Temperature monitoring during Laser Ablation by FBG sensors encapsulated within a metallic needle: Experiments on healthy swine tissue

Monitoring of local temperature in tissue undergoing Laser Ablation (LA) could be particularly beneficial to optimize treatment outcomes. A number of both invasive and non-invasive thermometric techniques may be employed to perform this task. Among others, Fiber Bragg Grating (FBG) sensors show the following valuable characteristics for temperature monitoring during LA: good sensitivity and accuracy, and immunity from electromagnetic interferences. The main drawbacks are their intrinsic invasiveness and the sensitivity to strain, which can entail measurement error for respiratory and patient movements. The aim of this work is to experimental assess the characteristics of an FBG sensor, housed within a metallic needle, employed in temperature monitoring of tissue undergoing LA. The use of a metallic needle allows neglecting errors due to patient movements, but induces an increase in sensor response time and a temperature overestimation due to direct absorption of laser light by the needle. The proposed sensor is tested during LA of ex vivo swine livers, and the tissue temperature measured by the FBG housed within the needle is compared to the temperature measured by an FBG without needle. This comparison showed that the needle induces a temperature overestimation, strongly dependent on the distance between sensor and laser applicator (e.g., about 2 °C at 6 mm, 4.4 °C at 4 mm). Furthermore, the needle causes an increase of response time (about 140 ms vs 40 ms). Since this response time is sufficient for the particular application and the overestimation can be reduced by using different techniques of data processing, the use of a needle to protect FBG seems to be a feasible solution to overcome the concern related to patient movements.