Metabolite and water apparent diffusion coefficients in the isolated rat heart: effects of ischemia

Rat Brain Global Ischemia-Induced Diffusion Changes Revisited: Biophysical Modeling of the Water and NAA MR "Diffusion Signal"

PURPOSE

To assess changes in intracellular diffusion as a mechanism for the reduction in water ADC that accompanies brain injury. Using NAA as a marker of neuronal cytoplasmic diffusion, NAA diffusion was measured before and after global ischemia (immediately postmortem) in the female Sprague-Dawley rat.

METHODS

Diffusion-weighted PRESS spectra, with diffusion encoding in a single direction, were acquired from large voxels of rat brain gray matter in vivo and postischemia employing either pairs of pulsed half-sine-shaped gradients (in vivo and postischemia, b_max  = 19 ms/μm² ) or sinusoidal oscillating gradients (in vivo only) with frequencies of 99.2-250 Hz. A 2D randomly oriented cylinder (neurite) model gave estimates of longitudinal and transverse diffusivities (D_L and D_T , respectively). In this model, D_L represents the "free" diffusivity of NAA, whereas D_T reflects highly restricted diffusion. Using oscillating gradients, the frequency dependence of D_T [D_T (ω)] gave estimates of the cylinder (axon/dendrite) radius.

RESULTS

A 10% decrease in D_L,NAA followed global ischemia, dropping from 0.391 ± 0.012 μm² /ms to 0.350 ± 0.009 μm² /ms. Modeling D_T,NAA (ω) provided an estimate of the neurite radius of 1.0 ± 0.6 μm.

CONCLUSION

Whereas the increase in apparent intraneuronal viscosity suggested by changes in D_L,NAA may contribute to the overall reduction in water ADC associated with brain injury, it is not sufficient to be the sole explanation. Estimates of neurite radius based on D_T (ω) were consistent with literature values.

Nuclear magnetic resonance footprint of Wharton Jelly mesenchymal stem cells death mechanisms and distinctive in‐cell biophysical properties in vitro

The importance of the biophysical characterization of mesenchymal stem cells (MSCs) was recently pointed out for supporting the development of MSC‐based therapies. Among others, tracking MSCs in vivo and a quantitative characterization of their regenerative impact by nuclear magnetic resonance (NMR) demands a full description of MSCs’ MR properties. In the work, Wharton Jelly MSCs are characterized in a low magnetic field (LF) in vitro by using different approaches. They encompass various settings: MSCs cultured in a Petri dish and cell suspensions; experiments‐ 1D‐T₁, 1D‐T₂, 1D diffusion, 2D T₁‐T₂ and D‐T₂; devices‐ with a bore aperture and single‐sided one. Complex NMR analysis with the aid of random walk simulations allows the determination of MSCs T₁ and T₂ relaxation times, cells and nuclei sizes, self‐diffusion coefficients of the nucleus and cytoplasm. In addition, the influence of a single layer of cells on the effective diffusion coefficient of water is detected with the application of a single‐sided NMR device. It also enables the identification of apoptotic and necrotic cell death and changed diffusional properties of cells suspension caused by compressing forces induced by the subsequent cell layers. The study delivers MSCs‐specific MR parameters that may help tracking MSCs in vivo.

Graphene oxide as a compatibilizer for polyvinyl chloride/rice straw composites

In this study, polyvinyl chloride (PVC)/rice straw (RS)/graphene oxide (GO) sustainable nanocomposite was prepared using the direct compounding method. Structural, morphological and mechanical properties of fabricated sustainable nanocomposites were compared with unfilled and RS-filled PVC compounds. Mechanical characteristics of PVC decreased with loading RS fibers. The main reason for the mechanical failure of PVC/RS composite is the incompatibility between PVC and RS fibers. GO nanosheets are used here to improve the compatibility between RS fibers and PVC macromolecules. Compared to the neat PVC, maximum strength of the RS/GO-loaded PVC composite increased up to 31%, with incorporating only 1 wt% of GO nanosheets. This enhancement in the mechanical characteristics of PVC/RS/GO nanocomposite can only be due to the role of GO nanosheets as a compatibilizer, as 1 wt% GO loading can only increase the mechanical strength of PVC compounds up to 9%. Fourier transform infrared spectroscopy results are used here to study the nature of these behaviors. It is suggested that the non-covalent and physical interactions between cellulose/hemicellulose portions of RS fibers and GO functional groups result in the enhancement of mechanical characteristics. Consequently, GO can be considered as a new compatibilizer for fabricating high performance PVC-based sustainable nanocomposites.

