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performed on two-dimensional slices through the body. EIT uses diffusion of current to deduce conductivity distribution, unlike methods such as magnetic resonance imaging (MRI), CT, positron emission tomography (PET), and single photon emission computed tomography (SPECT).

EIT is expected to have relatively poor resolution compared to MRI, CT, PET, and SPECT. Resolution is largely controlled by the number of electrodes that can be reasonably attached simultaneously to a patient. Schemes used to date have normally used electroencephalogram-style electrodes, relatively few in number and large in size. It is not yet known whether special belts or headbands having large numbers of small electrodes designed especially for EIT applications might considerably improve this situation. But, regardless of such possible advances in technology, it is not anticipated that EIT will ever "outresolve" methods like x-ray CT.

At the present time, EIT is the only method known that images electrical conductivity, although MRI and electromagnetic methods also have some potential to measure conductivity. So, for applications requiring knowledge of the distribution of this parameter through a body, EIT will continue to be an important method to consider for medical imaging, regardless of its resolving power.

On the other hand, EIT has some very attractive features. The technology for doing electrical impedance imaging is safe and inexpensive, and therefore could be made available at multiple locations (for example, at bedside) in hospitals. At the low current levels needed for this imaging technique, the method is not known to cause any long-term harm to the patient, and therefore could be used to do continuous (or frequent, but intermittent) monitoring of bedridden patients. Technology for acquiring data, and algorithms for inverting that data to produce images of conductivity/resistivity, have been developed to the point that real-time imaging could become routine today using a graphics workstation as the computing platform.

9.3 Present Status of EIT and Limitations

The impedance imaging problem is nonlinear and extremely ill posed, which means that large changes in interior properties can result in only small changes in the measurements. This implies that making a high-resolution image would require extremely accurate measurements, which in turn makes the problem of designing appropriate electronics very challenging.

At present there are two main approaches to the problem. The first,



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