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At least 307 records · Page 17Linked to original sources
PubMed1994

Inversion in the steady state: contrast optimization and reduced imaging time with fast three-dimensional inversion-recovery-prepared GRE pulse sequences.

PURPOSE: To evaluate the differences in contrast between 1-second delay and zero delay (for magnetization recovery) before the preparation radio-frequency pulse in three-dimensional, inversion-recovery (IR) fast gradient-echo (GRE) acquisitions. MATERIALS AND METHODS: Mathematical simulations and measurements of brain image contrast were performed with healthy volunteers and 10 patients. RESULTS: The zero-delay sequence generated T1-weighted contrast similar to that obtained with 1-second delay but was accompanied by a substantial reduction in imaging time. However, the zero delay prohibits full recovery of the longitudinal magnetization. Hence, the signal null characteristic of IR experiments is not easily observed, since it occurs (as a function of tissue T1) at very short inversion times (< 150 msec). CONCLUSION: T1-weighted contrast comparable with that of magnetization-prepared rapid GRE sequences with a 1-second delay and preparation time (TP) of 600-700 msec can be achieved in less time by using a zero delay and a shorter TP (400-500 msec).

Brain↗
PubMed1997

Comparative studies of phase-cycling schemes for multiple pi-pulse sequences.

Recently, a new phase cycling scheme was introduced by this laboratory for use in biological solid-state NMR experiments involving multiple pi-pulses with characteristics that suggested it may enhance the sensitivity of these kind of experiments (Y. Li and J. N. S. Evans, 1995, Chem. Phys. Lett. 241, 79 and Erratum, 1995, ibid. 246, 527; Y. Li and J. N. S. Evans, 1996, J. Magn. Reson. B 111, 296). The new sequence followed the supercycled concept proposed a decade ago for heteronuclear decoupling experiments. In this paper, more detailed experiments demonstrate that the claim of enhanced sensitivity was unfounded, and in fact the supercycle proposed differs little from the established XY-8 and XY-16 based supercycles.

Magnetic Resonance Spectroscopy↗
PubMed2005

Diffusion measurements for molecular capsules: pulse sequences effect on water signal decay.

Diffusion NMR and, more recently, diffusion ordered spectroscopy (DOSY) are gaining popularity as efficient tools for the characterization of supramolecular systems in solution. Here, using diffusion NMR of hydrogen-bond molecular capsules, we demonstrate that the use of different diffusion sequences may have a dramatic effect on exchanging peaks. In fact, we found that the signal decay of the water peak in [(1a)(6)(H(2)O)(8)] is monoexponential in the pulsed gradient spin-echo (PGSE) and stimulated echo (PGSTE) sequences and biexponential in the longitudinal eddy current delay (LED) and the bipolar longitudinal eddy current delay (BPLED) sequences, routinely used in modern DOSY experiments. By performing these diffusion measurements on molecular capsules, in which water is not part of the molecular capsules, we demonstrate that this phenomenon is observed only for water molecules that exchange between two sites that differ considerably in their diffusion coefficients. Degeneration of the LED or the BPLED sequences into PGSTE-type sequences by shortening the te period resulted in the disappearance of the extra slow diffusing component. The origin, as well as the implications of the different results obtained from conventional diffusion sequences, such as the PGSE and PGSTE as compared with the LED and BPLED sequences generally used in DOSY experiments, are briefly discussed.

Diffusion↗
PubMed1993

Time-of-flight techniques. Pulse sequences and clinical protocols.

Time-of-flight, the most commonly used angiographic sequence, relies on the inflow of bright, relaxed blood into a region whose magnetization is suppressed by radiofrequency pulses. Good angiographic contrast relies on pulse rates (i. e., repetition times) that are fast enough and pulse amplitudes (i. e., flip angles) that are large enough to saturate the magnetization of the background without saturating the magnetization of entering blood.

Humans↗
PubMed1988

In-plane vascular imaging: pulse sequence design and strategy.

