Bone marrow segmentation based on a combined consideration of transverse relaxation processes and Dixon oscillations

The aim of this study was to demonstrate that gradient‐echo sampling of single spin echoes can be used to isolate the signal from trabecular bone marrow, with high‐quality segmentation and surface reconstructions resulting from the application of simple post‐processing strategies. Theoretical expressions of the time‐domain single‐spin‐echo signal were used to simulate signals from bone marrow, non‐bone fatty deposits and muscle. These simulations were compared with and used to interpret signals obtained by the application of the gradient‐echo sampling of a spin‐echo sequence to image the knee and surrounding tissues at 1.5 T. Trabecular bone marrow has a much higher reversible transverse relaxation rate than surrounding non‐bone fatty deposits and other musculoskeletal tissues. This observation, combined with a choice of gradient‐echo spacing that accentuates Dixon‐type oscillations from chemical‐shift interference effects, enabled the isolation of bone marrow signal from surrounding tissues through the use of simple image subtraction and thresholding. Three‐dimensional renderings of the marrow surface were then readily generated with this approach – renderings that may prove useful for bone morphology assessment, e.g. for the measurement of femoral anteversion. In conclusion, understanding the behavior of signals from bone marrow and surrounding tissue as a function of time through a spin echo facilitates the segmentation and reconstruction of bone marrow surfaces using straightforward post‐processing strategies that are typically available on modern radiology workstations. Copyright © 2016 John Wiley & Sons, Ltd.     

In vivo measurements of irreversible and reversible transverse relaxation rates in human basal ganglia at 7 T: making inferences about the microscopic and mesoscopic structure of iron and calcification deposits

The goal of this study was to measure irreversible and reversible transverse relaxation rates in the globus pallidus and putamen at 7 T, and to use these rates to make inferences about the sub‐voxel structure of iron and calcification deposits. Gradient Echo Sampling of a Spin Echo (GESSE) data were acquired at 7 T on eighteen volunteers spanning a large range of ages (23–85 years), with calcifications in the globus pallidus incidentally observed in one volunteer. Maps of transverse relaxation rates were derived from the GESSE data, and the mean value of these rates in globus pallidus and putamen was estimated for each volunteer. Both irreversible and reversible transverse relaxation rates increased with the expected age‐dependent iron content in these structures, except for the individual with calcifications for whom extremely large reversible relaxation rates but normal irreversible relaxation rates were found in the globus pallidus. Given the sensitivity of irreversible and reversible transverse relaxation rates to microscopic and mesoscopic field variations, respectively, our findings suggest that joint consideration of these rates may yield information not only about the amount of iron and calcification deposited in the brain, but also about the sub‐voxel structure of these deposits, perhaps revealing certain aspects of their geometry and cellular distribution.     

Simultaneous spin‐echo and gradient‐echo BOLD measurements by dynamic MRS

This study aimed to dissociate the intravascular and extravascular contributions to spin‐echo (SE) and gradient‐echo (GE) blood oxygenation level‐dependent (BOLD) signals at 7 T, using dynamic diffusion‐weighted MRS. We simultaneously acquired SE and GE data using a point‐resolved spectroscopy sequence with diffusion weightings of 0, 600, and 1200 s/mm². The BOLD signals were quantified by fitting the free induction decays starting from the SE center to a mono‐exponential decay function. Without diffusion weighting, BOLD signals measured with SE and GE increased by 1.6 ± 0.5% (TE_SE = 40 ms) and 5.2 ± 1.4% (nominal TE_GE = 40 ms) during stimulation, respectively. With diffusion weighting, the BOLD increase during stimulation measured with SE decreased from 1.6 ± 0.5% to 1.3 ± 0.4% (P < 0.001), whereas that measured by GE was unaffected (P > 0.05); the post‐stimulation undershoots in the BOLD signal time courses were largely preserved in both SE and GE measurements. These results demonstrated the feasiblity of simultaneous SE and GE measurements of BOLD signals with and without interleaved diffusion weighting. The results also indicated a predominant extravascular contribution to the BOLD signal time courses, including post‐stimulation undershoots in both SE and GE measurements at 7 T.     

