Method for broadband in-SITU dielectric spectroscopy using microwave thermal ablation applicators and apparatus thereof

Abstract

The present invention relates to a method (100) for dielectric spectroscopy of a material or a tissue under test, comprising the following steps: measuring (110) the reflection coefficient (11) signal as a function of the frequency of said material or tissue under test, by a vector network analyzer (3), wherein a single applicator (5) is in contact with said material or a tissue under test and wherein the applicator (5) is operatively connected to the vector network analyzer (3); multiplying (120) the reflection coefficient (11) signal with the frequency behavior of an ultra-wide band (UWB) signal as a synthetic applicator excitation; converting (130) the signal of the previous step into the time domain by an Inverse Fourier Transform (IFT); suppressing (140) part of the converted signal of the converting step (130), according to the applicator (5) geometry; converting back (150) the signal of the suppressing step (140) to the frequency domain; and extracting (160) from the frequency signal converted the dielectric properties (162) and / or estimating (170) the dimension of the material or a tissue under test and / or of an ablated area of the material or a tissue under test thereof. The present invention also relates to an apparatus for dielectric spectroscopy (1).

Description

[0001]METHOD FOR BROADBAND IN-SITU DIELECTRIC SPECTROSCOPY USING MICROWAVE THERMAL ABLATION APPLICATORS AND APPARATUS THEREOF ***** The present invention relates to a method for broadband in-situ dielectric spectroscopy and tissue dimension evaluation, using microwave thermal ablation applicators, and the apparatus thereof. Field of the invention More specifically, the invention concerns a method and apparatus for an in- situ dielectric spectroscopy using microwave thermal ablation applicators used for treating local diseases of tissues. In the following the description will be addressed to applicators for the treatment of tumors, but it is clear that the same should not be considered limited to this specific use. Prior art Dielectric spectroscopy using microwave thermal ablation (MTA) applicators is a technique that measures the dielectric properties of materials and / or biological tissues by observing their response to electromagnetic waves, specifically microwaves. An MTA applicator is designed to deliver controlled microwave energy to targeted tissues, often for therapeutic purposes like tumor ablation or the like. During this process, the applicator can be also used to perform dielectric spectroscopy, which concerns analyzing the way tissues interact with the microwave signal injected. Specifically, by monitoring how the electromagnetic waves are absorbed or reflected by the tissue, dielectric spectroscopy can provide information about the dielectric properties, such as permittivity and conductivity. These properties disclose the electromagnetic properties of the material that can be an indication of the state of the tissue. This information allows for understanding the extent of tissue affected during ablation and the changes occurring in real-time due to the thermal treatment. Thus, this approach allows an accurate mapping of tissue changes and is beneficial for both diagnostics and treatment monitoring. Dielectric spectroscopy using microwave thermal ablation (MTA) applicators has been already the object of several studies. For instance, a method using an interstitial dipole antenna at 2.45 GHz has been investigated. Such a methodology relied on a rational function model to de- embed dielectric properties, which were fitted to a 2-pole Cole-Cole model. However, such analysis requires extensive pre-computation and solely examined the antenna’s reflection coefficient in the frequency domain. Moreover, this method yielded a 70% variance compared to reference open probe measurements and provided only an approximate assessment of the adjacent tissue volume. Similarly, it has been developed in the field an open-ended coaxial slot antenna at 5.8 GHz for both MTA and dielectric spectroscopy. Their de-embedding model, based on the Stuchly & Stuchly’s equivalent circuit of the open-ended coaxial probe, revealed high sensitivity. The results also highlighted that measurements could be affected by reflections if the material under test was not sufficiently large. Additionally, these analyses were only made in the frequency domain. The patent application WO2022271709A1 introduced a setup with multiple directional MTA applicators acting interchangeably as transmitters and receivers. The solution mapped changes in dielectric properties based on transmission coefficients during the ablation process but required specially designed applicators to avoid interference from healthy tissue. This approach involved several applicators which increased the invasiveness of the MTA procedure and relied exclusively on frequency-domain analysis. The patent application WO2018 / 140819A1 discloses an extra antenna attached to the MTA applicator's distal end. This added antenna cyclically measured power and impedance and compared them to initial procedure values, requiring a dedicated antenna and working within a bronchoscopic framework. Time-domain signal analysis in dielectric spectroscopy has been explored with a monopole antenna in a large tank. In this case it was attempted to mitigate noise through Inverse Fourier Transform (IFT) and windowing out late-time ripples from tank wall reflections. Despite the improvements, the approach remained focused on reflected signals and required large tanks, thus averaging dielectric properties over large volumes. Time-domain analysis has been used to estimate tumor dimensions in the patent application US2015005757A1, employing template signals to approximate the ablation size. Specifically, the application discloses the use of several pre- simulated signals in tumors as templates for tumor size estimation. As is known, in general, microwave thermal ablation (MTA) is a therapeutic technique that utilizes electromagnetic fields to treat malignant tissues. The antenna, when inserted into the center of a tumor, irradiates an electromagnetic field, which is absorbed by the surrounding tissue, causing a significant temperature increase to levels that result in immediate cell death. Presently, ultrasound (US) is used for intraoperative monitoring of MTA, but the high temperatures generate water vapor. Such water vapor interferes with the probe, making imaging not accurate. Post-procedural assessment of treatment success often requires computed tomography (CT) or magnetic resonance imaging (MRI), both of which are unsuitable for real-time monitoring due to high costs, electromagnetic interference, or health risks associated with ionizing radiation. Microwave imaging (MWI) systems have been explored as an alternative. An antenna array is placed externally over the treatment area. However, these systems can only approximate the ablated zone’s morphology, lacking the ability to provide enough resolution and penetration depth into the tissue. Since dielectric properties correlate directly with tissue type, condition, and temperature, the assessment of these properties via direct contact with the applicator would enable immediate insights into the tissue status and enhance the accuracy of MTA techniques. The prior art comprises also the article of Garrett JD, Fear EC. Average Dielectric Property Analysis of Complex Breast Tissue with Microwave Transmission Measurements; Sensors; 2015. The device herein disclosed measures the transmission coefficient (^^12) of the scattering matrix, with a complex measuring circuit. Also, the measure of transmission coefficient ^^12turns out to be inconvenient in algorithm terms, in achievable information, as well as in terms of circuital complexity. Scope of the invention In light of the above, it is, therefore, a purpose of the present invention to overcome the technical limitations of the prior art and, particularly, to measure dielectric properties in close proximity to the applicator, specifically an MTA applicator, within dimensions appropriate for microwave thermal ablation, ensuring accurate data collection. It is also the purpose of this invention to use the knowledge of dielectric properties thus gained to identify the tissue, e.g., tumor and ablated zone, as well to use them for antenna guidance into the tumor as a stand-alone feature or as an addition to ultrasound imaging based on the dielectric properties of the tissues through which the antenna passes. It is also the purpose of this invention to offer a reliable approach for assessing the real-time evolution of thermal ablation zones using even a single applicator, reducing the invasiveness of the procedure while improving diagnostic accuracy. It is additionally a purpose of the invention the possibility to determine the size of a tumor or a treated zone or ablated zone. Object of the invention These and other results are obtained according to the invention which refers to an