Transcranial magnetic stimulation coregistered with MRI: a comparison of a guided versus blind stimulation technique and its effect on evoked compound muscle action potentials.
METHODS
Compound muscle action potentials (CMAPs) elicited by transcranial magnetic stimulation (TMS) are characterized by enormous variability, even when attempts are made to stimulate the same scalp location. This report describes the results of a comparison of the spatial errors in coil placement and resulting CMAP characteristics using a guided and blind TMS stimulation technique. The former uses a coregistration system, which displays the intersection of the peak TMS induced electric field with the cortical surface. The latter consists of the conventional placement of the TMS coil on the optimal scalp position for activation of the first dorsal interossei (FDI) muscle.


RESULTS
Guided stimulation resulted in significantly improved spatial precision for exciting the corticospinal projection to the FDI compared to blind stimulation. This improved precision of coil placement was associated with a significantly increased probability of eliciting FDI responses. Although these responses tended to have larger amplitudes and areas, the coefficient of variation between guided and blind stimulation induced CMAPs did not significantly differ.


CONCLUSIONS
The results of this study demonstrate that guided stimulation improves the ability to precisely revisit previously stimulated cortical loci as well as increasing the probability of eliciting TMS induced CMAPs. Response variability, however, is due to factors other than coil placement.
Gugino
L D
LD
Department of Anesthesia, Brigham and Women's Hospital, Harvard Medical School, CWN-L1, 75 Francis Street, Boston, MA 02115, USA. vern@zeus.bwh.harvard.edu
Romero
J R
JR
Aglio
L
L
Titone
D
D
Ramirez
M
M
Pascual-Leone
A
A
Grimson
E
E
Weisenfeld
N
N
Kikinis
R
R
Shenton
M E
ME
D000328:Adult; D001921:Brain / Q000502:physiology*; D001931:Brain Mapping; D005260:Female; D006801:Humans; D008279:Magnetic Resonance Imaging; D008297:Male; D008875:Middle Aged; D018482:Muscle, Skeletal / Q000502:physiology*; D010812:Physical Stimulation; D011336:Probability; D012535:Scalp; D012684:Sensory Thresholds; D050781:Transcranial Magnetic Stimulation