The Blood Glucose By Using Rfid Information Technology Essay

The aim of the project is to measure the blood glucose by using RFID sensors without pricking the finger to make measurements at home. A general research is done to get a fair idea about how to measure the sugar levels, design the antennas and RFID sensors. A brief description is given about different methods to measure the sugar level. Various requirements for developing these methods are investigated. RFID sensors, probe antennas, dielectric properties of tissues are the important requirements for measuring the sugar level. Various electronic devices for measuring the sugar levels and probe antennas that are currently available in the market are compared, the applications and the working of the RFID sensors, various methods for measuring the sugar level, and dielectric properties of tissues at microwave frequencies, just to get an idea about their in the working of all these methods.

Diabetes is a common disease causing significant fatality rate. In 1995 the number of diabetes people was estimated to be 135 million in the year 2000, it was 154 million and in the year 2025, it is expected to top 300 million people with the main increase being in the developing countries. The projected increase in the developed countries is 42% but the developing countries; the increase is estimated to be 170%. In Malaysia, the first National Health and Morbidity Survey2 (NHMS 2) in 2000, the prevalence was reported to be higher at 10.5% the world Health Organisation (WHO) has estimated that in 2030.

There are two types of diabetes. Type1 diabetes is the most serious type and affects 5-10% of the diabetic population. This type of disease is usually that develops during childhood or adolescence and is characterized by a severe deficiency of insulin secretion. The most common form of diabetes is Type2 diabetes which affects 90-95% of the individuals with pre-diabetes by closely monitoring diet and exercise. Insulin and glucagon are hormones found in the body that maintain an exceptionally tight range of blood sugar levels in the body. The level of blood glucose in the body determines whether the pancreas secret glucagon or insulin. As the blood glucose level decreases the insulin secretion also decreases. Type1 diabetics must have insulin delivered by injection or a pump, while most Type2 diabetics can control their blood glucose through healthy eating, exercise, losing excess weight and taking oral medication [13].

Figure 2.1. Process of normalizing blood glucose levels in the body [13].

2.2. MICROWAVES

The electromagnetic spectrum covers a range of frequency from 0 to 10HZ, which represent different types of waves and is pictured bellow the common types of waves encountered are radio waves, UHF waves, infrared rays, microwaves, visible light and X-rays. Each type of wave has a different wavelength and frequency range in the electromagnetic spectrum. The frequency of microwaves inhabit in electromagnetic spectrum ranging from 300MHZ to 300GHZ, which is a subset of the radio portion of the spectrum [13].

The wave length of microwaves can be determined by equation:

Figure 2.2. The electromagnetic spectrum [14].

2.3. USES OF MICROWAVES IN BIOLOGICAL APPLICATIONS

Microwaves are present in society today more than ever before, they can be found daily in use with radars, satellite, cell phones and wireless internet connections. Today many new medical procedures are making use of microwave technology. The increased use of microwaves in daily life, the issue of human acknowledgment to this radiation has been concern for a number of years. The main concern has been with the amount of exposure from constant use of cell phones. This radiation is measured in SAR, which is the specific absorption rate or the time rate at which radio-frequency electromagnetic energy is imparted to an element of mass, of a biological tissue. According to the Institute of Electrical and Electronics Engineers standards there are different acceptable SAR levels for whole body or partial body radiation, which are dependent on the environment in which the radiation is taking place. These standards for exposure between 3MHZ and 6GHZ can be found in below table [13].

Type of environment

Whole body exposure

Partial Body Exposure

Wrists, Ankles, Hands, and Feet Exposure

Controlled Environment

0.4 W/kg

8 W/kg

20 W/kg

Uncontrolled Environment

0.08 W/kg

1.6 W/kg

4 W/kg

IEEE acceptable SAR levels for human exposure over the frequency range 3MHz – 6GHz [13].

Microwaves are becoming more commonly used in medical technology today. There are many different uses for microwaves, including the areas of tomography, therapeutic medicine, and thermograph [13].

2.4. PERMITTIVITY AND S-PARAMETERS

All materials are contain charged particles, so when some materials contact with electric or magnetic fields, it produce secondary fields. The magnetic and electric fields across a material result in polarization, conduction, magnetization of the particles in that material. Polarization of the particles results in the material acting as a dielectric [13].

