Optical-Fiber Measurement Systems for Medical Applications

After telecommunications, also medicine has been revolutionized by optical fibers. They were firstly used, in the early sixties, to visualize internal anatomical sites by illuminating endoscopes. The essential technological solution to obtain good quality images was the introduction of “cladding” during the fifties. The result was the development of minimally invasive tools that have become essential for medical diagnosis and surgery. But optical fibers offer the potential for much more than illumination or imaging tasks. For example, they can also be utilized to sense physiological parameters. The subject of present chapter is, therefore, a description of the design and measurement principles utilized in fiber optic sensors (FOSs) with a particular reference to biomedical applications. FOSs development started in the sixties, but the high component costs and the poor interest of the medical community delayed the industrial expansion. The cost reduction of key optical components allowing to realize even disposable or mono-patient FOSs, the increase of components quality, the development of miniaturization, and the availability of plug and play and easy-to-use devices are the main reasons of the growth that is taking place in the use of FOSs. Moreover, FOSs are characterized by some crucial advantages respect on the conventional transducers that allow to satisfy requirements for use in medical applications: they are robust, may have good accuracy and sensitivity, low zeroand sensitivity-drift, small size and light weight, are intrinsically safer than conventional sensors by not having electrical connection to the patient, large bandwidth, and show immunity from electromagnetic interference. This last feature allows to monitor parameters of physiological interest also during the use of electrical cauterization tools or in magnetic resonance imaging. At present, FOSs are used to measure physical variables (e.g., pressure, force, strain, and fluid flow) and also chemical variables (oxygen concentration in blood, pH, pO2, and pCO2). The simplest FOSs classification is based on the subdivision in intrinsic and extrinsic sensors. In an intrinsic sensor the sensing element is the optical fiber itself, whereas an extrinsic sensor utilizes the optical fiber as a medium for conveying the light, whose physical parameters are, in turn, related to the measurand. Due to different requirements for miniaturization and safety, in medical applications, these sensors are usually further divided in: invasive sensors, which are inserted into the body,