In vivo diffusion MRS investigation of non-water molecules in biological tissues

Diffusion MRS of non-water molecules offers great potential in directly revealing various tissue microstructures and physiology at both cellular and subcellular levels. In brain, ¹ H diffusion MRS has been demonstrated as a new tool for probing normal tissue microstructures and their pathological changes. In skeletal muscle, ¹ H diffusion MRS could characterize slow and restricted intramyocellular lipid diffusion, providing a sensitive marker for metabolic alterations, while ³¹ P diffusion MRS can measure ATP and PCr diffusion, which may reflect the capacity of cellular energy transport, complementing the information from frequently used ³¹ P MRS in muscle. In intervertebral disk, ¹ H diffusion MRS can directly monitor extracellular matrix integrity by quantifying the mobility of macromolecules such as proteoglycans and collagens. In tumor tissue, ¹³ C diffusion MRS could probe intracellular glycolytic metabolism, while ¹ H diffusion MRS may separate the spectrally overlapped lactate and lipid resonances. In this review, recent diffusion MRS studies of these biologically relevant non-water molecules under normal and diseased conditions will be presented. Technical considerations for diffusion MRS experiments will be discussed. With advances in MRI hardware and diffusion methodology, diffusion MRS of non-water molecules is expected to provide increasingly valuable and biologically specific information on tissue microstructures and physiology, complementing the traditional diffusion MRI of small and ubiquitous water molecules. Copyright © 2016 John Wiley & Sons, Ltd.

Proton diffusion tensor spectroscopy of metabolites in human muscle in vivo

PURPOSE

To study the apparent diffusivity and its directionality for metabolites of skeletal muscle in humans in vivo by (1) H magnetic resonance spectroscopy.

METHODS

The diffusion tensors were determined on a 3 Tesla MR system using optimized acquisition and processing methods including an adapted STEAM sequence with orientation-dependent diffusion weighting, pulse-triggering with individually adapted delays, eddy-current correction schemes, median filtering, and simultaneous prior-knowledge fitting of all related spectra.

RESULTS

The average apparent diffusivities, as well as the fractional anisotropies of taurine (ADCav=0.74 × 10(-3) s/mm(2) , FA=0.46), creatine (ADCav =0.41 × 10(-3) s/mm(2) , FA=0.33), trimethylammonium compounds (ADCav =0.48 × 10(-3) s/mm(2) , FA=0.34), carnosine (ADCav =0.46 × 10(-3) s/mm(2) , FA=0.47), and water (ADCav=1.5 × 10(-3) s/mm(2) , FA=0.36) were estimated. The diffusivities of most metabolites and water were significantly different from each other. Diffusion was found to be anisotropic and the diffusion tensors showed tensor correlation coefficients close to 1 and were hence found to be essentially coaligned. The magnitudes of apparent metabolite diffusivities were largely ordered according to molecular weight, with taurine as the smallest molecule diffusing fastest, both along and across the fiber direction.

CONCLUSION

Diffusivities, directional dependence of diffusion and fractional anisotropies of (1) H MRS-visible muscle metabolites were presented. It was shown that metabolites share diffusion directionality with water and have similar fractional anisotropies, hinting at similar diffusion barriers.

