Endomicroscopy - Biomedical Optical Imaging Technologies: Design and Applications - page 266

8.2.2.5 Beam Scanning Methods

Most endomicroscopy modalities require a scanning mechanism to sweep a focused
spot across the tissue surface. Beam scanning is either performed at the proximal
end of a fiber-optic bundle or at the distal tip of a single optical fiber. Clearly, the
scanning mechanism must be miniaturized in the latter case, where its dimensions
may impact the overall size of the endoscopic probe. Proximal scanning is relatively
straightforward to achieve by use of galvanometric scanners, resonant scanners,
DMD mirror arrays, translations stages, or other means [ 36 ]. Distal tip scanning
can be performed by translation of the entire probe, either in a linear or rotational
motion, with the translating force applied by a unit located external to the probe [ 10 ].
Alternatively, the probe can remain stationary and the beam exiting the delivery fiber
can be scanned by a miniaturized version of the common macroscopic scanners.
While the ability to perform optical sectioning endomicroscopy without minia-
turizing the scanning elements is an appealing feature of the fiber-bundle-based
platform, the discrete nature of individual cores within the bundle leads to the
appearance of the bundle structure within the image and can ultimately limit the
spatial resolution. Two distinct approaches have been demonstrated which replace
the fiber bundle with a single optical fiber and perform beam scanning through
miniaturized mechanisms at the distal tip. These mechanisms can be based on use
of piezoelectric actuators to sweep the fiber tip itself in a scanning pattern, which
may be a linear raster or spiral motion [ 19 , 38 , 39 ]. These standard quad piezos have
diameters ranging from 1 to 3 mm. Another option is to apply MEMS technology to
create tilting mirrors for beam scanning.
Conceptual schematics and pictures of the piezo scanning and electrostatic
MEMS approaches are shown in Figs. 8.6 and 8.8 , respectively. The unit shown in
Fig. 8.6 b[ 38 ] is based on modulating a four-electrode tube piezo with a sinusoidal
waveform. Each electrode pair is modulated with a 90 ı phase shift, generating a
spiral scanning path. Therefore, following [ 39 ], the applied electrode modulation
can be described with following functions:

C x D sin.r/ sin. /; (8.5a)

C y D sin.r/ cos. /; (8.5b)

where r corresponds to radial position while is a rotation angle.
The delivery fiber extending from the tube falls into resonance, allowing for a
larger movement of the fiber tip than the piezoelectric actuator itself, with operation
at low voltages (approx. 10V). The scanning range depends on the extended length
of the fiber, which in turn affects the resonant frequency. The typical scan range
is 100-300 m, while the resonant frequency is usually set between 500 Hz and
10 kHz. This frequency can be obtained using numerical methods [ 19 ]oranalytical
models. In general [ 38 , 40 ], a resonant frequency can be defined as:

s E

R
L 2

:

ˇ
4

f D (8.6)