When the electric field passes through the dielectric medium, the medium has an effect on the electric field called permittivity. Permittivity is essentially the ability of the material to transmit an electric field and is determined by the materials ability to polarize its particles under the influence of an electric field. The relative permittivity of a material is also called the dielectric constant. The relative permittivity is a ratio of the permittivity of a material to the permittivity of free space [13].

2.5. PERMITTIVITY AND BIOLOGICAL MODELING

Permittivity of human tissues shows noticeable changes with the change in applied microwave frequency as seen in below figure. Water is the most abundant molecule in the human body and has different content percentages in varying tissues in the body. Permittivity is a property dependent on water content due to the polarization of the water molecules in the presence of an applied electromagnetic field [13].

CHAPTER-3

3. VARIOUS TECHNIQUES FOR MEASURING THE SUGAR LEVEL

3.1. CONTINUOUS GLUCOSE MONITORING

Glucose Monitoring helps people who are having diabetics to manage the disease and avoid its accompanying problems. A person can use the results of glucose monitoring to make decisions about food, medication and physical activities. The most common way to check glucose levels involves pricking a fingertip with an automatic lancing device to hold a blood sample and the using a glucose level. Many types of glucose meters are available in the market, and all are exact and authentic if used properly some meters use a blood sample from a sensitive area than the fingertip. Continuous glucose monitoring (CGM) system uses a sensor inserted under the skin to check glucose levels in tissue fluid [1].

Figure 3.1. Lancing device and a glucose meter to measuring the glucose level [1]

A new method of assessing glucose control has been available for the past years called continuous glucose monitoring [2]. A continuous blood glucose monitor (CGM) determines blood glucose levels on an every few minutes. A typical system consists of:

A disposable glucose sensor placed under the skin, which is very tired for a few days until replacement.

A link from the sensor to a non-implanted transmitter which communicates to a radio receiver.

An electronic receiver work like a insulin pump that displays blood glucose levels with nearly continuous up days, as well as monitors rising and falling trends [3].

Figure 3.1.2. Lancing device and a glucose meter to measuring the glucose level [1]

Continuous blood glucose monitors measure the sugar level of tissue fluid. Lacking of continuous blood glucose monitors systems due to this fact are:

Continuous systems must be graduate with a traditional blood glucose measurement using current technology and therefore require both the CGM system and occasional “finger stick”.

Glucose levels in interstitial fluid lag temporally behind blood glucose values [3].

Patients therefore require traditional finger stick measurements for calibration typically twice per day and are often advised to use finger stick measurements to confirm high blood sugar before taking corrective action. Commonly, continuous blood glucose monitoring is not especially covered by health insurance in the United States in the same way that most other diabetic supplies are covered. Continuous blood glucose monitoring is an important component real-time automatic control of an insulin pump based on immediate blood glucose data from the sensor [3]. Continuous glucose monitoring is more expensive than conventional glucose monitoring, but they may enable better glucose control. CGM devices produced by Abbott, Dexcom, and Medtronic have been approved by the U.S.

3.2. RFID TO MEASURE THE GLUCOSE LEVELS

Diabetes is a major global disease, and with around 230 million people is diagnosed with this silent killer (American Diabetes Association, 2009). Awareness this Digital Angel had created syringe-implantable glucose-sensing RFID microchip for diabetic patients. The implantable bio-sensor chip prepared with a glucose microchip to painlessly scan it and determining their level of glucose concentration. The glucose data is exactly transmitted back by the RFID microchip to a wireless scanner which displays the level of the glucose. The product has received a patent form the U.S. Patent and trademark office and would be marketed by verichip [4]. In this case the implantable bio sensor chip consist of passive transponders, a sensor and an integrated circuitry therefore one can scan it without pain for checking their blood sugar levels. The chip transfer the data back to the wireless scanner displays the glucose levels [5].

Figure 3.2. Blood Glucose Monitoring System [5]

Commonly diabetics are demoralized by needle pricking system and the pain and costs connected with it keep them from testing their glucose levels as per the requirement which can at times prove to be necessary. In case of this glucose testing system the glucose sensors come with a tiny heating element and when the element warms up to around 130°C for 30 milliseconds it burns the outer level of dead skin cells which enables access to interstitial fluid beneath the layer to assess the body glucose levels [6].

