Cortical matrix remodeling as a hallmark of relapsing-remitting neuroinflammation in MR elastography and quantitative MRI.

Multiple sclerosis (MS) is a chronic neuroinflammatory disease that involves both white and gray matter. Although gray matter damage is a major contributor to disability in MS patients, conventional clinical magnetic resonance imaging (MRI) fails to accurately detect gray matter pathology and establish a clear correlation with clinical symptoms. Using magnetic resonance elastography (MRE), we previously reported global brain softening in MS and experimental autoimmune encephalomyelitis (EAE). However, it needs to be established if changes of the spatiotemporal patterns of brain tissue mechanics constitute a marker of neuroinflammation. Here, we use advanced multifrequency MRE with tomoelastography postprocessing to investigate longitudinal and regional inflammation-induced tissue changes in EAE and in a small group of MS patients. Surprisingly, we found reversible softening in synchrony with the EAE disease course predominantly in the cortex of the mouse brain. This cortical softening was associated neither with a shift of tissue water compartments as quantified by T2-mapping and diffusion-weighted MRI, nor with leukocyte infiltration as seen by histopathology. Instead, cortical softening correlated with transient structural remodeling of perineuronal nets (PNNs), which involved abnormal chondroitin sulfate expression and microgliosis. These mechanisms also appear to be critical in humans with MS, where tomoelastography for the first time demonstrated marked cortical softening. Taken together, our study shows that neuroinflammation (i) critically affects the integrity of PNNs in cortical brain tissue, in a reversible process that correlates with disease disability in EAE, (ii) reduces the mechanical integrity of brain tissue rather than leading to water accumulation, and (iii) shows similar spatial patterns in humans and mice. These results raise the prospect of leveraging MRE and quantitative MRI for MS staging and monitoring treatment in affected patients.

Low back pain in adolescent rowers: Association to muscle changes detected by magnetic resonance imaging.

Adult elite rowers are at risk of developing low back pain (LBP). However, LBP data on adolescent elite rowers is currently insufficient. Therefore, the aim of this study was to assess LBP prevalence, LBP intensity and training characteristics in male adolescent elite rowers and a healthy control group. Twenty rowers (mean age 15.8 ± 1.2 years) and a non-athletic control group matched by age and gender (n = 13) were prospectively enrolled and underwent LBP assessment with a validated questionnaire and magnetic resonance imaging (MRI) of the lumbar spine muscles, which included a T2-mapping sequence. From the quantitative image data, T2 relaxation times were calculated. The prevalence of LBP in the last 24 hours and 3 months in the rowing group was 55.0% and 85.0%, respectively, compared to 23.1% and 30.8% in the control group (p < 0.001). Rowers had significantly longer T2 relaxation times of the paraspinal muscles compared to controls (p ≤ 0.041). LBP intensity was associated with longer T2 relaxation times (p < 0.001). Adolescent rowers had a higher prevalence of LBP compared to an age-matched control group. The observed increase in T2 relaxation might be explained by muscle soreness due to strenuous exercise, which is correlated with short-term pain intensity.

Microscopic multifrequency MR elastography for mapping viscoelasticity in zebrafish.

PURPOSE: The zebrafish (Danio rerio) has become an important animal model in a wide range of biomedical research disciplines. Growing awareness of the role of biomechanical properties in tumor progression and neuronal development has led to an increasing interest in the noninvasive mapping of the viscoelastic properties of zebrafish by elastography methods applicable to bulky and nontranslucent tissues. METHODS: Microscopic multifrequency MR elastography is introduced for mapping shear wave speed (SWS) and loss angle (φ) as markers of stiffness and viscosity of muscle, brain, and neuroblastoma tumors in postmortem zebrafish with 60 µm in-plane resolution. Experiments were performed in a 7 Tesla MR scanner at 1, 1.2, and 1.4 kHz driving frequencies. RESULTS: Detailed zebrafish viscoelasticity maps revealed that the midbrain region (SWS = 3.1 ± 0.7 m/s, φ = 1.2 ± 0.3 radian [rad]) was stiffer and less viscous than telencephalon (SWS = 2.6 ± 0. 5 m/s, φ = 1.4 ± 0.2 rad) and optic tectum (SWS = 2.6 ± 0.5 m/s, φ = 1.3 ± 0.4 rad), whereas the cerebellum (SWS = 2.9 ± 0.6 m/s, φ = 0.9 ± 0.4 rad) was stiffer but less viscous than both (all p < .05). Overall, brain tissue (SWS = 2.9 ± 0.4 m/s, φ = 1.2 ± 0.2 rad) had similar stiffness but lower viscosity values than muscle tissue (SWS = 2.9 ± 0.5 m/s, φ = 1.4 ± 0.2 rad), whereas neuroblastoma (SWS = 2.4 ± 0.3 m/s, φ = 0.7 ± 0.1 rad, all p < .05) was the softest and least viscous tissue. CONCLUSION: Microscopic multifrequency MR elastography-generated maps of zebrafish show many details of viscoelasticity and resolve tissue regions, of great interest in neuromechanical and oncological research and for which our study provides first reference values.

