Estimate Of Relaxation Time Research Articles

Time of relaxation in dusty plasma model

Dust particles in plasma may have different values of average kinetic energy for vertical and horizontal motion. The partial equilibrium of the subsystems and the relaxation processes leading to this asymmetry are under consideration. A method for the relaxation time estimation in nonideal dusty plasma is suggested. The characteristic relaxation times of vertical and horizontal motion of dust particles in gas discharge are estimated by analytical approach and by analysis of simulation results. These relaxation times for vertical and horizontal subsystems appear to be different. A single hierarchy of relaxation times is proposed.

SU‐E‐J‐250: A Machine Learning Approach for Creating Texture‐Preserved MRI Tumor Models From Clinical Sequences

Purpose:We hypothesize that MRI texture‐based tumor outcome prediction models could be optimized via numerical simulations of image acquisitions. These simulations require knowledge of T1 and T2 relaxation times as inputs. The goal of this study is to evaluate the feasibility of using machine learning techniques to infer T1 and T2 tumor maps with accurate texture preservation for simulation inputs from clinical sequences.Methods:Clinical T1‐weighted (T1w) and T2‐weighted fat‐saturated (T2FS) scans, and measured T1 and T2 maps from eight patients with soft‐tissue sarcomas were used in this study. Measured T1 and T2 maps were computed using pulse sequences with variable flip angles and echo times, respectively. General regression neural networks (GRNNs) were trained on these data to infer T1 and T2 relaxation times from T1w and T2FS images. Four texture features were extracted to evaluate texture preservation: GLCM/Entropy, GLRLM/Gray‐Level Variance (GLV), GLSZM/Zone Size Variance (ZSV) and NGTDM/Complexity. The GRNN ability to estimate T1 and T2 relaxation times was assessed using leave‐one‐out cross‐validation.Results:The average T1 and T2 relaxation times within the tumor region of all patients were (1515 ± 542) ms and (226 ± 151) ms in the measured cases, and (1546 ± 546) ms and (249 ± 145) ms in the estimated cases, respectively. The average root‐mean‐square errors between measured and estimated relaxation times were 573 ms for T1 and 160 ms for T2. The average absolute percentage differences between measured and estimated GLCM/Entropy, GLRLM/GLV, GLSZM/ZSV and NGTDM/Complexity features were 5.1%, 0.02%, 0.0% and 16.2% for T1 maps, and 7.7%, 0.04%, 0.0% and 10.9% for T2 maps, respectively.Conclusion:From a texture preservation perspective, this work demonstrates the feasibility to create MRI numerical models using GRNNs from T1w and T2FS clinical scans. Further work is required to obtain higher accuracy for T1 and T2 absolute relaxation times.This work was supported by the Natural Sciences and Engineering Research Council (NSERC) of Canada under the scholarship CGSD3‐426742‐2012, as well as it was supported by the Canadian Institutes of Health Research (CIHR) under grant MOP‐136774.

Rheological studies of the fucose-rich exopolysaccharide FucoPol

In this work, the solution properties of the bacterial fucose-rich polysaccharide, FucoPol, were studied. The effect of pH (3.5–10.0) and ionic strength (0.02–1.0M NaCl) on the intrinsic viscosity and steady shear flow were evaluated using a central composite rotatable design of experiments and surface response methodology. FucoPol's intrinsic and apparent viscosities presented a quite low variation under a wide range of pH (3.5–8.0) and ionic strength (0.05–0.50M NaCl) values. FucoPol produced viscous solutions with shear-thinning behavior at different polymer concentrations (0.2–1.2wt.%). Flow curves were successfully described by the Cross model. The viscosity of the first Newtonian plateau varied from 0.01 to 2.47Pas for polymer concentrations from 0.2 to 1.2wt.%, and the dependence of the estimated relaxation time with polymer concentration suggests a large degree of interaction between FucoPol molecules. Given the results obtained, FucoPol is proposed as thickening agent for applications in which stability of the apparent viscosity under pH and ionic strength variations is required.

Quantification and significance of diffuse myocardial fibrosis and diastolic dysfunction in childhood hypertrophic cardiomyopathy.

