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Background: Clinical imaging modalities including optical coherence tomography (OCT) and diffusion tensor imaging (DTI) are vital in Multiple Sclerosis (MS), but their relationships during the different phases of Retinal ganglion cell (RGC) degeneration are not clear. We hypothesize that initial injury in optic nerve causes axonal degeneration leading to RGC loss in retina, which can be characterized by a combination of DTI and OCT. Our objective was to examine the correlation between noninvasive and histological data to chronicle the degeneration profile of RGCs in the retina and optic nerve in a mouse model of MS., Materials and Methods: Experimental Autoimmune Encephalomyelitis (EAE) was induced in 11 C57Bl/6 mice, with 8 mice reserved as controls. OCT and DTI was conducted 2-8 weeks after induction of EAE. The thickness of the retinal ganglion cell complex (GCC) was measured using OCT and compared to DTI indices measured in optic nerves. End-stage histology was used to quantify axon/myelin loss in the optic nerve and retinal thinning/RGC loss in the retina., Results: Significant changes in DTI-derived Axial Diffusivity (AD, -17.2%) and Trace Diffusivity (TR, -18.3%) began after 2 weeks of EAE. Later significant reductions in Fractional Anisotropy (FA) and AD, with increases in Radial Diffusion (RD) were apparent after 4 and 8 weeks. OCT-derived measures of GCC thickness were reduced after 4 weeks, and reached significant reduction after 8 weeks. Among EAE mice, DTI (FA, AD and RD measures) and OCT measures were all significantly correlated after 4 and 8 weeks. Among histology measures, RGC density (-23%), RGC size (-27%), and the number of SMI31+ axons (-54%) were reduced significantly. DTI measures of FA and AD along with GCC thinning were the best independent predictors of axon loss., Conclusions: DTI and OCT measures are tightly correlated during the chronic phase of axonal degeneration (4-8 weeks) in EAE mice. After 8 weeks of EAE, both OCT and DTI measures are strong predictors of axon loss in the Optic Nerve., (Copyright © 2018. Published by Elsevier B.V.)

Purpose: Diffusion tensor imaging (DTI) is commonly used to evaluate white matter integrity in multiple sclerosis (MS), but the relationship between DTI measures and functional changes during disease remains ambiguous. Using a mouse model of MS, we tested the hypothesis that DTI measures would correlate to the visual evoked potential (VEPs) dynamically at different disease stages., Methods: In vivo DTI, gadolinium-enhanced T1WI (Gd-T1WI) and VEPs were performed in 5 control and 25 mice after 2-12 weeks of experimental autoimmune encephalomyelitis (EAE). DTI indices, including fractional anisotropy (FA), axial and radial diffusivities (AD and RD), and Gd-T1WI enhancement, were measured in the optic nerve and tract (ON and OT), which were compared with measured VEPs., Results: Gd-T1WI showed a 3- to 4-fold enhancement over controls beginning after 2 weeks of EAE. Across the time course, we found progressive reductions in FA and increases in RD with increases in VEP latency and reductions in amplitude. Significant correlations between DTI (FA and RD) and VEP evolved; in control/early asymptomatic EAE mice, both FA and RD were highly correlated with VEP latency (but not amplitude), while in late EAE, both DTI indices were highly correlated with VEP amplitude (but not latency)., Conclusion: DTI measures FA and RD are associated to VEP latency in early stages of EAE but associated to VEP amplitude in later stages, suggesting that the patterns of DTI related to the functional decline may depend on the stage of disease progression.