Inversion-recovery MR elastography of the human brain for improved stiffness quantification near fluid-solid boundaries.

PURPOSE: In vivo MR elastography (MRE) holds promise as a neuroimaging marker. In cerebral MRE, shear waves are introduced into the brain, which also stimulate vibrations in adjacent CSF, resulting in blurring and biased stiffness values near brain surfaces. We here propose inversion-recovery MRE (IR-MRE) to suppress CSF signal and improve stiffness quantification in brain surface areas. METHODS: Inversion-recovery MRE was demonstrated in agar-based phantoms with solid-fluid interfaces and 11 healthy volunteers using 31.25-Hz harmonic vibrations. It was performed by standard single-shot, spin-echo EPI MRE following 2800-ms IR preparation. Wave fields were acquired in 10 axial slices and analyzed for shear wave speed (SWS) as a surrogate marker of tissue stiffness by wavenumber-based multicomponent inversion. RESULTS: Phantom SWS values near fluid interfaces were 7.5 ± 3.0% higher in IR-MRE than MRE (P = .01). In the brain, IR-MRE SNR was 17% lower than in MRE, without influencing parenchymal SWS (MRE: 1.38 ± 0.02 m/s; IR-MRE: 1.39 ± 0.03 m/s; P = .18). The IR-MRE tissue-CSF interfaces appeared sharper, showing 10% higher SWS near brain surfaces (MRE: 1.01 ± 0.03 m/s; IR-MRE: 1.11 ± 0.01 m/s; P < .001) and 39% smaller ventricle sizes than MRE (P < .001). CONCLUSIONS: Our results show that brain MRE is affected by fluid oscillations that can be suppressed by IR-MRE, which improves the depiction of anatomy in stiffness maps and the quantification of stiffness values in brain surface areas. Moreover, we measured similar stiffness values in brain parenchyma with and without fluid suppression, which indicates that shear wavelengths in solid and fluid compartments are identical, consistent with the theory of biphasic poroelastic media.

Reduction of breathing artifacts in multifrequency magnetic resonance elastography of the abdomen.

PURPOSE: With abdominal magnetic resonance elastography (MRE) often suffering from breathing artifacts, it is recommended to perform MRE during breath-hold. However, breath-hold acquisition prohibits extended multifrequency MRE examinations and yields inconsistent results when patients cannot hold their breath. The purpose of this work was to analyze free-breathing strategies in multifrequency MRE of abdominal organs. METHODS: Abdominal MRE with 30, 40, 50, and 60 Hz vibration frequencies and single-shot, multislice, full wave-field acquisition was performed four times in 11 healthy volunteers: once with multiple breath-holds and three times during free breathing with ungated, gated, and navigated slice adjustment. Shear wave speed maps were generated by tomoelastography inversion. Image registration was applied for correction of intrascan misregistration of image slices. Sharpness of features was quantified by the variance of the Laplacian. RESULTS: Total scan times ranged from 120 seconds for ungated free-breathing MRE to 376 seconds for breath-hold examinations. As expected, free-breathing MRE resulted in larger organ displacements (liver, 4.7 ± 1.5 mm; kidneys, 2.4 ± 2.2 mm; spleen, 3.1 ± 2.4 mm; pancreas, 3.4 ± 1.4 mm) than breath-hold MRE (liver, 0.7 ± 0.2 mm; kidneys, 0.4 ± 0.2 mm; spleen, 0.5 ± 0.2 mm; pancreas, 0.7 ± 0.5 mm). Nonetheless, breathing-related displacement did not affect mean shear wave speed, which was consistent across all protocols (liver, 1.43 ± 0.07 m/s; kidneys, 2.35 ± 0.21 m/s; spleen, 2.02 ± 0.15 m/s; pancreas, 1.39 ± 0.15 m/s). Image registration before inversion improved the quality of free-breathing examinations, yielding no differences in image sharpness to uncorrected breath-hold MRE in most organs (P > .05). CONCLUSION: Overall, multifrequency MRE is robust to breathing when considering whole-organ values. Respiration-related blurring can readily be corrected using image registration. Consequently, ungated free-breathing MRE combined with image registration is recommended for multifrequency MRE of abdominal organs.