apparatus able to measure the dielectric properties of biological tissue in situ and real time. The apparatus is made by any MTA applicator connected to a vector network analyzer (VNA) and comprises a new method to elaborate the data to retrieve the dielectric properties. Using the VNA when the applicator is inserted into the material or tissue under test (such as tissue or liquids), the reflection coefficient ^^11is recorded and from it the dielectric properties are reconstructed. It is, therefore, specific object of the present invention a method for dielectric spectroscopy of a material or a tissue under test, comprising the following steps: measuring the reflection coefficient signal as a function of the frequency of said antenna / applicator inserted into a material or tissue under test, by a vector network analyzer, wherein the single applicator is in contact with said material or a tissue under test and wherein the applicator is operatively connected to the vector network analyzer; multiplying the reflection coefficient signal with an ultra-wide band (UWB) signal as a synthetic applicator excitation; converting the signal of the previous step into the time domain by an Inverse Fourier Transform (IFT); suppressing the part of the converted signal of the converting step which does not represent the antenna- tissue interface, according to the applicator geometry and the material or tissue under test; converting back the signal of the suppressing step to the frequency domain; and extracting from the frequency signal converted the dielectric properties and / or estimating the dimension of the material or a tissue under test and / or of an ablated area of the material or a tissue under test thereof. Always according to the invention, said suppression step may comprise choosing a timestamp, to window out parts of the signal in the time domain. Still according to the invention, said suppression step may be carried out by the Kaiser-Bessel Derived (KBD) procedure and / or the Tukey and / or sine-tapered window (TUK) and / or other windows. Further according to the invention, the extraction step may comprise the following sub-steps: the rectified reflection coefficient obtained with the previous step now contains information only about the reflections occurring at the interface adherent to the applicator; and reconstructing the dielectric properties of the material or tissue under test just around the applicator. Advantageously according to the invention, the reconstruction step may be carried out by a Stuchly & Stuchly model and / or by the Marsland and Evans model and / or other methods developed for the open-probe technique. Further according to the invention, the ultra-wide band (UWB) signal acting as synthetic applicator excitation may be the spectrum of a Gaussian signal, and / or a monocycle, and / or other UWB signals. Always according to the invention, said measuring step may comprise the following sub-steps: recording calibration reflections, wherein the reflection coefficient of the applicator is recorded when immersed in at least three different calibration liquids, establishing baseline measurements; and recording material / tissue reflection, wherein ^^11is recorded when the antenna-probe is immersed in the test material, such as a tumor; the multiplication step may comprise the following sub-steps: multiplying calibration signal by the excitation signal step, wherein said multiplying is carried out after the recording calibration reflections; and multiplying test material signal by the excitation signal step, wherein the multiplying test material signal is carried out after said recording material / tissue reflection step; the Inverse Fourier Transform transformation may comprise the sub-steps: converting calibration signal for said multiplication calibration signal, wherein the converting calibration signal for said multiplication calibration signal is carried out after the multiplying calibration signal by the excitation signal step; and converting test material signal of the multiplying test material signal, wherein the converting test material signal is carried out after multiplying test material signal by the excitation signal step; wherein the recording material / tissue reflection is carried out after the converting calibration signal for said multiplication calibration signal step; said dimensions evaluating step may comprise the following sub-steps: averaging point by point in the time domain the converted calibration signals, and subtracting the obtained signal from the time-domain signal recorded inside material or tissue under test; determining the interface signal corresponding to the interface, wherein a valley in the interface signal corresponding to the main reflection is identified; determining the time of arrival (ToA) factor of the interface signal; calculating the velocity in the material under test ^^^^^^^^(^^) at a specified frequency ^^0, based on the dielectric properties of the reconstruction step and by the dielectric spectroscopy method; calculating the radial dimension of a portion of the material or tissue under test to estimate the dimension of the material or tissue under test. Still according to the invention, said measuring step may comprise the following sub-steps: recording the reflection coefficient of the applicator ^^11(^^^^^^^^) at the outset of ablation of the material or tissue under test by the applicator immersedin the tumor; recording the reflection coefficient ^^11 at a later time (^^^^^^^^ +during the ablation; the multiplication step may comprise the following sub-steps: multiplying the reflection signal from step of recording the reflection coefficient of the applicator with an excitation signal; multiplying the reflection signal from the step ofrecording the reflection coefficient at a later time with an excitationsignal; the Inverse Fourier Transform transformation may comprise the sub-steps: converting the signal from the multiplying step at ^^^^^^^^into the time-domain using theIFT; converting the signal from the multiplying step at ^^^^^^^^ + into the time-domainusing the IFT; said extraction step may comprise the following sub-steps: using the signal from the converting step as the template or reference signal for deriving the interface signal at the ablation zone of the material or tissue under test boundary; determining the part of the interface signal corresponding to the interface; determining the time of arrival (ToA) factor and carrying out the valley detection of the interface signal; calculating the velocity in the material under test ^^^^^^^^(^^) at a specified frequency ^^0, by the dielectric spectroscopy step; and calculating the radial dimension of a portion of the material or tissue under test based on the velocity in the material under test ^^^^^^^^(^^) and the time of arrival (ToA); wherein the method steps are repeated once the ablation zone fully envelops the tissue or the material under test. Advantageously according to the invention, after the step of measuring the reflection coefficient (^^11), a frequency re-selection is carried out, to select the specified frequency (^^0) for dielectric properties of the material or a tissue under test and / or of an ablated area of the material or a tissue under test thereof. Further according to the invention, said frequency selection step may comprise the following sub-steps: measuring the reflection coefficient ^^11as a function of the frequency of said material or tissue under test on a wide frequency range Δ^^; carrying out the dielectric spectroscopy; identifying the accurate frequency ranges in the obtained results; carrying out a test whether the measured dielectric is accurate at said specified frequency of the signal used, such that if the test is negative, a more appropriate specified frequency is chosen and a smaller frequency range from the reflection coefficient is extracted. It is also object of the present invention an apparatus for dielectric spectroscopy, comprising: a microwave generator for generating high power continuous wave single frequency microwave signal; an applicator, suitable for microwave thermal ablation through the microwave signal generated by the microwave generator; a vector network analyzer for sending an UWB low power signal, and for receiving the UWB signal reflected by a material or tissue under test and measuring the applicator reflection coefficient referred to said material or tissue under test; an electronic switch for routing the signals between the microwave generator, the vector network analyzer, and the applicator; a control logic unit, operatively connected to the microwave generator, the vector network analyzer, and the electronic switch, wherein the control logic unit is configured to control the microwave signal generated by the microwave generator, to be injected into the material or tissue under test, and the electronic switch, to allow the vector network analyzer to excite and then receive and processing the signal reflected by the material or tissue under test, wherein the control logic unit is configured to