Multiple Spin Echo Research Articles

Anisotropic spin diffusion and multiple spin echoes in3He-4He solutions

We propose a θ1-t1-θ2 pulse-NMR experiment to detect the spin-diffusion anisotropy, Δ=D∥-D⊥, in degenerate spin-polarized3He-4He mixtures, where D⊥ and d∥are the transverse and longitudinal spin diffusion coefficients. In such an experiment the nonlinearity of the dynamics produces multiple spin echoes (MSE). At the 3He concentration x3 ≈ 4% the spinrotation parameter vanishes (μM →0), so that the nonlinearity of the equations of motion is entirely due to the anisotropy. In this situation, detection of MSE amounts to observation of ΔD. For slight anisotropy, i.e. ΔD/D∥ ≲0.25, we use a perturbation scheme similiar to that developed by Einzel et al. (in that case, for small μ M and small demagnetizing field) to calculate the second and third echo heights. For larger anisotropy we numerically calculate the echo heights. We find that for ΔD/D∥ = 0.5 the heights are ≈ 2 % of the first echo, and should be detectable. The (θ1, θ2) tip-angle dependence of the ΔD echoes is different from that of the μM and demagnetization echoes, and furthermore, they occur at right angles to these echoes (in spin space). Thus, even when small spin-rotation and demagnetization effects are present, the θ1-t1-θ2 experiment provides a sensitive means of detecting the anisotropy.

Tirilazad prevention of reperfusion edema after focal ischemia in cynomolgus monkeys.

The purpose of the present investigation was to determine if post-ischemic treatment with the 21-aminosteroid lipid peroxidation inhibitor tirilazad mesylate (U-74006F) could affect reperfusion brain edema during the first 3h following a 3h period of middle cerebral artery occlusion-induced focal ischemia in cynomolgus monkeys. Adult female cynomolgus monkeys (N = 14) were subjected under halothane anesthesia to a 3h period of middle cerebral artery occlusion, followed by 3h of reperfusion. U-74006F, 3.0 mg/kg i.v. or citrate vehicle, was administered 10 min before beginning reperfusion. Multiple spin-echo (8 echoes: TE = 26.3 msec; TR = 3.0 secs; 2.35 Tesla) magnetic resonance imaging was performed every 30 min, beginning at 1h after reperfusion. Transverse relaxation rates (T2) for the caudate, putamen, cortex, insular cortex, parietal cortex and central white matter were calculated as an index of focal brain edema. After the final images, corresponding regions were removed for determination of water content by the wet weight/dry weight method. The T2 measurements strongly suggested the presence of post-reperfusion edema in all gray matter, but not white matter, regions at 1h after reperfusion in vehicle-treated animals. Significant attenuation of edema development was seen in the putamen and insular cortex in U-74006F-treated animals. An effect was also observed in the parietal cortex, but none in the caudate. The measurement of water content at 3h after reperfusion yielded similar results. These results showing the ability of U-74006F to attenuate post-reperfusion brain edema support the concept that lipid peroxidation is a significant mediator of reperfusion brain edema after focal ischemia. The therapeutic window for U-74006F's anti-edema effect appears to be at least 3h after the onset of focal ischemia since delaying treatment until just before reperfusion largely prevented subsequent edema in cortical regions and the putamen. The effects of U-74006F on edema may play a mechanistic role in the compound's reported neuroprotective efficacy in a variety of focal ischemia models.

Spin-spin interaction and multiple spin-echoes in AuIn2

We investigated several AuIn2 samples of similar purity by spin-echo techniques. The temperatures and fields ranged from 0.5 to 20 mK and from 0.06 to 0.8 T, respectively. Our results are: The Knight-shift changed from sample to sample within about 5%. Spin-echoes could not be found in all samples. T2 ranged from about 40 μs in one sample, to a temperature and field independent T2 = 83 μs for another sample, to a field and temperature dependent T2 which varied for this sample from 2 ms at low B/Tn-values (∼5 T/K) to 0.3 ms at B/Tn ∼300. We have shown that the temperature of the conduction electron system is the relevant temperature in the problem, and not the nuclear spin-temperature of In. From the amplitudes of multiple spin-echo structures we conclude that demagnetizing fields are not the only origin for their occurrence.