Fast tomoelastography of the mouse brain by multifrequency single-shot MR elastography.

PURPOSE: To introduce in vivo multifrequency single-shot magnetic resonance elastography for full-FOV stiffness mapping of the mouse brain and to compare in vivo stiffness of neural tissues with different white-to-gray matter ratios. METHODS: Viscous phantoms and 10 C57BL-6 mice were investigated by 7T small-animal MRI using a single-shot spin-echo planar imaging magnetic resonance elastography sequence with motion-encoding gradients positioned before the refocusing pulse. Wave images were acquired over 10 minutes for 6 mechanical vibration frequencies between 900 and 1400 Hz. Stiffness maps of shear wave speed (SWS) were computed using tomoelastography data processing and compared with algebraic Helmholtz inversion (AHI) for signal-to-noise ratio (SNR) analysis. Different brain regions were analyzed including cerebral cortex, corpus callosum, hippocampus, and diencephalon. RESULTS: In phantoms, algebraic Helmholtz inversion-based SWS was systematically biased by noise and discretization, whereas tomoelastography-derived SWS was consistent over the full SNR range analyzed. Mean in vivo SWS of the whole brain was 3.76 ± 0.33 m/s with significant regional variation (hippocampus = 4.91 ± 0.49 m/s, diencephalon = 4.78 ± 0.78 m/s, cerebral cortex = 3.53 ± 0.29 m/s, and corpus callosum = 2.89 ± 0.17 m/s). CONCLUSION: Tomoelastography retrieves mouse brain stiffness within shorter scan times and with greater detail resolution than classical algebraic Helmholtz inversion-based magnetic resonance elastography. The range of SWS values obtained here indicates that mouse white matter is softer than gray matter at the frequencies investigated.

Dynamic Perviousness Predicts Revascularization Success in Acute Ischemic Stroke.

BACKGROUND: The predictive value of thrombus perviousness in acute ischemic stroke (AIS), as measured by computed tomography (CT), has been intensively studied with conflicting results. In this study, we investigate the predictive potential of the novel concept of dynamic perviousness using three-dimensional (3D) volumetric evaluation of occlusive thrombi. METHODS: The full thrombus volume in 65 patients with a hyperdense artery sign on non-contrast CT (NCCT), who underwent mechanical thrombectomy (MT), was segmented. Perviousness maps were computed voxel-wise for the entire thrombus volume as thrombus attenuation increase (TAI) between NCCT and CT angiography (CTA) as well as between CTA and late venous phase CT (CTV). Perviousness was analyzed for its association with NIHSS at admission, Thrombolysis In Cerebral Infarction (TICI) score, and number of MT passes. RESULTS: The mean late-uptake TAI of thrombi with NIHSS scores greater than 21 at admission was approximately 100% higher than for lower scored NIHSS (p between 0.05 and 0.005). Concerning revascularization results, thrombi requiring less than four MT passes had ca. 80% higher group mean late-uptake TAI than clots requiring four or more passes (p = 0.03), and thrombi with TICI score III had ca. 95% higher group mean late-uptake TAI than thrombi with TICI II (p = 0.03). Standard perviousness showed no significant correlation with MT results. CONCLUSION: Standard thrombus perviousness of 3D clot volume is not associated with revascularization results in AIS. In contrast, dynamic perviousness assessed with a voxel-wise characterization of 3D thrombi volume may be a better predictor of MT outcomes than standard perviousness.

Dynamic Perviousness: A Novel Imaging Marker for Predicting Mechanical Thrombectomy Outcomes in Acute Ischemic Stroke.