Figure 3.2.1. Test Trip [6].

3.3. THE SEARCH OF NONINVASIVE GLUCOSE METERS

Blood Glucose Monitoring become most common place from the early 1980’s through the early 21st century, there was still blocking to its acknowledgment by many people, largely for the reason that, no matter how small the blood drop or how fast the test, there was no way to obtain a sample other than to stick a needle-sharp lancing device into part of the body to get blood.

The first electronic blood glucose device could be purchased in about 1970 for about $400. Unfortunately it had some reliability problems, basically from its rechargeable lead-acid batteries, and its use didn’t become widespread [7].

Figure 3.3: the electronic strip reader to appear was in about 1972 [7].

The next electronic strip reader to appear was in about 1972, called the Eyetone, and was manufactured by a Japanese company, Kyoto Dai-ichi (which later changed the company name to Ark-Ray). It also read Dextrostix, but used a plug-in AC adapter for power alternatively batteries. In about 1979, Kyoto Dai-ichi established an improved Dextrostix meter with a digital readout, called the Dextrometer [7].

Figure 3.3.1: Accu-Check bG in about 1982, Dextrostix meter with a digital readout [7]

Bochringer Mannheim, which had developed a parallel blood glucose test strip for visual color comparison called the Chemstrip bG, kept pace by introducing a meter to read the strips, The Accu-Check bG in about 1982. An early version was developed by the Bio Dynamics Company in Indianapolis and introduced as the stattek in 1974, and the company was quickly purchased by Boehringer [7].

Figure 3.3.2: Accu-Check 2 [7].

Some of the early GlucoScan meters had their own accuracy problems, but they continued the company until it was purchased by Johnson & Johnson in 1986 and it introduced entirely new technology in 1987 with the one touch meter and strip. The meter shown below at left is the one touch II the meter on the right is the one touch basic [7].

Figure 3.3.3: “Second Generation” blood glucose meters [7].

Early versions of the device had both efficiency safety problems, which embarrassed its early market acceptance.

Figure 3.3.4: Meters and Strips [7].

Meters and Strips have continued to disclose, with test times being reduced to only a few seconds, and blood samples as small as 0.3 microliters (Dextrostix used a drop of about 50 microliters, so the reduction in blood drop size has been about factor of 150) [7].

3.4. GLUCOSE TESTING

These below methods for non-invasive monitoring all remain in the process of study; there are limitations that exist for each of these methods. Because of the limitations that exist for various methods of non-invasive monitoring, a non-invasive method that would be available for everyday use by the public is far from realization. Everyday research organizations and institutions use new and different technology in hopes of creating a device that can be used by diabetic’s everyday as a daily non-invasive way for measuring their blood glucose levels [13].

Near Infrared Spectroscopy (NIR)

Raman Spectroscopy

Photo acoustic Spectroscopy

Scatter Changes

Polarization Changes

Mid-Infrared Spectroscopy

Absorption or emission data in the 0.7 to 0.5 μm region of the spectrum are compared to known data for glucose

Laser light is used to induce emission from transitions near the level excited

Laser excitation of fluids is used to generate an acoustic response and a spectrum as the laser is tuned

The scattering of light can be used to indicate a change in the material being examined

The presence of glucose in a fluid is known to cause a polarization preference in the light transmitted

Absorption or emission data in the 2.5 to 25μm region are examined and used to quantities glucose in a fluid

Table 3.4: Various techniques for non-invasive measurement of glucose using optical methods [13]

3.5. RFID SYSTEM WORK

An RFID system consists of a tag made up of a microchip with an antenna, and a reader with an antenna. The reader sends electromagnetic waves. The tag antenna is tuned to receive the electromagnetic waves. RFID is one of automatic technology to collect product place, time or transaction data quickly and easily without human intervention or error. RFID has been around since the late 60’s of the twentieth century but it is only recently technological advances have both brought down the cost and allowed its use for far more applications [9].

Figure 3.5: RFID tags [10]

This is how RFID works, step by step:

A processor controls the RFID sender/receiver controller.

An antenna sends high frequent energy with optional encoded information to the transponder.

In the transponder, the high frequent energy is being converted to an electrical charge, which is saved.

