Elsevier

Biological Psychiatry

Volume 50, Issue 1, 1 July 2001, Pages 58-61
Biological Psychiatry

Techniques and methods
Transcranial magnetic stimulation in therapy studies: examination of the reliability of “standard” coil positioning by neuronavigation

https://doi.org/10.1016/S0006-3223(01)01153-2Get rights and content

Abstract

Transcranial magnetic stimulation is investigated as a new tool in the therapy of depression and other psychiatric disorders. In almost all studies, the dorsolateral prefrontal cortex (DLPFC) has been selected as the target site for stimulation. Usually this region was determined by identifying the patient’s motor cortex, and from there the coil was placed 5 cm rostrally. The aim of our study was to test the reliability of this standard procedure. A neuronavigational system was used to relate the final coil position after applying the standard procedure to the individual cortical anatomy. In 7 of 22 subjects, the Brodman area 9 of the DLPFC was targeted correctly in this manner. In 15 subjects, the center of the coil was found to be located more dorsally (e.g., above the premotor cortex). The current method for locating the DLPFC is not precise anatomically and may be improved by navigating procedures taking individual anatomy into account.

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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