Longitudinal Relaxation Time Constants Research Articles

T1-mapping in the heart: accuracy and precision.

The longitudinal relaxation time constant (T1) of the myocardium is altered in various disease states due to increased water content or other changes to the local molecular environment. Changes in both native T1 and T1 following administration of gadolinium (Gd) based contrast agents are considered important biomarkers and multiple methods have been suggested for quantifying myocardial T1 in vivo. Characterization of the native T1 of myocardial tissue may be used to detect and assess various cardiomyopathies while measurement of T1 with extracellular Gd based contrast agents provides additional information about the extracellular volume (ECV) fraction. The latter is particularly valuable for more diffuse diseases that are more challenging to detect using conventional late gadolinium enhancement (LGE). Both T1 and ECV measures have been shown to have important prognostic significance.T1-mapping has the potential to detect and quantify diffuse fibrosis at an early stage provided that the measurements have adequate reproducibility. Inversion recovery methods such as MOLLI have excellent precision and are highly reproducible when using tightly controlled protocols. The MOLLI method is widely available and is relatively mature. The accuracy of inversion recovery techniques is affected significantly by magnetization transfer (MT). Despite this, the estimate of apparent T1 using inversion recovery is a sensitive measure, which has been demonstrated to be a useful tool in characterizing tissue and discriminating disease. Saturation recovery methods have the potential to provide a more accurate measurement of T1 that is less sensitive to MT as well as other factors. Saturation recovery techniques are, however, noisier and somewhat more artifact prone and have not demonstrated the same level of reproducibility at this point in time.This review article focuses on the technical aspects of key T1-mapping methods and imaging protocols and describes their limitations including the factors that influence their accuracy, precision, and reproducibility.

Magnetic Resonance Imaging Correlates of Dichotic Listening Performance in Multiple Sclerosis

Multiple sclerosis (MS) pathology can cause disruptions in central auditory processing. Here, we applied quantitative magnetic resonance imaging (MRI) techniques to investigate the impact of global neurodegenerative processes on dichotic listening performance in MS. We studied 28 subjects with clinically definite MS and 26 healthy controls using 3T MRI and a dichotic digits task (DDT) performed in the scanner. Subjects with MS displayed increased white matter lesions, prolonged water proton longitudinal relaxation time constants in normal appearing white matter, reduced gray matter volumes, and reduced corpus callosum areas compared with controls. No group differences were found for any of the DDT performance measures. In subjects with MS, corpus callosum area was strongly correlated with DDT performance. This relationship remained present after controlling for other measures of neuropathology, suggesting that callosal atrophy directly impacts dichotic listening performance in MS.

Active Trans-Plasma Membrane Water Cycling in Yeast Is Revealed by NMR

Plasma membrane water transport is a crucial cellular phenomenon. Net water movement in response to an osmotic gradient changes cell volume. Steady-state exchange of water molecules, with no net flux or volume change, occurs by passive diffusion through the phospholipid bilayer and passage through membrane proteins. The hypothesis is tested that plasma membrane water exchange also correlates with ATP-driven membrane transport activity in yeast (Saccharomyces cerevisiae). Longitudinal 1H2O NMR relaxation time constant (T1) values were measured in yeast suspensions containing extracellular relaxation reagent. Two-site-exchange analysis quantified the reversible exchange kinetics as the mean intracellular water lifetime (τi), where τi−1 is the pseudo-first-order rate constant for water efflux. To modulate cellular ATP, yeast suspensions were bubbled with 95%O2/5%CO2 (O2) or 95%N2/5%CO2 (N2). ATP was high during O2, and τi−1 was 3.1 s−1 at 25°C. After changing to N2, ATP decreased and τi−1 was 1.8 s−1. The principal active yeast ion transport protein is the plasma membrane H+-ATPase. Studies using the H+-ATPase inhibitor ebselen or a yeast genetic strain with reduced H+-ATPase found reduced τi−1, notwithstanding high ATP. Steady-state water exchange correlates with H+-ATPase activity. At volume steady state, water is cycling across the plasma membrane in response to metabolic transport activity.