carry out the method above, to reconstruct in real time the dielectric properties of the material or tissue under test. Always according to the invention, said apparatus may comprise a coaxial cable connecting said electronic switch and said antenna. Still according to the invention, the vector network analyzer may be configured to determine the reflection coefficient of said material or tissue under test. It is also object of the present invention a computer program comprising instructions that, when the program is executed by a computer, cause the computer to execute the steps of the method above. It is additionally object of the present invention a computer readable storage medium comprising instructions which, when executed by a computer, cause the execution of the method steps above by the computer. Brief description of the figures The present invention will be now described, for illustrative but not limitative purposes, according to its preferred embodiments, with particular reference to the figures of the enclosed drawings, wherein: Figure 1 illustrates a block diagram of an apparatus for dielectric spectroscopy according to the invention; Figure 2 illustrates the block diagram of Figure 1 where the steps for carrying out the dielectric spectroscopy are shown according to a first embodiment; Figure 3 illustrates a figure showing the recorded time signal with the windowing filter; Figure 4 illustrates a KBD window applied to a reflected signal; Figure 5 illustrates a TUK window applied to a reflected signal; Figure 6 illustrates a second embodiment of the method according to the invention; Figure 7 illustrates a third embodiment of the method according to the invention; and Figure 8 illustrates a flowchart describing the process of decision-making in case of radar-based approach for tissue formation dimension analysis. Detailed description In the various figures, similar parts will be indicated by the same reference numbers. Referring to Figure 1, a scheme of an apparatus for dielectric spectroscopy 1 according to the present invention is shown, which is designed to allow the examination of materials or tissues through the use of microwave signals, as better explained below. The apparatus for dielectric spectroscopy 1 essentially comprises a microwave generator 2, an applicator 5, which is an antenna-probe, a vector network analyzer 3, an electronic switch 4, and a control logic unit 7. The microwave generator 2 generates high-power continuous wave microwave signals, i.e., at a single frequency, used to ablate the tissues or the material under test through the antenna-probe 5. The microwave generator 2 could be based on various technologies such as solid-state or tube-based microwave sources. Connected to the microwave generator 2 is said applicator 5, which is specifically designed and adapted for microwave thermal ablation. The applicator 5, which could be e.g. a monopole, a dipole, or the like, emits the generated microwave energy towards the material or tissue under test. The suitability of the applicator 5 for thermal ablation allows for localized heating and destruction of targeted areas, which is particularly useful in medical applications such as cancer treatment. In fact, the cancer cells, as ablated, die immediately. The applicator 5, as mentioned above, is an antenna-probe and can be of several types, such as: - asymmetric dipoles (AD); - coaxial slot antennas; - open-ended coaxial slot antenna (OEC); - double slot antennas etc. Each type of antenna of the applicator 5 has its own technical advantages, depending on the specific application or tissue to be treated. The applicator 5 usually operates at frequencies used for MTA, which are 915 MHz, 2.45 GHz, and 5.8 GHz; it operates at the same frequency generated by generator 2. The applicator has a useful band, which at maximum covers a 1 GHz range. However, other operating frequencies can be considered. The vector network analyzer 3 issues a low-power signal and receives the signal reflected back from the material or tissue under test. The signal generated by the vector network analyzer 3 is different to that of the generator 2. The signal injected by the vector network analyzer 3 has a band much broader than that of the applicator 5 intended to ablate. A typical, although not limitative bandwidth of the signal injected by the vector network analyzer 3 is 10 ^^^^^^. By measuring and analyzing the signal reflected by the material or tissue under test, the vector network analyzer 3 determines the reflection coefficient ^^11characterizing the UWB signal transmitted by the vector network analyzer 3 along with that of the applicator 5. In this way, the vector network analyzer 3 allows recording the reflected signal on a UWB to allow also the determining of the dielectric properties of the material under test, as better explained below. ^^11is a component of the network scattering matrix ^^^^^^, well known in the literature. Specifically, the scattering parameters or ^^-parameters are the elements of a scattering matrix ^^^^^^, which describes the electrical behavior of linear electrical networks when undergoing various stimuli by electrical signals. The parameter ^^11represents the input voltage reflection coefficient at one port of a device. The advantage of measuring the reflection parameter ^^11of the scattering matrix is that it is possible to use a single antenna (as better explained below), i.e. only the applicator 5. The electronic switch 4 is suitable to route the signals between the microwave generator 2, the vector network analyzer 3, and the applicator 5. The electronic switch 4 may be designed using various technologies. In particular, it may be a semiconductor-based device. A coaxial cable 6 connects the electronic switch 4 to the antenna-probe 5, to transfer the microwave signal between components. The control logic unit 7 coordinates the functions of the microwave generator 2, vector network analyzer 3, and electronic switch 4. The control logic unit 7 controls the generation of the microwave signal by the microwave generator 2. Also, it allows for the collection and processing of the reflected signal detected by the vector network analyzer 3. The control logic unit 7 can be implemented in several ways. In the present embodiment, it was implemented as a computer program run by a personal computer. In general, it can be implemented also by a programmable microprocessor or a PLC, to run specific algorithms. The operation of the apparatus for dielectric spectroscopy 1 described above is as follows. To better disclose the operation of the apparatus 1 reference is made also to Figure 2, where a spectroscopy method 100 run by the control logic unit 7 can be seen. Specifically, to measure dielectric properties of semisolid or liquid materials or tissues under test, which, in the case at issue is a tissue within a patient P (and / o any tissue thereof) an antenna applicator 5 is used, with one end connected to the vector network analyzer 3, through the coaxial cable 6 and switch matrix 4, and the other end put in contact or immersed into the tissue of the patient P to be analyzed. The vector network analyzer 3, as said, measures the applicator 5 reflection coefficient ^^11. A model is then developed to de-embed the dielectric properties from the reflection coefficient ^^11as detected by the vector network analyzer 3. In the prior art two UWB antennas are deployed (not just one according to the invention). With the apparatus for dielectric spectroscopy 1 according to the invention, as better specified below, the measured signal in the frequency domain 110 is first multiplied by the frequency behavior of an excitation signal 120, which is preferably, in some present embodiments, a Gaussian signal, then it is converted in the time domain 130, filtered 140, converted back in the frequency domain 150 and divided by the same frequency behavior of an excitation signal used in the previous step 161. Accordingly, the apparatus for dielectric spectroscopy 1 does not need any operations to allow the signal of interest to emerge from the measured signal, but the useful time interval is isolated from the recorded signal. However, prior calibration of the de-embedding model is needed with measurements in well-characterized materials / liquids. Calibration consists in measuring the applicator 5 reflection coefficient in three or four different liquids (depending on the de-embedding model: Stuchly & Stuchly or Marsland & Evans) before entering the tissue. The operation of the apparatus for dielectric spectroscopy 1 can be seen, as said, in Figure 2, which comprises a flowchart of the method 100 for broadband in- situ dielectric spectroscopy. The operating method 100 according to the invention uses the analysis of the reflection coefficient ^^11in the time domain. In fact, vector network analyzer 3 measures the reflection coefficient ^^11as a function of the frequency (frequency-domain), and in this domain it is de-embedded. According to the invention, the measured reflection coefficient ^^11is received (step 110) and then first multiplied (step 120) with the frequency representation of an ultra- wide band (UWB) signal representing a synthetic applicator excitation. Specifically, in the multiplication step 120, the reflection coefficient ^^11is multiplied by the frequency behavior of a Gaussian pulse. In other embodiments other signals can be multiplied, e.g., monocycle or the like. Coming back to the flowchart, the obtained signal is converted (step 130) into the time domain using Inverse Fourier Transform (IFT). By the combined steps of multiplying the reflection coefficient ^^11with a frequency spectrum of a Gaussian pulse and carrying out the IFT, it is possible to locate the reflection between the antenna (i.e., applicator 5) feed and the surrounding material / tissue, that is a reflection occurring at the antenna’s geometrical structure. Therefore, the method 100 allows looking within the antenna ringing where the useful information (i.e. dielectric properties) is embedded; the antenna ringing is typically windowed out in the prior art. In the time domain it is possible to identify different pulses originating from reflections of the synthetic ultrawide-band (UWB) signal against different interfaces. The magnitude of these peaks in general depends on the dielectric contrast at the interfaces. These interfaces are native to the antenna itself (e.g. reflection against the antenna feed, against the distal tip, sleeve etc.) and the different tissue interfaces surrounding the antenna, as better explained below. The most prominent / visible reflections are those occurring at the interface between the applicator geometry (applicator 5 feed and tip) and the tissue adherent to the applicator 5, followed by smaller-magnitude reflections against interfaces between adjacent tissues, which can be located in the radial or axial direction from the applicator 5 (which is an antenna-probe) feed and which arrive later on in time. The information about the properties of the tissue adherent to the applicator 5 is embedded in the signal originating from the applicator geometry–tissue reflection. For this reason, in order to be able to get only that specific information, it is necessary to isolate it from the remaining time-domain signal components (those originating from interfaces between different tissue types). To perform the signal isolation in the terms disclosed above, a timestamp is chosen, after which different parts of the signal in the time domain are windowed out (step 140) using one of the windows from the signal processing theory. In particular, referring to Figure 3, an example of breakdown of the reflected signal recorded at the coaxial feed of the applicator 5 is shown. In the figure, time is in the abscissa, in seconds, and the recorded time signal (in Volts) is in the ordinate. More specifically, the signal is broken down considering the interfaces the signal encounters marked on the applicator 5 geometry. Specifically, Figure 3 illustrates the different signal components corresponding to reflections against different interfaces, wherein: - the signal “1” is the antenna-probe 5 feed tissue interface; - the signal “2” is the antenna-probe 5 tip-tissue interface; - the signal “3” is the antenna-probe 5 distal arm of the antenna interface, The windowing out or suppression step 140 may be carried out by several different methods. In some embodiments the Kaiser-Bessel derived (KBD) procedure may be applied. KBD window used in this embodiment isolates the signal of interest from both the reflections originating from the tissue boundaries reflections and those originating from e.g., connectors that were not compensated for during the pre- measurement setup calibration. A specific window length can be set (e.g., the exact part of the original signal can be preserved) and by changing the tuning parameter, that may increase the window’s slope. In some other embodiments the Tukey or sine-tapered window (TUK) may be applied. In general, TUK window is very flat in the time-domain, meaning that it is equal to “1”. The duration of the flat part is specified with “taper length”, and with ^^ the “cosine fraction” parameter ^^, i.e., the first and last 2 percent of the samples are set equal to parts of a cosine. This window is advantageous when analyzing transient data because the amplitude of the selected transient signal is less likely to be altered compared to some other commonly used windows (e.g., Hanning or Flattop). In this specific embodiment, half of the TUK window was used and added to a classical rectangular window to remove only the reflections arriving later in time (i.e. after the reflections against the geometry of the antenna). In any case, in other embodiments, other windows can be used, depending on the needs. Figures 4 and 5 respectively show the windowing operation of the reflected signal by a liver sample by the KBD window and the TUK window. In the figures, time is in the abscissa, in seconds, and the recorded time signal (in Volts) is in the ordinate. To determine the timestamp appropriately, it is necessary to know the geometry of the applicator 5 to verify if the most prominent peaks found in the time domain actually do correspond to the reflections against the feed of that applicator and its tip. This information is given by the time needed by the electromagnetic signal to travel along the applicator 5 length. After the suppression step 140 is performed, the signal is converted back to the frequency domain (step 150). In the frequency domain, the step of extraction 160 of dielectric properties is carried out. In the embodiment of Figure 2, the dielectric extraction 160 step comprises the sub-steps of dividing 161 the converted signal in the frequency-domain after the Fourier Transform step 150 by the frequency behavior of the used excitation signal, so as to obtain a purified or rectified reflection coefficient 161, which now contains information only about the reflections occurring at the interface adherent to the antenna-probe 5. The same procedure 120 - 161 has to be performed for the reflection coefficient measured with the applicator 5 inserted into the calibration materials as well, because although they are homogeneous there can be reflections against the boundaries of the container used to store the material. Finally, the newly obtained filtered and processed reflection coefficients, calibrated also in view of the materials and the material or tissue under test, are used in the de-embedding model to obtain dielectric properties of only the tissue adherent to the applicator 5, on a wide frequency band. As stated above, when considering the open-probe technique, the resulting dielectric properties are effective properties of the sensing volume of the probe. Since the applicator 5 radiates an electromagnetic field, and as such it reaches a tissue volume much greater than that of the open-probe itself, traditional de-embedding techniques would give as measurement results the effective properties of a significantly large volume. Coming back to Figure 2, in step 162 the dielectric properties of the tissue just around the applicator 5 are reconstructed. In some embodiments, the dielectric reconstruction step 162 is carried out by the so called Stuchly & Stuchly model, indicated in Figure 2 as S&S (see M. A. Stuchly and S. S. Stuchly, “Coaxial Line Reflection Methods for Measuring Dielectric Properties of Biological Substances at Radio and Microwave Frequencies-A Review,” IEEE Transactions on Instrumentation and Measurement, vol.29, no.3, Art. no.3, 1980, doi: 10.1109 / TIM.1980.4314902), whereby the measured dielectric ^^∗is calculated based on the reflection coefficients received (step 163). Therefore, in this case, the reflection coefficients are those which are received and then purified using the method according to the invention. The Stuckly & Stuckly model solves an equation, while in other approaches (e.g., those in the prior art), the solution is iteratively searched. The Stuckly & Stuckly is a method already proposed in the literature for the open probe sensor. In the technical context of the present invention such a method is particularly applied to elaborate the signal from the ablation antenna. To this end, the equivalent circuit model that is at the base of the Stuckly & Stuckly method is adapted to represent the antenna structure. In particular, the model consists of two parallel capacitances representing two fringing field, one in the dielectric of the antenna and one the field closing through the tissue adjacent to the radiating part of the antenna. In some embodiments, the dielectric reconstruction step 160 is carried by the Marsland and Evans model indicated in Figure 2 as M&E (see T. P. Marsland and S. Evans, “Dielectric measurements with an open-ended coaxial probe,” IEE Proc. H Microw. Antennas Propag. UK, vol.134, no.4, Art. no.4, 1987, doi: 10.1049 / ip- h-2.1987.0068), where still the measured dielectric ^^^^is calculated based on the reflection coefficients received (step 164). However, other methods for dielectric reconstruction from the reflection coefficient can be used as