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Application of fiber optic temperature measurement and sensor system in minimally invasive medical field

In recent decades, minimally invasive hyperthermia (i.e. radiofrequency ablation, laser ablation, microwave ablation, high-intensity focused ultrasound ablation, and cryoablation) has been widely recognized in the field of tumor resection. These techniques induce local temperature increase or decrease to remove tumors while maintaining the integrity of surrounding healthy tissues. Accurately measuring tissue temperature may be particularly beneficial for improving treatment outcomes, as it can serve as a clear endpoint for achieving complete tumor ablation and minimizing recurrence. Among the several temperature measurement technologies used in this field, fiber optic sensors (FOS) have several attractive characteristics: the high flexibility and small size of sensors and cables allow for the insertion of FOS into deep tissues; For this application, fiber Bragg gratings and frequency response (hundreds of kHz) are sufficient; Immunity to electromagnetic interference allows the use of FOS during thermal programs guided by magnetic resonance or computed tomography. In this review, the current status of the most commonly used FOS for temperature monitoring in thermal processes (such as fiber Bragg grating sensors; fluorescence sensors) is introduced, with a focus on their working principles and metrological characteristics. Include the basic physical principles of common ablation techniques to explain the advantages of using FOS in these programs.
Minimally invasive techniques have been widely recognized as an alternative to traditional surgery for cancer treatment and for treating patients who are not suitable for surgery. A special family of minimally invasive techniques is represented through a thermal ablation program, which triggers local temperature increment (laser ablation (LA), radiofrequency ablation (RFA), high-intensity focused ultrasound (HIFU), and microwave ablation (MWA)) or reduction (cryoablation) to kill the entire tumor while protecting surrounding healthy tissues. Their main advantage over traditional surgery lies in the possibility of ablation surgery through percutaneous, endoscopic, or extracorporeal guidance, thereby minimizing physical trauma to patients, avoiding adverse complications, reducing the need for general anesthesia, and treating patients who cannot undergo manual surgery. These elements may reduce the recovery time of patients, thereby lowering hospital costs.

Temperature monitoring is considered particularly beneficial for regulating the energy delivery during treatment. It has been shown that temperature can also serve as a clear endpoint to achieve complete tumor ablation and minimize recurrence. In addition, the effectiveness of high-temperature treatment planning tools in treatment management can be enhanced by accurately measuring feedback on tissue temperature. In recent decades, several temperature measurement techniques have been proposed to guide ablation based treatments in research, and recently in clinical settings. These methods can be divided into invasive (contact based) and non-invasive (non-contact based). In the case of non-invasive temperature measurement, temperature changes are measured without contact between the device and the internal body, and are inferred from images of temperature dependent tissue characteristics; The most famous methods are based on magnetic resonance imaging (MR), computed tomography (CT), ultrasound (US) imaging, and transverse wave elastography. Although there are obvious advantages associated with a lack of contact and the possibility of obtaining 3D temperature maps, image-based temperature measurement methods are not yet mature enough to be used as clinical tools for monitoring all thermal programs. In fact, MR thermometry is considered the current clinical gold standard in non-invasive thermometry, requiring specially designed sequences whose thermal sensitivity depends on the type of tissue, unless proton resonance frequency shift technology is used. In addition, MR scanners can only be operated with MR compatible devices; The CT thermometry method uses ionizing radiation (X-rays), so the first concern is related to the patient’s dose. In addition, its thermal sensitivity is tissue dependent and only preliminary studies have been conducted on its feasibility assessment in vivo; It looks very promising, but only within a temperature range of up to about 50 ° C; In addition, when the temperature approaches 60 ° C, using specific methods (such as temperature measurement based on changes in sound velocity with temperature) may result in poor accuracy of this technique, and thermal sensitivity depends on the properties of the tissue.

Invasive methods require sensors to be inserted into the target tissue, but more cost-effective imaging systems, and in some commercially available models, sensors are embedded in the energy delivery probes to minimize program invasiveness.

At present, the most commonly used sensors are thermocouples and fiber optic sensors (FOS). A thermocouple composed of two metal wires is inexpensive, quite precise (~1 ° C), and has a relatively short response time (which largely depends on the probe diameter and may be much shorter than 1 second). On the other hand, there are two main reasons that can lead to measurement errors: (i) the direct absorption of light through the metal wire when passing through LA, HIFU may cause significant temperature overestimation during ultrasound treatment, and the high thermal conductivity of the metal wire may also lead to overestimation of temperature (cryoablation) or underestimation (for high-temperature therapy). In addition, metal wires may cause significant image artifacts in CT or MR guided thermal programs.

