Vector magnetocardiography method and vector magnetocardiographic system

Abstract

A vector magnetocardiography method and a vector magnetocardiographic system carrying out the method. It is an object to provide an improved magnetocardiographic method, in particular in view of diagnosing ischemic heart disease. The method involves the calculation of the direction of an equivalent single current source ESCS or equivalent single magnetic source ESMS in relation to a reference direction which is independent of patient or heart position in order to compute a vector magnetocardiogram.

Claims

1. A vector magnetocardiography method, the method comprising the steps of: a. measuring direction and magnitude of the three orthogonal components of the magnetic field(s) generated by the heart of a subject during a period of heart activity using one or more magnetic field sensors, b. locating, using the data measured in step a, a reference source position for heart magnetic and/or electric activity, the reference source position being a point source suitable for representing the source of the magnetic and/or electric activity during said period of heart activity, c. evaluating a reference direction from the data measured in step a, reference direction being the mean direction of the magnetic moment and/or electric current in the heart muscle volume during a period of magnetic and/or electric heart activity with known direction of the magnetic moment and/or electric current in relation to the heart anatomy, d. calculating, from the data measured in step a, an equivalent single current source ESCS or an equivalent single magnetic source ESMS at the reference source position located in step b, the ESCS or ESMS representing an electric heart vector EHV or a magnetic heart vector MHV, in relation to the reference direction evaluated in step c, and e. registering the EHV and/or MHV, calculated in step d, during at least part of the heart activity in a vector magnetocardiogram.

2. The vector magnetocardiographic method according to claim 1, wherein a. in step b data measured during a strong magnetic and/or electric heart activity, preferably during the strongest magnetic and/or electric activity of a period of strong magnetic and/or electric heart activity, preferably during the R peak or T peak, is used for locating the reference source position, and/or b. in step c the reference direction of the magnetic field and/or electric current in the heart is evaluated during the R peak.

3. The vector magnetocardiographic method according to claim 1, wherein, in step b, the reference source position is located by computing a pseudocurrent map and/or a magnetic field map using the data measured in step a.

4. The vector magnetocardiographic method according to claim 1, wherein the difference in direction and/or magnitude between the EHV and/or MHV at different points of the vector magnetocardiogram are calculated, the different points representing different points of time during the heart sinus rhythm.

5. The vector magnetocardiographic method according to claim 4, wherein the difference in direction and/or amplitude between the EHV and/or MHV at T.sub.max and T.sub.end is calculated.

6. A vector magnetocardiographic system being adapted for carrying out the method according to claim 1.

7. A vector magnetocardiographic system according to claim 6, comprising a. one or more magnetic field sensors for measuring direction and magnitude of the three orthogonal components of the magnetic field(s) generated by the heart of a subject during a period of heart activity, b. a means for locating, using the data measured by the one or more magnetic field sensors, a reference source position for heart magnetic and/or electric activity, the reference source position being a point source suitable for representing the source of the magnetic and/or electric activity during said period of heart activity, c. a means for evaluating a reference direction from the data measured by the one or more magnetic field sensors, the reference direction being the mean direction of the magnetic moment and/or electric current in the heart muscle volume during a period of magnetic and/or electric heart activity with known direction of the magnetic moment and/or electric current in relation to the heart anatomy, d. a means for calculating, from the data measured by one or more magnetic field sensors, an equivalent single current source ESCS or an equivalent single magnetic source ESMS at the reference source position, the ESCS or ESMS representing an electric heart vector EHV or a magnetic heart vector MHV, in relation to the reference direction, and e. a means for registering the EHV and/or MHV during at least part of the heart activity in a vector magnetocardiogram.

Description

[0048] In the following, the invention is described in more detail by way of an example and the attached figures for illustration purposes only.

[0049] FIG. 1. Schematic illustration for locating a reference source position using a pseudocurrent map.

[0050] FIG. 2. Exemplary VMCG of a healthy subject established using the method of the invention.

[0051] FIG. 3. 2-D representation of the VMCG of FIG. 2 corresponding to an equivalent single current source (ESCS). The current units given are [A] 4 .sub.010.sup.6, where .sub.0 is the magnetic permeability of vacuum.

[0052] FIG. 4. 2-D representation of a VMCG of a patient with single vessel disease in the LAD. Current units of the axis are the same as in FIG. 3.

[0053] FIG. 1 shows a standard pseudocurrent map for a 2020 source grid, using 13 eigenvalues, for finding the optimal location of an ESMS. The pseudocurrent map was established from data measured during the R peak of the sinus rythm. The encircled area denotes the region of strongest electric activity during the R peak.

[0054] A VMCG of a healthy subject is depicted in FIG. 2. The large loop denoted with the reference numeral 1 represents the QRS complex, and includes the R peak 2, the smaller loop 3 represents the T peak.

