Biomedical Engineering Reference
In-Depth Information
Other well-established imaging technologies such as
MRI, positron emission tomography (PET), and ultra-
sound can assist digital mammography in the diagnosis
problem. For example, physicians frequently use ultra-
sound to help determine whether a lesion detected on
a mammogram is a malignant mass or a harmless cyst.
However, many of these techniques have real limitations.
MRI specificity is highly variable, ranging from 28 to
100 percent, depending on the interpretation technique
used and the patient population; PET scanners
are expensive and scarce, and ultrasound has trouble
seeing microcalcifications because of so-called speck-
ledtiny bright flecks on the image caused by scattered
echoes.
The imaging principle relies on the physical charac-
teristics of a cancer, such as its relative opacity to dif-
ferent wavelengths of light. This approach may need to
be helped by combining the imaging approach with the
usage of markers that exploit the biochemistry of cancer
(e.g., compounds that will bind to tumors only) and that
are picked up by imaging when such compounds react
to light.
they act as point sensors, either intrinsic or extrinsic,
depending on whether the modulation is produced in
the fiber sensor itself, or whether there is an external
transducer connected to the fiber. They can also be
used as spectral sensors, where the fibers act simply as
light guiders.
The design of fiber-optic probes must be such that no
thrombosis or inflammatory effects of the blood vessels
occurs. Of course, this is only for temporary use since
long-term implantable sensors are not feasible. Proper
probe encapsulation is a crucial factor, so potential
problems of selectivity, hysteresis, and stability must be
resolved. Contrary to physical sensors, chemical and
biochemical sensors cannot usually be hermetically en-
capsulated. There may be problems of interference with
other substances in the environments.
6.2.8.1 Fiber optics for circulatory
and respiratory systems
Fiber-optic sensors in the circulatory and respiratory
systems are becoming very popular in both invasive and
noninvasive approaches. Optical fibers can be used to
measure the oxygen saturation in the blood. This mea-
surement is necessary to monitor the cardiovascular and
cardiopulmonary systems. The simplest method of this
measurement is either through reflected or absorbed
light, which is collected at two different wavelengths
and the oxygen saturation is calculated on the basis
of isosbestic regions of Hb (hemoglobin) and OxyHb
(oxygenated hemoglobin) absorption.
Optical oximeters, which calculate oxygen saturation
via the light transmitted to the earlobes, toes, and fin-
gertips, have been developed primarily for neonatal care.
A difficulty in neonatal care is the importance of dif-
ferentiating between the light absorption due to arterial
blood and that due to all other tissues and blood in the
light path, which implies the use of multiple wave-
lengths. This challenge can be met using an oximeter.
This approach is based on the assumption that a change
in the light absorbed by the tissue during systole is
caused by the passage of arterial blood. Using two
wavelengths, it is possible to noninvasively measure the
oxygen saturation by using the pulsatile rather than the
absolute level of transmitted or reflected light intensity.
The detection of blood absorbance fluctuations that are
synchronous with systolic heart contractions is called
photoplethysmography.
6.2.8 Medical sensors
from fiber optics
Fiber optics is beginning to have great use in healthcare
for intracavity imaging and safe laser delivery. It is also
becoming greatly useful in monitoring physiological
functions. The optical fiber technology in healthcare
provides many advantages.
1.
The fibers are very small and flexible and they can be
inserted inside very thin catheters and hypodermic
needles. These are highly noninvasive techniques
useful for monitoring.
2.
Fibers are nontoxic, chemically inert, and very
safe for patients. They are practically immune
to electromagnetic interference from other
electronic sources, which is of great importance
for the patients.
3.
Because of interference immunity there is no
crosstalk between neighboring fibers, which allows
the usage of several sensors grouped together in
a single catheter.
The ideal fiber-optic sensors for medical applications
must have the following properties: (1) reliability,
(2) automated operations, and (3) simple implantation
and low-cost maintenance. The fiber-optic sensors in
medical applications are based on sensing processes
that are typical of other types of fiber sensors, espe-
cially in intensity and time domain modulation. Mostly,
Blood gases and blood pH
Monitoring of the blood pH and blood oxygen (pO
2
) and
carbon dioxide (pCO
2
) partial pressures to determine
