NMR Study of Water Distribution inside Tomato Cells: Effects of Water Stress

Tomato pericarp tissue was studied by low-field nuclear magnetic resonance (NMR) relaxometry. Two kinds of experiments were performed to investigate the correlation between multi-exponential NMR relaxation and the subcellular compartments. The longitudinal (T ₁) versus transverse (T ₂) relaxation times were first measured on fresh samples and then the transverse relaxation time was measured on samples exposed to water stress. Four signal components were found in all experiments. The results showed that all signal components corresponded to the water in different cell compartments, and that no signal from non-exchangeable protons was present. Moreover, we demonstrated that NMR relaxation is suitable for the continuous monitoring of water rebalancing between subcellular compartments of plant tissues.     

Spin-lattice relaxation and magnetization transfer in intracranial tumors in vivo: Effects of Gd-DTPA on relaxation parameters

Spin-lattice relaxation time T₁ and relaxation parameters in magnetization transfer (MT) imaging were measured in 11 intracranial tumors before and after injection of Gd-DTPA at 0.1 T by using the inversion recovery method and the saturation transfer technique, respectively. Preinjection T₁ relaxation times of the tumors were longer than those of white matter, but after Gd-enhancement the relaxation times of most tumors were in the same range as those of white matter. Gd-DTPA shortened the apparent relaxation time in the presence of off-resonance saturation pulse (T_1α) due to marked shortening of the relaxation time of mobile water (T_1w). Gd-DTPA decreased the magnetization transfer contrast (MTC) but did not influence on the magnetization transfer rate (R_wm). The parameters MTC and R_wm differed clearly between Gd-enhanced tumors and normal brain, whereas the relaxation time T_1α was in many Gd-enhanced tumors in the same range as in normal brain.     

New pulse sequences for T1− and T1/T2−contrast enhancing in NMR imaging

Improved pulse sequences DIFN (abbreviation of the words: DIFferentiation by N pulses), 90° − τ₁ − 180° − τ₁ − … 180° − τ_n, with optimised time intervals τ₁ for T₁ measurement and contrast enhancing in NMR imaging are presented. The pulse sequences DIFN have a better sensitivity to T₁ than the well-known pulse sequence SR. In contrast to the IR pulse sequence, the information given by the DIFN pulse sequence is more reliable, because the NMR signal does not change its sign. For a given time interval τ₀ ≤ (0.1 − 0.3) T₁′ the DIFN pulse sequences serve as T₁-filters. They pass the signal components with relatively short T₁ < T₁′ and suppress the components with relatively long T₁ < T₁′. The effects of the radiofrequency field inhomogeneity and inaccurate adjusting of pulse lengths are also considered. It is also proposed in this work to use the joint T₁T₂-contrast in NMR imaging obtained as a result of applying the DIFN pulse sequences in combination with the well-known Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence. The region of interest, where the contrast should be especially enhanced, is specified by the two times at which measurements are performed, which allow the amplitudes of pixels to reach some defined levels by spin-lattice and spin-spin relaxation.     

MRI rotating frame relaxation measurements for articular cartilage assessment

In the present work we introduced two MRI rotating frame relaxation methods, namely adiabatic T_1ρ and Relaxation Along a Fictitious Field (RAFF), along with an inversion-prepared Magnetization Transfer (MT) protocol for assessment of articular cartilage. Given the inherent sensitivity of rotating frame relaxation methods to slow molecular motions that are relevant in cartilage, we hypothesized that adiabatic T_1ρ and RAFF would have higher sensitivity to articular cartilage degradation as compared to laboratory frame T₂ and MT. To test this hypothesis, a proteoglycan depletion model was used. Relaxation time measurements were performed at 0 and 48 h in 10 bovine patellar specimens, 5 of which were treated with trypsin and 5 untreated controls were stored under identical conditions in isotonic saline for 48 h. Relaxation times measured at 48 h were longer than those measured at 0 h in both groups. The changes in T₂ and MT relaxation times after 48 h were approximately 3 times larger in the trypsin treated specimens as compared to the untreated group, whereas increases of adiabatic T_1ρ and RAFF were 4 to 5 fold larger. Overall, these findings demonstrate a higher sensitivity of adiabatic T_1ρ and RAFF to the trypsin-induced changes in bovine patellar cartilage as compared to the commonly used T₂ and MT. Since adiabatic T_1ρ and RAFF are advantageous for human applications as compared to standard continuous-wave T_1ρ methods, adiabatic T_1ρ and RAFF are promising tools for assessing cartilage degradation in clinical settings.     