The purpose of this study was to evaluate the presence of diffuse myocardial fibrosis in children and adolescents with hypertrophic cardiomyopathy (HCM) and to assess associations with echocardiographic and clinical parameters of disease. While a common end point in adults with HCM, it is unclear whether diffuse myocardial fibrosis occurs early in the disease. Cardiac magnetic resonance (CMR) estimation of myocardial post-contrast longitudinal relaxation time (T1) is an increasingly used method to estimate diffuse fibrosis. T1 measurements were taken using standard multi-breath-hold spoiled gradient echo phase-sensitive inversion-recovery CMR before and 15 min after the injection of gadolinium. The tissue-blood partition coefficient was calculated as a function of the ratio of T1 change of myocardium compared with blood. An echocardiogram and blood brain natriuretic peptide (BNP) levels were obtained on the day of the CMR. Twelve controls (mean age 12.8 years; 7 male) and 28 patients with HCM (mean age 12.8 years; 21 male) participated. The partition coefficient for both septal (0.27 ± 0.17 vs. 0.13 ± 0.09; p = 0.03) and lateral walls (0.22 ± 0.09 vs. 0.07 ± 0.10; p < 0.001) was increased in patients compared with controls. Eight patients had overt areas of late gadolinium enhancement (LGE). These patients did not show increased partition coefficient compared with those without LGE (0.27 ± 0.15 vs. 0.27 ± 0.19 and 0.22 ± 0.09 vs. 0.22 ± 0.09; p = 0.95 and 0.98, respectively). However, patients who were symptomatic (dyspnea, arrhythmia and/or chest pain) had higher lateral wall partition coefficient than asymptomatic HCM patients (0.27 ± 0.08 vs. 0.17 ± 0.08; p = 0.006). Similarly, patients with raised BNP (>100 pg/ml) had raised lateral wall coefficients (0.27 ± 0.07 vs. 0.20 ± 0.07; p = 0.03), as did those with traditional risk factors for sudden death (0.27 ± 0.06 vs. 0.18 ± 0.08; p = 0.007). Diffuse fibrosis, measured by the partition coefficient technique, is demonstrable in children and adolescents with HCM. Markers of fibrosis show an association with symptoms and raised serum BNP. Further study of the prognostic implication of this technique in young patients with HCM is warranted.

A Novel Statistical Approach for Brain MR Images Segmentation Based on Relaxation Times.

Brain tissue segmentation in Magnetic Resonance Imaging is useful for a wide range of applications. Classical approaches exploit the gray levels image and implement criteria for differentiating regions. Within this paper a novel approach for brain tissue joint segmentation and classification is presented. Starting from the estimation of proton density and relaxation times, we propose a novel method for identifying the optimal decision regions. The approach exploits the statistical distribution of the involved signals in the complex domain. The technique, compared to classical threshold based ones, is able to globally improve the classification rate. The effectiveness of the approach is evaluated on both simulated and real datasets.

Long-chain alkyl sulfonate micelle fission: a molecular dynamics study

In this study, we investigate micelle fission of long-chain alkyl sulfonate molecules using atomistic scale simulation. GROMACS software code with the united atom force field was applied. 0.5-μs parallel molecular dynamics simulation study was conducted for a surfactant/water system consisting of 192 sodium pentadecyl sulfonate and 40,553 water molecules. The large preassembled micelle was ruptured at Krafft above T = 323-K temperature, and we track two ellipsoid-like micelles over the course of the production run. To estimate the micelle shape, we determined the principal moments of inertia and the eccentricity, which proved that the micelles have a pronounced prolate spheroid shape, which agrees well with our previous experimental data. The mechanism of micelle fission was explored in detail. The aggregation number, ionization degree, and other parameters obtained from simulation were consistent with existing experimental finding. The determined parameters in addition to simple visual inspection of trajectories revealed monomer-micelle exchange—with the estimated relaxation time τ 1 = 10− 9s. We assume that the exchange process is conditioned by the unequal size of micelles leading to adjustment of aggregation number.