Characterization of the response of taurine protons to PRESS at 9.4 T for Resolving choline and Determining taurine T2

Point‐resolved spectroscopy (PRESS), characterized by two TEs (TE₁ and TE₂), can be employed to perform animal magnetic resonance spectroscopy (MRS) studies at 9.4 T. Taurine (Tau) and choline (Cho) are relevant metabolites that can be measured by MRS. In this work, the response of the J‐coupled protons of Tau as a function of PRESS TE₁ and TE₂ was characterized at 9.4 T to achieve two objectives. The first was to determine two TE₁ and TE₂ combinations that could be used to obtain T₂‐corrected measures of Tau (3.42 ppm) that were minimally influenced by J coupling. The second was to exploit the Tau J coupling to find a timing combination that minimized the 3.25‐ppm Tau signal to enable the Cho (3.22 ppm) resonance to be resolved from the overlapping Tau signal. The response of Tau protons was investigated both numerically and experimentally. It was numerically determined that the timings {TE₁, TE₂} = {17 ms, 10 ms} and {TE₁, TE₂} = {80 ms, 70 ms} yielded similar 3.42‐ppm Tau resonance areas (5% difference), rendering them suitable for Tau T₂ determination. {TE₁, TE₂} = {25 ms, 50 ms} was found to yield minimal 3.25‐ppm Tau signal, reducing its interference with Cho. The efficacy of the timings was demonstrated on phantom solutions and in vivo in four Sprague Dawley rats. LCModel was employed to analyse the in vivo spectra and Tau T₂ values were estimated by fitting the Tau peak areas obtained with {TE₁, TE₂} = {17 ms, 10 ms} and {TE₁, TE₂} = {80 ms, 70 ms} to a monoexponentially decaying function. An average Tau T₂ of 106 ms (standard deviation, 12 ms) was obtained. LCModel analysis of rat spectra obtained with {TE₁, TE₂} = {25 ms, 50 ms} demonstrated negligible levels of Tau signal, compared with that obtained with short TE.     

The interaction between apparent diffusion coefficients and transverse relaxation rates of human brain metabolites and water studied by diffusion‐weighted spectroscopy at 7 T

The dependence of apparent diffusion coefficients (ADCs) of molecules in biological tissues on an acquisition‐specific timescale is a powerful mechanism for studying tissue microstructure. Unlike water, metabolites are confined mainly to intracellular compartments, thus providing higher specificity to tissue microstructure. Compartment‐specific structural and chemical properties may also affect molecule transverse relaxation times (T₂). Here, we investigated the correlation between diffusion and relaxation for N‐acetylaspartate, creatine and choline compounds in human brain white matter in vivo at 7 T, and compared them with those of water under the same experimental conditions. Data were acquired in a volume of interest in parietal white matter at two different diffusion times, Δ = 44 and 246 ms, using a matrix of three echo times (T_E) and five diffusion weighting values (up to 4575 s/mm²). Significant differences in the dependence of the ADCs on T_E were found between water and metabolites, as well as among the different metabolites. A significant decrease in water ADC as a function of T_E was observed only at the longest diffusion time (p < 0.001), supporting the hypothesis that at least part of the restricted water pool can be associated with longer T₂, as suggested by previous studies in vitro. Metabolite data showed an increase of creatine (p < 0.05) and N‐acetylaspartate (p < 0.05) ADCs with T_E at Δ = 44 ms, and a decrease of creatine (p < 0.05) and N‐acetylaspartate (p = 0.1) ADCs with T_E at Δ = 246 ms. No dependence of choline ADC on T_E was observed. The metabolite results suggest that diffusion and relaxation properties are dictated not only by metabolite distribution in different cell types, but also by other mechanisms, such as interactions with membranes, exchange between “free” and “bound” states or interactions with microsusceptibility gradients. Copyright © 2014 John Wiley & Sons, Ltd.     

Quantification of GABA,glutamate and glutamine in a single measurement at 3 T using GABA‐edited MEGA‐PRESS

