Body Habitus Dominates Solver Choice as a Source of Uncertainty in MRI Safety Assessment of Active Implantable Medical Devices

MRI is increasingly critical for patients with active implantable medical devices (AIMDs), yet access depends on safety labeling derived from computational heating predictions under ISO/TS 10974 Tier 3. Published assessments have relied predominantly on a single electromagnetic solver class and one or two standard-BMI reference anatomies, leaving the relative contributions of solver choice, tissue property uncertainty, and patient anatomy to predictive variability uncharacterized within a common workflow. We performed a cross-platform evaluation of finite-difference time-domain (FDTD, Sim4Life) and finite element method (FEM, ANSYS HFSS) implementations of the full Tier 3 workflow for a deep brain stimulation system at 1.5 T, extending the analysis across more than 250 clinically realistic trajectories spanning standard male and female references (Duke, HBM, Ella), an elderly male (Glenn), and elevated-BMI models of both sexes (Fats, Ella BMI 30). FDTD and FEM agreed closely in standard anatomies, with Maximum Allowable B1+ limits converging near 2.6-3.0 uT. The elderly male model produced a comparable limit to Duke, indicating BMI rather than age drives heating variability. Elevated BMI reduced safe B1+ by 19-31% in both sexes, while sex at matched BMI had no significant effect. Geometric morphing approximated the native obese limit, whereas dielectric property sweeps failed to reproduce elevated-BMI heating distributions. Body habitus is the dominant source of predictive uncertainty in Tier 3 assessment, exceeding solver choice, dielectric assumptions, and sex. Anatomical diversity, including elevated-BMI female phenotypes, should be treated as a primary variable.