Biocompatibility and Biofunctionality of Magnetic Nanoparticles

Superparamagnetic nanoparticles are used as an imaging agent. Magnetic particle imaging (MPI) is applicable to not only diagnostics but therapy such as hyperthermia and drug delivery. Integrative therapeutic and diagnostic application is called as theranostics. In particular, magnetic relaxation is one of the most important factors for both imaging and hyperthermia treatment. The evaluation of magnetic relaxation property is necessary to design the materials and instrument applying an external magnetic field. Magnetic relaxation is characterized by two distinct modes. Brownian relaxation is occurred by rotation of the particles. Néel relaxation is induced by rotation of the magnetic moment. The mechanism of magnetic relaxation is influenced by the primary and secondary size of magnetic nanoparticles, surface coating agent, state of materials, viscosity of medium. In this study, the magnetic relaxation properties of Resovist and other commercial magnetic nanoparticles were measured at frequency of several kiro heltz up to 500 kHz and high magnetic field up to 50 Oe. The commercial iron oxide nanoparticles of both of superparamagnetic and ferrimagnetic features were used for the measurements. The hysteresis loop under an applied alternating magnetic field was measured for estimating magnetic loss and effective magnetic relaxation time. The materials were prepared in powder form, dispersed state in medium, or fixed state by agarose / epoxy bond. Magnetic nanoparticles dispersed in medium were rotated by an alternating magnetic field, and both Brownian and Néel relaxation could occur. In contrast, fixation regulated rotation of the particles and only Néel relaxation could occur. Therefore, hysteresis loop of alternating magnetic field in dispersed and fixed samples directly indicated the magnetic relaxation mechanisms of measured materials. The dependency of area of the hysteresis loop on the frequency of magnetic field indicated the effective frequency of Brownian and Néel relaxation. Relaxation time was also calculated. Experimental estimation of effective relaxation time confirms the influence of size distribution, concentration, state, surface coating agent and medium. Thus, evaluation of magnetic loss and magnetic relaxation time measured under the alternating field condition which is used for MPI is the effective method to evaluate the mechanisms of magnetic relaxation and optimum material property.

Relaxation time estimates for multipath catalytic reactions

Some results of studying the dynamic behavior of multipath catalytic reactions are given. Finitely and infinitely slow relaxations to stable stationary states were distinguished, and some relaxation time estimates are presented.

Optimal Configuration for Relaxation Times Estimation in Complex Spin Echo Imaging

Many pathologies can be identified by evaluating differences raised in the physical parameters of involved tissues. In a Magnetic Resonance Imaging (MRI) framework, spin-lattice T1 and spin-spin T2 relaxation time parameters play a major role in such an identification. In this manuscript, a theoretical study related to the evaluation of the achievable performances in the estimation of relaxation times in MRI is proposed. After a discussion about the considered acquisition model, an analysis on the ideal imaging acquisition parameters in the case of spin echo sequences, i.e., echo and repetition times, is conducted. In particular, the aim of the manuscript consists in providing an empirical rule for optimal imaging parameter identification with respect to the tissues under investigation. Theoretical results are validated on different datasets in order to show the effectiveness of the presented study and of the proposed methodology.

Correlation between brain volume change and T2 relaxation time induced by dehydration and rehydration: implications for monitoring atrophy in clinical studies.

Brain volume change measured from magnetic resonance imaging (MRI) provides a widely used and useful in vivo measure of irreversible tissue loss. These measurements, however, can be influenced by reversible factors such as shifts in brain water content. Given the strong effect of water on T2 relaxation, we investigated whether an estimate of T2 relaxation time would correlate with brain volume changes induced by physiologically manipulating hydration status. We used a clinically feasible estimate of T2 (“pseudo-T2”) computed from a dual turbo spin-echo MRI sequence and correlated pseudo-T2 changes to percent brain volume changes in 12 healthy subjects after dehydration overnight (16-hour thirsting) and rehydration (drinking 1.5 L of water).We found that the brain volume significantly increased between the dehydrated and rehydrated states (mean brain volume change = 0.36%, p = 0.0001) but did not change significantly during the dehydration interval (mean brain volume change = 0.04%, p = 0.57). The changes in brain volume and pseudo-T2 significantly correlated with each other, with marginal and conditional correlations (R2) of 0.44 and 0.65, respectively.Our results show that pseudo-T2 may be used in conjunction with the measures of brain volume to distinguish reversible water fluctuations and irreversible brain tissue loss (atrophy) and to investigate disease mechanisms related to neuro-inflammation, e.g., in multiple sclerosis, where edema-related water fluctuations may occur with disease activity and anti-inflammatory treatment.

