Biomedical Engineering Reference
In-Depth Information
A number of important trends can be observed from this equation. The eddy
current power increases with frequency and sphere diameter, while it decreases
with increasing resistivity. In an alternating magnetic field the induction of eddy
currents in the patient's body may lead to an unwanted heating at both cancerous
and healthy tissue reducing in this way the selectivity of the therapy. Therefore one
can minimize the eddy current power by using lower frequencies, smaller particles,
and higher resistivity materials.
If frequency of the external magnetic field could not cause significant eddy cur-
rent heat, the MNPs will produce heat by a large number of magnetic hysteresis
losses. For multi-domain ferro- or ferri-magnetic materials, heating is induced due
to hysteresis losses (Mornet et al. 2004 ). As limited by the applied magnetic field
strength in hyperthermia, MNPs can not be fully magnetized, leading to the greatest
heat production by magnetic hysteresis losses. Generally, only 25% of maximum is
available.
Hysteresis loss (Hergt et al. 2008 ) is proportional to the frequency, three power
of amplitude of the external magnetic field and hysteresis loop constant, which is
related to the kinds of material. Ignoring the heat caused by the eddy current effect
and the natural resonance, heat generated per unit volume due to ferromagnetic
hysteresis loss in the alternating magnetic field is
ò
(4)
FM P f d
=
m
HM
0
As the decrease of the size of nanoparticles within the single domain range, their
coercivity and remanence become bigger and would produce more heat. However,
when reduced to below critical size of nanoparticles, their coercivity and rema-
nence drop to zero. At this time, the MNPs show SPM without hysteresis, and
produce heat through Neel relaxation mechanism (Mornet et al. 2004 ; de Chatel
et al. 2009 ). Compared with paramagnetism, the magnetic moment of SPM parti-
cles is not of a single atom but of the entire particle, resulting in a much higher
susceptibility value than that for simple paramagnetism. The magnetic moments of
SPM nanoparticles are randomly reoriented by the thermal energy of their environ-
ment and do not display magnetism in the absence of a magnetic field. Unlike fer-
rimagnetic materials, they do not aggregate after exposure to a magnetic field.
Aggregation can hinder the body's efforts to remove the nanoparticles. Therefore,
SPM nanoparticles are ideal candidates for hyperthermia cancer treatment.
The magnetic susceptibility of SPM can be expressed as
i (5)
where, χ and depend on the frequency of external magnetic field. In this case,
SPM is determined by (Chan et al. 1993 )
χχχ
=+
'
''
P
=
µπ χ
f
''
H
2
(6)
SFM
0
Experiments show that SPM materials need smaller strength of electromagnetic
field than that of ferromagnetic, which therefore can improve cancer treatment
effect.
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