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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. Breast tissue which is
malignant (cancerous) has a much lower electrical impedance – or conducts electricity
much better – than normal tissue and benign (non-cancerous) tumors.
Sensors on the T-scan scanning probe
placed against the skin of the breast measure low-level electrical currents transmitted
through the body. The impedance of breast tissue under the scanning probe is
measured and then displayed in real-time as images on a monitor. These images will show
cancer (lower impedance) as bright, white spots on an otherwise gray background of normal
or non-cancerous breast tissue (higher impedance).
Technical Definition of
Electrical Impedance
Impedance is the measure of the degree
to which an electric circuit resists electric-current flow 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 flow 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 flow of 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.
Impedance Properties of
Normal and Cancerous Tissue
Neoplastic (cancerous) tissue causes
alterations in intracellular and extracellular fluid compartments, cell membrane surface
area, macromolecules, ionic permeability, and membrane associated water layers. These
histological and biochemical changes within the cancerous tissue give rise to measurable
changes in tissue electrical impedance. When a small alternating current is placed across
the breast, the focal increase in electrical conductance and capacitance of the cancer
tissue distorts the electric field within the breast. The resulting impedance map shows
the cancer as a focal brightness on the gray scale image of conductivity and capacitance
measured by an array of signal sensors on the skin surface.
Updated: January 27, 2000
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