Biomedical Engineering Reference
In-Depth Information
system: planar microcoils, microfluidic networks, onboard NMR spectrometer, and
a portable magnet. The microcoils are used for NMR detection and are arranged
in an array format for parallel measurements. The microfluidic networks facilitate
the handling and distribution of small volumes of samples. A small, portable
magnet (NdFeB, B
0
D
0:5 T) was employed to generate NMR field. The system
measured the T
1
relaxation time using inversion recovery pulse sequences; for
T
2
measurements, Carr-Purcell-Meiboom-Gill (CPMG) spin-echo pulse sequences
were used to compensate for the inhomogeneity of the polarizing magnetic field. To
generate versatile pulse sequences while using minimal electronic parts, we devised
a new circuit schematic for NMR electronics that has served as a blueprint for
subsequent NMR systems.
9.5.2
Optimal NMR Probe Design
Reducing sample volume requirements can lead to the effective increase of cell
concentrations. However, it can also lead to degradation of the signal-to-noise ratio
(SNR), as the absolute level of the NMR signal is proportional to the sample volume.
System miniaturization thus should be accompanied by measures to maintain or
enhance SNR to truly improve the detection sensitivity. In the second generation of
NMR system, we focused on improving SNR by engineering the NMR probes.
The SNR of a NMR probe can be expressed as
s
0
Q!
0
V
c
4k
B
Tf
;
SNR
D
M
0
(9.3)
where is the fraction of the coil volume (V
c
) occupied by the samples (filling
factor), M
0
is the nuclear magnetization of the sample,
0
is the vacuum permeabil-
ity, !
0
is the Larmor frequency, Q is the quality factor of an NMR coil, and f
is the bandwidth of a receiver electronics. For a given NMR setup (i.e., the same
magnets and electronics), SNR could be improved by increasing and Q, which are
properties of the NMR probes. Indeed, we have demonstrated a new probe design
that achieves both maximal .
25)[
28
]. In this design, the probe
consisted of a solenoidal microcoil embedded in a microfluidic structure (Fig.
9.7
a).
Solenoidal coils were chosen for their higher SNR than planar or birdcage coils
[
53
]. To increase , we adopted the cast-molding technique in device fabrication.
First, the coils were wound around polyethylene tubes and subsequently immersed
into a polymer (PDMS). After PDMS cure, the tubes were withdrawn to open up
the fluidic channels. With this design, the entire bore of the coil was available for
samples; in one example, the new probe displayed >350% larger SNR than a similar
coil wrapped around a tube (Fig.
9.7
b). Compared to the lithographically patterned
planar coils in our previous systems [
22
], the improvement in SNR was much more
significant (>20-fold enhancements); the solenoidal coil excited larger volumes of
1/ and high Q (
