Multichannel TMS Focality Is Getting Real About Hardware Limits
In the future of interventional psychiatry, “multichannel” or “multi-locus” TMS is framed less as a flashy upgrade and more as a workflow solution. Instead of physically moving a single coil to hit different cortical sites, multichannel TMS aims to steer the electric field electronically, shifting targets in milliseconds while the device stays in one placement. That promise matters because the practical bottleneck in many TMS paradigms is not whether we can stimulate a region. It is whether we can do it repeatedly, reliably, and with minimal targeting error when protocols demand multiple sites, rapid sequences, or tight timing across networks.
Where Standard TMS Still Wins And Where It Struggles
Single-channel figure-of-eight TMS can be quite focal in certain settings, especially when the target is superficial and coil placement can be optimized for that one spot. But when experiments or clinical concepts require moving across targets, repositioning introduces friction: time cost, operator variability, and cumulative localization error. The spatial profile of the induced field is also essentially fixed by coil geometry, which limits how much the shape of stimulation can be tuned beyond position and angle. In short, standard TMS is strong at one-target optimization and weaker at rapid multi-target workflows.
A Practical Definition Of Multichannel TMS Focality
The new study by Numssen and colleagues focuses on a deceptively simple question: if you keep a multichannel array in one realistic placement, how focal can it be when you try to stimulate many different cortical targets? Importantly, they define focality in an engineering-meets-neurophysiology way: deliver a target-strength electric field at the intended location while suppressing off-target field magnitude elsewhere. They also acknowledge a crucial reality check. Multichannel systems are constrained by maximum current rates, heating, and stimulator electronics. If an optimization ignores those limits, it can propose solutions that cannot be built or safely run.
Why The Modeling Choices Matter For Clinical Translation
Rather than relying on idealized heads, the authors used high-resolution finite-element head models derived from MRI data across nine individuals, then simulated fields across cortical targets. This matters because skull thickness, gyral folding, and scalp-to-cortex distance can meaningfully change the delivered field and the degree of off-target spread. In other words, focality is not a single number attached to a coil. It is a property that emerges from coil geometry plus the individual head.
Key Findings From The Focality Benchmarks
Across tested designs, the headline result is pragmatic: with one fixed placement, multichannel TMS achieved focality that was broadly comparable to repositioned single-channel TMS in the authors’ in-silico comparisons, especially when you evaluate focality using metrics that emphasize meaningful off-target overstimulation rather than tiny, widely distributed low-level fields. The nuances are clinically relevant. For superficial targets, optimized single-channel TMS placements tended to perform slightly better, which fits real-world intuition. But as targets became deeper, performance differences narrowed, and multichannel approaches looked more competitive. The study also found that adding channels can improve focality, but it pushes harder against current-rate constraints, meaning more channels is not a free lunch.
How The Optimization Works Without Becoming A Black Box
A key contribution is methodological. The team introduces an optimization approach that explicitly incorporates realistic current-rate constraints. Conceptually, the algorithm searches for the set of channel currents that preserves the required field magnitude at the target while minimizing the overall off-target field. They then evaluate outcomes using complementary quality metrics, including measures that capture whether the true maximum field occurs at the target rather than elsewhere and how much cortical area is stimulated above a meaningful fraction of target strength. This framing makes multichannel TMS focality actionable for developers and end users because it links focality to design constraints and use cases.
What Makes This Study Different For The Field
Many discussions of multi-locus TMS highlight electronic steering as the main innovation. This paper pushes the field toward standardization: compare systems under comparable constraints, quantify trade-offs with multiple metrics, and ground claims in individualized anatomy. That combination can help turn multichannel arrays from interesting prototypes into devices that can be evaluated, iterated, and eventually operationalized in rigorous research and potentially clinical workflows.
Clinical Implications And A Measured Outlook
If multichannel systems can reliably steer stimulation across targets without constant repositioning, the downstream implications are straightforward: faster mapping, tighter multi-site paradigms, and potential alignment with closed-loop strategies that depend on rapid retargeting. The more cautious implication is equally important. Performance depends on constraints such as current-rate limits and heating, as well as individual anatomy. Translation will likely require careful dose modeling, safety envelopes, and workflow integration with neuronavigation rather than assuming one-size-fits-all focality. The near-term story is not that multichannel TMS replaces standard coils tomorrow. It is that multichannel TMS focality is now being quantified in a way that makes engineering trade-offs visible, and that is exactly what a field needs before scaling from concept to platform.
Citations
Numssen O, Martin CW, Worbs T, et al. Optimizing and assessing multichannel TMS focality. Brain Stimulation (2026). https://www.brainstimjrnl.com/article/S1935-861X%2826%2900031-8/fulltext
Koponen LM, Nieminen JO, Ilmoniemi RJ. Multi-locus transcranial magnetic stimulation, theory and implementation. Brain Stimulation (2018). https://pubmed.ncbi.nlm.nih.gov/29627272/
Explore more at https://www.interventionalpsychiatry.org/