Suitability of tested materials to act as test objects

Materials have to satisfy both theoretical and practical requirements for calibrating a

multifrequency EIT system. Ideal test materials need to simulate the impedance properties of the different body tissues (see section 1.2.1.). They should be not only resistive, but also reactive.

Therefore, the impedance value of the test materials should vary with frequency. To resemble

brain tissues (see Table 1-2 for their impedance values), a characteristic frequency of some

hundreds of kHz, and an impedance modulus of some hundreds of Q.cm is required.

2.5.2.1. Assessment of inorganic materiais to act as test objects

For practical reasons, ideal materials should be inorganic, because they are stable, easy to

prepare and easy to handle. But none of the inorganic materials made of particles mixed with polyacrylamide gel produced significant reactivity. The maximum phase angle of non-biological materials was only 2.7° over 1 kHz -1 MHz. Therefore, inorganic materials do not appear to be

suitable for simulating the complex impedance characteristics of human tissues for calibrating

muitifrequency EIT systems. This was unfortunate as it would have been preferable if test objects could have been fabricated from stable inorganic materials, calibrated once, and then used long term without the need for re-calibration.

What needs to be addressed is why it so difficult to increase the phase angle or the ratio of reactance and resistance by increasing the electrical capacity of the tested polyacrylamide gel mixtures. In theory, a capacitor consists of two plates separated by an insulator, and its capacity

is calculated by the following formula 1.6 (see section 1.2.1.1):

= eoA/d (1.6)

where d and A are the thickness and surface area of the capacitor, respectively, i.e. the

polyacrylamide gel in our case, is the dielectric constant of the gel mixed with inorganic

particles, such as polystyrene, polymethylmethacrylate, silica or ballotini beads. Eq is the

capacitance of free space (8.86 x 10” '"^ F/cm). It might have been possible to increase the

phase angle or the ratio of resistance and reactance, if the thickness (d) was decreased by

increasing the concentration of the polyacrylamide gel; or the dielectric constant (e^ ) could have

been increased by mixing the gel with higher dielectric particles. However, this was not the case. One reason why the reactance of various inorganic materials could not be increased in the present experiments may be because inorganic particles have maximum dielectric constants of

and 1.005 in normal pressure air (Duck, 1990). In contrast, the dielectric constants of the cells of living tissues range from a few hundreds to a few thousands. The difference in the dielectric constant values between living tissues and inorganic particles is determined by the difference in

the complexity of the cellular structure, in particular the high capacity of the cellular membrane (see section 1.2.1.) and the homogeneous structure of the inorganic particles. To mix particles with such small dielectric constants into the polyacrylamide gel resulted in small increases in

phase angles.

On the other hand, a significant increase in reactance, for example, might be achieved by changing the surface area of interfaces (A). The results would depend upon the frequency of the applied current. However, within the frequency range for measuring changes in impedance of

living tissue of hundreds of kHz, such an approach could not be achieved.

2.S.2.2. Assessment of test objects formed with biological materiais

Test objects made of plant materials have been assessed because plant cells are practical and easy to obtain. Zhang et al., (1990) studied cucumber, onion bulbs, carrots, potato, green pepper, cabbage leaves, banana and mushrooms using a four-electrode method with

frequencies of 3 - 200 kHz. Their results showed that the capacitance of these plants were clearly frequency-dependent. Similar results were obtained in the present experiment: the

cucumber samples provided a large impedance change (33.4 to 3.9 Q.m) over a frequency

range of 1 kHz -1 MHz with a low central frequency of 10 kHz. It should be noted that in contrast to most animal cells, potassium and chloride ions are more common in plant cells (Raven at a!.,

1999). The ability to bind and transport in plant cells is 10 times higher than other ions

(Anderson and Beardall, 1994). Therefore, 0.05% KCI rather than NaCI solution, which had impedance values close to that of cucumber, was selected as the bathing solution for cucumber objects in the present study. This appeared to provide a stable medium.

Cucumbers cost less, are widely available, are easy to make samples and decay relatively slowly if they are kept in appropriate KCI solution. The impedances of the cucumber samples

used in the present experiments were stable (changed less than 15% or so) over several hours. It seems that cucumber samples may have met the electrical and practical criteria to make an

ideal test object which might be suitable for calibrating multifrequency systems, especially for lower frequencies.

However, cucumber objects had several limitations. Firstly, there was a variation in impedance

value up to 40% between cucumber samples. It will be necessary to measure the resistivity of

the cucumber samples and KCI bathing solution directly before any calibration studies. Secondly, the KCI solution for the cucumber was a resistive solution only, and therefore a large contrast in impedance existed between the cucumber object and the solution. This does not directly simulate the smoother changes in the biological tissues. The large interfacial impedance

contrast between a test object and its medium might pose difficulties in a tank intended to simulate smoother impedance transitions in physiological objects. Thirdly, it was noticed that resistance and phase angle of the cucumber drifted during the first hour after it was placed into the KCI solution. A possible explanation for this variation might be a change in the osmotic equilibrium of ions between the cucumber and the solution. Concentration of the KCI solution was selected to be 0.05% because it had the closest resistivity to that of cucumber at a frequency of 50 kHz. However, its osmotic pressure would have been lower than that of the

cucumber, as the concentration of normal saline is 0.09%. It took about an hour to obtain a stable resistance and phase angle after equilibrium occurred.

Samples of human blood are known to have dielectric properties (Foster and Schwan, 1989;

Fricke, 1925) similar to living tissues. Previous studies have found that whole blood samples have a central frequency of 1 - 2 MHz providing the impedance changes over the range of

frequencies normally applied in impedance measurement in human tissues. This was confirmed by the present results. Furthermore, because the sponge-blood samples provided small

impedance changes of 4.0 to 2.7 n.m over a range of frequencies, the small impedance

changes occurring during physiological activity were simulated more closely as both the test object and its bathing medium have significant capacitance. This is probably due to sponge

resemble the extracellular space in the living tissue (Nichoison and Sykova, 1998). Therefore, the sponge with packed blood cells resembled the structure of living tissues more closeiy and

provided impedance vaiues simiiar to that of iiving tissues. Thus the sponge-packed cell objects were reasonably stable over hours, as there was no equilibration between the sponge sample

and the blood cells. Further, the sponge could be melded or cut to any shape, could be handled more easily than gels, could be distributed in same format between labs, and could be stored

indefinitely.

However, the disadvantages of sponges are (1) the sponge-blood object has a high central

frequency over 1 MHz, which means that significant capacitance tends to occur at high frequencies. This is makes it more difficult to use sponges for testing a multifrequency EIT

system in the iow frequency range; (2) chemicals used for purposes such as rendering the material fire-retardant in polyurethane sponge may cause variation between samples; (3) preparation of the sponge-packed cell samples and whole blood solution requires the use of sterile handling procedures for the safety of the experimenters. This may be messy and difficult to carry out in non-hospital environments.

2.5.3. Direct measurement using a two-eiectrode method, and infiuence of eiectrode