resting motor threshold in TMS

The resting motor threshold (RMT) in transcranial magnetic stimulation (TMS) is defined as the minimum intensity of stimulation required to elicit a motor-evoked potential (MEP) of at least 50 µV peak-to-peak amplitude in a target muscle (typically the abductor pollicis brevis for upper limb studies) in at least 50% of trials (e.g., 5 out of 10 pulses) while the muscle is at rest. RMT is expressed as a percentage of the maximum stimulator output (% MSO) and varies across individuals due to factors like skull-to-cortex distance, coil type, and neural excitability.

Below, I address your questions regarding RMT, Tesla measurements, amperage, double-cone coil use for the left cerebellar hemisphere (lobule VII/Crus I), and portable TMS devices.

1. Understanding RMT and Converting 80-90% of RMT to Tesla

RMT is not directly measured in Tesla (the unit of magnetic field strength) but as a percentage of the stimulator’s maximum output. The magnetic field strength (in Tesla) depends on the TMS device, coil design, and stimulator settings. To estimate the Tesla value for 80-90% of RMT:

Typical TMS Device Output: A standard TMS device with a double-cone coil (e.g., Magstim or MagVenture) produces a magnetic field of approximately 1.4–2 Tesla at the coil surface at 100% MSO. In the cortex (1.5–2 cm below the scalp), this drops to about 0.5–0.8 Tesla due to field attenuation.
RMT Variability: RMT for a double-cone coil targeting the motor cortex is typically lower than for a figure-of-eight coil due to its ability to stimulate deeper structures. Studies report RMTs of ~40–60% MSO for the double-cone coil when stimulating the hand area (e.g., first dorsal interosseous muscle). For cerebellar stimulation, RMT may be higher due to the greater scalp-to-target distance (30–35 mm for lobule VII/Crus I).
Estimating Tesla for 80-90% RMT:
- Assume an RMT of 50% MSO for a double-cone coil (a reasonable average based on studies). At 100% MSO, the coil produces ~1.4 T at the surface and ~0.5 T in the cortex.
- At 50% MSO (RMT), the magnetic field is roughly half: ~0.7 T at the surface and ~0.25 T in the cortex.
- For 80–90% of RMT:
  - 80% of 50% MSO = 40% MSO → ~0.56 T (surface), ~0.2 T (cortex).
  - 90% of 50% MSO = 45% MSO → ~0.63 T (surface), ~0.225 T (cortex).
- These are rough estimates, as the exact magnetic field depends on the coil’s inductance, pulse waveform (monophasic or biphasic), and tissue properties. For cerebellar stimulation, the field strength at the target (3–3.5 cm deep) may be further reduced (~0.1–0.15 T), requiring higher % MSO.

Note: The exact RMT must be determined empirically for each individual by titrating the stimulator intensity to elicit MEPs. For cerebellar stimulation, RMT is often based on motor cortex stimulation (e.g., hand area) and adjusted for the cerebellar target, as direct MEP elicitation from the cerebellum is less common.

2. Amperage Supplied to the Coil

The amperage required to drive a TMS coil depends on the stimulator’s capacitor voltage, coil inductance, and desired magnetic field strength. Here’s an overview:

Typical Amperage: TMS devices deliver peak currents of 5,000–10,000 amperes (5–10 kA) to the coil to generate a 1.4–2 T magnetic field. For example, a MagVenture system with a double-cone coil may require ~7 kA at 100% MSO, driven by a capacitor charged to ~2 kV.
Amperage for 80–90% RMT:
- If RMT is 50% MSO, 80–90% of RMT corresponds to 40–45% MSO.
- Assuming a linear relationship between % MSO and current (a simplification), 40–45% MSO requires ~2,000–3,150 A (2–3.15 kA) for a coil producing 1.4 T at 7 kA.
- For cerebellar stimulation, higher intensities (e.g., 60–80% MSO) may be needed due to the deeper target, corresponding to ~4,200–5,600 A.
Coil-Specific Considerations: Double-cone coils have higher inductance than figure-of-eight coils due to their angled design, requiring more current to achieve the same magnetic field. The exact amperage depends on the coil’s specifications (e.g., number of windings, resistance). Manufacturers like MagVenture or Deymed provide coil-specific data sheets, but this information is rarely user-adjustable, as the stimulator controls current delivery.

Practical Note: TMS devices are calibrated to deliver the required current automatically based on the % MSO setting. Users do not manually set amperage; instead, they adjust the stimulator output (% MSO) to achieve the desired intensity (e.g., 80–90% RMT). For precise amperage, you’d need to consult the device’s technical manual or contact the manufacturer.

