Biomedical Engineering Reference
In-Depth Information
2.6
Functional Imaging Design
Besides the morphological information of the microvasculature that can be directly
extracted from 3-D OR-PAM images, important functional and metabolic param-
eters, such as sO
2
and blood flow, are also measureable with specially designed
imaging acquisition and data processing.
2.6.1
Spectroscopic Measurement of Oxygen Saturation
of Hemoglobin
Oxyhemoglobin (HbO
2
) and deoxyhemoglobin (HbR) are the two major forms
of hemoglobin, the predominant endogenous photoacoustic source in the visible
spectral range. HbO
2
and HbR have quite distinct optical absorption spectra and
thus can be spectrally differentiated to quantify sO
2
. Blood absorption imaged by
label-free OR-PAM can be expressed as [
19
]
ˆ.
i
/
/
a
.
i
/
F
D
ln 10
f
"
HbR
.
i
/
ŒHbR
C
"
HbO
2
.
i
/
ŒHbO
2
g
F;
(2.2)
where
a
.œ
i
/ is the blood absorption coefficient at wavelength œ
i
and "
HbR.HbO
2
/
.
i
/
and ŒHbR.ŒHbO
2
/ are the molar extinction coefficient and relative concentration
of HbR (HbO
2
), respectively. The detected photoacoustic signal amplitude ˆ.
i
/ is
proportional to the product of
a
.
i
/ and the optical fluence F . Assuming that F is
wavelength independent, we can ignore
ŒHbR
ŒHbO
2
"
HbR
.
1
/"
HbO
2
.
2
/
"
HbR
.
1
/"
HbO
2
.
2
/
1
ˆ.
1
/
ˆ.
2
/
:
D
(2.3)
Sequentially, sO
2
can be computed as
ŒHbO
2
ŒHbR
C
ŒHbO
2
:
sO
2
D
(2.4)
Here are two guidelines for selecting the proper optical wavelengths to mea-
sure sO
2
:
1. Wavelengths should be selected within the Q-band of hemoglobin absorption
spectrum (i.e., 550-600 nm) to ensure a sufficient signal-to-noise ratio (SNR).
2. Wavelengths where "
HBR
and "
HbO
2
have a pronounced difference in their ratio
(e.g., 561 nm is HbR dominant and 578 nm is HbO
2
dominant) are recommended.
Note that although a dual-wavelength measurement is adequate to calculate
sO
2
, using more wavelengths can potentially improve the measurement accuracy.
Besides, the total hemoglobin concentration (HbT) can also be calculated in relative
values, either by adding [HbR] and [HbO
2
] together or by directly measuring blood
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