Dinesh Babu Duraibabu

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.

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.

Underwater depth and temperature sensing based on fiber optic technology for marine and fresh water applications

Oceanic conditions play an important role in determining the effects of climate change and these effects can be monitored through the changes in the physical properties of sea water. In fact, Oceanographers use various probes for measuring the properties within the water column. CTDs (Conductivity, Temperature and Depth) provide profiles of physical and chemical parameters of the water column. A CTD device consists of Conductivity (C), Temperature (T) and Depth (D) probes to monitor the water column changes with respect to relative depth. An optical fibre-based point sensor used as a combined pressure (depth) and temperature sensor and the sensor system are described. Measurements accruing from underwater trials of a miniature sensor for pressure (depth) and temperature in the ocean and in fresh water are reported. The sensor exhibits excellent stability and its performance is shown to be comparable with the Sea-Bird Scientific commercial sensor: SBE9Plus.

An Optical Fibre Depth (Pressure) Sensor for Remote Operated Vehicles in Underwater Applications

A miniature sensor for accurate measurement of pressure (depth) with temperature compensation in the ocean environment is described. The sensor is based on an optical fibre Extrinsic Fabry-Perot interferometer (EFPI) combined with a Fibre Bragg Grating (FBG). The EFPI provides pressure measurements while the Fibre Bragg Grating (FBG) provides temperature measurements. The sensor is mechanically robust, corrosion-resistant and suitable for use in underwater applications. The combined pressure and temperature sensor system was mounted on-board a mini remotely operated underwater vehicle (ROV) in order to monitor the pressure changes at various depths. The reflected optical spectrum from the sensor was monitored online and a pressure or temperature change caused a corresponding observable shift in the received optical spectrum. The sensor exhibited excellent stability when measured over a 2 h period underwater and its performance is compared with a commercially available reference sensor also mounted on the ROV. The measurements illustrates that the EFPI/FBG sensor is more accurate for depth measurements (depth of ~0.020 m).

Fabry-Pérot based refractive index optical fiber sensor for measurement of oxygen concentration levels in hypoxic tumors during radiotherapy treatment

An optical fiber sensor based on Fabry-Pérot interferometer to measure the refractive index changes due to oxygenation level changes in hypoxic tumors for radiotherapy treatments is proposed. The sensors have an outer diameter of 220μm with a 20–30μm length air-cavity and a 30μm thickness end cap located at the tip of the sensor. The sensors are used to measure the phase change in the received optical spectrum when there is a change in refractive index using a Fast Fourier Transform based analysis method. The refractive index change is measured in order to determine the oxygenation level in hypoxic tumors. In this paper, different concentrations of iso-propanol solution are prepared to produce refractive index values between 1.3438 and 1.3655 in order to mimic the refractive index of hemoglobin. The sensors are coated with a 100 nm thick gold layer and a comparison is made with non-coated sensors. The coated sensors have a resolution in order of 10−3 RIU.

Novel miniature pressure and temperature optical fibre sensor based on an extrinsic Fabry-Perot Interferometer (EFPI) and Fibre Bragg Gratings (FBG) for the Ocean environment

A novel miniature sensor is proposed for accurate measurement of pressure (depth) and temperature changes in the ocean environment. The sensor is based on an optical fibre extrinsic Fabry Perot interferometer (EFPI) combined with a Fibre Bragg Grating (FBG). The EFPI provides pressure measurements while the Fibre Bragg Grating (FBG) provides temperature measurements. The FGB is post-inscribed into the EFPI using a femtosecond laser. The sensor is mechanically robust, corrosion resistant and suitable for use underwater. The combined pressure and temperature sensor system was mounted on-board a mini remotely operated underwater vehicle (ROV) in order to monitor the pressure changes at varying depths and compare with the reference pressure-depth sensor. The reflected optical spectrum of the sensor was monitored online and a pressure change caused a corresponding observable shift in the optical spectrum. The sensor exhibited excellent stability when measured over a 2 hour period underwater and its performance is compared with a commercially available reference sensor also mounted on the ROV. The comparison illustrates that the EFPI/FBG sensor is more accurate (~0.025m) with a resolution of ~0.005m, when compared to the reference sensor.

Intra-tissue pressure measurement during laser ablation with fiber-optic extrinsic Fabry-Perot sensor

We report the real-time measurement of intra-tissue parenchymal pressure during laser ablation (LA). Pressure detection is performed with an extrinsic Fabry-Perot interferometry (EFPI) probe, fabricated with a biocompatible all-glass design. Experiments have been performed with solid-state laser ablator, recording a peak pressure of 71.6 kPa. The pressure measurement methodology may allow fine control of LA procedure.

Fiber Bragg grating sensors for spatially resolved measurements in ex-vivo pancreatic laser ablation

Laser ablation (LA) is an emerging technology for mini-invasive treatment of tumors, and operates by damaging cancer cells by means of focused light-induced selective heating. In-situ temperature monitoring, with micro-sensors, may be particularly beneficial for both estimating in real-time the LA efficacy, and build prediction and controls for LA procedures. In this paper, we present our latest results on spatially resolved temperature monitoring, based on fiber Bragg grating (FBG). We used in-line FBG arrays based on 6 and 35 FBGs to achieve spatially resolved thermal sensing in LA applied to porcine pancreas. The main experimental results and their implications are hereby discussed.

Fibre optic sensors for temperature and pressure monitoring in laser ablation: experiments on ex-vivo animal model

Optical fibre sensors have been applied to perform biophysical measurement in ex-vivo laser ablation (LA), on pancreas animal phantom. Experiments have been performed using Fibre Bragg Grating (FBG) arrays for spatially resolved temperature detection, and an all-glass Extrinsic Fabry-Perot Interferometer (EFPI) for pressure measurement. Results using a Nd:YAG laser source as ablation device, are presented and discussed.