BACKGROUND: The predictive value of thrombus standard perviousness (SP) in acute ischemic stroke (AIS) for the technical success rates of mechanical thrombectomy (MT) or functional outcomes is not yet conclusive. We investigated the relationship between dynamic perviousness (DP) and revascularization results using time-dependent enhancement curve types determined with computed tomography (CT). METHODS: A retrospective analysis of 137 AIS patients was performed. DP was calculated as the thrombus attenuation increase (TAI) using three time points and categorized into four groups: (1) no enhancement (CNE); (2) late enhancement (CLE); (3) early enhancement with washout (CW); (4) early enhancement without washout (CNW). Associations with the technical success rate and functional outcomes were assessed. RESULTS: Late enhancement (CLE) had approximately two times higher odds for successful MT as compared to clots with other enhancement dynamics. The odds ratios (logistic regression model with CNW as the reference) for the TICI III scores were 4.04 (p = 0.067), 1.82 (p = 0.3), and 1.69 (p = 0.4) for CLE, CW, and CNE, respectively. The NIHSS scores at discharge and mRS scores at three months showed regression coefficients (linear regression model with CNW as reference) of -3.05 (p = 0.10), -1.17 (p = 0.51), and -1.24 (p = 0.47); and -1.30 (p = 0.097), -0.85 (p = 0.25), and -0.15 (p = 0.83) for CLE, CW, and CNE, respectively. CONCLUSIONS: Thrombi with late enhancement patterns showed a higher revascularization rate and better outcomes as compared to clots with early uptake or no washout.

Endovascular Embolization as a Stand-Alone Treatment of Head and Neck Paragangliomas with Long-Term Tumor Control.

BACKGROUND AND PURPOSE: The impact of therapeutic embolization as a stand-alone treatment of head and neck paragangliomas considered surgically high-risk remains insufficiently understood. The aim of this study was to investigate the procedural risks and long-term volumetric development in head and neck paragangliomas with high surgical risk following therapeutic endovascular embolization as a stand-alone treatment. MATERIALS AND METHODS: A retrospective database review of patients who underwent endovascular embolization as primary treatment for head and neck paragangliomas lacking appropriate curative treatment options at our institution (from January 2000 to February 2023) was conducted. Tumor volumetric analyses were performed before embolization and during follow-up. To assess the changes in tumor volume over time, the measurements were performed after embolization, first at 6 months and then on a yearly basis up to 6 years (mean follow-up time was 33.7 ± 24.4 months). Subgroup analyses were conducted for vagal and jugular/jugulotympanic paragangliomas. RESULTS: A total of 32 head and neck paragangliomas in 28 patients (mean age, 56.1 years ± 16.5 [standard deviation]; 18 female) with therapeutic embolization as stand-alone treatment were evaluated, of which 11 were vagal paragangliomas, 15 jugular/jugulotympanic paragangliomas, and 6 carotid body tumors. After a mean follow-up duration of 33.7 ± 24.4 months, tumor control was achieved in 75%, with significant median tumor volume reduction at 6 months (P = .02, n = 21). Vagal paragangliomas responded the most to embolization with a significantly decreased median volume from 22.32 cm3 to 19.09 cm3 (P = .008, n = 8). Transient complications occurred in 3.4%. CONCLUSIONS: Therapeutic embolization as a stand-alone treatment offers a low-risk control of tumor growth in surgically high-risk lesions, with a significant reduction in tumor volume after treatment. Among the different subtypes, vagal paragangliomas exhibited the strongest and longest regression of the tumor volume.

Dynamic Perviousness Has Predictive Value for Clot Fibrin Content in Acute Ischemic Stroke.

Dynamic perviousness is a novel imaging biomarker, with clot density measurements at multiple timepoints to allow longer contrast to thrombus interaction. We investigated the correlations between dynamic perviousness and clot composition in the setting of acute ischemic stroke. Thirty-nine patients with large vessel occlusion (LVO) undergoing mechanical thrombectomy (MT) were analyzed. Patients received a three-phase CT imaging pre-thrombectomy and histopathological analysis of retrieved clots. Clot densities for every phase and change in densities between phases were calculated, leading to four patterns of dynamic perviousness: no contrast uptake, early contrast uptake with and without washout and late uptake. Clots were categorized into three groups based on dominant histologic composition: red blood cell (RBC)-rich, fibrin/platelet-rich and mixed. Clot composition was correlated with dynamic perviousness using the Kruskal-Wallis test and Pearson's correlation analysis. The dynamic perviousness categories showed a significant difference between fibrin-rich clots when compared to RBC-rich plus mixed groups. The uptake without washout category had significantly fewer fibrin clots compared to the uptake with washout (p = 0.036), and nearly significantly fewer fibrin clots when compared to the no uptake category (p = 0.057). Contrast uptake with different patterns of contrast washout showed significant differences of the likelihood for fibrin-rich clots.

