Electric Impedance Tomography (EIT)
  • EIT is a technically difficult problem to solve in terms of developing

hardware for data capture and the algorithms to reconstruct the images.

  • clinical aplications
  • examining hardware
  • reconstrucctions algorithms

Reseña

Primeros Usos

  • The first reported use of electrical impedance tomography (EIT) was in geological

studies as much as 70 years ago

  • industrial process tomography (IPT), to detect air bubbles in process pipes

or to monitor mixing processes

  • it is not possible to reconstruct an image slice by slice
  • The change in conductivity anywhere in the domain can affect all measurements,

not just those on a ray path. In the early days of EIT development, assumptions
were made to simplify the mathematics of the reconstruction process; however, this
assumption limits the accuracy of the reconstructed images.

Papel de la EIT

  • EIT has the potential to be of clinical value and play an important role in

diagnostics and monitoring of a number of disease conditions, from the detection of
breast cancer (4) to monitoring brain function (5) and possibly stroke.

  • A technique that

does not use ionizing radiation and, in principle, is safe and cheap would be of great
value. At present, EIT does not have the spatial resolution of other methods like MRI
or computer tomography (CT). Its key advantage is its temporal resolution, which is
in the order of milliseconds.

Principios Basicos

  • Bioimpedance of a human body produces a great deal of information about

the changes of the body during different kinds of activities.

  • For example, the measured

impedance is dependant on the amount of water in the body. This kind of information
would be very useful, for example, for athletes. The measurement of bioimpedance
is not just limited to tissue water. It includes the characterization and identification
of cells based on their impedance, which differs between cells based on the size, orientation,
and membrane thickness, among other factors.

  • This phenomenon has been exploited in EIT, where the impedance information is used to form images of the

contents of the human body. In EIT, it is possible to inject a range of currents at different frequencies.

Propiedades Eléctricas del Tejido.

  • Tissue consists of cells and membranes that are thin but have a high resistivity as

well as electrically behaving as small capacitors.

  • The impedance of biological tissues comprises two components, the resistance and reactance.
  • The conductive characteristics

of body fluids provide the resistive component, whereas the cell membranes,
acting as imperfect capacitors, contribute a frequency-dependence

  • The biological tissue impedance measurements

can be made over a range of low (20 Hz) to high (1 MHz) frequencies.

  • Bioimpedance can therefore be used to measure volumes, shapes, or tissue electrical properties (8).
  • Another consideration, which has yet to be fully addressed inEIT, is the anisotropic

nature of human tissue. Refers to properties that differ based on the direction that is measured. For example, an anisotropic antenna is a directional antenna; the power level is not the same in all directions. Contrast with isotropic.

  • Aunque una de las suposiciones que se hacen para esta técnica es que el objeto que se reconstrute es de naturaleza isotropica. .Esto puede ser una suposición demasiado fuerte pero hay pocas investigaciones respeto a esto y en campos relacionados.

Breve historia de desarrollos clinicos en la EIT.

  • EIT was first developed for nonclinical applications.
  • The first clinical images obtained using EIT were produced by the Sheffield group

(9), who developed a system that used the electrical impedance of various tissues
within the human body to produce tomographic image maps of the resistivity distribution.

  • The Sheffield Algorithm, as it came to be known

(10), produces images in a two-dimensional plane. Later Metherall (11) from the
same group produced the first three-dimensional EIT image that used a sensitivity
approach.

  • However the progress toward EIT being adopted as a clinical tool is slow, as there are many technical problems that need to be addressed.

Imagenes de EIT.

  • EIT is

capable of creating two types of image.

  1. The first is known as difference imaging, where a change in ratio or percentage represents some physiological parameter like blood volume or cell size, The Sheffield group produced the first lung images using

this method. The images are obtained by measuring two data sets at different times
or frequencies at the boundary of the object. This is then subtracted and divided by
a reference data set.

  1. The second is known as absolute imaging, which produces an image of the absolute

conductivity or permitivity. This absolute method of imaging is technically more
difficult than the difference method, as the contact impedance of the electrodes cannot
be accurately characterized when making clinical measurements.

Obtención de Datos.

  • One of the most well known is the Sheffield EIT datacollecting

system. In fact, some of the first clinical images were produced by the
Sheffield MK1 system

Imagen EIT

Revisar

Revisar es un libro donde viene eso

  • The Sheffield system consisted of 16 electrodes with a single current source. A multiplexer was used to inject currents with different pairs of electrodes and measure voltage on adjacent pairs of electrodes.
  • The system is capable of measuring data at 30 frequencies between

2 kHz and 1.6 MHz at 25 frames per second. It also requires triaxial cables to achieve
the required bandwidth.

