Sensitivity in Fourier transform NMR spectroscopy of slowly relaxing systems
TL;DR: In this paper, the authors analyzed the effect of different spin-echo methods on the performance of 13C spectroscopy and found that among them, the most effective is likely to be a spin echo method in which a 90° pulse is applied at the maximum of an even-numbered echo.
About: This article is published in Journal of Molecular Spectroscopy.The article was published on 1970-08-01. It has received 148 citations till now. The article focuses on the topics: Pulse sequence & Free induction decay.
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TL;DR: In this paper, a simple method that alleviates the dynamic range problem and that facilitates the suppression of signals from protons that are not coupled to the lowy nucleus is described, where the low-y nucleus was assumed to be 13C.
1,316 citations
TL;DR: A new method for rapid NMR imaging dubbed FLASH ( fast low-a ngle sh ot) imaging is described which allows measuring times of the order of 1 s (64 × 128 pixel resolution) or 6 s (256 × 256 pixels), resulting in about a 100-fold reduction in measuring time without sacrificing spatial resolution.
1,227 citations
TL;DR: Extensions to real‐time MRI of blood vessels, diffusion coefficients, combination with two‐dimensional MR spectroscopy experiments, and other nuclei are discussed.
Abstract: Snapshot FLASH magnetic resonance imaging techniques have been developed to enable real-time imaging of MR parameters. The first realization of the method is based on a 64 × 128 FLASH tomogram acquired within 200 ms, using improved MR system hardware conditions. The soft tissue contrast obtained in FLASH MRI almost disappears by using flip angles of less than 5° and repetition times of 3 ms. This work describes extensions of FLASH MRI placing conventional MR experiments before the whole imaging sequence. This creates images of any desired contrast without changing the measuring time. Examples of inversion-recovery T1, spin-echo T2, chemical-shift-selective, and spectroscopic FLASH MRI are presented. Further extensions to real-time MRI of blood vessels, diffusion coefficients, combination with two-dimensional MR spectroscopy experiments, and other nuclei are discussed.
719 citations
TL;DR: Through shortened patient examination times, higher patient throughput, and lower cost per MRI examination, EPI may become a powerful tool for early diagnosis of some common and potentially treatable diseases such as ischemic heart disease, stroke, and cancer.
Abstract: Progress has recently been made in implementing magnetic resonance imaging (MRI) techniques that can be used to obtain images in a fraction of a second rather than in minutes. Echo-planar imaging (EPI) uses only one nuclear spin excitation per image and lends itself to a variety of critical medical and scientific applications. Among these are evaluation of cardiac function in real time, mapping of water diffusion and temperature in tissue, mapping of organ blood pool and perfusion, functional imaging of the central nervous system, depiction of blood and cerebrospinal fluid flow dynamics, and movie imaging of the mobile fetus in utero. Through shortened patient examination times, higher patient throughput, and lower cost per MRI examination, EPI may become a powerful tool for early diagnosis of some common and potentially treatable diseases such as ischemic heart disease, stroke, and cancer.
606 citations
TL;DR: In this paper, a steady state is established where there is a dynamic balance between the effect of the pulses and spin relaxation, and the deviation of the intensity of the free induction signal from its thermal equilibrium value is an exponential function of the pulse interval with time constant equal to the spin-lattice relaxation time.
Abstract: When a nuclear spin system is subjected to a repetitive sequence of strong radiofrequency pulses, a steady state is established where there is a dynamic balance between the effect of the pulses and spin relaxation. Under certain readily satisfied pulse conditions, the deviation of the intensity of the free induction signal from its thermal equilibrium value is an exponential function of the pulse interval with time constant equal to the spin–lattice relaxation time. The determination is unaffected by spin–spin relaxation provided that the interval between pulses is long enough to permit all transverse components of magnetization to be eliminated, and provided precautions are taken to inhibit spin‐echo formation. Through Fourier transformation of the transient response, high resolution spectra with many component resonances may be studied, and the spin–lattice relaxation times of the individual lines determined. The technique lends itself particularly well to repeated accumulation of the transient signal for the purpose of improving sensitivity. It has been applied to the problem of determining the spin–lattice relaxation rates of the eight different carbon‐13 resonances in 3,5‐dimethylcyclohex‐2‐ene‐1‐one. The results span a range from 2.6 to 39 sec, and are in good agreement with those obtained by applying 180°–t–90° sequences to the same sample.
470 citations
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TL;DR: In this paper, the effect of diffusion on free precession in nuclear resonance has been studied, and a new scheme for measuring the transverse relaxation time is described, which largely circumvents the diffusion effect.
Abstract: Nuclear resonance techniques involving free precession are examined, and, in particular, a convenient variation of Hahn's spin-echo method is described. This variation employs a combination of pulses of different intensity or duration ("90-degree" and "180-degree" pulses). Measurements of the transverse relaxation time ${T}_{2}$ in fluids are often severely compromised by molecular diffusion. Hahn's analysis of the effect of diffusion is reformulated and extended, and a new scheme for measuring ${T}_{2}$ is described which, as predicted by the extended theory, largely circumvents the diffusion effect. On the other hand, the free precession technique, applied in a different way, permits a direct measurement of the molecular self-diffusion constant in suitable fluids. A measurement of the self-diffusion constant of water at 25\ifmmode^\circ\else\textdegree\fi{}C is described which yields $D=2.5(\ifmmode\pm\else\textpm\fi{}0.3)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$ ${\mathrm{cm}}^{2}$/sec, in good agreement with previous determinations. An analysis of the effect of convection on free precession is also given. A null method for measuring the longitudinal relaxation time ${T}_{1}$, based on the unequal-pulse technique, is described.
5,630 citations
TL;DR: In this article, a spin echo method adapted to the measurement of long nuclear relaxation times (T2) in liquids is described, and the pulse sequence is identical to the one proposed by Carr and Purcell, but the rf of the successive pulses is coherent, and a phase shift of 90° is introduced in the first pulse.
Abstract: A spin echo method adapted to the measurement of long nuclear relaxation times (T2) in liquids is described. The pulse sequence is identical to the one proposed by Carr and Purcell, but the rf of the successive pulses is coherent, and a phase shift of 90° is introduced in the first pulse. Very long T2 values can be measured without appreciable effect of diffusion.
5,389 citations
TL;DR: In this paper, the application of a new Fourier transform technique to magnetic resonance spectroscopy is explored, which consists of applying a sequence of short rf pulses to the sample to be investigated and Fourier-transforming the response of the system.
Abstract: The application of a new Fourier transform technique to magnetic resonance spectroscopy is explored. The method consists of applying a sequence of short rf pulses to the sample to be investigated and Fourier‐transforming the response of the system. The main advantages of this technique compared with the usual spectral sweep method are the much shorter time required to record a spectrum and the higher inherent sensitivity. It is shown theoretically and experimentally that it is possible to enhance the sensitivity of high resolution proton magnetic resonance spectroscopy in a restricted time up to a factor of ten or more. The time necessary to achieve the same sensitivity is a factor of 100 shorter than with conventional methods. The enhancement of the sensitivity is essentially given by the square root of the ratio of line width to total width of the spectrum. The method is of particular advantage for complicated high resolution spectra with much fine structure.
1,551 citations
1,158 citations
01 Jan 1966
437 citations