Vortex Currents in a Torso Phantom --
Comparison of Magnetic and Electric Signal Strength

J Haueisen^a, M Liehr^ab, S Dutz^ac, U Leder^b, J Nenonen^d, ME Bellemann^3
a Biomagnetic Center, Department of Neurology, Friedrich-Schiller-University, Jena, Germany
^b Clinic of Internal Medicine III, Friedrich-Schiller-University, Jena, Germany
^c Department of Biomedical Engineering, University of Applied Sciences, Jena, Germany
^d Laboratory of Biomedical Engineering, Helsinki University of Technology, Espoo, Finland

Correspondence: J Haueisen, Biomagnetic Center, University Hospital Jena, Philosophenweg 3, D-07743 Jena, Germany.
E-mail: haueisen@biomag.uni-jena.de, phone +49 3641 935338, fax +49 3641 935355

1.  Introduction

Both active and passive vortex currents have been discussed for a long time as possible sources of differential information in the magnetocardiogram (MCG) and the electrocardiogram (ECG) [e.g. Roth et al., 1986; Wikswo et al., 1982; Barach, 1993]. Due to the closed loop, vortex currents would show up in the MCG but not in the ECG.

More recently, clinical observations indicated a diverging of MCG and ECG during exercise [Brockmeier et al., 1997].

In this study, consisting of two experimental setups, we quantify for the first time experimentally the influence of vortex currents on electric and magnetic data with the help of a torso phantom.

2.  Material and Methods

Both experiments (active and passive vortex currents) used a realistically shaped hollow torso phantom (Fig. 1) with 138 Ag/AgCl-electrodes, enabling simultaneous MCG and ECG recordings. For all measurements, the torso was filled with NaCl solution (conductivity; active: 0.16, passive: 0.1 S/m).

Active vortex currents were modeled with a set of twelve platinum dipoles arranged in a circle (diameter 55 mm) mounted inside the torso. Each dipole was fed with a sinusoidal current (0.5 mA, 20 Hz), and one by one the dipoles were switched on, starting with a single dipole and ending with all twelve dipoles switched on. Magnetic and electric measurements were performed in a magnetically shielded room (MCG: 99 channels, ECG 60 channels, Neuromag Ltd., Helsinki, Finland).

Passive vortex currents were modeled with a changing conductivity in a hollow, cylindrically shaped compartment inside the torso. The compartment boundaries consisted of two ionic exchange membranes that enabled different conductivities in the compartment without influencing the free ionic current flow. In this compartment, a single platinum dipole was mounted. The conductivity ratio between the cylindrically shaped compartment and the torso was increased from 0.25 to 100. The higher the conductivity ratio the more current flows in the cylinder wall and, thus, less current within the torso. In other words, the vortex current density was adjusted via the conductivity ratio. The magnetic field of the passive vortex currents was recorded with a 31-channel biomagnetometer (Philips, Hamburg, Germany). Electric potentials were measured with 63 electrodes.

Electric and magnetic signal strengths were determined at the peak of 20 consecutive periods of the sinusoidal signal. Mean values and standard deviations were calculated. The active vortex current values were normalized to the maximum, and the passive vortex current values were normalized to the value at the conductivity ratio of 1.

3.  Results

Figure 2 shows different behavior of the electric and magnetic signal strengths for the active vortex current experiment. When the whole circle of twelve dipoles is switched on, the electric signal strength is about a tenth of the signal strength of a single dipole. The standard deviations for the mean signal strengths over 20 periods are 0.6% for both magnetic and electric recordings.

Figure 2. Active vortex current model. Signal strength over the number of enabled dipoles.	Figure 3. Passive vortex current model. Signal strength over the conductivity ratio.

As shown in Fig. 3, we found that an increasing conductivity ratio causes a slight increase of the magnetic signal strength and a strong decrease of the electric signal strength. When the conductivity ratio increases from 1 to 100, the magnetic signal strength increases about 20% and the electric signal strength decreases about 80%. The standard deviations for the mean signal strengths over 20 periods are 0.05% for the magnetic recordings and 0.6% for the electric recordings.

4.  Discussion

In our experimental study, we modeled active vortex currents similar to those in [Wikswo et al., 1982] and passive vortex currents similar to those in [Roth et al., 1986]. Both active and passive vortex currents yielded a substantial decrease of the electric signal strength while the magnetic signal was not influenced (active) or increased (passive). In conclusion, we provide quantitative data for the differential influence of vortex currents on MCG and ECG recordings. Vortex currents might, at least in part, explain the experimentally observed differences between MCG and ECG studies.

References

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J.P. Barach, "Simulation of cardiac action currents having curl.", IEEE Trans. Biomed. Eng., 40:49-58, 1993.

B.J. Roth, J.P. Wikswo jr., "Electrically silent magnetic fields.", Biophys. J., 50:739-745, 1986.