Biophotonics_Concepts_to_Applications

Their main uses are for intraluminal PDT, such as in arteries or veins, in the
esophagus, in gastrointestinal tracts, in urinary tracts, or in bronchi in the lungs.
In addition to usingfibers to deliver the therapeutic light, opticalfibers also are
used to monitor the therapeutic lightfluence, the concentration of the tumor sen-
sitizer, and the oxygen levels produced by the PDT process [ 39 ]. A wide range of
opticalfiber types can be used in PDT depending on the medical application and the
desired light distribution pattern. Bare-endedfibers typically are silica glassfibers
with large NAs (normally less than 0.60) and core diameters ranging from 200 to
1000 μm. Thesefiber types allow the setting of a well-defined distance between the
fiber end and the tissue being treated or examined, and thus can enable precise
irradiations to be administered or accurate values of tissue properties to be
collected.

Example 6.11Consider the microlens diffuser device shown in Fig.6.22.

Assume the diffuser produces a spherical distribution of the light emerging

from thefiber. If P is the optical power level emitted from thefiber, what is

the irradiance at a distance R from the center of the microlens?

Solution: For a sphere of radius R, the surface area is 4πR^2. Therefore at a

distance R from the center of the microlens, the optical power per unit area

(the irradiance) is P/4πR^2.

6.5.3 Thermal Interaction

Thermal interactionsare characterized by an increase in tissue temperature in a
local region [ 43 , 44 ]. The temperature change is the main variable in these

Red PDT

light source

Transmission fiber

(typically 2 to 2.5 m)

Diffuser segment emits red light

along its length (10 to 70 mm)

Side-firing fiber

Red PDT

Delivery fiber with light source

spherically diffusing microlens

Fig. 6.22 Examples of light distribution schemes from opticalfibers for PDT

6.5 Light-Tissue Interaction Mechanisms 179