free ion content. Further factors affecting conductivity include temperature and other physiological factors, e.g. the respiratory cycle between in- and expiration when lung tissue becomes more conductive due to lower content of insulating air within its alveoli. After positioning surface electrodes through adhesive electrodes, an electrode belt or a conductive electrode vest around the body part of interest, alternating currents of typically a few milliamperes at a frequency of 10–100 kHz will be applied across two or more drive electrodes. The remaining electrodes will be used to measure the resulting voltage. The procedure will then be repeated for numerous "stimulation patterns", e.g. successive pairs of adjacent electrodes until an entire circle has been completed and image reconstruction can be carried out and displayed by a digital workstation that incorporates complex mathematical algorithms and a priori data. The current itself is applied using current sources, either a single current source switched between electrodes using a multiplexer or a system of voltage-to-current converters, one for each electrode, each controlled by a digital-to-analog converter. The measurements again may be taken either by a single voltage measurement circuit multiplexed over the electrodes or a separate circuit for each electrode. Earlier EIT systems still used an analog demodulation circuit to convert the alternating voltage to a direct current level before running it through an analog-to-digital converter. Newer systems convert the alternating signal directly before performing digital demodulation. Depending on indication, some EIT systems are capable of working at multiple frequencies and measuring both magnitude and phase of the voltage. Voltages measured are passed on to a computer to perform image reconstruction and display. The choice of current (or voltage) patterns affects the signal-to-noise ratio significantly. With devices capable of feeding currents from all electrodes simultaneously (such as ACT3) it is possible to adaptively determine optimal current patterns. If images are to be displayed in real time a typical approach is the application of some form of regularized inverse of a linearization of the forward problem or a fast version of a direct reconstruction method such as the D-bar method. Most practical systems used in the medical environment generate a 'difference image', i.e. differences in voltage between two time points are left-multiplied by the regularized inverse to calculate an approximate difference between permittivity and conductivity images. Another approach is to construct a finite element model of the body and adjust the conductivities (for example using a