Comparison of electrohysterogram signal measured by surface electrodes with different designs: A computational study with dipole band and abdomen models

Abstract

Non-invasive measurement of uterine activity using electrohysterogram (EHG) surface electrodes has been attempted to monitor uterine contraction. This study aimed to computationally compare the performance of acquiring EHG signals using monopolar electrode and three types of Laplacian concentric ring electrodes (bipolar, quasi-bipolar and tri-polar). With the implementation of dipole band model and abdomen model, the performances of four electrodes in terms of the local sensitivity were quantified by potential attenuation. Furthermore, the effects of fat and muscle thickness on potential attenuation were evaluated using the bipolar and tri-polar electrodes with different radius. The results showed that all the four types of electrodes detected the simulated EHG signals with consistency. That the bipolar and tri-polar electrodes had greater attenuations than the others, and the shorter distance between the origin and location of dipole band at 20 dB attenuation, indicating that they had relatively better local sensitivity. In addition, ANOVA analysis showed that, for all the electrodes with different outer ring radius, the effects of fat and muscle on potential attenuation were significant (all p < 0.01). It is therefore concluded that the bipolar and tri-polar electrodes had higher local sensitivity than the others, indicating that they can be applied to detect EHG effectively.

Figure 1

Simulated EHG signal sources with the dipole band moving from the fundus to cervix of the uterus, and returning to fundus. (a) Movement direction of the dipole band and the simulation position on the z axis. (b) The simulated EHG signal sources at different positions on z axis with moving dipole band. The horizontal axis from left to right (−400 mm to 400 mm) represents that the dipole band moves from the fundus to cervix of the uterus, and return to the fundus.

Figure 2

EHG signals recorded by the (a) monopolar, (b) bipolar, (c) quasi-bipolar, (d) tri-polar with the different positions and dipole band movement, as shown in the Fig. 1(a).

Figure 3

Comparison of Laplacian potential attenuations between the four types of electrodes without noise dipole (a,b,c) and with 6 noise dipoles (d,e,f). The performance with different outer ring radius was given separately. (a,d) outer ring radius 10 mm; (b,e) outer ring radius 15 mm; and (c,f) outer ring radius 20 mm. It can be seen that bipolar and tri-polar electrodes had larger attenuations than the other electrodes regardless of the outer ring radius.

Figure 4

Attenuation changes with different fat thickness 0–30 mm and a fixed muscle thickness of 8 mm, (a) without noise, (b) with 6 noise dipoles; with different muscle thickness 0–16 mm and a fixed fat thickness of 15 mm, (c) without noise, (d) with 6 noise dipoles.

Figure 5

Four types of electrode designs: (a) Monopolar, (b) Bipolar, (c) Quasi-bipolar with shorted disc and outer ring, (d) Tri-polar concentric ring electrodes.

Figure 6

Illustration of (a) the modelled human uterus as an inverted cone with a dipole band moving from the fundus to cervix of the uterus and then returning to the fundus, and (b) the simulated abdomen model of a pregnant women, including human uterus and three tissue layers (skin, fat and muscle).

Figure 7

Different arrangements of the approximation of Laplacian potential of concentric ring electrodes. (a) five-point arrangement, (b) quasi-bipolar arrangement, (c) nine-point arrangement. p1, p2, p3, and p4 have the same spacing r to p0, and p5, p6, p7, and p8 have the same spacing 2r to p0. (b) Laplacian potential of quasi-bipolar concentric ring electrode.