Abstract: In a magnetic resonance imaging system in which the read and phase directions are rotated with respect to the orthogonal gradient coil directions, a gradient coil is driven to generate a gradient waveform having a segment, either sinusoidal or linear, that approximates the sum of the simultaneous non-zero components of the read and phase gradient waveforms in the direction of the gradient coil. Resonant circuits, each including a gradient coil, generate simultaneous periodic gradient coil waveforms in which the integral over each waveform period is non-zero. First and second capacitive elements in the resonant power supply are electrically connected to the gradient coil through a switch and a bridged network, respectively. The bridged network selectively provides current flow paths between the coil and either of the terminals of the second capacitive element.

Abstract: A unitary, generally cylindrical member supports both the primary and shielding coils. By supporting both coils on a unitary member it is possible to take advantage of the antiphase relationship between the primary and shielding coils and thereby reduce the acoustic noise generated by the gradient coil assembly. The unitary member can be formed by starting with a mandrel to which the primary coil is applied, and then successively adding layers each having a cylindrical outer surface aligned with the same longitudinal axis. Each new cylindrical surface is formed by adding material and then removing some of the material while rotating the member about the longitudinal axis (e.g., turning on a lathe, or passing a blade past a still liquid skim coat). Additional primary and shielding coils are applied after each new aligned, cylindrical surface is formed.

Abstract: In a magnetic resonance imaging system in which the read and phase directions are rotated with respect to the orthogonal gradient coil directions, a gradient coil is driven to generate a gradient waveform having a segment, either sinusoidal or linear, that approximates the sum of the simultaneous non-zero components of the read and phase gradient waveforms in the direction of the gradient coil. Resonant circuits, each including a gradient coil, generate simultaneous periodic gradient coil waveforms in which the integral over each waveform period is non-zero. First and second capacitive elements in the resonant power supply are electrically connected to the gradient coil through a switch and a bridged network, respectively. The bridged network selectively provides current flow paths between the coil and either of the terminals of the second capacitive element.

Abstract: A unitary, generally cylindrical member supports both the primary and shielding coils. By supporting both coils on a unitary member it is possible to take advantage of the antiphase relationship between the primary and shielding coils and thereby reduce the acoustic noise generated by the gradient coil assembly. The unitary member can be formed by starting with a mandrel to which the primary coil is applied, and then successively adding layers each having a cylindrical outer surface aligned with the same longitudinal axis. Each new cylindrical surface is formed by adding material and then removing some of the material while rotating the member about the longitudinal axis (e.g., turning on a lathe, or passing a blade past a still liquid skim coat). Additional primary and shielding coils are applied after each new aligned, cylindrical surface is formed.

Abstract: A series topology is provided for the magnetic-field-generating coil and storage capacitor in the circuitry driving the coil of an MRI system. The coil and capacitor thus form a series resonant circuit that can deliver a sinusoidal current through the coil at a resonant frequency. A power source and switch are connected in series with the coil and capacitor, and current flow to the coil is initiated and interrupted by closing and opening the switch.

Abstract: Narrower conductive strips are provided in a segmented RF shield to reduce the gradient coil power loss attributable to the shield. Narrower conductive strips reduce the eddy current losses experienced by the gradient coil, thereby reducing the power required to operate the gradient coil, especially in high speed imaging where high frequencies up to 10 KHz are used. The conductive strips can be made sufficiently narrow to substantially reduce the gradient coil power loss attributable to the shield. When these strips are made in this manner, a third conductive layer may be placed over a transition region where there is a substantial change in the direction of the conductive strips. In such a transition region, RF current tends to travel in curved paths across the pattern of conductive strips instead of along the lengths of the strips, with resulting decrease in coil quality factor, Q. The third conductive layer provides a lower impedance path for RF currents in the transition region, to reduce RF power loss.