well, such as the full wave method, and the like. The same method steps can be applied to isolate other peaks in the time- domain signal at different time instants. This allows for the isolation of interfaces between different tissues or tissue types, such as ablated and non-ablated tissues, for instance. Consequently, a real-time system for monitoring the evolution of the thermal ablation area is realized. In fact, as stated above, it is known that the dielectric properties of treated tissue change during ablation, and for most tissues, these properties decrease due to water loss. By measuring the dielectric properties of tissue during microwave thermal ablation, the progress of the ablation can be estimated. Considering that the ablation zone advances slowly before encompassing the entire tumor, for example, and the required safety margin, it is necessary to isolate the signal corresponding only to the ablation zone (excluding adjacent non-ablated tumor and surrounding healthy tissue) in the time domain. The application of the suggested method for dielectric spectroscopy with the MTA applicator 5 addresses the current lack of real-time monitoring devices. Furthermore, it enables precise applicator 5 guidance into the targeted tissue based on changes in measured dielectric properties as the applicator 5 passes through different tissue types (such as skin and muscle). Knowledge of the dielectric properties of adjacent tissue indicates the type of tissue in which the applicator 5 is located and confirms if it has arrived at the desired position, namely, the tumor. It is worth mentioning that the suggested purified method of dielectric spectroscopy with the MTA antenna as applicator 5 is implementable with any type of interstitial MTA applicator 5. In some tests of the present invention, the method 100 has been tested with the most common applicator 5 types, including the asymmetric dipole, coaxial-slot antenna type I, and open-ended coaxial slot antenna. These applicators are designed to operate at both 2.45 and 5.8 GHz frequencies. Referring now to Figure 6, it is illustrated an additional embodiment of the invention. Specifically, in case of tumorous tissue and ablated area forming around the applicator 5, accurate, real-time dielectric spectroscopy of tissue adherent to the applicator 5 (the one we are proposing in the patent) can enable determination / estimation of the tumor and ablation area’s radial dimensions. The tumor and ablation area’s radial dimensions can be obtained by a radar- based technique, which determines the distance from the applicator 5’s feed of the different tissue interfaces, e.g. tumor-healthy tissue interface or embedded ablation area – tumorous tissue interface. The distance is calculated based on the knowledge of the time-of-arrival (ToA) of the signal reflected against the interface and of the electromagnetic wave’s velocity (^^^^^^^^) in the tissue at the specific frequency of radar operation, velocity which in turn is determined by the dielectric properties of the tissue in which the wave propagates (determined with the purified dielectric spectroscopy approach above illustrated): Once the applicator 5 is inserted into the tumor tissue, the tumor size estimation can be performed to determine if the applicator 5 is centered within the tumor to ensure the optimal coverage during the ablation treatment. To do so, the radar-based approach algorithm is adopted, as shown in said Figure 6. Specifically, Figure 6 illustrates a flowchart of the method 101 applied for measuring and processing reflection coefficients of an applicator 5 when immersed in various substances, aimed at characterizing dimensions of materials such as biological tissues. The method 101 comprises several steps and sub-steps, better disclosed below. The measured reflection step 110 comprises two sub-steps, namely: - recording calibration reflections 111 where the reflection coefficient ^^11of the antenna-probe 5 is recorded when immersed in at least three (or more) different calibration liquids, establishing baseline measurements; and - recording material / tissue reflection 112 where ^^11is recorded when the applicator 5 is immersed in the tissue or material under test, such as a tumor or any other tissue. The multiplication step 120 comprises the reflection signals from previous steps multiplied by the synthetic excitation signal, including multiplying calibration signals (step 121) from the step of recording calibration reflections 111 by the excitation signal, and multiplying test material signal with the excitation signal 122 for signals from recording material / tissue reflection 112 step. The Inverse Fourier Transform IFT transformation 130 converts these multiplied signals into the time domain using IFT, specifically converting calibration signal 131 for step 121 and converting test material signal 132 for step 122. As is well known, the Fourier transform (and then it’s inverse) gives a signal with points equally spaced in time. It is noted that in the prior art also other procedures are used, like the inverse chirp-z transform, which is a more flexible than FFT allowing non-uniform sampling of the signal. Such an approach was used in particular to evidence the peaks in the signal coming from different paths between two or more antennas (multipath), to allow fitting them with different Gaussian pulses, and time-gating them to isolate each pulse to elaborate each pulse separately. In the case at issue, a time gating is used to isolate the time interval in which the useful signal is located, thus simplifying considerably the process and the calculus. After the IFT transformation 130 step, in the present embodiment, the dimension estimation 170 comprises the following sub-steps. The signals obtained from the IFT transformation of calibration signal 131 step are averaged (step 171) point by point in the time domain to generate a “custom” template and the interface signal, which arises from the reflection against the boundary (step 172) between the tumor and healthy tissue, is derived by subtracting the “custom” template from the time-domain signal recorded inside the tumor (achieved after the IFT conversion step 132). According 172 step, the measured data is just one step 111 and the averaging is calculated among the signals recorded in the calibration liquids when transformed in the time domain to help elaborating the measured signal. The interface signal is obtained (step 172) and the valley or a peak in the interface signal corresponding to the main reflection is identified, specifying its time of arrival ToA (step 173) factor. The tumor radial dimension is then determined (step 175) based on the ToA and the velocity in the material under test ^^^^^^^^(^^) at a specified frequency ^^0, as outlined in the equation above (calculated in step 174). The velocity in the material under test ^^^^^^^^(^^) at a specified frequency ^^0is achieved based on the dielectric properties of the reconstruction step 162. With reference to the evaluation of the distance, it is clear that velocity and distance are easily obtained once the dielectric properties are known. The solution is also characterized by the way the dielectric properties and time are obtained, which are needed for the cited calculation, which are needed for the cited calculation. In other words, deriving the dielectric properties of the material to calculate the velocity in the material under test ^^^^^^^^(^^) at a specified frequency ^^0, and the time of arrival (ToA) and achieve the estimating tumor dimensions in the tissue under test. It is hereby noted that the sub-steps of the measured reflection step 110, the multiplication step 120, and Inverse Fourier Transform IFT transformation 130 are grouped under the functional stand point. Considering the temporal execution standpoint the sub-steps of recording calibration reflections 111, multiplying calibration signals 121 and the converting calibration signal 131 are executed earlier. Then the steps of recording material / tissue reflection 112, multiplying test material signal with the excitation signal 122, and converting test material signal 132 step 122 are executed immediately after, before the average and subtraction step 171. It is also noted that the creation of the average “custom” template can be performed before recording the signal of the material under test. Normally, three or four calibration standards are recorded, transformed to the time domain, and averaged point by point and stored in the memory so that it can be used afterwards for obtaining the interface signal between the tumor and healthy tissue. Referring to Figure 7, a further implementation of the method 102 is illustrated as a flowchart, which is utilized to ascertain the extent of the boundary between the ablated zone and the tumor. In this embodiment, the peak levels in the interface signals are monitored throughout the duration of the ablation. These peak levels are indicative of the dielectric contrast between the different tissues. The dielectric spectroscopy apparatus 1 carries out its measurements to the