Magnetic resonance (MR) angiography is a noninvasive method of obtaining images without contrast agents. Recent developments in sequence design have allowed images of moving spins to be obtained without a loss of signal by rephasing the spins with three or four gradient pulses to compensate for constant velocity or acceleration, respectively. At longer echo times (TE), this approach allowed for low readout gradients and high signal-to-noise ratios. Angiograms with a resolution of 300 micron were obtained. With additional sequences that allow some dephasing but minimal signal loss, separate images of arteries and veins were obtained. Phase information was used to estimate flow velocity. Application of the rephasing scheme to gradient-echo sequences allowed for ungated, fast MR angiograms. Acceleration correction was important for long TE sequences, but velocity-corrected, gradient-echo sequences with a very short TE were comparable to velocity- and acceleration-corrected, gradient-echo sequences with slightly longer TEs. With ungated three-dimensional, gradient-echo sequences, susceptibility artifacts were minimized and excellent contrast-to-noise ratios were obtained.

Angiography↗
PubMed2000

Measurement of spin-lattice relaxation times and concentrations in systems with chemical exchange using the one-pulse sequence: breakdown of the Ernst model for partial saturation in nuclear magnetic resonance spectroscopy.

A fundamental problem in Fourier transform NMR spectroscopy is the calculation of observed resonance amplitudes for a repetitively pulsed sample, as first analyzed by Ernst and Anderson in 1966. Applications include determination of spin-lattice relaxation times (T(1)'s) by progressive saturation and correction for partial saturation in order to determine the concentrations of the chemical constituents of a spectrum. Accordingly, the Ernst and Anderson formalism has been used in innumerable studies of chemical and, more recently, physiological systems. However, that formalism implicitly assumes that no chemical exchange occurs. Here, we present an analysis of N sites in an arbitrary chemical exchange network, explicitly focusing on the intermediate exchange rate regime in which the spin-lattice relaxation rates and the chemical exchange rates are comparable in magnitude. As a special case of particular importance, detailed results are provided for a system with three sites undergoing mutual exchange. Specific properties of the N-site network are then detailed. We find that (i) the Ernst and Anderson analysis describing the response of a system to repetitive pulsing is inapplicable to systems with chemical exchange and can result in large errors in T(1) and concentration measurements; (ii) T(1)'s for systems with arbitrary exchange networks may still be correctly determined from a one-pulse experiment using the Ernst formula, provided that a short interpulse delay time and a large flip angle are used; (iii) chemical concentrations for exchanging systems may be correctly determined from a one-pulse experiment either by using a short interpulse delay time with a large flip angle, as for measuring T(1)'s, and correcting for partial saturation by use of the Ernst formula, or directly by using a long interpulse delay time to avoid saturation; (iv) there is a significant signal-to-noise penalty for performing one-pulse experiments under conditions which permit accurate measurements of T(1)'s and chemical concentrations. The present results are analogous to but are much more general than those that we have previously derived for systems with two exchanging sites. These considerations have implications for the design and interpretation of one-pulse experiments for all systems exhibiting chemical exchange in the intermediate exchange regime, including virtually all physiologic samples.

Computer Simulation↗
PubMed2004

Polarization-transfer methods in solid-state magic-angle-spinning NMR: adiabatic CN pulse sequences.

An adiabatic double-quantum polarization-transfer experiment is described. It can be characterized as an adiabatic variant of the POST-C7 experiment. A continuous variation of the phase increment between pulses leads to the introduction of a fictitious Zeeman field that allows for an adiabatic passage through the recoupling condition. This results in a chemical-shift-offset-compensated adiabatic experiment, which leads to an efficient and broadbanded polarization transfer or to a double-quantum excitation. Similar variations of other C- or R-type experiments can be envisioned.