Magic-Angle Gradient-Coil Design

The target-field approach to gradient-coil design has been applied to the production of coils which will produce a linear gradient in the z component of the magnetic field, in a direction oriented at the magic angle to the z axis. The resulting coils, which consist of wires wound on the surface of a cylinder whose axis is parallel to the z direction, produce a pure linear gradient over a large fraction of the volume enclosed by the coil. Such magic-angle gradient coils may be usefully employed in two-dimensional NMR experiments requiring magnetic-field gradients, since magic-angle gradients effectively eliminate the long-range dipole coupling between spins, which can give rise to "intermolecular multiple-quantum coherences." An appropriate target field has been developed and this was used to produce a magic-angle gradient-coil design. A coil of 20 mm inner diameter was constructed to this design and tested using a 500 MHz NMR microscope. The resulting coil had an efficiency of 0.032 T m−1A−1 and an inductance of 3.2 μH. Using the magic-angle gradient coil, it was possible to largely eliminate the multiple spin echoes which occur in two-pulse NMR experiments applied to highly polarized liquids as a manifestation of the long-range dipolar coupling between spins.

Double quantum and cross correlation peaks in COSY NMR spectra

Recently, Warren and co-workers have shown that double quantum, gradient selective, 2D NMR experiments can give rise to apparently unreconcilable double quantum signals and intermolecular cross peaks. These experiments have been used to challenge the basic assumptions of room temperature NMR. We re-examine the gradient-induced multiple spin echo explanation, first discussed by Warren. It is shown that simple analytical explanations can be provided for (i) double quantum peaks, (ii) cross correlation peaks, (iii) the amplitudes of the observed cross peaks, and (iv) the disappearance of the cross peaks when the solvent and solute are placed in different tubes.

Optimization of MRI protocols and pulse sequence parameters for eigenimage filtering

The eigenimage filter generates a composite image in which a desired feature is segmented from interfering features. The signal-to-noise ratio (SNR) of the eigenimage equals its contrast-to-noise ratio (CNR) and is directly proportional to the dissimilarity between the desired and interfering features. Since image gray levels are analytical functions of magnetic resonance imaging (MRI) parameters, it is possible to maximize this dissimilarity by optimizing these parameters. For optimization, the authors consider four MRI pulse sequences: multiple spin-echo (MSE); spin-echo (SE); inversion recovery (IR); and gradient-echo (GE). The authors use the mathematical expressions for MRI signals along with intrinsic tissue parameters to express the objective function (normalized SNR of the eigenimage) in terms of MRI parameters. The objective function along with a set of diagnostic or instrumental constraints define a multidimensional nonlinear constrained optimization problem, which the authors solve by the fixed point approach. The optimization technique is demonstrated through its application to phantom and brain images. The authors show that the optimal pulse sequence parameters for a sequence of four MSE and one IR images almost doubles the smallest normalized SNR of the brain eigenimages, as compared to the conventional brain protocol.

Fast proton spectroscopic imaging of human brain using multiple spin‐echoes

We introduce a multi-echo multi-slice MR proton spectroscopic imaging method, which allows for a dramatic reduction of the measurement time by acquiring multiple spin-echoes within a single repetition time. In the multi-echo multi-slice experiment discussed in this paper, a threefold reduction in measurement time is obtained by sacrificing some spectral resolution. Signal-to-noise ratio and spatial resolution are preserved. Metabolite images of N-acetyl aspartate, and total choline+total creatine from multiple slices through the human brain are presented and compared with images obtained with a conventional single-echo multi-slice method.

Fast MRI by creating multiple spin echoes in a CPMG sequence.

A fast multislice imaging technique has been developed. RASTER (Rapid Acquisition with STimulated Echo Refocusing) is based on RARE (Rapid Acquisition with Relaxation Enhancement), and creates multiple spin echoes/each 180 degrees pulse utilizing stimulated echoes, and phase encode each differently. The sequence can be much faster than RARE while keeping the same spin echo image contrast. The main limitation of the technique is reduced signal-to-noise ratio.

Multiple Spin Echoes after Fast Repetition of Pulses on a Nuclear Spin- [formula omitted] System

Multiple spin echoes are studied with a two-RF-pulse sequence, applied to a nuclear spin- 1 2 system. These multiple echoes are formed when the repetition time is reduced to be comparable to the spin-lattice relaxation time T 1. Amplitudes of the echoes are dependent on repetition time, pulse widths, and the time separation between the two pulses, in accordance with the Bloch equation analysis. We propose the multiple echoes as an effective means of measuring very long spin-lattice relaxation times.

T1, T2 and proton density measurements in the grading of cerebral gliomas

The possibility that cerebral tumours may be graded by measuring T1 or T2 with magnetic resonance (MR) imaging was studied. A consecutive series of patients with subsequently verified gliomas was enrolled, and studied with MR. Patients who had prior surgical, chemotherapeutic or steroid treatment were excluded. Single slice multiple saturation recovery and multiple spin echo techniques were used to measure T1, T2 and proton density in the tumour. In 33 patients with cerebral gliomas there were 5 grade I, 12 grade II, 7 grade III and 9 grade IV. T1 and T2 values tended to be smaller in grade I gliomas than in grades II, III and IV gliomas. Relaxation parameters overlapped considerably in tumours with different grades. Proton density values did not show much change between different grades of gliomas. Relaxation parameters cannot be used to determine tumour grade reliably.

