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.     

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.     

Multi-components of T2 relaxation in ex vivo cartilage and tendon

The multi-components of T₂ relaxation in cartilage and tendon were investigated by microscopic MRI (μMRI) at 13 and 26 μm transverse resolutions. Two imaging protocols were used to quantify T₂ relaxation in the specimens, a 5-point sampling and a 60-point sampling. Both multi-exponential and non-negative-least-square (NNLS) fitting methods were used to analyze the μMRI signal. When the imaging voxel size was 6.76 × 10⁻⁴ mm³ and within the limit of practical signal-to-noise ratio (SNR) in microscopic imaging experiments, we found that (1) canine tendon has multiple T₂ components; (2) bovine nasal cartilage has a single T₂ component; and (3) canine articular cartilage has a single T₂ component. The T₂ profiles from both 5-point and 60-point methods were found to be consistent in articular cartilage. In addition, the depletion of the glycosaminoglycan component in cartilage by the trypsin digestion method was found to result in a 9.81–20.52% increase in T₂ relaxation in articular cartilage, depending upon the angle at which the tissue specimen was oriented in the magnetic field.     

On the strong field dependence and nonlinear response to gadolinium contrast agent of proton transverse relaxation rates in dairy cream

Dairy cream, as a suspension of lipid droplets in water, is a potentially useful magnetic resonance imaging (MRI) phantom material and an interesting material for studying fundamental relaxation mechanisms. Here we report a strong increase in the transverse relaxation rates with field strength for both the water and lipid protons in dairy cream. Also, studies at 4.7 T reveal a nonlinear response of transverse relaxation rates with increasing concentration of a common gadolinium (Gd)-based contrast agent, including an initial decrease of water relaxation rates as measured with Hahn spin echoes at the lower Gd concentrations. The results are treated within the framework of a model in which the magnetic susceptibility difference between the lipid droplets and the aqueous phase plays the prominent role for transverse relaxation. Second-order polynomial fits of the water proton transverse relaxation rate dependence on field strength and on Gd concentration at 4.7 T provided experimental parameters from which model parameters are extracted and compared with expectations available from the literature.     

Distributions of transverse relaxation times for soft-solids measured in strongly inhomogeneous magnetic fields

The single-sided NMR-MOUSE sensor that operates in highly inhomogeneous magnetic fields is used to record a CPMG ¹H transverse relaxation decay by CPMG echo trains for a series of cross-linked natural rubber samples. Effective transverse relaxation rates 1/T_2,short and 1/T_2,long were determined by a bi-exponential fit. A linear dependence of transverse relaxation rates on cross-link density is observed for medium to large values of cross-link density. As an alternative to multi-exponential fits the possibility to analyze the dynamics of soft polymer network in terms of multi-exponential decays via the inverse Laplace transformation was studied. The transient regime and the effect of the T₁/T₂ ratio in inhomogeneous static and radiofrequency magnetic fields on the CPMG decays were studied numerically using a dedicated C++ program to simulate the temporal and spatial dependence of the CPMG response. A correction factor T₂/T_2,eff is derived as a function of the T₁/T₂ ratio from numerical simulations and compared with earlier results from two different well logging devices. High-resolution T₁–T₂ correlations maps are obtained by two-dimensional Laplace inversion of CPMG detected saturation recovery curves. The T₁–T₂ experimental correlations maps were corrected for the T₁/T₂ effect using the derived T₂/T_2,eff correction factor.     

Magnetization recovery for signal enhancement: a fast imaging DEFT-based technique

This paper describes the development and application of a new fast MRI technique based on the DEFT principle. The sequence named MAgnetization RecoverY for Signal Enhancement (MARYSE) is composed of two completely symmetric gradient echoes separated by a 180 degrees refocusing pulse. The RF pulse scheme, 90 degrees x-180 degrees y-90 degrees -x enables restoration of the transverse magnetization along the longitudinal axis, and consequently artificially increases R1 relaxation rate. In this sequence, the period between the excitation pulse and the restoring pulse (Tem: transverse magnetization evolution time) is very short (< 10 ms). This makes possible a significant increase in signal-to-noise ratio, even with a relatively short repetition time (20 ms). Simulations were performed for different values of Tem and TR at definite T1 and T2 and for different values of T1 and T2 at constant Tem and TR. Relevant signal enhancement for species with long relaxation time constants as compared to classical gradient echo and fast spin-echo imaging was expected. In vitro studies on a fat/water phantom confirmed this simulation. Application of MARYSE to mouse brain imaging permitted to visualize almost completely cerebrospinal fluid of the ventricles, a signal usually partially saturated in fast gradient echo imaging.     

