Electrical impedance tomography (EIT) is a new imaging technique based on the different electrical conductivities of different areas of the body. Many potential applications are still in the experimental stage. Its use has been proven in testing lung function.
What is electrical impedance tomography?
Electrical impedance tomography has already established itself in pulmonary function diagnostics. Using electrodes, alternating electric currents of different frequencies and low amplitude are injected into the adjacent tissue. As a new non-invasive imaging technique for the examination of human tissue, electrical impedance tomography (EIT) has already established itself in pulmonary function diagnostics. For other applications, EIT is on the verge of a breakthrough. Using electrodes, alternating electric currents of different frequencies and low amplitude are fed into the adjacent tissue. Depending on the nature or functional state of the tissue, different conductivities result. These depend on the respective impedance (AC resistance) of the corresponding body area. Several electrodes are positioned on the body surface to be measured. While high-frequency alternating currents with small amplitude flow between two electrodes at a time, the electrical potential is measured at the other electrodes. The measurement is repeated continuously by varying the stimulating electrode pair as desired. The measured potentials produce a cross-sectional image that allows conclusions to be drawn about the composition and condition of the tissue under examination. In electrical impedance tomography, a distinction is made between absolute and functional EIT. Absolute EIT examines the composition of the tissue, whereas functional EIT measures different conductances depending on the particular functional state of the body area being measured.
Function, effect, and goals
As mentioned earlier, electrical impedance tomography is based on the different conductivity of different body areas, biological tissues or organs. Thus, there are well-conducting and poorly-conducting body areas. In the human body, conductivity is determined by the number of free ions. For example, a water-rich tissue with a high concentration of electrolytes is expected to have better conductivity than a fatty tissue. In addition, if there are functional changes in the organs, there may also be chemical changes in the tissue that affect conductivity. Absolute EIT is inaccurate because it depends on individual anatomy and poorly conducting electrodes. This often results in artifact formation. Functional EIT can significantly reduce these errors by subtracting the representations. The lungs are particularly suitable for examination by electrical impedance tomography because they have much lower conductivity than most other organs. This results in an absolute contrast to the other parts of the body, which has a positive effect on imaging. The conductivity of the lungs also changes cyclically depending on whether the patient is inhaling or exhaling. This is another reason to study the lungs in particular using EIT. Their varying conductivity during breathing suggests good results when testing lung function. Advances in digital technology make it possible for intensivists to have the data obtained from lung conductivity measurements processed so that lung function can be visualized directly at the patient’s bedside. Lung function monitors based on electrical impedance tomography have recently been developed and are already being used in intensive care medicine. Studies are currently being conducted to open up other possible applications for EIT. For example, this technology may play a role in the future as an adjunct diagnostic for mammography. It has been found that normal and malignant breast tissue have different conductivities at different frequencies. The same applies to additional diagnostics in gynecological cancer screening. Studies are also currently underway on the possible use of EIT in epilepsy and stroke. A future application for intensive medical monitoring of brain activity in severe brain pathologies is also conceivable.The good electrical conductivity of the blood also implies a possible application for imaging organ perfusion. Last but not least, electrical impedance tomography can also serve in the context of sports medicine to determine oxygen uptake (Vo2) or arterial blood pressure during exercise.
Risks, side effects, and hazards
Compared to other tomography methods, electrical impedance tomography has the advantage that it is completely harmless to the organism. No ionizing radiation is used, as in computed tomography. In addition, heating effects due to higher frequency alternating currents (10 to 100 kilohertz) with low current intensity can be avoided. In addition, because the equipment is also much cheaper and smaller than traditional tomography techniques, EIT can thus be used on patients for extended periods of time and provide continuous real-time visualization. Currently, however, the main disadvantage turns out to be the lower spatial resolution compared to other tomography techniques. However, there are ideas to improve the resolution of the images by increasing the number of electrodes. The quality of the images also still has shortcomings. However, quality improvement is gradually occurring through the increasing use of active surface electrodes. Another disadvantage is that the current does not remain in the body section to be examined, but is distributed in three-dimensional space following the least resistance. Therefore, image formation is also much more complicated than in classical computed tomography. Several two-dimensional representations in three-dimensional space are necessary to finally generate a three-dimensional image, which is then presented again in two dimensions. This results in the so-called “Inverse Problem”. The inverse problem states that the cause must be inferred from the present result. Mostly these problems are very difficult or even impossible to solve. Only in combination with other methods can the cause be clarified. Sufficient experience to evaluate the representations of EIT has yet to be gained through further studies.
