Sven Poeggel

Optical Fibre Pressure Sensors in Medical Applications

This article is focused on reviewing the current state-of-the-art of optical fibre pressure sensors for medical applications. Optical fibres have inherent advantages due to their small size, immunity to electromagnetic interferences and their suitability for remote monitoring and multiplexing. The small dimensions of optical fibre-based pressure sensors, together with being lightweight and flexible, mean that they are minimally invasive for many medical applications and, thus, particularly suited to in vivo measurement. This means that the sensor can be placed directly inside a patient, e.g., for urodynamic and cardiovascular assessment. This paper presents an overview of the recent developments in optical fibre-based pressure measurements with particular reference to these application areas.

Fiber-optic combined FPI/FBG sensors for monitoring of radiofrequency thermal ablation of liver tumors: ex vivo experiments.

We present a biocompatible, all-glass, 0.2 mm diameter, fiber-optic probe that combines an extrinsic Fabry-Perot interferometry and a proximal fiber Bragg grating sensor; the probe enables dual pressure and temperature measurement on an active 4 mm length, with 40 Pa and 0.2°C nominal accuracy. The sensing system has been applied to monitor online the radiofrequency thermal ablation of tumors in liver tissue. Preliminary experiments have been performed in a reference chamber with uniform heating; further experiments have been carried out on ex vivo porcine liver, which allowed the measurement of a steep temperature gradient and monitoring of the local pressure increase during the ablation procedure.

Recent Improvement of Medical Optical Fibre Pressure and Temperature Sensors

This investigation describes a detailed analysis of the fabrication and testing of optical fibre pressure and temperature sensors (OFPTS). The optical sensor of this research is based on an extrinsic Fabry–Perot interferometer (EFPI) with integrated fibre Bragg grating (FBG) for simultaneous pressure and temperature measurements. The sensor is fabricated exclusively in glass and with a small diameter of 0.2 mm, making it suitable for volume-restricted bio-medical applications. Diaphragm shrinking techniques based on polishing, hydrofluoric (HF) acid and femtosecond (FS) laser micro-machining are described and analysed. The presented sensors were examined carefully and demonstrated a pressure sensitivity in the range of sp = 2–10 nmkPa and a resolution of better than ΔP = 10 Pa (0.1 cm H2O). A static pressure test in 38 cmH2O shows no drift of the sensor in a six-day period. Additionally, a dynamic pressure analysis demonstrated that the OFPTS never exceeded a drift of more than 130 Pa (1.3 cm H2O) in a 12-h measurement, carried out in a cardiovascular simulator. The temperature sensitivity is given by k=10.7 pmK, which results in a temperature resolution of better than ΔT = 0.1 K. Since the temperature sensing element is placed close to the pressure sensing element, the pressure sensor is insensitive to temperature changes.

Intra-Tissue Pressure Measurement in Ex Vivo Liver Undergoing Laser Ablation with Fiber-Optic Fabry-Perot Probe.

We report the first-ever intra-tissue pressure measurement performed during 1064 nm laser ablation (LA) of an ex vivo porcine liver. Pressure detection has been performed with a biocompatible, all-glass, temperature-insensitive Extrinsic Fabry-Perot Interferometry (EFPI) miniature probe; the proposed methodology mimics in-vivo treatment. Four experiments have been performed, positioning the probe at different positions from the laser applicator tip (from 0.5 mm to 5 mm). Pressure levels increase during ablation time, and decrease with distance from applicator tip: the recorded peak parenchymal pressure levels range from 1.9 kPa to 71.6 kPa. Different pressure evolutions have been recorded, as pressure rises earlier in proximity of the tip. The present study is the first investigation of parenchymal pressure detection in liver undergoing LA: the successful detection of intra-tissue pressure may be a key asset for improving LA, as pressure levels have been correlated to scattered recurrences of tumors by different studies.

Miniature Optical fiber combined pressure- and temperature sensor for medical applications

An Optical fiber Pressure Sensor (OFPS also referred to as FOPS) is combined with an Optical fiber Bragg Grating (FBG) to create a pressure and temperature sensitive sensor in a single optical fiber. This sensor is novel in its design and fabrication and it is using quartz glass only. The novel design allows access to in-vivo biomedical pressure measurements, which have previously been impossible to undertake due to size and compatibility constraints. Therefore, it can be used in several medical applications such as bladder or lung pressure measurement. Because of the small diameter (200 um), applications within blood vessels are also possible. The sensor is ideally suited to medical applications as it has demonstrated the required high accuracy (2 mmHg) and is miniature in size. It is also possible to use the sensor in harsh medical environments e.g. within Medical Resonance Imaging (MRI) scanners and in the vicinity of Linear Accelerators (LinAcs).

