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Optical Fiber Techniques for Medical Applications 331

excitation. A krypton laser emitting about 0.25 W at treatment of the endometrium, in gynecology. Different
413 nm is suitable for this application. In order to see photosensitive drugs, such as aminolevulinic acid (ALA)
this red emission one has to attach to an imaging and lutetium texaphyrin, have been also tried for these
system an optical filter that transmits at 630 nm and applications.
blocks the exciting light at 413 nm.
3. Photodynamic therapy: It was found that the use of
HPD with lasers could serve not only for diagnosis of VIII. NOVEL FIBEROPTIC MEDICAL
cancer but also for therapeutic purposes. If rather than SYSTEMS
illuminating a tumor with UV light, one uses red light
(λ = 630 nm) of sufficient energy, the results are During the last 5 years there has been rapid advancement
strikingly different. Red light penetrates deeply into in the field of biomedical optics. The basic and preclini-
the tissue. HPD absorbs this red light, a series of cal research was described in Sections I–VII. There is a
photochemical reactions occur, and the end result is plethora of novel methods and medical systems and many
the release of some photoproduct (probably singlet of these are now being tried clinically. In this section we
oxygen), which kills the host malignant tissue. The discuss a few of these new developments.
method itself is called photodynamic therapy (PDT).
This is actually photochemotherapy, because it
A. Time Resolved Phenomena
involves the administration of a drug and the use of
optical radiation for triggering the process that cures There is great interest in the early detection of breast can-
2
the disease. Roughly 10–50 mW/cm of red light is cer. The problem is to detect tumors of a size of a few
needed for this photochemotherapy. millimeters in tissue of total thickness 40–100 mm, and to
try to distinguish between malignant and benign tumors.
The diagnostic method mentioned in (1) can be readily The traditional methods such as X-ray mammography or
adapted in endoscopy. A special endoscope could incor- ultrasound imaging do not always provide the necessary
porate a quartz fiber for UV illumination and a red trans- resolution, and they cannot distinguish between the types
mitting filter in front of the imaging fiberscope. With such of tumors. Magnetic resonance imaging is a very power-
an endoscope, tumors inside the body could be irradiated ful method but it is cumbersome and expensive. If there
by krypton laser light (or even by a Xe lamp with a UV is a small cancerous tumor inside the breast, the optical
transmitting filter), and using the red image formed, one absorption in it is higher, due to higher density of blood
could locate malignant tumors. This may be used for early vessels, and the optical scattering is also higher, due to
detection of lung cancer, when the malignant tumors are changes in the refractive index. Optical techniques could
too small to be detected by chest X-ray or by computed provide a way of distinguishing between two tissues. Such
tomography. techniques would be useful, not only for breast cancer but
Photodynamic therapy, mentioned in (2), is also adapt- for many other applications. One of these optical methods
ableforuseinconjunctionwithfiberopticsystems.Ahigh- is schematically described here.
intensityredlightcanbetransmittedthroughaquartzfiber, Let us consider an NIR laser that emits very short pulses
and delivered directly inside the tumor. This light may then (of the order of a few femtoseconds) in a narrow beam.
selectively destroy cancer cells. In the past, a Nd:YAG The beam impinges on one side of the breast and it en-
laser pumped or Ar laser pumped dye laser, emitting at counters scattering and absorption. Photons that are trans-
λ = 630 nm, was used for this purpose. There has also mitted are detected on the other side of the breast by a
been progress with the development of miniature semi- fast optical detector. There are three types of photons: (i)
conductor lasers, which emit continuously several watts ballistic photons which pass through the tissue in straight
at 630 nm. lines, without scattering, (ii) “snake” photons which pass
The endoscopic diagnosis and treatment of cancer has through the tissue in a zigzag path, with little scattering,
been reported in the esophagus, lungs, larynx, prostate, and (iii) diffuse photons, which suffer multiple scattering.
and cervix. Other photosensitive drugs are being tested, The ballistic photons will arrive at the detector much faster
including benzoporphyrin derivative and phthalocyanines. than the other photons. One may therefore place a very fast
Laser endoscopic photochemotherapy has been pro- shutter in front of the detector; use the shutter to let only
posed for noncancer applications, such as the treatment the ballistic photons through, and then turn the shutter off
of age-related degenerate macula. Another example is (i.e., optical gating). If the narrow beam is scanned across
photodynamic laser angioplasty, which would be used the breast, there will be a noticeable difference if it is
to treat coronary artery disease. Other examples are the transmitted through a tumor (which has higher absorption
treatment of rheumatoid arthritis, in orthopedics, and the and scattering coefficients) or through healthy tissue. This