Optical Fiber Sensors for Biomedical Applications

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664 L.C.L. Chin et al.utilize the tissue itself to modulate the illuminating light, whereby the collectedlight is the result of backscattering directly from the interrogated tissue or tissuefluorescence/Raman induced by an optical source. Indirect sensors employ an intermediaryin response to the tissue property of interest (e.g., temperature, enzymepresence). Indirect sensors can be subdivided into intrinsic, which utilize the fiberitself (core and/or cladding) as the sensing element, and extrinsic, which incorporatean additional sensing element at the fiber end (e.g., transducer or substrate).Examples include physical sensors that employ miniaturized transducers that modulatethe light in response to such physical parameters as temperature, pressure andradiation dose. Chemical or biosensors evaluate the change in a molecular reagentattached to the end of the fiber via spectroscopic or fluorometric measurement. Suchsensors have been utilized for the measurement of pH, glucose, and other intrinsicmetabolites. Figure 17.2 presents a diagrammatic overview of the classificationschemes for optical fiber sensors in biomedicine.The primary discussion in this chapter focuses on photonic sensors, since thisis the most relevant sensor type in the biophotonics community. Depending onthe clinical purpose (diagnostic or dosimetric), photonic sensors offer a range ofpotential modalities that provide unique physical measurements including whitelight spectroscopy, fluorescence, Raman scattering, optical coherence tomography(OCT), and polarimetry. Of particular importance for sensor design is the choiceof the appropriate modality for the given application. Table 17.1 lists the variousmodalities discussed in this chapter, including an overview of the relevantphysical measurements, their advantages and disadvantages, and their potentialapplications.Fig. 17.2 Classification schemes for optical fiber sensors in biomedicine
17 Optical Fiber Sensors for Biomedical Applications 665Table 17.1 Optical fiber sensor modalities to be discussed in this chapter, listing the physicalmeasurements, applications, advantages and disadvantagesModalityDetection/determination Applications Advantages DisadvantagesWhite lightspectroscopyFluorescenceRamanOCT• absolute lightintensity• optical properties• chromophoreconcentration• scatterer spacingand density• oxygen saturation• fluorescenceintensityoptical propertiesfluorophoreconcentration• metaboliteconcentration• optical properties• tissuecompositionalinformation• oxygen saturation• tissuemicrostructure• blood flow/Doppler• birefringence• diagnosis• oximetry• light intensitydosimetry• chromophore/drugconcentration• diagnosis• treatmentevaluation• fluorophore/drugconcentration• diagnosis• treatmentmonitoring/evaluation• diagnosis• treatmentmonitoring/evaluation• tissue functionalstatus (Dopplerblood flowmaps)• range ofsamplingvolumes• relativelyinexpensive• potentiallysimple dataprocessing• directmeasurement• wide array ofpossiblecontrast agents• high specificity• 3-D imaging• non-imaging• manycontributionsto the measuredsignal• uncertainsampling volume• auto fluorescencemay interfere• weak signals• long(er)integration times• need carefulanalysis toremoveconfoundingsignals• limitedpenetrationdepthThe choice of modality, in turn, guides a specific probe design that must considerthe accessibility of the target site. Superficial (exposed surface) tissue cantypically be examined using surface reflectance or endoscopic sensors, while internalstructures may require sensors suitable for interstitial insertion. Furthermore, inthe case of superficial targets, layered structures may require additional design considerationsfor spatial-depth discrimination. Such factors can be adjusted for usingappropriate source-detector geometry and fiber-tip design.While the specifics of actual sensors will be reviewed, we also examine thephysics behind sensor design parameters and their relation to light collection efficiencyand sampling volume in turbid media. By focusing on the physics of sensordesigns, it is hoped that the reader can extend the information of this chapter to aspecific biomedical application.
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