Experimental Study of the Magnetohydrodynamic (MHD) Effect with

Respect to Intracardiac ECG Signals

W. B. Buchenberg

, G. Hoppe

, R. Lorenz

, W. Mader

, P. Laudy

, C. Bieneck

and B. Jung

Dept. of Radiology, Medical Physics, University Medical Center, Freiburg, Germany

Freiburg Center for Data Analysis and Modeling, Albert-Ludwigs-University, Freiburg, Germany

CardioTek B.V., Maastricht-Airport, Netherlands

R&D, Schwarzer GmbH, Heilbronn, Germany

1 OBJECTIVES

To assess the status of the cardiac electrical system

for diagnosis and therapy, electro-physiologic (EP)

studies are an important tool for diagnosis and

therapy in patients with electrophysiological

disorders (Josephson, 2008); (Schneider, 2005).

Electrode catheters are inserted into the heart and

guided to the location of interest using X-ray

fluoroscopic images. Hence, the patient and the

investigator are exposed to X-rays and might

accumulate a high radiation dose. Therefore, it is of

great interest to use different imaging modalities

such as Magnetic Resonance Imaging (MRI) for

catheter guidance. However, the surface

electrocardiogram (ECG) signal is strongly affected

by the magnetohydrodynamic (MHD) effect

(Tenforde et al., 1983), (Tenforde, 2005); (Gupta et

al., 2008). Charged particles of an electrical

conductive fluid such as blood with a velocity

component v

perpendicular to the external magnetic

field B

are deflected by the Lorentz force F

. This

charge separation gives rise to a potential across the

great vessels. At the level of intracardiac catheter

use, the potential can be defined as U

MHD

= v

·d B

where d denotes the electrode distance. However,

only limited knowledge is available for intracardiac

ECG signals with respect to the MHD effect. A

recent animal experiment reported altered EP signals

inside the MR scanner (Tse et al., 2012). Therefore,

intracardiac signals acquired in a MR environment

have to be investigated in detail to characterize the

alteration caused by the MHD effect in order to

provide diagnostically valuable data.

The aim of this work is to establish an

experimental setup with common EP equipment to

simulate the MHD effect in a model system and to

analyse the pure MHD signal.

2 METHODS

All measurements were performed on a 1.5 T MR

scanner (Symphony, Siemens/Erlangen, Germany).

A closed flow circuit was established at the MR

scanner table (Figure 1b) and filled with a distilled

water-sodium-chloride mixture (electrical

conductivity at 23±1°C: 5.33±0.18 mS/cm) to

simulate the electrical conductivity of blood. A MR-

compatible Ventricular Assist Device (VAD)

(MEDOS, Stolberg, Germany) connected to a u-

shaped tube phantom (inner diameter: 22.1 mm) was

used for mimicking the pulsatile flow of the beating

heart (50 bpm). The tube was constructed using a

rapid prototyping method. A standard sized 6F EP

catheter (St. Jude Medical, Minnesota, USA) was

placed into a slit in the phantom wall (Figure 1a).

The slit allows for reproducible positioning of the

catheter as well as assuring the electrodes are

stationary. As shown in the upper schematics of

Figure 1a, d between the first two electrode pairs

(1/10, 2/9) are about equal, whereas d decreases

continuously from pairing 3/8 to 5/6. The EP

catheter is connected to a pre-amplifier (EP-Tracer,

CardioTek B. V., Netherlands) for measuring

potentials in the mV-range. A laptop was connected

for data registration. The electrodes of the surface

ECG were connected to the liquid as well as to the

ground to provide a reference for the pre-amplifier.

Bipolar measurements were carried out using the

lowest cut-off frequency (0.05 Hz) of the high-pass

hardware filters of the EP-Tracer.

3 RESULTS

Figure 2 shows representative curves of the MHD-

effect during pulsatile flow in a 1.5 T MR scanner.

The signal of electrodes 1/10 is smaller than the one

B. Buchenberg W., Hoppe G., Lorenz R., Mader W., Laudy P., Bieneck C. and Jung B..

Experimental Study of the Magnetohydrodynamic (MHD) Effect with Respect to Intracardiac ECG Signals.

 2013 SCITEPRESS (Science and Technology Publications, Lda.)

between the electrodes 2/9. However, this deviation

is within the limits of the experimental accuracy. A

linear decrease of the potential with a decrease in d

is observed (not shown). Additionally, it is important

to note the underlying unknown shift and scaling of

the voltage signal in the vertical direction. This is

due to the impact of the high pass filter of the pre-

amplifier on the MHD signal.

Figure 1: EP catheter positioning in the phantom and its

orientation with respect to the flow velocity v, the external

magnetic field B, and the Lorentz force F

(a), and

experimental setup (b).

4 DISCUSSION

The investigation of MHD potentials detected with

common EP equipment is important for developing

EP exam procedures in a MR environment.

Therefore, a MRI compatible flow circuit was

successfully established. The linear dependency

between the measured potential and d (as well as B

both not shown) predicted by theory was clearly

observed and validates the model system. Typical

electrode distances as between the electrodes 5/6

revealed a significant MHD potential which cannot

be neglected. The non-conductive walls of the flow

phantom are not expected to bias the outcome of the

measurement significantly since studies revealed

that vessel wall conductivity may be neglected

(Abdallah et al., 2008). Further investigations will

analyse the impact of different recording modalities

such as hardware filters on the detection of the MHD

signal. Furthermore, glycerol will be added to the

saltwater for simulating the viscosity and the density

of blood.

The time course of MR velocity data at the

catheter position (not shown) agrees very well with

the EP-Tracer data (apart from filter effects). Hence,

additional simple and quick MR flow measurements

at the location of interest during an EP exam may be

used to remove the MHD related potential from

intracardiac ECG signals representing an essential

step towards diagnostically valuable data.

Additionally, in vivo data, e.g. from animal models,

is required for the validation of these methods.

Figure 2: MHD potential time course.

ACKNOWLEDGEMENTS

EUROSTARS Program Grant #01QE1004D.

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ISMRM, oral presentation #206.