This makes the transponder answer with its own, unique encoded information.

The reader unit receives the transponder’s answer this information can be processed depending on the application.

3.6. RFID FOR ACTIVE MONITORING OF BLOOD GLUCOSE LEVELS

The most common method of monitoring blood glucose levels (finger sticks) depend on active patient participation and attachment to a schedule of monitoring activity. Radio Frequency Identification is not a single technology. A basic RFID system is collected of a tag to carry data and a reader (interrogator) with an antenna. The tag is made up of a chip (an integrated circuit) attached to an antenna. The most common devices operate at different radio frequency ranges: low frequency (LF) at about 128-135 kHz, high frequency at about 1356MHZ, and ultrahigh frequency (UHF) at 860-960MHZ. Each frequency range has different characteristics in terms of RFID performance [8].

A blood glucose sensor-enabled LF RFID tag would be implanted internal in a convenient but “private” location such as the inside of the upper arm. VeriMed is apparently still working on developing a glucose monitor integrated with its implantable, LF RFID tag. The VeriMed RFID tag is implanted using a large-gauge veterinary needle and requires no surgery and no external leads or connections. This RFID tag is used for companion animal identification under the Digital Angel brand [8].

Figure 3.6: Intermediate phase with “patch” communicating only to mobile device [8].

Recent developments may have already made the near term approach with an implantable sensor obsolete. A minimally invasive blood glucose sensor that measure tissue fluid has been developed by Georgetown University and SAIC. This process by using remotely read temperature sensor that is worn on the skin.

The Gentag RFID Company is working with SAIC to join a passive RFID device with a unique identification number and geo-location capabilities with glucose sensor. The aim is to produce a thin, patch, similar to a nicotine patch, incorporating both the sensor and the RFID transmitter that will sample blood glucose levels and transmit that data, securely, to the patient’s mobile device. Mobile phone manufactures are currently integrating a variety of NFC, HF, and UHF RFID readers into their devices [8].

Figure 3.6.1: Temperature sensor being read by a specially equipped mobile device with an add-on RFID interrogator [8].

3.7. AN IMPLANTABLE GLUCOSE SENSOR

The glucose sensor is a rice grain-sized solar powered implantable sensor, which hides underneath the skin for about a year at a stretch and measures the blood glucose levels consistently all the while the sensor send data to a variety of portable electronic devices such as cell phones and PDA’s alerting the patient each time the blood sugar levels go high [10].

Figure 3.7: Implantable glucose sensor powered by solar energy [10].

3.8. BLOOD GLUCOSE MONITORING

Blood Glucose Monitoring is a method of measuring the concentration of glucose in the blood. Basically important in the care of diabetes, a blood glucose test is performed by pricking the skin, or on the finger to draw blood, then applying the blood to a chemically active disposable “test-strip” [11].

Figure 3.8: Blood glucose testing, showing the size of blood drop required by modern meters [11].

Self monitoring blood glucose has been recommended for people with diabetes and their healthcare professionals in order to achieve a specific level of glycaemia control and to prevent hypoglylemia [12].

CHAPTER-4

4. DIELECTRIC PROPERTIES

4.1. DIELECTRIC STRENGTH

All insulating materials fail at some level of applied voltage, and ‘dielectric strength’ is the voltage a material can withstand before breakdown occurs. Dielectric strength is measured through the thickness of the material and is normally expressed as a voltage gradient. The value of dielectric strength for a case is also influenced by its temperature and ambient humidity, by any voids or foreign materials in the case, and by the conditions of test, so that it is often difficult to compare data from different sources. Test variables include electrode configuration and case geometry, and the frequency and rate of application of the test voltage [15].

Another test term sometimes used is ‘intrinsic dielectric strength’, which is the maximum voltage gradient a homogeneous substance will withstand in a uniform electric field. This shows the ability of an insulating material to resist breakdown, but practical tests produce lower values for a number of reasons [15].

4.2. DIELECTRIC CONSTANT AND PERMITTIVITY

The simplest capacitor structure is a pair of parallel conducting plates separated by a medium called the ‘dielectric’. The value of the capacitance between the plates is given by the equation [15]:

Where

A = the area of the plates

T = the separation between the plates

And ‘ε’ is the absolute permittivity of the dielectric, which is a measure of the electrostatic energy stored within it and therefore dependent on the material.