  • The Sheffield system is by no means the only system to be developed. Each group in the field has produced different versions of hardware. The reason for this is partly due to the requirement of the reconstruction method employed by these groups.
  • Some would argue that multiple current drives are needed, although this also increases the complexity of the hardware.
  • This has given rise to a range of hardware for clinical applications, including Rensselar’s Adaptive Current Tomographs (ACT4) system, which is a fourth-generation EIT high-precision multifrequency system. The ACT4 is a modular design that can support up to 72 electrodes, with an excitation frequency that is selectable from a discrete set in the range from 300 Hz to 1 MHz. The instrument is able to apply either voltage or currents to all the electrodes simultaneously and respectively measure the resulting currents or voltages. It can also control both

the phase and amplitude of the voltage or current excitation. (14). Rensselar’s system
is mainly used to image pulmonary function.

Imagenes con ACT4
Imagenes ACT4 ( Nota al ver esta página quitar el slah final)

[http://news.rpi.edu/update.do?artcenterkey=100&setappvar=page(1) ]

Importante

Knowing that tumors conduct electricity five to 10 times more readily than normal tissue, the team has developed the latest prototype of the adaptive current tomograph, ACT4. The system, based on an emerging technology called electrical impedance imaging, delivers real-time images of the electrical state of the body’s inside from measurements made on the outside.

  • The Dartmouth group (15) has produced a system for breast imaging. It is a multifrequency

modular system using digital signal processing (DSP) with 64 channels. It
measures magnitude and phase using methods implemented in software. The system
(Figure 1) has a circular electrode array with a frequency range of 10 kHz to 10 MHz. It has a reported signal-to-noise ratio of above 100 dB and collects high-speed multiplane
data at video rates of 30 fps. (frames per second)

Protocolos de Inyección de Corriente.

  • The methods of injecting current into the domain under investigation can vary. They

can be classified into adjacent, opposite, and optimum injection (2). The chosen
method again depends on the application. For imaging the human brain, the criterion
was to maximize the current density in the brain region.

Fuentes de Error calibración y validado.

  • It is of key importance that the hardware systems are calibrated to obtain the maximum signal

and minimum error.

  • The principal sources of error in the front end arise from common mode effects such as skin-electrode contact

impedance and stray capacitances (21–25). The final error for each measurement is dependent on the complex interaction of these effects, and will differ for each electrode combination.

Algoritmos de Reconstrucción.

  • At this point, it is

worth introducing some of the terminologies used in EIT. As previously indicated, the
most common method of obtaining an image of impedance or change in impedance is
made by measuring voltages on the surface of an object of known geometry, through
which a known current has been passed, to determine the distribution of conductivity
inside that object.

  • Many researchers reduce the complexity of the reconstruction problem by making a number of assumptions.

The first of these is to assume the reconstruction is quasistatic (29). This means that the effect of magnetic induction can be ignored in Maxwell’s equations.

Ecuaciones que dirigen la EIT

**One of the key questions asked in EIT is, Do we have a unique solution? Is it possible in EIT that more than one conductivity distribution could produce the same boundary voltage measurements V within their precision? The answer is yes. If measurements were made to infinite precision and the entire object surface was sampled continuously, the solution would be unique. However, there is loss of information in the imaging process because data is discretely

sampled and noisy.**

Lo que se requiere para obtener precision en las reconstrucciones de EIT

  • An accurate EIT reconstruction requires a model capable of

predicting the voltages on the electrodes for a given conductivity distribution. The
forward model must be capable of predicting the electric fields in the interior given
the conductivity.

Aplicaciones clínicas

Función pulmonar

  • EIT could be very useful for imaging ventilation and detection of blood clots in the lungs or pulmonary emboli.
  • Planar X-ray, X-ray CT, MRI, and radioisotope scintigraphy can all be used, with high spatial resolution, to image pulmonary ventilation and perfusion. However, the patient must be brought to the device and repeatedly exposed to radiation, which is undesirable. For these reasons, EIT is very useful for such an application as it can be brought to the bedside.

It has even been used during parabolic flights to determine regional ventilation and fluid shift signals in lateral posture during normo-, hyper-, and microgravity (61).

Problema importante

However, one problem is that as well as being sensitive to lung impedance changes, the technique is also sensitive to the ribs and to tissue movement, which can be large sources of artifact.

Ejemplo de Perro: The group at Rensselaer has developed an adaptive current tomography (ACT) system with which
they imaged a simulated pulmonary embolus in a dog, first ventilating one lung at a time and then occluding a major branch of the pulmonary artery. The region of the lungs that was ventilated but not perfused by blood had different electrical properties and, therefore, was observable in a time-varying conductivity map of the
thorax.

Volumen de sangre torácico

EIT may be used to measure cardiac output or isolated perfusion defects, suggesting a pulmonary embolism and the requirement of anticoagulants. Advantages and disadvantages of EIT over other techniques are similar to those for its use in pulmonary monitoring.