Blood longitudinal (T 1) and transverse (T 2) relaxation time constants at 11.7 Tesla

The goal of the study was to determine blood T(1) and T(2) values as functions of oxygen saturation (Y), temperature (Temp) and hematocrit (Hct) at an ultrahigh MR field (11.7 T) and explore their impacts on physiological measurements, including cerebral blood flow (CBF), blood volume (CBV) and oxygenation determination. T(1) and T(2) were simultaneously measured. Temperature was adjusted from 25 to 40°C to determine Temp dependence; Hct of 0.17-0.51 to evaluate Hct dependence at 25 and 37°C; and Y of 40-100% to evaluate Y dependence at 25 and 37°C. Comparisons were made with published data obtained at different magnetic field strengths (B(0)). T(1) was positively correlated with Temp, independent of Y, and negatively correlated with Hct. T(2) was negatively correlated with Temp and Hct, but positively correlated with Y, in a non-linear fashion. T(1) increased linearly with B(0), whereas T(2) decreased exponentially with B(0). This study reported blood T(1) and T(2) measurements at 11.7 T for the first time. These blood relaxation data could have implications in numerous functional and physiological MRI studies at 11.7 T.

Glioma cell density in a rat gene therapy model gauged by water relaxation rate along a fictitious magnetic field

Longitudinal and transverse rotating-frame relaxation time constants, T(1) (ρ) and T(2) (ρ) , have previously been successfully applied to detect gene therapy responses and acute stroke in animal models. Those experiments were performed with continuous-wave irradiation or with frequency-modulated pulses operating in an adiabatic regime. The technique called Relaxation Along a Fictitious Field (RAFF) is a recent extension of frequency-modulated rotating-frame relaxation methods. In RAFF, spin locking takes place along a fictitious magnetic field, and the decay rate is a function of both T(1ρ) and T(2ρ) processes. In this work, the time constant characterizing water relaxation with RAFF (T(RAFF) ) was evaluated for its utility as a marker of response to gene therapy in a rat glioma model. To investigate the sensitivity to early treatment response, we measured several rotating-frame and free-precession relaxation time constants and the water apparent diffusion coefficients, and these were compared with histological cell counts in 8 days of treated and control groups of animals. T(RAFF) was the only parameter exhibiting significant association with cell density in three different tumor regions (border, intermediate, and core tissues). These results indicate that T(RAFF) may provide a marker to identify tumors responding to treatment.

A method for rapid in vivo measurement of blood T1

We present a technique to measure the longitudinal relaxation time constant of venous blood (T(1b) ) in vivo in a few seconds. The MRI sequence consists of a thick-slab adiabatic inversion, followed by a series of slice-selective excitations and single-shot echo planar imaging readouts. The time intervals between excitations were chosen so that blood in macroscopic vessels is fully refreshed between excitations, making the blood signal follow an unperturbed inversion recovery curve. Static tissue, which experiences the inversion and all excitation pulses, quickly reaches a steady state at a low signal as a result of partial saturation. This allows blood-filled voxels to be discriminated from those containing static tissue, and to be fitted voxel-by-voxel to a simple inversion recovery model. The sequence was tested on a flow phantom with the proposed method, yielding T(1) values consistent to within 3% of those obtained using a conventional inversion recovery sequence with a spin-echo readout. The method was applied to seven adult volunteers and 18 neonates. The blood T(1) of the neonates (1799 ± 206 ms; range, 1393-2035 ms) was found to be more variable than that of adults (1717 ± 39 ms; range, 1662-1779 ms). A linear correlation between the inverse of T(1b) and the haematocrit was established in 12 neonates (R(2) = 0.90).

Rapid Acquisition of Multidimensional Solid-State NMR Spectra of Proteins Facilitated by Covalently Bound Paramagnetic Tags

