Water Compartmentalization Research Articles

18 O enrichment of sucrose and photosynthetic and nonphotosynthetic leaf water in a C3 grass-atmospheric drivers and physiological relations.

The 18 O enrichment (Δ18 O) of leaf water affects the Δ18 O of photosynthetic products such as sucrose, generating an isotopic archive of plant function and past climate. However, uncertainty remains as to whether leaf water compartmentation between photosynthetic and nonphotosynthetic tissue affects the relationship between Δ18 O of bulk leaf water (Δ18 OLW ) and leaf sucrose (Δ18 OSucrose ). We grew Lolium perenne (a C3 grass) in mesocosm-scale, replicated experiments with daytime relative humidity (50% or 75%) and CO2 level (200, 400 or 800 μmol mol-1 ) as factors, and determined Δ18 OLW , Δ18 OSucrose and morphophysiological leaf parameters, including transpiration (Eleaf ), stomatal conductance (gs ) and mesophyll conductance to CO2 (gm ). The Δ18 O of photosynthetic medium water (Δ18 OSSW ) was estimated from Δ18 OSucrose and the equilibrium fractionation between water and carbonyl groups (εbio ). Δ18 OSSW was well predicted by theoretical estimates of leaf water at the evaporative site (Δ18 Oe ) with adjustments that correlated with gas exchange parameters (gs or total conductance to CO2 ). Isotopic mass balance and published work indicated that nonphotosynthetic tissue water was a large fraction (~0.53) of bulk leaf water. Δ18 OLW was a poor proxy for Δ18 OSucrose , mainly due to opposite Δ18 O responses of nonphotosynthetic tissue water (Δ18 Onon-SSW ) relative to Δ18 OSSW , driven by atmospheric conditions.

P.32 Impact of age on muscle volume and T2-relaxation time during adulthood in mice using quantitative MRI

Overall ageing is associated with a loss of muscle volume and function over time; nevertheless, detailed changes of muscle tissue are incompletely understood. In this preliminary study, magnetic resonance imaging (MRI) was used to assess the effect of age on muscle volume and T2. A cohort of 13 C57Bl/6J Rj male mice was followed from 2.5 to 17 months of age. MRI acquisitions were done at 7 Tesla using a 1H surface cryoprobe. Muscle volume and T2 of the anterior compartment (TA+EDL: tibialis anterior + extensor digitorum longus) were assessed by cross sectional area (CSA) from high-resolution gradient-echo axial images and from a multi-slice multi echo MRI sequence, respectively. Student's paired t test and ANOVA were used for statistical analysis (p < 0.05). Body weight increased up to 12 months and stayed constant until 17 months of age. Muscle volume reached its maximum at 9 months of age. The maximum CSA of TA+EDL muscles, measured on a single slice, decreased significantly from 9 to 17 months of age (8±0.5 mm² at 9 m/o vs 7.1±0.4 mm² at 17 m/o). In parallel, muscle T2 increased significantly from 2.5 to 17 months of age (T2 at 2.5 m/o = 21.5 ±0.2 ms vs T2 at 17 m/o = 22.6±0.3 ms) and from 2.5 to 6 months of age (T2 at 6 m/o = 22.3±0.4 ms). A similar tendency was also observed between 9 and 17 months of age (T2 at 9 m/o = 22± 0.5; p = 0.054). Repeated acquisitions of high-resolution images allowed for longitudinal follow-up of muscle volume from 2.5 up to 17 months of age in mice. The onset of muscle ageing seems to be around 9 months whereas body weight still increases up to 12 months. Muscle T2 increased significantly with age over the timespan of the study. In young ages (2.5 to 6 m/o), T2 increase probably reflects slight changes of histological water compartmentation related to tissue maturation. Ongoing experiments will refine this longitudinal follow-up and provide data at older ages.

Total Body and Extracellular Water Measures Are Unrelated to Cramp Sensitivity in Euhydrated Cramp-Prone Individuals.