Purpose: To evaluate the feasibility of using diffusion tensor imaging (DTI) to characterize the temporospatial profile of axonal degeneration and its relation to blood-brain barrier (BBB) permeability., Materials and Methods: Longitudinal DTI was performed in Wallerian degeneration slow (WldS) mice following retinal ischemia. In parallel, gadolinium (Gd)-enhanced T 1 -weighted imaging (Gd-T 1 WI) was performed to evaluate BBB permeability in white matter during axonal degeneration. To confirm the in vivo findings, immunohistochemistry using SMI-31 and myelin basic protein (MBP) was performed to examine the axons and myelin, respectively, and Evans blue was used to evaluate the permeability of the BBB., Results: Reduced axial diffusivity was found in the optic nerve (ON, -15%, P = 0.0063) 1 week and optic tact (OT, -18%, P = 0.0077) 2 weeks after retinal ischemia, which were respectively associated with an 11% (P = 0.0116) and 25% (P = 0.0001) axonal loss. Increased radial diffusivity was found 1-2 weeks after the colocated decrease of axial diffusivity (35% increase, P = 0.0388 in the ON at week 2 and an 80% increase, P = 0.0015 in the OT at week 4). No significant changes were observed using Gd-T 1 WI (P = 0.13-0.75), although an approximately 1-fold increase in Evans blue staining intensity was found in the injured ON and OT starting 1 week after retinal ischemia., Conclusion: We demonstrated the utility of DTI to characterize anterograde-propagating axonal degeneration through the ON and OT following retinal damage. Evans blue staining revealed serum albumin accumulation at injured sites, although there was no BBB leakage detectable using Gd-T 1 WI., Level of Evidence: 2 J. Magn. Reson. Imaging 2017;45:482-491., (© 2016 International Society for Magnetic Resonance in Medicine.)

Visual deficits are commonly seen in patients with Alzheimer's disease (AD), but postmortem histology has not found substantial damage in visual cortex regions, leading to the hypothesis that the visual pathway, from eye to the brain, may be damaged in AD. Diffusion tensor imaging (DTI) has been used to characterize white matter abnormalities. However, there is a lack of data examining the optic nerves and tracts in patients with AD. In this study, we used DTI to analyze the visual pathway in healthy controls, patients with mild cognitive impairment (MCI) and AD using scans provided by the Alzheimer's Disease Neuroimaging Initiative (ADNI). We found significant increases in the total diffusivity and radial diffusivity and reductions in fractional anisotropy in optic nerves among AD patients. Similar but less extensive changes in these metrics were seen in MCI patients as compared to controls. The differences in DTI metrics between groups mirrored changes in the splenium of the corpus callosum, which has commonly been shown to exhibit white matter damage during AD and MCI. Our findings indicate that white matter damage extends to the visual system, and may help explain the visual deficits experienced by AD patients.

Diffusion tensor imaging (DTI) is achieved by collecting a series of diffusion-weighted images (DWIs). Signal averaging of multiple repetitions can be performed in the k-space (k-avg) or in the image space (m-avg) to improve the image quality. Alternatively, one can treat each acquisition as an independent image and use all of the data to reconstruct the DTI without doing any signal averaging (no-avg). To compare these three approaches, in this study, in vivo DTI data were collected from five normal mice. Noisy data with signal-to-noise ratios (SNR) that varied between five and 30 (before averaging) were then simulated. The DTI indices, including relative anisotropy (RA), trace of diffusion tensor (TR), axial diffusivity (λ║), and radial diffusivity (λ⊥), derived from the k-avg, m-avg, and no-avg, were then compared in the corpus callosum white matter, cortex gray matter, and the ventricles. We found that k-avg and m-avg enhanced the SNR of DWI with no significant differences. However, k-avg produced lower RA in the white matter and higher RA in the gray matter, compared to the m-avg and no-avg, regardless of SNR. The latter two produced similar DTI quantifications. We concluded that k-avg is less preferred for DTI brain imaging., (© 2013 Elsevier Inc. All rights reserved.)

