Endomicroscopy - Biomedical Optical Imaging Technologies: Design and Applications - page 261

8.2.2 Individual Components for Endomicroscopy

8.2.2.1 Light Sources

Each of the primary modes of endomicroscopy outlined above requires light sources
with specific properties. Point-scanning methods including confocal and nonlinear
microscopy, as well as optical coherence tomography, require a tightly focused
beam at the specimen in order to achieve high spatial resolution. This is typically
achieved by using a laser source with high beam quality (M 2 close to 1, a Gaussian
TEM 00 spatial mode) to enable generation of a near-diffraction-limited spot. Full-
field methods such as contact imaging can use an extended source, which can be
a lamp or LED. Nonlinear techniques require an ultrafast laser source to deliver
short ( 100 fs) pulses [ 27 - 30 ] with high peak power at high repetition rates
( 80-100 MHz). Titanium: Sapphire lasers are the most commonly used sources
for two-photon and second-harmonic imaging. They usually provide a tunable
output over approximately the 700-1,000-nm spectral range. Optical parametric
oscillators (OPO's) are used to generate picosecond and femtosecond pulses further
into the near-IR wavelength range for techniques including CARS microscopy.
Endoscopic optical coherence tomography typically requires a source with high
spatial coherence and low temporal coherence, the latter property being associated
with a broad spectral bandwidth. Ti:S lasers are therefore commonly used for
OCT imaging in the 700-1,000-nm region, but the majority of endoscopic work
to enable deep tissue imaging of the 1:3-m wavelength range uses broadband
superluminescent diodes, semiconductor optical amplifiers, or rapidly swept tunable
laser sources [ 10 , 31 ].

8.2.2.2 Light Delivery and Collection

Endomicroscopy requires the delivery to and collection of light from confined sites
within the body. This is achieved through use of fiber-optic materials which can be
classified either as single optical fibers or fiber-optic bundles comprising thousands
of single fibers. Use of single optical fibers for light transmission necessitates a
means for laterally scanning the emerging beam at the tissue surface.
An important consideration when using both single fibers and fiber bundles is
to ensure matching of the fiber NA to that of the miniature optics located at the
distal tip. This allows any scattering inside the optics in the illumination path to be
minimized and optical throughput to be maximized by matching the point spread
function in the image with the fiber mode. Single-mode fiber has an NA typically
around 0.10-0.15, while fiber-optic bundles have higher NA around 0.30-0.35.
In order for any miniature optics at the fiber's distal end to match the delivery
fiber's NA, the optical system needs to work at low magnification. Magnification is
determined by the ratio of NA at the tissue side to NA at the fiber side. For example,
using a miniature microscope objective with a relatively high NA of 1.0 (water)