A more usual way of writing the equation is to replace the absolute permittivity of the dielectric by the product term ε0εr, where e0 is the permittivity of free space, which has a value of 8.85Ã-10-12 Fm-1, and er is the relative permittivity, more usually called the ‘dielectric constant’. The dielectric constant is the ratio of the capacitance of a capacitor containing that material to the capacitance of the same electrode system with vacuum replacing the insulation as the dielectric medium. The dielectric constant of any given material varies with temperature, and for polymers a rapid increase begins near their glass transition temperature dielectric constants also vary as a function of frequency [15].

4.3. DIELECTRIC PROPERTIES OF MATERIALS

The absolute complex permittivity of a material is represented by the symbol, where = ′ − j. This is related to the dimensionless relative complex permittivity r, where r = ′r − j″r, by the expression = 0r, 0 being the permittivity of free space a fixed constant given approximately by0 = 8.85 x 10−12 F m−1. In general, depends on temperature and to a lesser extent, pressure. It is also frequency department, although ′ and ″ cannot vary independently with frequency, since their frequency variations are connected through the kramers-kronig relationship. A drop in ′ with increasing frequency is necessarily associated with a peak in [16].

A capacitor filled with a dielectric material has a real capacitance ′r times greater that would have a capacitor with the same electrodes in vacuum. The dielectric – filled capacitor would also have a power dissipation W per unit volume at each point when, resulting from an applied voltage, a sinusoidal electric field of frequency F and root-mean-square value E exists at that point. This power dissipation is given by W= 2Ï€fE2″. Where ″ is a measure of the energy dissipation per period [16].

4.3.1 Liquids

The permittivity in this table, except when a frequent is stated, is ‘static’ values, relating to frequencies high enough to exclude ionic conductivity. For non-polar liquids (r′ 2) the lower limit depends only upon ionic impurities, while the higher is usually above 10GHZ within these limit, the permittivity is constant, and the loss unlikely to exceed a few times10−4. For polar liquids, denoted by ‘P’, the lower frequency limit depends both upon purity and the intrinsic dissociation of the liquid, while the upper limit varies sharply with temperature. More extensive data and on many more liquids [16].

4.3.2. Water

Water is strongly polar, with a region of dispersion; at 20 °C centred around 17GHZ. It is also mostly dissociated, so that even de-ionized water cannot be treated as a dielectric at frequencies much below 1MHZ. Measurements at the high frequencies of the dispersion range contain, before about 1953 many errors shown by values, of′ and ″ mutually inconsistent with the simple Debye equations. The static relative permittivity as a function of temperature, within the range 0-60°C, is given with an accuracy of ±0.1 unit by

s = 88.15 – 41.4θ + 13.1θ2 − 4.6θ3

Where θ = Celsius temperature/100°C

The relaxation time as a function of temperature is as follows, with an accuracy of ±2%:

The ‘infinite frequency’ dielectric constant, ∞, occurs in the infra-red, and cannot be directly measured electrically. A value of 5.0 is appropriate for use with the foregoing dat. r decreases continuously from this value throughout the infra-red to a value of 1.8 in the optical region [16].

4.3.3. Gases and vapours

The values relate, excepting the final entry, to a pressure of one standard atmosphere, and hold for all frequencies below the start of the infra-red spectrum other values may be calculated over a limited range of temperature and pressure, for non-polar permanent gases, by assuming that (r -1) is proportional to density. This does not hold for polar gases, but if the polarity is strong (e.g. water vapour) a close approximation is

(r -1) pressure/(absolute temperature)2

Provided that the vapour is not near its condensation point, under the conditions either of the data used, or of the desired result. Values of relative permittivity may also be obtained from the data on refractive indices at radio frequencies by using the relation μrr = n2 which applies to non-absorbing gases. μr = 1 for all gases expect O2 where μr = 1 + 1.9 Ã- 10−6 [16].

The dielectric properties of the various materials used in semiconductors fabrication and packaging play an important role in achieving the desired performance of integrated is its permittivity. It is a measure of the ability of a material to be polarized by an electric field [17].