We describe a condensed data collection approach that facilitates rapid acquisition of multidimensional magic-angle spinning solid-state nuclear magnetic resonance (SSNMR) spectra of proteins by combining rapid sample spinning, optimized low-power radio frequency pulse schemes and covalently attached paramagnetic tags to enhance protein (1)H spin-lattice relaxation. Using EDTA-Cu(2+)-modified K28C and N8C mutants of the B1 immunoglobulin binding domain of protein G as models, we demonstrate that high resolution and sensitivity 2D and 3D SSNMR chemical shift correlation spectra can be recorded in as little as several minutes and several hours, respectively, for samples containing approximately 0.1-0.2 micromol of (13)C,(15)N- or (2)H,(13)C,(15)N-labeled protein. This mode of data acquisition is naturally suited toward the structural SSNMR studies of paramagnetic proteins, for which the typical (1)H longitudinal relaxation time constants are inherently a factor of at least approximately 3-4 lower relative to their diamagnetic counterparts. To illustrate this, we demonstrate the rapid site-specific determination of backbone amide (15)N longitudinal paramagnetic relaxation enhancements using a pseudo-3D SSNMR experiment based on (15)N-(13)C correlation spectroscopy, and we show that such measurements yield valuable long-range (15)N-Cu(2+) distance restraints which report on the three-dimensional protein fold.

Discrimination of intra- and extracellular 23Na + signals in yeast cell suspensions using longitudinal magnetic resonance relaxography

This study tested the ability of MR relaxography (MRR) to discriminate intra- ( Na i + ) and extracellular ( Na e + ) 23Na + signals using their longitudinal relaxation time constant ( T 1) values. Na +-loaded yeast cell ( Saccharomyces cerevisiae) suspensions were investigated. Two types of compartmental 23Na + T 1 differences were examined: a selective Na e + T 1 decrease induced by an extracellular relaxation reagent (RR e), GdDOTP 5−; and, an intrinsic T 1 difference. Parallel studies using the established method of 23Na MRS with an extracellular shift reagent (SR e), TmDOTP 5−, were used to validate the MRR measurements. With 12.8 mM RR e, the 23 Na e + T 1 was 2.4 ms and the 23 Na i + T 1 was 9.5 ms (9.4T, 24 °C). The Na + amounts and spontaneous efflux rate constants were found to be identical within experimental error whether measured by MRR/RR e or by MRS/SR e. Without RR e, the Na +-loaded yeast cell suspension 23Na MR signal exhibited two T 1 values, 9.1 (±0.3) ms and 32.7 (±2.3) ms, assigned to 23 Na i + and 23 Na e + , respectively. The Na i + content measured was lower, 0.88 (±0.06); while Na e + was higher, 1.43 (±0.12) compared with MRS/SR e measures on the same samples. However, the measured efflux rate constant was identical. T 1 MRR potentially may be used for Na i + determination in vivo and Na + flux measurements; with RR e for animal studies and without RR e for humans.

A dedicated spectrometer for dissolution DNP NMR spectroscopy

Using low temperature dynamic nuclear polarisation (DNP) in conjunction with dissolution makes it possible to generate highly polarised nuclear spin systems for liquid state applications of nuclear magnetic resonance spectroscopy. However, in its current implementation, which requires the transfer of the solute between two different magnets, the hyperpolarisation strategy is limited to spin systems with relatively long longitudinal relaxation time constants. Here we describe the design and construction of a dedicated spectrometer for DNP applications that is based on a magnet with two isocentres. DNP enhancement is carried out in the upper compartment of this magnet in a low temperature environment at 3.35 T, while a 9.4 T isocentre in the lower compartment is used for high resolution NMR spectroscopy. The close proximity (85 cm) of the two isocentres makes it possible to transfer the sample in the solid state with very little loss of spin polarisation. In first performance tests this novel experimental set-up proved to be superior to the strategy involving two separate magnets.

Long‐lived States in Multiple‐Spin Systems

Long-lived spin states are excited in molecules featuring more than two isolated coupled spins, including amino acids. The figure shows the exponential recovery with the longest time-constant in aspartic acid, T1max=5.842±0.004 s, and of the decay of the long-lived state, TLLS=10.9±0.2 s). An improvement in spin memory by a factor 2 compared to longitudinal spin-lattice relaxation time constants is obtained for most systems.