Earp, JE, Stearns, RL, Agostinucci, J, Lepley, AS, and Ward-Ritacco, CL. Total body and extracellular water measures are unrelated to cramp sensitivity in euhydrated cramp-prone individuals. J Strength Cond Res 36(9): 2653-2656, 2022-Spectral bioelectrical impedance analysis (BIA) is a valid and noninvasive tool for measuring total body water (TBW), intracellular water (ICW), and extracellular water (ECW). As altered hydration and electrolyte imbalance have been proposed as one of 2 etiologies for exercise-associated muscle cramps (EAMC), the purpose of this study was to determine if distribution of body water is related to cramp sensitivity in similarly hydrated cramp-prone individuals. To this end, 11 euhydrated subjects who regularly experience EAMC had their relative TBW, ICW, and ECW assessed using 8-pole spectral BIA. Subjects' cramp sensitivity was then assessed by electrically stimulating the tibial nerve at increasing frequencies until a muscle cramp occurred, allowing for the determination of the threshold frequency (TF) at which the cramp occurred. It was observed that TF was not significantly related to TBW ( r = 0.087, p = 0.368), ICW ( r = 0.105, p = 0.338), ECW ( r = 0.087, p = 0.368), or ECW:TBW ( r = 0.147, p = 0.280). As cramp etiology is poorly understood, these results add to a growing body of literature questioning the role of hydration and electrolyte imbalance in EAMC. Although fluid distribution may be unrelated to TF in those who commonly experience EAMC, additional research is needed to compare those who commonly experience cramps (athletes as well as individuals with specific neuropathies or pharmacologically induced cramps) with those who do not experience cramps and to determine if acute shifts in body water compartmentalization are related to changes in cramp sensitivity.

Water exchange between intra and extracellular compartments studied by nuclear magnetic resonance relaxometry and optical microscopy

Nuclear magnetic resonance (NMR) techniques, in particular 1H relaxometry, enable the acquisition of information on the water exchange between different compartments in real-time and in a non-invasive and non-destructive way. Therefore, these are currently the methods of choice for studying the diffusion of water in living tissues and cells. The multi-compartmental nature of these systems may generate multi-exponential decays of the nuclear magnetization vector components to be analyzed in terms of discrete or continuous distributions of the so-called relaxation times. We have evaluated the sensitivity of the continuous distributions of the transverse component relaxation time (T2) to detect and monitor, over time, the water diffusion caused by osmotic pressure variations in tumor cells suspended in isotonic and hypotonic/hypertonic culture media. Our results indicated that the T2 distribution shapes were capable of describing the water compartmentalization and exchange, with evolutions that strongly depended on the degree of tonicity alterations. In order to validate the NMR results we also monitored the samples by phase contrast optical microscopy finding consistency between the two methods. Moreover, a study of T2–T2 correlation function allowed us to observe the exchange between cytoplasmic water and intracellular 1H spin groups. Using a two-site exchange model we also discussed, as a function of the uncertainties on the intra-to-extra water fraction and on the T2 values, the computed intracellular average water lifetime at equilibrium, an important parameter recently proposed as indicator of tumor aggressiveness. The results support the idea that the information conveyed by the T2 parameter might be useful for an MRI diagnostic approach to diseases in which the fluid tonicity or the cellular membranes permeability are altered.

MR fingerprinting for water T1 and fat fraction quantification in fat infiltrated skeletal muscles.

