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 1. The transfer function is derived by applying a wide-band, low frequency (<100Hz) mechanical excitation at one location on the skull (via a piezo-tranducer 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 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 2 method resonant frequency of the skull bones is determined first, then a sinusoidal excitation at the resonant frequency is delivered through a piezo-transducer, and ICP is calculated directly from the phase difference between the excitatory signal and response detected with a second transducer. Yost and Cantrell 3 divided the process into two steps. In the first step, changes in the circumference of the cranium are calculated from the phase difference between a sinusoidal excitatory signal, delivered with a piezo-transducer, and the response that is received at a distance with another piezotransducer. In the second step, changes in ICP are calculated as a product of the changes in the cranium circumference and the elasticity constant of the skull that has been determined earlier by causing known changes in ICP while measuring the cranium circumference.
None of the aforementioned methods has been properly validated in relevant clinical populations, and their accuracy is unknown. One may assume however that it would be comparable to the ultrasound time-of-the-flight methods, and thus insufficient for a routine clinical use.
Luna Innovations Incorporated (NASDAQ: LUNA) developed EN-TACT system, an ultrasound device for monitoring compartment syndrome. The technology was claimed to have applications for raised intracranial pressure. Based on research from the NASA Ames research center, the company used ultrasound to measure skull diameter changes caused by ICP changes. However skull changes are tiny and only indirectly related to ICP, raising questions about the accuracy and calibration.
1 Mick, E.: US5074310 (1991).
2 Sinha , D.N.: US20006117089 (2000).
3 Yost,W.T., Cantrell, J.H.: US20046746410 (2004)