Generalized equation for describing the magnetization in spoiled gradient-echo imaging

The purpose of this study was to demonstrate a generalized equation for describing the magnetization in spoiled gradient-echo (SPGR) imaging in which the in-pulse relaxation and magnetization transfer (MT) effects are taken into account. First, the time-dependent Bloch equations for the two-pool exchange model with MT effect were reduced to an inhomogeneous linear differential equation, and then a simple equation was derived to solve it using a matrix operation. Second, the equations describing the magnetization before and after the radiofrequency (RF) pulse were derived based on the above solution for the RF-pulse excitation and evolution phases. Finally, a generalized equation describing the steady-state magnetization was derived. The validity of this equation was investigated by comparing with the transverse magnetization obtained by the regular Ernst equation and analytical solution in which the in-pulse transverse relaxation is considered. When the same assumption was made in our method, there were good agreements between them, indicating the validity of our method. The in-pulse transverse and longitudinal relaxations decreased the transverse magnetization compared to the case in which these effects were neglected, whereas MT increased it. In conclusion, we derived a generalized equation for describing the magnetization in SPGR imaging. This equation will provide a suitable basis for understanding the signal intensity in SPGR imaging and/or T₁ measurement using an SPGR sequence in cases in which the effect of in-pulse relaxation and/or MT cannot be neglected.     

Generalized RF Pulse Train with Insensitivity to B 1 Inhomogeneity

Adiabatic inversion recovery radiofrequency (RF) pulse techniques are used to address B ₁ inhomogeneity; however, the specific absorption rates of these techniques are significantly higher than that of non-adiabatic RF pulse techniques. In addition, time efficiency is poorer because of the required longer inversion recovery time. Therefore, an RF pulse train with three subpulses was previously developed and reported. The purpose of this article was to generalize the RF pulse train for tissues with different T ₁ relaxation times and in a different application. The RF pulse train B ₁ insensitivities and frequency responses were calculated with different T ₁ relaxation times and different subpulse durations using the Bloch equation. The previously reported optimal flip angle (FA) combination was used. When using the optimal FA combination, the RF pulse train B ₁ insensitivity did not change even if the T ₁ relaxation times and the subpulse durations did change. In other words, the optimal FA combination does not require adjustments according to the T ₁ and subpulse duration. The RF pulse train frequency responses with these subpulses can be dramatically improved even if the inherent subpulse frequency response is poor. This finding will facilitate RF pulse train technique implementation on magnetic resonance imaging scanners.     

A New Approach of Two-Dimensional the NMR Relaxation Measurement in Flowing Fluid

Driven-equilibrium fast saturation recovery (DEFSR), as a new method for two-dimensional (2-D) nuclear magnetic resonance (NMR) relaxation measurement based on pulse sequence in flowing fluid, is proposed. The two-dimensional functional relationship between the ratio of transverse relaxation time to longitudinal relaxation time of fluid (T ₁/T ₂) and T ₁ distribution is obtained by means of DEFSR with only two one-dimensional measurements. The rapid measurement of relaxation characteristics for flowing fluid is achieved. A set of the down-hole NMR fluid analysis system is independently designed and developed for the fluid measurement. The accuracy and practicability of DEFSR are demonstrated.     

MRICOM–MRI COntrast Modelling using 2D T1–T2 correlation spectra and relaxation signatures

Image contrast is calculated by inputting experimental 2D T₁–T₂ relaxation spectra into the ODIN software interface. The method involves characterising a magnetic resonance imaging pulse sequence with a “relaxation signature” which describes the sensitivity of the sequence to relaxation and is independent of sample parameters. Maximising (or minimising) the overlap between the experimental 2D T₁–T₂ relaxation spectra and the relaxation signature can then be used to maximise image contrast. The concept is illustrated using relaxation signatures for the echo planar imaging and Turbo spin-echo imaging sequences, together with in-vitro 2D T₁–T₂ spectra for liver and cartilage.     

