Multiple spin echos in cerebral dynamics: Evidence for spontaneous symmetry breaking during pulsation

Brain functions depend on cerebral transport mechanisms which are evidently propelled and regulated throughout the entire brain [1]. It is generally believed that the main driving force is the cardiac pulse [2] which, after arrival in the brain, travels effortlessly from the arterial into the veinous system seemingly leaving out what is in-between: namely the capillaries and the tissue, which both consist of a flow volume many magnitudes larger with a higher flow resistance than that of the blood vessels. If a pulsatile flow hits both the capillaries and the tissue [3], which are non-pulsatile regimes without vascular smooth musculature, then it should react like a small wave hitting a big inert mass. As a result, the wave should lose energy in a scattering process leading to damping, dispersion and reflection. However, the pulse exits the brain un-scattered [4] leaving us with the open question of what then drives cerebral dynamics. Here, we wanted to shed light on this mystery by studying microscopic parameters of cerebral dynamics. As such, we aimed to observe variations in Landau’s order parameter [5] which may be accessible via multiple spin echos (MSE) [6]. We found MSE oscillations which appeared in brain tissue simultaneously with the arterial pulse. Those oscillations showed remarkable time pattern which revealed the spontaneous character of the underlying physiological mechanism. Our finding can be interpreted as a spontaneous symmetry breaking of the order parameter [7]; a phenomenon also occurring in superfluids. Superfluidity in cerebral dynamics has been recently discussed [8] as a possible answer to what may drive cerebral flow. It would give an eloquent explanation for the still controversially discussed cerebral autoregulation [9] because in superfluidity, heat would drive the flow [10] and not the cardiac pulse. Furthermore, a superfluid would flow dispersion-free whereby membranes would be of no obstacle because superfluids leak through every pore [11, 12]. In Nuclear magnetic resonance (NMR), two radio-frequency (RF) pulses combined with a magnetic field gradient are sufficient to generate multiple spin echos (MSE) as shown in solid Helium in [6], superfluid Helium [13] and in water [14]. In solid Helium and water, multiple echos are considered a result of an asymmetry in the non-linear demagnetisation field. This asymmetry can be introduced or amplified by gradient fields [6, 14, 15], RF pulses [16], and local field inhomo∗Corresponding author: christian@kerskens.me geneities and anisotropies [17, 18, 19, 20]. Signal intensity depends mainly on the asymmetry via the dipolar correlation distance at which the bulk of dipolar interactions become visible and on the effective spin size which depends mainly on the thermodynamical phase via the spin order [21]. MSE are characterised by the demagnetisation field’s angle dependency which results in a signal cancellation at the so-called magic angle, distinguishing it from single spin echos (SSE). They have a common origin and contrast mechanism 1 peer-reviewed) is the author/funder. All rights reserved. No reuse allowed without permission. The copyright holder for this preprint (which was not . http://dx.doi. org/10.1101/219931 doi: bioRxiv preprint first posted online Nov. 15, 2017;