Abstract: A method of cancelling ghosts from NMR images. The method involves estimating a phase difference function .DELTA. (n.sub.1, n.sub.2) and using that function to solve a linear system of equations to find the magnitudes of the true object densities at the true image and ghost locations x(n.sub.1,n.sub.2) and x(n.sub.1,n.sub.2 +N/2), respectively, where the image has dimensions N.times.N.sub.s. Experimental values of .DELTA. (n.sub.1, n.sub.2) for a variety of objects indicate that its variation along n.sub.1 is considerably larger than along n.sub.2. Thus, for each column n.sub.1, the phase difference function .DELTA. (n.sub.1, n.sub.2) can be modelled as a one-dimensional function of n.sub.2 with two parameters .alpha. (n.sub.1) and .beta. (n.sub.1), which are estimated from the pixels in the 2-D FFT processed reconstructed image Y(n.sub.1,n.sub.2). These parameters are then used to estimate .DELTA. (n.sub.1, n.sub.2), which is ultimately used to de-ghost the image.

Abstract: A method to recover MRI signals resulting from the application of time varying gradients. The raw MRI signal is demodulated, low pass filtered, and digitized using an A/D converter sampling linearly at the Nyquist rate. The samples from the A/D converter are compiled into a vector, which is multiplied by a least squares estimator matrix (H*H).sup.- H*, where H is a matrix whose mkth element, in the case of a sinusoidal gradient, is given by ##EQU1## This multiplication is repeated for each vector of samples from the A/D converter, and the vectors resulting from the multiplication are accumulated to form the columns of a matrix, the rows of which are Fourier transformed to obtain the MRI image.

Abstract: A method of high-speed magnetic resonance imaging in which the object to be imaged is placed in a high static magnetic field. Nuclear spins are excited in an image area by applying a pulse of radio frequency magnetic field. Optionally, a first gradient field, termed a slice selection gradient, may be applied in conjunction with the RF excitation pulse such that spins are excited only in a selected plane of the object. Following an encoding pulse sequence and rephasing of the nuclear spins by the application of a 180.degree. radio frequency pulse, mutually orthogonal phase-encoding and readout gradients are alternatively applied in the image plane to effect a traversal through spatial frequency domain (k-space). In a preferred embodiment of the invention, the readout gradient is applied as a continuous sinusoidal wave, resulting in a slight overlap between the phase-encoding and readout gradients. If phase errors vary slowly in time, only a partial k-space trajectory is required.

Abstract: A method of high-speed magnetic resonance imaging in which the object to be imaged is placed in a high static magnetic field. Nuclear spins are excited in an image area in a selected plane of the object by superimposing a slice-selection gradient along an axis and applying a pulse of radio frequency magnetic field. Following an encoding pulse sequence and rephasing of the nuclear spins by the application of a 180.degree. radio frequency pulse, mutually orthogonal phase-encoding and readout gradients are alternately applied in the image plane to effect a traversal through spatial frequency domain (k-space). In a preferred embodiment of the invention, the readout gradient is applied as a continuous sinusoidal wave, resulting in a slight overlap between the phase-encoding and readout gradients. If phase errors vary slowly in time, only a partial k-space trajectory is required.

Abstract: A method of high-speed imaging is employed in which less than 100% of the spatial frequency domain (k-space) is sampled. In one embodiment, the trajectory extends over the k-space origin, and the information acquired from the extension is used to compensate for any phase errors. If the same number of points are collected as in a full k-space acquisition, signal bandwidth is maintained, and spatial frequency response is increased. In a second embodiment, two or more partial k-space acquisitions are performed and then pieced together in a "mosaic" prior to Fourier transformation. In a further embodiment, partial k-space acquisitions in the direction of the readout gradient are combined with interleaved acquisitions in the direction of the phase-encoding gradient to avoid discontinuities.