What causes the hyperintense T2-weighting and increased short T2 signal in the corticospinal tract?

The corticospinal tract (CST) appears hyperintense on both T₂-weighted images and myelin water maps. Here, an extended multiecho T₂ relaxation sequence with echoes out to 1120 ms was used to characterize the longer T₂ times present in the CST. The T₂ distribution from the CST was compared to other white matter structures in 14 healthy subjects. The intra-/extracellular T₂ peak of the CST was broadened relative to other white matter structures and often split into two distinct peaks. In the CST, it appeared that the intracellular and extracellular water environments had unique T₂ times, causing the intracellular water peak to be pushed down into the myelin water T₂ regime and the extracellular peak to be pushed up to longer T₂ times. The conventional myelin water T₂ limits of 5-40 ms resulted in an artificial increase in myelin water fraction (MWF), causing the CST to be bright on myelin water images. When the upper limit for MWF was decreased to 25 ms, the CST regions exhibited MWF values similar to those found for adjacent anterior and posterior regions. The CST has unique magnetic resonance characteristics, which should be taken into consideration when being examined, especially when compared to pathological tissue.     

Magnetic Resonance Imaging of Gases: A Single-Point Ramped Imaging with T1 Enhancement (SPRITE) Study

A pure phase-encoding MRI technique, single-point ramped imaging withT₁enhancement, SPRITE, is introduced for the purpose of gas phase imaging. The technique utilizes broadband RF pulses and stepped phase encode gradients to produce images, substantially free of artifacts, which are sensitive to the gasT₁andT^*:₂relaxation times. Images may be acquired from gas phase species with transverse relaxation times substantially less than 1 ms. Methane gas images,¹H, were acquired in a phantom study. Sulfur hexafluoride,¹⁹F, images were acquired from a gas-filled porous coral sample. High porosity regions of the coral are observed in both the MRI image and an X-ray image. Sensitivity and resolution effects due to signal modulation during the time-efficient acquisition are discussed. A method to increase the image sensitivity is discussed, and the predicted improvement is shown through 1D images of the methane gas phantom.     

Optimized gradient spoiling of UTE VFA-AFI sequences for robust T1 estimation with B1-field correction

Quantifying T₁ relaxation times is a challenge because inhomogeneities of the B₁ field have to be corrected to obtain proper values. It is a particular challenge in tissues with short T₂^⁎ values, for which conventional MRI techniques do not provide sufficient signal. Recently, a B₁-field correction technique called AFI (Actual Flip angle Imaging) has been introduced that can be combined with UTE (ultra-short echo-time) sequences, which have much shorter echo times compared to conventional MRI techniques, allowing quantification of signal in short T₂^⁎ tissues. A disadvantage of AFI is that it requires very long relaxation delays between repetitions to minimize the influence of imperfect spoiling of transverse magnetization on signal behavior. In this work, we propose a novel spoiling scheme for the AFI sequence that efficiently provides accurate B₁ correction maps with strongly reduced acquisition time. We validated the method with both phantom and preliminary in vivo results.     

On the measurement of multi-component T2 relaxation in cartilage by MR spectroscopy and imaging

The multicomponent T₂ relaxation in bovine nasal cartilage (BNC) was investigated by nuclear magnetic resonance spectroscopy using the Carr-Purcell-Meiboom-Gill (CPMG) sequence and microscopic magnetic resonance imaging (μMRI) method using a CPMG-SE imaging sequence. All experimental data were analyzed by the non-negative least square (NNLS) procedure. Only one T₂ component was found in BNC by both experimental methods (about 113 and 170 ms before and after being enzymatically digested by trypsin). Several experimental and specimen-related factors were investigated in this study, and it was found that some of them could produce artificial multi-component T₂, including the use of the standard MSME imaging sequence at certain imaging gradients.     

