Chapter 7 – Complementary Experimental Tools  303

7.7.2  ELECTRICAL AND THERMAL PROPERTIES OF TISSUES

Biological tissue contains both free and bound electrical charges and so has both electrically

conductive and dielectric characteristics, which varies widely between different tissue types

compared to other biophysical parameters. A comparison of, for example, the attenuation

coefficients of clinical x-​rays used in computer-​assisted tomography (CAT)/​computerized

tomography (CT) scanning, a biophysical workhorse technology in modern hospitals (see

the following section of this chapter), between the two most differing values from different

tissues in the human body (fat and bone), indicates only a difference by a factor ~2. Blood

and muscle tissue essentially have the same value, thus not permitting discrimination at all

between these tissue types on x-​ray images. The resistivity of different tissue types, however,

varies by over two orders of magnitude and so offers the potential for much greater discrim­

ination, in addition to a frequency dependence on the electrical impendence permitting even

finer metrics of discrimination.

Electrical impedance spectroscopy (EIS), also known as dielectric spectroscopy, in its sim­

plest form consists of electrodes attached across a tissue sample using sensitive amplification

electronics to measure the impedance response of the tissue with respect to frequency of

the applied AC voltage potential between the electrodes, which has been applied to a var­

iety of different animal tissues primarily to explore the potential as a diagnostic tool to dis­

criminate between normal and pathogenic (i.e., diseased) tissues. The cutting edge of this

technology is the biomedical tool of tissue impedance tomography, discussed later in this

chapter. A good historical example of EIS was in the original investigations of the generation

of electrical potentials of nerve fibers utilizing the relatively large squid giant axon. The axon

is the central tube of nerve fibers, and in squid these can reach huge diameters of up to 1 mm,

making them relatively amenable for the attachment of electrodes, which enabled the elec­

trical action potential of nervous stimuli to first be robustly quantified (Hodgkin and Huxley,

1952). But still these days similar EIS experiments are made on whole nerve fibers, albeit at

a smaller length scale than for the original squid giant axon experiments, to probe the effect

of disease and drugs on nervous conduction, with related techniques of electrocardiography

and electroencephalography now accepted as clinical standards.

Different biological tissues also have a wide range of thermal conductivity properties.

Biophysical applications of these have included the use of radio frequency (RF) heating, also

known as dielectric heating in which a high-​frequency alternating radio or microwave heats

a dielectric material through an induced dipole resonance; this is essentially how microwave

ovens work. This has been applied to the specific ablation of tissue, for example, to destroy

diseased/​dying tissue in the human body and to enable reshaping of damaged collagen tissue.

7.7.3  BULK MAGNETIC PROPERTIES OF TISSUES

Biological tissues have characteristic magnetic susceptibility properties, which is significantly

influenced by the presence of blood in the tissue due to the iron component of hemoglobin

in red blood cells (see Chapter 2), but can also be influenced by other factors such as the

presence of myelin sheaths around nerve fibers and of variations in the local tissue biochem­

istry. The technique of choice for probing tissue magnetic properties involves using magnetic

resonance to typically map out the variation of susceptibility coefficients χm across the extent

of the tissue:

(7.18)

M

H

m

= χ

where

M is the magnetization of the tissue (i.e., the magnetic dipole moment per unit volume)

H is the magnetic field strength

The technique of magnetic resonance imaging (MRI) is described in fuller detail later in this

chapter.