Optical Fiber-Based MR-Compatible Sensors for Medical Applications: An Overview

1 Unit of Biomedical Robotics and Biomicrosystems, Center for Integrated Research, Università Campus Bio-Medico di Roma, Via Álvaro del Portillo, 21, Rome 00128, Italy; E-Mails: ti.supmacinu@acimrof.d (D.F.); ti.supmacinu@onipid.g (G.D.P.)

Find articles by Fabrizio Taffoni

Domenico Formica

Find articles by Domenico Formica

Paola Saccomandi

2 Unit of Measurements and Biomedical Instrumentation, Center for Integrated Research, Università Campus Bio-Medico di Roma, Via Álvaro del Portillo, 21, Rome 00128, Italy; E-Mails: ti.supmacinu@idnamoccas.p (P.S.); ti.supmacinu@anehcs.e (E.S.)

Find articles by Paola Saccomandi

Giovanni Di Pino

3 Institute of Neurology, Campus Bio-Medico University, and Fondazione Alberto Sordi-Research Institute for Ageing, Center for Integrated Research, Università Campus Bio-Medico di Roma, Via Álvaro del Portillo, 200, Rome 00128, Italy

Find articles by Giovanni Di Pino

Emiliano Schena

Find articles by Emiliano Schena

* Author to whom correspondence should be addressed; E-Mail: ti.supmacinu@inoffat.f; Tel.: +39-0622-5419-610.

This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution license (http://creativecommons.org/licenses/by/3.0/).

Abstract

During last decades, Magnetic Resonance (MR)—compatible sensors based on different techniques have been developed due to growing demand for application in medicine. There are several technological solutions to design MR-compatible sensors, among them, the one based on optical fibers presents several attractive features. The high elasticity and small size allow designing miniaturized fiber optic sensors (FOS) with metrological characteristics (e.g., accuracy, sensitivity, zero drift, and frequency response) adequate for most common medical applications; the immunity from electromagnetic interference and the absence of electrical connection to the patient make FOS suitable to be used in high electromagnetic field and intrinsically safer than conventional technologies. These two features further heightened the potential role of FOS in medicine making them especially attractive for application in MRI. This paper provides an overview of MR-compatible FOS, focusing on the sensors employed for measuring physical parameters in medicine (i.e., temperature, force, torque, strain, and position). The working principles of the most promising FOS are reviewed in terms of their relevant advantages and disadvantages, together with their applications in medicine.

Keywords: fiber optic sensors, MR-compatibility, interferometry, fiber Bragg grating, hyperthermia, respiratory monitoring, MR-compatible robotic assistive device

1. Introduction

Historically, the first application of optical fibers to the medical field enabled the illumination of internal organs during endoscopic procedures. During the years, the same technology has been adopted to perform other tasks, such as laser treatments, and to develop transducers for monitoring parameters of interest for both therapeutic and diagnostic purposes. Although forty years have passed since the introduction of Fiber Optic Sensors (FOS) [1] and despite some advantages of FOS with respect to other mature technologies, only during the last decade has their market grown notably, thanks to the improvement of key optical components and to the decrease of the costs [2]. Currently, FOS are exploited to monitor different chemical and physical parameters of medical interests [3,4]. These sensors are commonly grouped in two categories [5]: intrinsic sensors, where the optical fiber is, by itself, the sensing element; extrinsic sensors, where the optical fiber behaves as a medium for conveying the light, whose characteristics (e.g., intensity, frequency, phase) are modulated by the measurand. Sensors of this second class allow deploying the basic components of FOS (e.g., light source, photodetector) away from the sensing element, in order to develop small size sensors and hybrid solutions [6].

In addition to the already well-established applications in industrial and medical fields, immunity from electromagnetic interferences, together with good metrological characteristics and small size, make FOS attractive for several applications that take place inside, or close to, the magnetic resonance scanner.

From the introduction of Magnetic Resonance Imaging (MRI), in the early seventies, its importance in clinical imaging has exceeded even the most optimistic hopes of researchers. The constant increase of examinations based on this technique and the introduction of new procedures performed under MRI-guidance in clinical practice have promoted research on new sensors to be applied in this scenario. Among the Magnetic Resonance (MR)-compatible exploitations, FOS can be useful both to improve surgical procedure outcomes and for patient monitoring. Examples of those applications range from the measurement of the temperature of patients undergoing MRI-guided hyperthermic procedures [7], to the assessment of deflection and force on needles during MRI-guided procedures [8], to the estimation of physiological parameters (e.g., heart rate and respiratory monitoring) [9]. Possible exploitations of such sensors for research protocols are innumerable.

The ASTM standard F2503 covers MR safety-related interactions concerning the use of devices inside the MR environment. The standard distinguishes between “MR safe”, “MR conditional”, and “MR unsafe”. “MR-safe” is an item that poses no known hazards in all MR environments; “MR-conditional” is an item that has been demonstrated to pose no known hazards in a specified MR environment under specified use conditions; “MR-unsafe” is an item that is known to pose hazards in all MR environments. Despite the ASTM subcommittee for the F04.15.11 MR Standards decided to withdraw the “MR compatible” term, it is still commonly used in medical and engineering practice. The differentiation between the terms “MR-safety” and “MR-compatibility” is crucial [10,11]. MR compatible indicates that a device, when used in the MR environment, is “MR safe” and has been demonstrated to neither significantly affect the quality of the diagnostic information, nor have its operations affected by the MR device.

