Measuring brain beats: cardiac-aligned fast fMRI signals

Abstract

Blood and cerebrospinal fluid (CSF) pulse and flow throughout the brain, driven by the cardiac cycle. These fluid dynamics, which are essential to healthy brain function, are characterized by several noninvasive magnetic resonance imaging (MRI) methods. Recent developments in fast MRI, specifically simultaneous multislice (SMS) acquisition methods, provide a new opportunity to rapidly and broadly assess cardiac-driven flow, including CSF spaces, surface vessels and parenchymal vessels. We use these techniques to assess blood and CSF flow dynamics in brief (3.5 minute) scans on a conventional 3T MRI scanner. Cardiac pulses are measured with a photoplethysmograph (PPG) on the index finger, along with fMRI signals in the brain. We retrospectively analyze the fMRI signals gated to the heart beat. Highly reliable cardiac-gated fMRI temporal signals are observed in CSF and blood on the timescale of one heartbeat (test-retest reliability within subjects R2>0.50). Cardiac pulsations with a local minimum following systole are observed in blood vessels, with earlier extrema in the carotid and basilar arteries and in branches of the anterior, posterior and middle cerebral arteries and extrema ~200 ms later in the superior sagittal, transverse and straight sinuses. CSF spaces in the ventricles and subarachnoid space showed cardiac pulsations with a local maximum following systole instead. Similar responses are observed, with less temporal detail, in slower resting state scans with slice timing retrospectively aligned to the cardiac pulse in the same manner. The SMS measurements rapidly, noninvasively and reliably sample brain-wide fMRI signal pulsations aligned to the heartbeat. The measurements estimate the amplitude and phase of cardiac driven fMRI pulsations in the CSF relative to those in the arteries, which is thought to be an estimate of the local intracranial impedance. Cardiac aligned fMRI signals can provide new insights about fluid dynamics or diagnostics for diseases where these dynamics are important.