In vivo characterization of brain tumor biomechanics: magnetic resonance elastography in intracranial B16 melanoma and GL261 glioma mouse models.

Introduction: Magnetic Resonance Elastography (MRE) allows the non-invasive quantification of tumor biomechanical properties in vivo. With increasing incidence of brain metastases, there is a notable absence of appropriate preclinical models to investigate their biomechanical characteristics. Therefore, the purpose of this work was to assess the biomechanical characteristics of B16 melanoma brain metastases (MBM) and compare it to murine GL261 glioblastoma (GBM) model using multifrequency MRE with tomoelastography post processing. Methods: Intracranial B16 MBM (n = 6) and GL261 GBM (n = 7) mouse models were used. Magnetic Resonance Imaging (MRI) was performed at set intervals after tumor implantation: 5, 7, 12, 14 days for MBM and 13 and 22 days for GBM. The investigations were performed using a 7T preclinical MRI with 20 mm head coil. The protocol consisted of single-shot spin echo-planar multifrequency MRE with tomoelastography post processing, contrast-enhanced T1- and T2-weighted imaging and diffusion-weighted imaging (DWI) with quantification of apparent diffusion coefficient of water (ADC). Elastography quantified shear wave speed (SWS), magnitude of complex MR signal (T2/T2*) and loss angle (φ). Immunohistological investigations were performed to assess vascularization, blood-brain-barrier integrity and extent of glucosaminoglucan coverage. Results: Volumetric analyses displayed rapid growth of both tumor entities and softer tissue properties than healthy brain (healthy: 5.17 ± 0.48, MBM: 3.83 ± 0.55, GBM: 3.7 ± 0.23, [m/s]). SWS of MBM remained unchanged throughout tumor progression with decreased T2/T2* intensity and increased ADC on days 12 and 14 (p<0.0001 for both). Conversely, GBM presented reduced φ values on day 22 (p=0.0237), with no significant alterations in ADC. Histological analysis revealed substantial vascularization and elevated glycosaminoglycan content in both tumor types compared to healthy contralateral brain. Discussion: Our results indicate that while both, MBM and GBM, exhibited softer properties compared to healthy brain, imaging and histological analysis revealed different underlying microstructural causes: hemorrhages in MBM and increased vascularization and glycosaminoglycan content in GBM, further corroborated by DWI and T2/T2* contrast. These findings underscore the complementary nature of MRE and its potential to enhance our understanding of tumor characteristics when used alongside established techniques. This comprehensive approach could lead to improved clinical outcomes and a deeper understanding of brain tumor pathophysiology.

Advanced muscle imaging in adolescent elite rowers utilizing diffusion tensor imaging: Association of imaging findings with stroke typology.

PURPOSE: Muscular overuse injuries are a common health issue in elite athletes. Changes in the muscular microenvironment can be depicted by Diffusion Tensor Imaging (DTI). We hypothesize that the biomechanics of different stroke typologies plays a role in muscle injury and tested our hypothesis by magnetic resonance imaging (MRI) examination of the lumbar spine muscles of adolescent rowers utilizing DTI. METHODS AND MATERIALS: Twenty-two male elite rowers (12 sweep, 10 scull rowers) with a mean age of 15.8 ± 1.2 years underwent 3-Tesla MRI of the lumbar spine 6 hours after cessation of training. Apparent diffusion coefficient (ADC) and fractional anisotropy (FA) were calculated for the erector spinae and multifidus muscle. Student's t-test was used to test differences of DTI parameters between sweep and scull rowers and a Pearson correlation was utilized to correlate the parameters to training volume. RESULTS: ADC values in the erector spinae and multifidus muscle were significantly higher (p = 0.039) and FA values significantly lower (p < 0.001) in sweep rowers compared to scull rowers. There was no significant association between DTI parameters and training volume (r ≤ -0.459, p ≥ 0.074). CONCLUSIONS: Our DTI results show that lumbar spine muscle diffusivity is higher in sweep rowers than in scull rowers. Altered muscle diffusivity is suggestive of microscopic tissue disruption and might be attributable to biomechanical differences between stroke typologies.