Measurement/INS; and estimation of relaxation times in magnetic resonance imaging of brain tissue in patients with multiple sclerosis (MS)

Measurement/INS; and estimation of relaxation times in magnetic resonance imaging of brain tissue in patients with multiple sclerosis (MS)

MSE-MRI sequence optimisation for measurement of bi- and tri-exponential T2 relaxation in a phantom and fruit

The transverse relaxation signal from vegetal cells can be described by multi-exponential behaviour, reflecting different water compartments. This multi-exponential relaxation is rarely measured by conventional MRI imaging protocols; mono-exponential relaxation times are measured instead, thus limiting information about of the microstructure and water status in vegetal cells. In this study, an optimised multiple spin echo (MSE) MRI sequence was evaluated for assessment of multi-exponential transverse relaxation in fruit tissues. The sequence was designed for the acquisition of a maximum of 512 echoes. Non-selective refocusing RF pulses were used in combination with balanced crusher gradients for elimination of spurious echoes. The study was performed on a bi-compartmental phantom with known T2 values and on apple and tomato fruit. T2 decays measured in the phantom and fruit were analysed using bi- and tri-exponential fits, respectively. The MRI results were compared with low field non-spatially resolved NMR measurements performed on the same samples.The results demonstrated that the MSE-MRI sequence can be used for up to tri-exponential T2 quantification allowing for estimation of relaxation times from a few tens of milliseconds to over a second. The effects of the crusher moment and the TE value on T2 measurements were studied both on the bi-compartmental phantom and on the fruit tissues. It was demonstrated that the sequence should be optimised with regard to the characteristics of the tissue to be examined by considering the effects of water molecular diffusion in the presence of both imaging gradients and gradients produced by susceptibility inhomogeneities.

Liver fat volume fraction quantification with fat and water T1 and T2* estimation and accounting for NMR multiple components in patients with chronic liver disease at 1.5 and 3.0 T

To validate a magnitude-based method for fat volume fraction (FVF) quantification in the liver without any dominant component ambiguity problems and with the aim of transferring this method to any imaging system (clinical fields of 1.5 and 3.0T). MR imaging was performed at 1.5 and 3.0T using a multiple-angle multiple-gradient echo sequence. A quantification algorithm correcting for relaxation time effects using a disjointed estimation of T1 and T2* of fat and water and accounting for the NMR spectrum of fat was developed. Validations were performed on fat-water emulsion at 1.5 and 3.0T and compared with (1)H-MRS. This was followed by a prospective in-vivo comparative study on 28 patients with chronic liver disease and included histology. Phantom study showed good agreement between MRI and MRS. MR-estimated FVF and histological results correlated strongly and FVF allowed the diagnosis of mild (cutoff = 5.5%) and moderate steatosis (cutoff = 15.2%) with a sensitivity/specificity of 100%. FVF calculation worked independently of the field strength. FVF may be a relevant biomarker for the clinical follow-up of patients (1) with or at risk of NAFLD (2) of steatosis in patients with other chronic liver diseases. • Non-invasive techniques to diagnose non-alcoholic fatty liver diseases (NAFLD) are important. • Liver fat volume fraction quantified using MRI correlates well with histology. • Fat volume fraction could be a relevant marker for NAFLD clinical follow-up. • Disjointed relaxation time estimation could potentially identify factors contributing to NAFLD.

Rapid multi‐echo measurement of brain metabolite T2 values at 7 T using a single‐shot spectroscopic Carr–Purcell–Meiboom–Gill sequence and prior information

We present a method for the robust and accurate estimation of brain metabolite transverse relaxation times (T2 ) from multiple spin-echo data acquired with a single-shot Carr-Purcell-Meiboom-Gill (CPMG) spectroscopic sequence. Each acquired echo consists of a small number of complex time-domain data points. The amplitudes of the spectral components in each echo are calculated by solving a set of linear equations in which previously estimated frequencies and linewidths serve as prior information. These priors are obtained from a short MRS experiment in which a large number of time-domain data points are acquired, and are subsequently estimated using linear prediction with singular value decomposition (LPSVD) processing. We show that this process can be used to accurately and rapidly measure the T2 values for the main singlet resonances in single-volume MRS measurements in the brain. The proposed method can be generalized to any set of MRS experiments comprising repeated measurements of amplitude changes, e.g. as a function of an experimental parameter, such as TE, inversion time or diffusion weighting.

