Dielectric Properties (800 - 2,400 MHz) of Human Brain Measured Less Than 10 Hours Post Mortem

A series of animal experiments indicated that the dielectric properties of brain tissue decrease within a few hours after death. Measurements of the dielectric properties of human brain tissue reported so far were mainly performed on tissue samples more than 20 hours after death. We have measured dielectric properties at 800 - 2,400 MHz in 19 brains from humans within 3 - 10 hours after death. All measurements took place on the excised whole brain before the routine autopsy procedure. Relative permittivity as well as conductivity were measured at 4 well defined positions on the cortex of the temporal lobe at the right as well as at the left side. Based on autopsy results, brains of deceased patients with neurological diseases were excluded from the study. An open-ended coaxial probe (HP85070B) in combination with a vector network analyser (HP8722C) after standard air/short/water calibration was used as measurement system. The 19 samples consisted of 8 male and 11 female brains. The average age of the patients at the time of death was 70.3 years (min. 47.5 years, max. 81.8 years) and the measurements were performed at a mean post mortem time of 6.8 hours (min. 3 hours, max. 10 hours). The tissue temperatures during the measurements varied between different brains and were between 18 °C and 25 °C. Figure 1 shows the mean and standard deviation of the measured electric conductivity in the frequency range 800 - 2,400 MHz, when taking into account all measured positions on all brains (160 different measurements). Figure 2 illustrates the dependence of conductivity on the time after death at 900 MHz. For this figure the 19 brains were divided into 3 groups corresponding to post mortem time when measurements were made. The mean value of conductivity at 900 MHz for each group was calculated from all measurement positions, and these mean values were normalised to a tissue temperature of 22 °C using a temperature coefficient of 1 %/°C.