TECHNICAL FIELD AND BACKGROUND

The present disclosure relates to dielectric measurements, in particular to a method for measuring dielectric properties of a tissue sample.

Information regarding the electrical properties of tissues samples is desired for investigation and simulation purposes in biomedical applications of electromagnetic (EM) sensors. The tissue sample typically comprises biological material, e.g. soft tissues, organs, and/or bone. While available literature mostly deals with ex-vivo characterization of isolated tissues, knowledge on dielectric properties of these tissues in their original environment is needed for an accurate design and analysis of sensors operating in proximity of the human body. Also the electrical properties of other samples, in particular soft samples, either organic or inorganic are required for a variety of applications

Dielectric properties of samples can be measured in various ways. One method comprises placing the sample inside a hollow conductive waveguide. The waveguide can be arranged for propagating electromagnetic waves at a frequency of interest. The waveguide can be connected to an analyser for generating the electromagnetic waves at the frequency of interest. The electromagnetic waves can traverse the sample and interact with the sample according to the dielectric properties of the sample. For example, a transmitted and/or reflected part of the electromagnetic can be influenced by the interaction with the sample. The interaction of the electromagnetic waves with the sample can be measured for determining the dielectric properties. For example the transmitted and/or reflected signals can be measured.

However, tissue samples are typically not well suited to be measured in a waveguide. For example, the tissue may change composition or shape. To alleviate this, the sample can be frozen. However, this may also change the dielectric properties of the sample compared to in -vivo

circumstances.

Accordingly, there remains a desire for a method to accurately determine the dielectric properties of a tissue sample while better

maintaining in-vivo conditions.

SUMMARY

A first aspect of the present disclosure provides a method for measuring dielectric properties of a tissue sample according to claim 1. The method comprises providing a hollow conductive waveguide for propagating electromagnetic waves at a frequency of interest inside the waveguide. The sample is provided inside the waveguide for having the electromagnetic waves traverse the sample and interact with the sample according to the dielectric properties of the sample. The waveguide is connected to an analyser for generating the electromagnetic waves at the frequency of interest and measuring the interaction of the electromagnetic waves with the sample for determining the dielectric properties. The sample is cut in a predetermined shape and size to tightly fit within an inner contour of a container. In other words, the sample exactly fits into the container contacting the inner contour on all sides. An outer contour of the container tightly fits to fill a cross-section of the waveguide. In other words, there is no spacing between the container and the surrounding waveguide.

Advantageously, the container can be used for maintaining the sample in a well defined shape while preventing liquid from escaping the sample. In this way the measurement can be more reproducible while the in-vivo conditions of the sample can be better maintained. By cutting the sample to tightly fit the container, it can be prevented that (air) gaps are formed between the container wall and the sample. By preventing the air gaps, the dielectric properties can be more accurately determined. The tight fitting container may also help to better keep the sample together in the original condition, e.g. prevent pockets of liquid forming. Furthermore by providing the container with an outer contour that tightly fits to fill a cross- section of the waveguide, it can be prevented that part of the

electromagnetic waves pass without traversing the sample. Also

reproducibility of aligning the sample in the waveguide can be improved. In this way, the accuracy of the measurement can be further improved.

By using a rectangular box as the container, the sample can be more easily cut to shape. For example, the sample can be cut in the form of a rectangular block tightly fitting the container in the form of a rectangular box. By using an elongate box, having different length and width

dimensions, and having the electromagnetic waves traverse the box with the electric component along the smaller dimension, the influence of the side portion of the box on the dielectric measurement can be minimized. In addition, by minimizing the wall thickness of the container, the influence of the container can be further minimized. Accordingly, it can be preferred to have a maximum wall thickness less than 2 mm, preferably less than 1 mm, more preferably less than 0.5 mm, e.g. between 0.1 to 1.0 mm

The sample can be conveniently cut by use of a punching machine.

The punching machine can accurately and reproducibly determine at least two of the dimensions of the sample shape. The third dimension can be accurately determined e.g. by shaving, grinding, or polishing. By using a template with a predetermined cavity that fits the sample shape, the third dimension can be accurately reproduced e.g. by cutting part of the sample protruding from the cavity. By smoothing an interface of the sample, it can better fit into the container. In addition, the reflective and transmissive properties can be more accurately modelled.

