The influence of age and skull conductivity on surface and subdermal bipolar EEG leads

Abstract

Bioelectric source measurements are influenced by the measurement location as well as the conductive properties of the tissues. Volume conductor effects such as the poorly conducting bones or the moderately conducting skin are known to affect the measurement precision and accuracy of the surface electroencephalography (EEG) measurements. This paper investigates the influence of age via skull conductivity upon surface and subdermal bipolar EEG measurement sensitivity conducted on two realistic head models from the Visible Human Project. Subdermal electrodes (a.k.a. subcutaneous electrodes) are implanted on the skull beneath the skin, fat, and muscles. We studied the effect of age upon these two electrode types according to the scalp-to-skull conductivity ratios of 5, 8, 15, and 30 : 1. The effects on the measurement sensitivity were studied by means of the half-sensitivity volume (HSV) and the region of interest sensitivity ratio (ROISR). The results indicate that the subdermal implantation notably enhances the precision and accuracy of EEG measurements by a factor of eight compared to the scalp surface measurements. In summary, the evidence indicates that both surface and subdermal EEG measurements benefit better recordings in terms of precision and accuracy on younger patients.

Figure 1

Reported conductivity values of live skull samples temporarily removed during epileptic surgery plotted against patient age [5]. The thick blue trend with circles graphs raw data and the thin gray trend with dots graphs the least squares fit. Reproduced from [6].

Figure 2

The midsagittal views show the bipolar electrode locations of the surface and subdermal (i.e., on the skull) measurement locations at the apex C _Z and the occipital cortex O _Z. The EEG electrode dimensions are 1 mm × 1 mm × 1 mm. (a) The sagittal slice of the Visible Human Man displays all four locations. (b) The sagittal slice of the Visible Human Woman also shows the surface and subdermal locations.

Figure 3

Measurement sensitivity distributions of the Visible Human Man mapped in the logarithmic scale: ((a)–(d)) surface electrodes placed on the scalp solved according to the scalp-to-skull conductivity ratio mentioned in the subcaption and ((e)–(h)) subdermal insulated needle electrodes inserted through the skin placing the measuring tip on the skull surface solved according to the scalp-to-skull conductivity ratio mentioned in the subcaption. Scalp-to-skull conductivity ratios are specified in each subcaption: ((a), (e)) 5 : 1, ((b), (f)) 8 : 1, ((c), (g)) 15 : 1, and ((d), (h)) 30 : 1.

Figure 4

Measurement sensitivity distributions of the Visible Human Woman mapped in the logarithmic scale: (a)–(d) surface electrodes placed on the scalp solved according to the scalp-to-skull conductivity ratio mentioned in the subcaption and (e)–(h) subdermal insulated needle electrodes inserted through the skin placing the measuring tip on the skull surface solved according to the scalp-to-skull conductivity ratio mentioned in the subcaption. Scalp-to-skull conductivity ratios are specified in each subcaption: (a), (e) 5 : 1, (b), (f) 8 : 1, (c), (g) 15 : 1, and (d), (h) 30 : 1.