γ‐Aminobutyric acid (GABA) and glutamate (Glu), major neurotransmitters in the brain, are recycled through glutamine (Gln). All three metabolites can be measured by magnetic resonance spectroscopy in vivo, although GABA measurement at 3 T requires an extra editing acquisition, such as Mescher–Garwood point‐resolved spectroscopy (MEGA‐PRESS). In a GABA‐edited MEGA‐PRESS spectrum, Glu and Gln co‐edit with GABA, providing the possibility to measure all three in one acquisition. In this study, we investigated the reliability of the composite Glu + Gln (Glx) peak estimation and the possibility of Glu and Gln separation in GABA‐edited MEGA‐PRESS spectra. The data acquired in vivo were used to develop a quality assessment framework which identified MEGA‐PRESS spectra in which Glu and Gln could be estimated reliably. Phantoms containing Glu, Gln, GABA and N‐acetylaspartate (NAA) at different concentrations were scanned using GABA‐edited MEGA‐PRESS at 3 T. Fifty‐six sets of spectra in five brain regions were acquired from 36 healthy volunteers. Based on the Glu/Gln ratio, data were classified as either within or outside the physiological range. A peak‐by‐peak quality assessment was performed on all data to investigate whether quality metrics can discriminate between these two classes of spectra. The quality metrics were as follows: the GABA signal‐to‐noise ratio, the NAA linewidth and the Glx Cramer–Rao lower bound (CRLB). The Glu and Gln concentrations were estimated with precision across all phantoms with a linear relationship between the measured and true concentrations: R¹ = 0.95 for Glu and R¹ = 0.91 for Gln. A quality assessment framework was set based on the criteria necessary for a good GABA‐edited MEGA‐PRESS spectrum. Simultaneous criteria of NAA linewidth <8 Hz and Glx CRLB <16% were defined as optimum features for reliable Glu and Gln quantification. Glu and Gln can be reliably quantified from GABA‐edited MEGA‐PRESS acquisitions. However, this reliability should be controlled using the quality assessment methods suggested in this work.     

Adiabatic multi‐echo 31P spectroscopic imaging (AMESING) at 7 T for the measurement of transverse relaxation times and regaining of sensitivity in tissues with short T2* values

An adiabatic multi‐echo spectroscopic imaging (AMESING) sequence, used for ³¹P MRSI, with spherical k‐space sampling and compensated phase‐encoding gradients, was implemented on a whole‐body 7‐T MR system. One free induction decay (FID) and up to five symmetric echoes can be acquired with this sequence. In tissues with low T₂* and high T₂, this can theoretically lead to a potential maximum signal‐to‐noise ratio (SNR) increase of almost a factor of three, compared with a conventional FID acquisition with Ernst‐angle excitation. However, with T₂ values being, in practice, ≤400 ms, a maximum enhancement of approximately two compared with low flip Ernst‐angle excitation should be feasible. The multi‐echo sequence enables the determination of localized T₂ values, and was validated with ³¹P three‐dimensional MRSI on the calf muscle and breast of a healthy volunteer, and subsequently applied in a patient with breast cancer. The T₂ values of phosphocreatine, phosphodiesters (PDE) and inorganic phosphate in calf muscle were 193 ± 5 ms, 375 ± 44 ms and 96 ± 10 ms, respectively, and the apparent T₂ value of γ‐ATP was 25 ± 6 ms. A T₂ value of 136 ± 15 ms for inorganic phosphate was measured in glandular breast tissue of a healthy volunteer. The T₂ values of phosphomonoesters (PME) and PDE in breast cancer tissue (ductulolobular carcinoma) ranged between 170 and 210 ms, and the PME to PDE ratios were calculated to be phosphoethanolamine/glycerophosphoethanolamine = 2.7, phosphocholine/glycerophosphocholine = 1.8 and PME/PDE = 2.3. Considering the relatively short T₂* values of the metabolites in breast tissue at 7 T, the echo spacing can be short without compromising spectral resolution, whilst maximizing the sensitivity. Copyright © 2013 John Wiley & Sons, Ltd.     

BOLD background gradient contributions in diffusion‐weighted fMRI—comparison of spin‐echo and twice‐refocused spin‐echo sequences

The interaction (‘cross terms’) between diffusion‐weighting gradients and susceptibility‐induced background gradient fields around vessels has an impact on apparent diffusion coefficient (ADC) measurements and diffusion‐weighted functional magnetic resonance imaging (DFMRI) experiments. Monte‐Carlo (MC) simulations numerically integrating the Bloch equations for a large number of random walks in a vascular model were used to investigate to what extent such interactions would influence the extravascular signal change as well as the ADC change observed in DFMRI experiments. The vascular model consists of a set of independent, randomly oriented, infinite cylinders whose internal magnetic susceptibility varies as the state changes between rest and activation. In such a network, the cross terms result in the observation of a functional increase in ADC accompanied by a descending percent signal change with increasing diffusion weighting. It is shown that the twice‐refocused spin‐echo sequence permits sufficient yet not total suppression of such effects compared to the standard Stejskal‐Tanner spin‐echo diffusion weighting under experimentally relevant conditions. Copyright © 2010 John Wiley & Sons, Ltd.     

