For questions regarding this document contact Ann H. Costello Ph.D., D.M.D . at 301-796-6493 or by email at


U.S. Department of Health and Human Services

Food and Drug Administration

Center for Devices and Radiological Health

Office of Device Evaluation

Division of Ophthalmic, Neurological and Ear, Nose and Throat Devices

Neurodiagnostic and Neurotherapeutic Devices Branch


For the purposes of this guidance document, the following definitions and recommendations for measurement are used:

Frequency: The number of pulses per second expressed in Hertz (Hz).

Magnetic Field Strength: Magnetic field strength is expressed as B, the magnetic flux density, in units of Tesla. The time rate of change of B determines the current density level induced in the cortex; therefore, dB/dt is used to express the strength of the field induced by the magnetic field at a given point. The units for magnetic field flux (i.e., dB/dt) are Tesla/second. Magnetic field strength is determined by measuring the voltage induced across a small pickup loop placed at the location of interest.

Motor Threshold (MT)/MT Intensity: The motor threshold level is the minimum stimulator setting, in Standard Motor Threshold (SMT) units, that induces an observable motor response by the patient in 50% of the applied pulses, usually as observed by movement of the thumb. “MT level” is determined with the rTMS treatment coil positioned over a specific location within the motor strip, called the motor threshold location (MT location).10 The MT location may be used as an anatomic reference point for navigating the coil to the rTMS treatment location. The MT level is used as a reference point for setting the rTMS treatment intensity, usually expressed as a percent multiple of the MT level, e.g. 120% MT.

Pulse Width: When your rTMS output is a damped sinusoidal wave, the pulse width is defined as the time (duration) from the initial peak to next peak of the wave; it is also described as the period of the sinusoidal wave. For monophasic pulse shapes, we recommend reporting pulse width as the time between the rising phase and falling phase of the wave measured at a standard amplitude (e.g., 10% of total amplitude). The pick-up loop for making this measurement should be located at the same distance from the coil as the target tissue in the brain.

SMT Unit: So that measurement of stimulator magnetic field output may be standardized, the SMT unit is suggested. 1.0 SMT is the output setting of a rTMS device that corresponds to an induced electric field of 130V/m at a point located at the fixed distance of the target along the central axis of the coil from the surface of the scalp into the cortex. This induced electric field is measured with a pick-up loop with the dipole oriented along the front-to-back (i.e., normally anterior-posterior) axis of the treatment coil.

Stimulation Volume: Stimulation volume defines the region of cortical tissue within the magnetic field that is above the threshold of cortical stimulation, i.e., the 3-dimensional volume within which the induced electric field achieves a value greater than or equal to 80% of the electric field at the 2.0 cm reference point. For example, for a treatment at the 120% MT level, the field at the boundary of the stimulation volume is equivalent to the MT level and all tissue within the stimulation volume is above the MT level.

1 Format for Traditional and Abbreviated 510(k)s

2 Wassermann, E, et al. “Risk and safety of repetitive transcranial magnetic stimulation: report and suggested guidelines from the International Workshop on the Safety of Repetitive Transcranial Magnetic Stimulation, June 5–7, 1996,” Electroencephalography and clinical Neurophysiology 108 (1998) 1–16.

3 Rossi S et al. “Safety, ethical considerations, and application guidelines for the use of transcranial magnetic stimulation in clinical practice and research”, Clinical Neurophysiology 120 (2009) 2008–2039.



6 We recommend that you request the Agency’s review of your protocols prior to initiating clinical studies for your device. This process may help ensure that any issues are addressed prior to submitting your premarket submission. To request the Agency’s review of your protocols, you can submit a pre-submission to the Agency.

7 See http://www.cssrs.columbia.edudisclaimer icon

8 Although final labeling is not required for 510(k) clearance, final labeling must comply with the requirements of 21 CFR Part 801 before a medical device is introduced into interstate commerce. In addition, final labeling for prescription medical devices must comply with 21 CFR 801.109. Labeling recommendations in this guidance are consistent with the requirements of Part 801.


10 Rothwell JC, Hallett M, Berardelli A, Eisen A, Rossini P, Paulus W, Magnetic stimulation: motor evoked potentials. Electroenceph Clin Neurophysiol Supp 1999; 52:97-103.