Endomicroscopy - Biomedical Optical Imaging Technologies: Design and Applications - page 269

Electrical Wire Injection Mold

Optical Fiber Outer Sheath GRIN Lens

MEMS Die

Fig. 8.9 Schematic and photo of an OCT endoscope's tip using MEMS scanner. Note that outer
diameter of the encapsulated (to avoid obstruction outer layer is not shown) system is 4 mm [ 46 ]

from few to over 20 ı [ 46 ]. Implemented MEMS scanners have dimensions of 2-3
mm including the reflective mirror surface and actuating mechanism. An example
of the MEMS implementation for OCT probe is shown in Fig. 8.9 . When operating
at their resonant frequencies, MEMS scanners can be run at lower power, with
consumptions in the Watts range. Unfortunately these systems remain quite large,
increasing the overall endoscope size and making it difficult to package the system
for passage through a standard biopsy channel. MEMS scanners also usually require
custom fabrication and are expensive to prototype.
In addition to the lateral (XY) scanning mechanisms described above, axial
scanning methods have been developed to achieve imaging at different depths within
tissue. These include translating the entire probe tip [ 47 ], using hydraulic suction to
physically pull tissue toward or away from the objective lens [ 13 ], or translating the
last lens of the miniature objective system [ 48 ].

8.2.2.6 Miniature Optics

Miniature objectives are one of the most important components of endomicroscopes.
They need to provide high NA over a sufficiently large FOV to make endoscopic
application practical. Objective systems can be divided in two groups: those that
use gradient refractive index (GRIN) materials and those based on miniature multi-
lens microscope objectives. Both groups have been broadly used for most imaging
modalities. Note that with a lens of small outer diameter, in order to maintain
high NA (and consequently efficient light collection and high resolution), either
the working distance or FOV are usually reduced. However, for smaller lenses,
aberrations actually scale down, allowing an improvement in the ratio between lens
diameter and effective FOV compared to classical larger microscope objectives.

GRIN Lens-Based Miniature Objectives

GRIN lenses are rod-shaped components. Most commonly, the index of refraction
varies radially, an effect obtained by ion exchange processes during fabrication. The
index of refraction has a maximum value on axis and falls off quadratically to a