Effect of glycogen loading on skeletal muscle cross‐sectional area and T2 relaxation time

This study was performed to investigate if glycogen loading of skeletal muscles, by binding water, would effect the cross‐sectional area (CSA) and if an altered water content would alter the transverse relaxation time (T2) measured by magnetic resonance imaging (MRI). Five healthy volunteers participated in a programme with 4 days of extremely carbohydrate‐restricted meals followed by 4 days of extremely high carbohydrate intake. The CSA and T2 of thigh and calf muscles were related to the intramuscular glycogen content evaluated at days 4 and 8. An increase in glycogen content from 281 to 634 mmol kg^–1 dry wt increased the CSA of the vastus muscles by 3.5% from 78 ± 11 to 80 ± 12 cm² and the thigh circumference by 2.5% from 146 ± 20 to 150 ± 23 cm². Calf circumference increased non‐significantly by 4% from 78 ± 15 to 82 ± 19 cm². Mono‐exponential T2 decreased in m tibialis anterior from 27.8 ± 1.2 to 26.9 ± 1.7 ms, did not change in m. vastus lateralis 26.5 ± 1.9 ms/26.6 ± 1.3 ms or in m. gastrocnemius 29.5 ± 1.0 ms/29.8 ± 1.9 ms. Glycogen loading increased the signal intensity mainly at different echo times (TE) 15 and 30 ms. The study shows that increased glycogen filling in the muscles increases muscle CSA and that this can be detected by MRI. The signal intensity increased the most at shorter TEs suggesting a more tight intracellular binding of water in glycogen loaded muscles.     

The in vivo J‐difference editing MEGA‐PRESS technique for the detection of n‐3 fatty acids

In this study, we present a method for the detection of n‐3 fatty acid (n‐3 FA) signals using MRS in adipose tissue in vivo. This method (called oMEGA‐PRESS) is based on the selective detection of the CH₃ signal of n‐3 FA using the MEGA‐PRESS (MEshcher–GArwood Point‐RESolved Spectroscopy) J‐difference editing technique. We optimized the envelope shape and frequency of spectral editing pulses to minimize the spurious co‐editing and incomplete subtraction of the CH₃ signal of other FAs, which normally obscure the n‐3 FA CH₃ signal in MR spectra acquired using standard PRESS techniques. The post‐processing of the individual data scans with the phase and frequency correction before data subtraction and averaging was implemented to further improve the quality of in vivo spectra. The technique was optimized in vitro on lipid phantoms using various concentrations of n‐3 FA and examined in vivo at 3 T on 15 healthy volunteers. The proportion of n‐3 FA estimated by the oMEGA‐PRESS method in phantoms showed a highly significant linear correlation with the n‐3 FA content determined by gas chromatography. The signal attributed to n‐3 FA was observed in all subjects. Comparisons with the standard PRESS technique revealed an enhanced identification of the n‐3 FA signal using oMEGA‐PRESS. The presented method may be useful for the non‐invasive quantification of n‐3 FA in adipose tissue, and could aid in obtaining a better understanding of various aspects of n‐3 FA metabolism. Copyright © 2014 John Wiley & Sons, Ltd.     

In vivo detection and automatic analysis of GABA in the mouse brain with MEGA‐PRESS at 9.4 T

The goals of this study were to develop an acquisition protocol and the analysis tools for Meshcher–Garwood point‐resolved spectroscopy (MEGA‐PRESS) in mouse brain at 9.4 T, to allow the in vivo detection of γ‐aminobutyric acid (GABA) and to examine whether isoflurane alters GABA levels in the thalamus during anesthesia. We implemented the MEGA‐PRESS sequence on a Bruker 94/20 system with ParaVision 6.0.1, and magnetic resonance spectra were acquired from nine male wild‐type C57BL/6 J mice at the thalamus. Four individual scans were obtained for each mouse in a 2‐h time course whilst the mouse was anesthetized with isoflurane. We developed an automated analysis program with improved correction for frequency and phase drift compared with the standard creatine (Cr) fitting‐based method and provided automatic quantification. During MEGA‐PRESS acquisition, a single voxel with a size of 5 × 3 × 3 mm³ was placed at the thalamus to evaluate GABA to Cr (GABA/Cr) ratios during anesthesia. Detection and quantitative analysis of thalamic GABA levels were successfully achieved. We noticed a significant decrease in GABA/Cr during the 2‐h anesthesia (by linear regression analysis: slope < 0, p < 0.0001). In summary, our findings demonstrate that MEGA‐PRESS is a feasible technique to measure in vivo GABA levels in the mouse brain at 9.4 T.     

