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Magnetocardiography in Non-Invasive Cardiology

Magnetocardiography (MCG) is a method for investigating electrophysiological processes in the heart while analyzing parameters of its magnetic field recorded by non-invasive (and non-contact) way at observation points in the air (usually above a patient's chest). History of the magnetocardiography takes its origin in 1963, when Baule G. and McFee R. made first measurements of parameters of human heart magnetic field in Syracuse University [1,2].

Since that time the MCG research method has covered a significant way in its evolution and still continues its dynamic development nowadays (see, e.g., [3-5]). The MCG examination is aimed at analysis of electrical activity in the myocardium. The same purpose is inherent in electrocardiography (ECG). Since magnetocardiography and electrocardiography are based on the same electrophysiological sources, the data they bear complement each other and are interrelated. A new, more informative content of MCG is associated with its extreme sensitivity to tangential components of the heart excitation wave and less (compared to ECG) dependence of magnetic field measured parameters at an observation point upon effect of multi-layered conducting medium containing the field source. Currently many experimental and clinical works have proved much higher spatial resolution of magnetocardiography as compared to electrocardiography, i.e. its much higher sensitivity to local currents. These currents, usually low in magnitude, emerge at interfaces of myocardium portions having different electro-physical properties, in other words, with different duration of action potentials of their sells. Such currents are much more legible in magnetic field than in electric one. This advantage becomes especially obvious when there are two excitation fronts, close in magnitude and opposite in direction. In this case they compensate for each other in electric field, but in magnetic field they will be explicitly displayed. Keeping this in mind, Baule's and McFee's comment on MCG method becomes clear, that "magnetocardiography allows to reveal such components of heart electro-motive force which otherwise would stay "silent" [1,2]. Sensitivity to local currents allows to assess homogeneity of myocardium in electrical terms.

We can say that similar to the fact that appearance of electro-physical examination in the 60-ties introduced into practical work the criteria based on assessment of conduction rate in different portions of a heart conducting system, duration of refractory periods, the magnetocardiography introduces into "bloodless" clinical electrocardiology a very important notion - electric homogeneity of myocardium.

To recap, a conclusion can be made that magnetocardiography possesses easiness and safety for patients inherent in non-invasive methods, along with carefulness and accuracy of direct methods of myocardium electrophysiology. It should be recognized that such an advantageous combination has to be "reimbursed for" by some technical complexities. Magnetic field of the Earth is on the order of 10-4 T, and magnetic field of human heart makes up tens of picoTesla ((10-50)´10^-12 T). It is clear that to record such a weak field an extremely sensitive sensor is needed. Such a sensor is a superconductive quantum interferometer (SQUID) [7] operating at low temperatures, for that it must be placed into liquid helium (or liquid nitrogen). Besides, special technical methods shall be used to reduce effect of external noise on MCG signal. These all makes the MCG systems a little bit more expensive and requirements to the level of maintenance personnel higher.

A reasonable question might arise - whether it is possible to obtain information inherent in magnetocardiogram by other, less expensive and simple method (even with some loss in accuracy and details), e.g., using electrocardiographic surface mapping? As it is well known, this method to some extent allows to judge the myocardium homogeneity also. Responding to this question it should be pointed out that comparison of diagnostic capabilities of MCG and ECG of mapping was performed in a lot of studies. In an absolute majority of them it was shown that diagnostic sensitivity of MCG is much higher, sometimes several times higher. This also refers to diagnostics of both arrythmogenic substrate and ischemic changes of myocardium, localization of accessory pathways and to actopic focuses. This enables us to believe that MCG and ECG mapping are methods of basically different level of diagnostic sensitivity. As to comparison of practical utility of these methods, then, as it is known, electrographic mapping entails a series of problems associated with need of fixing a large number of electrodes. And very often in this case it is hard to get a good quality signal due to higher electric skin resistance, big hair coverage on a chest, skin diseases, burns, etc.. All the above makes the ECG mapping rather hard both for a patient and personnel and constrains its spread. In case of MCG examination the sensor does not touch skin, and therefore it's not necessary even to undress a patient.