To develop a fast MR fingerprinting (MRF) sequence for simultaneous estimation of water T1 (T1H2O ) and fat fraction (FF) in fat infiltrated skeletal muscles. The MRF sequence for T1H2O and FF quantification (MRF T1-FF) comprises a 1400 radial spokes echo train, following nonselective inversion, with varying echo and repetition time, as well as prescribed flip angle. Undersampled frames were reconstructed at different acquisition time-points by nonuniform Fourier transform, and a bi-component model based on Bloch simulations applied to adjust the signal evolution and extract T1H2O and FF. The sequence was validated on a multi-vial phantom, in three healthy volunteers and five patients with neuromuscular diseases. We evaluated the agreement between MRF T1-FF parameters and reference values and confounding effects due to B0 and B1 inhomogeneities. In phantom, T1H2O and FF were highly correlated with references values measured with multi-inversion time inversion recovery-stimulated echo acquisition mode and Dixon, respectively (R2 > 0.99). In vivo, T1H2O and FF determined by the MRF T1-FF sequence were also correlated with reference values (R2 = 0.98 and 0.97, respectively). The precision on T1H2O was better than 5% for muscles where FF was less than 0.4. Both T1H2O and FF values were not confounded by B0 nor B1 inhomogeneities. The MRF T1-FF sequence derived T1H2O and FF values in voxels containing a mixture of water and fat protons. This method can be used to comprehend and characterize the effects of tissue water compartmentation and distribution on muscle T1 values in patients affected by chronic fat infiltration.

Impact of enrichment with egg constituents on water status in gluten-free rice pasta – nuclear magnetic resonance and thermogravimetric approach

Effects of freeze-dried egg white, yolk and whole egg enrichment on water behaviour in fresh pasta dough, dried and cooked rice pasta with respect to control samples were studied by 1H nuclear magnetic resonance (NMR) relaxometry and thermogravimetric analysis. Enrichments caused lower mobility of water (T2) localised within the starch-protein matrix in fresh dough as well as dried pasta. Water compartmentalization was also downgraded in cooked products. Water fractions with different T2 values were linked to temperature peaks at the first derivative of the thermogravimetric (DTG) curve. From the DTG curve strong interaction of water molecules with proteins of egg white was revealed. Egg proteins also influenced viscoelastic properties of dough, and enhanced the firmness and chewiness of cooked pasta. Structural changes induced by various types of enrichment were reflected in the different molecular mobility at the water-matrix interface (T1). The enrichments also altered the colour and cooking properties.

Building nanopore-supported 2D vesicles in surfaces

Water compartmentalization in a two-dimensional (2D) configuration is a challenging and thriving field. Here, we form fluid-fluid structures called 2D vesicles in which aqueous solutions are encapsulated within small oil barriers into a mesoporous thin film that allows them to communicate with a surrounding aqueous environment through predefined paths. Each area in the 2D space can also be programmed to specifically achieve different chemical processes. The framework of these nanopore-supported 2D vesicles has exciting applications in advanced surface developments and multi-device technologies.

P.96 - Effects of water compartmentation and distribution on skeletal muscle T1 values assessed by quantitative NMR imaging

NMR imaging, and particularly T1-weighted acquisitions, can be used for the diagnostic of NMDs taking advantage of the high contrast between healthy and fatty infiltrated muscles. However, very little is known about the effects of tissue water compartmentation and distribution on muscle T1 values. With the development of fast T1 mapping sequences, it is now possible to monitor this parameter within scan times compatible with clinical research. The goal of this study was to investigate the variations of skeletal muscle T1 values under various physiopathological conditions using a fast T1 mapping sequence. NMR imaging of the lower limbs was performed at 3T on 10 healthy volunteers at rest and after a 10' plantar flexion bout and at rest on 43 patients suffering from different neuromuscular pathologies: Becker muscular dystrophy (BMD: N = 30) and inclusion body myositis (IBM: N = 13). The T1 mapping sequence consisted in the acquisition of a 1000 radial spokes FLASH echo train after magnetization inversion (TE/TR = 2.75/5.08 ms, resolution = 1.8x1.8 mm2, 5 slices, Tacq = 50s). Water T2 and fat fractions were also quantitatively assessed with the “tri-exponential fitting” and “Dixon” reference methods. After exercise, a significant 7.2% increase of T1 was measured on the 10 volunteers in the gastrocnemius lateralis and medialis, corresponding to the muscle groups recruited by plantar flexion of the foot. T1 and water T2 values measured at the different time-points before and after this paradigm were correlated (R = 0.75, p < 0.05). Combining FF and T1 mapping, we measured higher T1s in the muscle of the IBM group compared to the BMD group, and global T1 was systematically increased when water T2 was increased. The sensitivity of the T1 and T2 approaches were equivalent to discriminate between the different muscle conditions. The next step will be to develop a bi-compartment fitting approach in order to simultaneously estimate water T1 and fat fractions from this T1 mapping sequence.

