Biomedical Engineering Reference
In-Depth Information
This NLDS procedure is repeated to obtain dispersion measures for all points on the
sample. The results are shown in
Figure 14.11D
, where the negative log of the standard
deviation of
n
is superposed on the topological map. Note that the dark red hue of the
fluorescent bead indicates higher dispersion compared to the background and clear bead,
which appear in a light yellow hue. These dispersion results provide complementary
information to the absorption map in
Figure 14.11B
.
Δ
14.5.2 Red Blood Cells
The second experiment conducted to demonstrate proof of the NLDS principle examines
normal, fully oxygenated RBCs. A sample of whole blood was obtained from a healthy
donor and processed as described previously
[52]
. The RBCs were plated on a silvered
coverglass and imaged at ambient room temperature and pressure. All procedures were
adhered to approved Institutional Review Board protocols.
We followed the same signal-processing method described earlier to analyze an isolated
RBC. Again, the topological map was obtained from the linear phase (SDPM) analysis, as
shown in
Figure 14.12A
, illustrating the well-known round biconcave shape of a healthy
RBC. The attenuation-based spectral information is shown in
Figure 14.12B
as the true
color spectroscopic map. Here, the RBC appears colorless, which is in fact in good
agreement with typical appearance of isolated RBCs when observed with a phase-contrast
microscope. These results indicate that the absorption incurred by a single RBC is not
sufficient to impart a visible spectral modulation. The lack of spectroscopic information
hinders accurate quantification of important biomarkers, namely hemoglobin (Hb)
concentration (
C
Hb
) and oxygen saturation (SO
2
). This observation is also reflected in
previous findings, where quantification of
C
Hb
and SO
2
of individual RBCs have shown
large uncertainties
[56,66]
. In contrast, the NLDS technique is capable of providing higher
sensitivity as shown in
Figure 14.12C
by the good contrast between the RBC and the
background. In fact, based on the current system's phase sensitivity (
B
10 mrad),
C
Hb
as
low as
B
0.5 g/dl may be detected (normal RBC
C
Hb
:30
36 g/dl). Compared with
attenuation-based spectral techniques
[62]
, NLDS is two orders of magnitude more
sensitive.
The full potential of NLDS is realized by analyzing the detailed dispersion spectra,
allowing the accurate quantification of
C
Hb
. The procedure entails using the oxygenated-Hb
dispersion coefficients in a matrix inversion with the computed dispersion
[55]
.
Figure 14.12D
shows a typical spectra along with the best fit corresponding to
C
Hb
7.7 g/dl,
in good agreement with expected values of healthy RBCs
[51,67]
.
Figure 14.13
shows a
group of four RBCs with concentrations of (from left to right) 34.6
5
31.9 g/dl. The average concentration across the entire cell was
C
Hb
5
33.4
6
6
9.1, 36.0
6
11.9,
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