Bruker Instruments, Inc.

Other

David G Cory

Werner E Maas

Frank H Laukien

Joel T Lewandowski

Grum Teklemariam

Jianyu Lian

Kevin D Lilly

Peter Sprenger

A magnet assembly primarily for use in MRI applications is disclosed. The magnet assembly is composed of a single disk and ring permanent magnet set connected to a C-type magnet yoke and return fixture with asymmetrically arranged poles. Together, the permanent magnets create a strong magnetic field that has a large static gradient in the central enclosure. The magnet assembly is primarily designed for diffusion based MRI scanning but can perform routine MRI scanning as well.

Compact inhomogeneous permanent magnetic field generator for magnetic resonance imaging

A gradient magnetic field generator is provided for generating a spatially varying gradient magnetic field for use with a nuclear magnetic resonance spectroscopy probe having a rotatable sample container. The gradient field generator has a plurality of straight line conductive segments which lie parallel to one another and perpendicular to a plane within which lies a rotation axis about which the sample container rotates. The straight line conductive segments each conduct a current which generates a component of the overall gradient magnetic field. The conductive segments preferably lie in a cylindrical distribution about a stator within which the sample container is rotated. The appropriate currents for the conductive segments may be determined by finding a solution for the Jacobian which defines the magnetic field variations in the three-dimensional space of the stator. Finding an appropriate solution is simplified by presuming the cylindrical distribution of conductive segments and allowing restriction due to the size and shape of the stator, and the physical space between the stator and an inner surface of the probe housing.

Apparatus and method for the generation of gradient magnetic fields for

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

A small sample imaging apparatus replaces the dewar which normally passes along the axis of an NMR imaging probe. The small sample imaging apparatus incorporates an integral RF coil and capacitor resonant circuit. The coil and capacitor are positioned adjacent to each other and remotely at the end of a projecting stalk. Stray inductance effects are thereby avoided and the stalk physically positions the resonant circuit at substantially the properly centered location. The stalk which positions the coil and capacitor may be formed from a length of rigid coaxial cable which carried RF energy to the resonant circuit. When large samples are imaged, the conventional probe coil insert can be utilized in the normal manner. However, when smaller samples are images, the adapter can be attached to the probe and the adaptor resonant circuit can be utilized.

NMR probe for imaging small samples

A plurality of measurements are made with a probe containing a material sample with a known frequency spectrum. The probe is positioned in one spot in the magnet bore in the presence of a field gradient along a predetermined axis. Prior to each measurement, the gradient strength is increased by a predetermined constant amount. The resulting measurement values are mathematically manipulated to yield a set of values which can be used to set the shim coil currents based on known equations for the field generated by each shim coil. The method can be used with a material sample with a simple NMR spectrum or a complex NMR spectrum. If a sample with a complex NMR spectrum is employed in the measurement, the spectral complexity can be removed by deconvolution during the manipulation of the measurement values.

Method for automatically shimming a high resolution NMR magnet

A multiple-pulse RF pulse sequence applied to abundant nuclei after excitation and in the presence of a spatial magnetic field gradient significantly reduces the resonant line width thereby increasing image resolution relative to other known pulse sequences. The RF pulse sequence is constructed from selected sets of known six-pulse cycles called dipolar decoupled inversion pulses. The dipolar decoupled inversion pulses are, in turn, constructed from standard solid echo pulse pairs. The magnetic field gradient is periodically varied during application of the RF pulses to preserve imaging information.

Method for increasing resolution in a NMR solids imaging system

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).

Method for creating a z-rotation using radial pulses in NMR experiments

In an NMR solid-state imaging experiment, second averaging is carried out during the experiment to reduce the effects of unwanted interactions that obscure the results of an interaction of interest. The second averaging is arranged so that the second averaging interaction and the interaction of interest are along the same axis, but are separated in time. The second averaging interaction is further designed to introduce a multiple of .pi./2 phase offset between applications of the interaction of interest. In this way, even when the sum of the actual resonance frequency and the second averaging frequency are zero, the spin dynamics are still modulated and a second averaging takes place. An additional advantage is provided in that the resonance frequency can be sampled in such a fashion that the second averaging frequency does not appear in the observed resonance frequency.

Method for improving the resolution of solid-state NMR multiple-pulse

In an NMR solid-state imaging experiment, a pulsed gradient is interleaved between subcycles of a line-narrowing RF pulse sequence in such a manner that the RF and gradient fields do not overlap in time, the subcycles of RF modulation which occur between gradient pulses are either cyclic or anti-cyclic and symmetric. The line-narrowing pulse sequence can be broken into subcycles, each of which is dipolar decoupling to at least a zero-order approximation. The gradient pulses are intercalated between a selected set of the subcycles which are symmetric. In this manner the prior art zero-order decoupling of the line narrowing interaction and the gradient interaction is extended to include the zero order and all odd order terms in the Magnus expansion of the average dipolar Hamiltonian. If the selected subcycle is dipolar decoupling to second order, then the gradient will also be decoupled to second order.

The quadrature channels of an RF generator in an NMR spectrometer are aligned relative to one another by applying pulse sets to pairs of channels that are 180.degree. out of phase. The effect of the pulses in one channel is the opposite of the pulses in the other channel. As a result, the magnetization of the sample returns to the direction in which it started when the amplitudes of the pulses applied to each channel are equal. The magnetization returns to the initial direction even in the presence of RF inhomogeneities. The characteristic magnetization pattern can be easily observed and used to adjust the relative pulse amplitudes equal and the absolute pulse amplitudes to .pi./2.

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David G Cory