RF pulse cascade for the generation of NMR spectra

Details RF pulse cascade for the generation of NMR spectra RF pulse cascade for the generation of NMR spectra

1992-06-01

1994-02-08

Arana, Louis

Electricity: measuring and testing

Particle precession resonance

Using a nuclear resonance spectrometer system

324307, G01V 300

Patent

active

052851591

DESCRIPTION:

BRIEF SUMMARY
The invention relates to a method for generating a spectrum of nuclear magnetic resonance signals by radiating a sequence of n RF pulses onto a sample which is present in a homogeneous static magnetic field, in which the envelope of the nuclear magnetic resonance signal response in the frequency space is approximately a rectangular function.
The capacity for the excitation or inversion of nuclear spins over a selected band of frequencies has become an important part in many experiments in modern nuclear magnetic resonance. In high resolving NMR spectroscopy it is often desirable to restrict the width of the relevant frequency domains, especially in two and three dimensional spectroscopy. Frequency selective excitation is also an integral component of a whole class of experiments in which two or three dimensional spectra are compressed into one dimension. The perhaps most important application of selective excitation are the methods for NMR imaging. In this context, two especially important problems are to be mentioned, namely the inversion over a well defined rectangular window in the frequency spectrum and the excitation of transverse magnetization with minimal phase dispersion proceeding from longitudinal magnetization, again over a rectangular window in the frequency spectrum.
Among the numerous methods that were proposed by different authors for rectangular inversion and excitation, a large number are based on the use of composite pulses that consist of trains of hard pulses which are phase shifted against one another, but have rectangular envelopes and constant amplitudes. Other processes for rectangular inversion and excitation use amplitude and/or phase modulation. In the further discussion there are to be considered only pulses with purely amplitude modulated envelopes. For the spin inversion, it is possible to use radio frequency (RF) pulses whose envelopes can be described by simple analytic functions, such as, for example, Gaussian, Sinc or Hermite functions. The only known amplitude modulated pulse for in-phase excitation with an envelope that can be described by analytical function is a Gaussian pulse with 270.degree. on-resonance flip angle. Although with this pulse the above problems can be partly solved, in the case of demanding applications, due to residual phase dispersion and the deficient selectivity, there still remain unsolved problems.
Latest strivings for the optimization of formed pulses were based on the division of an arbitrary envelope in the time domain into many discrete intervals and the variation of the RF amplitude in each individual interval until the signal response approximates a target function. For this purpose, various degrees of refinements were introduced, among others, the optimal regulating technique of Conolly et al. in "Proceedings of the Fourth Annual Meeting of the Society of Magnetic Resonance in Medicine, London, August 1985, pp. 958 ff", the conjugated gradient method of Murdoch et al. in "J. Magn. Reson." 74, 226 (1987) and the linearization of the Bloch equations according to Ngo et al. in "Magn. Reson. Med." 5, 217 (1987).
These processes lead, to be sure, to excellent results. Those of Ngo et al. approach rather close to an ideal signal response. The resulting envelopes, however, require between 100 and 500 independent parameters for their definition, so that their practical application becomes extremely complicated.
An object of the present invention is, therefore, to present a process for generating a spectrum of nuclear magnetic resonance signals by radiating a sequence of n RF pulses onto a sample which is located in a homogeneous static magnetic field, in which the envelope of the nuclear magnetic resonance signal response in the frequency space is approximately a rectangular function, in which, by a sequence of n RF pulses with as few adjustable parameters as possible, a nuclear magnetic resonance spectrum is generated with rectangular characteristics in frequency space.
This problem is solved according to the invention by the means that the n RF

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Proceedings of the Fourth Annual Meeting of the Society of Magnetic Resonance in Medicine, London, Aug. 1985, p. 958 ff, Conolly et al.: Selective Pulse Design Via Optimal Control Theory.
Journal of Magnetic Resonance, 74, 226-263 (1987) Murdoch et al.: Computer-Optimized Narrowband Pulses for Multislice Imaging.
Magnetic Resonance in Medicine, 5, 217-237 (1987) Nago et al.: General Solution to the NMR Excitation Problem for Noninteracting Spins.

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