Carbon Isotopes↗
PubMed1998

Influence of High Orientational Order on the Shape of the Echo Response from a Hahn Pulse Sequence

The amplitude modulations in the simulations of the Hahn echo responses from cholestane spin labels in samples characterized by a high degree of orientational order are shown to arise from the use of "soft" pulses. Soft pulses have a limited spectral range and cover only a small portion of the CW-ESR spectra, so that not all the spins are on-resonance. The magnetization vectors of the off-resonance spins only partially tilted away from the laboratory z axis, the direction of the applied static magnetic field. They thus contribute oscillating components to the magnetization in the xy plane. The contribution from the off-resonance spins to the Hahn echo formation is significant in highly oriented samples, but cancels out in samples exhibiting a small degree of order. Experimental echo responses obtained from CSL molecules embedded in rigid matrices of eggPC bilayers and the liquid crystalline materials ZLI and MBBA confirm the theoretical predictions. Copyright 1998 Academic Press.

Journal Article↗
PubMed2003

The depletion of protein signals in metabonomics analysis with the WET-CPMG pulse sequence.

Nuclear magnetic resonance (NMR) spectroscopy is a powerful analytical tool capable of providing a comprehensive metabolic profile of biofluids such as urine, plasma, and serum. Unfortunately, when measuring serum and plasma, the high protein concentration can obscure the signals originating from low molecular weight metabolites. We evaluated the use of different parameters within the Carr-Purcell-Meiboom-Gill (CPMG) pulse train of fast spin-echoes to remove the macromolecular signal contribution in one-dimensional proton (1H) NMR spectra. Experimental parameters such as the refocusing delay in the CPMG pulse train, pulse miscalibration, and recycle time were examined to assess the ability to remove the protein signals from the spectrum without causing a deleterious effect on the signals originating from free, low molecular weight metabolites. The 1H-NMR spectra of a variety of serum samples spiked with 2'-deoxyadenosine were acquired using various acquisition parameters. Our results show that the delay used in the CPMG spin-echo and the combination of the acquisition pulse flip angle and recycle time are the two major factors affecting the observed metabolite signal amplitudes in the resulting 1H-NMR spectrum.

Body Fluids↗
PubMed1994

Low-energy transvenous cardioversion defibrillation of atrial tachyarrhythmias in the canine: an assessment of electrode configurations and monophasic pulse sequencing.

Prevention of recurrent atrial fibrillation and flutter remains a difficult clinical problem. Consequently, development of an easily implantable automatic atrial cardioverter defibrillator is appealing. In this context we have examined the feasibility of intracavitary low-energy shocks delivered via transvenously positioned electrodes for termination of induced atrial tachyarrhythmias in canine models. This study extends these observations with use of single-pathway (5 msec pulse duration) and dual-pathway sequential (5/5 msec, 0.2 msec separation) shocks of varying leading edge voltages (100 to 400 V) in a closed-chest canine talc-pericarditis model. Bipolar 9.5 French electrode catheters (electrode surface areas, 0.62 cm2) were positioned at the superior vena cava-right atrium (SVC-RA) junction (labeled SVC) and right ventricular (RV) apex, with a subcutaneous plate over the chest wall. For single-pathway shocks, overall treatment effectiveness was comparable among the three vectors tested (RV apex to SVC, 35%; RV apex to subcutaneous plate, 17%; and SVC to subcutaneous plate, 35%). Furthermore, there was no evident relationship between leading edge voltage and shock effectiveness. In contrast, although each of the dual-pathway shock vector combinations tested also showed similar overall effectiveness, there was an apparent dose-response effect as leading edge voltage increased. The SVC (common) to RV apex (pulse 1) and subcutaneous plate (pulse 2) achieved 60% effectiveness at 400 V (approximately 4 joules). Thus this study provides additional evidence favoring feasibility of low-energy transvenous atrial cardioversion defibrillation. However, further refinement of energy delivery is essential for the implantable automatic atrial cardioverter defibrillator concept to become clinically accepted.

Animals↗
PubMedDate not supplied

Use of a variable electrical pulsing sequence in rabbit oocyte activation.

