
been detected with a high- T
c
SQUID (Drung et al.
1996) having a white noise of o10 fTHz
1/2
.
4. Magnetocardiography (MCG)
Although the maximum amplitude of MCG signals is
1–2 orders of magnitudes higher than typical MEG
signals it is necessary to achieve a similarly good sig-
nal-to-noise ratio, as the relevant signal components
may be of comparably small amplitude. Clinically
interesting applications are seen in the field of risk
stratification of sudden heart death and myocardial
vitality.
One way to derive relevant information from mag-
netic field maps is to recognize specific patterns or
features in the field maps that can be considered as a
‘‘fingerprint’’ or ‘‘signature’’ of certain physiological
or pathophysiological functions in which the physi-
cian is interested and wants to discriminate. A com-
mon approach is then to quantify these signatures by
derived characteristic factors. For instance, patients
who suffer from coronary artery heart disease, and
thus have a high risk of a sudden heart death, show
distinctive alterations in magnetic field maps com-
pared to those of normal subjects. The factors men-
tioned above reflect these alterations and give a
valuable indication as to this risk.
5. Source Localization
Many applications demand a ‘‘localization’’ of the
physiological or pathophysiological function looked
for. Technically, this requires a reconstruction of the
current density distribution, the ‘‘source,’’ that gen-
erated the measured magnetic field pattern. Mathe-
matically, this inverse problem, i.e., the calculation of
the source distribution with the help of measured field
values, is a so-called ill-posed problem, because prin-
cipally no unique solution exists. However, with the
aid of appropriate models an acceptable approxima-
tion of the source may be derived. The most common
model is the equivalent current dipole, which is of
good value in mapping localized brain functions such
as evoked cognitive responses.
A more sophisticated algorithm (Fuchs et al. 1995
pp. 320–5) models the current density distribution of
the investigated brain or heart function by many
small current dipoles distributed over the cortex or
the heart ventricle only at such positions that come
into question on physiological grounds. In this way,
the possible solutions of the inverse problem are ef-
fectively constrained. The proper anatomic geometry
is derived from magnetic resonance imaging.
6. Towards Unsh ielded SQUID Systems
In order to suppress noise due to magnetic fields
generated, for example, from power line hum, electric
street cars, etc., biomagnetic systems are commonly
operated in shielded rooms with thick walls consist-
ing of several layers of mu-metal. Such shielded
rooms make SQUID systems very expensive and
prohibit a wider distribution of biomagnetic meth-
ods. Therefore, many approaches have been tried to
reduce the need for passive shielding.
One way is to use gradiometer concepts, i.e., in-
stead of measuring with one magnetometer the mag-
netic flux at one position, one determines the
difference of magnetic flux of two adjacent positions
(i.e., the approximation of the gradient of the mag-
netic field). As the source to be investigated is close to
the sensors it provides a strong gradient as compared
to a distant interfering source and thus the source
signals are enhanced with respect to the interference.
Gradiometric configurations may be achieved with
appropriately formed pick-up coils: vertical or planar
gradiometers. Another way is to use two magneto-
meters and subtract their signals from each other
electronically, thus forming ‘‘electronic’’ gradiome-
ters. Similarly ‘‘software’’ gradiometers may be
designed.
A modern solution is to configure the SQUIDs to
form a reference system that provides all signal in-
formation for a sophisticated algorithm, which then
generates the appropriate compensation signals to
each of the SQUIDs in the measurement plane of a
multichannel system. At sites with only moderate in-
terference, such systems have successfully demon-
strated an unshielded performance. However, in
urban or clinical environments a passive shield may
still be necessary. For a detailed analysis of the topic
of interference discrimination see Vrba (1996 pp.
117–78).
With HTS SQUIDs unshielded operation is even
more difficult owing to the sensitivity to trapped and
moving flux lines in the high-T
c
material, particularly
if the sensors are cooled and moved in the earth’s
magnetic field.
See also: SQUIDs: The Instrument; SQUIDs: Non-
destructive Testing
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2
Cu
3
O
7x
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Fuchs M, Wagner M, Wischmann H -A, Do
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ssel O 1995 Cor-
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SQUIDs: Biomedical Applications