By freezing the sample before cutting and keeping the sample frozen until insertion into the container, the liquid part of the sample can be maintained. After the sample is inserted in the container, it can be unfrozen while the liquids can remain in the sample. The sample can be inserted e.g. via an open side. By optionally sealing the open side, the liquid can be better maintained. For example, a resin can be used for sealing the sample.

Preferably, the dry resin has similar dielectric properties as the material of the container, or even comprise the same material.

It is typically desired to provide a sample thickness close to a quarter or three-quarter times the wavelength of the electromagnetic wave in the sample to more accurately model the interaction and retrieve the dielectric properties. However, the wavelength may be dependent on the dielectric properties and not be a priori known. By producing the container using rapid prototyping, the sample dimensions can be conveniently adjusted. This can be used to accommodate specific samples and/or used iteratively to approach better thickness conditions.

By using a dielectric material for the walls of the container, such as plastic, the electromagnetic waves may suitably traverse the walls of the container to reach the sample. Plastic may also provide a suitable retention of liquid content. To more accurately determine the dielectric properties of the sample, the analysis may comprise a mathematical model to correct for the interaction of the electromagnetic waves with the container, e.g. using known dielectric properties and dimensions of the container. Preferably, the model includes the influence of the front and back sides of the container, e.g. according to a model as described by Okamura et al. (IEEE Transactions On Microwave Theory And Techniques, Vol. 47, No. 5, May 1999). The model may also include the influence of the sides of the container abutting the waveguide, e.g. according to a relative contribution of the sides to the total cross-section. BRIEF DESCRIPTION OF DRAWINGS

These and other features, aspects, and advantages of the apparatus, systems and methods of the present disclosure will become better understood from the following description, appended claims, and accompanying drawing wherein:

FIG 1 schematically illustrates a method for measuring dielectric properties of a tissue sample;

FIG 2 schematically illustrates a perspective view of an embodiment of a sample section;

FIG 3 schematically illustrates a cut-out view of part of the sample section including a container therein;

FIGs 4A and 4B show photographs of tools for cutting a sample into shape;

FIG 5A shows a photograph of a sample and corresponding container;

FIG 5B shows a photograph of a sample being inserted in a waveguide;

FIGs 6 through 8 shows graphs for example measurements;

FIG 9 shows another embodiment of a sample cell.

DESCRIPTION OF EMBODIMENTS

Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly

understood by one of ordinary skill in the art to which this invention belongs as read in the context of the description and drawings. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. In some instances, detailed descriptions of well-known devices and methods may be omitted so as not to obscure the description of the present systems and methods. Terminology used for describing particular embodiments is not intended to be limiting of the invention. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. The term "and/or" includes any and all combinations of one or more of the associated listed items. It will be understood that the terms "comprises" and/or "comprising" specify the presence of stated features but do not preclude the presence or addition of one or more other features. It will be further understood that when a particular step of a method is referred to as subsequent to another step, it can directly follow said other step or one or more intermediate steps may be carried out before carrying out the particular step, unless specified otherwise. Likewise it will be understood that when a connection between structures or components is described, this connection may be established directly or through intermediate structures or components unless specified otherwise. All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety. In case of conflict, the present specification, including definitions, will control.

In the last decades, there has been a significant increase of microwave sensors for biomedical applications e.g., for non-invasive diagnostics, and for therapy. See for example E. C. Fear, S. C. Hagness, P. M. Meaney, M. Okoniewski, M. A. Stuchly, "Enhancing breast tumor detection with near field imaging", IEEE Microwave Magazine, March 2002; or M.M. Paulides,J. F. Bakker, A. P. Zwamborn and G. C. van Rhoon, "A head and neck hyperthermia applicator: theoretical antenna array design", Int J Hyperthermia. 2007 Feb;23(l):59-67.