Phantom and in vivo study of the look–locher T1 mapping method

This paper describes and tests the LL-EPI method for obtaining quantitative T₁ estimates in a few seconds thereby allowing dynamic T₁ studies. It is shown that the method works even when there is an inflow into the imaged volume, e.g., in a vessel. No calibration is needed. The method has been tested in a phantom study with several different scan parameter set-ups, with and without inflow. The method shows robustness and individual scan parameters and inflow rates do not influence the ability to calculate the Gd-DTPA concentration. Linearity prevail between the measured 1/T₁ and the Gd-DTPA concentration in the range 150 < T₁ < 2500 ms. In a dynamic Gd-DTPA phantom study, it was shown that the dynamic LL-EPI T₁ mapping technique was three times more sensitive than the signal from a T^*₂-weighted EPI sequence. In an in vivo study, dynamic T₁ mapping of the Gd-DTPA uptake in a meningioma was performed. Inspection of the uptake curves indicates that the method is feasible in clinical perfusion studies.     

In vivo NMR T2 relaxation of experimental brain tumors in the cat: A multiparameter tissue characterization

Experimental gliomas (F98) were inoculated in cat brain for the systematic study of their in vivo T₂ relaxation time behavior. With a CPMG multi-echo imaging sequence, a train of 16 echoes was evaluated to obtain the transverse relaxation time and the magnetization M(0) at time t = 0. The magnetization decay curves were analyzed for biexponentiality. All tissues showed monoexponential T₂, only that of the ventricular fluid and part of the vital tumor tissue were biexponential. Based on these NMR relaxation parameters the tissues were characterized, their correct assignment being assured by comparison with histological slices. T₂ of normal grey and white matter was 74 ± 6 and 72 ± 6 msec, respectively. These two tissue types were distinguished through M(0) which for white matter was only 0.88 of the intensity of grey matter in full agreement with water content, determined from tissue specimens. At the time of maximal tumor growth and edema spread a tissue differentiation was possible in NMR relaxation parameter images. Separation of the three tissue groups of normal tissue, tumor and edema was based on T₂ with T_2(normal) < T_2(tumor) < T_2(edema). Using M(0) as a second parameter the differentiation was supported, in particular between white matter and tumor or edema. Animals were studied at 1–4 wk after tumor implantation to study tumor development. The magnetization M(0) of both tumor and peritumoral edema went through a maximum between the second and third week of tumor growth. T₂ of edema was maximal at the same time with 133 ± 4 msec, while the relaxation time of tumor continued to increase during the whole growth period, reaching values of 114 ± 12 msec at the fourth week. Thus, a complete characterization of pathological tissues with NMR relaxometry must include a detailed study of the developmental changes of these tissues to assure correct experimental conditions for the goal of optimal contrast between normal and pathological regions in the NMR images.     

A Comparative Study of Water T1 and T2 Nmr Relaxation Times in Healthy and Pathological Blood Fluid

Water protons T₁ and T₂ relaxation times in samples of whole blood, obtained from healthy people and from patients affected by Macrocytic Anemia on one side and Lymphatic and Myeloid Leukemia on the other, have been measured with the FT NMR technique at 80 Mhz and at 25 °C. No significant difference with respect to the value of the spin lattice relaxation time parameter measured for the healthy control group is experimentally evident in the case of the Macrocytic Anaemia while the spin spin relaxation time increases in magnitude. On the reverse both the leukemic cases present a significant (p < 0.001) increase in the relaxation times with respect to the control group. The experimental relaxation data belonging to the anaemic case show a linear correlation with the red cells volume while that obtained for the two leukaemic cases appear linearly correlated with the total white cell numbers. From the relaxation data an estimate of the amount of water tightly bound to the white cells membrane can be determined which results roughly thirty times lower than that bound to the red cells membrane. In this work is also presented a step by step outline of the water relaxation behavior which starts with the pure water and ends with the water in the whole blood supported by relaxation experiments done on the isolated blood main components.     

