Spectral Analysis of Mechanical and Respiratory Influences on Width of Subarachnoid Space Assessed with Non-invasive Method of Near-infrared Transillumination/back Scattering Sounding

The technique of transillumination has been known for a long time now [1–10]. Initially only white light was used and later also infrared radiation. Due to poor light propagation through the skulls of adult individuals, the use of white light transillumination was limited mostly to paediatric cases [1,3]. Ultimately, interpretation difficulties put an end to the clinical use of the technique in its traditional form. Application of near-infrared radiation (NIR) as information medium in the technique did not prove successful in the past, because of very strong attenuation of propagation of the NIR by the blood in the superficial circulation of the head skin [6,7]. In our team, we have managed to work out a new technical solution allowing to overcome this major obstacle and thus opening opportunities for application of the NIR-T/BSS technique in clinical practice [11,12]. Combined special design of the electronic data acquisition equipment and dedicated algorithms of signal processing have made it possible to extract components of the returning signal which are fully coupled with and dependent upon the longand short-term changes in the instantaneous width of the subarachnoid space (SAS). An important factor affecting the width of the SAS is continuous cerebrovascular pulsation generated by contractions of the heart and the resulting pulsatile changes in the filling with blood of the intracranial arteries and brain as a whole. There are a variety of factors exerting influence on instantaneous condition of the intracranial vascular bed, among them also positional or gravitational ones. The influence of some of internal or physiological and external factors on quality and quantity of the cerebrovascular pulsation can be assessed with spectral analysis of the NIR-T/BS signals acquired in specially designed experimental procedure. Analysis of the frequency spectrum of quantitatively measured physiological processes affecting the SAS width reveals two general frequency ranges: heart rate (HR) frequency range, and lower frequency range – related closely to periodic physiological phenomena, like respiration, peristaltic movements of the bowels, head movements of a period longer than that of HR, etc. In this study we have focused our attention on mechanical processes, which – as external factors – exert influence on the SAS width and magnitude of cerebrovascular pulsation, and whose influence can be detected and estimated quantitatively using spectral analysis of the transillumination signals defined in part II. In physiological conditions, the feasibility of assessment of influence of external mechanical factors of different frequencies on the SAS width is highly limited. Methods enabling such assessment seem to be experiments with rhyth-