| Electrical
Impedance Tomography |
INTROUCTION to Electrical Impedance Tomography
To begin with, the word tomography
can be explained with reference to 'tomo' and 'graphy'; 'tomo' originates from
the Greek word 'tomos' which means section or slice, and 'graphy' refers to representation.
Hence tomography refers to any method which involves reconstruction of the internal
structural information within an object mathematically from a series of projections.
The projection here is the visual information probed using an emanation which
are physical processes involved. These include physical processes such as radiation,
wave motion, static field, electric current etc. which are used to study an object
from outside. Medical
tomography primarily uses X-ray absorption, magnetic resonance, positron emission,
and sound waves (ultrasound) as the emanation. Nonmedical area of application
and research use ultrasound and many different frequencies of electromagnetic
spectrum such as microwaves, gamma rays etc. for probing the visual information.
Besides photons, tomography is regularly performed using electrons and neutrons.
In addition to absorption of the particles or radiation, tomography can be based
on the scattering or emission of radiation or even using electric current as well. When
electric current is consecutively fed through different available electrode pairs
and the corresponding voltage, measured consecutively by all remaining electrode
pairs, it is possible to create an image of the impedance of different regions
of the volume conductor by using certain reconstruction algorithms. This imaging
method is called impedance imaging. Because the image is usually constructed in
two dimensions from a slice of the volume conductor, the method is also called
impedance tomography and ECCT (electric current computed tomography), or simply,
electrical impedance tomography or EIT. Electrical
Impedance Tomography (EIT) is an imaging technology that applies time-varying
currents to the surface of a body and records the resulting voltages in order
to reconstruct and display the electrical conductivity and permittivity in the
interior of the body. This technique exploits the electrical properties of tissues
such as resistance and capacitance. It aims at exploiting the differences in the
passive electrical properties of tissues in order to generate a tomographic image. Human
tissue is not simply conductive. There is evidence that many tissues also demonstrate
a capacitive component of current flow, and therefore, it is appropriate to speak
of the specific admittance (admittivity) or specific impedance (impedivity) of
tissue rather than the conductivity; hence, electric impedance tomography. Thus,
EIT is an imaging method which maybe used to complement X-ray tomography (computer
tomography, CT), ultrasound imaging, positron emission tomography (PET), and others.
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