Water compartmentalization and hydration state of patients attending to a cardiac rehabilitation program.

Introduction: Alterations in cardiovascular functions can change body water distribution. Therefore it would be interesting to include the control of this parameter in programs of cardiac rehabilitation (PCR) so as to keep body water in a healthy range. Objective: To evaluate the effects of PCR on patients’ hydration. Method: Study was performed in two groups of patients suffering from a cardiovascular event: 1) PCR: 135 patients attending to a PCR (105 men, 30 women); 2) NOPCR: 70 patients not attending to a PCR (50 men, 20 women). Body water and phase angle were measured by bioelectrical impedance (TANITA MC-980MA multifrequency). Results: In PCR group, total water in men was higher (43.77±0.63 vs. 40.44±0.72 kg) (p<0.05) than NOPCR group, and as for women, total water (37.13±1.49vs. 32.54±1.52 Kg) and intracellular water also increased with regard to NOPCR group (20.39±0.94 vs. 17.02±0.81 kg) (p<0.001 and p<0.01 respectively). In addition, a major phase angle exists in left body (4.98±0.23 vs. 6.11±0.18 kg in men and 3.87±0.17 vs. 5.15±0.33 kg in women) and both legs in the PCR group (4.89±0.21 vs. 6.05±0.19 kg in men and 4.02±0.29 vs. 5.21±0.31 kg) (p<0.05). Conclusions: Subjects attending PCR showed a higher amount of total and intracellular water and higher phase angle, revealing a better state of hydration, and also a better integrity of the cell membrane and distribution of water between compartments.

Water compartmentalization, cell viability and morphology changes monitored under stress by 1H-NMR relaxometry and phase contrast optical microscopy

The quasi-continuous distributions of spin-lattice (T1) and spin-spin (T2) relaxation times of a population of cells kept under stress conditions for about 400 h has been obtained by 1H-NMR relaxometry. The comparison with the results obtained from the analysis of cell viability and morphological variations by phase contrast optical microscopy and performed with acquisition times, duration and conditions matching those of the relaxometry experiments allowed us to identify three main phases of the processes related to the exchanges of water and remodeling of cellular compartments: between 0 and 40 h the intra- and extra-cellular compartments are characterized by a T2 of ~250 ms and of ~2 s respectively and the cell population half-life has been experimentally evaluated to be ~45 h; after 40 and until 100 h cells NMR parameter changes suggest that the confined water is increasing according to the progressive fragmentation of the cells membrane; over 100 h almost all cells are non-viable and the constancy of the main NMR parameters reflects the reaching of the final equilibrium of the system. In conclusion our observations validated the use of NMR as a non-destructive, non-invasive powerful technique for monitoring the progression of cellular processes involving compartments water exchange and reorganization.

Influence of water compartmentation and heterogeneous relaxation on quantitative magnetization transfer imaging in rodent brain tumors.

The goal of this study was to investigate the influence of water compartmentation and heterogeneous relaxation properties on quantitative magnetization transfer (qMT) imaging in tissues, and in particular whether a two-pool model is sufficient to describe qMT data in brain tumors. Computer simulations and in vivo experiments with a series of qMT measurements before and after injection of Gd-DTPA were performed. Both off-resonance pulsed saturation (pulsed) and on-resonance selective inversion recovery (SIR) qMT methods were used, and all data were fit with a two-pool model only. Simulations indicated that a two-pool fitting of four-pool data yielded accurate measures of pool size ratio (PSR) of macromolecular versus free water protons when there were fast transcytolemmal exchange and slow R1 recovery. The fitted in vivo PSR of both pulsed and SIR qMT methods showed no dependence on R1 variations caused by different concentrations of Gd-DTPA during wash-out, whereas the fitted kex (magnetization transfer exchange rate) changed significantly with R1 . A two-pool model provides reproducible estimates of PSR in brain tumors independent of relaxation properties in the presence of relatively fast transcytolemmal exchange, whereas estimates of kex are biased by relaxation variations. In addition, estimates of PSR in brain tumors using the pulsed and SIR qMT methods agree well with one another. Magn Reson Med 76:635-644, 2016. © 2015 Wiley Periodicals, Inc.

