A method and apparatus for measuring an impedance of a tissue region inside a body of a patient. A first guidewire is advanced into a tissue region of a body of a patient, wherein the first guidewire comprises one or more electrodes electrically coupled to the tissue region. A second guidewire is advanced into the body, wherein the second guidewire comprises one or more electrodes electrically coupled to the tissue region of the body of the patient and spaced from the first guidewire. An error correction electrode is electrically coupled to the body. A voltage is applied across the first and the second guidewires. The impedance of the tissue region between the first and the second guidewires is measured while using an electrical pathway created by the error correction electrode.
