1. Field of the Invention
This invention pertains to the general field of electroencephalography. In particular, it provides a new device for marking the scalp of a patient where electrodes are attached to monitor brain activity.
2. Description of the Prior Art
The procedure involved in obtaining an electroencephalograph (EEG) requires that specific points be located on the scalp of a patient for attachment of electrodes by means of a conductive paste for short-term recording and an adhesive (typically collodion) for long-term recording (longer than 24 hours). The specific electrode sites are selected uniformly throughout the world according to the standards specified by the International 10/20 System. Simply stated, this system requires that the distance between the nasion N and the inion I along the top of the scull of a patient (referenced as L1 in FIG. 1 for purposes of illustration) be divided into segments ten, thirty and fifty percent away from the ends, and that each corresponding point be marked on the scalp. Similarly, the distance (L2) passing through the fifty percent point of L1 between the tragus T of each ear is divided into segments corresponding to ten, thirty and fifty percent away from each tragus, and each corresponding point is marked on the scalp. Obviously, the fifty percent point normally corresponds to the point of intersection with L1 and is already marked. Once these positions are established, the two lines (L3 and L4 in FIG. 1) joining the ten-percent points in L1 on both sides of the scalp along the plane containing the four ten-percent points in L1 and L2 are themselves divided into segments corresponding to ten, thirty and fifty percent of each line from the ten-percent points in L1, and each corresponding point is marked on the scalp. Again, the fifty percent points correspond to the points of intersection with L2 and are already marked. Thus, four quadrants become defined between L1, L2, L3 and L4. The midpoint of each of these four quadrants is then found and marked, corresponding to the thirty-percent points of the lines intersecting it, as illustrated in FIG. 1. Thus, a whole lattice of electrode locations with predetermined spacings is found and marked.
A total of 21 electrode locations need to be identified and marked in preparation for an electroencephalograph. This process is obviously time consuming and prone to errors, especially considering the nonideal conditions (asymmetries in the patient's head, the presence of hair, the patient's tendency to move during the procedure) under which these measurements are carried out. Therefore, attempts have been made at simplifying the procedure to shorten the time spent in positioning the electrodes.
U.S. Pat. No. 3,998,213 describes a self-adjustable electrode holder that consists of a cap containing a set of electrodes arranged according to the International 10/20 System. The cap is formed with elastic straps that follow the framework described in FIG. 1, wherein each electrode is attached to the 10/20 System points described above. By using elastic material to manufacture the cap, each strap is able to stretch to conform to different-size heads and retain the 10/20 percent relationship between the electrode locations. Thus, a ready-to-go assembly is provided that only requires good positioning of the cap and connection of the electrode wires to EEG equipment.
The problem with this device is that it can only be used with the set of electrodes incorporated into it. Because of their attachment to the straps in the cap, which are necessarily pushed away from the scalp by the hair of the patient, it is more difficult to maintain electrical contact with the scalp with these electrodes than with standard, free-standing electrodes. In addition, each set of electrodes needs to be washed and sterilized between uses, which is normally accomplished by scrubbing in a detergent medium and, if necessary, a sterilizing chemical. Since the electrodes described in the patent are incorporated into the cap, the entire device must be sterilized, for which the normal procedure is not suitable. The alternative of treating the electrode cap as a disposable unit is obviously too expensive to be considered viable.
Moreover, the cap is usable only for short-term monitoring because of patient's discomfort due to tightness and perspiration induced by the unavoidable temperature rise associated with its use. Poor contact results from this temperature problem, which in turn causes sweat-induced artifacts in the EEG procedure that require frequent adjustments in the equipment's electrical impedence. Thus, the preferred method for long-term monitoring is the application of individual electrodes with collodion, which is not practical using available devices. Therefore, from a practical point of view the cap described in the referenced patent does not fully provide a solution to the problems associated with placing EEG electrodes according to the International 10/20 System. A need remains for a device that simplifies the procedure and that is compatible with utilizing a laboratory's own existing electrodes.