Bruker Instruments, Inc.

Werner E Maas

David G Cory

Frank H Laukien

Joel T Lewandowski

A method of reducing radiation damping during free induction decay in NMR measurements of samples having a narrow line width uses the active switching of the quality factor value of the coil circuit of an NMR detection probe. After application of an excitation pulse to the sample, data acquisition is accomplished in periodic samples. The Q of the coil circuit is set to a high value while each sample is being taken, but is reduced significantly in between samples by detuning the coil circuit. Minimization of the high-Q state of the coil circuit and maximization of the difference between the high Q value and the low Q value greatly decrease the detrimental effects of radiation damping on free induction decay. The coil circuit Q is modified automatically by the application of a Q switching signal generated by a controller, such as a computer which controls other aspect of the NMR experiment.

Method for the reduction of radiation damping during signal acqusition

A nuclear magnetic resonance cross polarization probe uses a dual-coil arrangement in which a single-turn inner coil is surrounded by a solenoid coil. The inner coil is tuned to the frequency of a relatively high Larmor frequency nuclei type, such as proton. The solenoid coil is tuned to a lower Larmor frequency nuclei type. An inner sample region surrounded by the inner coil has a first magnetic field component induced by an electrical signal at the relatively high frequency in the inner coil. An electrical signal at the lower frequency is input to the solenoid coil and results in the generation of a magnetic field alternating at the lower frequency. This field induces a current in the inner coil at the lower frequency that, in turn, induces a magnetic field component in the inner region at the lower frequency. Because the structure of the relatively high magnetic field component and the lower magnetic field component in the inner region are both dictated by the shape of the inner coil, a good spatial matching of the fields is obtained resulting in a good Hartman-Hahn match across the sample volume.

NMR probe for cross-polarization measurements

B.sub.1 (RF) gradient echo pulse sequences are combined with frequency-selective pulse sequences to selectively suppress a solvent resonance signal by preventing the formation of an echo for the solvent resonance, while allowing the formation of an echo for the sample resonances under study. The RF gradient pulses may be planar or radial pulses. A pulse sequence for suppressing a solvent resonance signal in NMR experiments comprises a pair of (RF) gradient pulses which sandwich a selective inversion pulse sequence. In accordance with one embodiment of the invention, the RF gradient pulses are anti-symmetric (have opposite phase) and the selective inversion pulse sequence comprises a homogeneous frequency-selective inversion pulse, such as a .pi. pulse, applied in time between the two RF gradient pulses. In accordance with another embodiment of the invention, the RF gradient pulses are symmetric (have the same phase) and the selective inversion pulse sequence comprises a pulse sequence which has an excitation profile with a null at the frequency of the resonance to be suppressed.

Method for suppressing solvent resonance signals in NMR experiments

A composite RF pulse is created from a sequence of conventional homogeneous RF pulses and conventional gradient RF pulses and the composite pulse generates a gradient magnetic field with a spatially varying amplitude, but a spatially independent phase. In one embodiment of the invention, the pulse sequence consists of four conventional gradient RF pulses interspersed with two conventional homogeneous RF pulses. In another embodiment of the invention, a conventional gradient RF pulse is combined with a conventional homogeneous RF pulse and the pulse pair is repeated in order to generate an effective magnetic field with a spatially varying amplitude, but a spatially independent phase.

Pulse sequence and method for creating a radio-frequency magnetic field

Coherence transformation selectivity is improved by using combinations of homogeneous RF pulses and 'radial' RF pulses that have a uniform RF field strength throughout the sample, but whose phase (relative to the detection coil phase) has a spatial dependence such that all possible phase differences are equally represented throughout the sample. The use of radial pulses allows the spin coherences in the sample to evolve in spatial waves and the observation, or suppression, of a given coherence can be selected by using a receiver coil which has a predetermined symmetry relative to the symmetry of the spatial wave.

Method for improving selectivity in NMR experiments involving coherence

An NMR probe is designed to generate both a homogeneous RF field over the sample volume and, alternatively, a 'radial' field comprising two orthogonal gradient fields generated simultaneously in the transverse plane or a linear gradient field. The homogeneous field is generated by means of a known homogeneous coil construction, such as a Helmholtz coil or modified Helmholtz coil. The radial field can be generated by means of an inverted Helmholtz coil, either modified or unmodified, and the linear field can be generated by a Golay type coil, which coils are positioned coaxially with the homogeneous coil. The two coils are connected in parallel to the RF signal generator and switching can be accomplished either by means of an active switch or by detuning one of the coil resonant circuits when the other coil is in use.

NMR probe which includes B1, gradient coils

A composite RF pulse for NMR experiments is created by applying to a sample a radial pulse followed by a .pi. homogeneous pulse. The radial pulse has a uniform RF field strength throughout the sample and a phase relative to the detection coil phase with a spatial dependence such that all possible phase differences are equally represented throughout the sample. The composite pulse converts the radial RF pulse into a spatially-varying z rotation. The creation of a spatially-varying composite z pulse based on a radial pulse allows for a simple and direct application of a radial pulse in a manner analogous to many known B.sub.0 gradient NMR experiments (such as multiple-quantum filters, quadrature detection, and solvent suppression).

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Billerica, MA, United States of America

Werner E Maas