Abstract: System for non-invasive measuring of an intracranial reserve space (ICRS) parameter of a mammalian subject, comprising a multi-frequency ultrasound probe configured, beginning at a start time, to emit and receive ultrasound waves into and from the subject's head and to produce a signal of brain tissue pulsation; an instrument configured to non-invasively partially occlude an internal jugular vein (IJV) starting at the start time and including a second ultrasound probe producing a second signal; and a computer system configured to receive the signal, the second signal and the start time, the computer system also configured, using one or more processors, to derive from the signal an intracranial brain tissue pulsation waveform and from the second signal images of the IJV, and to determine a length of time from the start time to a subsequent time at which the waveform is sufficiently compressed so as to exhibit a predefined decline in variability.