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.
The paper presents our recent results towards the development of a miniaturized all-fiber probe for laser induced thermal ablation of tumor cells, which combines the optimal delivery of a near-infrared high power ablating beam, a low power visible aiming beam and fast Bragg grating (FBG) temperature sensors. Specific combiner and probe end-cap based on dual cladding fibers have been developed to allow the simultaneous handling of the laser beams and of the signal that feeds the temperature sensor. Moreover, a very fast FBG interrogation system has been implemented to track abrupt temperature variations during medical treatment.
Thulium-doped fiber lasers are gaining in popularity since they emit at about 2 μm, a wavelength particularly interesting
for many industrial, sensing and medical applications, and, moreover, in the so-called “eye-safe” spectral region. Despite
the many advantages, however, thulium-doped fiber lasers with power high enough to allow practical applications have
still limited deployment mainly due the high cost per emitted watt. The paper investigates alternative paths to high power
CW emission at about 2 μm by exploring two complementary approaches: the development of specific pump combiners
and the study of new pumping schemes that take advantage of co-doped fibers. The developed pump combiners are
based on fused fiber technology and are characterized either by the use of “non-standard” fiber dimensions to allow
pumping through an ytterbium-doped fiber laser or by a large number of input ports (up to 39) to provide adequate levels
of pump power through the efficient coupling of several fiber pigtailed diodes with emission wavelength suitable for
pumping thulium. On the other hand, a co-doped ytterbium-thulium fiber is also studied to analyze the possibility of
using ytterbium ions as pump source for thulium ions. The use of ytterbium, either as co-dopant or as laser source, is
particularly interesting because it allows taking advantage of the remarkable advancements made in the pump diodes for
such a laser system, and specifically of the favorable cost per emitted watt. Preliminary experimental results have
demonstrated the feasibility of the proposed approaches and have shown that the joint use of the “ad-hoc” pump
combiners and of the ytterbium-thulium co-doping can lead to the development of lasers with power suitable for
industrial applications, although the efficiency needs further improvements.
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