Biomedical Engineering Reference
In-Depth Information
In our OR-PAM system, motor scanning also contributes to the Doppler
bandwidth broadening. The measured flow speed is actually a vector combination of
the flow speed and the motor-scanning speed [Fig.
2.5
b]. Thus, bidirectional motor
scanning can be used to determine the flow direction. The measured flow speeds
under the two scanning directions are
q
v
f
v
f mC
D
C
v
2
m
C
2
v
f
v
m
cos
(2.7)
q
v
f
C
v
2
m
2
v
f
v
m
cos ;
v
f mC
D
(2.8)
where
v
m
is the motor-scanning speed and is the angle subtended by
v
mC
and
v
f
. Solving the simultaneous equations (
2.7
)and(
2.8
), we can get the true flow
speed via
s
v
f m
C
C
v
f m
2
v
2
m
2
v
f
D
:
(2.9)
By comparing
v
f mC
and
v
f m
, we can further determine the flow direction.
Figure
2.5
c-e shows the in vivo label-free Doppler OR-PAM imaging of a mouse
ear [
15
]. A 1
2 mm
2
ROI was bidirectionally scanned with a motor speed of
0.75 mm/s and a step size of 0:625 m. Two optical wavelengths (560 and 570 nm)
were used to image the vascular anatomy [Fig.
2.5
c] and sO
2
[Fig.
2.5
d]. The flow
speed is pseudocolored in Fig.
2.5
e, and the positive and negative flow directions are
labeled red and blue in Fig.
2.5
f, respectively. Determination of flow direction also
helps distinguish arterioles from venules because the paired arteriole and venule are
expected to have opposite flow directions [Fig.
2.5
d, f].
2.7
Laser Safety
Similar to other scanning optical microscopy technologies, OR-PAM can improve
SNR by increasing the pulse energy. However, for in vivo applications, laser
parameters (such as optical wavelength, pulse duration, exposure duration, and
exposure aperture) are regulated by laser safety standards of the American National
Standards Institute (ANSI) [
25
].
2.7.1
Laser Safety for Ophthalmic Imaging [
11
,
25
]
We have demonstrated OR-PAM imaging of the anterior segment of a living mouse
eye [
11
]. The experimental parameters and ANSI-defined constants are listed as
follows:
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