Biomedical Engineering Reference
In-Depth Information
real tissues there is a distribution of these dielectric dispersions
across the spectrum, which can be represented mathematically by
a superposition ofdispersions.
ε
(
ω
)
=
ε
∞
+
ε
1
[1
−
exp(
−
t
/τ
1
)]
+
ε
2
[1
−
exp(
−
t
/τ
2
)]
+
...
(3.8)
According to SFT, measurements of a planar slab, as illustrated
in Fig. 3.16, reveal both conduction and magnetic processes in
biological tissue. Both are forms of imaginary impedance; one is
positive impedance, while the other is negative. Hence if both
processes are present, then the measured result will be overall
impedance, thedifference betweenthe two.
Thevariousdispersions,bothpermittivityandconductivityasso-
ciated with a particular tissue, are the result of differing underlying
biophysical mechanisms. It is common that there exist three or
moredispersionstermedalpha,beta,gammaanddelta(
α
,
β
,
γ
,
δ
)in
tissues.Forinstance,relativepermittivitydecreaseswithincreasing
frequency in three or more major steps.
10
2
Hz)
is associated with polarisation within membrane surfaces.
α
-dispersion (
∼
β
-
10
6
Hz) occurs due to capacitive charging of cell
membranes in tissues, with a small component due to protein
dipolar relaxation.
∼
dispersion (
10
10
Hz) is due to the dipolar
relaxation of water molecules. Study of underlying mechanisms for
dielectric dispersion is an active area of research (Fig. 3.17).
The clinical uses of measuring the intrinsic electrical and
magnetic properties of biological tissues include impedance tomog-
raphy, X-ray tomography and nuclear magnetic resonance imaging.
What surprises in AD 2011 is the comparison between dielectric
and diamagnetic measurements and theory. A general assumption
within the bioelectromagnetics (BEM) community is that for many
biological tissues there are no differences to the diamagnetic
measurement of free space. This assumption is certainly incorrect
within the DNA with its various structural forms. Yet BEM clinicians
and researchers have largely ignored this line of research to date.
There appears to be a wealth of practical uses for magnetic
fields in clinical situations, including bone refracture, perfusion and
prophylactics againststroke, amongstmany.
While these macroscopic measurements and dielectric theory
have discovered much about biological dielectrics, the underlying
γ
∼
-dispersion (
