Efficient and Reliable Assessment of the Maximum Local Tissue Temperature Increase at the Electrodes of Medical Implants under MRI Exposure

Aiping Yao, Earl Zastrow, Esra Neufeld, and Niels Kuster, Bioelectromagnetics, Volume 40, Issue 6, September 2019, pp. 422–433, online 19 July 2019; doi: 10.1002/bem.22208

Standard risk evaluations posed by medical implants during magnetic resonance imaging (MRI) includes (i) the assessment of the total local electromagnetic (EM) power (P) absorbed in the vicinity of the electrodes and (ii) the translation of P into a local in vivo tissue temperature increase ∆T (P2∆T) in animal experiments or simulations. We investigated the implant/tissue modeling requirements and associated uncertainties by applying full‐wave EM and linear bioheat solvers to different implant models, incident field conditions, electrode configurations, and tissue models. Results show that the magnitude of the power is predominately determined by the lead, while the power distribution, and the P2∆T conversion, is determined by the electrode and surrounding tissues. P2∆T is strongly dependent on the size of the electrode, tissue type in contact with the electrode, and tissue inhomogeneity (factor of >2 each) but less on the modeling of the lead (<±10%) and incident field distribution along the lead (<±20%). This was confirmed by means of full‐wave simulations performed with detailed high‐resolution anatomical phantoms exposed to two commonly used MRI clinical scenarios (64 and 128 MHz), resulting in differences of less than 6%. For the determination of P2∆T, only the electrode and surrounding tissues must be modeled in great detail, whereas the lead can be modeled as a computationally efficient simplified structure exposed to a uniform field. The separate assessments of lead and electrode reduce the overall computational effort by several orders of magnitude. The errors introduced by this simplification can be taken into consideration as part of the uncertainty budget.

The scientific and technical impact of the study can be summarized as:

  • A parameter study was performed to identify the minimal modeling requirement for reliable assessment of P2∆T at the electrodes
  • The results demonstrate that the lead can be greatly simplified whereas the electrodes and the local tissues must be modeled and evaluated in great detail
  • Evaluations in homogeneous tissue simulating materials may greatly underestimate the maximum temperature increase in vivo