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. 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 degree Centigrade 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 (approximately 1 cm 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 selection of the
rotating or sinusoidal field parameters (field frequency and amplitude).
The work indicates that it may be feasible to combine low-field MRI
with a magnetic hyperthermia system using superparamagnetic iron oxide
nanoparticles.