Moreover, constant advances in semiconductor technology and the availability of a consolidated information and communication infrastructure connected in a Wireless Body Sensor Network (WBSN), allows for the large scale use of wearable devices such as patient monitoring sensors, personal digital assistants and other wireless devices for health and care self- management. See for example Vallejo M, Recas J, del Valle PG, Ayala JL, "Accurate Human Tissue Characterization for Energy-Efficient Wireless On- Body Communications," Sensors, 2013, 13(6):7546-7569.

For designing a microwave sensor, it is essential to predict the propagation characteristics of the electromagnetic (EM) field in the intervening issues. In WBSNs the body acts as a communication channel for the propagation of EM waves, and the evaluation of the power budget requires estimating the specific absorption rate (SAR) in the human tissues. To understand the interaction between the EM radiation and the

interacting tissues, a detailed knowledge of the dielectric properties of biological tissues is required. Also the dielectric properties of for example clothes (textiles) and other materials in the neighborhood may influence the propagation characteristics of the electromagnetic field.

Extensive literature has been published on the characterization of biological tissues for EM dosimetry. Gabriel et. al. marks the first attempt toward a systematic approach with the permittivity values made available for a broader frequency range based on the parametric model in. See for example

- C. Gabriel, S. Gabriel and R. Corthout, "The Dielectric properties of

biological tissues: I. Literature survey," Phys. Med. Biol., vol 41, 1996.

- C. Gabriel, R.W. Lau and S. Gabriel, "The Dielectric properties of

biological tissues: II. Measurements in the frequency range 10Hz to 20 GHz," Phys. Med. Biol., vol 41, 1996.

- C. Gabriel, R.W. Lau and S. Gabriel, "The Dielectric properties of

biological tissues: III. Parametric Models for the dielectric spectrum of tissues," Phys. Med. Biol., vol 41, 1996. - C. Gabriel, "Dielectric properties of body tissues in the frequency range 10 Hz - 100 GHz," L'Istituto di Fisica Applicata "Nello Carrara",

Florence, http://niremf.ifac.cnr.it/tissprop, Accessed October 30, 2013

In more recent publications a study on the change of the dielectric properties with the tissue age is reported. See for example A Peyman, C Gabriel, E H Grant, G Vermeeren and L Martens, "Variation of the dielectric properties of tissues with age: the effect on the values of SAR in children when exposed to walkie-talkie devices", Phys. Med. Biol. 54 (2009) 227-241.

However, in most of the published studies measurements are carried out ex-vivo on the tissues isolated from their natural environment, such as in where the tissues are cleaned from blood and other biological fluids and immerged in a Grant water bath for coaxial probe measurements. Gabriel work was extended upon by using in-vivo measurements, but it appears that in-vivo measurement results are difficult to interpret especially if the tissue of interest is located far from the body surface.

Therefore, missing in the above characterization is the influence of the structure of the tissue. A tissue sample normally does not consist of a single dielectric material but it is a mixture of several types. For instance, bone consists of calcium, marrow, blood and other fluids. The properties of the mixture are denoted as macroscopic properties while the properties of a single material are denoted as microscopic.

Since microwave sensors operates in proximity of living tissues, the knowledge of microscopic properties is mostly not sufficient for an accurate prediction of the sensor functioning and macroscopic properties should be retrieved.

The invention is described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. The description of the exemplary embodiments is intended to be read in connection with the accompanying drawings, which are to be considered part of the entire written description. In the drawings, the absolute and relative sizes of systems, components, layers, and regions may be exaggerated for clarity. Embodiments may be described with reference to schematic and/or cross-section illustrations of possibly idealized embodiments and intermediate structures of the invention. In the

description and drawings, like numbers refer to like elements throughout. Relative terms as well as derivatives thereof should be construed to refer to the orientation as then described or as shown in the drawing under discussion. These relative terms are for convenience of description and do not require that the system be constructed or operated in a particular orientation unless stated otherwise.

FIG 1 schematically illustrates a method for measuring dielectric properties ε of a sample 1. The figure includes a perspective view "A" of the container 2 with an indication of axis coordinates X,Y, and Z.