Sexual Dimorphism in Extracellular Matrix Composition and Viscoelasticity of the Healthy and Inflamed Mouse Brain.

Magnetic resonance elastography (MRE) has revealed sexual dimorphism in brain stiffness in healthy individuals and multiple sclerosis (MS) patients. In an animal model of MS, named experimental autoimmune encephalomyelitis (EAE), we have previously shown that inflammation-induced brain softening was associated with alterations of the extracellular matrix (ECM). However, it remained unclear whether the brain ECM presents sex-specific properties that can be visualized by MRE. Therefore, here we aimed at quantifying sexual dimorphism in brain viscoelasticity in association with ECM changes in healthy and inflamed brains. Multifrequency MRE was applied to the midbrain of healthy and EAE mice of both sexes to quantitatively map regional stiffness. To define differences in brain ECM composition, the gene expression of the key basement membrane components laminin (Lama4, Lama5), collagen (Col4a1, Col1a1), and fibronectin (Fn1) were investigated by RT-qPCR. We showed that the healthy male cortex expressed less Lama4, Lama5, and Col4a1, but more Fn1 (all p < 0.05) than the healthy female cortex, which was associated with 9% softer properties (p = 0.044) in that region. At peak EAE cortical softening was similar in both sexes compared to healthy tissue, with an 8% difference remaining between males and females (p = 0.006). Cortical Lama4, Lama5 and Col4a1 expression increased 2 to 3-fold in EAE in both sexes while Fn1 decreased only in males (all p < 0.05). No significant sex differences in stiffness were detected in other brain regions. In conclusion, sexual dimorphism in the ECM composition of cortical tissue in the mouse brain is reflected by in vivo stiffness measured with MRE and should be considered in future studies by sex-specific reference values.

Microscopic multifrequency magnetic resonance elastography of ex vivo abdominal aortic aneurysms for extracellular matrix imaging in a mouse model.

An abdominal aortic aneurysm (AAA) is a permanent dilatation of the abdominal aorta, usually accompanied by thrombus formation. The current clinical imaging modalities cannot reliably visualize the thrombus composition. Remodeling of the extracellular matrix (ECM) during AAA development leads to stiffness changes, providing a potential imaging marker. 14 apolipoprotein E-deficient mice underwent surgery for angiotensin II-loaded osmotic minipump implantation. 4 weeks post-op, 5 animals developed an AAA. The aneurysm was imaged ex vivo by microscopic multifrequency magnetic resonance elastography (µMMRE) with an in-plane resolution of 40 microns. Experiments were performed on a 7-Tesla preclinical magnetic resonance imaging scanner with drive frequencies between 1000 Hz and 1400 Hz. Shear wave speed (SWS) maps indicating stiffness were computed based on tomoelastography multifrequency inversion. As control, the aortas of 5 C57BL/6J mice were examined with the same imaging protocol. The regional variation of SWS in the thrombus ranging from 0.44 ± 0.07 to 1.20 ± 0.31 m/s was correlated fairly strong with regional histology-quantified ECM accumulation (R2 = 0.79). Our results suggest that stiffness changes in aneurysmal thrombus reflect ECM remodeling, which is critical for AAA risk assessment. In the future, µMMRE could be used for a mechanics-based clinical characterization of AAAs in patients. STATEMENT OF SIGNIFICANCE: To our knowledge, this is the first study mapping the stiffness of abdominal aortic aneurysms with microscopic resolution of 40 µm. Our work revealed that stiffness critically changes due to extracellular matrix (ECM) remodeling in the aneurysmal thrombus. We were able to image various levels of ECM remodeling in the aneurysm reflected in distinct shear wave speed patterns with a strong correlation to regional histology-quantified ECM accumulation. The generated results are significant for the application of microscopic multifrequency magnetic resonance elastography for quantification of pathological remodeling of the ECM and may be of great interest for detailed characterization of AAAs in patients.

Mechanical properties of murine hippocampal subregions investigated by atomic force microscopy and in vivo magnetic resonance elastography.

