(temporal region, back of the head, or through the eye), as the bones strongly attenuate the transmission of the ultrasound at these frequencies. TCD is primarily a technique for diagnosing various intracranial vascular disorders such as emboli, stenosis, or vasospasm, and can be used to identify patients who are at risk of developing cerebral ischemia in early phases of traumatic brain injury or stroke. ICP can be estimated from the TCD measurements because it impedes the blood flow and consequently decreases the velocity of blood flow. Besides the mean velocity, pulsatility index (which is the difference between peak systolic and end diastolic velocity, divided by mean flow velocity), a fraction of the cycle in systole and slopes of the TCD waveforms have been correlated with ICP. The estimates are however insufficiently accurate with the margin of error of ±10 - 15 mmHg. Physiosonics, Inc. used transcranial Doppler ultrasound to measure ICP indirectly by assessing the elasticity of the biological material in a defined part of the brain. However, the elasticity in the brain is highly dependent on many other variable individual factors apart from ICP, including arterial blood pressure, state of cerebral blood flow auto-regulation, and the level of edema. Therefore, this approach would require calibration and expert positioning. Skull bones Methods from this group attempt to derive ICP from mechanical properties of the skull bones rather than of the intracranial content. The underlying assumption is similar to that of the ultrasound time of the flight techniques: that the skull is not completely rigid so that changes in ICP result in a small but measurable skull expansion which creates additional stress within the skull bones and modifies their mechanical properties. The transfer function is derived by applying a wide-band, low frequency (<100 Hz) mechanical excitation at one location on the skull (via a piezo-transducer or an impact hammer) and comparing its spectrum to that of a signal received at another location on the upper half of skull. It is proposed that the measurement to be self-calibrated by obtaining the frequency response spectrum from a point on the base of the skull of the same subject, which is assumed not to be affected by ICP, or alternatively, pre-calibrated on subjects with normal ICP. Other methods from this group vary this basic approach of Mick in different ways. In Sinha's method resonant frequency of the skull bones is determined first, then a