In one embodiment, the method comprises providing a hollow conductive waveguide 3 for propagating electromagnetic waves EM at a frequency of interest "f inside the waveguide 3. The frequency of interest typically comprises a certain bandwidth, i.e. a range of frequencies. The method further comprises providing the sample 1 inside the waveguide 3 for having the electromagnetic waves EM traverse the sample 1 and interact with the sample 1 according to the dielectric properties ε of the sample 1. The method further comprises connecting the waveguide 3 to an analyser 5 for generating the electromagnetic waves EM at the frequency of interest and measuring the interaction of the electromagnetic waves EM with the sample for determining the dielectric properties ε. Advantageously, the sample 1 is cut in a predetermined shape and size to tightly fit within an inner contour of a container 2 for preventing liquid escaping the sample 1.

An outer contour of the container tightly fits to fill an inside cross- section X,Y of the waveguide 3. In other words, the outer dimensions of the container exactly match the inner dimensions of the waveguide, e.g. within an accuracy of less than 0.2 mm, preferably less than 0.1 mm, more preferably even less. For example an accuracy between 1 to 50 pm can be desired. The more accurate the fit of the container in the waveguide, the better it can be ensured that all EM waves travel through the container and sample and the more reproducible the alignment of the container in the waveguide. Alignment of the container along a length of the waveguide can be improved by e.g. providing a ridge inside the waveguide against which the container can be contacted. For example a ridge or corner 3 lr as described below with reference to FIG 3.

It will be appreciated that the present methods provides particular benefit to samples having a relatively large liquid content which needs to be maintained. For example, tissue samples such as brain and muscles typically comprise about 75% water and bone typically comprises about 22% water. In one embodiment, a tissue sample as used herein may be defined, as any solid or semi-solid sample having a liquid content by weight of more than 10%, more than 20%, or even more than 50%. The liquid content is typically water. Alternatively, or in addition, other liquids can be present, e.g. oil. In a further embodiment, the tissue sample is organic, i.e. comprising biological material. In particular for organic tissue samples, it is important to retain the in -vivo structure including liquid content to provide a meaningful and reproducible measurement.

A sample that retains shape by itself without container, e.g. a sample that is self-supporting, may be considered solid or semi-solid. The degree of solidity may depend on the amount of liquid content. Accordingly the liquid content of solid or semi-solid sample is typically less than 90%, less than 80%, less than 70%, or less than 60%, e.g. 50%. In a solid or semisolid tissue sample natural occurring inhomogeneities of the tissue can remain intact without mixing. On the other hand, a liquid sample, e.g.

aqueous solution, is not considered solid or semi-solid and cannot be "cut" in any shape.

In one embodiment, the sample 1 is cut in the form of a rectangular block tightly fitting the container 2 in the form of a rectangular box, e.g. plastic box. In other words, the outer dimensions of the sample exactly match the inner dimensions of the sample cell, e.g. within an accuracy of less than 0.1 mm, preferably less than 0.05 mm, more preferably even less. For example an accuracy between 1 to 10 pm can be desired. The more accurate the cutting, the better it can be ensured that no air gaps are formed between the walls of the container and the sample.

In one embodiment, the sample 1 is frozen before cutting into shape and kept frozen while cutting up to and including insertion of the cut sample 1 into the container 2.

In one embodiment, the container 2 comprises a dielectric material, preferably the container is made entirely of the dielectric material. In one embodiment, the container 2 is a plastic container. The dielectric properties and liquid retaining properties of plastic are found particularly suitable for the present purposes.

In one embodiment, the container 2 is initially open on one side 2a for inserting the sample and wherein the open side 2a is sealed with a resin after insertion of the sample. Preferably an acrylic resin is used. For example, cyanoacrylate is an acrylic resin that rapidly polymerises in the presence of water, specifically hydroxide ions.

In one embodiment, the container 2 comprises a maximum wall thickness "d" of less than one millimetre, preferably less than 0.5 mm, e.g. 200 pm. To retain liquid, the wall may also need a minimum thickness, e.g. more than 10 pm, preferably more than 50 pm. In one embodiment, the container walls are arranged (i.e. having suitable material and thickness) to provide a water vapour transmission rate (WVTR) of less than 100, preferably less than 10, or even less than 1 gram per square meter per day. The lower the WVTR, the better the retention of moisture in the sample.

