The harmonic residual index: a spectral purity metric for magnetic hyperthermia RF generators

Abstract

RF generators are core components of magnetic hyperthermia (MH) systems, yet output quality is rarely characterized beyond nominal frequency and amplitude. Harmonic distortion can introduce parasitic components that complicate reproducibility and accurate field characterization; MH-specific spectral quality indices remain limited. This study proposes two indices for RF generator output quality assessment in MH: the harmonic residual index ( HRI) and a simulation-derived harmonic specific absorption rate (SAR) contribution index ( Lambda & strns;SAR). A sinusoidal RF generator (50-200 kHz, 115 W) was evaluated with three coil configurations at 10%-100% power. Spatial magnetic-field measurements were acquired using a passive search coil, and HRI was computed pointwise to generate harmonic distortion maps. THD was computed from the same spectra for benchmarking. Spatial mean HRI ( HRI & horbar;) and THD ( THD & horbar;) were modelled using weighted power-law fitting. Inductive heating experiments (120-166.5 mg ml-1 MNP) evaluated the reliability of experimental calorimetry, motivating a shift toward simulation-based dosimetry. Finite element method (FEM) simulations of a simplified neck model driven by measured harmonics estimated relative harmonic SAR contributions and Lambda & strns;SAR. HRI & horbar; ranged from 0.17 +/- 0.04 to 16.41 +/- 3.48 (RMSE = 0.301-1.523) and increased monotonically with power level across all coils, while THD showed smaller variations and less spatially structured behaviour. Experimental heating trials yielded marginal temperature elevations (<= 2.5 degrees C) with quantization errors, demonstrating that standard calorimetry is metrologically insufficient for SAR derivation at clinical field intensities. Conversely Lambda & strns;SAR ranged from 2.23E + 4 to 2.70E + 6, indicating low harmonic contributions relative to the fundamental under the tested conditions. HRI enables spatially resolved quantification of RF generator spectral impurities in MH systems and provides greater spatial sensitivity than THD under the tested conditions. Combined with FEM screening of harmonic SAR contributions, the framework supports reproducible, standardized characterization of RF generator output quality for MH instrumentation quality assurance.