Focused Impedance Measurement

Focused Impedance Measurement

The Focused Impedance Measurement (FIM) is a recent technique for measurement of tissue impedance in the human body with improved zone localization compared to conventional methods. This method was proposed and developed by Department of Biomedical Physics and Technology of University of Dhaka under the close supervision of Prof. Khondkar Siddique-e-Rabbani; who first introduced the idea and FIM is considered widely as the brainchild of his. For the simplicity and accuracy offered by FIM, it may prove to be a potential replacement of other earlier methods like Four Electrode Impedance Measurement (FEIM) and Electrical impedance Tomography (EIT) in many practical measurement situations. FIM can be considered as the technique that bridges FEIM and EIT.

Many biological parameter and processes can sensed and monitored using their impedance as marker . Measurement of electrical bioimpedance is relatively easy to do. It requires little equipment, can be done with simple and small instruments. Due to the simplicity of the equipment and with the advantage that measurement technique is non-invasive it may be made low cost.

Measurement of electrical impedance to obtain physiological or diagnostic information has been of interest to researchers for many years. Conventional Four Electrode or Tetra-polar Impedance Measurement (TPIM) is a simple technique, where current is driven through a pair of electrodes while potential is measured across another pair in order to eliminate the inclusion of contact impedance, but the zone of sensitivity is not well defined and many include organs other that those of interest, thus making the interpretations difficult and unreliable. It needs to be mentioned that for such impedance measurement on the human body, ac driving currents of frequencies greater than 10 kHz are usually employed because of various consideration; tissue electrolysis and unstable contact potential at dc at very low frequencies, and neuro-muscular stimulation at frequencies less than 10 kHz being the main effects that need to be avoided. At the relatively high measurement frequencies, skin electrode impedance are low (because of capacitive reactance) and this allows measurement of tissue impedance in the bulk of the body, even if measurements are made using skin surface electrodes.

A remarkable progress in impedance measurements was the introduction of Electrical Impedance Tomography (EIT) by a group in Sheffield, UK, using which a 2D distribution of tissue impedance in the plane of the electrodes was obtained through back-projection technique similar to that used in X-ray and CT. This used 16 electrodes around the thorax in the transverse plane for its initial applications. The human body has uneven geometry and unequal distribution of conductivity inside, which again varies from person to person and between phases of normal body activity making an accurate analytical treatment difficult. Electrical impedance in tissues is a function of variables such as ion concentrations, cell geometry, extra-cellular fluids, intra-cellular fluids and organ geometry, so it is easy to imagine that sensitivity to different changes in these variables may be detectable. Unfortunately it is difficult to established a link between measured impedance change and a change in one or more of the input variables. This tagged electrical bioimpedance measurements as in general- has good sensitivity but poor specificity.

Conventional four-electrode impedance measurements (FEIM) cannot localize a zone of interest in a volume conductor. On the other hand, electrical impedance tomography (EIT) system offers an image with reasonable resolution, but is complex and needs many electrodes. By placing two FEIM systems perpendicular to each other over a common zone at the center and combining the two results, it is possible to obtain enhanced sensitivity over this central zone. This is the basis of the focused impedance measurement (FIM). This new method may be useful for impedance measurements of large organs like stomach, heart, and lungs. Being much simpler in comparison to EIT, multifrequency systems can be simply built for FIM. Besides, FIM may have utility in other fields like geology where impedance measurements are performed.