In one embodiment, the cutting comprises use of a punching machine for determining two dimensions X,Y of the shape of the sample and wherein a third dimension Z of the shape is determined by shaving.

In one embodiment, the cutting comprises smoothing an smooth interface X,Y of the sample.

In one embodiment, the container 2 is produced by rapid prototyping, also known as 3D printing or additive layer manufacturing.

In one embodiment, the determining of the dielectric properties "ε" of the sample 1 comprises correcting for an interaction of the

electromagnetic waves EM with the container 2.

FIG 2 schematically illustrates a perspective view of an embodiment of a sample section 30. FIG 3 schematically illustrates a cut-out view of part of the sample section 30 including the container 2 therein.

In one embodiment, the waveguide 3 comprises a sample section 30 for holding the container 2 with the sample and guiding the

electromagnetic waves via the sample section 30 through the sample. The sample section 30 comprises a conductive gutter 33 and a removable conductive roof 34. The roof 34 can be used for closing the gutter 33 and completing the waveguide 3. In one embodiment, the gutter 33 and roof 34 are arranged for tightly enclosing the sample 1 therein between. In one embodiment, the gutter 33 has a U-shaped cross section.

In one embodiment, the roof 34 is clamped onto the gutter 33 for holding the container 2 by said clamping.

In one embodiment, the container 2 comprises a rectangular elongate box having a width X, length Y, and thickness Z. The width X and length Y tightly fit a rectangular cross-section X,Y of the waveguide 3. The width X is smaller than the length Y. In a further embodiment, the electromagnetic waves are generated in the waveguide 3 having an electric component E of the electromagnetic waves primarily or exclusively along the width dimension X of the container, i.e. along the smaller dimension of the cross-section. Also a direction of the H field is illustrated.

In one embodiment, the sample section 30 comprises a narrowed section, e.g. corner 31r, for abutting the square shaped sample container 2. In this way the container 2 can be reproducibly positioned at the reference plane 31p.

In one embodiment, the sample section 30 comprises a gutter 33 enclosed between flanges 31 and 32. The flanges may be used to connect the sample section 30 to the rest of the waveguide, e.g. by screws or other fastening means. In one embodiment, the sample section 30 comprises a section, e.g. part of the first flange 31, for holding a clamping mechanism 35 to clamp the roof 34 onto the gutter 33. The clamping mechanism 35 may e.g. comprise a lever to conveniently apply a force onto the roof 34. It will be appreciated that the roof 34 may distribute the applied force. In the following, an example application of the present methods and systems is provided. In particular, the application describes ex-vivo measurement of the dielectric properties of human or animal tissues preserving their original environment. The technique presented here can, however, also be applied for the measurement of the dielectric properties of other materials. The technique can in particular be advantageously used for soft samples. The accuracy of most documented approaches for dielectric measurements depends greatly on mechanical properties of the sample such as roughness, flatness, presence of air gaps and fluid leakage. In one example, a measurement setup is outlined based on transmission-line methods which allow accurately characterizing fluids and semisolids. The measurements described below have been carried out at X-band to derive the permittivity of human bone. Those skilled in the art will appreciate that the technique may also be used in other bands in which waveguides are used.

Several methods are available in the open literature for the characterization of dielectric materials, mainly divided in resonant and non- resonant methods. The latter have the advantages to provide results for a broad frequency range and therefore were deemed to be more suitable for our purposes. In this type of methods the material under test is inserted in a piece of transmission line and the reflection and transmission are measured. In particular, for the present work a rectangular waveguide was chosen, because of the availability of standard calibration procedures and the easy sample shape. From the measured S-parameters, the electromagnetic material parameters, the complex-valued permittivity and permeability, can be determined using an inversion algorithm.

Several procedures have been presented in the open literature for the inversion. In the following we present the results using the Nicolson- Ross-Weir (NRW) algorithm. See for example M. Nicolson and G. F. Ross. Measurement of the intrinsic properties of materials by time-domain techniques. IEEE Trans. Instrumentation and Measurement, 19, 377 382, 1970; or W. B. Weir. Automatic measurement of complex dielectric constant and permeability at microwave frequencies. Proc. IEEE, 62, 33 36, 1974.

