Experimental demonstration of diffusion limitations on resolution and SNR in MR microscopy

Magnetic resonance microscopy images at cellular resolution (< 10 microns) are limited by diffusion. SNR and spatial resolution suffer from the dephasing of transverse magnetization caused by diffusion of spins in strong gradients. Such effects may be reduced by using phase encoding instead of frequency encoding readout gradients. Demonstration of the benefits of phase encoding are lacking, and the conditions in which it is preferred are not clearly established. We quantify when phase encoding outperforms a readout gradient with emphasis on the detrimental effects of diffusion on SNR and resolution. A 15.2T MRI scanner, with 1 T/m gradients, and micro solenoid RF coils < 1 mm in diameter, were used to quantify diffusion effects on resolution and SNR of frequency and phase encoded acquisitions. Frequency and phase encoding resolution and SNR per square root time were calculated and measured for images at the diffusion limited resolution. The point-spread-function was measured for phase and frequency encoding using additional constant time gradients with voxels 3-15 microns. The effect of diffusion during the readout gradient on SNR was experimentally demonstrated. The achieved resolutions of frequency and phase encoded acquisitions were measured via the point-spread-function. SNR per square root time and actual resolution were calculated for a wide range of gradient amplitudes, diffusion coefficients, and relaxation properties. The results provide a practical guide on how to choose between phase and frequency encoding. Images of excised rat spinal cord at 10 x 10 microns in-plane demonstrate benefits of phase encoding in the form of higher measured resolution and SNR vs the same image acquired with a conventional readout. We demonstrate the extent to which phase encoding outperforms readout gradients in SNR and resolution over a wide range of voxel sizes, sample, and hardware properties.Comment: 36 pages, 9 figures, 1 table, and 4 supplemental figures. Submitted to Journal of Magnetic Resonance; cleaned up metadata, fixed heading typ

Relayed nuclear Overhauser enhancement sensitivity to membrane Cho phospholipids

Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/155956/1/mrm28258_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/155956/2/mrm28258.pd

Non-invasive Predictors of Human Cortical Bone Mechanical Properties: T2-Discriminated 1H NMR Compared with High Resolution X-ray

Recent advancements in magnetic resonance imaging (MRI) have enabled clinical imaging of human cortical bone, providing a potentially powerful new means for assessing bone health with molecular-scale sensitivities unavailable to conventional X-ray-based diagnostics. To this end, 1H nuclear magnetic resonance (NMR) and high-resolution X-ray signals from human cortical bone samples were correlated with mechanical properties of bone. Results showed that 1H NMR signals were better predictors of yield stress, peak stress, and pre-yield toughness than were the X-ray derived signals. These 1H NMR signals can, in principle, be extracted from clinical MRI, thus offering the potential for improved clinical assessment of fracture risk

Cognitive reactivity: cultural adaptation and psychometric testing of the Persian version of the Leiden Index of Depression Sensitivity Revised (LEIDS-R) in an Iranian sample

Cognitive reactivity (CR) to the experimental induction of sad mood has been found to predict relapse in recovered depressed patients. The Leiden Index of Depression Sensitivity Revised (LEIDS-R) is a self-report measure of CR. The aim of the present study was to establish the validity and reliability of the Persian version of the LEIDS-R. The participants were recovered depressed and non-depressed Iranian individuals (n = 833). The analyses included content validation, factor analysis, construct validity, and reliability testing. Preliminary construct validation analysis confirmed that factor analysis was appropriate for the Persian version of the LEIDS-R. Factor analysis displayed similar factor loadings to the original English version. The total internal consistency of the translated version, which was assessed using Cronbach’s alpha coefficient, was equal to 0.90. The test-retest reliability of the total score was equal to that of the test-retest conducted after a two-week interval at 0.94. Content validity, face validity, and construct validity, as well as reliability analysis were all found to be satisfactory for the Persian version of the LEIDS-R. The Persian version of the LEIDS-R appears to be valid and reliable for use in future studies, and has properties comparable to the original version and to that obtained in previous studies

Characterization of hyperpolarization-activated cyclic nucleotide-gated channels in oligodendrocytes

