Biomedical Engineering Reference
In-Depth Information
17.6.2 Coherent Bundles for Imaging
Optical fibers can be bundled together in an aligned fashion such that the orders of the
fibers at both ends are identical. Such a fiber bundle is called a coherent bundle or an
ordered bundle. Note that the coherence here means the correlation between the spatial
positions of the fibers at both ends and has nothing to do with the light coherence. The
important property of coherent bundles is their capability to transmit images through a
flexible channel. If an image is projected onto one end of a bundle, a replicate of the image
is produced at the other end. Coherent bundles of optical fibers are the key components in
endoscopes.
In endoscopic applications, an internal organ is imaged and viewed outside the body in a
minimally invasive fashion. An incoherent (nonordered) bundle of optical fibers is used to
illuminate the portion to be imaged inside the human body. A coherent (ordered) bundle of
optical fibers is used to transmit an image of the target portion. A white light source is usu-
ally used for the illumination so an accurate color image of the tissue can be obtained.
The quality of image transmission is mainly determined by two factors: light collection
and image resolution. The collection power of each individual optical fiber is limited by
the diameter of its core and the numerical aperture (NA). A large NA and core diameter
allow a good transmission of light from the illuminated object to the eye of the physician.
The image resolution indicates how fine details can be seen and is limited by the core diam-
eter,
, of the cores of the individual fibers. The resolution, in number of discernible lines
per millimeter, is approximately
d
/2. The smaller the core diameter, the better the image
resolution. Due to the limited resolution, a straight line in an object may appear zigzagged
in the image.
Cladding of each individual optical fiber is required to minimize or avoid crosstalking
among the fibers. Unclad fibers were used in early endoscopes and had poor imaging quality.
Light in unclad fibers may leak from the core and cross into other fibers. The crosstalk causes
an overlay of various portions of an image during transmission and results in a blurred image.
The requirements of high light collection and good image resolution have conflicts.
A large core diameter allows good light transmission but gives poor resolution. A thick
cladding layer avoids crosstalk but limits light collection and image resolution. A tradeoff
has to be made. In practice, the core diameter is usually 10-20
d
m
m, and the cladding thick-
ness is of the order of 1.5-2.5
m.
Several other factors may deteriorate the image obtained with a coherent bundle. Some
stray light may transmit through the cladding layers into the cores. The stray light would
add an undesirable background that reduces the image contrast. Defective fibers in a coher-
ent bundle would cause a serious problem. If a defective fiber does not transmit any light, a
static dark spot appears in the image. If the ordering of the fibers is not identical at both
ends, image distortion will degrade the images.
Lenses may be added at both ends of an imaging coherent bundle to adjust the magnifi-
cation. Although the distance between the objective lens and the fiber bundle can be
adjusted in principle to adjust the focusing of the coherent bundle, a fixed focus with a large
depth of focus is often used for simplicity. A digital image acquisition board can be used to
capture the image in a computer for display, or a VCR can be used to record the images in
real time while the images are displayed on a monitor.
m
