Fetal Magnetocardiography

Chapter 1 Fetal Magnetocardiography Maria J. Peters, Jeroen G. Stinstra, Siarhei Uzunbajakau, and Narayanan Srinivasan ABSTRACT Fetal magnetocardiography is a non-invasive method to study the fetal heart: the patient (i.e., the mother) is not even touched. A fetal magnetocardiogram (MCG) is the registration of a component of the magnetic field generated by the electrical activity of the fetal heart. Usually the component of the magnetic field that is perpendicular to the maternal abdomen is measured. Fetal MCGs show the typical features that are found in ECGs of adults (i.e. a P-wave, QRS-complex and T-wave). To enable the discrimination between pathological and healthy fetuses, values of the duration of these waveforms are collected in several research groups. These durations can be used as a reference. Measurements show that MCGs of fetuses with severe congenital heart disease have an abnormal shape. Hence, fetal MCGs may be of help in the early intra-uterine detection of congenital heart anomalies and the progress of the disease. Fetal magnetocardiography can also be used to classify fetal arrhythmias. The fetal MCG is a very weak signal (about 10−13 tesla) compared with fields that are present in a hospital. The Earth’s magnetic field, for example, is about 5 × 10−5 tesla. The only magnetic field sensor that is sensitive enough to measure fetal MCGs is a SQUID. This sensor has to be cooled in liquid helium. The vessel containing the helium and the sensor is positioned near the maternal abdomen. At present, fetal MCGs are measured within magnetically shielded rooms Maria J. Peters, Jeroen G. Stinstra, Siarhei Uzunbajakau Faculty of Science and Engineering, University of Twente, 7500 AE Enschede, The Netherlands. Narayanan Srinivasan Biomedical Engineering Research Centre, Nanyang Technological University, 639 798 Singapore. Advances in Electromagnetic Fields in Living Systems, Volume 4, edited by James C. Lin, Springer Science+Business Media, New York, 2005. 1 2 Maria J. Peters et al. in order to avoid disturbing fields. Signal processing techniques, such as filtering and averaging, are used to enhance the signal–to-noise ratio. The electrical activity in the heart gives rise to currents in the fetus and maternal abdomen. These currents also contribute to the fetal MCG. In order to estimate this influence, simulations are carried out and discussed in the last section of this chapter on fetal MCG. 1. INTRODUCTION Fetal magnetocardiography involves the acquisition and interpretation of the magnetic field near the maternal abdomen due to the electrical activity of the fetal heart. A fetal magnetocardiogram (MCG) can be recorded reliably from the 20th week of gestation onward. Fetal magnetocardiography is a truly non-invasive technique in which the body is not even touched. It can be used to classify arrhythmias and to diagnose certain congenital heart de- fects. Hence, it may help to provide optimum care for the patient. However, until now, fetal MCGs have mainly been measured in research laboratories. Apparently, the application of fetal magnetocardiography in a clinical setting is hampered by the fact that the measuring instrument and its exploitation are still rather expensive and skilled personnel is needed to carry out the measurements. However, it is expected that these problems will be overcome in the near future. A recording of a component of the magnetic field caused by the electrical activity of the fetal heart as a function of time is called a fetal MCG. This magnetic field is the result of the synchronous activity of many heart cells; the magnetic field of a single cell cannot be measured. At least 104 heart cells forming a dipole layer have to be active simultaneously. The first fetal MCG was measured in 1974 [Kariniemi et al. 1974]. The subsequent 25 years saw a slow and steady growth of interest in fetal magnetocardiography. During that period, the development of biomagnetometer systems was primarily dedicated to the study of the adult brain and heart. However at the beginning of the nineties, fetal MCG regained some popularity and instruments specially designed for measuring the fetal MCG became commercially available. An example of such a fetal magnetocardiograph is shown in Fig. 1. Currently, several groups in the world are developing systems for measuring fetal MCGs in an unshielded environment as well as collecting MCGs from healthy fetuses and of fetuses with a multitude of heart diseases. Several aspects of fetal magnetocardiography will be considered in this chapter. In section 2, a short description of the electrical activity of the heart will be followed by a discussion on other methods for fetal surveillance and subsequently, fetal MCGs of healthy fetuses will be characterized. In section 3, the detection of fetal arrhythmias and congenital heart