Biomedical Engineering Reference
In-Depth Information
of
5m. According to the geometry of a focused Gaussian beam, the beam
diameter on the skin surface is
30 m.
Rule 1: Single Pulse Limit
The MPE for a single laser pulse is
D
2:0
10
2
J=cm
2
;
D
2
C
A
10
2
MPE
SP
(2.17)
where C
A
, the wavelength correction factor, is unity when is between 400 and
700 nm.
Rule 2: Average Power Limit
Different from the anterior-eye-segment imaging, where all laser pulses overlap
on the retina due to light defocusing, only
24 adjacent laser pulses overlap on the
skin surface. With a PRF of 600 Hz, the exposure time t is 0.04 s. So, the MPE for
the pulse train is
D
0:5
J=cm
2
:
D
1:1
C
A
t
0:25
MPE
train
(2.18)
Thus, the MPE/pulse for the pulse train is
D
2:1
10
2
J=cm
2
:
MPE
average
D
MPE
train
24
(2.19)
Rule 1 is slightly more rigorous, so the overall MPE for each pulse is 2:0
10
2
J=cm
2
. Knowing that the beam diameter on the skin surface is
30 m,
the maximum permissible single laser pulse energy for OR-PAM skin imaging is
computed to be
141 nJ. Thus, our experimentally used laser pulse energy (
40 nJ)
is well within the ANSI limits.
2.8
Recent Technical Advances
Although traditional OR-PAM has demonstrated broad biomedical applications,
its imaging speed is still slow compared with the mainstream optical microscopy
technologies. Moreover, integration of OR-PAM with other optical microscopy for
multicontrast imaging, as an original motivation for developing OR-PAM, has not
been realized yet. Thus, recent technical developments of OR-PAM are focused on
these two aspects.
2.8.1
New Scanning Mechanism
The integration of OR-PAM with existing optical microscopy has been hampered,
mainly due to the incompatible scanning mechanisms. It is challenging to trans-
plant the fast optical scanning widely used in optical microscopy technologies to
OR-PAM, which requires scanning the ultrasonic-optical dual foci.
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