Focused Impedance Measurement - Use of Electrical Impedance in Medical Diagnosis

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".

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