Which statement about how impedance is determined is correct?

• A. It is measured directly
• B. It is calculated, not measured
• C. It is estimated by boundary conditions
• D. It is unrelated to tissue properties

Answer: (B)

Impedance in this context comes from how tissue properties (like conductivity and permittivity) interact with the geometry and boundary conditions of the region you’re studying. To determine it, you set up the governing equations for the electrical field, insert the tissue properties and the geometry, apply the boundary conditions (how the electrodes or boundaries constrain the field), and solve to relate voltage and current. That means impedance is obtained by calculation from the model, not simply read off from a single direct measurement.

Thinking of it this way helps: you can measure voltage and current, but the impedance of the tissue is what you get when you use the model to connect those measurements to the underlying properties and shape. The idea that impedance is simply measured directly ignores the role of tissue properties and geometry. Saying it’s just estimated by boundary conditions is incomplete because boundary conditions are part of the model, but the full determination relies on both the material properties and the geometry. Saying it’s unrelated to tissue properties is clearly incorrect.