Background: Diffusion tensor imaging (DTI) suggests the presence of white matter abnormality at the prodromal stage in human Alzheimer's disease (AD).Objective: To use a mouse model of AD to determine whether the white matter abnormality detected by in vivo DTI is associated with functional deficits and axon damage.Methods: Amyloid-β1-42 (Aβ1-42) was injected into the left lateral ventricle in mice. Two months after the injection, in vivo DTI and visual evoked potential (VEP) recordings were performed, followed by immunohistochemistry of phosphorylated neurofilament and myelin basic protein.Results: DTI of Aβ1-42-treated mice showed a significant increase of radial diffusivity in white matter including the optic nerves and tracts. The abnormality was associated with decreased amplitude and increased latency of VEP. Immunohistochemistry confirmed a significant loss of axons and myelin integrity.Conclusion: White matter damage induced by Aβ1-42 in mice can be detected non-invasively by DTI. [ABSTRACT FROM AUTHOR]

Abstract: Keyhole diffusion tensor imaging (keyhole DTI) was previously proposed in cardiac imaging to reconstruct DTI maps from the reduced phase-encoding images. To evaluate the feasibility of keyhole DTI in brain imaging, keyhole and zero-padding DTI algorithms were employed on in vivo mouse brain. The reduced phase-encoding portion, also termed as the sharing rate, was varied from 50% to 90% of the full k-space. Our data showed that zero-padding DTI resulted in decreased fractional anisotropy (FA) and decreased mean apparent diffusion coefficient (mean ADC) in white matter (WM) regions. Keyhole DTI showed a better edge preservation on mean ADC maps but not on FA maps as compared to the zero-padding DTI. When increasing the sharing rate in keyhole approach, an underestimation of FA and an over- or underestimation of mean ADC were measured in WM depending on the selected reference image. The inconsistency of keyhole DTI may add a challenge for the wide use of this modality. However, with a carefully selected directive diffusion-weighted image to serve as the reference image in the keyhole approach, this study demonstrated that one may obtain DTI indices of reduced-encoding images with high consistency to those derived with full k-space DTI. [ABSTRACT FROM AUTHOR]

Abstract: The changes of directional diffusivities derived from diffusion tensor imaging (DTI), i.e. decreased axial diffusivity (λ ||) and increased radial diffusivity (λ ⊥), have shown significant correlation with axonal and myelin damage, respectively. However, after formalin fixation, reduced sensitivity of λ || in detecting axonal damage in tissue has raised the concern of applying DTI ex vivo. In order to distinguish whether death or the fixation process diminishes the sensitivity of DTI in detecting lesions, in vivo, pre-fixed postmortem, and fixed postmortem DTI were conducted on mouse optic nerves 3 and 14 days after transient retinal ischemia. Our data showed that, from in vivo to pre-fixed postmortem, λ || and λ ⊥ decreased by 50 to 70% in both healthy and injured optic nerves (3 and 14 day injury). From pre-fixed postmortem to fixed postmortem, λ || and λ ⊥ decreased by 40 to 50% in normal and 3-day injured optic nerves, but only by 15 to 25% in 14-day injured optic nerves. Consequently, for the 14-day injured optic nerves, the differences between healthy and injured nerves were not preserved after fixation: the 40% decreased λ|| and 200% increased λ⊥ in injured nerves as compared to the normal nerves were measured in vivo and pre-fixed postmortem, but after the fixation process, 300% increased λ⊥ and insignificant changes in λ|| were found in injured nerves as compared to the normal nerves. This study clarified that fixation process, but not death, could change the sensitivity of DTI in detecting injury. [Copyright &y& Elsevier]