Magnetic field and tissue dependencies of human brain longitudinal 1H2O relaxation in vivo

Brain water proton (1H2O) longitudinal relaxation time constants (T1) were obtained from three healthy individuals at magnetic field strengths (B0) of 0.2 Tesla (T), 1.0T, 1.5T, 4.0T, and 7.0T. A 5-mm midventricular axial slice was sampled using a modified Look-Locker technique with 1.5 mm in-plane resolution, and 32 time points post-adiabatic inversion. The results confirmed that for most brain tissues, T1 values increased by more than a factor of 3 between 0.2T and 7T, and over this range were well fitted by T1 (s)=0.583(B0)0.382, T1(s)=0.857(B0)0.376, and T1(s)=1.35(B0)0.340 for white matter (WM), internal GM, and blood 1H2O, respectively. The ventricular cerebrospinal fluid (CSF) 1H2O T1 value did not change with B0, and its average value (standard deviation (SD)) across subjects and magnetic fields was 4.3 (+/-0.2) s. The tissue 1/T1 values at each field were well correlated with the macromolecular mass fraction, and to a lesser extent tissue iron content. The field-dependent increases in 1H2O T1 values more than offset the well-known decrease in typical MRI contrast reagent (CR) relaxivity, and simulations predict that this leads to lower CR concentration detection thresholds with increased magnetic field.

Fast Local Backbone Dynamics of Encapsulated Ubiquitin

Backbone dynamics of ubiquitin confined within AOT reverse micelles have been evaluated based on analysis of 15N NMR relaxation data. Results indicate that upon encapsulation the protein experiences a slight overall increase in the value of the order parameter, S2, indicating a restriction in the average amplitude of fast local N-H bond vector motion. The largest increases in S2 upon encapsulation were concentrated in the region of beta-sheet 2 and, additionally, at the transitions of secondary structure motifs and loop regions. In addition, statistical analysis of the residue average ratio of the 15N longitudinal and transverse NMR relaxation time constants indicates that chemical exchange contributions to relaxation are consistent with previous aqueous studies. Earlier studies have demonstrated that native protein structure can be maintained in the encapsulated state. These results presented here establish that the dynamical behavior of encapsulated ubiquitin is likewise nativelike and adds important new observations regarding the enhancement of protein stability under confinement.

Use of a Paramagnetic Core to Affect Longitudinal Nuclear Relaxation in DendrimersA Tool for Probing Dendrimer Conformation

The longitudinal relaxation time constants (T 1 ) of the protons in a series of dendrimers that alternatively had paramagnetic ([Fe 4 S 4 (SR) 4 ] 2- , R = dendron) and diamagnetic (tetraphenylmethane) cores were compared. The T 1 values of the phenyl and benzyl protons in the paramagnetic core dendrimers were attenuated compared to those of analogous protons in the diamagnetic core dendrimers. This observation indicated that protons in each set of topologically different repeat units (generation) of the dendrimer approach the core of the molecule closely in space. This conclusion is consistent with the computed radial density distributions of the different generations calculated from molecular dynamics simulations. In addition, by comparing T 1 values of protons at two slightly different temperatures, the terminal groups in both sets of dendrimers were concluded to be, on average, more mobile than the other generations within the dendrimers. This conclusion is consistent with the computed mean square displacement correlation functions for the different generations also calculated from molecular dynamics simulations.

Comparison of backbone dynamics of reduced and oxidized Escherichia coli glutaredoxin-1 using 15N NMR relaxation measurements.

NMR-based structure determination of Escherichia coli glutaredoxin-1 in its reduced and oxidized forms revealed only subtle structural differences between the two forms. In an effort to characterize the role dynamics may play in the functioning of the protein, the backbone dynamics of both the reduced and oxidized forms of E. coli glutaredoxin-1 have been characterized using inverse-detection two-dimensional 15N-1H NMR spectroscopy. Longitudinal (T1) and transverse (T2) 15N relaxation time constants and steady-state [1H]-15N NOEs were measured for a majority of the protonated backbone nitrogen atoms. These data were analyzed by using a model-free formalism to determine the generalized order parameter (S2), the effective correlation time for internal motions (tau(e)), 15N exchange broadening contributions (R(ex)), and the overall molecular rotational correlation time (tau(m)). Sedimentation equilibrium measurements showed the reduced protein to be monomeric whereas the oxidized form could be fit to a monomer-dimer equilibrium. In order to try and assess the effect of dimerization on the dynamical parameters, the measurements on the oxidized protein have been carried out at two concentrations with very different monomer/dimer ratios. There is increased motion on both nano-picosecond and micro-millisecond time scales in the reduced form relative to the oxidized form, consistent with a more rigid oxidized protein. The increase in motion in the reduced protein correlates with its decreased thermodynamic stability. The role of the observed differences in the dynamic behavior in the two forms, particularly in the active site, in glutaredoxin-1's role as a protein disulfide reductant is discussed.

