Optical fibre sensors for distributed temperature monitoring during mini-invasive tumour treatments with laser ablation.

The advent of mini-invasive surgical techniques to treat tumours has increased the number of successful recoveries, while minimizing the patient discomfort. These techniques usually rely on the generation of a localized thermal gradient, either heating (High Intensity Focused Ultrasound, Radiofrequency Ablation, Microwave Ablation, and Laser Ablation), or cooling (Cryoablation) the tissue at cytotoxic levels. The aim of this Master's Thesis has been the study of novel optical fibre temperature sensors for the real-time monitoring of percutaneous Laser Ablation (LA) treatments. Fibre Optic sensors are particularly attractive for LA because they can be integrated into the applicator, do not introduce artefact due to interactions with the laser beam; moreover, being all non-conductive, they cannot cause electrocution and can be used under the Nuclear Magnetic Resonance procedures employed to position the applicator and monitor the treated tissue evolution. Two types of sensors suitable multi-point measurements along a single fibre have been investigated and compared: 1) Arrays of multiplexed Fibre Bragg Gratings (FBGs) for quasi-distributed sensing; 2) Optical Frequency Domain Reflectometry (OFDR) in Single-Mode Fibres (SMFs) for fully distributed sensing. FBGs are optical devices with notch filter response, so that they propagate all the wavelengths except for some centred at the so-called Bragg wavelength, which are reflected toward the source because of the backscattering phenomena. The Bragg wavelength shifts linearly with temperature, allowing estimating temperature variations from spectral response changes. Multi-point sensing to recover the temperature distribution in the treated region can be obtained by cascading FBGs with different Bragg wavelengths. For this work, arrays up to 15 FBGs with 1 mm spacing have been written by femto-second laser in single-mode telecom-grade fibres. FBG arrays can be interrogated with tunable lasers obtaining 0.1 °C resolution with a ~2 mm spatial resolution. SMFs can also be used as a truly distributed temperature sensor by measuring the Rayleigh scattering, which is dependent on the local temperature, as a function of the position. The Rayleigh scattering can be measured with an advanced instrument called optical backscatter reflectometer, which allows evaluating the temperature with sub-millimetre spatial resolution. Both types of sensors have been first characterized and calibrated in a climatic chamber. Then, in order to investigate their distributed sensing capabilities, a specific setup using a metallic cantilever devised to generate linear thermal gradients has been built. Both the FBG array and the OFDR have allowed recovering a temperature distribution profile in agreement with a reference measure obtained from thermocouples. Finally, both optical fibre sensors have been used to monitor a laser ablation test on ex-vivo bovine liver. Again, both recovered temperature profiles have been found in agreement with reference measures obtained with a thermographic camera, demonstrating that both sensors represent promising tools for monitoring of the temperature during LA treatments.