Thursday October 05, 2006
Electrical Impedance Tomography


Editors' note: We try to keep our visitors to be posted with new developing technologies applicable in ICU. The following technology is very interesting, portable at bedside and basically an another enhanced way of looking at structural as well as functional anatomy of desired organ - along with CT, MRI, EEG, echocardiogram, EEG etc


Electrical Impedance Tomography (EIT), is a medical imaging technique in which an image of the conductivity or permitivity of part of the body is inferred from surface electrical measurements. Although investigations with CT have taught us that ALI and ARDS are heterogeneous diseases and provide important information about alveolar collapse and reversal of atelactasis, it cannot be applied roultinly in ICU with ALI / ARDS patients.

EIT can produce images by placing electrodes around the anatomy of interest and studying the preferential paths of current flow. Computer reconstruction techniques are employed to generate images, which although of poor resolution, can give functional information in real time. EIT measures the distribution of impedance in a cross-section of the body. This is possible because the electrical resistivities of different body tissues varies widely from 0.65 ohm m for cerebrospinal fluid to 150 ohm m for bone.

Before data can be recorded a series of electrodes are attached to a subject in a transverse plane. These are linked to a data acquisition unit, which outputs data to a PC. By applying a series of small currents to the body a set of potential difference measurements can be made from non-current carrying pairs of electrodes. Since electric currents applied to the body take the paths of least impedance, where the currents flow depends on the subject's conductivity distribution. For example, the heart is full of blood. Blood conducts electricity well, so the heart has a low impedance. The lungs are filled with air. Air does not conduct electricity well, so the lungs have relatively high impedance. Therefore images can be reconstructed from the data using a variety a methods.

Proposed applications include monitoring of lung function, detection of cancer in the skin and breast and location of epileptic foci.

All applications are currently considered experimental.