Comparison of backbone and tryptophan side-chain dynamics of reduced and oxidized Escherichia coli thioredoxin using nitrogen-15 NMR relaxation measurements

The backbone and tryptophan side-chain dynamics of both the reduced and oxidized forms of uniformly 15N-labeled Escherichia coli thioredoxin have been characterized using inverse-detected two-dimensional 1H-15N NMR spectroscopy. Longitudinal (T1) and transverse (T2) 15N relaxation time constants and steady-state (1H)-15N NOEs were measured for more than 90% of the protonated backbone nitrogen atoms and for the protonated indole nitrogen atoms of the two tryptophan residues. These data were analyzed by using a model free dynamics formalism to determine the generalized order parameter (S2), the effective correlation time for internal motions (tau e), and 15N exchange broadening contributions (Rex) for each residue, as well as the overall molecular rotational correlation time (tau m). The reduced and oxidized forms exhibit almost identical dynamic behavior on the picosecond to nanosecond time scale. The W31 side chain is significantly more mobile than the W28 side chain, consistent with the positions of W31 on the protein surface and W28 buried in the hydrophobic core. Backbone regions which are significantly more mobile than the average include the N-terminus, which is constrained in the crystal structure of oxidized thioredoxin by specific contacts with a Cu2+ ion, the C-terminus, residues 20-22, which constitute a linker region between the first alpha-helix and the second beta-strand, and residues 73-75 and 93-94, which are located adjacent to the active site. In contrast, on the microsecond to millisecond time scale, reduced thioredoxin exhibits considerable dynamic mobility in the residue 73-75 region, while oxidized thioredoxin exhibits no significant mobility in this region. The possible functional implications of the dynamics results are discussed.

Backbone dynamics of the Bacillus subtilis glucose permease IIA domain determined from 15N NMR relaxation measurements.

The backbone dynamics of the uniformly 15N-labeled IIA domain of the glucose permease of Bacillus subtilis have been characterized using inverse-detected two-dimensional 1H-15N NMR spectroscopy. Longitudinal (T1) and transverse (T2) 15N relaxation time constants and steady-state (1H)-15N NOEs were measured, at a spectrometer proton frequency of 500 MHz, for 137 (91%) of the 151 protonated backbone nitrogens. These data were analyzed by using a model-free dynamics formalism to determine the generalized order parameter (S2), the effective correlation time for internal motions (tau e), and 15N exchange broadening contributions (Rex) for each residue, as well as the overall molecular rotational correlation time (tau m). The T1 and T2 values for most residues were in the ranges 0.45-0.55 and 0.11-0.15 s, respectively; however, a small number of residues exhibited significantly slower relaxation. Similarly, (1H)-15N NOE values for most residues were in the range 0.72-0.80, but a few residues had much smaller positive NOEs and some exhibited negative NOEs. The molecular rotational correlation time was 6.24 +/- 0.01 ns; most residues had order parameters in the range 0.75-0.90 and tau e values of less than ca. 25 ps. Residues found to be more mobile than the average were concentrated in three areas: the N-terminal residues (1-13), which were observed to be highly disordered; the loop from P25 to D41, the apex of which is situated adjacent to the active site and may have a role in binding to other proteins; and the region from A146 to S149. All mobile residues occurred in regions close to termini, in loops, or in irregular secondary structure.

Some considerations concerning susceptibility, longitudinal relaxation time constants and motion artifacts in In vivo human spectroscopy

Artifacts due to localized susceptibility effects, variations in the spin-lattice relaxation time constants of signals and a mixing of signals arising from tissue motions can contaminate otherwise credible results. The sources and magnitude of some of these are discussed and their likely impact assessed, so that the necessity of incorporating additional measurements in an individual study can be demonstrated.

On the maximum of the spectral density of the amplitude fluctuations of a maser oscillator

The spectral density of the amplitude fluctuations of a maser oscillator shows a peak when the saturation factor is high enough. An analytical expression is given for the Fourier frequency at which the maximum arises, in terms of the saturation factor and the transverse and longitudinal relaxation time constants.