Optical Fiber Sensors for Biomedical Applications

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702 L.C.L. Chin et al.directly recover the intrinsic fluorescence and information on tissue optical propertiesfrom the measured distorted spectrum. Future advancement of this concept willlikely allow for fluorescent corrections without the necessity of added reflectancecomponents, thereby greatly simplifying detection technology and computationalanalysis.17.6 Optical Coherence Tomography FibersUnlike other fiber sensors described in this chapter, OCT forms truly depth-resolvedtomographic cross-sectional images, with a depth resolution in the 1–20 μm range(cf. “depth-sensing” discussions of various sensors above). Essentially, an OCTimaging sensor forms the sample arm of an OCT interferometer and typically consistsof a single mode optical fiber terminated with a focusing element (e.g., aGRIN or a ball lens) and beam deflecting element (e.g., a retro-reflecting prismor an angle cleaved distal optic) for a side-viewing sensor. The overall principle canbe appreciated from the layout of a catheter-based fast OCT imaging system presentedby Tearney et al. [57], as shown in Fig. 17.21. Specifics of the distal sensordesign, combination of radial scanning and longitudinal fiber pull-back approach,and integration with the rest of the OCT imager can be enabled. Using the system inFig. 17.21 and its later variants, the authors were able to generate impressive subsurfaceimages of the superficial layers of the gastro-intestinal and broncho-pulmanorytracts, as well as intravascular images of atherosclerotic plaques in coronary arteries[58]. Several other groups have also developed their own versions of intravascularand endoscopic OCT imaging systems, all using their own unique implementationsof OCT fiber sensors. For example, Fig. 17.22 shows an endoscopic sensordeveloped by Yang et al. [59] for esophageal imaging in the gastrointestinal tract.Cr 4+ :fosteritelaserDetectorTransimpedanceamplifierBandpassfilterDemodulator50/50SamplearmReferencearmComputerFramegrabberGratingPhasecontroldelay lineGalvanometercontrollerVideosynchronizationcircuitryCatheterendoscopeGalvanometermirrorOptical fiber(angle cleaved)Grin lens(angle polished)PrismTransparentouter sheathS-VHSrecorderMonitorFig. 17.21 (a) layout of the OCT system. (b) Second-generation OCT catheter-endoscope. Byadjusting the angle cleaving the optical fiber and the angle polishing the GRIN lens, normal incidencesare avoided and thus internal reflections can be minimized. (c) Photograph of an OCTcatheter-endoscope at the distal end of the sensor. (Reprinted with permission from [57])
17 Optical Fiber Sensors for Biomedical Applications 703Fig. 17.22 An OCT optical fiber sensor adapted for biomedical imaging during human gastrointestinalendoscopy. (a) the proximal end of the endoscope, showing the linear translation motor thatpulls the OCT fiber to-fro by ∼3 mm to enable longitudinal cross-sectional imaging (b) the distalend of the endoscope, with the OCT probe protruding from the instrument channel (c) close-up ofthe OCT fiber sensor within its transparent sheath, with OF = optical fiber, L = focusing (GRIN)lens, P = side-reflecting prism, and A = adhesive sealant (d) clinical imaging showing OCT probein contact with human esophageal tissue. (Reprinted with permission from [59] and[74])A particularly important design consideration for OCT sensors is the avoidanceof normal interfaces and retro-reflective surfaces, which in addition to reducingsignal strength by causing photon loss, can inadvertently interfere with the referencebeam and generate spurious and artificial lines or streaks in the resultantimage. Thus, care must be taken to minimize the number of elements in an OCTsensor, as each one will likely contribute at least one (and often two) normalinterfaces whose reflections could fall within the coherence ranging depth of thesystem and thus cause artifacts in the image. The use of angle-cleaved surfaces toavoid normal incidence, refractive-index matching compounds, and careful opticaldesign to minimize the number of interfaces are thus essential for successfuloperation of an OCT fiber sensor. Figures 17.21(b, c) and 17.22 illustrate theseconcepts.A significant limitation of OCT is its shallow imaging depth, which is typically1–3 mm in nontransparent mammalian tissues. While a huge variety of excitingbiomedical applications are still possible within this depth range (for example, endoscopicand intravascular imaging as described above, where the pathologic changesof interest occur within 1–2 mm of the inner surface), many scenarios would benefitfrom deep tissue imaging with OCT. Fortunately, owing to the technique’s intrinsiccompatibility with optical fiber technology, this can indeed be done in the contextof an interstitial OCT fiber sensor.
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