Transfer function analysis of respiratory and cardiac pulsations in human brain observed on dynamic magnetic resonance images.

Magnetic resonance (MR) imaging provides a noninvasive, in vivo imaging technique for studying respiratory and cardiac pulsations in human brains, because these pulsations can be recorded as flow-related enhancement on dynamic MR images. By applying independent component analysis to dynamic MR images, respiratory and cardiac pulsations were observed. Using the signal-time curves of these pulsations as reference functions, the magnitude and phase of the transfer function were calculated on a pixel-by-pixel basis. The calculated magnitude and phase represented the amplitude change and temporal delay at each pixel as compared with the reference functions. In the transfer function analysis, near constant phases were found at the respiratory and cardiac frequency bands, indicating the existence of phase delay relative to the reference functions. In analyzing the dynamic MR images using the transfer function analysis, we found the following: (1) a good delineation of temporal delay of these pulsations can be achieved; (2) respiratory pulsation exists in the ventricular and cortical cerebrospinal fluid; (3) cardiac pulsation exists in the ventricular cerebrospinal fluid and intracranial vessels; and (4) a 180-degree phase delay or inverted amplitude is observed on phase images.

The respiratory modulation of intracranial cerebrospinal fluid pulsation observed on dynamic echo planar images.

Pressure changes in cerebrospinal fluid (CSF) that occur with respiration rhythms have been studied in animals and humans for more than 100 years. This phenomenon has been recently validated in vivo on MR images by applying spectral analysis to signal-time curves at selected regions of interest. However, selecting regions of interest requires knowledge of physiology and anatomy, and manual selection is time consuming. We postulate that CSF pulsation is passively modulated by intra-thoracic pressure that is secondary to respiration, and this pulsation can be observed as a flow-related enhancement on MR images. To investigate the spatiotemporal patterns of respiratory rhythms in human brains, we conducted a study on MR scanning of 12 healthy volunteers who performed normal-breathing and breath-holding experiments during scanning. Spectral analysis, spectroscopic images, independent component analysis and signal measurements in selected regions were applied to dynamic MR images acquired from these volunteers. Through independent component analysis, respiratory rhythms were found at the vicinity of ventricles and CSF areas in nine subjects in normal-breathing experiments. In breath-holding experiments, respiratory rhythm suppression and vessel dilation were observed in 8 and 10 subjects, respectively. Information obtained from this study further elucidates the respiratory modulation of CSF in vivo.