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

326 Optical Fiber Techniques for Medical Applications

an arterial stenosis (narrowing) are also studied with the Pressure may be measured via mechanical transduc-
same technique. ers attached to the optical fiber. For example, a reflective
Bloodflowmayalsobemeasuredbyadyedilutiontech- surface may be attached to the distal end of a fiber by
nique. As an example, sodium fluorescein may be injected flexible bellows [Fig. 11(b)]. Light sent through the fiber
into the blood. This dye has a yellow-green luminescence is reflected back through the same fiber. The light output
when excited by blue light. An optical fiber is inserted into depends on the shape of the reflective surface, which in
an artery, and the same fiber transmits both the excitation turn depends on pressure. Both temperature and pressure
light and the emitted luminescence. The dye dilution in the have been measured in vivo inside blood vessels.
blood can be deduced from the decrease in luminescence
with time. This in turn may be used to calculate blood 2. Chemical Sensors
flow.
In this case, the miniature transducers, which are attached
There are other physical effects that can be directly
to the end of the optical fiber, are sensitive to chemi-
measured with optical fibers. For example, light scattering
cal changes in the sample of interest (e.g., blood). The
from tissues can be easily measured. The scattering from
basic design of an indirect chemical sensor is shown in
diseased tissues may be significantly different from that
Fig. 11(c). A special reagent is trapped inside a porous
from healthy tissue.
polymer or sol-gel layer. Light is sent through one fiber,
and reflected light or luminescence from the reagent is
2. Chemical Sensors
transmitted back. The reagent is allowed to interact with
Oxygen in blood is carried by the red blood cells. Oxygen blood, for example, through diffusion. This, in turn, is
saturation is the ratio (in percentage) between the oxy- manifested as a change in luminescence (or change in
gen content in a blood sample and the maximum carrying reflected light). Sensors for measuring pH are based on
capacity. In arteries the blood is normally more than 90% a dye indicator. Some dyes luminesce under UV excita-
saturated, and in the veins only about 75%. There is a large tion, and the luminescence intensity is determined by the
differencebetweenhemoglobinandoxyhemoglobininab- pH of the sample. For other dyes the absorption spectra
sorption of light at λ = 660 nm. On the other hand, there is are determined by the pH. In practice, fiberoptic sensors
little difference at λ = 805 nm. Light at λ = 660 nm, emit- have been used for measuring pH of blood in the physio-
ted from a semiconductor source (or incandescent lamp), logical/pathological range (pH = 6.8–7.7) with accuracy
is sent through one optical fiber. The light scattered back better than 0.01 pH units.
from blood is transmitted through a second fiber, and its in- A different sensor, of similar construction, is used for
tensity I 660 is measured. Two other fibers are used for mon- monitoring the partial pressure of oxygen pO 2 in blood. In
this case, one may again use a dye that fluoresces. In some
itoring the intensity I 805 for calibration. The ratio I 660 /I 805
is used to determine the oxygen saturation in blood. Mea- dyes the fluorescence emission decreases with increase in
surements have been routinely performed through a thin pO 2 (quenching). The fluorescence intensity is therefore a
catheter of diameter 2 mm. direct measure of pO 2 . Similar sensors have also been used
for measuring the partial pressure of CO 2 , pCO 2 , in blood.
The feasibility of using optical fibers as biomedical sen-
C. Indirect Sensors
sors has been established. Some technical problems (e.g.,
1. Physical Sensors response time, calibration, and shelf life) need further at-
tention. Some of the sensors described in Section V have
In this case, a transducer (optode) is attached to the distal
already been introduced into clinical use. A new family of
tip of an optical fiber catheter, in order to perform physical
indirect chemical sensors incorporates biomolecules such
measurements. Two measurements are of special impor-
as enzymes or antibodies. These sensors can monitor the
tance: temperature and pressure.
body’s levels of glucose or penicillin and may soon be used
There are several schemes for measuring temperature.
to measure metabolic substances, toxins, and microorgan-
One scheme is based on the change in luminescence of
isms in the body.
a phosphor with temperature. The phosphor powder is
attached to the distal end of a plastic clad silica fiber. A
pulse of UV light is sent through the fiber and excites the
VI. INTEGRATED FIBEROPTIC SYSTEMS
phosphor. The visible luminescence is returned through
the same fiber, and its decay time is measured. The decay
A. Power Fibers for Medical Systems
time is dependent on temperature, and its measurement
is translated into a temperature reading. The accuracy is Therapeutic applications of lasers, and in particular surgi-
◦
better than 0.1 C near room temperature. cal applications, call for using relatively high laser power.