Biomedical Engineering Reference
In-Depth Information
MRI is based on the principal of nuclear magnetic relaxation of water protons
[3, 5]. The protons align with an applied magnetic field in two ways in an MRI
scanner: parallel (low energy state) and antiparallel (high energy state). The population
difference between these two states is very small with more protons (in ppm level)
being in low energy state but is proportionally increased with the field strength. This
is the reason why an MRI scanner with a higher field has a better sensitivity. If a res-
onance electromagnetic radio-frequency (RF) pulse is applied transversely, the
protons absorb the energy and then jump to an antiparallel state. correspondingly, the
longitudinal magnetization vector paralleling to external magnetic field is decreased
(
M
z
in Fig. 2.2b) while giving rise to transverse magnetization (
M
xy
in Fig. 2.2b) [3, 6,
16]. The subsequent relaxation occurs when the RF is turned off, allowing protons to
return to the origin state. Two relaxation processes are monitored: longitudinal mag-
netization recovery (
T
1
-recovery) and transverse magnetization decay (
T
2
-decay), to
generate a bright (
T
1
-weighted) and a dark (
T
2
-weighted) image, respectively. The
image contrast depends on local variation of relaxation time, resulting from the
proton density and the physiological environment of specimen [3]. Here,
T
2
repre-
sents the time it takes to drop the transverse magnetization to its 36.8% original mag-
nitude. In the presence of MNPs, the
T
2
of the surrounding protons is shortened due
to the impact of secondary magnetic field produced by MNPs. as a result, MNPs can
act as a
T
2
contrast agent to give out a “darker” image. The sensitivity of a
T
2
contrast
agent is indicated by the relaxation rate,
R
2
= 1/
T
2
(s
−1
), and the relaxivity,
r
2
=
R
2
/
concentration (mM
−1
·s
−1
) [5]. Based on a quantum mechanical outer-sphere theory,
the relaxation rate of the MNPs contrast agent in solution is defined by
=
(
)
22
*
22
256
πγ /
VMr
DLr
405
1
s
(2.2)
(
)
T
1
+
/
2
where
γ
is the proton gyromagnetic ratio,
V
*
is the volume fraction,
M
s
is saturation
magnetization,
r
is the radius of MNP core,
D
is the diffusivity of water molecules,
and
L
is the thickness of an impermeable surface coating [17, 18]. Therefore, an ideal
MNP contrast agent is expected to possess high saturation magnetization (high
M
s
),
large size (large
r
), and thin coating (small
L
). It is also favorable that these super-
paramagnetic NPs are monodisperse (size distribution in diameter with standard
deviation <10%) with an overall hydrodynamic size letter than 50 nm to avoid the
nonspecific uptake and have a long circulation time [19].
2.3
organIc solutIon phase syntheses of IoMnps
Organic solution phase reactions have been proven to be ideal for synthesizing
IOMNPs in high quality. In a typical synthesis, an iron precursor is thermally decom-
posed in an organic solution and then generates the IOMNPs through the nucleation
and growth steps (Fig. 2.3a) [3, 8, 20-22]. Iron acetylacetonate (Fe(acac)
3
/Fe(acac)
2
)
or iron oleate complex is the common precursor for the synthesis. an organic solvent
Search WWH ::
Custom Search