THE SQUARE-WAVE APPROACH TO IMPEDANCE MEASUREMENT OF BRAIN-TISSUE WATER DYNAMICS

Electrical properties of living soft tissue have been used to analyze their structure and function. Presently, the 'admittance locus' method , with the sine-wave signal of changing frequency, is the most informa tive continuous method for analyzing extra- and intracellular water co ntent in brain tissue. Using the square-wave signal in lieu of the sin e-wave signal, we can avoid cumbersome and costly measurements and fac ilitate real-time data processing. An isolation-calibration device was developed for the present study in order to condition and stabilize e lectrical current through the brain cortex. This device was also used for impedance calibration before and after the experiments. We propose a simple algorithm for data analysis on the basis of equivalent circu it approach, which allows to develop a computer program for data proce ssing. Preliminary experiments on rat brains were carried out with a 0 .2-0.5 mm stainless-steel tetrapolar electrode system. These studies s howed good linearity between stimulating currents (L = 5-30 mu A) thro ugh the external electrodes in the brain cortex and a drop in voltage which was measured by 2 inner electrodes. The results of the device an d the program accuracy tests allow us to choose the optimal range for the working current. We can recommend this method for usage in animal experiments.