Continuous transcranial magnetic stimulation during positron emission tomography: A suitable tool for imaging regional excitability of the human cortex

In six healthy volunteers, (H2O)-O-15 positron emission tomography (PET) wa s employed to evaluate rate-dependent functional activation of the left pri mary sensorimotor hand area (SM1(HAND)) during subthreshold repetitive tran scranial magnetic stimulation (rTMS). Using an eight-shaped coil, continuou s trains of rTMS were delivered during nine 50-s (H2O)-O-15 PET scans. Nine different stimulation frequencies were used, ranging from 1 to 5 Hz. Stimu lus intensity was set at 10% below active motor threshold. During three add itional PET scans, an ineffective rTMS was applied via another eight-shaped coil, which was held 10 cm above the vertex. Statistical parametric mappin g was employed to assess relative differences in normalized regional cerebr al blood flow (rCBF) across conditions. Compared with ineffective rTMS, sub threshold rTMS increased normalized rCBF in the stimulated SM1(HAND). Moreo ver, the increase in rCBF in the left SM1(HAND) showed a linear positive re lationship with the rate of rTMS, indicating a rate-dependent functional ac tivation of the stimulated SM1(HAND). These data demonstrate that, by varyi ng the variables of rTMS across scans, continuous rTMS during (H2O)-O-15 PE T provides a noninvasive tool to study the regional excitability profile of a distinct cortical area. (C) 2001 Academic Press.