Biomedical Engineering Reference
In-Depth Information
Fig. 2.1 Comparison
of acoustic-resolution
photoacoustic microscopy
(AR-PAM) and
optical-resolution
photoacoustic microscopy
(OR-PAM). ( a ) Schematic
of AR-PAM. ( b ) Schematic
of OR-PAM. ( c )AR-PAM
image of the cortical
vasculature in a living adult
mouse with both the scalp
and the skull intact.
( d ) OR-PAM image of the
cortical vasculature in a living
adult mouse with the scalp
removed and the skull intact.
Scale bars: 1 mm
where 0 and NA denote the center wavelength of the photoacoustic wave and the
numerical aperture of the ultrasonic detector, respectively. A lateral resolution of
45 m has been achieved with a center frequency of 50 MHz and an NA of 0.44 [ 5 ].
This resolution, although adequate for many biomedical applications, cannot resolve
fine structures such as capillaries (diameter: 4-9m).
To resolve single capillaries acoustically, an ultrasonic center frequency of
greater than 400 MHz is required; however, at such a frequency, strong ultrasonic
attenuation limits the penetration depth to less than 100 m[ 6 ]. A more practical
way to improve the lateral resolution is to use fine optical focusing. As shown in
Fig. 2.1 b, the optical excitation beam can be focused to a diffraction-limited spot
inside the acoustic focus. Thus, the size of the optically excited region, rather than
that of the ultrasonically detected region, determines the lateral resolution. Using
this configuration (referred to as OR-PAM), the lateral resolution of PAM can be
improved from 45 to 5m[ 6 ]orevenbetter[ 7 ], at the expense of penetration depth.
Figures 2.1 c and d present a vivid comparison of the penetration and resolution of
AR-PAM and OR-PAM [ 12 , 17 ]. OR-PAM can clearly resolve as small as single
capillaries in an adult mouse brain through the intact skull [Fig. 2.1 d], while AR-
PAM can only resolve cortical vessels thicker than 50 m[Fig. 2.1 c]. However,
OR-PAM requires the scalp to be removed, while AR-PAM is able to penetrate both
the scalp and the skull.
Based on the principle of OR-PAM, we list a couple of general guidelines that
one may consider to follow when designing an OR-PAM system:
1. Higher optical resolution always accompanies with a smaller optical focus zone.
Balance the trade-off by choosing a proper optical condenser lens or microscope
objective according to the desired lateral resolution.
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