NMR study of tomato pericarp tissue by spin-spin relaxation and water self-diffusion

Pericarp tissues of tomato varieties Quest and Cameron were studied by low-field nuclear magnetic resonance (NMR) at a controlled temperature of 20°C. The spin-spin relaxation times and the water diffusion coefficients were measured with Carr-Parcell-Meiboom-Gill and pulsed field gradient multi-spin-echo (PFGMSE) NMR sequences. Four relaxing components were extracted from the spin-spin relaxation. The components withT ₂=11 ms,T ₂=65 ms,T ₂=430 ms andT ₂=1500 ms were related to the nonexchangeable protons and water proton in each cell compartment (i.e., cell wall-extracellular space, cytoplasm and vacuole, respectively). In contrast to the relative intensities, theT ₂ values appeared insensitive to variety and harvest period. The difference in relative intensity was related to the size of the pericarp cell. The water self-diffusion coefficients for each cell compartment were determined simultaneously with the PFGMSE sequence. The water self-diffusion coefficients for the vacuole and cytoplasm were not affected by the harvest date or variety. However, the water self-diffusion in the cell wall-extracellular space was significantly different between the two varieties.     

7Li NMR analysis on perovskite structured Li0.15La0.28TaO3

The nanocrystalline perovskite material Li_0.15La_0.28TaO₃ has been synthesized by alkoxide-free Pechini type sol gel method. ⁷Li NMR measurements were carried out using a Bruker Avance 300 spectrometer at 116 MHz over the temperature range 150 to 400 K. Longitudinal spin-lattice relaxation times (T₁) measured by saturation recovery and longitudinal relaxation times in the rotating frame (T_1ρ) measured using the pulse sequence (π/2–spin lock τ acquisition) with lock radio-frequency field υ = 62.5 kHz and the T₂ relaxation time measured by Hahn echo are presented. The static Hahn-echo spectra show two different lithium sites in this perovskite oxide. Further, the relaxation measurements T₁ and T_1ρ show two different types of lithium cations with fast and slow dynamics.     

Optical solitons in dense resonant media

Exact single-soliton solutions of the modified system of Maxwell-Bloch equations, in which the dipole-dipole interactions of the atoms of a dense resonant medium are taken into account, are obtained. Two propagation regimes are analyzed: “coherent,” where the pulse duration is much shorter than both relaxation times (T _p ? T ₁, T ₂), and “incoherent,” where the pulse duration falls between the relaxation times (T ₂ ? T _p ? T ₁). It is predicted, for the first time, that soliton propagation of an ultrashort pulse is possible in a dense resonant absorbing medium in an incoherent interaction regime. The differences between the amplitude and phase characteristics of the solitons considered and the corresponding characteristics of the solitons for McCall-Hahn self-induced transparency are noted.     

Characterization of proton NMR relaxation times in normal and pathological tissues by correlation with other tissue parameters

To help understand which tissue parameters best account for the water proton NMR relaxation times, the longitudinal relaxation time (T₂), the transverse relaxation time (T₂), and the water content of 16 tissues from normal adult rats were measured at 10.7 MHz and 29°C. Regression analyses between the above and other tissue parameters were performed. These other tissue parameters included: the amounts of various organic and inorganic components, protein synthetic rate, oxygen consumption rate, and morphological composition. In addition, the differences in T₁, T₂, and water content values between normal liver and malignant tumor (Morris #7777 a transplantable hepatoma) were studied to help understand how a disease state can be detected and characterized by NMR spectroscopy. The results of this study and information from the literature allow the following generalizations to be made about tissue T₁ and T₂ values: (1) Each normal tissue has rather consistent and characteristic T₁ and T₂ relaxation times which are always shorter than the T₁ and T₂ of bulk water; (2) tissues with higher water content tend to have longer T₁ relaxation times; (3) tissue T₂ values are not, however, as well correlated with water content as T₁ values; (4) tissues with shorter T₁ values have higher calculated hydration fractions, greater amounts of rough endoplasmic reticulum, and a greater rate of protein synthetic activity; (5) tissues with higher lipid content, associated with intracellular non-membrane bounded lipid droplets, tend to have longer T₂ values; (6) tissues with greater overall surface area, whether in the form of cellular membranes or intracellular or extracellular fibrillar macromolecules, tend to have shorter T₂ values; (7) the differences between T₁ and T₂ values between tumor and normal tissues correlated with differences in the volume fraction (amounts) of extracellular fluid volumes and in the amounts of membrane and fibrillar surface area in the cells. The above generalizations should be useful in predicting T₁ and T₂ changes associated with specific tissue pathologies.     