Mature oligodendrocytes (OLG) are the myelin-forming cells of the central nervous system. Recent work has shown a dynamic role for these cells in the plasticity of neural circuits, leading to a renewed interest in voltage-sensitive currents in OLG. Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels and their respective current (Ih) were recently identified in mature OLG and shown to play a role in regulating myelin length. Here we provide a biochemical and electrophysiological characterization of HCN channels in cells of the oligodendrocyte lineage. We observed that mice with a nonsense mutation in the Hcn2 gene (Hcn2ap/ap) have less white matter than their wild type counterparts with fewer OLG and fewer oligodendrocyte progenitor cells (OPCs). Hcn2ap/ap mice have severe motor impairments, although these deficits were not observed in mice with HCN2 conditionally eliminated only in oligodendrocytes (Cnpcre/+; Hcn2F/F). However, Cnpcre/+; Hcn2F/F mice develop motor impairments more rapidly in response to experimental autoimmune encephalomyelitis (EAE). We conclude that HCN2 channels in OLG may play a role in regulating metabolism

Quantitative analysis of mouse corpus callosum from electron microscopy images

This article provides morphometric analysis of 72 electron microscopy images from control (n=4) and hypomyelinated (n=2) mouse corpus callosum. Measures of axon diameter and g-ratio were tabulated across all brains from two regions of the corpus callosum and a non-linear relationship between axon diameter and g-ratio was observed. These data are related to the accompanying research article comparing multiple methods of measuring g-ratio entitled ‘A revised model for estimating g-ratio from MRI’ (West et al., NeuroImage, 2015)

MRI-derived bound and pore water concentrations as predictors of fracture resistance

AbstractAccurately predicting fracture risk in the clinic is challenging because the determinants are multi-factorial. A common approach to fracture risk assessment is to combine X-ray-based imaging methods such as dual-energy X-ray absorptiometry (DXA) with an online Fracture Risk Assessment Tool (FRAX) that includes additional risk factors such as age, family history, and prior fracture incidents. This approach still does not adequately diagnose many individuals at risk, especially those with certain diseases like type 2 diabetes. As such, this study investigated bound water and pore water concentrations (Cbw and Cpw) from ultra-short echo time (UTE) magnetic resonance imaging (MRI) as new predictors of fracture risk. Ex vivo cadaveric arms were imaged with UTE MRI as well as with DXA and high-resolution micro-computed tomography (μCT), and imaging measures were compared to both whole-bone structural and material properties as determined by three-point bending tests of the distal-third radius. While DXA-derived areal bone mineral density (aBMD) and μCT-derived volumetric BMD correlated well with structural strength, they moderately correlated with the estimate material strength with gender being a significant covariate for aBMD. MRI-derived measures of Cbw and Cpw had a similar predictive ability of material strength as aBMD but did so independently of gender. In addition, Cbw was the only imaging parameter to significantly correlate with toughness, the energy dissipated during fracture. Notably, the strength of the correlations with the material properties of bone tended to be higher when a larger endosteal region was used to determine Cbw and Cpw. These results indicate that MRI measures of Cbw and Cpw have the ability to probe bone material properties independent of bone structure or subject gender. In particular, toughness is a property of fracture resistance that is not explained by X-ray based methods. Thus, these MRI-derived measures of Cbw and Cpw in cortical bone have the potential to be useful in clinical populations for evaluating fracture risk, especially involving diseases that affect material properties of the bone beyond its strength

Data from: Loss of mTORC2 signaling in oligodendrocyte precursor cells delays myelination

Myelin abnormalities are increasingly being recognized as an important component of a number of neurologic developmental disorders. The integration of many signaling pathways and cell types are critical for correct myelinogenesis. The PI3-K and mechanistic target of rapamycin (mTOR) pathways have been found to play key roles. mTOR is found within two distinct complexes, mTORC1 and mTORC2. mTORC1 activity has been shown to play a major role during myelination, while the role of mTORC2 is not yet well understood. To determine the role of mTORC2 signaling in myelinogenesis, we generated a mouse lacking the critical mTORC2 component Rictor in oligodendrocyte precursors (OPCs). Targeted deletion of Rictor in these cells decreases and delays the expression of myelin related proteins and reduces the size of cerebral white matter tracts. This is developmentally manifest as a transient reduction in myelinated axon density and g-ratio. OPC cell number is reduced at birth without detectable change in proliferation with proportional reductions in mature oligodendrocyte number at P15. The total number of oligodendrocytes as well as extent of myelination, does improve over time. Adult conditional knock-out (CKO) animals do not demonstrate a behavioral phenotype likely due in part to preserved axonal conduction velocities. These data support and extend prior studies demonstrating an important but transient contribution of mTORC2 signaling to myelin development