Global separation of plant transpiration from groundwater and streamflow.

Current land surface models assume that groundwater, streamflow and plant transpiration are all sourced and mediated by the same well mixed water reservoir--the soil. However, recent work in Oregon and Mexico has shown evidence of ecohydrological separation, whereby different subsurface compartmentalized pools of water supply either plant transpiration fluxes or the combined fluxes of groundwater and streamflow. These findings have not yet been widely tested. Here we use hydrogen and oxygen isotopic data ((2)H/(1)H (δ(2)H) and (18)O/(16)O (δ(18)O)) from 47 globally distributed sites to show that ecohydrological separation is widespread across different biomes. Precipitation, stream water and groundwater from each site plot approximately along the δ(2)H/δ(18)O slope of local precipitation inputs. But soil and plant xylem waters extracted from the 47 sites all plot below the local stream water and groundwater on the meteoric water line, suggesting that plants use soil water that does not itself contribute to groundwater recharge or streamflow. Our results further show that, at 80% of the sites, the precipitation that supplies groundwater recharge and streamflow is different from the water that supplies parts of soil water recharge and plant transpiration. The ubiquity of subsurface water compartmentalization found here, and the segregation of storm types relative to hydrological and ecological fluxes, may be used to improve numerical simulations of runoff generation, stream water transit time and evaporation-transpiration partitioning. Future land surface model parameterizations should be closely examined for how vegetation, groundwater recharge and streamflow are assumed to be coupled.

QuantitativeT2: interactive quantitative T2 MRI witnessed in mouse glioblastoma.

The aim of this study was to establish an advanced analytical platform for complex in vivo pathologies. We have developed a software program, QuantitativeT2, for voxel-based real-time quantitative T2 magnetic resonance imaging. We analyzed murine brain tumors to confirm feasibility of our method for neurological conditions. Anesthetized mice (with invasive gliomas, and controls) were imaged on a 9.4 Tesla scanner using a Carr-Purcell-Meiboom-Gill sequence. The multiecho T2 decays from axial brain slices were analyzed using QuantitativeT2. T2 distribution histograms demonstrated substantial characteristic differences between normal and pathological brain tissues. Voxel-based quantitative maps of tissue water fraction (WF) and geometric mean T2 (gmT2) revealed the heterogeneous alterations to water compartmentalization caused by pathology. The numeric distribution of WF and gmT2 indicated the extent of tumor infiltration. Relative evaluations between in vivo scans and ex vivo histology indicated that the T2s between 30 and 150ms were related to cellular density and the integrity of the extracellular matrix. Overall, QuantitativeT2 has demonstrated significant advancements in qT2 analysis with real-time operation. It is interactive with an intuitive workflow; can analyze data from many MR manufacturers; and is released as open-source code to encourage examination, improvement, and expansion of this method.

MRI investigation of subcellular water compartmentalization and gas distribution in apples

Water status and distribution at subcellular level in whole apple fruit were evaluated by Magnetic Resonance Imaging (MRI) measurement of the multi-exponential transverse (T2) relaxation of water protons. Apparent microporosity, also estimated by MRI, provided mapping of gas distribution in fruit tissues. Measuring for the first time the multi-exponential relaxation of water and apparent tissue microporosity in whole fruit and combining these with histological measurements provided a more reliable interpretation of the origins of variations in the transverse relaxation time (T2) and better characterization of the fruit tissue. Measurements were performed on 54 fruits from 3 different cultivars. Fruits of different sizes were selected for each cultivar to provide tissues with cells of different dimensions. Macrovision measurements were carried out on parenchymal tissue from all fruits to investigate the impact of cell size on T2 value.The results showed that the MRI transverse relaxation signal is well fitted by a tri-exponential decay curve that reflects cell compartmentalization. Variations in cell size partially explained the different T2 observed. This study highlighted the heterogeneity of apple tissues in terms of relaxation parameters, apparent microporosity and cell morphology and in relation to specific variations between fruit of different cultivars.

