Techniques and methodsTranscranial magnetic stimulation in therapy studies: examination of the reliability of “standard” coil positioning by neuronavigation
Introduction
Several groups have investigated the therapeutic efficacy of repetitive transcranial magnetic stimulation (rTMS) above the dorsolateral prefrontal cortex (DLPFC) in depression and other psychiatric disorders George et al 1999, Post et al 1999. The DLPFC has been selected as a target area based on neuroimaging findings such as the reduction of prefrontal glucose metabolism (Soares and Mann 1997); however, the DLPFC has not been experimentally proven to be the most effective target for therapeutic rTMS. To place the coil over the suggested position (i.e., above Brodman areas [BA] 9 and 46 as functionally relevant parts of the DLPFC), George et al (1995) and Pascual-Leone et al (1996) proposed a “standard procedure,” which was then applied by nearly all investigators in this field. First, the motor cortex was localized by evoking a response of contralateral hand muscles, for instance, the abductor pollicis brevis muscle (APB). Then the coil was moved 5 cm rostrally, presumably targeting the DLPFC. The measure of 5 cm was derived from the Talairach atlas George et al 1995, Talairach and Tournoux 1988. This method of coil placement is easy to perform but does not account for individual variations in the distance between motor areas and the DLPFC. To determine the precision of this method, we used a neuronavigational system in conjunction with magnetic resonance imaging.
Section snippets
Methods and materials
We tested the standard procedure for coil placement over the DLPFC in 22 subjects (12 women, age range 21–61, 10 depressed patients and 12 healthy subjects). All subjects gave their written informed consent after the procedure had been fully explained. The protocol was approved by the local ethics committee. A neuronavigational system commonly used in neurosurgery (Surgical Tool Navigator [STN], Zeiss Oberkochen) was adapted to navigate the coil according to the individually determined anatomy
Results
In all subjects, the standard procedure resulted in placement of the midpoint of the coil above the area of the middle frontal gyrus (MFG); however, the precise position varied considerably between subjects, ranging from the premotor cortex (PMC) to the DLPFC (Table 1). In 7 of the 22 subjects, the coil was placed over the PMC (BA 6). In five subjects, the coil was placed above the borders of the PMC and the DLPFC, above the borders of BA 6 to BA 8 respectively. In three subjects, the coil was
Discussion
In our study, we investigated the accuracy of the standard procedure for coil positioning, which is commonly applied in treatment trials with rTMS above the DLPFC. We found a reliable positioning of the coil above the MFG; however, in 15 of 22 (68%) of our subjects, the coil was not placed above BA 9 in the DLPFC as intended. Instead, it was located above BA 6 or BA 8, the PMC or the FEF, respectively. Thus, the stimulation sites were located more posteriorly and superiorly relative to BA 9.
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