4.4. Dielectric properties of tissues at microwave frequencies

This section deals with some basic techniques of dielectric spectroscopy and the dielectric spectrum of a biological tissue. The dielectric properties of materials are obtained from their measured complex relative permittivity, which being a relative quantity, has o unit, it is expressed as

Where j = √-1 is the relative permittivity, a measure of the charge displacement and consequent energy stored in the material, and….. is the out-of-phase loss factor, a measure of the electrical energy dissipated. In a perfect dielectric material, losses are due to displacement currents and the loss factor…….can be expressed in terms of a displacement electrical conductivity……

The total conductivity of the material…is given by

………………

And is related to the loss factor through the expression

………………..

Where….. is displacement electrical conductivity,……….is ionic conductivity.

In practice it is only possible to measure the total conductivity… Where present…, which is frequency independent, can only be obtain from dielectric spectral analysis. In the loss factor expression,…..is the permittivity of free space and .. the angular frequency of the field. The dielectric properties of a biological tissue are a measure of the interaction of electromagnetic radiation with its component at the cellular and molecular level. The main features of the dielectric spectrum of a biological tissue are as follows [18]:

The dielectric properties of tissues are highly frequency and temperature dependent. Their dielectric spectrum consists of three main regions known as α, β and γ dispersions, characteristic referred to a occurring at low, intermediate and high frequency in the frequency range from hertz to gigahertz. The low-frequency dispersion in the hertz to kilohertz range is combine with ionic diffusion, extends over 3-4 frequency decades centred in the hundreds of kilohertz region, and is due mainly to the polarisation of cellular leaf and organic macromolecules [18].

The γ dispersion, in the gigahertz region, is due to the molecular polarisation of tissue water. The study of both α and β dispersions is outside the scope of this project as the frequency range investigated extends from 50 to 2000MHZ, revealing the toil end of the β dispersion and a good part of the γ dispersion [18].

4.5. QUICK CAREFUL METHOD FOR MEASURING THE MICROWAVE DIELECTRIC PROPERTIES OF SMALL TISSUE SAMPLEST

The developing techniques of passive and active microwave imaging for disease exposure and treatment monitoring require proper knowledge of body tissue dielectric properties at the lower microwave frequency 1.5-6GHZ. The relative permittivity, or dielectric constant, and the loss factor, or equivalent conductivity of tissue, dispose the radiation or spatial response patterns of antennae used to induce or respond to microwave radiation in a viewed tissue volume, and the distance through tissue which radiation is transmitted. Various measurement techniques are available for high frequency dielectric measurements. Sample-cell terminated techniques are available for high frequency dielectric measurements in the 100MHZ to 1GHZ region, but are difficult to apply accurately at higher, microwave, frequencies. The use of an open-ended coaxial line terminated by an extended tissue sample is popular because the method allows measurements to be made over a wide frequency range. This method can be applied at the lower microwave frequencies, but requires complicated measuring equipment, careful arrangement and the use of fairly large tissue samples. The method is also affected to tissue in homogeneity and gives a measurement of dielectric behaviour strongly weighted to the part of the sample adjacent to the coaxial line end [19].

CHAPTER-5

5. AN OVERVIEW OF DIELECTRIC PROPERTIES MEASURING TECHNIQUES

5.1. DIELECTRIC PROPERTIES OF ANIMAL TISSUE IN VIVO

Information on the dielectric properties of tissue is of importance for several reasons. At the molecular level, this information helps to develop an understanding of the basic biophysical interaction mechanisms of electromagnetic fields with living matter. The permittivity data is needed at the macro scope level for doss metric calculations. Doss metre facilitates the determination of the specific absorption rate (SAR) of the radio frequency (RF) or microwave (MW) radiation in an irradiated biological body or its model. For doss metre purpose, knowledge of the differences in the permittivity of the same tissue type in various species is important as experiments to determine the biological effects of RF and MW radiation are carried out on various animals and the results are scaled and extrapolated to fit a human model [20].

The permittivity of biological substances and tissue has been investigated over many years. Many tissues has were measured by cook (1951), followed by extensive work by Schwann and his colleagues (Schwann 1957).

A convenient and accurate method for measurements of the tissue permittivity in vivo was developed, and recently modified and improved. This method was used to determine the permittivity of various tissues of cats and rats at frequencies between 0.1 and 10GHZ [20].

