2 seconds to the water content sensor 11 in the preferred embodiment the frequency spectrum for measuring brain water content without interference from other sources is optimally measured between 400 and 600 mhz however other ranges may be useful depending on the probe length the direct output from the spectrum analyzer 88 is coupled to the digital computer 86 and a second output is coupled to an analog to digital convertor 90 this allows display of the resonant frequency of the water content sensor 11 determined from the direct output as well as heart beat to heart beat changes in frequency and standing wave ratio swr from the digital to analog converter 90 the outputs from the spectrum analyzer 88 and the digital to analog convertor 90 are plotted on a display 92 the display 92 is preferably a high resolution monitor but any display device may be used the digital computer 86 contains software necessary to simultaneously display the pulsatile waveform outputs from the icp sensor 70 and the water content probe 11 on the display 92 as will be explained below the brain water content and blood congestion alter the resonant frequency of the water content probe 11 and provides an indication of the real time read out of apparent tissue water content and the stiffness of.