In the presence of alternating-sinusoidal or rotating magnetic fields,
magnetic nanoparticles will act to realign their magnetic moment with
the applied magnetic field. The realignment is characterized by the
nanoparticle's time constant,
τ.
As the magnetic field frequency is increased, the nanoparticle's
magnetic moment lags the applied magnetic field at a constant angle for a
given frequency,
Ω, in rad s
−1.
Associated with this misalignment is a power dissipation that increases
the bulk magnetic fluid's temperature which has been utilized as a
method of magnetic nanoparticle hyperthermia, particularly suited for
cancer in low-perfusion tissue (e.g., breast) where temperature
increases of between 4 and 7 °C above the ambient
in vivo
temperature cause tumor hyperthermia. This work examines the rise in the
magnetic fluid's temperature in the MRI environment which is
characterized by a large DC field,
B0. Theoretical
analysis and simulation is used to predict the effect of both
alternating-sinusoidal and rotating magnetic fields transverse to
B0. Results are presented for the expected temperature increase in small tumors (
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radius) over an appropriate range of magnetic fluid concentrations
(0.002¿0.01 solid volume fraction) and nanoparticle radii (1¿10 nm). The
results indicate that significant heating can take place, even in
low-field MRI systems where magnetic fluid saturation is not
significant, with careful the goal of this work is to examine, by means
of analysis and simulation, the concept of interactive fluid
magnetization using the dynamic behavior of superparamagnetic iron oxide
nanoparticle suspensions in the MRI environment. In addition to the
usual magnetic fields associated with MRI, a rotating magnetic field is
applied transverse to the main
B0 field of the MRI.
Additional or modified magnetic fields have been previously proposed for
hyperthermia and targeted drug delivery within MRI. Analytical
predictions and numerical simulations of the transverse rotating
magnetic field in the presence of
B0 are investigated to demonstrate the effect of
Ω,
the rotating field frequency, and the magnetic field amplitude on the
fluid suspension magnetization. The transverse magnetization due to the
rotating transverse field shows strong dependence on the characteristic
time constant of the fluid suspension,
τ. The analysis shows that as the rotating field frequency increases so that
Ωτ
approaches unity, the transverse fluid magnetization vector is
significantly non-aligned with the applied rotating field and the
magnetization's magnitude is a strong function of the field frequency.
In this frequency range, the fluid's transverse magnetization is
controlled by the applied field which is determined by the operator. The
phenomenon, which is due to the physical rotation of the magnetic
nanoparticles in the suspension, is demonstrated analytically when the
nanoparticles are present in high concentrations (1¿3% solid volume
fractions) more typical of hyperthermia rather than in clinical imaging
applications, and in low MRI field strengths (such as open MRI systems),
where the magnetic nanoparticles are not magnetically saturated. The
effect of imposed Poiseuille flow in a planar channel geometry and
changing nanoparticle concentration is examined. The work represents the
first known attempt to analyze the dynamic behavior of magnetic
nanoparticles in the MRI environment including the effects of the
magnetic nanoparticle spin-velocity. It is shown that the magnitude of
the transverse magnetization is a strong function of the rotating
transverse field frequency. Interactive fluid magnetization effects are
predicted due to non-uniform fluid magnetization in planar Poiseuille
flow with high nanoparticle concentrations.