Biomedical Engineering Reference
In-Depth Information
deposits, while the susceptibility artifacts also distort the background image [23,
24].
Consequently, signifi cant efforts have recently been made on the development
of T
1
MRI contrast agents based on magnetic nanoparticles of other metal oxides.
These involve mostly manganese and rare earth-based oxides. For example, T
1
MRI
contrast agents based on biocompatible MnO nanoparticles have recently been
reported by Hyeon
et al.
[24]. This material facilitated the acquisition of good-
quality T
1
-weighted MR images of the brain, liver, kidney, and spinal cord, showing
very fi ne anatomic structure in animal models. It was also noted that the MnO
nanoparticles could be easily conjugated with a tumor-specifi c antibody and hence
utilized for selective tumor imaging.
Small rare earth (Gd or Dy) oxide nanocrystals have also shown great promise
as contrast agents for MRI, because they can provide a large number of unpaired
electrons per unit of contrast agent, as well as the small particle size required for
low
r
2
/
r
1
values. Typically, small gadolinium oxide (Gd
2
O
3
) nanocrystals can be
prepared via the polyol route by thermal decomposition of, for example, Gd(NO
3
)
3
in the presence of diethylene glycol [25 - 27] . Diethylene glycol - capped Gd
2
O
3
nanoparticles have been shown to demonstrate both
r
1
and
r
2
relaxivities almost
twice as high as the corresponding Gd-chelate-based agents in aqueous solutions
[27]. Similar solution-based thermal decomposition methods were employed to
prepare nanocrystalline Gd
2
O(CO
3
)
2 *
H
2
O and Gd
2
O
3
particles, which have also
shown promising positive- and negative-contrast effects [28, 29]. However, rare
earth oxides are highly reactive and must be coated with an appropriate shell in
order to be utilized in physiological media. For example, when paramagnetic
Gd
2
O
3
cores were protected and stabilized by encapsulation within a polysiloxane
shell, the
r
1
values of these materials were found to be higher than those of
positive contrast agents based on gadolinium chelates [30]. In another approach
to improve stability and biocompatibility, gadolinium oxide nanoparticles were
embedded in albumin microspheres and used as prototype contrast agents for
multimodal X - ray and MRI studies [31] .
4.2.2
Magnetic Metal- and Alloy-Based Nanoparticles as Contrast Agents
Transition metal nanoparticles such as pure Fe and Co, or metallic alloys such as
FeCo, have been envisaged as potentially promising T
1
and T
2
contrast agents.
These metallic nanoparticles tend to have a larger magnetic moment than their
iron oxide counterparts [32] .
Typical examples of metal alloy-based nanoparticles are face-centered cubic (fcc)
FePt nanoparticles, which can be synthesized via the pyrolysis of iron(III) ethoxide
and platinum(II) acetylacetonate. Relaxometry studies have shown that FePt par-
ticles have a higher T
2
effect than superparamagnetic iron oxide, indicating that
the former might serve as superior negative contrast agents for MRI [33].
Iron metal-based nanoparticles can also be prepared by the laser-induced pyroly-
sis of iron carbonyl (Fe(CO)
5
) vapors, the process resulting in the partial oxidation
