Transcranial Magnetic Stimulation

A related registration and tracking application is the generation of functional brain mapping using a trans-cranial magnetic stimulation device. This coil generates magnetic field impulses which stimulate underlying nerve cells in a focused volume. By measuring responses to the stimulations and tracking the position of the coil relative to the MR scan we generate functional maps of the brain in a low-cost, non-invasive, and accurate manner. The LEDs placed on the face in the image below are used to track the position of the patient during the procedure, so that the patient's head does not have to be fixed.

Motor Cortex Mapping

At the beginning of a TMS motor mapping procedure, the patient lies down on a cot and electrodes are placed on the muscles that are to be mapped. We routinely map the index finger (FDI), forearm (FCR), biceps, jaw muscle, and leg (ATM). A ground electrode is also attached near the collar bone. These electrodes are connected to the machine (Cadwell Laboratories Inc., Washington USA) that controls the firing of the coil and reads the muscle responses.

Once the coil is positioned as desired, the patient is asked to relax, as the operator counts backwards from five. At the end of the count-down, the magnetic coil is fired and responses from the muscle electrodes are read. A graph of muscle response (in uV) versus time (in ms) is generated for each muscle being mapped. An example graph (waveform) of the forearm muscle appears below.

Each waveform is coded semi-automatically to extract the times corresponding to the takeoff, peak, trough, and recovery of the response. The two measures most often used in the analysis of motor mapping data are latency and amplitude. The latency of the response is defined as time of the takeoff (in ms) and the amplitude is the absolute difference between the peak and trough (in uV). When the magnet is placed directly over the ``hotspot'' of a muscle (or region of cortex that controls that muscle), the amplitude of the response should be large and the latency of the response should be small, as compared to the responses when the coil is place away from the hotspot.

A typical motor mapping session consists of approximately 120 stimulations at various positions on the head. For each stimulation, for each muscle, we get a latency and amplitude value, and these results are combined to produce motor cortex maps like those that appear below.

The figures below illustrate motor response amplitude maps for four muscles. Green indicates small response. Red corresponds to the largest response. The muscles mapped are (a) index finger, (b) forearm, (c) biceps, and (d) jaw.

The first image below illustrates the responses of the index finger as arrows at each individual stimulation point. Gray arrows correspond no response, and once again, green arrows indicate small responses, while red arrows indicate the the largest response. The second image shows a color coded map of the positions of the minimum latencies and largest responses. Red, yellow, and green are finger, fcr, and biceps respectively, and blue and purple are left oris and right oris respectively.

Visual Suppression Mapping

In addition to the TMS motor mapping procedures, we have also begun experiments in visual suppression mapping. When a volunteer is stimulated in the region of the visual cortex, excitations of the neurons briefly inhibit interpretation of the affected visual field.

During a visual suppression mapping session, the volunteer sits in a chair and puts his chin in a chin rest, in front of a monitor. Each visual suppression trial begins with a small dot appearing in the center of the screen. The volunteer is instructed to fixate on the dot, and, when ready, to press the space bar on the computer. At this point, the computer flashes three capital letters in the center of the screen and 90 ms later, fires a train of four pulses on the magnetic coil. If the magnet was directed toward the area of visual cortex, then a momentary blind spot in the visual field will occur. The volunteer is asked to report the three letters that he saw on the monitor. Afterwards, the computer reports the actual three letters. Any discrepancy is considered to be a response and is noted in the results. For visual cortex mapping we treat the responses as binary and thus directly map the points that caused positive suppression in the left and/or right fields of view to the MRI surface points.

In the figures below, red arrows correspond to a suppression in the right visual field, and blue arrows correspond to a suppression in the left visual field. Green arrows indicate no response as the subject correctly reported all three letters.

Selected Publications

G. Potts, L. Gugino, M.E. Leventon, W.E.L. Grimson, R. Kikinis, W. Cote, E. Alexander. J. Anderson, G.J. Ettinger, L. Aglio, M. Shenton, "Visual Hemifield Mapping Using Transcranial Magnetic Stimulation Coregistered with Cortical Surfaces Derived from Magnetic Resonance Images" J Clin Neurophysiology, 15(4):344-350, 1998. [html]

G.J. Ettinger, M.E. Leventon, W.E.L. Grimson, R. Kikinis, V. Gugino, W. Cote, L. Sprung, L. Aglio, M. Shenton, G. Potts, E. Alexander. "Experimentation with a Transcranial Magnetic Stimulation System for Functional Brain Mapping." In CVRMED/MRCAS, Grenoble, France, 1997. [color postscript 10.0M]

G.J. Ettinger, W.E.L. Grimson, M.E. Leventon, R. Kikinis, V. Gugino, W. Cote, M. Karapelou, L. Aglio, M. Shenton, G. Potts, E. Alexander. "Non-invasive Functional Brain Mapping Using Registered Transcranial Magnetic Stimulation." IEEE. Workshop on Mathematical Methods in Biomedical Image Analysis, San Francisco, CA, 1996. [color postscript 6.5M] [bw postscript 2.7M]

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Last updated Feb 5, 1999.
Michael Leventon