For Clinicians
TMS Information for Clinicians
Transcranial magnetic stimulation (TMS) is a non-invasive neuromodulation treatment that uses a magnetic field to induce neuronal depolarization in targeted cortical regions. Depending on the anatomic target and stimulation parameters, the net effect may be stimulatory or inhibitory.
How does a TMS machine work?
A TMS machine sends an electrical current through a wire coil, generating a magnetic field. When the coil is placed next to the patient’s head over a selected cortical target, the magnetic field induces electrical effects in nearby neurons, leading to depolarization.
Clinical effect depends on target selection and machine settings.
What part of the brain is targeted?
Different TMS targets are associated with different clinical effects. One of the most common targets in depression treatment is the left dorsolateral prefrontal cortex (DLPFC).
In OCD, common deep TMS targets include the anterior cingulate cortex (ACC) and medial prefrontal cortex (mPFC). These deeper targets require a broader magnetic field.
What is motor threshold (MT)?
Motor threshold is a scalar measure, expressed as a percentage, of the magnetic field strength required to elicit a motor evoked potential, typically by stimulating the motor cortex and observing contralateral thumb movement.
Most TMS protocols aim for a treatment intensity of approximately 110–120% of motor threshold. The SAINT protocol uses 90% MT.
How do we target specific brain areas?
5.5 cm method
The motor strip is identified by producing isolated movement in the contralateral thumb. The coil is then moved 5.5 cm anteriorly to estimate the prefrontal target.
Beam method
Uses tragus-to-tragus, nasion-to-inion, and head circumference measurements to calculate DLPFC location.
10–20 method
EEG-based localization. More labor intensive and more often used in research settings.
Anatomic MRI targeting
Uses MRI with stereotactic guidance to identify a target based on anatomy.
Functional MRI targeting
Based on network models and connectivity patterns, including DLPFC–subgenual cingulate relationships. This is the targeting approach used in the SAINT protocol.
What are the different types of coils?
- Figure-eight coil
- H-coil
- Double cone coil
One of the main practical tradeoffs is depth versus focality. A larger magnetic field, such as with the H-coil, may allow deeper penetration but generally with reduced target specificity.
What are the different types of TMS?
rTMS
Repetitive TMS, or rTMS, is the form of TMS most commonly used in clinical practice. It delivers repeated pulse trains at a specified intensity, frequency, and duration, followed by intertrain rest periods.
High-frequency left DLPFC rTMS
High-frequency stimulation, often referred to as “standard TMS,” is typically delivered over the left DLPFC and is generally considered stimulatory.
- 10 Hz
- 100–120% MT
- 4-second trains
- Approximately 3000 pulses per session
- Often delivered over 75 trains
Low-frequency right DLPFC rTMS
Low-frequency stimulation, typically 1 Hz or less, is often delivered over the right DLPFC and is generally considered inhibitory.
- 900–1200 pulses per session
- Approximately 15–20 minutes
Deep TMS
Deep TMS uses an H-coil rather than a figure-eight coil, allowing stimulation of deeper structures at the cost of reduced focality.
Practical summary
From a clinician perspective, the key variables in TMS are:
- cortical target
- coil type
- motor threshold
- stimulation frequency
- treatment intensity
- pulse count
- targeting method
Referral / Contact
If you would like to discuss whether a patient may be appropriate for TMS, please contact our office.