Errors in 1H‐MRS estimates of brain metabolite concentrations caused by failing to take into account tissue‐specific signal relaxation

Accurate measurement of brain metabolite concentrations with proton magnetic resonance spectroscopy (¹H‐MRS) can be problematic because of large voxels with mixed tissue composition, requiring adjustment for differing relaxation rates in each tissue if absolute concentration estimates are desired. Adjusting for tissue‐specific metabolite signal relaxation, however, also requires a knowledge of the relative concentrations of the metabolite in gray (GM) and white (WM) matter, which are not known a priori. Expressions for the estimation of the molality and molarity of brain metabolites with ¹H‐MRS are extended to account for tissue‐specific relaxation of the metabolite signals and examined under different assumptions with simulated and real data. Although the modified equations have two unknowns, and hence are unsolvable explicitly, they are nonetheless useful for the estimation of the effect of tissue‐specific metabolite relaxation rates on concentration estimates under a range of assumptions and experimental parameters using simulated and real data. In simulated data using reported GM and WM T₁ and T₂ times for N‐acetylaspartate (NAA) at 3 T and a hypothetical GM/WM NAA ratio, errors of 6.5–7.8% in concentrations resulted when TR = 1.5 s and TE = 0.144 s, but were reduced to less than 0.5% when TR = 6 s and TE = 0.006 s. In real data obtained at TR/TE = 1.5 s/0.04 s, the difference in the results (4%) was similar to that obtained with simulated data when assuming tissue‐specific relaxation times rather than GM–WM‐averaged times. Using the expressions introduced in this article, these results can be extrapolated to any metabolite or set of assumptions regarding tissue‐specific relaxation. Furthermore, although serving to bound the problem, this work underscores the challenge of correcting for relaxation effects, given that relaxation times are generally not known and impractical to measure in most studies. To minimize such effects, the data should be acquired with pulse sequence parameters that minimize the effect of signal relaxation.     

Simultaneous detection of valine and lactate using MEGA‐PRESS editing in pyogenic brain abscess

Valine and lactate have been recognized as important metabolic markers to diagnose brain abscess by means of MRS. However, in vivo unambiguous detection and quantification is hampered by macromolecular contamination. In this work, MEGA‐PRESS difference editing of valine and lactate is proposed. The method is validated in vitro and applied for quantitative in vivo experiments in one healthy subject and two brain abscess patients. It is demonstrated that with this technique the overlapping lipid signal can be reduced by more than an order of magnitude and thus the robustness of valine and lactate detection in vivo can be enhanced. Quantification of the two abscess MEGA‐PRESS spectra yielded valine/lactate concentration ratios of 0.10 and 0.27. These ratios agreed with the concentration ratios determined from concomitantly acquired short‐T_E PRESS data and were in line with literature values. The quantification accuracy of lactate (as measured with Cramér‐Rao lower bounds in LCModel processing) was better for MEGA‐PRESS than for short‐T_E PRESS in all acquired in vivo datasets. The Cramér‐Rao lower bounds of valine were only better for MEGA‐PRESS in one of the two abscess cases, while in the other case coediting of isoleucine confounded the quantification in the MEGA‐PRESS analysis. MEGA‐PRESS and short‐T_E PRESS should be combined for unambiguous quantification of amino acids in abscess measurements. Simultaneous valine/lactate MEGA‐PRESS editing might benefit the distinction of brain abscesses from tumors, and further categorization of bacteria with reasonable sensitivity and specificity.     