Emulation of petroleum well-logging D-T2 correlations on a standard benchtop spectrometer

An experimental protocol is described that allows two-dimensional (2D) nuclear magnetic resonance (NMR) correlations of apparent diffusion coefficient D_app and effective transverse relaxation time T_2,eff to be acquired on a bench-top spectrometer using pulsed field gradients (PFG) in such a manner as to emulate D_app − T_2,eff correlations acquired using a well-logging tool with a fixed field gradient (FFG). This technique allows laboratory-scale NMR measurements of liquid-saturated cored rock to be compared directly to logging data obtained from the well by virtue of providing a comparable acquisition protocol and data format, and hence consistent data processing. This direct comparison supports the interpretation of the well-logging data, including a quantitative determination of the oil/brine saturation. The D − T₂ pulse sequence described here uses two spin echoes (2SE) with a variable echo time to encode for diffusion. The diffusion and relaxation contributions to the signal decay are then deconvolved using a 2D numerical inversion. This measurement allows shorter relaxation time components to be probed than in conventional diffusion measurements. A brief discussion of the numerical inversion algorithms available for inverting these non-rectangular data is included. The PFG-2SE sequence described is well suited to laboratory-scale studies of porous media and short T₂ samples in general.     

Use of a modified polysaccharide gel in developing a realistic breast phantom for MRI

A polysaccharide material, TX-151, has been used together with water, NaCl, and Al powder to create a tissue equivalent gel to make a realistic, inexpensive, conveniently moldable, temporally stable tissue equivalent MRI phantom. Various phantom compositions were studied for variations in gelling time and relaxation times. Gd-DTPA added as a T₁ (and T₂) modifier and aluminum powder added to decrease T₂ permitted phantoms to be made with a range of relaxation times comparable to human tissues. We have used this polysaccharide gel to create breast phantoms for testing breast coils and evaluating different MRI imaging sequences available for diagnosis. The breast phantoms consisted of a layer of Crisco, a good model for adipose tissue, surrounding the TX-151 gel. Some of these phantoms were created with a silicone implant encapsulated in the gel to simulate an augmented breast. More sophisticated phantoms can easily be developed by additions of other materials to this polysaccharide gel.     

Low-molecular weight lanthanide contrast agents: Evaluation of susceptibility and dipolar effects in red blood cell suspensions

Red blood cell (RBC) suspensions, containing low-molecular weight (LMW) dysprosium (Dy) and gadolinium (Gd) chelates, were selected as a two-compartment system for the evaluation of the magnetic dipolar and susceptibility contributions to the transverse (T₂) relaxation of solvent water protons. The influence of RBC geometry and degree of metal chelate compartmentalization on T₂ was investigated by variation of the osmolality and hematocrit (HC), respectively. The T₂-relaxation ability of Dy-chelates was markedly improved in RBC suspensions, in comparison to aqueous solutions, due to the presence of susceptibility effects that more than compensated for the low dipolar relaxation efficacy. Despite a smaller susceptibility effect, the Gd-chelates were still the most efficacious in shortening T₂ due to their comparatively larger dipolar relaxation contribution. The results obtained with the Dy-chelates allowed the evaluation of the relative contributions of susceptibility and dipolar mediated relaxation for the Gd-chelates. The RBC geometry and degree of compartmentalization influenced strongly the T₂ relaxation efficacy of Dy-chelates, as opposed to the Gd-chelates. Hemolysis eliminated the susceptibility effect, essentially removing the T₂ relaxation ability of Dy-chelates. The T₂ relaxation efficacy of Gd-chelates was improved by hemolysis due to enhancement of the dipolar relaxation. As a conclusion, RBC suspensions have clearly been shown to be a suitable ex vivo model with which to distinguish the different contrast mechanisms of LMW Dy- and Gd-based MRI contrast agents.     

