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Encyclopedia of Physical Science and Technology EN011L-523 August 10, 2001 11:17

332 Optical Fiber Techniques for Medical Applications

transillumination technique may therefore give rise to an The first application of OCT was in ophthalmology,
image of a tumor. This time resolved imaging technique where fine structures in the eye (i.e., macular hole in
is somewhat similar to X-ray imaging, but it may provide the retina) have been observed. The system was then
higher resolution and it does not suffer from the health adapted for endoscopic applications. The interferometer
hazards of X-rays. was equipped with optical fibers, and one of them was in-
Instead of scanning the beam across the breast one may serted into a thin endoscope. By attaching a rotating prism
use two bundles of optical fibers. In each of them the to the distal tip of the fiber, or by scanning the fiber, one
individual fibers at the proximal end are tightly arranged. may obtain a cross sectional image. Endoscopic OCT has
At the distal ends the fibers are spread, and they surround been tried clinically to study the gastrointestinal tract and
the breast. A very short pulse emitted from a diode laser or the esophagus. It is being tried as well for intravascular
a dye laser is focused on one of the fibers in the proximal imaging, in cardiology, to study plaque inside the arteries.
end of one of the bundles. The optical energy transmitted Endoscopic OCT offers physicians the ability to view mi-
through the breast is collected by the distal ends of the croscopic structures inside the body in real time (even at a
second fiber bundle and transmitted to the proximal end. video rate) and with very high resolution. This is a form of
The energy collected at each of these fibers is measured, “optical biopsy” which provides images of tissue in situ,
one by one, using a fast optical detector. The focused laser with no need to remove tissue specimens for examination.
beam is moved to a different fiber in the first bundle, and
the process is repeated. From the analysis of the results
one may obtain the image of a tumor in the breast. IX. OUTLOOK
This and similar imaging techniques have been tried for
imaging of breast cancer and brain cancer. Early results of During the past decade there has been rapid progress in
this technique are very promising. There are many tech- the development of optical fibers and fiberoptic systems.
nical hurdles that have to be overcome before this method Foremost was the development of silica-based glass fibers,
could be used routinely. It is expected that the spatial reso- which have extremely low transmission losses. These
lution will be of the order of 1 mm, adequate for the early fibers are the major building blocks of future commu-
detection of breast cancer. nication systems. Other optical fibers were developed for
special applications, such as fibers with good transmission
in the mid-IR or in the deep-UV parts of the spectrum.
B. Optical Coherent Tomography
Among these we may also include power fibers that are
Ultrasound imaging is based on bouncing ultrasound capable of transmitting laser beams of high intensities.
waves off tissue and determining the delay time of waves With the development of the various optical fibers, there
reflected from internal structures in the tissue. This can be has been an increasing recognition of the enormous pos-
translated to 2D images with a resolution of a few hundred sibilities for utilizing them in medical applications. Three
microns. Optical coherence tomography (OCT) is based major uses of fiber—optical endoscopy, diagnostics, and
on the bouncing of light waves from tissue, and measur- power transmission—have been reviewed here.
ing how long it takes for the backscattered light from mi- In the past, endoscopes were rigid, and their uses were
crostructures in tissues to return. Because of the very short limited. A drastic change occurred when fiberscopes (es-
delay times involved, conventional electronics cannot be pecially the flexible ones) were included in endoscopes.
used, and one has to apply other techniques. In its sim- Rigid endoscopes are still widely used in many applica-
plest form, an OCT system is similar to a Michaelson tions because they do provide an excellent image, it is
interferometer, which includes an NIR laser source with easier to steam sterilize them, and they are cheaper. As
a relatively broad band of wavelengths. One beam from mentioned earlier, there are videoscopes based on CCDs
the interferometer focuses on a tissue sample, whereas that are also used in endoscopy. These are still bulky and
a second beam provides a reference. The reflected light expensive, but progress is being made to reduce their size
from some depth in the sample interferes with the refer- and cost.
ence signal, only if the depth is identical to the length of An endoscopist could get a clear image of an internal
the reference arm. The reference arm is scanned and a de- organ, and perform surgical operations such as biopsy or
tector records the interference patterns between the two the removal of the gallbladder, by inserting medical in-
beams, which are actually generated by reflections from struments through ancillary channels. A computer, such
different depths in the tissue. The analysis of the interfer- as the one used in robot–assisted surgery, could control
ence patterns of the two beams can be used to obtain 2D the surgical instruments. The first operations carried out
images of the tissue, at a depth of a few millimeters, with with a laser-plus-robot system were the minimally inva-
a resolution of a few micrometers. sive coronary bypass operations (with rigid telescope).