanted monitors and leadless pacemakers. Tier 3 is required for larger and more complex devices, while Tier 4 is only infrequently performed due to its complexity. All mathematical simulations must be verified by actual measurements in a tissue-mimicking phantom similar to that pictured above and described in ASTM F2182. 
In addition to RF-heating, ISO/TS 10974 describes similar tiered procedures for evaluating gradient-induced implant heating. Methods for measuring gradient-induced vibration of implants are also included.

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References
     Al-Dayeh L, Rahman M, Venook R. Practical aspects of MR imaging safety test methods for MR conditional active implantable medical devices. Magn Reson Imaging Clinic N Am 2020: 28:559-571.  [DOI Link]
     ASTM F2182-19e2, Standard Test Method for Measurement of Radio Frequency Induced Heating On or Near Passive Implants During Magnetic Resonance Imaging, ASTM International, West Conshohocken, PA, 2019, www.astm.org
     International Electrotechnical Commission. IEC 60601-2-33:2010: Medical Electrical Equipment - Part 2-33: Particular Requirements for the Basic Safety and Essential Performance of Magnetic Resonance Equipment for Medical Diagnosis. 3rd ed. with amendments. International Electrotechnical Commission; 2015. (accessed September 2020)
     ISO/TS 10974:2018. Assessment of the safety of magnetic resonance imaging for patients with an active implantable medical device (2nd Ed). International Standards Organization, Geneva, 2018.
     Nordbeck P, Ertl G, Ritter O. Magnetic resonance imaging safety in pacemaker and implantable cardioverter defibrillator patients: how far have we come? Eur Heart J 2015; 36:1505-1511. [DOI Link]
     Winter L, Seifert F, Zilberti L, et al. MRI-related heating of implants and devices: a review. J Magn Reson Imaging 2021; 53:1646-1665 [DOI Link]
Related Questions
     What causes implants to heat up during MR imaging?
     What is B1+ rms? Is it a better metric for energy deposition than SAR? 
     How can tissue temperature be measured using MRI?