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During thermal procedures, the monitoring of tissue temperature is useful to improve therapy success. The aim of this study is the feasibility assessment of a Fiber Bragg Grating (FBG)-based probe, which contains six FBGs, to obtain distributed temperature measurement in tissue undergoing laser ablation (LA). Among different thermometric techniques, FBG sensors show valuable characteristics, even though their sensitivity to strain entails measurement error for patient respiratory movement. We performed: i) the static calibration of the FBG-based probe to estimate the thermal sensitivity of the six FBGs; ii) the estimation of the response time of the FBGs. All FBGs have a thermal sensitivity of 10 pm·°C-1 and a time constant in the order of < 250 ms. Additionally, we performed a preliminary estimation of the error due to the strain and caused by respiratory movements. Experiments were carried out by simulating a typical respiratory movement on ex vivo swine liver. The measurement error was

Temperature monitoring in tissue undergone Laser Ablation (LA) may be particularly beneficial to optimize treatment outcome. Among many techniques, fiber Bragg grating (FBG) sensors show valuable characteristics for temperature monitoring in this medical scenario: good sensitivity and accuracy, and immunity from electromagnetic interferences. Their main drawback is the sensitivity to strain, which can entail measurement error for respiratory and patient movements. The aims of this work are the design, the manufacturing and the characterization of a needle-like probe which houses 4 FBGs. Three FBGs have sensitive length of 1 mm and are used as temperature sensors; one FBG with length of 10 mm is used as reference and to sense eventual strain. The optical fiber housing the FBGs was encapsulated within a needle routinely used in clinical practice to perform MRI-guided biopsy. Two materials were used for the encapsulation: i) thermal paste for the 3 FBGs used for temperature monitoring, to maximize the thermal exchange with the needle; ii) epoxy resin for the reference FBG, to improve its sensitivity to strain. The static calibration of the needle-like probe was performed to estimate the thermal sensitivity of each FBG; the step response was investigated to estimate the response time. FBGs 1 mm long have thermal sensitivity of 0.01 nm·°C(-1), whereas the reference FBG presents 0.02 nm·°C(-1). For all FBGs, the response time was in the order of 100 ms. Lastly, experiments were performed on ex vivo swine liver undergoing LA to i) evaluate the possible presence of measurement artifact, due to the direct absorption of laser light by the needle and ii) assess the feasibility of the probe in a quasi clinical scenario.

Laser ablation (LA) is a minimally invasive procedure used to remove cancer by inducing hyperthermia. It is based on the interaction between laser light and tissue: the absorbed light is converted into heat causing a tissue temperature increase. The amount of damaged volume depends on temperature and time exposure of the tissue to the hyperthermia. As a consequence, the monitoring of tissue temperature during LA could be particularly beneficial to optimize treatment outcomes. Thermocouples are one of the most employed transducer for temperature measurement. Their main drawback is related to the strong light absorption of the two metallic wires which constitute a thermocouple. The light absorption causes an overestimation of actual temperature, in literature known as artifact. This work aims at assessing this artifact on ex vivo swine pancreases undergoing LA. The artifacts have been estimated at the three laser powers (1.6 W, 2 W and 5 W) and at two distances from the optical applicator. In particular, the artifact decreases with the distance from the optical applicator and depends on P: at 1.6 W and 2 W it is negligible at 12 mm of distance, on the other hand at 5 W it is significant also at 15 mm (1.7 °C). Summing up: the artifact is strongly influenced by the distance between the thermocouple and the optical applicator, and by the laser power; also at high distance from the applicator it can cause error which are not acceptable for the application of interest (e.g., at 5 W and 10 mm the error is about 4 °C). Although the use of thermocouples entails the concern related to the artifact, it must be considered that proper model can be employed to correct the measurement error. © 2015 IEEE.