Biomedical Engineering Reference
In-Depth Information
Fig. 2.7
Schematic of the laser-scanning optical-resolution photoacoustic microscopy setup
One intermediate solution is to relax the acoustic focus and keep it stationary,
while the diffraction-limited optical focus is optically scanned inside the enlarged
acoustic focus. This leads to the development of laser-scanning OR-PAM (LSOR-
PA M ) [
26
]. The configuration of the LSOR-PAM system is similar to that of
the traditional mechanical-scanning-based OR-PAM (Fig.
2.7
). The pulsed laser
beam generated from the same type of wavelength-tunable laser system is spatially
filtered by an iris, and the beam diameter is expanded to 8 mm by a pair of
condenser lens. The expanded laser beam is then attenuated and steered by a 2-D
galvanometer scanner (6230H, Cambridge Technology) before entering an objective
lens (AC254-060-A1, Thorlabs) for optical focusing. An unfocused ultrasonic
transducer (V312, Olympus NDT; center frequency: 10 MHz; bandwidth: 80%;
active element diameter: 6 mm) is placed 30 mm away from the object with an
oblique angle of
15
ı
, which allows to overlap the optical illumination without
blockage.
LSOR-PAM enables fast scanning with designable scanning patterns, which is
particularly useful when studying the behavior of a single microvessel. However,
unlike traditional OR-PAM, LSOR-PAM has a limited field of view (FOV) (6 mm
in diameter in this case), which is determined by the ultrasonic beam size of the
unfocused transducer. Moreover, the SNR of LSOR-PAM is lower than that of
traditional OR-PAM because of the reduced sensitivity of the unfocused ultrasonic
transducer at the optical focus.
To improve the imaging speed of OR-PAM while maintaining a high SNR, Rao
et al. developed a hybrid-scanning OR-PAM (HSOR-PAM) system [
27
]. In HSOR-
PA M ( F i g .
2.8
), the pulsed laser beam is scanned along the B-scan direction by
a one-dimensional (1-D) galvanometer scanner (6220H, Cambridge Technology).
A glass plate is submerged in the water tank to reflect the photoacoustic waves to a
cylindrically focused ultrasonic transducer (25 MHz bandwidth, focal length 15 mm,
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