Written and maintained by CASRAI Editorial Board
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General device information from regulator and manufacturer sources, not medical advice. Decisions about treatment belong to a patient and a qualified clinician.
Short answer
A TMS coil is the coil in a transcranial magnetic stimulation system that turns a brief electrical current into a pulsed magnetic field aimed at the head. In the United States the coil is treated as one component of a repetitive transcranial magnetic stimulation (rTMS) system classified under 21 CFR 882.5805 as a Class II device. The two coil designs people search for most are the figure-8 coil and the H-coil.
Definition and scope
21 CFR 882.5805 describes a repetitive transcranial magnetic stimulation system as an external device that produces pulsed magnetic fields to induce electrical currents in the brain, names major depressive disorder as its indication, and requires the system to be designed to avoid inducing seizures. FDA’s product classification entry for the same device (product code OBP, named “Transcranial magnetic stimulator”) lists it as Class 2 and not eligible for third-party review. The coil is the part that shapes where the field goes; the stimulator, the cabling, the positioning arm and the software make up the rest of the regulated system. This page covers coil design and how makers document it, not how a treatment is delivered.
How the coil is used in practice
FDA’s classification entry says the magnetic pulses induce electrical currents in the cerebral cortex; the coil is the part positioned against the head that delivers them. FDA’s special controls guidance for rTMS systems makes a point that explains why coil geometry matters at all: the magnetic field decreases quickly with distance from the face of the treatment coil, so any loss of contact or change in position can change how much stimulation reaches the target. For that reason the guidance asks manufacturers to describe design features such as sensors and alarms that confirm proper contact and position.
Figure-8 coil vs H-coil
Coil shape is the main design variable that search results discuss. Both designs sit inside the same regulatory framework; the table below separates what a regulator states from what a manufacturer reports.
| Item | Figure-8 coil | H-coil | Source type |
|---|---|---|---|
| Regulatory home | Component of an rTMS system under 21 CFR 882.5805 | Same | FDA / eCFR |
| Reported field shape | Used by the manufacturer as the comparison point (the H-coil is called broader and deeper than it) | Described by one manufacturer as a broader, deeper field than the figure-8 coil | Manufacturer (BrainsWay knowledge center) |
| Strength of the comparison | Cross-trial comparison written by a manufacturer; its authors call for head-to-head studies | Manufacturer | |
Note the limit of that evidence. The claim that an H-coil reaches deeper comes from a vendor and compares results across separate trials, so treat it as a manufacturer-reported design difference, not an established ranking of designs. Whether a particular coil is cleared for a particular indication has to be read from that device’s own FDA clearance record.
How FDA expects the coil field to be characterised
The 2011 FDA guidance “Class II Special Controls Guidance Document: Repetitive Transcranial Magnetic Stimulation System” (issued July 26, 2011) is the special control named in 21 CFR 882.5805(b). It asks submitters to characterise the pulse (shape, timing, width and amplitude), the spatial distribution of the field, output linearity, and the relationship between scalp and brain field change. Its summary table asks for the peak magnetic field at 2 cm from the coil, the maximum rate of field change (dB/dt), and the coil surface temperature at maximum output. It also asks for the physical unit of the amplitude setting and how it relates to the output unit the guidance defines (the SMT).
Questions to ask a manufacturer
This checklist is derived from the items FDA asks makers to characterise. It is a documentation checklist, not a purchasing recommendation.
- What is the peak magnetic field at 2 cm from the coil face, and at what output setting?
- What is the maximum dB/dt, and how was the spatial field distribution measured?
- What is the coil surface temperature at maximum output?
- How does the device detect loss of contact or position change (sensors, alarms)?
- Which FDA clearance or De Novo record covers this exact system, and which indications does it list?
For background on how such records are structured, see what a medical device is, FDA De Novo classification and the eSTAR submission template for 510(k) and De Novo.
Where it matters for labs, clinics and supply decisions
For staff who document or maintain equipment, the practical consequence of the regulatory framing is that a coil is evaluated as part of a named system. Records, labeling and clearance checks follow the system, so a coil is not interchangeable across systems just because the shapes look alike. Related neurostimulation devices have their own pages, including the bone stimulator and the cranial electrotherapy stimulation device.
Common misunderstandings
The coil is the whole device
The regulation describes a system. The coil shapes the field but depends on the stimulator and the positioning and monitoring features around it.
Deeper means better
Depth and breadth are design characteristics. A manufacturer’s report that one coil design reaches deeper does not by itself establish better outcomes, and the cited comparison is across separate trials.
Any TMS coil is cleared for any condition
The classification regulation names major depressive disorder as the indication. Other uses depend on each device’s own clearance record.
FAQ
What is a figure-8 coil?
It is the conventional TMS coil shape named in manufacturer literature as the comparison point for the H-coil, which one manufacturer says induces a broader and deeper field.
What is an H-coil in TMS?
It is a coil design that one manufacturer reports produces a broader, deeper field than a figure-8 coil. The comparison is vendor-authored and across separate trials.
Is a TMS coil an FDA-regulated device on its own?
The regulation, 21 CFR 882.5805, covers the rTMS system. The coil is a component of that Class II system.
Sources
- eCFR, 21 CFR 882.5805, Repetitive transcranial magnetic stimulation system: ecfr.gov (checked 2026-10-31).
- FDA Product Classification, Transcranial magnetic stimulator (OBP): accessdata.fda.gov (checked 2026-10-31).
- FDA, Repetitive Transcranial Magnetic Stimulation (rTMS) Systems, Class II Special Controls Guidance: fda.gov (checked 2026-10-31).
- BrainsWay knowledge center, coil design and neuroanatomical variability (manufacturer source): brainsway.com (checked 2026-10-31).