However the invention is not limited to this procedure. For such algorithm, the accurate knowledge of the sample thickness is important, since the method is not well-behaved for samples thicknesses multiple of one-half wavelength in very low-loss materials.

First, verification of the procedure has been carried out through full-wave simulations using the commercial software package CST of standard microwave materials in waveguide. Then the method has been applied to the data collected on actual tissue samples, as described in next section.

One embodiment, e.g. similar to that described in FIG 1, for the measurement set-up is based on a standard WR90 waveguide, with the two ports connected to a Agilent PNA Network Analyser, such that all four S- parameters Sl l, S21, S12 and S22 are measured. A metallic sample holder, consisting of a waveguide section of thickness 4 mm, is placed at the reference plane of the waveguide. For this method it is important that the sample has the same dimensions as the waveguide in height and width (no air gaps) and that the thickness is well known.

Ex-vivo measurements can be performed on a frozen sample in order to contain the fluids and semisolid materials but the permittivity and permeability are temperature dependent. Thus frozen samples do not represent the in-vivo tissue.

Biological tissues consist of solid and fluid part and the confinement in a sample holder is therefore cumbersome. The same holds for other samples comprising soft parts, in particular fluid parts.

Measurements carried out on frozen samples often do not give reproducible results. Another approach is to in case the frozen samples in a thin layer of glue. Such sample can be measured at room temperature, once the sample is thawed. However, also in such case the repeatability of the measurement results is in general rather poor, both because of the softness of the sample and because of the difficulty of maintaining the same shape for all samples. In fact, the gluing process yields to very rough sides and varying thickness of the glue surrounding the sample.

In the method as described herein, a sample holder is used. The sample to be measured is at least partly encapsulated by this sample holder In an embodiment described here in more detail, a thin (200μιη) plastic 3D- printed sample holder is used for encasing the sample, placed in the original metallic waveguide section. Moreover, accurate fitting of the sample to the sample holder was ensured by using very precise instrumentation for sample preparation (see FIGs 4A and 4B). This allows the sample to thaw without any leakage and retains the structure of the tissue.

In one embodiment, the sample preparation comprises the following steps:

- Selecting an area of interest from a material or product, for example human or animal tissue;

- Extracting a sample from the area of interest;

- Positioning the sample in a sample holder such that the sample fits well within the sample holder;

- Placing the sample holder comprising the sample into a waveguide for measuring the dielectric properties.

- One or more of the first three steps may be performed on solidified, in particular frozen, material and/or sample.

The external dimensions of the sample holder are determined by the dimensions of the waveguide, more in particular the cross section. The four external surfaces of the sample holder facing the walls of the waveguide should fit well to the waveguide to obtain good results. The same holds for the sample and the sample holder. The sample should fit well to the sample holder.

The selection of an area of interest from a material or product may be based on visual appearance, including the structure, or any other criteria. The selection may for example be the selection of a specific type of tissue. The selection may for example be a specific human or animal bone or a part of such bone.

The rectangular sample holder 2 shown in FIG 1 is open at one side in order to allow the sample to be put into the sample holder. After the sample is placed in the sample holder, this side may be closed, preferably by a material that has similar dielectric properties as the sample holder. The sample holder may be closed for example by a glue. It will be appreciated that such a sample holder is in particular suited for liquid material.

The internal dimensions of the sample holder can be adapted to the sample as extracted from the area of interest or the sample as extracted from the area of interest can further be adapted to the internal dimensions of the sample holder by for example sawing, carving, cutting, and/or polishing.

The inventors found the following steps very suitable for the preparation of human or animal tissue samples (in the first eight steps the sample remains frozen):

(1) Make X-ray photo of the frozen tissue.

(2) Select an area of interest.

(3) Position indicators in the tissue indicating the selected area.

(4) Extract the selected area of interest from the tissue.

(5) Select one or more samples in the area of interest.

(6) Extract the one or more samples out of the area of interest.

(7) Cut the one or more samples for fitting precisely in a sample holder.

(8) Put a sample in a very thin, 3D-printed sample holder.

(9) Close and isolate the sample holders with glue (Figure 3).

(10) Bring the sample to room temperature.