Relationships between MR transverse relaxation parameters R*(2), R(2) and R'(2) and hepatic iron content in thalassemic mice at 1.5 T and 3 T

Assessment of hepatic iron concentration is important in the management of patients with thalassemia. The goal of this study was to investigate the relationships between the three MR transverse relaxation rates, R*(2), R(2) and R'(2), and hepatic iron content in a mouse model of thalassemia at 1.5 and 3 T field strengths. A GESFIDE (gradient-echo sampling of free induction decay and echo) pulse sequence was used to measure the three parameters efficiently in a single scan in a study examining the livers of normal and thalassemic mice, including a subgroup of the latter that were subjected to periodic transfusions. The results showed that R*(2), R(2) and R'(2) all correlated closely with liver iron concentration at both 1.5 T and 3 T, with correlation coefficients ranging from 0.72 to 0.79. High degrees of correlation (r = 0.93-0.99) were also observed among the three MR parameters at both field strengths. It can be concluded that the three rates could all be effective for assessing hepatic iron concentration and that imaging at higher fields may not offer any advantages over that at lower fields.     

Characterization of gradient echo signal decays in healthy and cancerous prostate at 3T improves with a Gaussian augmentation of the mono‐exponential (GAME) model

A biomarker of cancer aggressiveness, such as hypoxia, could substantially impact treatment decisions in the prostate, especially radiation therapy, by balancing treatment morbidity (urinary incontinence, erectile dysfunction, etc.) against mortality. R₂^* mapping with Mono‐Exponential (ME) decay modeling has shown potential for identifying areas of prostate cancer hypoxia at 1.5T. However, Gaussian deviations from ME decay have been observed in other tissues at 3T. The purpose of this study is to assess whether gradient‐echo signal decays are better characterized by a standard ME decay model, or a Gaussian Augmentation of the Mono‐Exponential (GAME) decay model, in the prostate at 3T. Multi‐gradient‐echo signals were acquired on 20 consecutive patients with a clinical suspicion of prostate cancer undergoing MR‐guided prostate biopsies. Data were fitted with both ME and GAME models. The information contents of these models were compared using Akaike's information criterion (second order, AIC_C), in skeletal muscle, the prostate central gland (CG), and peripheral zone (PZ) regions of interest (ROIs). The GAME model had higher information content in 30% of the prostate on average (across all patients and ROIs), covering up to 67% of cancerous PZ ROIs, and up to 100% of cancerous CG ROIs (in individual patients). The higher information content of GAME became more prominent in regions that would be assumed hypoxic using ME alone, reaching 50% of the PZ and 70% of the CG as ME R₂^* approached 40 s^?1. R₂^* mapping may have important applications in MRI; however, information lost due to modeling could mask differences in parameters due to underlying tissue anatomy or physiology. The GAME model improves characterization of signal behavior in the prostate at 3T, and may increase the potential for determining correlates of fit parameters with biomarkers, for example of oxygenation status.     

Optimized MEGA‐SPECIAL for in vivo glutamine detection in the rat brain at 14.1 T

Glutamine has multiple roles in brain metabolism and its concentration can be altered in various pathological conditions. An accurate knowledge of its concentration is therefore highly desirable to monitor and study several brain disorders in vivo. However, in recent years, several MRS studies have reported conflicting glutamine concentrations in the human brain. A recent hypothesis for explaining these discrepancies is that a short T₂ component of the glutamine signal may impact on its quantification at long echo times. The present study therefore aimed to investigate the impact of acquisition parameters on the quantified glutamine concentration using two different acquisition techniques, SPECIAL at ultra‐short echo time and MEGA‐SPECIAL at moderate echo time. For this purpose, MEGA‐SPECIAL was optimized for the first time for glutamine detection. Based on the very good agreement of the glutamine concentration obtained between the two measurements, it was concluded that no impact of a short T₂ component of the glutamine signal was detected. Copyright © 2014 John Wiley & Sons, Ltd.     

