Biomedical Engineering Reference
In-Depth Information
Endoscopes may be specially built for imaging of various organs. The following is a list
of commonly used endoscopes: angioscopes for veins and arteries, arthroscopes (or ortho-
scopes) for the joints, bronchoscopes for the bronchial tubes, choledoschoscopes for the bile
duct, colonoscopes for the colon, colposcopes for the vagina, cystoscopes for the bladder,
esophagoscopes for the esophagus, gastroscopes for the stomach and intestines, laparo-
scopes for the peritoneum, laryngoscopes for the larynx, and ventriculoscopes for the ven-
tricles in the brain.
17.6.3 Diagnostic and Sensing Fiber Probes
In terms of sensors for minimally invasive measurements into the body, the use of fiber
optic probes began to thrive beginning in the 1980s. To understand the potential capability
of these probes, a sensor is first defined along with some of the requirements for a good
sensor. A sensor is a device that transforms an input parameter or measurand into another
parameter known as the signal. For instance, a displacement membrane on the tip of a fiber
probe could transform a pressure signal into a light intensity change that is then depicted as a
change in voltage from a light detector. The requirements for any good sensor are specificity
or the ability to pick out one parameter without interference from the other parameters,
sensitivity or the capability to measure small changes in a given measurand, accuracy or
closeness to the true measurement, and low cost. As with most sensors, fiber optics have to
trade off these parameters to within some limit. For example, you may be able to get 95 per-
cent accuracy at a reasonable cost, but obtaining 99 percent accuracy might require a huge
increase in cost, so one must trade off what might be clinically acceptable given the cost. Fiber
optic probes offer the potential to meet the preceding sensor requirements, as well as provide
for miniaturization; good biocompatibility for the visible and near-infrared wavelengths;
speed, since light is used; and safety, since no electrical connections to the body are required.
All fiber optic probes transmit light into the body, and the light, directly or indirectly,
interacts with the biological parameter of interest, be it a biological fluid, tissue, pressure,
or body temperature. The interaction causes a change in the light beam or beams that travel
back to the detection system. The returning signal can be physically separated from the
input signal in a separate fiber or fibers, or it can be separated if the output beam is at a sep-
arate wavelength from the input beam, as is the case for fluorescence probes. Fiber optic
sensors can be used for each of the preceding chemically and physically based measure-
ments described in Sections 17.3 and 17.4. However, rather than try to discuss each fiber
optic probe for each application, the rest of this section will be focused on separating the
fiber probes into two classes: indirect and direct fiber optic sensors. Direct fiber optic
sensors are defined as those probes in which the light interacts directly with the sample.
For instance, absorption measurements can theoretically be made to determine such things
as blood oxygenation or to quantify blood analytes with either two side firing fiber optic
probes separated at a fixed distance, as shown in Figure 17.22a, or an evanscent wave fiber
optic, which has the cladding stripped off so some of the light travelling down the core
travels out of the fiber, interacts with the sample, and then returns to the fiber core, as
shown in Figure 17.22b.
In Figure 17.22c, a fiber optic bundle is depicted that could be used to transmit and
receive light
to and from tissue to distinguish between normal and cancerous or