The hippocampus is a very heterogeneous brain structure with different mechanical properties reflecting its functional variety. In particular, adult neurogenesis in rodent hippocampus has been associated with specific viscoelastic properties in vivo and ex vivo. Here, we study the microscopic mechanical properties of hippocampal subregions using ex vivo atomic force microscopy (AFM) in correlation with the expression of GFP in presence of the nestin promoter, providing a marker of neurogenic activity. We further use magnetic resonance elastography (MRE) to investigate whether in vivo mechanical properties reveal similar spatial patterns, however, on a much coarser scale. AFM showed that tissue stiffness increases with increasing distance from the subgranular zone (p = 0.0069), and that stiffness is 39% lower in GFP than non-GFP regions (p = 0.0004). Consistently, MRE showed that dentate gyrus is, on average, softer than Ammon´s horn (shear wave speed = 3.2 ± 0.2 m/s versus 4.4 ± 0.3 m/s, p = 0.01) with another 3.4% decrease towards the subgranular zone (p = 0.0001). The marked reduction in stiffness measured by AFM in areas of high neurogenic activity is consistent with softer MRE values, indicating the sensitivity of macroscopic mechanical properties in vivo to micromechanical structures as formed by the neurogenic niche of the hippocampus.

Biomechanical properties of the hypoxic and dying brain quantified by magnetic resonance elastography.

Respiratory arrest is a major life-threatening condition leading to cessation of vital functions and hypoxic-anoxic injury of the brain. The progressive structural tissue changes characterizing the dying brain biophysically are unknown. Here we use noninvasive magnetic resonance elastography to show that biomechanical tissue properties are highly sensitive to alterations in the brain in the critical period before death. Our findings demonstrate that brain stiffness increases after respiratory arrest even when cardiac function is still preserved. Within 5 min of cardiac arrest, cerebral stiffness further increases by up to 30%. This early mechanical signature of the dying brain can be explained by water accumulation and redistribution from extracellular spaces into cells. These processes, together, increase interstitial and intracellular pressure as revealed by magnetic resonance spectroscopy and diffusion-weighted imaging. Our data suggest that the fast response of cerebral stiffness to respiratory arrest enables the monitoring of life-threatening brain pathology using noninvasive in vivo imaging. STATEMENT OF SIGNIFICANCE: Hypoxia-anoxia is a life-threatening condition eventually leading to brain death. Therefore, monitoring vital brain functions in patients at risk is urgently required during emergency care or treatment of acute brain damage due to insufficient oxygen supply. In mouse model of hypoxia-anoxia, we have shown for the first time that biophysical tissue parameters such as brain stiffness changed markedly during the process of death.

The influence of body temperature on tissue stiffness, blood perfusion, and water diffusion in the mouse brain.

While hypothermia of the brain is used to reduce neuronal damage in patients with conditions such as traumatic brain injury or stroke, little is known about how temperature affects the biophysical properties of in vivo brain tissue. Therefore, we measured shear wave speed (SWS), apparent diffusion coefficient (ADC), and cerebral blood flow (CBF) in the mouse brain at different body temperatures to investigate the relationship between temperature and tissue stiffness, water diffusion, and blood perfusion in the living brain. Multifrequency magnetic resonance elastography (MRE), diffusion-weighted imaging (DWI), and arterial spin labeling (ASL) were performed in seven mice while increasing and recording body temperature from hypothermia (28-30 °C) to normothermia (36-38 °C). SWS, ADC, and CBF were analyzed in regions of whole brain, cortex, hippocampus, and diencephalon. Our results show that SWS decreases while ADC and CBF increase from hypothermia to normothermia (whole brain SWS: -6.2%, ADC: +34.0%, CBF: +80.2%; cortex SWS: -10.1%, ADC: +30.9%, CBF: +82.4%; all p > 0.05). We found a significant inverse correlation between SWS and both ADC and CBF in all analyzed regions except diencephalon (whole brain SWS-ADC: r = -0.8, p < 0.005; SWS-CBF: r = -0.84, p < 0.005; cortex SWS-ADC: r = -0.74, p < 0.05; SWS-CBF: r = -0.65, p < 0.05). These results show that in vivo brain stiffness is inversely correlated with temperature, extracellular water mobility, and microvascular blood flow. Regional differences indicate that cortical areas are more markedly affected by hypothermia than central regions such as diencephalon. Temperature should be considered as a confounder in elastographic measurements, especially in preclinical settings. STATEMENT OF SIGNIFICANCE: Hibernating mammals lower their body temperature and metabolic activity. A hypothermic state can also be induced for medical purposes to reduce the risk of neural damage in patients with neurological disease or injury. However, little is known how physical soft-tissue properties of the in-vivo brain such as water diffusion, blood perfusion or mechanical parameters correlate with each other when temperature changes. Our study demonstrates for the first time that those quantitative imaging markers are tightly linked to changes in body temperature. While water diffusion and blood perfusion are reduced during hypothermia, brain stiffness significantly increases, suggesting that multiparametric quantitative MRI should be used for the noninvasive assessment of brain metabolic activity.

