IMAGING TECHOLOGIES
Transcranial Magnetic Stimulation (TMS)
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What is it? What does it measure?
Transcranial Magnetic Stimulation (TMS) is a non-invasive technique used to induce electric currents in neurons of the brain. It can be used to modulate brain activity with millisecond temporal accuracy. TMS can be targeted to a rather small cortical area. Thus TMS provides direct information about the the dynamics of evoked and spontaneous neural activity and the location of functions in the brain.
TMS and magnetoencephalography (MEG) are like mirror images of each other. TMS induces currents in the brain via magnetic fields, while MEG detects magnetic fields generated by neural currents. Both TMS and MEG are non-invasive, because magnetic fields pass effortlessly through tissues such as skin and skull.
How does it work? What equipment is needed?
TMS is applied by feeding very brief electrical current pulses through a coil that is placed above the brain area to be stimulated. This induces weak electrical currents in the underlying neurons. To localize the stimulated brain area accurately, an additional navigator device is needed.
Multimodal imaging
Combining several imaging technologies is called multimodal imaging. To understand what kinds of effects TMS has, investigators often record behavioral measures such as reaction time, peripheral signals from muscles such as electromyogram (EMG), and brain activity before, after, and/or during TMS. The most natural companion to TMS is EEG, which provides information about the electric currents induced by TMS and how ongoing processes were changed as a result of TMS. However, TMS strongly disturbs normal EEG amplifiers, and therefore specialized TMS-compatible EEG equipment is needed. We are also interested in investigating ways to utilize anatomical MRI and diffusion tensor imaging (DTI) data to predict which brain areas are activated by TMS. Presently, the combination of TMS, MEG, EEG, fMRI, and structural data is under further investigation within our Center for Functional Imaging Technologies. It is expected that deeper understanding of the relationship of the hemodynamic and electrophysiological signals will lead to combined models which employ TMS, fMRI, MEG, and EEG jointly to estimate the sites and dynamics of brain activity.
Temporal resolution
Milliseconds. TMS allows real-time modulation of brain activity.
Spatial resolution
From about 7 millimeters to several centimeters, depending on the coil and target area. Note that TMS stimulates superficial areas more strongly than deep areas.
What are some features/benefits of TMS?
TMS is noninvasive.
The data can be collected in a seated position or lying down.
The measurement environment is silent except for the occasional clicks from the TMS coil, which facilitates especially auditory studies.
TMS has an extremely high temporal resolution (milliseconds) and also provides a good spatial resolution.
Most superficial areas on the skull convexity can be stimulated.
What are its limitations?
Subjects and patients that consider participating in a TMS study should know that TMS is not suitable for everyone. To see a list of our screening questions, click here. People who have electrically, magnetically, or mechanically activated implants (such as cardiac pacemakers or deep brain stimulation devices) should not have TMS because these devices may malfunction as a result of the TMS pulses. Also, those with metal or magnetic pieces in their head (such as clips on blood vessels in the brain or other metal fragments) should not have TMS because the magnetic pulses may cause the metal to move and/or heat up. However, persons with metal dental fillings may have TMS. It is recommended that women who are pregnant do not participate in TMS research. As an extremely rare adverse effect, even healthy individuals can experience a seizure during TMS. This likelihood is slightly elevated in subjects with epilepsy and some other clinical conditions. However, when TMS has been operated within the National Institutes of Health ( NIH) safety guidelines, there have not been any seizures in healthy subjects. In the unlikely event that a seizure would occur, it would stop immediately because the investigator would stop TMS. All subjects that have experienced a seizure during TMS have recovered fully. A study staff member trained in managing any potential problems will be next to you throughout the experiment to make sure that you are feeling fine.
Due to increased distance between the coil and the target it is difficult to stimulate deep brain areas.
TMS does not provide structural/anatomical information. Therefore TMS is combined with MRI data into a composite image of function overlaid on anatomy.
Research and clinical applications
TMS is a very powerful research tool. TMS has a unique role in understanding how the brain works, because it can be used to disengage a brain area for a fraction of a second, allowing scientists to understand its functional role. This distinguishes TMS from other noninvasive brain imaging techniques such as fMRI, EEG, MEG, and optical imaging - these can only detect brain activations.
Clinically, TMS is used to treat depression. It is also used to localize brain areas important for motor functions and speech, which should be avoided by the surgeon in planning for removal of brain tumors. Several other applications (e.g., migraine, epilepsy, Parkinson's disease, schizophrenia) are being investigated.
At the Martinos Center, researchers use TMS often in conjunction with EEG, MRI, fMRI, and optical imaging to obtain maps of brain activity in cognitive neuroscience studies designed to investigate the workings of the normal and damaged brain.
by T. Raij, updated 05/2009
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