Experimental Measurement and Theoretical Assessment of Fast Lanthanide Electronic Relaxation in Solution with Four Series of Isostructural Complexes

The rates of longitudinal relaxation for ligand nuclei in four isostructural series of lanthanide(III) complexes have been measured by solution state NMR at 295 K at five magnetic fields in the range 4.7-16.5 T. The electronic relaxation time T(le) is a function of both the lanthanide ion and the local ligand field. It needs to be considered when relaxation probes for magnetic resonance applications are devised because it affects the nuclear relaxation, especially over the field range 0.5 to 4.7 T. Analysis of the data, based on Bloch-Redfield-Wangsness theory describing the paramagnetic enhancement of the nuclear relaxation rate has allowed reliable estimates of electronic relaxation times, T(1e), to be obtained using global minimization methods. Values were found in the range 0.10-0.63 ps, consistent with fluctuations in the transient ligand field induced by solvent collision. A refined theoretical model for lanthanide electronic relaxation beyond the Redfield approximation is introduced, which accounts for the magnitude of the ligand field coefficients of order 2, 4, and 6 and their relative contributions to the rate 1/T(le). Despite the considerable variation of these contributions with the nature of the lanthanide ion and its fluctuating ligand field, the theory explains the modest change of measured T(le) values and their remarkable statistical ordering across the lanthanide series. Both experiment and theory indicate that complexes of terbium and dysprosium should most efficiently promote paramagnetic enhancement of the rate of nuclear relaxation.

MR Spectroscopy of the Prostate at 3T: Measurements of Relaxation Times and Quantification of Prostate Metabolites using Water as an Internal Reference

We performed single-voxel magnetic resonance spectroscopy (MRS) of the human prostate at 3 tesla using a surface coil to measure prostate water, choline (Cho), creatine (Cr), and citrate (Cit) relaxation times T1, T2, and to estimate concentrations of Cho, Cr, and Cit in healthy volunteers. In nine of 17 healthy volunteers, we performed experiments to estimate relaxation time, and we used the spectra of the other eight to compute metabolite concentrations. Spectra were processed by LCModel and AMARES (advanced method for accurate, robust, and efficient spectral fitting) algorithms. T1 and T2 values were obtained by monoexponential fitting of the spectral intensities. Metabolite concentrations were estimated using prostate tissue water as an internal concentration reference. Relaxation times are reported for prostate water (T1, 2163±166 ms; T2, 110±18 ms), Cho (T1, 987±71 ms; T2, 239±24 ms), Cr (T1, 1128±149 ms; T2, 188±20 ms), and Cit (T1, 476±70 ms; T2, 228±42 ms). Mean concentrations in healthy prostate were Cho, 2.6±0.3 mM, Cr, 5.8±1.3 mM, and Cit, 26.9±5.5 mM. We observed metabolite relaxation times and concentrations consistent with published values of healthy volunteers at 1.5 and 3T. T1 values increased and T2 slightly decreased with magnetic field strength. Our preliminary patient results indicate that water-referenced quantitative MRS of the human prostate is a promising tool for monitoring therapeutic effects and detecting tumor relapse, i.e., in situations when Cit intensity is small or undetectable.

7Li+ NMR quadrupolar splitting in stretched hydrogels: developments in relaxation time estimation from z‐spectra

(7)Li NMR z-spectra were recorded from the cation constituted in gelatin gels that were held stretched. The system has been studied previously, but we revisited the disparity that was noted between estimates of some of the relaxation times of spin-states of various ranks and orders made using a global data-fitting strategy and estimates made from data acquired by using multiple-quantum-filter pulse sequences. The global data fitting was performed with a probability approach along with the Markov chain Monte Carlo (MCMC) method applied to z-spectra from (7)Li(+) dissolved in (1)H(2)O using more refined experimental methods than hitherto. We also present a more extensive explanation of the MCMC method as it applies in the present NMR context. We achieved much closer agreement between the estimates of relaxation times made by using the two methods of analysis and attribute the previous discrepancies to spectral drift and z-spectrum asymmetry.

Measurement of magnetic nanoparticle relaxation time.