[0055] FIG. 3 shows a 2-D representation of the VMCG of FIG. 2 corresponding to an equivalent single current source (ESCS). Such a representation is especially useful to facilitate angle measurements, i.e. a change in direction of MHV/EHV. The Q, R, S, T and even P waves are visible. The P wave at 125 ms is denoted with numeral 5, the R peak at 200 ms with numeral 2, T.sub.max at 339 ms with numeral 6. The current directions during the entire sinus rythm are congruent with literature. Each segment 4 of the VMCG corresponds to an MHV for identical time intervals during the sinus rhythm, the length of each segment representing the amplitude of the magnetic field, and the orientation the direction in relation to the reference direction.

[0056] FIG. 4 is a 2-D representation of a VMCG of a subject with single vessel disease in the left anterior descending artery (LAD), confirmed by angiography. The current direction during the T-wave (T.sub.max 6 at 381 ms) is about 180 o compared to the R peak (at 200 ms).

REFERENCES

[0057] [1] George E Burch. The history of vectorcardiography. Medical History, 29(55):103-131, 1985.

[0058] [2] David Cohen, J C Norman, F Molokhia, and W Hood. Magnetocardiography of direct currents: St segment and baseline shifts during experimental myocardial infarction. Science, 172(3990):1329-1333, 1971.

[0059] [3] Harold W Draper, Catherine J Pener, Friedman W Stallman, David Litmann, and Hubert V Pipberger. The corrected orthogonal electrocardiogram and vectorcardiogram in 510 normal men (frank lead system). Circulation, 30(6):853-864, 1964.

[0060] [4] Ernest Frank. An accurate, clinically practical system for spatial vectorcardiography. Circulation, 13(5):737-749, 1956.

[0061] [5] Wolfgang Haberkorn, Uwe Steinho, Martin Burgho, Olaf Kosch, Andreas Morguet, and Hans Koch. Pseudocurrent density maps of electrophysiological heart, nerve or brain function and their physical basis. Biomagnetic Research and Technology, 4, 2006.

[0062] [6] Helena Hnninen et al., Multichannel magnetocardiography and body surface potential mapping in exercise-induced myocardial ischemia. University of Helsinki, 2002.

[0063] [7] Helena Hnninen, Panu Takala, Petri Korhonen, Lasse Oikarinen, Markku Mkijrvi, Jukka Nenonen, Toivo Katila, and Lauri Toivonen. Features of st segment and t-wave in exercise-induced myocardial ischemia evaluated with multichannel magnetocardiography. Annals of medicine, 34(2):120-129, 2002.

[0064] [8] Helena Hnninen, Panu Takala, Markku Mkijrvi, Juha Montonen, Petri Korhonen, Lasse Oikarinen, Jukka Nenonen, Toivo Katila, and Lauri Toivonen. Detection of exercise-induced myocardial ischemia by multichannel magnetocardiography in single vessel coronary artery disease. Annals of noninvasive electrocardiology, 5(2):147-157, 2000.

[0065] [9] Hyun Kyoon Lim, Namsik Chung, Kiwoong Kim, Young-Guk Ko, Hyukchan Kwon, Yong-Ho Lee, Jin-Bae Kim, Jung Rae Cho, Jin-Mok Kim, In-Seon Kim, et al. Reproducibility of quantitative estimate of magnetocardiographic ventricular depolarization and repolarization parameters in healthy subjects and patients with coronary artery disease. Annals of biomedical engineering, 35(1):59-68, 2007.

[0066] [10] Jaakko Malmivuo and Robert Plonsey. Bioelectromagnetism: principles and applications of bioelectric and biomagnetic fields. Oxford university press, 1995.

[0067] [11] J T Nenonen, M S Hmalinen, and R J Iimoniemi. Minimum-norm estimation in a boundary-element torso model. Medical and Biological Engineering and Computing, 32(1):43-48, 1994.

[0068] [12] J W Park and F Jung. Qualitative and quantitative description of myocardial ischemia by means of magnetocardiography. Biomedizinische Technik. Biomedical engineering, 49(10):267-273, 2004.

[0069] [13] Kirsten Tolstrup, Bo E Madsen, Jose A Ruiz, Stephen D Greenwood, Judeen Camacho, Robert J Siegel, H Caroline Gertzen, J-W Park, and Peter A Smars. Non-invasive resting magnetocardiographic imaging for the rapid detection of ischemia in subjects presenting with chest pain. Cardiology, 106(4):270-276, 2006.

[0070] [14] Satsuki YAMADA and Iwao YAMAGUCHI. Magnetocardiograms in clinical medicine: unique information on cardiac ischemia, arrhythmias, and fetal diagnosis. internal Medicine, 44(1):1-19, 2005.