Vascular origins of low‐frequency oscillations in the cerebrospinal fluid signal in resting‐state fMRI: Interpretation using photoplethysmography

Abstract

In vivo mapping of cerebrovascular oscillations in the 0.05-0.15 Hz remains difficult.Oscillations in the cerebrospinal fluid (CSF) represent a possible avenue for noninvasively tracking these oscillations using resting-state functional MRI (rs-fMRI), and have been used to correct for vascular oscillations in rs-fMRI functional connectivity. However, the relationship between low-frequency CSF and vascular oscillations remains unclear. In this study, we investigate this relationship using fast simultaneous rs-fMRI and photoplethysmogram (PPG), examining the 0.1 Hz PPG signal, heart-rate variability (HRV), pulse-intensity ratio (PIR), and the second derivative of the PPG (SDPPG). The main findings of this study are: (a) signals in different CSF regions are not equivalent in their associations with vascular and tissue rs-fMRI signals; (b) the PPG signal is maximally coherent with the arterial and CSF signals at the cardiac frequency, but coherent with brain tissue at 0.2 Hz; (c) PIR is maximally coherent with the CSF signal near 0.03 Hz; and (d) PPGrelated vascular oscillations only contribute to 15% of the CSF (and arterial) signal in rs-fMRI. These findings caution against averaging all CSF regions when extracting physiological nuisance regressors in rs-fMRI applications, and indicate the drivers of the CSF signal are more than simply cardiac. Our study is an initial attempt at the refinement and standardization of how the CSF signal in rs-fMRI can be used and interpreted. It also paves the way for using rs-fMRI in the CSF as a potential tool for tracking cerebrovascular health through, for instance, the potential relationship between PIR and the CSF signal.

Increased very low frequency pulsations and decreased cardiorespiratory pulsations suggest altered brain clearance in narcolepsy

“…Furthermore, the NT1 group had lower Var frac in MREG card in the anterior corni of the lateral ventricles. Since the cardiac pulsation is known to drive CSF flow also in the ventricular system 71 – 73 , these present results may reflect the altered flow of CSF along the aqueduct and through the lateral ventricles in narcolepsy type 1 patients. A recent study showed that interstitial brain volume increases during sleep, while noradrenaline activity decreased this volume during wakefulness, indicating that declining noradrenaline-driven vasomotor tone during sleep is conducive to CSF flow through the brain 25 .…”

Humming (Simple Bhramari Pranayama) as a Stress Buster: A Holter-Based Study to Analyze Heart Rate Variability (HRV) Parameters During Bhramari, Physical Activity, Emotional Stress, and Sleep

“…Better HRV parameters compared to sleep may indicate that the "humming" state is unique and can sustain respiratory components through a resonance frequency even lower than 0.1 Hz. The use of sound vibrations at the lower frequency may have a potentially influencing role in CSF flow that could be studied further [ 14 , 32 ]. The lower stress index and SDNN confirm the unique RR variations during humming.…”

“…Reducing stress is one of the critical enablers for increased cognition. Recent research highlighting the area of cerebrospinal fluid (CSF) flow further supports this claim [ 14 , 15 ]. The use of sound vibration further enhances cognition, focus, and attention [ 16 ].…”

Real-Time Phase-Contrast MRI to Monitor Cervical Blood and Cerebrospinal Fluid Flow Beat-by-Beat Variability

“…In addition, a transfer of LF waves via the IC arterio–venous capacitive coupling [ 14 ] would be rather more sensitive to the systolic AF modulation, since the major role of the capacitive coupling is compensating the systolic arterial water-hammer effect. Further evidence supporting this hypothesis is the poor presence of LF waves in the CSFF series [ 27 ]. Clearly, the CSF compartment is only capacitively coupled to AF and VF.…”