specific dielectric properties of the tissue surrounding the antenna-probe 5, enabling it to precisely gauge the distances between various interfaces. In a manner similar to estimating tumor dimensions as illustrated in Figure 6, this method 102 tracks the expansion of the ablated area around the feed of the applicator 5. The method 102, as shown in Figure 7, adopts a different approach for acquiring the interface signal of the ablation zone / tumor (and in general a material or tissue under test) compared to the tumor size estimation method. In this case, the template signal used is derived from the signal recorded in the same area during a previous time step 113, which is stored temporarily, as opposed to using the recording from the calibration materials. The method 102 applied for monitoring the expansion of the ablation area is disclosed in a block diagram in Figure 7 and comprises the following steps: - recording 113 the reflection coefficient of the applicator 5, namely ^^11(^^^^^^^^), at the outset of ablation when the applicator 5 (the antenna) is immersed in the tumor; -recording 114 the reflection coefficient ^^11 (^^^^^^^^ + at a later time ^^^^^^^^ + ^^1during ongoing ablation; - multiplying 123 the reflection signal from step 113 with an excitation signal; - multiplying 124 the reflection signal from step 114 with an excitation signal, following the same procedure as in step 123; - converting 133 the signal from step 123 into the time-domain using the IFT; - converting 134 the signal from step 124 into the time-domain using the IFT; - using the signal from the converting step 133 as the template for deriving the interface signal at the ablation zone-tumor boundary (step 171a); - deriving the interface signal 172 by subtracting the template signal from step 171a from the time-domain signal recorded at ^^^^^^^^ (signal from step 134);- identifying 173 the main reflection in the interface signal, marked by a distinct valley or peak, to ascertain its time of arrival (ToA); - calculating 175 the radial dimension of the ablation zone using the ToA and the velocity in the material under test ^^^^^^^^(^^0), according to the specified equation - The velocity 174 is obtained from the dielectric spectroscopy procedure 100 (Fig.2) The method 102 continues to monitor the ablation zone until it adequately encompasses the entire tumor, plus a necessary safety margin. This is verified by comparing the dimensions of the tumor and the ablation zone. Once the ablation zone fully envelops the tumor, an observable shift in the interface signal’s characteristic occurs, with a prominent peak emerging rather than a valley in the identification step 173. Likewise Figure 6, the measured reflection step 110, the multiplication step 120, and Inverse Fourier Transform IFT transformation 130 are grouped under the functional stand point. From the temporal execution standpoint the sub-steps of recording 113, multiplying 123 the reflection signal from step 113 with an excitation signal, and converting 133 the signal from step 123 into the time-domain using the IFT steps are executed earlier. Then the steps of recording 114 the reflectioncoefficient ^^11 at a later time ^^^^^^^^ + during ongoing ablation, multiplying 124 thereflection signal from step 114 with an excitation signal, and converting 134 the signal from step 124 into the time-domain using the IFT are executed immediately after, before the interface signal at the ablation zone-tumor boundary (step 171a). As previously reported, to determine the interfaces’ distances, the recorded reflection coefficient of the applicator 5 immersed into the tumor / ablation area is multiplied with an excitation signal (in this case, an ultra-wide band (UWB) signal) and by means of IFT, the signal is transformed into the time domain. The UWB signal has to be carefully chosen since its central frequency ^^0is the frequency at which the interface reflection is the most prominent, and hence detectable during the time-domain analysis. Therefore, it is important to ensure accurate dielectric properties knowledge at the radar’s frequency ^^0. In some cases, the dielectric spectroscopy with a certain antenna (i.e., the applicator 5) geometry is inaccurate at a specific frequency or frequency range due to its design. To enable radar-based technique for distance estimation regardless of the latter limitation, the UWB signal can be chosen with a frequency content such that the central frequency falls in a frequency band in which the applicator 5 is suitable for the tissue. The suggested approach can be adopted under the condition that the frequency range used for measurements of the reflection coefficient ^^11is adequately broad and sampled. When re-using the already recorded reflection coefficient ^^11, a smaller frequency range is selected. For example, in case of 0-10 GHz measurement performed with a UWB signal with the central frequency at 5 GHz, a 0-6 GHz range can be extracted andmultiplied with a UWB signal with the central frequency ^^0 = 3 ^^^^^^ and thentransformed into the time domain for radar-based analysis of the different dimensions. When doing this re-usage, it is important to have accurate dielectric properties at the new central frequency of the UWB signal, and that the width of the signal is still narrow enough to maintain peak-detection correlated to the tissue interface (e.g., in case of closeness of different applicator 5 (antenna) and tissue interfaces, a too narrow bandwidth UWB signal can cause overlapping of different interface signals which in turn causes information loss). A block diagram describing the process 180 of decision-making or selection in case of radar-based approach for tissue formation dimension analysis is shown in Figure 8, wherein after the measurement of the coefficient ^^11step (step 181), in place of step 110 of Figure 2 for instance, on a range Δ^^ say, for example, of 0- 10GHz, the purified dielectric spectroscopy (step 182) is carried out according the procedure described above and illustrated in Figure 2. Then it is identified (step 183) the accurate frequency ranges in the obtained results. Then a test is made (step 184) whether the measured dielectric property ^^∗is accurate at the frequency ^^0of the UWB signal used. If the test is positive, the radar based analysis is continued (step 185, which implies carrying out the step 174, namely the calculation of the velocity in the material under test ^^^^^^^^(^^0), disclosed in Figure 6 and 7); otherwise, a more appropriate frequency ^^0is chosen (step 186) and extract (step 187) a smaller frequency range (2^^0) from until a suitable frequency ^^0is obtained. The proposed dielectric spectroscopy technique has already been verified through numerical simulations with various types of MTA antennas, demonstrating successful de-embedding of dielectric properties even with small dimensions or spherical tumor shapes. Thus, this approach ensures accurate property retrieval and allows clinicians to tailor treatment strategies more effectively. Advantages An advantage of the present invention is that it offers real-time, in situ monitoring of ablation procedures using time-domain-based dielectric spectroscopy with an MTA applicator. This technique can be applied to any MTA procedure with various applicator types without requiring additional setup for the clinician, modifying only the console of existing devices. An additional advantage is that the present invention does not need dedicated design of the MTA applicator, being applicable to any MTA applicator design. Another advantage of the present invention is that it complements standard imaging by confirming the position of the applicator in the body and verifying whether the antenna has reached the tumor during insertion. It is also an advantage of the present invention that it provides accurate data on dielectric properties directly adjacent to the applicator, giving physicians crucial real-time feedback on treatment progress. If dielectric properties decline in a certain pattern, it indicates successful tissue ablation, while a strong increase suggests potential problems like blood vessel leakage or water cooling failure. Additionally, a drastic decrease in properties may point to tissue perforation caused by the ablation. This information on dielectric properties improves measurements for more accurate delineation of tumor boundaries, thereby improving treatment quality and reducing complications. Another advantage of the invention is that the invention makes ablation therapy more efficient, less invasive, and more cost-effective for healthcare systems globally. By improving treatment precision and minimizing complications, this technique reduces the need for repeating procedures and ensures better resource utilization. The present invention has been described for illustrative but not limitative purposes, according to its preferred embodiments, but it is to be understood that modifications and / or changes can be introduced by those skilled in the art without departing from the relevant scope as defined in the enclosed claims.