Apparent diffusion coefficients of metabolites in patients with MELAS using diffusion-weighted MR spectroscopy

BACKGROUND AND PURPOSE

DW-MR spectroscopy can detect the diffusion coefficients of NAA, Cr, PCr, and Cho and can, therefore, provide some useful information. The aims of this study were to probe the mechanisms underlying the pathogenesis of MELAS and to see whether DW-MR spectroscopy is a useful technique for other diseases besides cerebral infarction.

MATERIALS AND METHODS

Fifteen healthy volunteers and 10 patients with MELAS were enrolled in the study. All were scanned on a 3T whole-body MR imaging scanner. Fifteen ADCs of the singlet metabolites in the gray matter of the healthy subjects, 10 ADCs of the singlet metabolites in the lesions, and 8 ADCs of the singlet metabolites in the nonaffected areas were used in the statistical analysis, respectively.

RESULTS

The metabolite ADCs of the nonaffected areas and the lesions in the patients were higher than those of the frontal gray matter in the healthy controls. There were significant differences between the metabolite ADCs of the nonaffected areas in patients and those in the healthy controls, and it was the same with the metabolite ADCs of the lesions and those of the healthy controls.

CONCLUSIONS

The increased ADC values of the metabolites reveal that MELAS is a mitochondrial neuronopathy and involves the entire brain. DW-MR spectroscopy is a very useful noninvasive technique, which can show some valuable information that conventional MR imaging cannot display. Thus, it can be applied to brain diseases besides cerebral infarction.

High-energy phosphate transfer in human muscle: diffusion of phosphocreatine

The creatine kinase (CK) reaction is central to muscle energetics, buffering ATP levels during periods of intense activity via consumption of phosphocreatine (PCr). PCr is believed to serve as a spatial shuttle of high-energy phosphate between sites of energy production in the mitochondria and sites of energy utilization in the myofibrils via diffusion. Knowledge of the diffusion coefficient of PCr (D(PCr)) is thus critical for modeling and understanding energy transport in the myocyte, but D(PCr) has not been measured in humans. Using localized phosphorus magnetic resonance spectroscopy, we measured D(PCr) in the calf muscle of 11 adults as a function of direction and diffusion time. The results show that the diffusion of PCr is anisotropic, with significantly higher diffusion along the muscle fibers, and that the diffusion of PCr is restricted to a ∼28-μm pathlength assuming a cylindrical model, with an unbounded diffusion coefficient of ∼0.69 × 10(-3) mm(2)/s. This distance is comparable in size to the myofiber radius. On the basis of prior measures of CK reaction kinetics in human muscle, the expected diffusion distance of PCr during its half-life in the CK reaction is ∼66 μm. This distance is much greater than the average distances between mitochondria and myofibrils. Thus these first measurements of PCr diffusion in human muscle in vivo support the view that PCr diffusion is not a factor limiting high-energy phosphate transport between the mitochondria and the myofibrils in healthy resting myocytes.

Evaluation of extra- and intracellular apparent diffusion coefficient of sodium in rat skeletal muscle: effects of prolonged ischemia

The mechanism of water and sodium apparent diffusion coefficient (ADC) changes in rat skeletal muscle during global ischemia was examined by in vivo 1H and 23Na magnetic resonance spectroscopy (MRS). The ADCs of Na+ and water are expected to have similar characteristics because sodium is present as an aqua-cation in tissue. The shift reagent, TmDOTP5(-), was used to separate intra- and extracellular sodium (Na+i and Na+e, respectively) signals. Water, total tissue sodium (Na+t), Na+i, and Na+e ADCs were measured before and 1, 2, 3, and 4 hr after ischemia. Contrary to the general perception, Na+i and Na+e ADCs were identical before ischemia. Thus, ischemia-induced changes in Na+e ADC cannot be explained by a simple change in the size of relative intracellular or extracellular space. Na+t and Na+e ADCs decreased after 2-4 hr of ischemia, while water and Na+i ADC remained unchanged. The correlation between Na+t and Na+e ADCs was observed because of high Na+e concentration. Similarly, the correlation between water and Na+i ADCs was observed because cells occupy 80% of the tissue space in the skeletal muscle. Ischemia also caused an increase in the Na+i and an equal decrease in Na+e signal intensity due to cessation of Na+/K+-ATPase function.

