Correlative study between computerized transverse scanning and stereoimpedoencephalography in space-occupying lesions of the brain.

Stereoimpedoencephalography (SIEG) is routinely used by the authors during stereotactic investigations of glial brain tumours prior to stereotactic implantation of isotopes or prior to conventional surgery: the brain tissue impedance is measured along a probe track according to the four electrodes impedance technique. The correlation between the tissue structures and the corresponding impedance values has been studied by comparison with the CATScan data reported on the stereotactic schedules; impedance values are influenced by changes in arterial pCO2 and also by other unknown factors; in order to avoid these uninterpretable changes of the impedance, the recorded values have been normalized with respect to the average impedance along a control track in the normal hemisphere. As a rule, the impedance profile exhibits a "hole" at the site of the pathological processes like tumours as well as surrounding oedema, and the delimitation of the pathological area is very similar to that obtained by the CATScan. There is a good linear correlation between the values of normalized impedance and the x-ray absorption as given by the computer listing of the CATScan. Lastly, statistical values of normalized impedances are determined for the main structures of the brain, showing that the tumour tissue has an impedance equal to half the impedance of the normal tissue.