An in vivo study of the orientation‐dependent and independent components of transverse relaxation rates in white matter

Diffusion‐weighted imaging (DWI) provides information that allows the estimation of white‐matter (WM) fibre orientation and distribution, but it does not provide information about myelin density, fibre concentration or fibre size within each voxel. On the other hand, quantitative relaxation contrasts (like the apparent transverse relaxation, ) offer iron and myelin‐related contrast, but their dependence on the orientation of microstructure with respect to the applied magnetic field, B₀, is often neglected. The aim of this work was to combine the fibre orientation information retrieved from the DWI acquisition and the sensitivity to microstructural information from quantitative relaxation parameters. The in vivo measured quantitative transverse relaxation maps (R₂ and ) were decomposed into their orientation‐dependent and independent components, using the DWI fibre orientation information as prior knowledge. The analysis focused on major WM fibre bundles such as the forceps major (FMj), forceps minor (FMn), cingulum (CG) and corticospinal tracts (CST). The orientation‐dependent R₂ parameters, despite their small size (0–1.5 Hz), showed higher variability across different fibre populations, while those derived from , although larger (3.1–4.5 Hz), were mostly bundle‐independent. With this article, we have, for the first time, attempted the in vivo characterization of the orientation‐(in)dependent components of the transverse relaxation rates and demonstrated that the orientation of WM fibres influences both R₂ and contrasts.     

A rapid inversion technique for the measurement of longitudinal relaxation times of brain metabolites: application to lactate in high‐grade gliomas at 3 T

The aim of this study was to develop a time‐efficient inversion technique to measure the T₁ relaxation time of the methyl group of lactate (Lac) in the presence of contaminating lipids and to measure T₁ at 3 T in a cohort of primary high‐grade gliomas. Three numerically optimized inversion times (TIs) were chosen to minimize the expected error in T₁ estimates for a given input total scan duration (set to be 30 min). A two‐cycle spectral editing scheme was used to suppress contaminating lipids. The T₁ values were then estimated from least‐squares fitting of signal measurements versus TI. Lac T₁ was estimated as 2000 ± 280 ms. After correcting for T₁ (and T₂ from literature values), the mean absolute Lac concentration was estimated as 4.3 ± 2.6 mm . The technique developed agrees with the results obtained by standard inversion recovery and can be used to provide rapid T₁ estimates of other spectral components as required. Lac T₁ exhibits similar variations to other major metabolites observable by MRS in high‐grade gliomas. The T₁ estimate provided here will be useful for future MRS studies wishing to report relaxation‐corrected estimates of Lac concentration as an objective tumor biomarker. Copyright © 2016 John Wiley & Sons, Ltd.     

2,3‐Butanedione monoxime increases speed of relaxation in single muscle fibres of frog

The effects of 2,3‐butanedione monoxime (BDM) on intracellular Ca²⁺ transient and cross‐bridge function were studied in frog single fibres from the anterior tibialis muscle of Rana temporaria (sarcomere length, 2.2 μm; temperature, 2–4 °C). The fluorescent dye fluo‐3 was used to monitor the intracellular free calcium concentration ([Ca²⁺]_i) during isometric contractions. BDM (1–5 mM ) reduced the amplitude of the Ca²⁺ transient during twitches, but this effect was too small to explain the marked inhibition of BDM on twitch force. [Ca²⁺]_i reached at the end of 1‐s tetanic stimulation was not significantly affected by BDM (1.0 and 1.8 mM ) while the maximum tetanic tension was substantially reduced. The rate of relaxation during isometric tetanus was increased by BDM whereas the rate of decay of the Ca²⁺ transient was reduced in the presence of BDM. The results strongly suggest that BDM, under the experimental conditions used, mainly affects the contractile machinery resulting in altered performance of the cross‐bridges. These effects of BDM were evaluated in terms of the cross‐bridge model of 17 which was fitted to the experimental force–velocity data in the presence and absence of BDM.     

Anisotropy of spin relaxation of water protons in cartilage and tendon

Transverse spin relaxation rates of water protons in articular cartilage and tendon depend on the orientation of the tissue relative to the applied static magnetic field. This complicates the interpretation of magnetic resonance images of these tissues. At the same time, relaxation data can provide information about their organisation and microstructure. We present a theoretical analysis of the anisotropy of spin relaxation of water protons observed in fully hydrated cartilage. We demonstrate that the anisotropy of transverse relaxation is due almost entirely to intramolecular dipolar coupling modulated by a specific mode of slow molecular motion: the diffusion of water molecules in the hydration shell of a collagen fibre around the fibre, such that the molecular director remains perpendicular to the fibre. The theoretical anisotropy arising from this mechanism follows the ‘magic‐angle’ dependence observed in magnetic‐resonance measurements of cartilage and tendon and is in good agreement with the available experimental results. We discuss the implications of the theoretical findings for MRI of ordered collagenous tissues. Copyright © 2009 John Wiley & Sons, Ltd.