5.1.1. MATERIALS AND METHODS

The measurement method is based on determination of the input reflection coefficient depends on the tissue permittivity, frequency and dimensions of the coaxial line. This method was selected as the best suited to measure tissues in vivo. The probe is very small and the tissue has to be available to the probe from one side only [20]. The selected measurement method compares favourably with other methods.

The probe dimensions are selected to optimise the measurement certainly within the frequency range and tissue of interest. The optimum capacitance of the open line, which depends solely upon the line dimensions (when only the fundamental TEM mode propagates in the line), is shown in below figure as a function of frequency for two types of tissues. This figure also shown is the capacitances for various typical coaxial lines. Other factors such as the propagation of higher order modes in the coaxial line may additionally affect the choice of the probe.

To measure the input reflection coefficient of the probe at frequencies between 0.1 and 10GHZ a computer controlled system based on a network analyser was used. The system consisted of an HP8410B network analyzer, HP8745A and HP8743A relay actuator, an HP59313A A/D converter, and a frequency counter, all controlled through the HP-IB bus by an HP 9825 desktop computer [20].

Commonly, and in previous in vivo measurements a standard calibration and error correction of the network analyser was used. In that technique, three known terminations (a short circuit, an open circuit and a matched load) are placed at the reference plane, the plane where the sample is later located. The input reflection coefficients obtained for the three terminations are used to correct for the system imperfections, are used to correct for the system imperfections, i. e, directivity, frequency tracking, and reflections within the system including those of the connectors.

The in vivo measurements were performed upon anaesthetised animals, three cats and five rats. The cats had an average weight of 2.7kg. Each was premeditated with Atrovet 0.25ml 1M. For each animal and each tissue the probe was placed in at least three, and frequently up to five, locations. For each of the locations, at least 10 data points were obtained, at a given test frequency, at changing the frequency within a narrow range [20].

5.2. DIELECTRIC PROPERTIES OF BODY TISSUES

Studies of the ways in which tissues interact with electromagnetic energy are of importance not only to the continuing developments in electrical impedance imaging, but also to other medical areas such as radiofrequency and microwave hyperthermia, impedance pneumography and plethysmography. Measurements of the dielectric properties of biological materials have also provided important contributions to the biophysical and physiological sciences. The relative permittivity of biological tissue typically decreases with increasing frequency in three major steps which are designated as the α-, β-, and γ-dispersions, and an idealised representation of this is shown below figure [21].

The α-dispersion is generally considered to be associated with interfacial polarisations associated with electrical double layers and surface ionic conduction effects at thin layer of tissue boundaries. The β-dispersion in above figure is shown to have two components and, although such fine detail is not normally so evident, the purpose of representing it in this way is to indicate that this dispersion arises from at least two contributions, namely the capacitive shorting-out of membrane resistances, and rotational relaxations of bio macromolecules. The γ-dispersion arises from the relaxation of bulk water in the tissue.

Comparatably little work has been reported for the dielectric properties of tissue in the frequency range up to a few kHz, but a good example of the α-dispersion has been obtained by Singh et al (1979) for in vitro measurements on freshly excised kidney and in vivo measurements using external electrodes for normal breasts and breasts with malignant tumours. Some of these results are shown in below figure. Another good example of the α-dispersion for muscle tissue were obtained by Schwa (1954) and an important aspect of this work was the observation that the permittivity and resistivity measured at 1KHZ decreased steadily after excision of the tissue. This effect is consistent with the steady loss of integrity and physiological visibility of the cell is thin layer of tissues.

The α-dispersion is essentially associated with the heterogeneous structure inherent in membrane-electrolyte structures. At the boundary between two dissimilar dielectrics there is a build-up of charge and this gives rise to interfacial, or Maxwell-Wagner, polarisations. The magnitudes of these polarisations are dependent on the conductivity, permittivity and geometry of the separate components of the heterogeneous structure. For frequency higher than about 100 MHZ, where the capacitive shorting-out affects of the cell membrane resistances begins to take effect, the dielectric characteristics of tissues can be expected to reflect the properties of the inter- and intra cellular electrolytes and, in particular, to exhibit a dielectric dispersion associated with the relaxation of water dipoles. That this is the case can be seen from below two figures, where the relative permittivity and resistivity values of brain, fat and muscle for frequencies above 100MHZ are shown alongside those exhibited by 0.9% saline solution, which corresponds to the ionic strength of cellular fluid. For example, which in man can have water contents ranging from 73 to 77.6 wt%, exhibits a much larger permittivity and conductivity compared to that for whose water content can range from 5 to 20 wt% [21].

