An efficient and accurate new method for locating the F3 position for prefrontal TMS applications

Abstract

The International 10-20 system is a method for standardized placement of electroencephalogram (EEG) electrodes. The 10-20 system correlates external skull locations with the underlying cortical areas. This system accounts for variability in patient skull size by using certain percentages of the circumference and distances between four basic anatomical landmarks. This 10-20 system has recently been used in transcranial magnetic stimulation (TMS) research for locating specific cortical areas. In the treatment of depression (and some types of pain), the desired placement of the TMS coil is often above the left dorsalateral prefrontal cortex (DLPFC) which corresponds to the F3 location given by the 10-20 system. However, for an administrator with little experience with the 10-20 system, the numerous measurements and calculations can be excessively time-consuming. Additionally, with more measurements comes more opportunity for human error. For this reason we have developed a new, simpler and faster way to find the F3 position using only three skull measurements. In this paper, we describe and illustrate the application of the new F3 location system, provide the formulas used in the calculation of the F3 position, and summarize data from 10 healthy adults. After using both the International 10-20 system and this new method, it appears that the new method is sufficiently accurate; however, future investigations may be warranted to conduct more in dept analyses of the method's utility and potential limitations. This system requires less time and training to find the optimal position for prefrontal coil placement and it saves considerable time compared to the 10-20 EEG system.

Figure 1

Illustration of the Beam F3 Location system in practice. [1] measurement of the distance from tragus to tragus and marking of the midpoint [2] measurement of the distance from nasion to inion (the midpoint is marked here as well and the vertex is the point where the two lines containing the midpoints meet) [3] measurement of head circumference. Once these three measurements are attained, they are entered into the software package which provides two output values (values X and Y) [4] a point along the circumference is marked X cm from the midline [5] F3 is marked as a point along the line running from the vertex through the point created in the previous step Y cm from the vertex [6] the “x” at the top of the head is the vertex, the other “x” is the F3 location

Figure 2

Screen-shot of the F3 Locator program. The user enters 1) the distance from tragus to tragus, 2) the distance from nasion to inion, and 3) the head circumference. The program outputs two values that are used to locate F3.

Figure 3

Diagram of the electrode placement positions and labels from the 10–20 EEG system.