Nanoparticle relaxation time measurements have many applications including characterizing molecular binding, viscosity, heating, and local matrix stiffness. The methods capable of in vivo application are extremely limited. The hypothesis investigated by the authors was that the relaxation time could be measured quantitatively using magnetic spectroscopy of nanoparticle Brownian motion (MSB). The MSB signal (1) reflects the nanoparticle rotational Brownian motion, (2) can be measured from very low nanoparticle concentrations, and (3) is a function of the product of the drive frequency and the relaxation time characterizing Brownian motion. To estimate the relaxation time, the MSB signal was measured at several frequencies. The MSB signal for nanoparticles with altered relaxation time is a scaled version of that for reference nanoparticles with a known relaxation time. The scaling factor linking the altered and reference MSB measurements is the same factor linking the altered and reference relaxation times. The method was tested using glycerol solutions of varying viscosities to obtain continuously variable relaxation times. The measured relaxation time increased with increasing viscosity of the solution in which the nanoparticles resided. The MSB estimated relaxation time matched the calculated relaxation times based on viscosity with 2% average error. MSB can be used to monitor the nanoparticle relaxation time quantitatively through a scale space correlation of the MSB signal as a function of frequency.

Markov-chain Monte Carlo estimation of distributed Debye relaxations in spectral induced polarization

There is growing interest in the link between electrical polarization and physical properties of geologic porous media. In particular, spectral characteristics may be controlled by the same pore geometric properties that influence fluid permeability of such media. Various models have been proposed to describe the spectral-induced-polarization (SIP) response of permeable rocks, and the links between these models and hydraulic properties have been explored, albeit empirically. Computation of the uncertainties in the parameters of such electrical models is essential for effective use of these relationships. The formulation of an electrical dispersion model in terms of a distribution of relaxation times and associated chargeabilities has been demonstrated to be an effective generalized approach; however, thus far, such an approach has only been considered in a deterministic framework. Here, we formulate a spectral model based on a distribution of polarizations. By using a simple polynomial descriptor of such a distribution, we are able to cast the model in a stochastic manner and solve it using a Markov-chain Monte Carlo (McMC) sampler, thus allowing the computation of model-parameter uncertainties. We apply the model to synthetic data and demonstrate that the stochastic method can provide posterior distributions of model parameters with narrow bounds around the true values when little or no noise is added to the synthetic data, with posterior distributions that broaden with increasing noise. We also apply our model to experimental measurements of six sandstone samples and compare physical properties of a number of samples of porous media with stochastic estimates of characteristic relaxation times. We demonstrate the utility of our method on electrical spectra with different response characteristics and show that a single metric of relaxation time for the SIP response is not sufficient to provide clear insight into the physical characteristics of a sample.

Quantification of myocardial perfusion based on signal intensity of flow sensitized MRI

Summary A new method to quantify myocardial perfusion was developed based on slice select (MS) and non-select (MG) inversion recovery acquisitions at a single inversion time. A modified Bloch equation was solved to obtain an analytical expression for perfusion (P) in terms of ΔMSG =M S-MG The average myocardial perfusion of healthy C57BL/6 mice measured using this technique (P=5.7±0.4 ml/g/min) agreed with that measured using traditional techniques and it had a high reproducibility with mean standard deviation of 3.6% between repeated measures. Perfusion maps of ischemia-reperfusion mice showed significantly low perfusion (P=1.6±0.3 ml/g/min) in the infarcted regions compared to that of remote regions (P=4.1±0.3 ml/g/min,p=0.004). Background The arterial spin labeling technique based on the T1 relaxation time of tissue (T1 method) can be used to quantify myocardial perfusion without the use of exogenous contrast materials[1]. However accurate estimation of T1 relaxation times in the heart, especially in mice is difficult and can require long scan time. As an alternative, we developed a method to quantify myocardial perfusion based on the signal intensity(SI method)of flow sensitized MRI. Methods Myocardial tissue was modeled as intra and extra vascular compartments with fast exchange of spins in between them[1]. The flow sensitization was achieved by slice select (MS) and non-select (MG) inversion recovery acquisitions at a single inversion time. A steady state gradient echo image (Msa) was also acquired to normalize receiver characteristics. A modified Bloch equation was solved for this acquisition scheme to obtain an analytical expression for perfusion (P) in terms of ΔMSG = MS-MG as follows: