Optical Fiber Sensors and their Applications in Medical Field

Introduction

The use of optical fiber sensors in medical field is based on their ability to transmit light over great distances with low power loss and the interaction of the light with a measurand system.

These sensors are electrically passive and hence immune to electromagnetic disturbances. Optical fibres are geometrically flexible and corrosion resistant. They can be miniaturized and are most suitable for telemetry applications.

Optical fiber sensors find important use in various medical applications; they can be used to measure physical variables like pressure, temperature, strain and displacement. Other optical fibres are used to sense chemicals plus other uses discussed later in this post.

The optical transducers are based on glass, or plastic fibres about 100 to 250 μm in diameter.

Basically, in an optical fiber sensor, the sensing element is attached to one end of the fiber. It varies on one parameter like the intensity, phase, polarization, wavelength of transit time of light through the fiber. Sensors that vary on the intensity of light are simple and require only a source and a detector.

Types of Optical Fiber Sensors

Optical fiber sensors and transducers can be classified into 3 main types as follows:

• Physical sensors
• Photometric sensors
• Chemical sensors

Physical Sensors

Physical parameters like pressure and temperature can be measured by optical fiber sensors. These sensors are based on the attachment of an optical transducer at the end of an optical fiber.

Temperature Sensing

The sensing element is deposited directly on the cleaved end of the optical fiber and the temperature is deduced from the phase of the reflected light or its spectrum. There are various kinds of sensors for temperature and pressure measurement in oil wells. The sensing element can be semiconductor, a phosphor, or a metal or a liquid crystal.

The figure below shows a metal deposited at one end of the fiber:

Temperature ranging from -100 °C to 400 °C can be measured. Some fibers measure temperature along the length of the fiber, and give an average of the output.

The greatest advantage of fiber optic, temperature sensors is their complete immunity to Electromagnetic Interference (EMI), Radio Frequency Interference (RFI), high voltage and electrical interference.

In the temperature measurement system above, the light is emitted by an LED, transmitted to and from crystal via optical fibres and measured by a photodetector. No metal parts are used in the probe design, resulting in transparency of the probe to electromagnetic fields.

Typical Medical applications of Optical fiber Temperature sensors

The production of localized and controlled hyperthermia i.e. elevated temperatures in the range of 42 -45 °C or higher for cancer treatment by electromagnetic energy, either the Radio frequency (RF), or Microwave frequency range, poses a difficult temperature measurement problem. Traditional temperature sensors, such as thermistors or thermocouples have metallic components and connecting wires which perturb the incident electromagnet (EM) fields and may cause localized heating spots and the temperature readings may be erratic due to interference. This problem is effectively, overcome by using temperature sensors based on fiber optics. These optical fiber devices utilize externally induced changes in the transmission characteristic of the optical fibers and offer typical advantages of optical fibres such as flexibility, small dimensions and immunity to electromagnetic interference.

Pressure Sensors

Measurement of intracranial and intracardiac pressure can be performed by using fiber optic sensors. For intracranial pressure measurement, the device is based on a pressure balancing system. Here static pressure is to be monitored and a sensor based on the deflection of a cantilever mirror attached to membrane. Deflection of the membrane causes the light emitted from centre optical fiber to be reflected differentially towards either of the two collecting fibers located on each side of the control fiber. The ratio of the light collected by two different fibers is sensed and suitable feedback air pressure is applied to the interior of the probe through the pneumatic connecting tube, balancing the membrane to its null position and providing readout of the balancing pressure.

With a few changes in the design, this same principle is also applied for monitoring intravascular pressure.

Photometric Sensors

The principle of operation of optical fiber photometric sensors is based on the light emanating from a fiber end that is scattered or fluoresced back into the fiber, allowing measurement of the returning light as an indication of the optical absorption or fluorescence of the volume at the fiber tip.
The variations in the returning light are sensed using a photodetector. Such sensors monitor the variations in amplitude or frequency of the reflected light. Examples of medical applications of the photometric sensors include:

• Oximeter – the working of this instrument is based on the amplitude measurement. This instrument measures the oxygen saturation of blood based on the fact that haemoglobin and Oxyhaemoglobin have different absorption spectra.
• Dye densitometry applied in blood flow measurement is also categorized as a photometric optical fiber sensor. Here, a dye i.e. indocyanine green is injected into the blood and its concentration monitored by its absorption at an appropriate wavelength. The time variation of dye concentration is then used to calculate the cardiac output by dilution techniques.
• Blood flow measurement using Doppler frequency shift can also employ fiber-optic light guide to send light from a laser onto the skin surface. Doppler frequency shift enables us to get the information on the blood flow.

Chemical Sensors

The optical sensing of chemical species is based on the interaction of these entities with light. When light strikes a substance, a variety of interaction may occur between the photons of the electromagnetic radiation and the molecules of the substance. These interactions involve an exchange of energy and may lead to absorption, transmission, emission, scattering or reflection of light. The quantized nature of this energy transfer produces important information about the composition of the system and this forms the basis of the spectroscopic method of chemical analysis.

We have 2 types of optical fiber sensors used in chemical measurement.

• Chemical sensors – here a chemical transduction system is interfaced to the optical fiber at its end. In action, interaction with the analyte leads to change in optical properties of the reagent phase, which is probed and detected through the fiber optic. The optical property measured can be absorbance, reflectance or luminescence.
• Spectroscopic – here, the optical fiber functions only as a light guide conveying light from the source to the sampling area and from the sample to the detector. The light interacts with species being sensed.

You can also read: Spectrophotometry Instrumentatl Method of Chemical Analysis

Advantages of Optical Fiber Sensors as Applied in Medicine

• The sensors do not involve any electrical connection to the patient body, thereby ensuring patient safety.
• They are non-electrical, hence free from electrical interference.
• More than one chemical species can be measured with a single sensor by using more than one probe detection wavelength. This offers an economical advantage over other sensors.
• There is a high degree of mechanical flexibility associated with the fiber optic and this combined with its reduced size, allows access to inaccessible areas of the body.