Biomedical Engineering Reference
In-Depth Information
needle, it can be inserted into other body parts. If the probe has a large enough
focal depth to obtain information through a blood vessel wall, it may be able
to reach inside organs located deep within the body, such as the pancreas
and brain. At present, many of the techniques discussed above are still only
proposals, but they are expected to be realized in the near future.
There is an increasing need for minimally invasive and highly accurate
techniques for diagnosis confirmation. Noninvasive cancer screening technolo-
gies, such as positron-emission tomography (PET), X-ray computed tomog-
raphy (CT), and magnetic resonance imaging (MRI) have become more and
more sensitive and are capable of detecting small cancerous lesions, and so
are increasingly commonly used in clinics. These techniques can detect “sus-
picious” tissues. It may be possible to reach a diagnosis from the obtained
image of advanced and large cancerous tissues, but it is dicult for small sus-
picious tissues. Generally, for patients with positive results, a doctor performs
a biopsy to confirm the diagnosis and decide on a treatment plan. This biopsy
includes resections of the suspicious tissue, and the process becomes more
invasive if the tissue is located in a body part that is inaccessible with an en-
doscope. Hence, an optical biopsy using a miniaturized Raman probe provide
feasible alternative or further support for confirming a diagnosis. This is the
case, not only for cancer but also for other diseases such as atherosclerosis.
2.2 Optical Properties of Fibers and Devices
There are generally two types of fiber-optic Raman probe. One is a probe with
a confocal type head, as shown in Fig. 2.1A. Although it is rather large in
size, various types of optical filter can be used because there is enough room
to arrange filters and lenses for effectively removing the Raman scatter and
fluorescence generated in the optical fiber. The other type is a miniaturized
fiber-optic probe, in which the filters and lenses are directly mounted on the
end of the optical fiber, as shown in Fig. 2.1B. In order to reduce the size,
the head structure must be quite simple. The following section describes the
properties of optical fibers and other devices to construct the latter type of
fiber-optic Raman probe.
2.2.1 Glass and Fused Silica Optical Fibers
Since there are many different optical fibers that are commercially available, it
is impossible to cover all of them in this chapter. Therefore, this section will de-
scribe the optical properties of glass and fused silica fibers that can be used as
for miniaturized fiber-optic Raman probes. A major di
culty in developing a
sensitive fiber-optic Raman probe is to suppress the fiber spectral background
(FSB) generated in the optical fibers themselves [14-30]. This background is
orders of magnitude larger than the Raman signal from the tissues to be mea-
sured. The FSB consists of Raman scattered light and fluorescence generated
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