Spin-polarized 3He-4He solutions: Longitudinal spin diffusion and nonlinear spin dynamics.

Pulsed-NMR techniques have been used to investigate longitudinal spin diffusion and nonlinear spin dynamics in dilute, spin-polarized $^{3}\mathrm{\ensuremath{-}}^{4}$He solutions between 4 and 400 mK. Solutions with $^{3}\mathrm{He}$ concentrations ${\mathit{x}}_{3}$=3.5\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}4}$ and 19.4\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}4}$ were forcibly polarized to as much as 65% and 25%, respectively, with a 9.2-T magnetic field. A technique for measuring the coefficient of longitudinal spin diffusion ${\mathit{D}}_{\mathrm{?}}$ is described, and the results of the technique are compared with recent theoretical calculations. Throughout a temperature range that covers both degenerate and nondegenerate behaviors, theory and experiment are found to be in excellent quantitative agreement for ${\mathit{x}}_{3}$=3.5\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}4}$, and in somewhat weaker agreement for ${\mathit{x}}_{3}$=19.4\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}4}$. The presence of strong molecular fields in this system is confirmed by the observation of multiple spin echoes, but it is found that they are not adequately described by recent theory. In addition, the observation of a novel, extremely long-lived oscillation with a lifetime on the order of 10 s is reported. These oscillations are found to have sensitive temperature and magnetic-field-gradient dependencies. A simple model which implies that this behavior is driven by a nonlinear instability is presented, and the results of numerical simulations based on this model are examined in an attempt to gain further insight into the spin dynamics of the system.

A pulsed NMR study of 3He-4He solutions

We report a study of the Leggett-Rice effect in dilute 3He-4He solutions at a Larmor frequency of 71 MHz. We have measured the height and phase of the spin echoes following 2-pulse NMR sequences of three solutions with 3He concentrations of 0.56%, 1.4%, and 4.0%. The theory of Leggett fits the data extremely well and yields values of the parameter ΜM0 and the transverse diffusion coefficient D. We are also able to evaluate ΜM0 and D by analyzing multiple spin echoes and find good consistency between the two methods. From values of λ/(1+F 0 a ) at various concentrations, we are able to determine the scattering length for 3He quasiparticles, a, quite accurately. We find a = −0.97±0.03 A.

Multiple spin echoes in a high-resolution spectrometer

The Hahn spin-echo pulse sequence 90°—τ-90° normally results in a single spin echo at time 2τ; however, several workers (e.g., Einzel et al., Phys. Rev. Lett. 53, 2312, 1984; Bowtell et al., J. Magn. Reson. 88, 643, 1990) have reported that its application to liquid samples with strong signals in high magnetic fields can result in multiple spin echoes (MSEs) at times 2τ, 3τ, 4τ, …. In this paper a simplified explanation of the MSE phenomenon is offered, and it is shown that extended trains of strong multiple echoes can be produced easily in a conventional high-resolution spectrometer.

A spatial editing of NMR spectra by use of heteronuclear multiple spin echoes

NMR mu ltiple spin echoes (MSEs), observed when the Hahn spin-echo pulse sequence 90°-7-90’ (1) is applied to samples with strong equilibrium polarizations (for example, 3He or water) in a linear field gradient (2-d)) may also be seen in heteronuclear experiments. This allows a novel form of spatial editing of heteronucleus NMR spectra, restricting signals to those from heteronuclei spatially coincident with high proton concentrations. The sequence 90°-7-90° normally produces a single spin echo at time 7 following the second pulse. However, mu ltiple spin echoes, occurring at times nr (n = 2, 3, 4, . . . ) following the second pulse, are observed when the same sequence is applied to samples with large equilibrium magnetizations in a linear field gradient, G . The echoes arise from the partial cancellation of the applied field gradient by the dipolar field generated by the z component of the nuclear magnet ism (5). Heteronuclear mu ltiple spin echoes ( HMSEs ) can be observed at times n ( yn / y x )7 for signals of dilute heteronuclei X in samples with high proton concentrations if the heteronucleus signal is measured following a 90°-r-90’ sequence applied to protons. F igure 1 shows the first two of a train of such echoes obtained for a solution of Na2HP04 in HZ0 using the sequence (I):

Magnetic resonance imaging analysis of extremely slow flow in a model shunt system.

