Biomedical Engineering Reference
In-Depth Information
methods for addressing background variations in T
2
and expanding the dynamic
range can be applied in a variety of detection coil confi gurations, including the
multiplexed detection hardware introduced by H. Lee in the Weissleder group,
which will be discussed in greater detail below [23].
A team led by I. Koh in the Josephson laboratory at MGH has demonstrated
that a combination of methods can be used to increase the projected sensitivity of
a MRSw assay by fi ve orders of magnitude [19]. The model system used to dem-
onstrate these methods consisted of superparamagnetic nanoparticles or mic-
roparticles decorated with the Tag peptide, which is from the HA of the human
infl uenza virus. The addition of an anti-Tag antibody led to clustering of the
peptide-decorated particles. The method employed CLIO nanoparticles that were
30 nm in diameter, did not settle, had 20-30 attached peptides per nanoparticles,
an
R
2
of 50 s
− 1
m
M
− 1
, a magnetization of 86.6 emu g
− 1
Fe, with 8000 iron atoms per
nanoparticle, and a concentration of 2.8
1 0
− 9
for a T
2
of 100 ms. The micropar-
ticles used were 1000 nm in diameter, settled less than 5% in aqueous solution
[20] , had 3
×
1 0
5
peptides per particle, an
R
2
of 43 s
− 1
m
M
− 1
, a magnetization of
105 emu g
− 1
Fe, 2.8
×
1 0
− 15
for a measured T
2
of 100 ms. Koh
et al.
characterized the performance of these
nanoparticles in terms of EC
50
and projected sensitivity. For simplicity, the pro-
jected sensitivity will be discussed at this point [19].
When the nanoparticles (NP) and microparticles (MP) were titrated with anti-
Tag antibody, the T
2
values decreased for the NP and increased for the MP, which
corresponded to the NP being within the motional averaging regime and the MP
being within the visit-limited regime [19]. The NP and MP exhibited projected
sensitivities of 26 n
M
and 0.41 n
M
, respectively. The
×
1 0
9
Fe atoms per particle, and a concentration of 5.1
×
60 - fold increase in sensitivity
for MP arose from the larger mass of iron per unit conjugated peptide that cor-
responded to a much larger
R
2
relaxivity on a per particle basis [19, 20]. Previous
studies with viral targets have suggested that crosslinking agents with a greater
binding valency could lead to increased sensitivity [46], and this was confi rmed by
Koh
et al.
, who increased the valency of their bivalent antibody target to a tetrava-
lent target with the addition of an antibody that selectively bound the Fc region
of the anti-Tag antibody. This increased the projected sensitivity for the MP to
0.0002 n
M
, or by a factor of 2000 [19].
The use of MP allows for an additional method for sensitivity enhancement.
Investigations conducted by Baudry and coworkers in Paris showed that the reac-
tion rate between reactive groups on magnetic MP could be greatly accelerated by
magnetic fi eld-induced self-assembly of the MP into linear chains [21-23] or fractal
agglomerates [20]. For Koh
et al.
, an alignment of the magnetic dipoles of indi-
vidual MP during incubation in a 0.47 T bench-top magnet led to spatial confi ne-
ment of the MP and increased reaction kinetics. During incubation in the magnet,
the T
2
increased due to the linear self-assembly of the MP. In order to distinguish
the magnetic- fi eld induced T
2
changes from analyte-induced T
2
changes, the
sample was removed from the magnet for a few minutes prior to T
2
measurement.
If analyte was present, the MP remained clustered, but if no analyte was present
then the MP would disperse due to Brownian motion. This method, termed
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