Cardiac and respirationinduced brain deformations in humans quantified with highfield MRI Abstract   Microvascular blood volume pulsations due to the cardiac and respiratory cycles induce brain tissue deformation and, as such, are considered to drive the brainu0027s waste clearance system. We have developed a highfield magnetic resonance imaging (MRI) technique to quantify both cardiac and respirationinduced tissue deformations, which could not be assessed noninvasively before. The technique acquires motion encoded snapshot images in which various forms of motion and confounders are entangled. First, we optimized the motion sensitivity for application in the human brain. Next, we isolated the heartbeat and respirationrelated deformations, by introducing a linear model that fits the snapshot series to the recorded physiological information. As a result, we obtained maps of the physiological tissue deformation with 3xe2x80xafmm isotropic spatial resolution. Heartbeat and respiration induced volumetric strain were significantly different from zero in the basal ganglia (median (25xe2x80x9375% interquartile range): 0.85xc2xb710xe2x88x923 (0.39xc2xb710xe2x88x923xe2x80x931.05xc2xb710xe2x88x923), pxe2x80xafxe2x80xaf0.0008 and xe2x88x920.28xc2xb710xe2x88x923 (xe2x88x920.41xc2xb710xe2x88x923xe2x80x930.06xc2xb710xe2x88x923), pxe2x80xafxe2x80xaf0.047, respectively). Smaller volumetric strains were observed in the white matter of the centrum semi ovale (0.28xc2xb710xe2x88x923 (0xe2x80x930.59xc2xb710xe2x88x923) and xe2x88x920.06xc2xb710xe2x88x923 (xe2x88x920.17xc2xb710xe2x88x923xe2x80x930.20xc2xb710xe2x88x923)), which was only significant for the heart beat (pxe2x80xafxe2x80xaf0.02 and pxe2x80xafxe2x80xaf0.7, respectively). Furthermore, heartbeat induced volumetric strain was about three times larger than respiration induced volumetric strain. This technique opens a window on the driving forces of the human brain clearance system.

Imaging brain function with simultaneous BOLD and viscoelasticity contrast: fMRIfMRE Abstract   Magnetic resonance elastography (MRE) is emerging as a new tool for studying viscoelastic changes in the brain resulting from functional processes. Here, we demonstrate a novel time series method to generate robust functional magnetic resonance elastography (fMRE) activation maps in response to a visual task with a flashing checkerboard stimulus. Using a singleshot spinecho (SSSE) pulse sequence, the underlying raw images inherently contain bloodoxygenlevel dependent (BOLD) contrast, allowing simultaneous generation of functional magnetic resonance imaging (fMRI) activation maps from the magnitude and functional magnetic resonance elastography (fMRE) maps from the phase. This allows an accurate comparison of the spatially localized stiffness (fMRE) and BOLD (fMRI) changes within a single scan, eliminating confounds inherent in separately acquired scans. Results indicate that tissue stiffness within the visual cortex increases 6xe2x80x9311% with visual stimuli, whereas the BOLD signal change was 1xe2x80x932%. Furthermore, the fMRE and fMRI activation maps have strong spatial overlap within the visual cortex, providing convincing evidence that fMRE is possible in the brain. However, the fMRE temporal SNR      (   tSN   R   fMRE     )      maps are heterogeneous across the brain. Using a dictionary matching approach to characterize the time series, the viscoelastic changes are consistent with a viscoelastic response function (VRF) time constant of 12.1xe2x80xafs    xc2xb1    3.0xe2x80xafs for a firstorder exponential decay, or a shape parameter of 8.1xe2x80xafs    xc2xb1    1.4xe2x80xafs for a gammavariate.