3. Double-Cone Coil for Left Cerebellar Hemisphere (Lobule VII/Crus I)

The double-cone coil is well-suited for cerebellar stimulation due to its ability to penetrate deeper brain structures (up to 3–4 cm) compared to figure-of-eight coils. Here’s how it applies to targeting the left cerebellar hemisphere (lobule VII/Crus I):

Anatomical Considerations:
- Lobule VII/Crus I is located in the lateral posterior cerebellum, approximately 30–35 mm from the scalp.
- The scalp-to-cerebellar cortex distance is ~1.5 times greater than the scalp-to-motor cortex distance, requiring higher stimulation intensities.
- The left cerebellar hemisphere is targeted by placing the coil over the left posterior scalp, typically 3 cm lateral and 1 cm inferior to the inion (based on cerebellar-brain inhibition [CBI] protocols).
Coil Positioning:
- The double-cone coil should be angled to conform to the head’s curvature, with the coil’s center over the target region. Neuronavigation (e.g., using MRI-guided systems) is recommended for precise targeting of lobule VII/Crus I.
- The induced current should flow in a postero-anterior direction to optimize stimulation efficacy.
Stimulation Parameters:
- Intensity: Studies suggest intensities of 60–80% MSO for cerebellar stimulation with a double-cone coil, often referenced to the motor cortex RMT. For lobule VII/Crus I, intensities may need to be higher (e.g., 70–90% MSO) due to depth, but should not exceed safety limits (e.g., 60% MSO for double-cone coils in some protocols to avoid discomfort or seizures).
- Pulse Type: Biphasic pulses are commonly used for cerebellar stimulation, as they are more efficient for deeper targets.
- Frequency: For cerebellar studies, single-pulse or low-frequency (e.g., 1 Hz) repetitive TMS (rTMS) is often used to assess CBI or modulate connectivity. High-frequency rTMS (>5 Hz) may be used for therapeutic purposes but requires caution.
Effectiveness: The double-cone coil (e.g., Deymed’s model) is more effective than figure-of-eight or batwing coils for cerebellar stimulation, eliciting CBI at tolerable intensities. However, discomfort is common due to scalp muscle activation, and intensities above 60% MSO may exceed moderate discomfort levels (3.5/7 on a discomfort scale).

Safety Considerations:

Cerebellar stimulation can cause discomfort, dizziness, or, rarely, vasovagal syncope.
Safety guidelines recommend limiting double-cone coil intensity to 60% MSO for leg or cerebellar stimulation to minimize risks like seizures.
Ensure participants meet TMS safety criteria (e.g., no metal implants, no history of seizures).

4. Portable TMS Devices Available on the Market

Traditional TMS devices (e.g., Magstim, MagVenture) are large, non-portable systems requiring high-voltage power supplies and cooling systems. However, portable or compact TMS devices are emerging, though their availability for cerebellar stimulation with a double-cone coil is limited. Here’s an overview:

Challenges for Portability:
- Double-cone coils require high peak currents (5–10 kA) and voltages (~2 kV) to generate sufficient magnetic fields, which typically necessitates bulky capacitors and power supplies.
- Cerebellar stimulation requires precise targeting and higher intensities, making portable devices less practical unless paired with neuronavigation.
Available Portable/Compact Devices:
- MagVenture MagPro Compact:
  - A smaller version of MagVenture’s TMS system, designed for research.
  - Supports double-cone coils and biphasic pulses.
  - While more compact than full-size systems, it still requires a stable power source and is not truly “portable” (e.g., not battery-operated).
  - Not explicitly marketed for cerebellar stimulation but can be adapted with appropriate coils.
  - Availability: Available through MagVenture distributors; contact for pricing and specifications (https://www.magventure.com/).[](https://pmc.ncbi.nlm.nih.gov/articles/PMC4737243/)
- Deymed DuoMAG XT:
  - A compact TMS system supporting double-cone coils, noted for effective cerebellar stimulation.
  - Offers biphasic pulses and is more portable than traditional systems, though still requires a power outlet.
  - Suitable for research settings; may support cerebellar protocols with neuronavigation.
  - Availability: Available through Deymed (https://www.deymed.com/); contact for details.
- NeuraLite (Investigational):
  - A newer, portable TMS device under development for home or clinical use, focusing on low-intensity stimulation.
  - Limited information on coil types; unlikely to support double-cone coils or high-intensity cerebellar stimulation.
  - Availability: Not widely available; primarily for research or clinical trials.
- Low-Intensity Devices (e.g., NeuroStar Portable):
  - Some companies are developing battery-powered, low-intensity TMS devices for depression or pain management.
  - These typically use smaller coils (not double-cone) and produce weaker fields (<0.5 T), insufficient for cerebellar stimulation.
  - Availability: Limited to specific indications; check with manufacturers like Neuronetics (https://neurostar.com/).
Limitations:
- No fully portable, battery-operated TMS device currently supports double-cone coils for high-intensity cerebellar stimulation due to power requirements.
- Portable devices are often designed for superficial cortical stimulation (e.g., prefrontal cortex) and may not reach the 3–3.5 cm depth needed for lobule VII/Crus I.
- Neuronavigation, critical for precise cerebellar targeting, is typically separate and non-portable, further limiting field use.

