L-Coil vs. M-Coil: How the Two TMS Coil Designs Differ

The two most common TMS coil families—figure-of-8 and M-coil—differ in how they shape the magnetic field, and in how deep that field is designed to reach. A figure-of-8 coil concentrates its field into a small, focal area, roughly the size of a coin, while an M-coil design spreads the induced current across a broader region of tissue. Both designs are grounded in the same electromagnetic physics described in our overview of how a TMS coil works; the difference is purely one of field geometry.

This distinction matters for anyone trying to understand TMS equipment, because coil design is often described using language like "deep TMS," which can be misread as meaning the field reaches further into the brain. In physical terms, it doesn't—it reaches a broader area at a similar depth. Below is a plain look at the geometry, field shape, and design trade-offs of each coil type.

What does a figure-of-8 coil look like, and how does it shape the field?

The L-coil, a figure-of-8 coil, is built from two adjacent circular wire loops, wound so that current flows in opposite directions around each loop and meets at the center junction. Where the two loops intersect, the magnetic fields from each loop add together, creating a concentrated, focused field at that junction point. Away from the junction, the fields largely cancel each other out.

The practical result is a coil that produces a well-defined, focal field—concentrated over an area often compared to the size of a coin. This focality is a direct consequence of the winding geometry: two opposing loops meeting at a point naturally produce a peak in field strength right at that intersection, with a sharp drop-off elsewhere.

What does an M-coil look like, and how does it shape the field?

M-coil designs use a more complex winding pattern, often incorporating multiple loops arranged in layers or angled segments rather than two simple adjacent circles. Instead of concentrating current at a single junction point, the windings are arranged to distribute the induced field across a wider volume of tissue.

The result is a broader, more diffuse field pattern. Rather than a sharply focused hot spot, the M-coil produces a lower-intensity effect spread over a larger surface area. This is a deliberate geometric trade-off: spreading the same total field energy across more tissue naturally reduces the intensity at any single point while increasing the width of coverage.

What are the practical design trade-offs between focal and diffuse coils?

Choosing between a focal and a diffuse coil is fundamentally an engineering trade-off between precision and coverage, and each has genuine advantages depending on what a given design is trying to achieve.

A focal coil like the L-coil offers precision: because the field is concentrated in a small area, it lends itself to targeting a specific, well-defined region of the cortex, and works well alongside precise positioning tools like neuronavigation. A diffuse coil like an M-coil trades that precision for breadth: it can influence a wider area in a single placement, which may simplify coil positioning since exact targeting matters less when the field itself covers more ground.

Neither trade-off is inherently better in an absolute sense—it depends on the design goal. This is why some TMS systems, including Ampa One, offer both an L coil (a focal, figure-of-8 design) and an M coil (a diffuse design), allowing the coil geometry to be matched to the target rather than locking the system into a single field shape.

Coil Comparison at a Glance

Ampa One L-coil

Ampa One M-coil

Winding design

Two adjacent loops meeting at a center junction

Multiple loops or angled segments distributing current

Field shape

Focal, concentrated at the junction point

Diffuse, spread across a wider area

Coverage

Small, coin-sized area

Broader region of cortex

Effective depth

Roughly 1.5 cm

Roughly 2.5 cm

Design strength

Precision targeting

Wider single-placement coverage

Frequently Asked Questions

Is one coil design newer or more advanced than the other? Both designs are well-established coil geometries in TMS engineering, developed to serve different purposes. Neither is a newer version of the other; they represent parallel approaches to shaping a magnetic field, each suited to different targeting goals.

Can a single TMS system use both coil types? Yes. Because coil geometry is a separate engineering component from the stimulator and positioning system, a TMS platform can be designed to accept multiple coil types. Ampa One, for example, offers both an L coil and an M coil so the field shape can be matched to the target area.

Does a more focal coil mean a stronger pulse? Not necessarily. Focality refers to how concentrated the field is in a given area, not the peak strength of the pulse generated by the stimulator. A focal and a diffuse coil can be driven by pulses of similar strength; the difference is how that field energy is spread across tissue.

Why does "M-coil" sometimes get called "deep TMS"? That naming reflects the coil's ability to influence a broader area of tissue in a single placement, which has informally been described as "deep." As explained above, this refers to breadth of coverage rather than additional physical depth, since the underlying field-distance physics is the same for all coil types.

How does coil choice relate to neuronavigation? Coil focality and positioning technology are related but distinct considerations. A more focal coil generally benefits more from precise, repeatable targeting, which is why focal coils are often paired with camera-based or MRI-based neuronavigation systems. See our article on camera-based TMS targeting for more detail.

Does coil design affect how a treatment protocol like accelerated TMS is delivered? Coil geometry and protocol scheduling are separate design elements. Protocols like an accelerated pulse schedule govern timing and session frequency, while coil design governs field shape. Our article on accelerated TMS as a differentiator covers the protocol side of this distinction.