5.2. SKIN

The dielectric properties of skin are largely determined by the stratum corneum which has a thickness of the order 15µm and consists of layers of dead epidermal cells. The dielectric properties viewing considerable regional variability over the body, with the impedance being lowest in areas such as the palms where sweat ducts are in abundance. Rosendale (1945) measured the properties of wet, freshly excised, skin approximately 1mm thick at frequencies of 1KHZ and 10KHZ and obtained values for the effective capacitance and resistance of the stratum corneum of 4.6 nF cm-2 and 34.9 kΩ cm2, respectively.

The average in vivo electrical properties of skin over the frequency range 1HZ to 1MHZ, determined by Yamamoto and Yamamoto (1976), are shown in below figure.

An understanding of these properties was approached in terms of the inhomogeneous structure and composition of skin and the way in which this varies from the skin surface into the underlying dermis and subcutaneous tissue. As a unsure proposal the origin of these dispersions was considered to be located within the corneal cells [21].

5.2.2. BLOOD

The relationship between the resistivity (σ) of whole blood and haematocrit (H) is shown in below figure and follows the equation

σ = 68 exp (0.025 H %) Ω cm

No difference is found between the resistivity and haematocrit relationship for adult normal blood, neonatal and placental blood, indicating that factors such as membrane resistance are more important than the presence of fatal haemoglobin, for example. Deduce, time-expired bank blood exhibits a different relationship (Geddes and Sadler 1973) of the form [21]:

σ = 53.2 exp (0.022 H %) Ω cm

5.3. DIELECTRIC PROPERTIES OF BRAIN TISSUE BETWEEN 0.01 AND 10GHZ

Dielectric permittivity and conductivity are reported for grey and white matter from dog brain tissue between 0.01 and 10GHZ. Between 0.01 and ~ 1GHZ, the permittivity decreases and conductivity increases as a power law of frequency. Above 1GHZ, the conductivity increases quadratic ally with frequency due to dipolar reorientation of free water molecules in tissue; for studies on the biological effects of radiofrequency and microwave radiation, accurate information about the dielectric properties of tissues is important, for at least two reasons. On a macroscopic level, theses dielectric properties determine the energy deposition patterns in tissue upon irradiation by an electromagnetic field. The need for accurate dielectric data on brain tissue is particularly acute. Several studies have modelled the human head as a homogeneous for brain tissue, or as several concentric spheres representing skull and superficial tissue as well as brain, and have solved the electromagnetic field equations assuming plane wave irradiation [22].

Dielectric measurements at 100MHZ and below were performed on a Boonton model 250A RX meter, using a variable capacitance conductivity cell and measurements techniques described elsewhere (Schwan 1963). Measurements at UHF and microwave frequencies were performed using the Roberts and Von Hippel technique (1946), in which the sample-filled coaxial transmission line is terminated in a short circuit. Most of measurements below 7GHZ were made on a General Radio model 900 precision slotted line. A thin (0.15cm) nylon disc confined the sample next to the short, and a water Jacket surrounded the cell to control the temperature. A Vernier scale was used to measure the sample thickness to within 0.003cm. For these slotted line measurements, an additional computer routine corrected the data for the presence of the nylon disc by treating the nylon-filled line as a short length of transmission line of characteristic impedance transformations required by transmission line theory. A microwave network network analyser system (consisting principally of a Hewlett-Packard model 8410B analyser, 8743 reflection test unit, 8620C sweep oscillator and 5342A frequency counter) covers a wider frequency range, and can measure reflection coefficients more conveniently than with a slotted line. For these measurements, a sample holder was constructed from precision 7mm coaxial line was otherwise similar to that used for the slotted line measurements. A water jacket was mounted on the outside for temperature control. This procedure accurately compensates for reflections from the plastic disc supporting the sample, and for most of the residual errors arising within the instrument [22]. The tissue dielectric data obtained using the network analysers were in excellent argument with the slotted line results.