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.

Use of electrical impedance in medical diagnosis

Electrical impedance is a measurement of how electricity travels though a given material. Every tissue has different electrical impedance determined by its molecular composition. Some materials have high electrical impedance while others have low electrical impedance. For example, breast tissue which is malignant (cancerous) has a much lower electrical impedance – or conducts electricity much better – than normal tissue and benign (non-cancerous) tumours.

Impedance is the measure of the degree to which an electric circuit resists electric current when a voltage is impressed across its terminals. Impedance, expressed in ohms, is the ratio of the voltage impressed across a pair of terminals to the current between those terminals. In direct-current (DC) circuits, impedance corresponds to resistance. In alternating current (AC) circuits, impedance is a function of resistance, inductance, and capacitance. Inductors and capacitors build up voltages that oppose the current. This opposition, called reactance, must be combined with resistance to find the impedance. The reactance produced by inductance is proportional to the frequency of the alternating current, whereas the reactance produced by capacitance is inversely proportional to the frequency.

Electrical impedance measurements exploit the differences in the electrical properties of materials to detect inhomogeneities. Electrodes placed on the surface of an object are used to determine the electrical characteristics of the volume of that object. By injecting known amounts of current and measuring the resulting electrical potential field at points on the boundary of the body, it is possible to "invert" such data to determine the conductivity or resistivity of the region of the body probed by the currents. Since this method is often used in principle to image changes in dielectric constant at higher frequencies, the term "impedance”, is used instead of "conductivity" or "resistivity".

Computerized Focused Impedance Measurement

For efficient experimentation and medical use of this new technique, computerization was felt to be a necessity. The first computerization of Focused Impedance Measurement was done by Saiful Arefin Khaled of Biomedical Research Lab of University of Dhaka. A user friendly, windows-based software for data acquisition, storage and display of FIM data was developed. Using this software a large amount of experimental data could be analyzed very easily in a short time. Furthermore using this software, real time graphical presentation of data could be viewed in any microcomputer. This development immensely helped further researches in this field.