Four possible types of pulses for self-induced transparency

We have found four types of possible steady-state solutions to the coupled Maxwell-Bloch equations which describe extremely intense pulse propagation in a resonant medium. This is possible due to the replacement of the usual slowly varying envelope approximation by the assumption that (c/V-1)/ωτ « 1, where V is the pulse speed and τ is the pulse duration. We ignore possible non-resonant losses and set T₁ = T₂ = ∞.     

Measurement of APT using a combined CERT-AREX approach with varying duty cycles

The goal is to develop an imaging method where contrast reflects amide-water magnetization exchange, with minimal signal contributions from other sources. Conventional chemical exchange saturation transfer (CEST) imaging of amides (often called amide proton transfer, or APT, and quantified by the metric MTR_asym) is confounded by several factors unrelated to amides, such as aliphatic protons, water relaxation, and macromolecular magnetization transfer. In this work, we examined the effects of combining our previous chemical exchange rotation (CERT) approach with the non-linear AREX method while using different duty cycles (DC) for the label and reference scans. The dependencies of this approach, named AREX_double,vdc, on tissue parameters, including T₁, T₂, semi-solid component concentration (f_m), relayed nuclear Overhauser enhancement (rNOE), and nearby amines, were studied through numerical simulations and control sample experiments at 9.4 T and 1 μT irradiation. Simulations and experiments show that AREX_double,vdc is sensitive to amide-water exchange effects, but is relatively insensitive to T₁, T₂, f_m, nearby amine, and distant aliphatic protons, while the conventional metric MTR_asym, as well as several other APT imaging methods, are significantly affected by at least some of these confounding factors.     

A study of the effective line width in lithium ferrites using magneto microwave Kerr effect

A new technique to determine the effective line width ΔH_eff using the dimensional resonance phenomenon observed in magneto-microwave Kerr-effect experiment is discussed. This effect depends on the Permeability tensor components and the damping parameter α. The components of permeability tensor μ have been calculated theoritically and the damping parameter α is determined from experimental data. The measurements have been made at 24 GHz on three samples of lithium ferrite and their effective line widths have been obtained both in the partially magnetised and the saturated states of magnetisation. It has been found that ΔH_eff for two states of magnetisation are different. Further, the value of ΔH_eff obtained are higher than expected at this frequency. This suggests the possibility of additional relaxation mechanisms at dimensional resonance.     

Relaxation-Time and Diffusion NMR Microscopy of Single Neurons

Relaxation-time and diffusion-weighted NMR micrographs have been obtained for single neurons isolated from Aplysia californica. These images allow the nucleus and cytoplasm to be clearly differentiated, in contrast to proton spin-density images, which appear relatively homogenous. Images of the spatial distribution of T₁ and T₂ relaxivities and the diffusion coefficient (D), as well as average values for T₁, T₂, and D in the cytoplasm and nucleus, were calculated from sets of appropriately weighted images. In all cases, water in the nucleus had relaxation and diffusion properties markedly differing from those of cytoplasmic water, which in turn had properties which were distinct from those of free water. Additionally, the cytoplasmic T₂ was observed to triple following cell death, which is attributed to cytoplasmic dilution as water enters the cell. The work presented represents the first effort at a consistent exploration of the spatial distribution of NMR characteristics of water within intact single cells. These studies have implications both for modeling the NMR characteristics of water in neuronal tissues based on an understanding of the characteristics of water in different cell compartments and for understanding water/macromolecule interactions within cells. NMR microscopy studies such as these may help form a foundation for understanding and interpreting NMR characteristics measured from large assemblies of cells, i.e., spectroscopy and imaging of living tissues.     