diseases by means of fetal MCG will be discussed. As fetal MCG signals are weak, an extremely sensitive sensor is needed. The most sensitive sensor for magnetic fields that fulfills the requirements is a SQUID. It has a sensitivity of some femtotesla (1 fT = 10−15 T) and is discussed√ in section 4. Magnetic disturbances in urban surroundings are roughly 106 − 107 fT/ Hz in the frequency range of fetal MCGs. The noise power spectrum shows also high peaks at specific frequency components, as for instance the frequency generated by power lines. A comparison with other magnetic field strengths is given in Fig. 2, making it clear that measures have to be taken to reduce the disturbances. In section 5, tools to Fetal Magnetocardiography 3 Figure 1. A multichannel fetal magnetocardiograph manufactured by CTF systems inc. enhance the signal-to-noise ratio will be discussed, i.e., magnetically shielded rooms and flux transformers. The subject of section 6 is signal processing. In order to estimate the influence of the tissues surrounding the fetal heart on the fetal MCG, models are composed that describe the fetoabdominal volume conductor. These models are used in simulations discussed in the sections 7 and 8. Magnetic field (tesla) 10−4 Magnetic field of the earth 10−5 −6 10 Magnetic disturbance in − 10 7 hospital 10−8 10−9 10−10 − 10 11 MCG adult 10−12 MCG fetus 10−13 10−14 SQUID sensor noise 10−15 Figure 2. A comparison of magnetic field due to a fetal heart with magnetic fields of other sources. 4 Maria J. Peters et al. 2. THE FETAL HEART Blood rich in oxygen and nutrients travels along the umbilical vein from the placenta to the fetal body. About half of the blood travels to the fetal liver; the rest goes via a vessel, called the ductus venosus, to the inferior vena cava. There the oxygenated blood from the placenta mixes with deoxygenated blood from the lower part of the fetal body. This mixture continues to the right atrium. In an adult heart, the blood from the right atrium enters the right ventricle and is pumped to the lungs. In the fetus, however, the lungs are nonfunctional and the blood largely bypasses them. A large part of the blood from the fetal right atrium is shunted directly to the left atrium through an opening in the atrial septum, called the foramen ovale. The rest passes into the right ventricle, where it enters the pulmonary circuit. The blood that enters the left atrium through the foramen ovale moves to the left ventricle and is pumped into the aorta. A part of the blood in the aorta reaches the myocardium, the brain, the lower regions of the body, and a part passes through the umbilical arteries to the placenta where it is reoxygenated. At the time of birth, important adjustments have to occur as the placenta ceases to function and the newborn starts to breathe. The anatomy of the heart is sketched in Fig. 3. The heart muscle, like other tissues in the human body consists of cells surrounded by a membrane. The electrical behavior of a single cardiac cell has been recorded by measuring the potential difference between an intracellular and an extracellular microelectrode. At rest, the intracellular potential is about 90 mV lower than the extracellular potential. Any process that abruptly changes the res ting potential above a critical value (the threshold) results in an action potential. An action potential is a rapid change in the intracellular potential, called the depolarization of the cell, and is followed by a return to the resting potential, called the repolarization of the cell. Typical action potentials recorded from a cell in the ventricles and from a cell in the sinoatrial (SA) node are depicted in Fig. 4. The various phases of the cardiac action Sympathetic nerve Parasympathetic nerve Aorta Superior veno casa Left pulmonary artery Sinoatrial node Left atrium Right atrium Bundle of His Atrioventricular Left ventricle node Right ventricle Figure 3. The conduction system of the heart. Fetal Magnetocardiography 5 Ventricle +20 1 2 0 −20 − 40 0 3 −60 −80 4 −100 SA node +20 0 −20 0 −40 3 −60 4 −80 −100 Figure 4. Typical action potentials r ecorded from a cell in the ventricles and from a cell in the sinoatrial (SA) node in an adult heart. potential are associated with changes in the permeability of the cell membrane, mainly to sodium, potassium, and calcium ions. The five phases are: 1) Phase 0 indicates the rapid depolarization and is related almost exclusively to a rapid influx of Na+ ions. 2) Phase 1 is a rapid partial repolarization achieved by the efflux of K+ ions. 3) Phase 2 the plateau phase, achieved by a balance between the influx of Ca++ ions and an efflux of K+ ions. 4) Phase 3 indicates the repolarization. 5) Phase 4 indicates the period between repolarization and the beginning of the ne xt action potential.

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