Thermal ablation, using radiofrequency, microwave, and laser sources, is a common treatment for hepatic tumors. Sensors allow monitoring, at the point of treatment, the evolution of thermal ablation procedures. We present optical fiber sensors that allow advanced capabilities for recording the biophysical phenomena occurring in the tissue in real time. Distributed or quasi-distributed thermal sensors allow recording temperature with spatial resolution ranging from 0.1 mm to 5 mm. In addition, a thermally insensitive pressure sensor allows recording pressure rise, supporting advanced treatment of encapsulated tumors. Our investigation is focused on two case studies: (1) radiofrequency ablation of hepatic tissue, performed on a phantom with a stem-shaped applicator; (2) laser ablation of a liver phantom, performed with a fiber laser. The main measurement results are discussed, comparing the technologies used for the investigation, and drawing the potential for using optical fiber sensors for "smart"-ablation.
Radiofrequency thermal ablation (RFTA) induces a high-temperature field in a biological tissue having steep spatial (up to 6°C/mm) and temporal (up to 1°C/s) gradients. Applied in cancer care, RFTA produces a localized heating, cytotoxic for tumor cells, and is able to treat tumors with sizes up to 3 to 5 cm in diameter. The online measurement of temperature distribution at the RFTA point of care has been previously carried out with miniature thermocouples and optical fiber sensors, which exhibit problems of size, alteration of RFTA pattern, hysteresis, and sensor density worse than 1 sensor/cm. In this work, we apply a distributed temperature sensor (DTS) with a submillimeter spatial resolution for the monitoring of RFTA in porcine liver tissue. The DTS demodulates the chaotic Rayleigh backscattering pattern with an interferometric setup to obtain the real-time temperature distribution. A measurement chamber has been set up with the fiber crossing the tissue along different diameters. Several experiments have been carried out measuring the space-time evolution of temperature during RFTA. The present work showcases the temperature monitoring in RFTA with an unprecedented spatial resolution and is exportable to in vivo measurement; the acquired data can be particularly useful for the validation of RFTA computational models.
We present a miniature and biocompatible fiber-optic sensing system, for specific application in monitoring of the
radiofrequency thermal ablation (RFA) process. The sensing system is based on combination of Extrinsic Fabry-Perot
Interferometry (EFPI) sensor for pressure detection, and Fiber Bragg Grating (FBG) for temperature measurement. The
dual pressure/temperature measurement shows an extremely low cross-sensitivity. Measurements have been performed
ex-vivo on porcine liver, recording several RFA procedures in different location. Maximum values of 164°C and 162 kPa
have been recorded on the ablation point.
The incoming restoration works of Duomo di Milano main spire requires a continuous structural health monitoring of the
cupola supporting it. For reasons mainly connected to the lightning hazard, fiber optic sensors have been selected, based
on FBG technology. Strain of the lower part of the vaulting-rigs inside the octagonal cupola is the measurement of
interest. Being the expected signals very small and the thermal disturbances very important, a thermal characterization of
two types of commercial strain gauges was carried out in laboratory with a thermal chamber and a block of the same
marble used for the Duomo construction. This allowed to find a relationship later used to compensate any thermal
effects, leading to the extraction of the mechanical load contribution only. An uncertainty analysis gave a result of 5 to
10 μm/m in the tested temperature range -5 °C to +40 °C. The future work will expand the monitoring system to more
measurement points and it is expected this can provide an important diagnostic tool during restoration operations.
The use of FBG based sensors for the monitoring of the pantograph-catenary interaction is very attractive due to the
insensitivity of fiber optic sensors to the electromagnetic disturbances and due to their ability to be electrically insulated.
In fact, the monitoring of pantograph-catenary interaction with traditional sensors needs a complicated set-up to
electrically insulate sensors, to power the signal conditioning devices and to transmit the signal to the data acquisition
system, and to avoid interferences between the measurement signals and electromagnetic disturbances typically
generated by continuous sparking and eventual arcing phenomena caused by contact loss between the pantograph
collector and the contact wire of overhead line. In this work the application of a commercial FBG accelerometer on a
pantograph of an underground train, instrumented for experimental in-line tests, is analyzed. In particular, a comparison
between a traditional capacitive accelerometer and a FBG accelerometer is presented to highlight the proper working of
the fiber optic sensor during in-line tests and to take the use of this kind of fiber optic sensor into consideration for
monitoring aim in the pantograph-catenary interaction, simplifying the measurement set-up. The first results show that
this approach is promising.