Claims

CLAIMS 1. Method (100) for dielectric spectroscopy of a material or a tissue under test, comprising the following steps: measuring (110) the reflection coefficient (^^11) signal as a function of the frequency of said material or tissue under test, by a vector network analyzer (3), wherein a single applicator (5) is in contact with said material or a tissue under test and wherein the applicator (5) is operatively connected to the vector network analyzer (3); multiplying (120) the reflection coefficient (^^11) signal with the frequency behavior of an ultra-wide band (UWB) signal as a synthetic applicator excitation; converting (130) the signal of the previous step into the time domain by an Inverse Fourier Transform (IFT); suppressing (140) part of the converted signal of the converting step (130), according to the applicator (5) geometry; converting back (150) the signal of the suppressing step (140) to the frequency domain; and extracting (160) from the frequency signal converted the dielectric properties (162) and / or estimating (170) the dimension of the material or a tissue under test and / or of an ablated area of the material or a tissue under test thereof.

2. Method (100; 101; 102) according to claim 1, characterized in that said suppression step (140) comprises choosing a timestamp, to window out parts of the signal in the time domain.

3. Method (100; 101; 102) according to the preceding claim, characterized in that said suppression step (140) is carried out by the Kaiser-Bessel Derived (KBD) procedure and / or the Tukey and / or sine-tapered window (TUK) and / or other windows.

4. Method (100) according to any one of the preceding claims, characterized in that the extraction (160) step comprises the following sub-steps: dividing (161) the converted signal by the used excitation signal, to obtain a rectified reflection coefficient (^^11), which now contains information only about the reflections occurring at the interface adherent to the applicator (5); andreconstructing (162) the dielectric properties of the material or tissue under test just around the applicator (5).

5. Method (100) according to the preceding claim, characterized in that the reconstruction (162) step is carried out by a Stuchly & Stuchly model (163) and / or by the Marsland and Evans model (164) and / or other methods developed for the open-probe technique.

