Influence of body parameters on gastric bioelectric and biomagnetic fields in a realistic volume conductor

Abstract

Electrogastrograms (EGG) and magnetogastrograms (MGG) provide two complementary methods for non-invasively recording electric or magnetic fields resulting from gastric electrical slow wave activity. It is known that EGG signals are relatively weak and difficult to reliably record while magnetic fields are, in theory, less attenuated by the low-conductivity fat layers present in the body. In this paper, we quantified the effects of fat thickness and conductivity values on resultant magnetic and electric fields using anatomically realistic torso models and trains of dipole sources reflecting recent experimental results. The results showed that when the fat conductivity was increased, there was minimal change in both potential and magnetic fields. However, when the fat conductivity was reduced, the magnetic fields were largely unchanged, but electric potentials had a significant change in patterns and amplitudes. When the thickness of the fat layer was increased by 30 mm, the amplitude of the magnetic fields decreased 10% more than potentials but magnetic field patterns were changed about four times less than potentials. The ability to localize the underlying sources from the magnetic fields using a surface current density measure was altered by less than 2 mm when the fat layer was increased by 30 mm. In summary, results confirm that MGG provides a favorable method over EGG when there are uncertain levels of fat thickness or conductivity.

Conflict of interest statement

The authors report no conflicts of interest.

© 2012 Institute of Physics and Engineering in Medicine

Figures

Figure 1

Anatomically realistic torso model constructed from Visible Human data. Shown are (a) the default model and (b) the same model with the fat layer increased by 30 mm. The torso models have three boundary element surfaces representing the stomach (red), muscle (brown) and fat and skin (grey) surfaces.

Figure 2

Contour plots of a (a) potential or (b) magnetic field, with red contours and blue contours representing positive and negative values, respectively. The location of the maximum field value is indicated by the red circle, the minimum field value by the blue triangle and the midpoint between these two locations by the green square. The orientation of the maxima and minima relative to the coordinate system was represented by the black lines.

Figure 3

The normalized amplitudes averaged over 110 electrodes/sensors for the models with varying fat conductivity when using (a) a single dipole or (b) multiple dipoles. The * denotes the model with the default level of fat thickness.

Figure 4

The comparison between the field (potential and magnetic) fall-off and power of the distance from a dipole to a sensor/electrode at t=56 s when using (a), (c) a single dipole or (b), (d) multiple dipoles.

Figure 5

Changes in field parameters (mid-point locations and orientations) due to changes in fat layer thickness when using (a),(c) a single dipole and (b),(d) multiple dipoles. The * denotes the model with the default level of fat thickness.

Figure 6

The normalized amplitudes averaged over 110 electrodes/sensors for the models with varying fat thickness when using (a) a single dipole or (b) multiple dipoles. The * denotes the model with the default fat conductivity value.

Figure 7

Changes in field parameters (mid-point location and orientation) due to changes in conductivities using (a),(c) a single dipole source or (b),(d) multiple dipole sources. The * denotes the model with the default fat conductivity value.

Figure 8

SCD estimates of the (a) horizontal and (b) vertical positions of the underlying sources. The black lines represent the dipole locations. The blue and red (dot) lines represent the maximum SCD approximations of the positions for model 01 and model 05, respectively. The average difference between the two models was 1.4 mm.