In specific configurations, fiber optic technology allows for overcoming these obstacles: due to their construction (glass or polymer), FOS is not easily overvalued due to light absorption, and has low thermal conductivity (silicon glass is an excellent insulation). In addition, MR compatible FOS can be used in CT and MR guided hot programs. These characteristics make FOS technology particularly attractive for temperature monitoring during heat treatment processes.

There are several types of FOS, which are based on different working principles and are usually divided into two inherent categories, with optical fibers forming sensing elements; Externally, fiber optic is only used as a medium for transmitting light to and from individual components or spaces. Among a large number of FOS, only two are widely used for temperature measurement during heat treatment processes, namely fiber Bragg grating sensors (FBG) and fluorescence sensors. In addition to the valuable features listed, FBG can also perform distributed, quasi distributed, and multi-point measurements, allowing for temperature measurement at different points in the organization by inserting a single small-sized element (such as an optical element). Fiber with an outer diameter of several hundred micrometers.

This article reviews the latest technologies of FOS (especially FBG and fluorescence sensors) used for temperature monitoring in heat treatment. Throughout the article, important descriptions of the main advantages and disadvantages of these two sensors are provided, while considering different heat treatments. For clarity, the product is divided into two main parts: the first part describes the basic physical principles of the most commonly used thermal procedures and the importance of temperature monitoring during these processing processes; In the second part, the measurement principles, advantages and disadvantages of FBG and fluorescence sensors, as well as their applications in the field of interest, are described. Fiber optic sensors used for temperature monitoring in heat treatment processes have key advantages in performance, size (sensing area and wiring), and integration possibilities compared to their electronic counterparts (such as microelectromechanical systems (MEMS)) in terms of working principles and metrological characteristics. The fluorescence based temperature measurement method was first commercialized in 1978; Fluorescent optical systems have always supported thermal measurement in hyperthermia, especially in the past decade. Recently, new developments in FBG sensors, particularly the manufacturing method based on wire drawing towers, have reduced the cost and spatial resolution of FBG sensors to 0.5 to 2 sensors/cm within the same optical fiber. Emerging technologies allow for “ultra dense” sensing, reducing spatial resolution to below millimeters: two noteworthy examples are fiber Bragg gratings, which extend the FBG principle, and distributed sensing systems based on scanning wavelength interferometry for Rayleigh scattering analysis.

Fluorescence based sensor operating guidelines.

The operating principle of fluorescence based sensors, combined with optical fibers, is based on the measurement of fluorescence lifetime. There was no research on fluorescence based temperature measurement in the 1990s, during which the principle of fluorescence attenuation in phosphor materials was applied in optical fibers.

External fluorescence examination is based on the measurement of fluorescence decay time, which is induced by fluorescent materials such as ruby, emerald, th, or several rare earth materials. A typical temperature measurement system based on external fluorescence is proposed. Using a square wave pattern internally modulated and coupled with a light source inside a standard fiber to excite phosphors; The probe is a doped region on the tip of a Cr 3+sapphire fiber, spliced into a quartz fiber and encapsulated in an alumina sheath. High speed photodetectors are used to record the decay time of fluorescent materials. Usually, temperature values are extracted from the sensor output through two steps: exciting the sensing element with light pulses; After this stimulus, the fluorescence signal decays exponentially. The time constant of the exponential trend depends on temperature, so it can be considered as an indirect measurement of temperature. Due to exponential decay being limited to a few μ s. Fluorescent lens sensors typically have fast responses.

In addition, most rare earth materials are compatible with operations from room temperature to over 200 ° C, as well as operations below -40 ° C. Materials; The operating temperature range of the system is -100-290 ° C, with an accuracy of 0.1 ° C. The attractive features of fluorescence based systems include detection speed, accuracy, and the possibility of using fiber optic probes as disposable units, and therefore several patents have been developed for combining one or more fiber optic temperature sensors in thermal ablation.
Working principle of fiber Bragg grating (FBG).

Fiber Bragg grating (FBG) sensors are the most commonly used method in modern fiber optic sensing. FBG is a wavelength selective notch filter that can reflect narrow spectra near a single peak wavelength; When temperature changes are applied to FBG structures, the FBG spectrum shifts with a nearly perfect constant sensitivity. Therefore, the wavelength corresponding to the maximum value of the spectral intensity of the reflection is called the Bragg wavelength( λ B) can be used to estimate temperature. Due to the narrow spectral reflection of FBG and its transparency to all other wavelengths, multiple FBG arrays can be deployed on the same fiber, each with a different central wavelength, thus utilizing wavelength division multiplexing (WDM). In this configuration, FBG based systems have gained a new dimension of biomedical sensing, as they allow multiple micro sensors to be hosted on the same optical fiber, maximizing sensing capabilities. The cost of FBG sensors is approximately $35 or lower. However, the system used to interrogate sensors is more expensive.