After the sample is brought to the temperature at which the sample has to be characterized, for example room temperature, the sample is placed in the waveguide for performing the dielectric measurement.

Tools suitable for step 7 are shown in FIGs 4A and 4B. The oversized samples are cut with a very sharp planning tool. The four knives are positioned in a rectangular shape and aligned on the plate under the planning tool using an alignment tool, such that the sample size is within lOpm accuracy. All the fluid material is preserved. The material is put in dry-ice before the next stage. The sample 1 is inserted in the sample holder 2 as shown in FIG 5A. After calibration the sample holder 2 containing the sample of bone is inserted into a sample section 30 of the waveguide 3 (see FIG 5B) and the S- parameters are measured.

The system of equations obtained from the NRW algorithm when measuring all four scattering parameters is over determined if the measurement setup is perfectly symmetrical. However, because of manufacturing inaccuracies in the sample holder shape and the presence of air gaps, the setup may appear non-reciprocal.

As a first approach, the NRW algorithm can be applied to the geometrical average <S22,S11> and <S21,S12> to calculate the permittivity and the magnetic permeability of the sample. This approach, however is considered not to be preferred. A second approach that is preferred by the inventors is based on a multilayer simulation of the sample holder together with the sample inside the waveguide, plus a gap correction, e.g. a dielectric gap correction. The multilayer simulation applies the transfer-matrix method for the fundamental mode propagation only. See for example Born, M.; Wolf, E., Principles of optics: electromagnetic theory of propagation, interference and diffraction of light. Oxford, Pergamon Press, 1964.

The material of the sample holder is known (plastic with permittivity about 2.5), as are its dimensions (sample holder wall thickness of about 200 pm). The dimensions of the sample are also known. The unknown permittivity and permeability of the sample can be obtained by fitting the simulated S 11 and S21 to the measured geometrical average <S22,S11> and <S21,S12>. The implemented transfer-matrix method may not account for the thickness of the sample holder along the shortest dimension of the waveguide. Therefore, the results can be corrected using a gap correction, which corresponds for ε to the harmonic mean and for μ to the arithmetic mean of the sample and of the gap dielectric properties. See for example W. B. Westphal. "Techniques for measuring the permittivity and permeability of liquids and solids in the frequency range 3c/s to

50kmc/s," Tech. Report XXXVI, Lab. for Insulation Research, MIT, July 1950.

To validate the approach, first the accuracy of the single mode transfer-matrix method in predicting the S-parameters of the measured structure was evaluated. The S-parameters calculated with the transfer- matrix method were compared with the S-parameters obtained by a full wave simulator (CST) and with the measurements for two known materials: rexolite with nominal permittivity 2.53 and Eccostock HIK500f with nominal permittivity 10. In FIG 6, top, the real and imaginary parts of the Sl l for rexolite inside the sample holder are compared. Very good

agreement between the transfer-matrix method, the full wave approach and the measurements can be observed for both the real and imaginary part. For the HIK500f material, the dielectric contrast with the plastic sample holder is stronger and the approximation in the dielectric gap correction leads to a discrepancy between full wave results/ measurements and single-mode calculations, as shown in FIG 6, bottom.

To evaluate the accuracy of this approach in predicting the dielectric properties of the sample, the approach was first validated on the basis of the S-parameters calculated by CST for several lossless materials with relative permittivity between 2 and 16, inserted in a sample holder of wall thickness of about 200 pm. This size was used both in the multilayer simulation as in the gap correction. The sample thickness was 4 mm and height was 9.67. In the frequency range 8 to 12 GHz the permittivity could be predicted with an error of less than 5% away from the resonances.

However, the effect of resonance in the measurement of biological tissues can be deemed negligible because of the high dielectric losses and absorption in the tissue itself.

A further validation of the approach was performed by predicting the permittivity of known materials from the measured scattering parameters. The materials were rexolite with ε=2.53, Eccostock HIK500f(6) with ε=6 and HIK500f(10) with ε=10. For the HIK500f materials the permittivity is defined with an error of 5%. The results are shown e.g. in FIG 7.