Variability in oocyte activation sensitivity to electrical stimuli was shown in two types of oocytes (i.e., oocytes with a whole first polar body: w-PB 1 and those with a fragmented PB 1: f-PB 1), of a similar post-ovulatory age. In order to initiate the normal activation display (i.e., extrusion of the second polar body), the w-PB 1 oocytes required, on average, 3.6 +/- 0.2 pulses and the f-PB 1 oocytes 2.9 +/- 0.1 pulses (p = 0.18). From both experimental series carried out in this work, the average haploid activation rates were 68% and 70% for w-PB1 and f-PB1 oocytes, respectively. Oocyte type did not affect the haploid embryo developmental ability observed at 24 h of culture (8-cell stage: 33-35% in Series 1 and 23-26% in Series 2), nor at 32 h of culture (16-cell stage: 77-93% and morula stage: 34-41%; Series 2). Therefore, in further experiments, the f-PB 1 oocytes may also be used as potential forerunners of haploid embryos, almost up to the morula stage.

Animals↗
PubMed1998

Pulse sequences for steady-state saturation of flowing spins.

It is useful to be able to suppress the NMR signal from spins in a flowing fluid, for example for "black-blood" visualization of blood vessels in vivo, for the suppression of flow artifacts, and for the estimation of tissue perfusion by continuous labeling of inflowing arterial spins. This work considers the flow of fluid through a region in which it is subjected to a train of saturation pulses. Computer simulations and in vitro measurements show that a train of equal-duration spoiler pulses produces less effective suppression than does a train of pulses of geometrically increasing duration. It is shown analytically that a long train of ideal equal-duration spoiler pulses converts initial magnetization (0, 0, M0) into a combination of longitudinal and transverse magnetization equal to 0. 29 (-M0, 0, M0) and is therefore unsatisfactory for continuous saturation.

Algorithms↗
PubMed1993

[Analysis of cerebral blood flow dynamics and evaluation of extracranial-intracranial arterial bypass function using magnetic resonance angiography with presaturation pulse sequence].

Magnetic resonance (MR) angiography was applied to the study of blood flow dynamics and evaluation of extracranial-intracranial arterial bypass function in patients who had received bypass surgery. Three-dimensional gradient-echo acquisitions with selective presaturation of individual vessels were used to determine the direction of blood flow. Presaturation causes signal loss within the territory supplied by the presaturated artery, without affecting vessels not coursing through the presaturation slab. The patency of arterial bypass graft could be demonstrated as properly with MR angiography as with conventional angiography. In addition to that, MR angiography with presaturation pulse clearly demonstrated the direction of blood flow and the territory supplied by arterial bypass graft. The results were correlated with those from transcranial Doppler sonography and conventional angiography. We concluded that MR angiography is a noninvasive useful method for the evaluation of dependency on post-surgical collaterals and bypass function.

Aged↗
PubMed1984

The dependence of nuclear magnetic resonance (NMR) image contrast on intrinsic and pulse sequence timing parameters.

In Nuclear Magnetic Resonance (NMR) the image pixel value is governed by at least three major intrinsic parameters: the spin density N (H), the spin-lattice relaxation time T1, and the spin-spin relaxation time T2. The extent to which the signal is weighted toward one or several parameters is related to the history of the spin system preceding detection. On the simplifying, though not generally warranted assumption that the spin density does not vary significantly in soft tissues, relative tissue contrast can be predicted quantitatively provided the relaxation times are known. Signal intensities and contrast were computed on the basis of the Bloch equations and experimentally determined relaxation times as a function of pulse timing parameters and the data compared with those in images recorded at 0.5T field strength. Significant deviations from the equal density hypothesis were found for gray and white substance. Notably partial saturation but also spin echo and inversion-recovery images are not in full accordance with predictions made on the basis of relaxation times alone.

Brain↗
PubMed1995

Heteronuclear NMR pulse sequences applied to biomolecules.