Natural Variation in Resistance to Desiccation and Heat Shock Protein Expression in the Land SnailTheba pisanaalong a Climatic Gradient

Land snails frequently encounter desiccating conditions, and their survival depends on a suite of morphological, physiological, and molecular adaptations to the specific microhabitat. Strategies of survival can be determined by integrating information from various levels of biological organization. In this study, we used a combination of physiological parameters related to water economy and molecular factors (stress protein expression) to investigate the strategies of survival adopted by seven populations of the Mediterranean-type land snail Theba pisana from different habitats. We analyzed water compartmentalization during aestivation and used experimental desiccation to compare desiccation resistance. We also measured the endogenous levels of heat shock proteins (HSPs) Hsp72, Hsp74, and Hsp90 under nonstress conditions and analyzed the HSP response to desiccation in two populations that differed mostly in their resistance to desiccation. We revealed significant intraspecific differences in resistance to desiccation that seem to be determined by the speed of recruitment of the water-preserving mechanisms. The ability to cope with desiccating conditions was correlated with habitat temperature but not with the rainfall gradient, implying that in the coastal region, temperature is likely to have a major impact on desiccation resistance rather than precipitation. Also, higher desiccation resistance was correlated with higher constitutive levels of Hsp74 in the foot tissue. HSPs were upregulated during desiccation, but the response was delayed and was milder in the most resistant population compared to the most susceptible one. Our study suggests that T. pisana populations from warmer habitats were more resistant to desiccation and developed distinct strategies of HSP expression for survival, namely, the maintenance of high constitutive levels of Hsp70 together with a delayed and limited response to stress.

Non-Destructive Global and Localized 2D T1/T2 NMR Relaxometry to Resolve Microstructure in Apples Affected by Watercore

Apples can be considered as having a complex system formed by several structures at different organization levels: macroscale (>100 μm) and microscale (<100 μm). This work implements 2D T1/T2 global and localized relaxometry sequences on whole apples to be able to perform an intensive non-destructive and non-invasive microstructure study. The 2D T1/T2 cross-correlation spectroscopy allows the extraction of quantitative information about the water compartmentation in different subcellular organelles. A clear difference is found as sound apples show neat peaks for water in different subcellular compartments, such as vacuolar, cytoplasmatic and extracellular water, while in watercore-affected tissues such compartments appear merged. Localized relaxometry allows for the predefinition of slices in order to understand the microstructure of a particular region of the fruit, providing information that cannot be derived from global 2D T1/T2 relaxometry.

Dynamic effect of total solid content, low substrate/inoculum ratio and particle size on solid-state anaerobic digestion

Among all the process parameters of solid-state anaerobic digestion (SS-AD), total solid content (TS), inoculation (S/X ratio) and size of the organic solid particles can be optimized to improve methane yield and process stability. To evaluate the effects of each parameter and their interactions on methane production, a three level Box-Behnken experimental design was implemented in SS-AD batch tests degrading wheat straw by adjusting: TS content from 15% to 25%, S/X ratio (in volatile solids) between 28 and 47 and particle size with a mean diameter ranging from 0.1 to 1.4mm. A dynamic analysis of the methane production indicates that the S/X ratio has only an effect during the start-up phase of the SS-AD. During the growing phase, TS content becomes the main parameter governing the methane production and its strong interaction with the particle size suggests the important role of water compartmentation on SS-AD.

