Biomedical Engineering Reference
In-Depth Information
FIGURE 4.45: (SEE COLOR INSERT) Recording and analysis of atrial mag-
netic fields. (A) The sensor arrangement of a 33 triple sensor (99-channel) magne-
tometer. Superimposed on sensor array is the antero-posterior view of the left atrium
by electroanatomic mapping. (B) Signal—averaged magnetic field density on each
magnetometer channel over cardiac cycle. The onset and end of atrial signal are de-
termined automatically using filtering technique. (C) Spatial distribution of the mag-
netic field B z component over the middle part of atrial complex interpolated from
the measurement using multipole expansion. The blue color indicates flux out of the
chest (-) and red color flux into the chest (+). The step between two consecutive lines
is 200 fT. (D) Pseudocurrent map derived by rotating magnetic field gradients by
90 â—¦ . The red-yellow color indicates the area of the top 30% of strongest currents,
and the large arrow indicates their mean direction. Zero angle direction is pointing
from subject's right to left and positive clockwise. From [Jurkko et al., 2009].
non-invasive localization of ischemic areas in patients with coronary artery disease
is possible when proper regularization is applied.
4.2.4.2 Fetal MCG
Magnetic field measurement is a promising technique for evaluation of fetal well-
being. Fetal biomagnetic signals are unaffected by poor electrical conductivity of the
vernix caseosa, a waxy substance which forms on the fetal skin at about 25 weeks'
gestation and impedes the transmission of fetal bioelectric signals. Fetal magnetocar-
diography (fMCG) has the potential to provide beat-to-beat fetal heart rate in case
of normal rhythm and its disturbances. Amplitude changes in fMCG and fetal heart
rate acceleration make fetal movement assessment possible by means of a technique
called actocardiography [Zhao and Wakai, 2002].
Various groups reported good quality fMCG recordings in which P, QRS, and T
waves may be distinguished even without signal averaging. However, usually after
bandpass filtering and maternal MCG removal, the averaged waveforms are com-
puted after aligning fetal complexes, e.g., by means of autocorrelation. This kind of
analysis was used for finding fetal cardiac repolarization abnormalities [Zhao et al.,
2006].
For the determination of fetal heart rate variability (fHRV), methods similar to the
ones described in the section concerning fECG may be used. In [Wilson et al., 2008]
another approach was applied. The aim of the study was determination of the fetal
 
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