Coupling of Low Frequency Hemodynamic Oscillations Between the Brain and Spinal Cord.

In Functional Magnetic Resonance Imaging (fMRI), the primary contrast is Blood Oxygen Level Dependent (BOLD) signal. Systemic Low Frequency Oscillations (sLFO) are BOLD signals between 0.01 and 0.1 Hz originating from systemic physiological processes. While sLFO signals in the brain have been shown to travel with blood in numerous studies, their behavior in the spinal cord (SC) remains unexplored. This study characterizes the coupling between brain-sLFO and SC-sLFO signals. Understanding brain-SC-coupling is pivotal for unraveling the vascular continuity of the central nervous system, which plays a crucial role in SC-injury pathophysiology. BOLD signal extraction involved registering structural masks to fMRI space to obtain average time series from the brain, SC, and superior sagittal sinus. The sLFOs of the time series were cross-correlated to determine vascular delays and analyzed for band power. It is found that the SC-sLFO signal comprises two components relative to the brain, showing opposite correlation polarity and varying delays. These findings suggest that highly oxygenated blood arrives at the spinal cord before arriving at the brain, and some component of the brain's venous output circulates to or near to the spinal cord later, likely due to unique arterial and venous pathways connecting the central nervous system. This insight offers a valuable imaging marker for future studies on the effects of SC injury on brain-SC interconnectivity.