Investigation of intracranial media ultrasonic monitoring model
Introduction
Various methods of non-invasive human intracranial pressure (ICP) measurement have appeared since 1980. However, the need for new non-invasive ICP monitoring technologies that are reliable and suitable for clinical application still exists [1]. The main problem is to find suitable parameter of human cerebrospinal system which would be a stable and repeatable function related to the ICP or cerebral perfusion pressure (CPP). In addition, this function need to be independent of factors such as arterial blood pressure (ABP) or cerebral blood flow autoregulation.
Recently, a new method [2] for the non-invasive measurement of intracranial volume or pressure has been created that is based on the ultrasonic time-of-flight measurement technique. It is capable of measuring the acoustic properties of the intracranial media (IM) such as ultrasound speed and ultrasound attenuation. The aim of this study is to answer the following questions:
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what is the relationship between the characteristics of the ultrasonic signal (time-of-flight, period of oscillation) measured non-invasively by the time-of-flight technique and the acoustic properties of the IM?
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how does it reflect the ICP changes or physiological state of the human brain?
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what is the influence that the skull bones (SB)/external tissues (ET) make on the time-of-flight measurement results and how to minimise that influence?
Section snippets
Theoretical model
The idea of measuring the changes of intracranial component volumes non-invasively is based on the transmission of a broadband ultrasonic signal through the human head and monitoring such signal parameters as the time-of-flight and the oscillation period [2], [3], [4]. As all intracranial components (brain tissue, cerebrospinal fluid (CSF), blood) have different acoustic properties (ultrasound speed, frequency dependent attenuation), changes of their content inside the acoustic path will
Influence of skull bones and external tissues
The influence of SB and ET must be taken into account while performing non-invasive transintracranial ultrasonic measurements. The variation of ET acoustic properties caused by the hemodynamics or swelling in these tissues influences the change of measurement results, separately to the state of the IM.
The methods of non-invasive measuring of the acoustic properties of IM with a compensation of SB/ET influence are shown in Fig. 3. Signal delays in the SB/ET are evaluated by measuring the
Results
A new non-invasive ultrasonic Vittamed monitor based on the models of the ultrasound speed in cerebral parenchymal acoustic path measurement was designed and tested in an intensive care unit (ICU). The example of simultaneous long term (more than 3 h) ICP monitoring with a new non-invasive Vittamed monitor and with a Camino V420 invasive monitor for a head injury patient in the ICU is shown in Fig. 4. The non-invasive ICP data are calculated from the time-of-flight measured data using the
Conclusions
The investigation of the human IM physiological monitoring model allows us to develop the concepts for non-invasive ICP/CPP measurements:
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The intraventricular or supraventricular parenchymal acoustic path which crosses the human head is used for non-invasive measurements. The parenchymal arterioles that are located in this path are responsible for cerebral blood flow autoregulation [12]. The ultrasound speed inside the parenchymal acoustic path mainly depends on the blood volume inside this path.
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