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
We present methods to compute the imbalance in a gradiometer of arbitrary shape due to imperfections in its geometry, eddy currents induced in the radio-frequency interference shield, and screening currents induced in the modules of the superconducting quantum interference devices (SQUIDs). As an example, the methods are applied to evaluate the maximum expected initial imbalance of second-and third-order axial gradiometers in a measuring setup designed for fetal magnetocardiography. Mechanical imperfections in this specific setup appear to have the largest effect: the field imbalance is 2 10 2 ; the first-order gradient imbalance is 10 3 m; the second-order gradient imbalance is 10 4 m 2 . In the example, the imbalance caused by the other effects is one order smaller.
Abstract-The optimum geometry of a third-order gradiometer for operation in unshielded environments is discussed. The optimization result depends on the specific signal and noise conditions. The fetal heart is considered as an example of the signal source. We optimized the gradiometer such that the signal-to-noise ratio is maximized in an averaged sense for all relevant environmental noise conditions and distances to the signal source. The resulting design consists of two second-order gradiometers that can be combined to form a third-order gradiometer in noisy environments, whereas a single second-order gradiometer can be used in lownoise environments. The gradiometer can provide the signal-tonoise ratio that allows detection of fetal heart signals in all relevant environmental noise conditions.
Abstract-We derive expressions for the magnetic flux in a circular loop due to eddy currents and thermal noise in coaxial metallic disks. The eddy currents are induced by an applied field that changes sinusoidally in time. We give expressions for the eddy current noise when the frequency of the applied field is very low as well as when it is very high. We combine these expressions to obtain one that is valid over the whole frequency range. The theoretical results agree well with experimental ones obtained by means of a superconducting quantum interference device (SQUID) magnetometer system. We also studied the flux due to thermal noise; again, the theoretical results show fair agreement with the experimental ones.
Abstract-To use fetal magnetocardiography for diagnostic purposes, it is important to know the requirements for the instrument. One of the questions to be answered is how sensitive the fetal magnetocardiograph must be. In this paper the requirements will be discussed and a highly sensitive magnetocardiograph, that is optimized for fetal magnetocardiography in a magnetically shielded room, will be presented. Keywords-fetal magnetocardiography, fetal arrhythmia I. INTRODUCTIONThe fetal magnetocardiograph is intended to measure magnetic fields arising from currents generated in the fetal heart. These fields are extremely weak and can only be detected by means of a superconducting quantum interference device (SQUID) cooled by liquid helium. Usually, the mother is lying in supine position underneath the vessel (i.e. cryostat) containing the SQUID immersed in liquid helium, as illustrated in Fig. 1. Fetal magnetocardiograms (MCGs) can be measured reliably from the 20 th week of gestation onward. However, customarily the data are averaged, because in the raw data often the P-wave and the T-wave are not discernible and the duration of the various waves cannot be extracted from the raw data. A signal can be averaged if the R-peak signal is clearly observable in the raw data and if enough heart cycli are measured that are correlated.Fetal MCG can be used for the detection and classification of arrhythmias and the study of congenital heart diseases [1,2]. Though others techniques do exist (fetal ECG, ultrasound), they either do not provide information about the electrophysiology of the fetal heart, the reliability is low, or they have a low resolution making it difficult to apply them for certain clinical applications.The most common arrhythmia in a fetus are isolated extrasystoles. In order to diagnose them it is enough to measure the fetal heart rate with a beat-to-beat accuracy. The same is true for the diagnosis of tachycardia and bradycardia. To distinguish atrioventricular blocks in second-or thirddegree ones, it is necessary to see the P-waves and the QRScomplexes in the raw data. The same is true for atrial flutter and extrasystoles (in order to be able to determine whether the extrasystoles have a ventricular or supraventricular origin). In our experience, the P-wave is somewhat larger in case of a heart block than in normals. This may be due to hypertrophy of the atria. Precise determination of the atrioventricular relationship in cases of supraventricular tachycardia is a major element on which the choice of an appropriate antiarrhythmic agent is based. All ultrasonographic approaches are overestimating the PRinterval because isometric contraction times and electromechanical delays are included. Moreover, the inaccuracy in the PR-interval determination is about 30 ms [3]. With fetal MCG it is possible to determine the PRinterval with an accuracy of 5 ms and the RR-interval with 2 ms. This requires that the fetal MCG should be recorded at several positions above the maternal abdomen because if the fetal MCG is re...
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