But how to obtain all the data contained in magnetocardiogram? To this end, experts developed and built a technology for recording and analyzing parameters of heart magnetic field consisting of three sequential stages.

• The 1^st stage comprises magnetocardiogram recording and its entering into computer according to developed algorithms of measurements.

• The 2^nd stage comprises signal conversion aimed at determining required parameters of heart magnetic field, and then to solve an inverse problem of magnetostatics and compute parameters and distribution of sources of biomagnetic signal generating this field, and finally, present this information for subsequent "medical" analysis.

• The 3^rd stage can be called "medical". It involves formulation of diagnostics conclusion based on developed diagnostically important criteria and signatures, and taking a clinical decision based on this conclusion.

In its turn the "medical" stage also consists of a series of successive stages corresponding to level of analysis of MCG-signal. There are a morphological analysis of MCG, qualitative and quantitative assay of dynamic maps of magnetic field distribution, analysis of source parameters generating this field. Each stage expands and complements to diagnostic information, and at each stage new and original methods were developed for processing and analysis of MCG-signal and “know-how”. Such a logical sequence of actions and a complex approach to solving the problem allow to obtain valid diagnostically valuable information under conditions of a usual room which is non-shielded or shielded against external noise.

1.2. Clinical Experience

Using developed algorithms for MCG signal processing a doctor can estimate at any moment of cardio-cycle a number of excitation sources in myocardium, their location, intensity, spread, direction of excitation fronts, as well as changes in all these parameters during the entire cardio-cycle or its certain period.

During few years already this information has been used by our medical colleagues and has been helping to solve real diagnostic problems of cardiological ward.

Main tasks and clinical situations resolution of which use MCG are the following:

1. MCG is applied for localization of the additional conducting pathways (ACP) and determining effectiveness of radio frequency ablation of these pathways. Preliminary out-surgery non-invasive ACP locating allows to reduce time for a surgery.
2. Determining presence and degree of myocardium ischemia, including its early stage.

Clinical research shows that MCG in rest possesses high sensitivity to chronical ischemia even among patients with stable or slightly changed ECG in rest. MCG examination allows to solve the issue of whether there is an indication to medication antianginal therapy, specify indications to coronaroangiography.

3. Determining effectiveness of antianginal therapy, including period after cardiac infarction. The examination helps to address the issue of dose change or change in antianginal medicine.
4. Determining risk of paroxism of ventriclar tachycardia. The examination allows to solve a question of antiarythmic therapy, specify indications for invasive EPE.
5. Determining nature of a disease resulted in heart size increase (cardio megalium). The examination allows in controversial cases to solve the issue of main disease, and this effects tactics of treatment.

All these problems are addressed with an array of original qualitative and quantitative criteria which, in different situations, produce sensitivity up to 90%.

Uncertainty in spatial localization of the accessory pathways is less than 5 mm.

1.3. MCG Niche Among Other Instrumental Research Methods

MCG is part of complex examination of cardiological patients that significantly increases its effectiveness. As a rule, it is impossible to diagnose the primary case on the basis of MCG-examination only. However, when the diagnosis is certain already, in case of dynamic observation the MCG is able (it's experts' opinion) not only to complement, but also to replace other tests which are unpleasant or hazardous for a patient. So, MCG can replace repetitive electro-physiological examination (EPE) while deciding on effectiveness of surgical destruction or pharmacological blockage of ACP, as applied to clinically clear cases, and estimating effectiveness of anti-arythmic therapy. MCG can also replace repetitive load test, especially when there are relative contraindications against the load. Experts hope that in the future some part of coronarography and radioisotopic examination can be replaced by magnetocardiography.

Conclusion

Analysis of operation of singlechannel magnetocardiograph under conditions of a real clinic shows that algorithms for MCG data processing being used give valid results in case of locating arythmogenic zones in human heart [16]. They also allow to reveal and assess degree of ischemic and non-ischemic damage of myocardium [18]. As it was described above the solution of these tasks assumes implementation of three stages of analysis of heart magnetic parameters - recording, analysis and medical interpretation of MCG-data.

