Method for selective excitation of NMR signals

Details Method for selective excitation of NMR signals Method for selective excitation of NMR signals

1990-07-24

1993-03-23

Tokar, Michael J.

Electricity: measuring and testing

Particle precession resonance

Using a nuclear resonance spectrometer system

G01R 3320

Patent

active

051967952

DESCRIPTION:

BRIEF SUMMARY
The present invention relates to a method for selective excitation of NMR signals by irradiating upon a sample, which is located within a homogeneous static magnetic field, an rf pulse sequence comprising at least one selective Gaussian pulse.
It is a requirement of many NMR spectroscopic methods that the nuclear spin resonances under observation must be excited in a frequency-selective way. Initially, selective excitation was achieved by simply applying rectangular rf pulses of very long duration and low power, which were described as "soft" pulses. However, the application of such pulses is connected with the disadvantage that their frequency spectrum shows side lobes which are far remote from the resonance frequency thus leading to the excitation of undesirable nuclear spin resonances. It has been found that a much better frequency behavior can be achieved if the pulses have a more favorable shape, instead of being rectangular. Although pulses with very sophisticated phase and amplitude modulation have been developed in the course of the past few years, the Gaussian pulse still has been found to be particularly favorable due to its relatively short duration and simple shape.
However, just as all the other excitation pulses used in high-resolution NMR, the 90.degree. Gaussian pulse also provides the disadvantage that it leads to strong phase response in the resulting transverse magnetization. Although it would of course be possible theoretically, when recording unidimensional spectra, to effect a first-order phase correction, such correction cannot be carried out in practice without having to accept an important shift in the spectrum base line. And in the case of more complex pulse experiments, where coherent transmission takes place, such correction is no longer possible because then not only in-phase, but also anti-phase magnetizations are encountered.
From a paper by Bruhschweiler et al. published in J. Magn. Reson. 78, 178 (1988) a method for refocusing the magnetization has been known where a soft 90.degree. pulse is followed by a hard 180.degree. pulse and the latter is then followed by the data-logging step at the end of a certain time delay equal approximately to 60% of the duration of the (truncated) Gaussian pulse. However, such extension of the pulse length is undesirable as it results in an increase of the relaxation-based losses. In addition, the proposed solution is not suited for refocusing coupled spins. Instead, the phase error will even be increased in such cases because the hard 180.degree. pulse inverts the polarizations of the coupled partners, thus preventing the couplings from being refocused. Consequently, this method is absolutely of no use if the area of interest contains a single multiplet only which is the case in the selective version of the NOESY experiment for small flip angles. One might consider to effect refocusing by a soft 180.degree. pulse, instead of a hard 180.degree. pulse, as in this case any dephasing caused by couplings would be reversed, just as any dephasing resulting from offsets. Yet, the application of a soft 180.degree. pulse creates as many problems as are solved by it. Firstly, it leads again to an increase of the duration of the sequence. And secondly, every magnetization, which relaxes during the first pulse in the Z direction or which has not been excited by the first pulse, may be excited by the refocusing pulse so that an EXORCYCLE phase shift of the second pulse would become important, which would extend the overall duration of the experiment. Consequently, this method--which has been described for example by Bodenhausen et al. in J. Magn. Reson. 27, 511 (1977)--is not suitable for application in all cases.
A paper by Friedrich et al. published in J. Magn. Reson. 75, 390 (1987) describes the use of a "half Gaussian pulse" as means for eliminating phase errors occurring in selective excitation. Although the "half Gaussian pulse" as such does not generate any constant-phase magnetization, constant-phase spectra can be obtained by adding the results o

REFERENCES:
patent: 4502008 (1985-02-01), Ohuchi
An Improved Sequence for Broadband Decoupling: WALTZ-16, Journal of Magnetic Resonance, vol. 52, 1983, pp. 335-338, Shaka et al.
Combined Use of Hard and Soft Pulses for Decoupling in Two Dimensional NMR Spectroscopy, Journal of Magnetic Resonance, vol. 78, 1988, pp. 178-185, Broschweiler et al.

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