Multimodal validation of temporal interference in a 3D-printed pediatric head phantom

BACKGROUND: Transcranial temporal interference stimulation (tTIS) is a technique for generating electric-field envelopes within deep brain structures. Yet, its application in human subjects is limited, particularly in pediatric cohorts. OBJECTIVE: To evaluate the validity and accuracy of tTIS envelope reconstruction across beta, gamma, and ripple frequency bands in deep brain structures using a 3D-printed pediatric phantom. METHODS: A phantom resembling the conductivity properties of a 3-year-old child's head was implanted with dipoles at six deep-brain locations. Sinusoidal signals were delivered via two optimized scalp electrode pairs, using either a high-frequency carrier (HF-C; f₁ = 2-9 kHz; f₂ = 2.02-9.17 kHz) or a low-frequency carrier (LF-C; f₁ = 20 Hz; f₂ = 40-190 Hz) as a non-equivalent reference condition. Signals were recorded by an oscilloscope connected to dipoles. High-density electroencephalography was acquired as a validation tool for reconstructing stimulation-induced field patterns. Time-frequency analysis and permutation t-tests assessed differences between target and control dipoles within region of interest (ROI). RESULTS: HF-C showed smaller Euclidean distances (∼7-23 mm) between target and reconstructed maxima, along with more spatially confined envelope distributions across all targets and frequency bands, with better performance at 7-9 kHz carriers. The LF-C reference produced substantially larger deviations (∼50-64 mm). Permutation t-tests confirmed greater power in ROI compared with non-ROI regions (p < 0.05). CONCLUSION: HF-C produced more spatially confined reconstructed envelopes at higher kHz within targeted structures compared with lower kHz conditions. These results indicate improved spatial confinement of envelope reconstruction in the 7-9 kHz range within the phantom.