FBG has a constant sensitivity within the measurement range required for thermal ablation (i.e. 30-100 ° C), with a typical value of~10 pm · ° C-1. Engraved on the same fiber optic with 5 FBG sensors, suitable for RF ablation; Each FBG has an effective length of 0.5 cm, a sensing capacity of 1 FBG/cm, and a distance of 1.8 nm between each peak wavelength; This result corresponds to one of the latest examples of FBG sensing in thermal ablation. The response of five FBG arrays during heating and cooling processes is shown in Figure 4. The possibility of performing WDM and integrating multiple sensors into a single fiber with narrow density is a key advantage of FBG sensors compared to fluorescence based sensors. By using an interrogator to detect FBG spectra and applying post-processing, the temperature of each sensor can be retrieved with an accuracy of 0.1 ° C.
With many of the latest advancements in manufacturing technology, the technology behind FBG sensors is rapidly evolving. The most noteworthy aspect is that the consolidation of FBG array’s wire drawing tower manufacturing is based on industrialized so-called wire drawing tower gratings (DTGs) that expose fibers to UV light through phase masks, providing significant metrological advantages over traditional FBG manufacturing techniques. DTG can be manufactured through precise positioning: corresponding one-to-one to the Bragg wavelength of each sensor composing the array, and along the geometric position of the optical fiber; This is essential in hyperthermia to provide reliable temperature pattern reconstruction. Due to the fact that the DTG manufacturing process does not require peeling and repainting of fiber buffer layers, maintaining the original strength and thickness, the mechanical strength also increases. In addition, DTG is typically manufactured on bending insensitive fibers. Currently, commercial DTG arrays achieve a density of 1 FBG/cm on a single fiber.

A new technology for FBG manufacturing has recently been established, which uses femtosecond laser for point by point recording. This technology has the potential to improve sensing capabilities, as it can soon manufacture highly reflective FBGs with a length of<1mm for dense array packaging. Main applications. During LA, RFA, and recently during MWA and cryoablation, FBG is mainly used to monitor tissue temperature.

FBG work guidelines. FBG manifests as a continuous FBG chain, with each FBG having a different peak wavelength. The most interesting configuration is linear FBG (LCFBG), where the Bragg wavelength varies linearly in space.

The manufacturing length of FBG is 1.5 centimeters to 5 centimeters, and the bandwidth range is 5 nanometers to 50 nanometers. From the perspective of metrology, LCFBG manifests as a series of sensors; Its spectrum comes from the entire temperature mode of all sensors. The application of LCFBG in spatial resolution temperature measurement is still in a relatively early stage. By using LCFBG instead of standard FBG arrays, the spatial resolution has decreased to much less than 1mm, and is mainly limited by the decoding system’s ability to analyze temperature patterns from the LCFBG spectrum. The backward reflection spectrum from FBG can be recorded by the same interrogator used for uniform FBG, but customized software can be developed to decode the signal and estimate temperature, as there is currently no commercially available software available.

Main applications. FBG is becoming increasingly popular in the field of tissue temperature monitoring in recent thermal programs, especially RFA. Display a spatial resolution of 75 microns on a length of 1.5 centimeters. However, decoding techniques are mainly used for monotonic temperature modes. The current research aims to develop fast decoding algorithms for non monotonic temperature modes, such as those commonly obtained in thermal ablation.

The working principle of Rayleigh scattering distributed sensing.

Distributed Temperature Sensing (DTS) adopts a different approach from previous technologies, as it uses standard optical fibers as sensors; Decoding is a process performed in the time or frequency domain by measuring the backscatter Rayleigh pattern,. At present, the DTS gold standard instrument used for dense spatial resolution thermal measurement is based on the operating principle of scanning wavelength interferometry. This DTS system is capable of recording the Rayleigh backscattering characteristics of the source sensing fiber and analyzing them with sub millimeter spatial accuracy. These sensors were developed using standard single-mode optical fibers (at negligible cost), but they require expensive interrogators to analyze and record signals.

Performance depends on a close trade-off between spatial resolution, accuracy, effective length, and sampling time. Achieved a spatial resolution of 200 microns and an accuracy of approximately 0.5 ° C, with a measurement rate of 1 Hz. Due to the use of standard optical fibers in the system, there is no need to manufacture any structure, so low-cost disposable probes can be developed; On the other hand, the cost of the interrogator is at least one order of magnitude higher than other fiber optic sensing systems and ablation devices. Rayleigh scattering distributed sensing systems have been adopted in medical scenarios, although they are promising solutions for distributed temperature or thermal gradient measurement.

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