The influence of noise on MDEV inversion given by eq.(8).

<p>The right-hand side and the left-hand side of eq.(7) are calculated from experimental data at individual drive frequencies from 30 Hz to 60 Hz (note, for demonstration purposes, experiments were performed in a volunteer with 7 excitation frequencies ranging from 30 to 60 Hz in increments of 5 Hz). The triangles correspond to unfiltered curl components, and the circles are obtained by applying the filter described in the Methods section (see also fig. 4c). Data from different frequencies are color-coded with different filling patterns (open symbols: 45–60 Hz; solid gray: 40 Hz; horizontal line pattern: 35 Hz; vertical line pattern: 30 Hz). The slopes of the fit lines correspond to modulus . According to eq.(8), the fit lines are forced to run through the origin, resulting in severe underestimation of . A better implementation of least-squares inversion would account for an offset as done by the fit yielding (here only the consistent high-frequency data [blue symbols] are considered). The noise-related offset is suppressed by an appropriate noise filter as used in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0071807#pone-0071807-g004" target="_blank">figure 4c</a> yielding .</p

Sequence timing diagram for acquisition of wave fields (three Cartesian motion components) in 30 slices at eight time steps () during a vibration period 1/<i>f</i>.

<p>Sequence timing diagram for acquisition of wave fields (three Cartesian motion components) in 30 slices at eight time steps () during a vibration period 1/<i>f</i>.</p

Enhanced Adult Neurogenesis Increases Brain Stiffness: <i>In Vivo</i> Magnetic Resonance Elastography in a Mouse Model of Dopamine Depletion

<div><p>The mechanical network of the brain is a major contributor to neural health and has been recognized by in vivo magnetic resonance elastography (MRE) to be highly responsive to diseases. However, until now only brain softening was observed and no mechanism was known that reverses the common decrement of neural elasticity during aging or disease. We used MRE in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine hydrochloride (MPTP) mouse model for dopaminergic neurodegeneration as observed in Parkinson’s disease (PD) to study the mechanical response of the brain on adult hippocampal neurogenesis as a robust correlate of neuronal plasticity in healthy and injured brain. We observed a steep transient rise in elasticity within the hippocampal region of up to over 50% six days after MPTP treatment correlating with increased neuronal density in the dentate gyrus, which could not be detected in healthy controls. Our results provide the first indication that new neurons reactively generated following neurodegeneration substantially contribute to the mechanical scaffold of the brain. Diagnostic neuroimaging may thus target on regions of the brain displaying symptomatically elevated elasticity values for the detection of neuronal plasticity following neurodegeneration.</p></div

Changes in MDEV maps resulting from increasing noise suppression in a single transverse slice.

<p> and <i>φ</i> are reconstructed from: (a) unsmoothed and unfiltered wave data; (b) smoothed wave data prior to curl calculations; (c) smoothed wave data prior to curl calculations and subsequent noise filter (wave number limit of 100 m⁻¹. The processing steps used for (c) were applied to the rest of the paper.</p

For illustration, one central slice of the T1-weighted template data (T1w), maps and <i>φ</i> maps are selectively shown with all four anatomical regions, i.e., the head of caudate nucleus (HCN), thalamus (TH), corpus callosum genu (CCG), and white matter (WM).

<p>Regions of HCN, TH, and CCG were manually selected while the WM region was automatically segmented from the T1w.</p

Regional differences estimated using non-normalized elastographic maps of subjects obtained in the range of drive frequencies from 30 Hz to 60 Hz.

<p>The boxplot depicts the lower and upper quartiles and the 50^th percentile (median) from the 23 subject group. The full data range is presented by whiskers.</p

Preprocessed wave fields used for MDEV inversion in a single slice.

<p>Shown are the curl components (real parts of after Fourier transformation).</p

Experimental setup of MRE of the brain: a) Nonmagnetic driver placed at the end of the patient table.

<p>b) Passive actuator integrated in the head coil and connected to a) by a carbon fiber piston. The main vibration direction is indicated by black arrows.</p

Variation of MRE parameters due to MPTP in the hippocampus of mice and results of cell counts in the DG.

<p>MPTP induced a transient increase of brain elasticity and viscosity (a, b and c) at 6 dpi, while the phase angle φ (d) remained unchanged (mean±SEM). Histologically, a transient MPTP-induced increase of new precursor cells (BrdU+/Nestin/GFP+) at 3 dpi (e) and of new neurons (BrdU+/NeuN+) at 6 dpi (f) as percentage of all newborn cells (BrdU+) was found (mean±SEM). *p<0.05, **p<0.01.</p

Representative images of the magnitude MRE signal, shear waves and the magnitude complex modulus (|G*|) in a mouse.

<p>The green line demarcates the chosen region of interest in the hippocampus.</p