• Correlation between MEMRI and DTI was found in P301L tau mice. • Axonal transport deficits in tauopathy were correlated with the loss of myelin. • Myelin loss may contribute to the axonal degeneration in tauopathy. Diffusion Tensor Imaging (DTI) and Manganese Enhanced MRI (MEMRI) are noninvasive tools to characterize neural fiber microstructure and axonal transport. A combination of both may provide novel insights into the progress of neurodegeneration. To investigate the relationship of DTI and MEMRI in white matter of tauopathy, twelve optic nerves of 11-month-old p301L tau mice were imaged and finished with postmortem immunohistochemistry. MEMRI was used to quantify Mn2+ accumulation rates in the optic nerve (ON, termed ONAR) and the Superior Colliculus (SC, termed SCAR), the primary terminal site of ON in mice. We found that both ONAR and SCAR revealed a significant linear correlation with mean diffusion (mD) and radial diffusion (rD) but not with other DTI quantities. Immunohistochemistry findings showed that ONAR, mD, and rD are significantly correlated with the myelin content (Myelin Basic Protein, p < 0.05) but not with the axonal density (SMI-31), tubulin density, or tau aggregates (AT8 staining). In summary, slower axonal transport appeared to have less myelinated axons and thinner remaining axons, associated with reduced rD and mD of in vivo DTI. A combination of in vivo MEMRI and DTI can provide critical information to delineate the progress of white matter deficits in neurodegenerative diseases. [ABSTRACT FROM AUTHOR]

Abstract White matter abnormalities, revealed by Diffusion Tensor Imaging (DTI), are observed in patients with Alzheimer's Disease (AD), representing neural network deficits that underlie gradual cognitive decline in patients. However, how DTI changes related to the development of Amyloid beta (Aβ) and tau pathology, two key hallmarks of AD, remain elusive. We hypothesized that tauopathy induced by Aβ could initiate an axonal degeneration, leading to DTI-detectable white matter abnormalities. We utilized the visual system of the transgenic p301L tau mice as a model system. Aβ was injected in Lateral Geniculate Nucleus (LGN), where the Retinal Ganglion Cell (RGC) axons terminate. Longitudinal DTI was conducted to detect changes in the optic tract (OT) and optic nerve (ON), containing the distal and proximal segments of RGC axons, respectively. Our results showed DTI changes in OT (significant 13.2% reduction in axial diffusion, AxD vs. vehicle controls) followed by significant alterations in ON AxD and fractional anisotropy, FA. Histology data revealed loss of synapses, RGC axons and cell bodies resulting from the Aβ injection. We further tested whether microtubule-stabilizing compound Epothilone D (EpoD) could ameliorate the damage. EpoD co-treatment with Aβ was sufficient to prevent Aβ-induced axon and cell loss. Using an acute injection paradigm, our data suggest that EpoD may mediate its protective effect by blocking localized, acute Aβ-induced tau phosphorylation. This study demonstrates white matter disruption resulting from localized Aβ, the importance of tau pathology induction to changes in white matter connectivity, and the use of EpoD as a potential therapeutic avenue to prevent the axon loss in AD. Highlights • Aβ injection into the Lateral Geniculate Nucleus of p301L mice precipitated degeneration of Retinal Ganglion Cells. • Degeneration induced by Aβ was detectable by Diffusion Tensor Imaging early in OT and later in ON, confirmed by histology. • Treating mice with microtubule stabilizer EpoD ameliorated Aβ-induced tauopathy, axonal damage, and neuronal loss. [ABSTRACT FROM AUTHOR]

Abstract: Myelin damage, as seen in multiple sclerosis (MS) and other demyelinating diseases, impairs axonal conduction and can also be associated with axonal degeneration. Accurate assessments of these conditions may be highly beneficial in evaluating and selecting therapeutic strategies for patient management. Recently, an analytical approach examining diffusion tensor imaging (DTI) derived parameters has been proposed to assess the extent of axonal damage, demyelination, or both. The current study uses the well-characterized cuprizone model of experimental demyelination and remyelination of corpus callosum in mouse brain to evaluate the ability of DTI parameters to detect the progression of myelin degeneration and regeneration. Our results demonstrate that the extent of increased radial diffusivity reflects the severity of demyelination in corpus callosum of mouse brain affected by cuprizone treatment. Subsequently, radial diffusivity decreases with the progression of remyelination. Furthermore, radial diffusivity changes were specific to the time course of changes in myelin integrity as distinct from axonal injury, which was detected by βAPP immunostaining and shown to be most extensive prior to demyelination. Radial diffusivity offers a specific assessment of demyelination and remyelination, as distinct from acute axonal damage. [Copyright &y& Elsevier]