Differential in vivo urodynamic measurement in a single thin catheter based on two optical fiber pressure sensors

Abstract. Urodynamic analysis is the predominant method for evaluating dysfunctions in the lower urinary tract. The exam measures the pressure during the filling and voiding process of the bladder and is mainly interested in the contraction of the bladder muscles. The data arising out of these pressure measurements enables the urologist to arrive at a precise diagnosis and prescribe an adequate treatment. A technique based on two optical fiber pressure and temperature sensors with a resolution of better than 0.1  cm H2O (∼10  Pa), a stability better than 1  cm H2O/hour, and a diameter of 0.2 mm in a miniature catheter with a diameter of only 5 Fr (1.67 mm), was used. This technique was tested in vivo on four patients with a real-time urodynamic measurement system. The optical system presented showed a very good correlation to two commercially available medical reference sensors. Furthermore, the optical urodynamic system demonstrated a higher dynamic and better sensitivity to detect small obstructions than both pre-existing medical systems currently in use in the urodynamic field.

Fiber-Optic EFPI Pressure Sensors for In Vivo Urodynamic Analysis

A fiber optic sensing system for pressure measurement in invasive urodynamics is presented and experimentally validated. Probes are based on extrinsic Fabry-Perot interferometry (EFPI) principle, with an all-glass biocompatible design having 0.2-mm thickness. Pressure sensitivity of 1.0-1.6 nm/kPa with high stability and reduced thermal sensitivity has been achieved, leading to 0.3-cmH2O pressure accuracy. The EFPI probes have been embedded in medical catheters with 4F size for dual bladder and abdominal pressure measurement throughout the full course of a urodynamic analysis. Pressure traces have been compared with a PICO2000 urodynamic instrument, showing improved accuracy and higher responsivity to local pressure variations. Tests have been carried out in vivo on seven patients; the main highlights are reported, showing the premises for an EFPI-based diagnostic tool.

Femtosecond-Laser-Based Inscription Technique for Post-Fiber-Bragg Grating Inscription in an Extrinsic Fabry–Perot Interferometer Pressure Sensor

In this paper, a novel fiber Bragg grating inscription technique based on a femtosecond laser is presented. The grating was inscribed in close proximity to the tip of an extrinsic Fabry-Perot interferometer (EFPI)-based optical fiber pressure sensor. This therefore represents an optical fiber pressure and temperature sensor (OFPTS) for simultaneous pressure and temperature measurement for use in exactly the same physical location. The temperature measurement can also be used to compensate thermal drift in the EFPI sensor. The Bragg wavelength can be tailored precisely to any given wavelength in the optical spectrum and the degree of reflection can be adjusted to suit the FPI spectrum. The OFPTS has a diameter of 200 μm and is fully biocompatible. Furthermore, the sensor shows a high stability after grating inscription, of better than 0.5% in 20 min. The small size and high stability makes the sensor especially interesting for volume restricted areas, like blood vessels or the brain.

Novel FBG femtosecond laser inscription method for improved FPI sensors for medical applications

A novel fibre Bragg grating (FBG) post-inscription technique using a femto second laser (FSL), used to modify an optical fibre pressure sensor (OFPS) based on an extrinsic Fabry Perot Interferometer (EFPI) is presented. The resultant sensor is an optical fibre pressure and temperature sensor (OFPTS), able to measure temperature and pressure simultaneously in precisely the same location within the optical fibre. Hence the temperature measurement can be used to accurately compensate any thermal fluctuations in the pressure measurements, leading to an improved long term stability. The Bragg-wavelength can be tailored to coincide with any part of the Fabry-Perot Interferometer (FPI) spectrum (e.g. define the FBG at a valley of the FPI spectrum). We use a modified femtosecond laser, point-by-point inscription method for precise and controlled placement of the fibre Bragg grating. Our technique can be readily adapted to commercial production methods for optical fibre sensors as it greatly mitigates the alignment problems associated with femtosecond laser inscription of gratings in optical fibres. The sensor presented in this paper is entirely fabricated with quartz glass, which makes it fully bio-compatible and can be used for biomedical application. The sensors achieved a high sensitivity of 1.3 nm over kPa resulting in a resolution of ~ 1mmHg and a temperature sensitivity of ~ 10.7pm over K. After the inscription, the sensors still demonstrated a stability of better than 0.1% in 30min. The small diameter of only 200μm allows biomedical in-vivo application in volume restricted areas (e.g. blood vessels or the brain) for simultaneous temperature and pressure measurements.

Low drift and high resolution miniature optical fiber combined pressure- and temperature sensor for cardio-vascular and other medical applications

A fiber optic Extrinsic Fabry-Perot Interferometer (EFPI) including an internal fully integrated Fibre Bragg Grating (FBG) for application in cardiovascular pressure detection is presented. The combination of EFPI and FBG sensing elements allows the measurement of temperature and pressure at the same time, as well as the compensation of cross-effects. This sensor is entirely fabricated of quartz glass and is a result of a novel design and fabrication process. The sensor was tested in simulated in-vivo biomedical pressure devices, i.e. a cardiovascular pressure simulator that replicates human pressure patterns related to heartbeat. A resolution of 0.1mmHg and a drift of <; 1mmHg in over 1hour at a frequency of 50Hz was achieved. Tests were performed at the Polymer Core Center, within Cleveland Clinic (Ohio, USA). The small sensor overall diameter of 200μίη allows in-vivo application for temperature-compensated pressure detection of aortic, arterial and ventricular pressure patterns.