Current concepts in heteronuclear multidimensional NMR spectroscopy are reviewed. Methods to improve the sensitivity and the efficiency of data collection include constant time, compression through the overlap of chemical shift evolution and dephasing and rephasing periods, and dual or time-shared evolution. Two classes of three-dimensional and four-dimensional triple-resonance experiments applied to proteins are considered. The first class correlates 1H, 15N, and 13C signals of the protein backbone. The second class correlates both backbone and side-chain signals. Application of triple resonance to RNA is also discussed. Heteronuclear cross polarization (HCP) is considered as an alternative to INEPT transfer, and its application to nucleic acids is presented. Finally, two methods of employing pulsed field gradients (PFGs) are reviewed.

Magnetic Resonance Spectroscopy↗
PubMed1997

Improvements for measuring 1H-1H coupling constants in DNA via new stripe-COSY and superstripe-COSY pulse sequences combined with a novel strategy of selective deuteration.

Three bond proton-proton vicinal coupling constants are of potential value for analyzing sugar conformations in DNA. However, self-cancellation in antiphase cross peaks and modulation of peak splittings by transverse cross relaxation can alter the apparent coupling constants such that they do not accurately reflect the sugar conformations. Transverse cross relaxation is most effective between strongly coupled geminal proton pairs. Here we report the use of stereospecific deuteration at the H2" position in the A5 and A6 residues in the 12 base pair DNA sequence [d(CGCGAATTCGCG)2] as a means of investigating the effect of transverse cross relaxation on P.E.COSY type cross peaks. Deuteration of the H2" proton is expected to reduce the transverse cross relaxation rate by the square of ratio of the proton to deuteron gyromagnetic ratios, i.e., by a factor of 42. Additionally, a striking eight- to ninefold increase in the signal intensity was observed for cross peaks involving the remaining H2' proton resulting from diminished dipolar relaxation. Further improvements in signal-to-noise ratio were realized by collecting P.E.COSY spectra in strips, using an experiment referred to as stripe-COSY, employing selective excitation pulses which reduced the number of required t1 increments by a factor of four. A final improvement was achieved by employing selective time-shared homonuclear decoupling during the acquisition period, in an experiment referred to as superstripe-COSY, to collapse splittings due to passive couplings. Collectively, these approaches provide P.E. COSY-type spectra with two to three orders of magnitude increased sensitivity per unit time and that are relatively free from artifacts.

DNA↗
PubMed2002

Multislice double inversion pulse sequence for efficient black-blood MRI.

Over the last several years there has been a rapidly growing interest in high-resolution MRI of the vascular wall to assess the extent of atherosclerotic lesions. Vessels of particular clinical relevance are the carotid and coronary arteries. Currently, the preferred imaging sequence for these studies is a "black-blood" technique based on the double-inversion scheme to null the blood signal. A critical drawback of the black-blood technique, however, has been its single-slice nature, as there is only one point in time during the recovery of the blood magnetization from inversion at which the signal is completely nulled. Consequently, the total scan time can become prohibitively long, particularly when an imaging protocol includes several series of these datasets. In this work, a multiple-slice double-inversion technique is described that can reduce the scan time by a factor of two or more. It is demonstrated in vivo with examples from carotid and coronary arteries that one can acquire multiple slices with sufficient nulling of blood, following a single set of inversion pulses.

Adult↗
PubMed1999

MRI of liver metastases: limitation of spleen-liver model in optimizing pulse sequences.

The spleen-liver model, as a predictor for contrast-to-noise ratio (C/N) in liver metastases, was verified for seven sequences in 22 patients with 70 colorectal metastases. Optimization of conventional spin-echo, T1-magnetization-prepared gradient-echo and fat frequency-selective presaturation inversion-recovery fast spin echo can be done using the spleen-liver model. C/N of liver-spleen and liver-metastases, however, differed significantly on our T1 gradient-echo and T2-weighted fast spin-echo images, with and without fat-selective saturation.

Adult↗