Shunt malfunction is common and its diagnosis may require invasive testing that may be inaccurate or result in complications. Magnetic resonance imaging (MRI) may prove to be a useful noninvasive test of shunt function as it has been shown that MRI is capable of measuring cerebrospinal fluid (CSF) flows from 2 ml/h to 40 ml/h in model systems. Since flows in functioning shunt systems can be less than 2 ml/h, MRI must be sensitive enough to detect flow in this range in order to be a valid test for shunt function. Continuing previous studies, we have studied MRI flow-related enhancement at flow rates from 0 to 2 ml/h. Multiple spin echo scans (TR2000, TE20) were made through a specialized section of tubing in a model shunt system. The intensity of the MRI signal at points known to demonstrate maximal flow-related enhancement was measured. A linear relationship was demonstrated between signal intensity and flow as low as 0.8 ml/h. These results add support to the concept that MRI is sensitive enough to detect the lowest flows present in functioning shunt systems and therefore may be useful as a noninvasive test of shunt function.

Multiple spin echoes in spin polarized Fermi liquids

Multiple spin echoes (MSEs) in3He and in3He-4He mixtures have been analyzed using a theoretical technique proposed by Einzel et al. In both systems MSEs are generated by Fermi liquid interactions, but for3He, MSEs are also generated at higher temperatures by the dipolar demagnetizing field. The theory is compared with the experimental results for3He and with our measurements for3He-4He mixtures. We have improved on the fit to the second echo heights for3He given by Einzel et al. by taking account of both the Leggett-Rice effect and the dipolar interaction in the calculation. Good fits were also found for the first and second echo heights for the3He-4He data. The third echo height is larger than expected if we assume that the total magnetization remains uniform in space. We have developed the theory for the case when this approximation is not made, and we find good agreement for the first five echoes.

Tissue Characterization of Intracranial Tumors by MR Imaging

The main purpose of this in vivo study was to provide a detailed description of the T1- and T2-relaxation processes in intracranial tumors at 1.5 T. A total of 100 patients were investigated. Optimal experimental conditions were carefully observed, including the use of long TR values. T1 determination was based on a partial saturation inversion recovery sequence covering 12 or 6 data points. T2 determination involved a multiple spin echo sequence with 32 echoes. Calculations included biexponential analysis of the 12-point T1 data and all T2 data obtained. The results were evaluated in accordance with histopathology. The T1- and the T2-relaxation times of the prevailing tumor types were significantly different (p less than 0.0005). However, biologic scatter and overlap between tumor types were considerable. In particular, no discrimination between benign and malignant tumor growth was possible. Biexponential evaluation did not increase the specificity, although a biexponential relaxation behavior was recognized in 37% of the T2 curves. The results indicate that tissue heterogeneity is responsible for most of the scatter in the relaxation times. It is concluded that tissue characterization by MR imaging, based solely on relaxation time measurements, seems to be of no value in the differentiation of intracranial tumors.

Multiple spin echoes in multicomponent liquids

Multiple spin echoes (MSE) can occur in pulsed NMR experiments carried out on liquid samples in high magnetic fields. The extra echoes are generated by the dipolar demagnetizing field, which introduces nonlinear terms into the Bloch equation. Previously it was found that the theory which was developed to describe similar MSE in solid 3He is in good agreement with data obtained for samples of water in a field of 11.7 T. In this paper the behavior of the MSE for samples containing different spin components each with their own chemical shift is described. In this situation, interference between the magnetization of the different components leads to “beating” in the amplitudes of the MSE. The theory is extended to obtain equations which describe the evolution of the separate components. When the degree of nonlinearity is small, these equations can be solved analytically; the resulting solutions give a good description of experimental data obtained in a field of 11.7 T from samples of methyl acetate and from a mixture of acetone and water. As the demagnetizing field increases in size, the nonlinearity becomes more severe and it becomes necessary to solve the equations numerically. Numerical solutions to simulate the behavior which can be expected in fields considerably greater than 11.7 T have been obtained.

Multiple spin echo formation (abstract)

Multiple spin echo formation for quadrupole nuclei (I≳1/2) in magnetics having inhomogeneous hyperfine and quadrupole interactions was investigated using two and three pulses technique. It was shown that the ‘‘forbidden’’ echo appearance is possible for the time intervals comparable with the characteristic times of the quardupole and hyperfine broadening. The amplitude drop when the time interval between exciting pulses increases is determined not by relaxation time (as usual) but by inhomogeneity having the larger characteristic time (HFI or quadrupole). Multiple spin echo signal for the two pulses technique is due to the transitions with quantum numbers m2≳m1.1 However, when three pulses technique with ‘‘unusual’’ exciting pulses sequence (τ12≳τ23) is used transitions with m1<m2 take part in generation of some additional echo signals. This is confirmed by amplitude and time dependences for the echo signals.

Diffusion measurements by pulsed field-gradient multiple spin echoes

Diffusion measurements by pulsed field-gradient multiple spin echoes