In this light, fiber optic technology is particularly suitable to develop “MR-compatible” sensors, since its immunity from electromagnetic fields allows: (1) being safe; (2) not affecting image quality; (3) maintaining unaltered sensors' functionalities. Furthermore, the material used to fabricate the optical fibers does not perturb magnetic fields inside the MR-scanner, which is crucial factor for the preservation of the quality of diagnostic information.

This paper provides an overview of “MR-compatible” FOS, focusing especially on sensors employed for measuring temperature, force, torque, strain, and position during several medical procedures. Throughout the paper we offer a critical review of the most promising and widespread techniques. For the sake of clarity we arranged them in three groups: (i) FOS based on fiber Bragg grating technology; (ii) intensity-based FOS; (iii) FOS based on interferometric techniques. Moreover, measuring principles, possible medical applications, advantages and weaknesses of each method are provided and discussed.

2. MR-Compatible FBG-Based Sensors

2.1. Working Principle

MR-compatible sensors based on fiber Bragg grating (FBG) technology, developed with different possible designs, allow sensing of temperature variations and strain. The introduction of the FBG in the field of thermal and mechanical measurements started with the research of Hill et al., who used electromagnetic waves to locally modify the refractive index of the optical fiber core [12]. About ten years later, the study of Meltz et al., promoted the diffusion of FBGs, providing the description of a more effective, holographic technique for grating formation [13]. Thanks to the characteristics of photosensitivity technology and its inherent compatibility with optical fibers, FBGs were introduced in different fields not only related to telecommunications [14], but also to the design of FOS [15]. Despite several valuable characteristics of the FBG sensors, their spread was delayed because of the high cost and the difficulties of the manufacturing, which have been overcome only during the 1990s. The last decade saw several research groups developing sensors based on FBG. The characteristics of these sensors, their fabrication process, and their applications in medicine have been already described in detail in different reviews [4,16].

The working principle of an FBG is based on radiation reflection caused by the Bragg grating: when a fiber optic, which houses an FBG, is interrogated with a polychromatic radiation, only a narrow range of wavelengths are reflected by the FBG. The central wavelength of such range, called Bragg wavelength (λB) can be expressed as a function of the effective refraction index of the core (ηeff) and of the spatial period of the grating (Λ) as follows:

λ_B = 2 · Λ · η_eff

The influence of both temperature and strain on Λ and η_eff allows the design of sensors for monitoring temperature and strain, and other physical parameters related to them (e.g., pressure, force, vibrations, and flow). Specific solutions for FBG-based transducers can be adopted in order to make them selectively sensitive to strain or temperature. Usually a reference FBG is added to the main sensor in order to attenuate the influence of undesired effects, thus improving the repeatability of the measurement system [17].

FBG technology allows the development of sensors with good metrological characteristics, such as good accuracy, large bandwidth, large dynamic range, and high strain and thermal sensitivity (typical values range from 0.64 pm/με to 1.2 pm/με, and from 6.8 pm/°C to 13 pm/°C, respectively). Moreover, this technology offers the advantages of multiplexing, because it is possible to write multiple gratings with different Bragg wavelengths on a single fiber. On the other hand, the measurement chain should adopt an expensive device to detect the wavelength of the reflected radiation (i.e., an optical spectrum analyzer), in order to avoid the decrease of performances (e.g., resolution and accuracy).

2.2. Medical Applications

The main characteristics that make FBG technology particularly suitable for applications in medicine are biocompatibility, wide bandwidth, and small size. Furthermore, the immunity of fiber optics to electromagnetic fields, and the negligible interference with the electromagnetic fields used in MRI, make this technology very attractive for developing “MR-compatible” sensors.

Some research groups have proposed FBG-based sensors for monitoring temperature in MRI, which is of main importance for different applications: for example Rao and colleagues developed a measurement chain for cardiac output estimation, with resolution of 0.2 °C and accuracy of 0.8 °C [18]. This technology is also employed to measure tissue temperature during MRI-guided hyperthemic treatments. Temperature plays indeed a crucial role during hyperthermia, and its monitoring can be useful to drive the physician in the adjustment of thermal exposure. However, the metrological performance of commonly used temperature sensors are affected by the electromagnetic fields used during the procedure to induce hyperthermia (e.g., the artifacts due to self-absorption of thermocouples [19]. To overcome this problem, Webb and coworkers proposed a measurement system with five FBGs, that allows them to perform temperature measures during hyperthermia treatment of kidney and liver in alive rabbits [20]. Similarly, other groups assessed the feasibility of using FBGs for temperature monitoring in swine pancreatic tissue undergoing hyperthermia [7,21,22]. In a subsequent study the same authors, in the attempt to improve spatial resolution, measured tissue temperature using 12 small size FBGs (1 mm length) to improve [23], and adopted an ad hoc designed MR-compatible polymethylmethacrylate (PMMA) mask to precisely arrange the optical applicator and the FBGs inside the tissue, as shown in Figure 1A . This technology has been also used to monitor temperature during cryosurgery of prostate [24] and liver [25], where the MRI compatibility was experimentally assessed.