variant of Levenburg–Marquart method) to fit the measured data. This is more challenging as it requires an accurate body shape and the exact position of the electrodes. Much of the fundamental work underpinning Electrical Impedance was done at Rensselaer Polytechnic Institute starting in the 1980s. See also the work published in 1992 from the Glenfield Hospital Project (reference missing). Absolute EIT approaches are targeted at digital reconstruction of static images, i.e. two-dimensional representations of the anatomy within the body part of interest. As mentioned above and unlike linear x-rays in Computed Tomography, electric currents travel three-dimensionally along the path of least resistivity (figure 1), which results in partial loss of the electric current applied (impedance transfer, e.g. due to blood flow through the transverse plane). This is one of the reasons why image reconstruction in absolute EIT is so complex, since there is usually more than just one solution for image reconstruction of a three-dimensional area projected onto a two-dimensional plane. Another difficulty is that given the number of electrodes and the measurement precision at each electrode, only objects bigger than a given size can be distinguished. This explains the necessity of highly sophisticated mathematical algorithms that will address the inverse problem and its ill-posedness. Further difficulties in absolute EIT arise from inter- and intra-individual differences of electrode conductivity with associated image distortion and artifacts. It is also important to bear in mind that the body part of interest is rarely precisely rotund and that inter-individual anatomy varies, e.g. thorax shape, affecting individual electrode spacing. A priori data accounting for age-, height- and gender-typical anatomy can reduce sensitivity to artifacts and image distortion. Improving the signal-to-noise ratio, e.g. by using active surface electrodes, further reduces imaging errors. Some of the latest EIT systems with active electrodes monitor electrode performance through an extra channel and are able to compensate for insufficient skin contact by removing them from the measurements. Another potential solution to problem with electrode-skin contact is contactless EIT technique which uses voltage excitation and capacitive coupling instead of direct contact with the skin. Capacitively coupled electrodes are more comfortable for the patient but maintaining a constant and equal coupling capacitance for all electrodes is challenging in real measurements. Time difference EIT bypasses most of these issues by recording measurements in the same individual between two or more physiological states associated with linear conductivity changes. One of the best examples