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.

A fibre Bragg grating (FBG) array-based sensing probe has been installed on a device for radiofrequency thermal ablation (RFTA). The probe is made of five FBGs with 0.5 cm active area and 1 cm spacing, to provide quasi-distributed thermal pattern measurements. Multiple experiments have been conducted on porcine liver, reporting the temperature pattern along the ablation axis. Thermal maps allow an estimation of the RFTA efficiency and spatial extension in the liver.Introduction: Radiofrequency thermal ablation (RFTA) is a medical procedure for post-screening treatment of lung, hepatic and kidney tumours via hyperthermia [1]. The concept of RFTA is to exploit RF irradiance to generate a well-confined high-temperature region at the focal point of the tumour. Heat is generated by RF irradiance from a thin RFTA needle (1-4 mm, percutaneous) to the target and then propagated by conduction in a symmetrical way. The mortality rate of tumour cells is a function of temperature value and its persistence: temperatures higher than 42°C are cytotoxic [2], whereas 52°C for 1 min is a typical reference value; over 60°C cell death is nearly instantaneous. As the real-time thermal profile in the ablated region plays a fundamental role in the tumour cell mortality, the quasi-distributed measurement of temperature in the tissue throughout the ablation is a key asset. Previous literature shows thermal measurements with a thermocouple [3], which, however, alters both the RF irradiance and the tissue, and with an infrared thermal camera [4], which, however, requires a line of sight with the ablation region, typically unavailable in RFTA.In this Letter, we report the thermal measurement of the RFTA pattern in porcine liver, based on a fibre Bragg grating (FBG) array [5,6]. The FBG array has been directly mounted on the ablation device, providing real-time temperature measurement at 5 points (1 sensor/cm).

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.

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.

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