Brain maturation is associated with increasing tissue stiffness and decreasing tissue fluidity.

Biomechanical cues guide proliferation, growth and maturation of neurons. Yet the molecules that shape the brain's biomechanical properties are unidentified and the relationship between neural development and viscoelasticity of brain tissue remains elusive. Here we combined novel in-vivo tomoelastography and ex-vivo proteomics to investigate whether viscoelasticity of the mouse brain correlates with protein alterations within the critical phase of brain maturation. For the first time, high-resolution atlases of viscoelasticity of the mouse brain were generated, revealing that (i) brain stiffness increased alongside progressive accumulation of microtubular structures, myelination, cytoskeleton linkage and cell-matrix attachment, and that (ii) viscosity-related tissue fluidity decreased alongside downregulated actin crosslinking and axonal organization. Taken together, our results show that brain maturation is associated with a shift of brain mechanical properties towards a more solid-rigid behavior consistent with reduced tissue fluidity. This shift appears to be driven by several molecular processes associated with myelination, cytoskeletal crosslinking and axonal organization. STATEMENT OF SIGNIFICANCE: The viscoelastic properties of brain tissue shape the environment in which neurons proliferate, grow, and mature. In the present study, novel tomoelastography was used to spatially map tissue mechanical properties of the in-vivo mouse brain during maturation. In vivo tomoelastography was also combined with ex vivo mass spectrometry proteomic analysis to identify the molecules which shape the biomechanical properties of brain tissue. With the combined technique, we observed that brain maturation is associated with a shift of brain mechanical properties towards a more solid-rigid behavior consistent with reduced tissue fluidity which is driven by multiple molecular processes. We believe that this shift of brain mechanical properties discovered in our study reflects a fundamental biophysical signature of brain maturation.

Sensitivity of multifrequency magnetic resonance elastography and diffusion-weighted imaging to cellular and stromal integrity of liver tissue.

Microscopic structural alterations of liver tissue induced by freeze-thaw cycles give rise to palpable property changes. However, the underlying damage to tissue architecture is difficult to quantify histologically, and published data on macroscopic changes in biophysical properties are sparse. To better understand the influence of hepatic cells and stroma on global biophysical parameters, we studied rat liver specimens freshly taken (within 30 min after death) and treated by freeze-thaw cycles overnight at either -20 °C or -80 °C using diffusion-weighted imaging (DWI) and multifrequency magnetic resonance elastography (MRE) performed at 0.5 T in a tabletop MRE scanner. Tissue structure was analyzed histologically and rheologic data were analyzed using fractional order derivatives conceptualized by a called spring-pot component that interpolates between pure elastic and viscous responses. Overnight freezing and thawing induced membrane disruptions and cell detachment in the space of Disse, resulting in a markedly lower shear modulus µ and apparent diffusion coefficient (ADC) (µ[-20 °C] = 1.23 ±â€¯0.73 kPa, µ[-80 °C] = 0.66 ±â€¯0.75 kPa; ADC[-20 °C] = 0.649 ±â€¯0.028 µm2/s, ADC[-80 °C] = 0.626 ±â€¯0.025 µm2/s) compared to normal tissue (µâ€¯= 9.92 ±â€¯3.30 kPa, ADC = 0.770 ±â€¯0.023 µm2/s, all p < 0.001). Furthermore, we analyzed the springpot-powerlaw coefficient and observed a reduction in -20 °C specimens (0.22 ±â€¯0.14) compared to native tissue (0.40 ±â€¯0.10, p = 0.033) and -80 °C specimens (0.54 ±â€¯0.22, p = 0.002), that correlated with histological observations of sinusoidal dilation and collagen distortion within the space of Disse. Overall, the results suggest that shear modulus and water diffusion in liver tissue markedly decrease due to cell membrane degradation and cell detachment while viscosity-related properties appear to be more sensitive to distorted stromal and microvascular architecture.