Effect of physiological Heart Rate variability on quantitative T2 measurement with ECG-gated Fast Spin Echo (FSE) sequence and its retrospective correction

Object

Quantitative T₂ measurement is applied in cardiac Magnetic Resonance Imaging (MRI) for the diagnosis and follow-up of myocardial pathologies. Standard Electrocardiogram (ECG)-gated fast spin echo pulse sequences can be used clinically for T₂ assessment, with multiple breath-holds. However, heart rate is subject to physiological variability, which causes repetition time variations and affects the recovery of longitudinal magnetization between TR periods.

Materials and methods

The bias caused by heart rate variability on quantitative T₂ measurements is evaluated for fast spin echo pulse sequence. Its retrospective correction based on an effective TR is proposed. Heart rate variations during breath-holds are provided by the ECG recordings from healthy volunteers. T₂ measurements were performed on a phantom with known T₂ values, by synchronizing the sequence with the recorded ECG. Cardiac T₂ measurements were performed twice on six volunteers. The impact of T₁ on T₂ is also studied.

Results

Maximum error in T₂ is 26% for phantoms and 18% for myocardial measurement. It is reduced by the proposed compensation method to 20% for phantoms and 10% for in vivo measurements. Only approximate knowledge of T₁ is needed for T₂ correction.

Conclusion

Heart rate variability may cause a bias in T2 measurement with ECG-gated FSE. It needs to be taken into account to avoid a misleading diagnosis from the measurements.     

Fast and high-resolution MRI on the basis of interleaved-spiral technique at 4.7 T and its application for imaging of ischemic rat brain

The interleaved-spiral magnetic resonance imaging (MRI) technique was implemented and optimized on a Bruker Biospec 47/30 scanner. The method gives rise to high-resolution images with a time saving factor of up to 8, as compared to the conventional approach. A multifunctional pulse sequence for the fast interleaved-spiral MRI was composed. These functions include spin intensity imaging, transverse relaxation timeT ₂ and apparent diffusion-weighted imaging. The method was used to obtain the dynamic responses of a rat brain during ischemia.     

The interpretation of multi-exponential water proton transverse relaxation in the human and porcine eye lens

We report results of 1H NMR transverse relaxation experiments on human and porcine eye lenses. Several authors have reported that transverse relaxation is not mono-exponential when observed by the Carr-Purcell-Meiboom-Gill (CPMG) sequence and have interpreted the results by postulating the presence of "pools" of water molecules in different binding environments that do not exchange rapidly on the NMR timescale. We have compared CPMG data for intact lenses with results for lens homogenates and have combined a CPMG spectroscopic pulse train with NMR micro-imaging to study the nature of the transverse relaxation process in human and porcine lenses. Fast exchange of water protons with the lens proteins (crystallins) leads to an enhanced transverse relaxation rate that varies linearly with protein concentration. At the resolution of NMR micro-imaging the transverse relaxation process is mono-exponential. The results show that the multi-exponential CPMG data observed spectroscopically for whole lenses reflect spatial variations in crystallin content through the lens rather than the presence of distinct "bound" and "free" water pools.     

Simultaneous measurement of total water content and myelin water fraction in brain at 3 T using a T2 relaxation based method

PurposeThis work demonstrates the in vivo application of a T₂ relaxation based total water content (TWC) measurement technique at 3 T in healthy human brain, and evaluates accuracy using simulations that model brain tissue. The benefit of using T₂ relaxation is that it provides simultaneous measurements of myelin water fraction, which correlates to myelin content.MethodsT₂ relaxation data was collected from 10 healthy human subjects with a gradient and spin echo (GRASE) sequence, along with inversion recovery for T₁ mapping. Voxel-wise T₂ distributions were calculated by fitting the T₂ relaxation data with a non-negative least squares algorithm incorporating B₁⁺ inhomogeneity corrections. TWC was the sum of the signals in the T₂ distribution, corrected for T₁ relaxation and receiver coil inhomogeneity, relative to either an external water standard or cerebrospinal fluid (CSF). Simulations were performed to determine theoretical errors in TWC.ResultsTWC values measured in healthy human brain relative to both external and CSF standards agreed with literature values. Simulations demonstrated that TWC could be measured to within 3–4% accuracy.ConclusionIn vivo TWC measurement using T₂ relaxation at 3 T works well and provides a valuable tool for studying neurological diseases with both myelin and water changes.