Biomedical Engineering Reference
In-Depth Information
the potential of low - fi eld MRI now beginning to be realized [179], it is possible
that materials with high relaxivity at low fi eld will be used to generate contrast in
the micro- Tesla (
T) fi eld range.
More substantial deviations from standard SPM theory, marked by an increase
in the low-frequency relaxivity and the absence of a mid-frequency maximum, have
also been observed for other complex aggregated magnetic materials, such as
core-shell iron-iron oxide nanoparticle clusters [34]. These observations have been
interpreted in terms of an increased magnetocrystalline anisotropy energy associ-
ated with inter-particle interactions within the aggregates, as in the case of
magnetic nanoparticle clusters formed from synthetic polyelectrolytes [118], or
long- chain DNA [126] .
Corti and coworkers [180] have recently reported the preparation of spherical
and tetrapod-like particles, via the phase transfer of particles produced by a non-
polar route, into an aqueous solution on coating with amphiphilic polymers.
Relaxivity measurements as a function of magnetic fi eld in the clinical range
(0.15-5 T; 5-212 MHz) were used to provide an insight into the contribution
of primary particle size and shape to the surface and interparticle anisotropic
interactions. This is a good example of a problem that could benefi t from the extra
information that the low-frequency relaxation behavior, which can be assessed
using NMRD, provides. A similar observation could be applied to other interesting
fi ndings recently reported by the same group [181], where PEI-coated superpara-
magnetic nanoparticle aggregates were studied.
μ
4.4.4
General Application of Relaxation Time Measurements
Many reports have been made over the past ten years where relaxivity (
r
1
and
r
2
),
measurements at single applied magnetic fi elds were used to characterize poten-
tial nanoparticulate contrast agents. Usually, the materials characterization in
these studies is completed by other methods, such as magnetometry or the MRI
imaging of phantoms, cells, or animals. The results of some representative studies
are outlined here.
Recently, Wan and coworkers [182] reported a simple method for the production
of monodisperse (16.5
3.5 nm), triethylene glycol - stabilized, magnetite nanopar-
ticles via an aqueous route, with relaxivity values of 14.14 and 82.7 s
− 1
m
M
− 1
for
r
1
and
r
2
, respectively, being reported. The
r
2
/
r
1
ratio of 5.85 indicated that
the agents could have potential as negative contrast agents. Optical staining of
normal rat and C6 glioma cells showed a preferential uptake of the particles into
the cancer cells; hence, these materials might have potential applications for
cellular imaging.
Qin and coworkers [183] recently reported the stabilization of iron oxide nanopar-
ticles in aqueous solution with pluronic copolymers (PF127). Both, magnetometry
and TEM were used to demonstrate that the primary particles were monodisperse
and superparamagnetic, with a core size of 20 nm. The hierarchical structure of
the coating resulted in stable, dispersed 71 nm composites which showed a very
±