6. Method (101) according to any one of the preceding claims, characterized in that the ultra-wide band (UWB) signal is the spectrum of a Gaussian signal, and / or a monocycle, and / or other UWB signals.

7. Method (101) according to any one of the preceding claims, characterized in that said measuring step (110) comprises the following sub-steps: - recording calibration reflections (111), wherein the reflection coefficient (^^11) of the applicator (5) is recorded when immersed in at least three different calibration liquids, establishing baseline measurements; and - recording material / tissue reflection (112), wherein ^^11is recorded when the antenna-probe (5) is immersed in the test material, such as a tumor; in that the multiplication step (120) comprises the following sub-steps: - multiplying (121) calibration signal by the excitation signal (121) step, wherein said multiplying (121) is carried out after the recording calibration reflections (111); and - multiplying test material signal (112) by the excitation signal (122) step, wherein the multiplying test material signal (112) is carried out after said recording material / tissue reflection (112) step; in that the Inverse Fourier Transform transformation (130) comprises the sub- steps: - converting calibration signal (131) for said multiplication calibration signal (121), wherein the converting calibration signal (131) for said multiplication calibration signal (121) is carried out after the multiplying (121) calibration signal by the excitation signal (121) step; and - converting test material signal (132) of the multiplying test material signal (122), wherein the converting test material signal (132) is carried out aftermultiplying test material signal (112) by the excitation signal (122) step; wherein the recording material / tissue reflection (112) is carried out after the converting calibration signal (131) for said multiplication calibration signal (121) step; in that said dimensions evaluating (170) step comprises the following sub- steps: - averaging (171) point by point in the time domain the converted calibration signals (131), and subtracting the obtained signal from the time-domain signal recorded inside material or tissue under test (132); - determining (172) the interface signal corresponding to the interface, wherein a valley or a peak in the interface signal corresponds to the main reflection is identified; - determining the time of arrival (ToA) (173) factor of the interface signal; - calculating (175) the radial dimension of a portion of the material or tissue under test based on the dielectric properties of the reconstruction step (162); and - calculating (174) the velocity in the material under test ^^^^^^^^(^^) at a specified frequency (^^0), by the dielectric spectroscopy method (100), to estimate the dimension of the material or tissue under test.

8. Method (102) according to any one of the preceding claims, characterized in that said measuring step (110) comprises the following sub-steps: - recording (113) the reflection coefficient of the applicator (5) (^^11(^^^^^^^^)); -recording (114) the reflection coefficient ^^11at a later time ((^^^^^^^^ +^^1)); in that the multiplication step (120) comprises the following sub-steps: - multiplying (123) the reflection signal from step of recording (113) the reflection coefficient (^^11(^^^^^^^^)) with an excitation signal; - multiplying (124) the reflection signal from the step of recording (114) the reflection coefficient ^^11 (^^^^^^^^at a later time with an excitation signal;in that the Inverse Fourier Transform transformation (130) comprises the sub- steps: - converting (133) the signal from the multiplying step (123) into the time- domain using the IFT;- converting (134) the signal from the multiplying step (124) into the time- domain using the IFT; in that said extraction (170) step comprises the following sub-steps: - using (171a) the signal from the converting step (133) as the template or reference signal for deriving the interface signal at the ablation zone of the material or tissue under test boundary; - determining (172) the part of the interface signal corresponding to the interface; - determining the time of arrival (ToA) (173) factor and carrying out the valley detection of the interface signal; - calculating (174) the velocity in the material under test ^^^^^^^^(^^) at a specified frequency ^^0, by the dielectric spectroscopy step (162); and - calculating (175) the radial dimension of a portion of the material or tissue under test based on the velocity in the material under test ^^^^^^^^(^^) and the time of arrival (ToA); wherein the method (102) steps are repeated in real time during the procedure as well as once the ablation zone fully envelops the tissue or the material under test.

9. Method (102) according to any one of the preceding claims, characterized in that after the step of measuring (110) the reflection coefficient (^^11), a frequency re-selection (180) is carried out, to select the specified frequency (^^0) for dielectric properties of the material or a tissue under test and / or of an ablated area of the material or a tissue under test thereof.

10. Method (102) according to any one of the preceding claims, characterized in that said frequency selection (180) step comprises the following sub-steps: measuring (181) the reflection coefficient (^^11) as a function of the frequency of said material or tissue under test on a wide frequency range Δ^^; carrying out the dielectric spectroscopy (182); identifying (183) the accurate frequency ranges in the obtained results; carrying out a test (184) whether the measured dielectric (^^∗) is accurate at said specified frequency (^^0) of the signal used, such that if the test is negative, amore appropriate specified frequency (^^0) is chosen (186) and a smaller frequency range (2^^0) from the reflection coefficient (^^11) is extracted (187).

11. Apparatus for dielectric spectroscopy (1), comprising: a microwave generator (2) for generating a high power continuous wave single frequency microwave signal; an applicator (5), suitable for microwave thermal ablation through the microwave signal generated by the microwave generator (2); a vector network analyzer (3) for sending an UWB low power signal, and for receiving the UWB signal reflected by the applicator (5) inserted into a material or tissue under test, to measure the reflection coefficient referred to said applicator in the material or tissue under test; an electronic switch (4) for routing the signals between the microwave generator (2), the vector network analyzer (3), and the applicator (5); a control logic unit (7), operatively connected to the microwave generator (2), the vector network analyzer (3), and the electronic switch (4), wherein the control logic unit (7) is configured to control the microwave signal generated by the microwave generator (2), to be injected into the material or tissue under test, and the electronic switch (4), to allow the correct timing for processing by the vector network analyzer (3) of the signal reflected by the material or tissue under test, wherein the control logic unit (7) is configured to carry out the method of any one of the preceding claims, to reconstruct in real time the dielectric properties of the material or tissue under test.

12. Apparatus for dielectric spectroscopy (1) according to the preceding claim, characterized in that it comprises a coaxial cable (6) connecting said electronic switch (4) and said antenna (5).

13. Apparatus for dielectric spectroscopy (1) according to any one of claims 11 or 12, characterized in that the vector network analyzer (3) is configured to determine the reflection coefficient (^^11) of said applicator in the material or tissue under test.

14. Computer program comprising instructions that, when the program is executed by a computer, cause the computer to execute the steps of the method according to any one of the claims 1-10.

15. Computer readable storage medium comprising instructions which, when executed by a computer, cause the execution of the method steps by the computer according to any one of the claims 1-10.

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