In EIT of the thorax, it is possible to perform cardiosynchronous averaging and produce dynamic impedance images describing different times in the cardiac cycle

Cáncer de Seno.

The properties of many tumors, in particular, those exhibiting malignancy, differ significantly from the surrounding
tissue, and separation and characterization of contrasting tissues can deliver early diagnosis.

Buena página de EIT sobre cancer de seno

Hipertermia.

  • Reduction of tumor size is possible in certain malignancies using temperature treatment or hyperthermia. This involves concentrated heating of the tumor tissue to approximately 47◦C without damaging the surrounding tissue.
  • A noninvasive form of thermometry would be valuable to ensure that the heating remains focused only on the malignant region. Because there is a linear relationship between temperature and tissue resistivity [2%/◦C (68)], EIT may be useful in this application. However, it has been found that there are artifacts in the resistance images owing to other physiological factors and that they are significant compared with the temperature-dependent changes.

Funcion Gastro Intestinal.

  • En esta aplicación és en la que ha habido mejores resultados.
  • Dynamic EIT images are much more desirable, however, because the technique avoids the use of radioactive isotopes and intubation is uncomfortable. Instead, electrodes can be attached easily around the abdomen, where there are no bony structures to obstruct current flow, and EIT can be used to follow meals such as porridge (74) or meals labeled with saline.

Imagenes de Cerebro

  • In terms of clinical applications of EIT for the human brain, there are two areas that have shown great potential. The first is application for epilepsy. Focal epilepsy is a functional abnormality often related to a structural abnormality, such as a lesion, and an individual’s seizure always originates from the same cerebral focus.
  • For patients with intractable epilepsy, surgery is required to remove that part of the brain and accurate localization of the epileptic focus is crucial before surgical excision.
  • As mentioned above, the repetitive activity owing to a focal seizure can cause local ischemia, detectable using EIT.
  • Continuous monitoring is necessary because the occurrence of seizures is so unpredictable; therefore, EIT may be better suited than fMRI for localization of epileptic foci.
  • Patients have their epilepsy-suppressing medication suspended for a week prior to surgery, during which time they are monitored with EEG to gain as much electrical information as possible during seizure activity. This technique, known as telemetry, is quite successful for localization of superficial foci, but less so for foci situated deeper in the brain. For these, EEG electrodes are often implanted deep in the brain, a process that can cause irreversible damage. EIT may provide a more powerful, as well as less invasive, alternative for their localization.
  • As a means of validating the use of EIT on the human brain, the UCL groupperformed a number of measurements from human volunteers who were subject to visual, motor, and somatosensory stimuli to assess the applicability of EIT to the problem of imaging functional activity in the human brain (80). Impedance increases (conductivity decreases) and impedance decreases (conductivity increases) in the brain can occur as a result of changes in blood volume, blood having a lower impedance (higher conductivity) than the surrounding tissue.

Variaciones de la EIT

Magnetic Induction Tomography.

Magnetic Induction Tomography Graz group

Magnetic Resonance Electrical Impedance Tomography

  • The second variation is a combination of MRI and EIT known as MREIT (magnetic resonance electrical impedance tomography). In MREIT, a reconstruction of the cross-sectional current density and conductivity (or resistivity) images of human body with high spatial and contrast resolution can be obtained.

Puntos Importantes.

  1. Key potential clinical uses for EIT are gastrointestinal function, breast, and brain imaging; however, further development is needed before it can be adopted as a clinical tool in regular use.
  2. A higher level of investment is needed for hardware development to maximize the performance of theses systems and hence reduce theCMRRerrors.
  3. The use of electrodes presents a considerable challenge in terms of accurate placement and minimizing the effects of contact impedance. The new variations presented in this review could have considerable advantages, as the need to deal with variations in geometric shape and the possibility of noncontacting solutions would greatly reduce the technical difficultly associated with EIT systems.
  4. EIT is a nonlinear problem; however, at present only reconstruction algorithms that use linear approximations have produced clinically useful images.

Problemas no resueltos y futuro de la técnica.

  1. There is still a need to use the best available algorithms when reconstructing clinical data, and there are still groups using algorithms that have welldefined weakness in them. One of the key problems is the use of 2-D solution

to solve a 3-D problem.

  1. The issues of anisotropic properties of tissues must be fully addressed.
  2. Researchers must incorporate all available prior information, including the

boundary shape and electrode positions.

  1. Investigation of the use of hybrid methods to solve the forward problem is needed. If the forward problem is well defined, then the solution to the inverse will be simpler.
  2. Development of EIT or one of its variations as a viable clinical tool is also needed. Currently, the best hope for this appears to be breast imaging; there is also great potential for its use in gastrointestinal monitoring.
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