Chemistry Reference
In-Depth Information
−1
−1
value is between 60 and 70 nm:
r
varies between 190 mM
s
(70
2
−1
−1
(60 nm at 17.6 T). Large 1 H transverse
relaxation rate enhancements in aqueous suspensions are also
observed for other paramagnetic Ln
nm at 7 T) and 675 mM
s
particles (with Ln = Gd,
Er) [117]. Although these particles show paramagnetic behavior,
the magnitude of this effect is comparable with the effect caused
by superparamagnetic iron oxide particles [118] but only at high
external magnetic field. Since there is a tendency to perform MRI
exams at higher magnetic fields, lanthanide oxide nanoparticles
have favorable relaxivity properties for these higher fields.
Other routes for the preparation of gadolinium oxide based
contrast agents have been recently developed. Gadolinium oxide
nanoparticles in carbon nanotubes were synthesized for MRI
application [119]. They were obtained after thermal treatment of
gadolinium acetate entrapped in single-wall carbon nanohorns
(SWNHs) with holes (NHox). Gd
O
2
3
-NHox dispersed in agarose gel
induces a significant positive contrast enhancement as revealed by
T
O
2
3
T1-weighted images of the phantoms. However, no
r
value was given
1
1
probably because the variation of 1/
as a function of gadolinium
concentration is not linear. Although the enhancement induced
by Gd
T
1
O
-NHox appears sufficient for MRI, their application as
in
2
3
vivo
contrast agent seems to be limited due to the lack of colloidal
stability in biological fluid and the toxicity of carbon nanotubes.
Since the colloidal stability of paramagnetic nanoparticles is a
prerequisite for their application as
contrast agent for MRI,
the encapsulation of gadolinium oxide nanoparticles (5 nm) in the
protein apoferritin has been proposed [120]. The protein shell
prevents bulk aggregation of the particles, rendering them water
soluble. These nanoparticles exhibit longitudinal and transverse
relaxivities higher than those of classic Gd(III)-complexes (10-25
and 70 times, respectively). Moreover, the
in vivo
ratio is drastically
altered by changing the frequency, thus giving the possibility to tune
the response in terms of image contrast. The external apoferritin
shell also affords the opportunity of covalently grafting organic
molecules for incorporating new functionality or for increasing the
plasma half-life. However, no data on
r
/
r
2
1
in vivo
imaging experiments
are available when this chapter was written.
succeeded in synthesizing porous and hollow
gadolinium oxide particles [121]. They demonstrated that hollow
spheres produced better positive contrast enhancement (
Yeh
et al.
r
= 17.7
1