The average measured permittivity values were: for rexolite without a sample holder 2.536, with sample holder 2.532, for HIK500f(6) with sample holder 6.11 (no measurements were available without a sample holder), for HIK500f(10) without sample holder 10.28, with sample holder 10.76. Apart for the HIK500f(10) in the holder (7% deviation), all other results are within 5% of the expected values.

Finally, an example of the predicted permittivity from the measured scattering parameters of a human bone with all fluids, prepared according to the procedure described above, obtained assuming μ _Γ =1, is given in FIG 8.

FIG 9 shows another embodiment of a container 2. In the embodiment, the container 2 comprises a solid material, e.g. produced by a rapid prototyping process, e.g. layerwise manufacturing or 3D printing. In the embodiment, the container 2 comprises two halves 2a, 2b with a spherical cut out in the centre. The container 2 can be used for measuring a spherical sample 1. As before, the sample 1 is cut in a predetermined shape and size to tightly fit within an inner contour of a container 2 for preventing liquid escaping the sample 1, wherein an outer contour of the container 2 tightly fits to fill a cross-section of the waveguide (not shown here). Also other variations of the shape of the sample 1 and/or container 2 are conceivable.

A method and measurement setup for the ex-vivo dielectric characterization of biological tissues and for the dielectric characterization of other materials, in particular soft materials, has been described. It will be appreciated that the present disclosure offer particular benefit for

measurement of samples that include a relatively large liquid content such as biological samples, in particular soft-tissue samples. In general, the disclosure may find application in the measurement of samples containing liquid content, in particular where a need exists that the liquid content be contained or preserved for accurate and/or reproducible measurement.

Several methods may be envisaged in order to further improve the accuracy in the derived dielectric properties. For example the fit of measured with simulated scattering parameters may benefit from full wave simulations (e.g. carried out using a commercial software tool such as CST) which account for all the modes in the dielectric sample and also allows easily including the effect of tolerances in the shape and thickness of the sample holder and of the dielectric sample. With the use of such a simulator there may be no need for the gap correction as the correct geometry can be incorporated into the simulator. An analytical approach based on the cascade of multi-mode equivalent networks of the different waveguide sections and dielectric discontinuities, may also be pursued.

For the purpose of clarity and a concise description, features are described herein as part of the same or separate embodiments, however, it will be appreciated that the scope of the invention may include

embodiments having combinations of all or some of the features described. For example, while embodiments were shown for a particular waveguide, also alternative ways may be envisaged by those skilled in the art having the benefit of the present disclosure for achieving a similar function and result. Electrical and/or optical components may be combined or split up into one or more alternative components. Of course, it is to be appreciated that any one of the above embodiments or processes may be combined with one or more other embodiments or processes to provide even further improvements in finding and matching designs and advantages.

While the present systems and methods have been described in particular detail with reference to specific exemplary embodiments thereof, it should also be appreciated that numerous modifications and alternative embodiments may be devised by those having ordinary skill in the art without departing from the scope of the present disclosure. For example, embodiments wherein devices or systems are disclosed to be arranged and/or constructed for performing a specified method or function inherently disclose the method or function as such and/or in combination with other disclosed embodiments of methods or systems. Furthermore, embodiments of methods are considered to inherently disclose their implementation in respective hardware, where possible, in combination with other disclosed embodiments of methods or systems. Furthermore, methods that can be embodied as program instructions, e.g. on a non-transient computer- readable storage medium, are considered inherently disclosed as such embodiment.

Finally, the above-discussion is intended to be merely illustrative of the present systems and/or methods and should not be construed as limiting the appended claims to any particular embodiment or group of embodiments. The specification and drawings are accordingly to be regarded in an illustrative manner and are not intended to limit the scope of the appended claims. In interpreting the appended claims, it should be understood that the word "comprising" does not exclude the presence of other elements or acts than those listed in a given claim; the word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements; any reference signs in the claims do not limit their scope; several "means" may be represented by the same or different item(s) or implemented structure or function; any of the disclosed devices or portions thereof may be combined together or separated into further portions unless specifically stated otherwise. The mere fact that certain measures are recited in mutually different claims does not indicate that a combination of these measures cannot be used to advantage. In particular, all working combinations of the claims are considered inherently disclosed.