Source of low-frequency fluctuations in functional MRI signal

To investigate the source of native low-frequency fluctuations (LFF) in functional MRI (fMRI) signal.

Phase analysis was performed on tissue-segmented fMRI data acquired at systematically varying sampling rates.

LFF in fMRI signal were both native and aliased in origin. Scanner instability did not contribute to native or aliased LFF. Aliased LFF arose from cardiorespiratory processes and head motion. Native LFF did not arise from cardiorespiratory processes, but did so, at least in part, from head motion. Motion correction reduced native LFF, but did not eliminate them. The residual native LFF in motion-corrected fMRI data showed a systematic phase difference among different tissue structures. The native LFF in fMRI signals of cerebral blood vessels and CSF were synchronous, and preceded those of gray and white matter, indicating that the vascular fluctuations lead the metabolic fluctuations.

The primary physiologic source of native LFF in fMRI signal is vasomotion.