Correction of errors caused by imperfect inversion pulses in MR imaging measurement of T1 relaxation times

Spin-lattice (T₁) relaxation times were measured by an inversion-recovery magnetic resonance imaging method with a slice-selective inversion pulse (SIP), a non-selective rectangular inversion pulse (RIP), or a B₁-insensitive adiabatic inversion pulse (AIP). Data analysis either assumed perfect inversion (two-parameter fit) or allowed for imperfect inversion (three-parameter fit). Imperfect inversion pulses caused low T₁ values in phantoms with a two-parameter fit, while three-parameter T₁ estimates were accurate over the range 430–2670 ms. A difference of ∼10% between two-parameter and three-parameter T₁ values in normal human brain tissue was attributed to B₁ inhomogeneity with the slice-selective inversion pulse and rectangular inversion pulse, to the slice profile with the slice-selective inversion pulse, and to T₂ effects for the adiabatic inversion pulse. Any T₁ method that relies on accurate flip angles may have a significant systematic error in vivo. Phantom accuracy does not ensure accuracy in vivo, because phantoms may have a more homogeneous B₁ field and a longer T₂ than do biological samples.     

Estimation of water content and water mobility in the nucleus and cytoplasm of Xenopus laevis oocytes by NMR microscopy

NMR microscopy is a noninvasive approach for studying cell structure and properties. Spatially resolved measurement of the relaxation times T₁ and T₂ provided information on the water proton spin density and water mobility in different parts of Xenopus laevis oocytes. The spin-lattice relaxation time T₁ was determined using a saturation-recovery sequence and the common spin-echo sequence with increasing repetition times, while the transverse relaxation time T₂ was measured by means of the spin-echo sequence with varying echo times. From the relaxation times, the mole fractions of possible reorientational correlation times τ_c for different types of intracellular water were calculated according to a simple two-phase model. The values for T₁, T₂, and proton spin density (i.e., water content) are: nucleus ⪢ animal cytoplasm > vegetal cytoplasm. Based on the estimation of τ_c, nearly 90% of the nuclear water and 74.4% of the water of the animal pole was considered as free mobile water, whereas 55.5% of the water of the vegetal pole appeared as bound water.     

T2 Relaxation of peripheral nerve measured in vivo

It is demonstrated that multi-exponential transverse (T₂) relaxation components can be estimated from multi-echo images of peripheral nerve. Three T₂-relaxation components with T₂ values ± standard deviations (populations ± standard deviations) of 19 ± 7 ms (26 ± 9%), 63 ± 31 ms (29 ± 11%) and 241 ± 24 ms (45 ± 7%) have been identified in vivo in the sciatic nerve of the amphibian Xenopus laevis. The longer-lived component, not identified previously in vivo, provides a significant contrast-to-noise ratio (CNR) between nerve and muscle in the latter-echo images. It is shown that the CNR can be further improved by the averaging of selected images from the multi-echo set.     

Saving transverse magnetization

A magnetization storage sequence, ALT-1 (alternating longitudinal and transverse components), is reported. The ALT-1 sequence is a hybrid of two types of storage sequences, the Carr–Purcell type and store-and-restore sequences. During incremental storage periods within the ALT-1 sequence, essentially half of the initially transverse magnetization is stored along the z-axis and the other half is prolonged by an echo-generating pulse. The portions of initial magnetization that are stored as longitudinal components or transverse components are alternated by a π/2 pulse during the cycle. Both transverse components of the initial magnetization are treated the same in the ALT-1 sequence and orientational (phase) information of the initial magnetization is kept during the storage period. The ALT-1 sequence can preserve magnetization more effectively than a published class of modified Carr–Purcell type sequences, because essentially half of the magnetization during incremental storage periods is not subjected to relaxation from T₂ effects.