Diffusion characteristics of large molecules assessed by proton MRS on a whole-body MR system

Methods for examinations of diffusion of large molecules of the size of fatty acids or triglycerides were developed for whole body MR units. Samples of aliphatic molecules were examined to study the influence of chain length. Feasibility under in vivo conditions was tested on lard samples at 37 degrees C and on human subjects Three stimulated echo sequences with maximum b-values of 2000 s/mm(2), 20000 s/mm(2), and 80000 s/mm(2) were used to assess a wide range of mobility. Sequence timing was optimized to minimize relaxation losses of fatty tissue. Apparent diffusion coefficients (ADC) were determined from five spectra with different diffusion weighting. In-vitro experiments were performed on butanol, decanol, and oleic acid to study the influence of chain length. In vivo conditions were mimicked using lard at 37 degrees C representing a composition of substances of various chain lengths. Subcutaneous fat and tibial bone marrow were studied in three healthy volunteers. ADC of muscular lipids of the lower leg was determined in two subjects. ADC values of pure aliphatic substances were in the range between 3.2 x 10(-5) mm(2)/s for oleic acid and 37.8 x 10(-5) mm(2)/s for butanol. In vivo investigations revealed ADC values of 1.11-1.24 x 10(-5) mm(2)/s for tibial bone marrow and 1.21-2.05 x 10(-5) mm(2)/s for subcutaneous fat. Diffusion coefficients of extra- and intramyocellular lipids were 1.83-3.65 x 10(-5) mm(2)/s and 2.22-3.60 x 10(-5) mm(2)/s, respectively. The proposed technique enables determination of ADC values of relatively large molecules and of lipid tissue compartments under in vivo conditions. Diffusion properties in several human lipid compartments are reported for the first time. Incoherent voxel motion influences the in vivo results to an unknown degree because of high motion sensitivity. In vitro experiments revealed ADC values depending on the chain length of the substances, indicating a residual dependence of measured ADC's on sequence timing.

In vivo study of microcirculation in canine myocardium using the IVIM method

The intravoxel incoherent motion (IVIM) method was implemented in closed-chest dogs to obtain measurements on microcirculation in the left ventricular wall in vivo. Specifically, it enabled us to measure the mean microflow velocity (400 +/- 40 microm/s) and the vascular volume fraction (VVF) (11.1% +/- 2.2%), and observe the directional preference of capillary orientation. The apparent diffusion coefficients (ADCs) of water along and perpendicular to myofibers were also measured. With vasodilatation by adenosine infusion, a 25% increase in the VVF and a 7% increase in the mean microflow velocity were observed, while no change in the ADC was detected. A 28.5% decrease of the ADC was observed postmortem.

The effect of orientation on quantification of muscle creatine by 1H MR spectroscopy

Creatine is a central energy metabolite whose N-CH3 group can be detected with 1H MR spectroscopy (1H MRS) with relatively high sensitivity. Prior studies suggest that muscle fiber orientation can influence the appearance of other resonances attributed to total creatine (CR). Our purpose was to determine whether muscle fiber orientation affects muscle CR concentration quantification by 1H MRS with the commonly used N-CH3 resonance at 3.0 ppm. Skeletal muscle CR was quantified with water-referenced 1H MRS in normal subjects with different forearm muscle orientations relative to the static magnetic field at 1.5T. There were no significant differences in mean total [CR] in two different series of experiments separately including two orthogonal orientations and four orientations (0 degrees, 30 degrees, 60 degrees, 90 degrees) of the forearm relative to the static field using either short (TE = 15 ms) or long (TE = 100 ms) echo times for voxels containing or centered on the same tissues. Subtle differences in CR line-width and T2 correction factors were observed with orientation. These observations are consistent with the primary hypothesis that careful water-referenced [CR] quantification, accounting for T2 effects and using the N-CH3 peak at 3.0ppm, is not affected by muscle orientation.