Electrical Impedance Tomography
Published on Feb 12, 2016
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.
X-RAY COMPUTED TOMOGRAPHY
Projection radiography suffers from the loss of depth information and the difficulties of detecting structural details that are partly hidden by overlying images of body areas that are not of interest. This problem is solved by selectively recording an image of a single plane in the body; the result is called a tomogram. An early technique of producing sectional views is motion tomography. By defined motions of the X-ray tube and the film during exposure, images are produced in which all but one predetermined plane are blurred. Thus, the projection shows a single plane with an added, approximately constant background intensity caused by the other planes. Contrast enhancement is not possible with this technique.
X-ray computerized tomography is a scheme for imaging human body cross-sections with very high resolution, so that the physician can view the structure of internal organs for diagnostic purpose. The cross-section to be imaged is illuminated using a source of X-rays. As the X-rays propagates through the tissues photons are continually lost from the beam either due to absorption or due to scattering which accounts for a constant at each point called the linear attenuation coefficient μ(x,y) of the tissues. The intensity of the exciting X-rays are collected using a ring of detectors placed at the opposite side of the X-ray source. The source detector arrangement is then rapidly rotated around the body and data for various angles are taken.
This probed visual information is called the projection data which are actually the line integral of the attenuation coefficient of the
tissues. The problem here is to mathematically reconstruct the image function μ(x,y) from the measured projection data. Drawbacks of this method are radiation hazards, comparatively costly, only transverse sections can be imaged and a single parameter is available for imaging.
MAGNETIC RESONANCE IMAGING
MRI is a clinically important medical imaging modality due to its’ exceptional soft tissue contrast. This technique is based on the fundamental property that protons inherently possess a magnetic moment and spin. When placed in a magnetic field the protons align either parallel or anti-parallel to the magnetic field. In MRI, also referred to as Nuclear Magnetic Resonance (NMR), the patient is placed inside a strong magnetic field that is usually generated by a large bore superconducting magnet. Nuclear Magnetic Resonance is utilized to obtain images as a function of proton spin density and relaxation times.
Application of a short pulse of circularly polarized radio frequency radiation, whose magnetic field vector is perpendicular to the applied magnetic field of flux density B0, causes the net magnetic component M to tilt away from the z-axis and rotate in the xy plane at frequency ω0, called Larmour frequency. This represents a source of detectable radiation at the frequency ω0. The recovery of the z-component gives rise to a signal, which is characterized by two decay mechanisms that are represented by the longitudinal and transverse relaxation times, T1 and T2 respectively. T2 relaxation, which is usually considerably faster than T1 relaxation, is due to a loss in phase coherence between neighbouring nuclei, caused by the local variations in the magnetic field, which arise from a local, tissue dependent, variation in magnetic susceptibility as well as a non-uniform magnetic field.
T1 relaxation is a result of the precessing nuclei losing their associated potential energy due to the coupling with the magnetic moments of the surrounding nuclei. Because of the nature of interaction, T1 provides information about vibrational motion in the lattice, which in biological tissues is usually water. These relaxation times are characteristic of different tissue types and can be measured by applying suitable sequences of RF pulses and measuring the radio frequency NMR signals with a receiver coil.
Data are collected by applying a current to the object through electrodes connected to the surface of the object and then making measurements of the voltage on the object surface through the same or other electrodes. Although conceptually simple, technically this can be difficult. Great attention must be paid to the reduction of noise and the elimination of any voltage offsets on the measurements. The currents applied are alternating currents usually in the range 10 KHz to 1 MHz.
Since, tissue has a complex impedance; the voltage signals will contain in-phase and out-of-phase components. In principle, both of these can be measured. In practice, measurement of the out-of-phase (capacitive) component is significantly more difficult because of the presence of the unwanted (stray capacitance between various parts of the voltage measurement system, including the leads from the data collection apparatus to the electrodes. These stray capacitances can lead to appreciable leakage currents, especially at the higher frequencies, which translate into systematic errors no the voltage measurements. The signal measured on an electrode, or between a pair of electrodes, oscillates at the same frequency as the applied current.
Various data collection schemes have been proposed. Most data are collected from a two-dimensional (2D) configuration of electrodes. The simplest data-collection electrodes (often an adjacent pair) and measure the voltage difference between other adjacent pairs. Although in principle voltage could be measured on electrodes through which current is simultaneously flowing, the presence of an electrode impedance generally unknown, between the electrode and the body surface means that the voltage measured is not actually that on the body surface. However, in many systems, measurements from electrodes through which current is flowing are simply ignored.
Electrode impedance is generally not considered to be a problem when making voltage measurements on electrodes through which current is not flowing, provided a voltmeter with sufficiently high input impedance is used, although since the input impedance is always finite, every attempt should be made to keep the electrode impedance as low as possible. Using the same electrode for driving current and making voltage measurements even at different times in the data collection cycle, means that at some point in the data-collection apparatus wires carrying current and wires carrying voltage signals will be brought close together in a switching system, leading to the possibility of leakage currents. Thus, separate sets of electrodes are used for driving and measuring in order to reduce this problem.
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