Micro-compartment specific T2⁎ relaxation in the brain

MRI at high field can be sensitized to the magnetic properties of tissues, which introduces a signal dependence on the orientation of white matter (WM) fiber bundles relative to the magnetic field. In addition, study of the NMR relaxation properties of this signal has indicated contributions from compartmentalized water environments inside and outside the myelin sheath that may be separable. Here we further investigated the effects of water compartmentalization on the MRI signal with the goal of extracting compartment-specific information. By comparing MRI measurements of human and marmoset brain at 7T with magnetic field modeling, we show that: (1) water between the myelin lipid bilayers, in the axonal, and in the interstitial space each experience characteristic magnetic field effects that depend on fiber orientation (2) these field effects result in characteristic relaxation properties and frequency shifts for these compartments; and (3) compartmental contributions may be separated by multi-component fitting of the MRI signal relaxation (i.e. decay) curve. We further show the potential application of these findings to the direct mapping of myelin content and assessment of WM fiber integrity with high field MRI.

Probing tissue microstructure with restriction spectrum imaging: Histological and theoretical validation.

Water diffusion magnetic resonance imaging (dMRI) is a powerful tool for studying biological tissue microarchitectures in vivo. Recently, there has been increased effort to develop quantitative dMRI methods to probe both length scale and orientation information in diffusion media. Diffusion spectrum imaging (DSI) is one such approach that aims to resolve such information based on the three-dimensional diffusion propagator at each voxel. However, in practice, only the orientation component of the propagator function is preserved when deriving the orientation distribution function. Here, we demonstrate how a straightforward extension of the linear spherical deconvolution (SD) model can be used to probe tissue orientation structures over a range (or "spectrum") of length scales with minimal assumptions on the underlying microarchitecture. Using high b-value Cartesian q-space data on a rat brain tissue sample, we demonstrate how this "restriction spectrum imaging" (RSI) model allows for separating the volume fraction and orientation distribution of hindered and restricted diffusion, which we argue stems primarily from diffusion in the extraneurite and intraneurite water compartment, respectively. Moreover, we demonstrate how empirical RSI estimates of the neurite orientation distribution and volume fraction capture important additional structure not afforded by traditional DSI or fixed-scale SD-like reconstructions, particularly in gray matter. We conclude that incorporating length scale information in geometric models of diffusion offers promise for advancing state-of-the-art dMRI methods beyond white matter into gray matter structures while allowing more detailed quantitative characterization of water compartmentalization and histoarchitecture of healthy and diseased tissue.

The γ parameter of the stretched-exponential model is influenced by internal gradients: Validation in phantoms

In this paper, we investigate the image contrast that characterizes anomalous and non-Gaussian diffusion images obtained using the stretched exponential model. This model is based on the introduction of the γ stretched parameter, which quantifies deviation from the mono-exponential decay of diffusion signal as a function of the b-value. To date, the biophysical substrate underpinning the contrast observed in γ maps, in other words, the biophysical interpretation of the γ parameter (or the fractional order derivative in space, β parameter) is still not fully understood, although it has already been applied to investigate both animal models and human brain. Due to the ability of γ maps to reflect additional microstructural information which cannot be obtained using diffusion procedures based on Gaussian diffusion, some authors propose this parameter as a measure of diffusion heterogeneity or water compartmentalization in biological tissues. Based on our recent work we suggest here that the coupling between internal and diffusion gradients provide pseudo-superdiffusion effects which are quantified by the stretching exponential parameter γ. This means that the image contrast of Mγ maps reflects local magnetic susceptibility differences (Δχm), thus highlighting better than T2∗ contrast the interface between compartments characterized by Δχm. Thanks to this characteristic, Mγ imaging may represent an interesting tool to develop contrast-enhanced MRI for molecular imaging.The spectroscopic and imaging experiments (performed in controlled micro-beads dispersion) that are reported here, strongly suggest internal gradients, and as a consequence Δχm, to be an important factor in fully understanding the source of contrast in anomalous diffusion methods that are based on a stretched exponential model analysis of diffusion data obtained at varying gradient strengths g.