Surface Gradient Analysis of Atrial Activation
from Magnetocardiographic Maps

Jukka Nenonen^ac, Juha Montonen^c, Raija Koskinen^b,c

^aLaboratory of Biomedical Engineering, Helsinki University of Technology, Espoo, Finland
 ^bBioMag Laboratory, Helsinki University Central Hospital, Helsinki, Finland
^cDivision of Cardiology, Helsinki University Central Hospital, Helsinki, Finland

Correspondence: JT Nenonen, Helsinki University of Technology, Laboratory of Biomedical Engineering, P.O. Box 2200, 02015 HUT, Espoo, Finland. E-mail: jukka.nenonen@hut.fi, phone +358 9 451 3171, fax +358 9 451 3182

1.  Introduction

Progress in treatment of cardiovascular diseases has increased the demand for non-invasive localization and determination of the size of abnormally functioning regions in the heart. During the recent years, magnetocardiographic (MCG) source imaging with multichannel SQUID systems has received increasing interest. Although MCG has not yet been established as a routine clinical tool, successful results have been reported in clinically important problems, such as assessment of the risk of life-threatening arrhythmias, detection and characterization of myocardial ischemia, and non-invasive localization of cardiac activation [Koch 2001; Nenonen et al., 2003].

The MCG field distributions can be converted to pseudo-current distributions, because tangential components of the magnetic field show a maximum immediately above an electrically activated region [Cohen and Hosaka, 1976; Kandori et al., 2001]. In the following, we discuss the method and its application in normal atrial excitation in multichannel MCG.

2.  Methods

Because the biomagnetic fields are quasistatic (Ñ´B = 0), it follows directly that ¶B_z/¶x = ¶B_x/¶z and ¶B_z/¶y = ¶B_y/¶z. In other words, measurements with planar gradiometers, detecting ¶B_z/¶x and ¶B_z/¶y, yield also information about the tangential field components B_x, B_y. Cohen and Hosaka [1976] utilized this fact when they introduced so called arrow map vectors by rotating the planar gradients of the B_z component by 90 degrees: v = (¶B_z/¶y)e_x - (¶B_z/¶x)e_y. Here, e_x and e_y are the perpendicular unit vectors on the sensor array plane. The resulting arrow map provides a useful zero-order approximation (pseudo-current pattern) for the underlying current sources [Kandori et al., 2001; Koch 2001].

In this study, arrow maps were evaluated from MCG data recorded in the magnetically shielded room of the BioMag Laboratory at the Helsinki University Central Hospital from a healthy male volunteer. A 99-channel cardiomagnetometer was used (Neuromag Ltd., Helsinki, Finland). The sensor array has 33 triple-sensor thin-film units, with a magnetometer (B_z) integrated on top of two perpendicular planar gradiometers. A 64-channel ECG was recorded simultaneously with the MCG. Isofield patterns and gradient arrow maps are interpolated in 169 points on the sensor array surface using the minimum-norm estimation [Hänninen et al., 2000; Nenonen et al., 2003].

3.  Results

MCG distributions and gradient arrow maps of the atrial activation are displayed in Figure 1. The results show that the initial right atrial activation is oriented from the right shoulder of the subject to left and down. A clear transition of the field pattern and gradient distribution is seen approximately at 60 ms from the P-wave onset, which corresponds well with the physiological knowledge of the normal activation in the right atrium. The total P-wave duration was about 110 ms. In the same subject, multichannel ECG did not reveal a clear change in the map patterns throughout the whole P-wave.

Figure 1.   Left: The P-wave in the central magnetometer and five selected time points. Upper row: MCG field distributions of the B_z component. Postitive values indicate the field toward the chest, and negative out of the chest. The contour step is 0.2 pT. Lower row: The white arrows represent the rotated surface gradients, yielding a pseudo-current pattern of the underlying current density in the heart. Darkest red indicates largest gradient magnitudes, and the red-green boundary shows where the gradient magnitude is 50% of the maximum.

4.  Discussion

The gradient arrow maps have been utilized in MCG data analysis in various studies. For example, the properties of planar gradiometers were utilized by Hänninen et al. [2000], who defined the rotation of the MCG pattern as the orientation of the maximum surface gradient (¶B_z/¶x, ¶B_z/¶y). The rotation angle of the maps between the rest and post-exercise were used to identify the patients in three subgroups of single-vessel coronary artery disease. The same method as in the present study has recently been applied in analyzing the ST-segment in ischemic patients [Kandori et al., 2001], and the end of the T-wave in patients with congenital long-QT syndrome [Kandori et al., 2002]. In addition, the method can be utilized in obtaining an initial estimate in reconstructing source current distributions in the heart [Koch 2001].

Our initial results seem to indicate that the MCG is more sensitive than the ECG to separate the right and left atrial activations. In addition, the method appears potential in determining the duration and spatial sequence of excitation in each atrium, e.g., in testing the influence of various drugs to treat atrial arrhythmias. The analysis was however applied only in one normal subject, and a larger group of subjects and patients need to be analyzed to evaluate and validate the method.

References

Cohen D, Hosaka H. Magnetic field produced by a current dipole. Journal of Electrocardiology, 9, 409-417, 1976.

Hänninen H, Takala P, Mäkijärvi M, Montonen J, Korhonen P, Oikarinen L, Nenonen J, Katila T, Toivonen L. Detection of exercise induced myocardial ischemia by multichannel magnetocardiography in single vessel coronary artery disease. Annals of Noninvasive Electrocardiology, 5, 147-157, 2000.

Kandori A, Kanzaki H, Miyatake K, Hashimoto S, Itoh S, Tanaka N, Miyashita T, Tsukada K. A method for detecting myocardial abnormality by using a total current-vector calculated from ST-segment deviation of a magnetocardiogram signal. Medical & Biological Engineering & Computing, 39, 21-28, 2001.

Kandori A, Shimizu W, Yokokawa M, Maruo T, Kanzaki H, Nakatani S, Kamakura S, Miyatake K, Murakami M, Miyashita T, Ogata K, Tsukada, K. Detection of spatial repolarization abnormalities in patients with LQT1 and LQT2 forms of congenital long-QT syndrome. Physiological Measurement, 23, 603-614, 2002.

Koch H. SQUID magnetocardiography: status and perspectives. IEEE Transactions on Applied Superconductivity, 11-1/1, 49-59, 2001.

Nenonen J, Montonen J, Mäkijärvi M. Principles of magnetocardiographic mapping. In: Cardiac Mapping (Second Edition), Shenasa M, Borggrefe M, Breithardt G, editors. Blackwell Publishing Inc./Futura Division, 2003, 119-129.