Recommendation: For cerebellar stimulation, consider compact systems like the Deymed DuoMAG XT or MagVenture MagPro Compact with a double-cone coil. These are the closest to “portable” while supporting the required intensity. For true portability, you may need to wait for advancements in low-power, high-efficiency TMS technology. Contact manufacturers directly for availability, as these devices are often sold through specialized distributors.

5. Practical Steps for Your Setup

To implement TMS with a double-cone coil targeting the left cerebellar hemisphere (lobule VII/Crus I):

Determine RMT:
- Use the motor cortex (e.g., left M1 hand area) to establish RMT by eliciting MEPs in the right abductor pollicis brevis. Start at 30% MSO and increase until 50 µV MEPs are elicited in 5/10 trials.
- Expect RMT to be ~40–60% MSO with a double-cone coil.
Adjust for Cerebellar Stimulation:
- Position the double-cone coil 3 cm left and 1 cm inferior to the inion, angled to conform to the scalp. Use neuronavigation if available to target lobule VII/Crus I.
- Set intensity to 80–90% of motor cortex RMT (e.g., 32–54% MSO if RMT is 40–60% MSO) or titrate based on CBI elicitation. Avoid exceeding 60% MSO for safety.
Estimate Magnetic Field and Current:
- At 80–90% RMT, expect a magnetic field of ~0.56–0.63 T at the coil surface and ~0.1–0.15 T at the cerebellar target.
- The stimulator will deliver ~2–5 kA to the coil, depending on the % MSO and coil inductance.
Select Equipment:
- Use a TMS system supporting double-cone coils (e.g., MagVenture, Deymed).
- Consider compact systems like Deymed DuoMAG XT for semi-portable use.
- Pair with neuronavigation (e.g., Brainsight) for precision.
Safety and Monitoring:
- Screen participants for TMS contraindications.
- Monitor for discomfort, dizziness, or syncope during cerebellar stimulation.
- Use ear protection due to the loud clicking of the coil.

6. Limitations and Future Directions

Tesla and Amperage Precision: Exact Tesla and amperage values require device-specific calibration, as coil inductance and tissue properties vary. Manufacturers may provide simulation tools or technical data for precise calculations.
Cerebellar RMT: RMT is typically defined for motor cortex, not cerebellum. Cerebellar stimulation studies often use motor cortex RMT as a reference, which may underestimate the intensity needed for lobule VII/Crus I.
Portable Devices: The field is moving toward portable TMS, but current options are limited for deep brain stimulation. Emerging technologies (e.g., miniaturized capacitors, wireless power) may enable portable cerebellar TMS in the future.
Critical Perspective: While TMS is FDA-approved for depression and OCD, cerebellar stimulation is largely experimental. Claims of deep brain stimulation efficacy should be scrutinized, as field strength drops significantly with depth, and non-specific scalp effects may confound results.

Summary

80–90% RMT in Tesla: For a double-cone coil with RMT at 50% MSO, 80–90% RMT corresponds to ~0.56–0.63 T at the coil surface and ~0.1–0.15 T at the cerebellar target (lobule VII/Crus I).
Amperage: Expect ~2,000–5,600 A, depending on the stimulator and intensity (40–80% MSO). Exact values require device-specific data.
Double-Cone Coil for Cerebellum: Effective for lobule VII/Crus I at 60–80% MSO, positioned 3 cm left and 1 cm inferior to the inion. Use neuronavigation for precision.
Portable Devices: Compact systems like Deymed DuoMAG XT or MagVenture MagPro Compact are the closest to portable but require power outlets. Fully portable devices for cerebellar stimulation are not yet available.
Next Steps: Contact MagVenture or Deymed for device specifications, and consider neuronavigation for accurate targeting.

For further details, consult manufacturer technical manuals or recent reviews on cerebellar TMS (e.g., Hardwick et al., 2014). If you need assistance with specific device recommendations or protocols, please provide more details about your setup or research goals

usametov/rmt.md