CORTICAL LOCATION OF 10-20 SYSTEM ELECTRODES ON NORMALIZED CORTICAL MRI SURFACES.

P. Vitali^1,2, G. Avanzini², L. Caposio³, E. Fallica², L. Grigoletti², E. Maccagnano³, B. Rigoldi², 
G. Rodriguez¹, F. Villani^2
1Clinical Neurophysiology (DIMI), University of Genova,
V.le Benedetto XV,6, Genova, ITALY I-16132
²Divisions of Clinical Neurophysiology and ³Neuroradiology, Istituto Nazionale Neurologico “C. Besta”,
Via Celoria 11, Milan, ITALY  I-20133

Abstract: The 10-20 system locate the electrodes on the scalp using standard cranial landmarks. It assumes a consistent correlation between the electrodes and the cortical structures. This basic concept is limited by cortical gyration variability.

In this study, on data obtained in a clinical setting , the variability of 10-20 electrodes projection on the cortical structures have been evaluated.

Using high resolution MR data, scalp (with electrodes) and cortical surfaces were obtained from 10 epileptic patients. The electrode positions on the scalp were projected orthogonally on the cortical surface. Each brain volume was normalized to the MNI template, thus obtaining normalized coordinates of electrode projections.

Based on our patients data, we have been able to define the exact relationship between the 10-20 electrodes and the cortical structures. Coordinates normalization allowed to average data among subjects and to evaluate inter-subject variability.

INTRODUCTION

The 10-20 system locate the electrodes on the scalp using standard cranial anatomical landmarks and proportional measurements. One basic assumption for the 10-20 system is that a consistent correlation exists between the electrodes scalp positions and the underlying cortical structures.

Validation of this assumption was first addressed by Jasper [1] on cadaver brains, and later on by several studies using standard X-rays [2] [3], CT scans [4], and, more recently, MRI [5] [6] [7].

Because of the variability of brain dimension and shape, measures performed in different subjects are difficult to compare. In our study 10-20 electrodes locations were normalized on the absolute average space using a standard brain template.

The aim of the study was to verify the variability of 10-20 electrode projections on the cortical structures in a clinical setting.

METHODS

10 patients affected by drug-refractory partial epilepsy were evaluated for pre-surgical purposes by prolonged video-EEG recording. 32 scalp electrodes were placed using collodion, according to the extended 10-20 system. On each electrode a paramagnetic fiducial marker was applied.

High resolution MRI scan (T1-weighted, 1 mm isotropic voxel) was acquired for each subject with electrodes and fiducial markers on the scalp.

Scalp and cortical surfaces were obtained from each MR volume using MRIcro freeware [8]; by rotating simultaneously the two surfaces, the 19 standard electrode positions on the scalp were projected orthogonally on the cortex.

Using SPM99 [9], each brain volume was then normalized to the MNI template [10], thus obtaining normalized coordinates of the electrode projections.

Based on the assumption that homologous electrodes were symmetrically positioned on the two hemispheres, homologous electrodes normalized coordinates were merged and averaged among subjects.

 

Figure 1. Top-lateral view of the scalp and brain surfaces. The cross indicates C4 electrode position and cortical projection.

 

Figure 2. Cortical surface (same subject of fig. 1), before and after normalization to the MNI template. Cross on Cz.

RESULTS

Average and standard deviations of the coordinates (according to the Analyze standard) of the electrodes projections on the cortical surface, normalized to the standard MNI template, are reported in table 1 and shown in fig. 3.

TABLE I
Normalized coordinates of electrodes projection on the cortex

Electrodes	Average x, y, z (mm)	Standard deviation  x, y, z (mm)
Fz	1      42     57	6     5     4
Cz	-1    -18     79	4     7     3
Pz	-7    -78     58	10     9     9
F1(2)	30     65     16	7     5     8
F3(4)	50     31     36	8    11  10
F7(8)	60     27     -1	5    12     9
C3(4)	57    -13    54	5    13     6
T3(4)	70    -12     -8	4    10     9
P3(4)	54    -66    39	7     8    12
T5(6)	62    -64    -9	5     9    10
O1(2)	35    -98      6	7     7    11

Figure 3. Cortical surface of a normalized brain with average of normalized electrodes projections on the cortex. Standard deviations bigger then 1 cm are indicated by the major axis.

The anterior-posterior position of C3-C4 electrodes resulted to have the maximal standard deviation (13 mm, thus sometimes projecting out of the pre-central gyrus).

DISCUSSION

Based on the data obtained in a clinical setting from 10 epileptic patient, it has been possible to define the exact relationship between the 10-20 electrodes and the cortical structures. Coordinates normalization allowed to average data among subjects and to evaluate the inter-subject variability.

The maximal anterior-posterior variability of C3-C4 position may depend on variable location of the rolandic fissure respect to the standard cranial landmarks.

Due to the variability in the electrode/cortical relationship observed in our clinical setting, it is important to obtain the exact location of the electrodes in relation to the patient anatomy. This issue is most important when the source of electrophysiological activities must be located in a realistic head model.

Acknowledgments: We thank Dr. Chris Rorden for making MRIcro software freely available and for his helpful support.

REFERENCES

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[8] www.psychology.nottingham.ac.uk/staff/cr1/mricro.html

[9] www.fil.ion.ucl.ac.uk/spm/spm99.html

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