Logical sequence of actions and a complex approach to analysis of the problem solution at all the stages allows to obtain valid, diagnostically valuable information using single- or multichannel MCG-system under conditions of a usual room which is non-shielded or shielded against external noise.

REFERENCES

1. Baule G., Mc Fee R. Detection of the magnetic field of the heart , Amer.Heart J., 1963, V.66, N 1, p. 95 - 96.

2. Baule G., Mc Fee R. Theory of magnetic detection of the heart’s electrical activity, J.Appl.Phys., 1965, V.36, N 6, p. 2066 - 2073.

3. Biomagnetism : Clinical Aspects, Proceedings of the 8th International Conference on Biomagnetism, M.Hoke et al (Eds), EM, 1992, p.904.

4. Biomagnetism : Fundamental Research and Clinical Applications, Proceedings of the 9th International Conference on Biomagnetism, Elsevier, IOS Press, 1995, Baumgartner et al (Eds), p.846.

5. BIOMAG 96 Abstracts, Santa Fe, NM, USA, 1996, p. 378.

6. Primin M.A., Gumenyuk-Sychevsky V.I., Nedaivoda I.V.. Methods and Algorithms for Locating Magnetic Field Source, Kiev, “Naukova Dumka”, 1992, p.96.

7. Weak Superconductivity. Quantum Interferometers, Their Application /Edited by. B.B. Shwarts, S.Phoner. - M.:Mir, 1980. -p.256.

8. Primin M., Gumeniuk - Sychevskij V., Nedayvoda I. Algorithms of data processing for distributed multichannel squidgradiometer system in the case of magnetic field source localization // Cryogenics.- 1992.-32.-p. 529-532.

9. Primin M., Gumeniuk - Sychevskij V., Nedayvoda I. Preliminary determination of measurement device position in biomagnetic research: using a reference source // Int. J. Of Applied Electromagn. In Materials. - 1993. - 3. - p.263-268.

10. Primin M. A., Nedaivoda I. V. Algorithms of Biomagnetic Data Processing // Electron. Modeling. - 1998. -¹1 - p.103-113.

11. Primin M., Nedayvoda I., Gapeliuk A., Matlashov A. Determination of coordinates of magnetometric system sensors: using a reference source / Biomagnetism: fundamental research and Clinical Applications // Ed. C. Baumgarther, L. Deecke, G. Stroink, S.J. Williamson. - Elsevier, Amsterdam, 1995. - p.467-470.

12. Primin M., Gumeniuk - Sychevskij V., Nedayvoda I. Mathematical models and algorithms of information conversion in spatial analysis of weak magnetic fields / Int. J. Applied Electromagn. In Materials. - 1994. - 5. - p.311-319.

13. Primin M., Nedayvoda I. Mathematical model and measurement algorithms for a dipole source location // Int. J. Applied Electromagn. and Mechanics. - 1997. - 8. - p.119-131.

14. Primin M. A., Nedaivoda I. V. New Algorithms of Biomagnetic Data Processing// US&M. - 1995. - ¹3. - p.3-11.

15. MurashkoV.V., Strutynsky A.V.. Electrocardiology . M.: Medpress, 1998.- p. 312.

16. Gapelyuk A., Schirdewan A., Primin M. et.al. Evaluation of MCG localization results: the importance of invasively measured electrophysiological time intervals, Biomag 96, Abstracts, Santa Fe, USA, 1996, p.214.

17. Primin M. A., Hedaivoda I. V. Mathematical Model and Method for Building Spatial-Time Spectrum of Magnetic Cardio Signal // Electron. Modeling. - 1998. -¹3. - p.109-118.

18. Gapelyuk A., Schirdewan A., Primin M. et.al. Magnetocardiographic evaluation of repolarization process in coronary artery disease patients prone to malignant ventricular tachycardia. World congress of medical Physics and Biomedical Engineering, Nice, France, p.16.

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