Abstract: A cavity resonator suitable for use in a high-speed nuclear magnetic resonance (NMR) imaging device comprises a plurality of transmission lines aligned parallel to a common axis and magnetically coupled to produce an essentially uniform dipole magnetic field perpendicular to the axis. The transmission lines are two conductor lines, with the first conductors being arranged inwardly of a second conductor common to all lines. The common conductor is symmetrically arranged about the axis of the resonator and constitutes a shield to contain the magnetic field within the cavity. The plurality of transmission lines are conductively isolated from each other but spaced in sufficiently close proximity to each to provide for strong mutual magnetic coupling amongst all transmission lines. A single inductive coupling loop may be provided for driving the resonator with a linearly polarized magnetic field and for receiving NMR information from the resonator. The use of a pair of inductive coupling loops spaced 90.degree.

Abstract: A method of high-speed magnetic resonance imaging in which the object to be imaged is placed in a high static magnetic field. Nuclear spins are excited in an image area in a selected plane of the object by superimposing a slice-selection gradient along an axis and applying a pulse of radio frequency magnetic field. Following an encoding pulse sequence and rephasing of the nuclear spins by the application of a 180.degree. radio frequency pulse, mutually orthogonal phase-encoding and readout gradients are alternately applied in the image plane to effect a traversal through spatial frequency domain (k-space). If phase errors vary slowly in time, only a partial k-space trajectory is required. Chemical shift is eliminated either by the application of a suppression pulse of RF field or by tailoring the frequency spectrum of the rephasing pulse so as to preclude rephasing of one or other of the chemical moieties.

Abstract: A modular function generator for generating a plurality of analog waveforms for use in an NMR imaging system. The function generator includes a plurality of waveform synthesizer modules which are interconnected by a main bus to a central controller CPU. Each synthesizer is adapted to produce one analog waveform for the NMR imaging system. A synthesizer module includes a memory and an address sequencer which produces a sequence of addresses which control the retrieval of data from the memory. Each data world of the memory contains several fields. An instruction field and a general data field control the address sequencer so that the sequence of addresses is, in turn, controlled by the memory data. Other fields from the memory define analog and digital outputs produced by the synthesizer module. The fields are generated by program control means which are organized to have data segments which define a set of elementary analog waveform segments and subroutines which are controlled by a main program.

Abstract: In order to increase the efficiency with which NMR information is received, the peak amplitude of the received NMR signal is reduced. This reduction is obtained by varying the phase of the nuclear spin systems which produce the NMR signal. The phase variation may be obtained by applying a spatially inhomogeneous rf pulse. It may alternatively be obtained by applying a tailored rf pulse having either an amplitude spectrum or a phase spectrum which varies with frequency. It may alternatively be obtained by applying gradient pulses. The phase variation may be produced at one or more of several times during the data acquisition. In addition to reducing the peak amplitude of the NMR signal, the phase variation results in reduced noise from an FFT performed during processing.

Abstract: Fourier images based on multiple NMR echoes may be obtained using the method and apparatus disclosed. A sequence of pulses and gradients are applied to the object to be imaged. The pulses begin with an excitation pulse, which is followed by a series of refocusing pulses. In order to compensate for phase error in the refocusing pulses, the refocusing pulses are applied along alternating X and Y axes in the rotating frame of reference. The echoes are then coherently added to reserve phase information. Alternate echoes may be time reversed and their complex conjugate may be taken to permit coherent addition. Alternatively, the even echoes may be added separately from the odd echoes to produce separate data sets. The pulse and gradient switching sequence provides sign reversals such that any baseline component will be cancelled by the coherent addition.

Abstract: The phase encoding of NMR information is performed using rf pulse sequences. The rf pulse sequences produce a substantially continuous phase variation cross the resonant frequency range of the atoms of an object in order to perform phase encoding. Some of the pulses may be tailored pulses, with each tailored rf pulse including frequencies which cover a bounded continuous frequency range. Either the phase or the amplitude of each tailored rf pulse varies with frequency across this continuous range. If the amplitude is linearly variant across the range, the phase is constant, but if the phase is linearly variant across the range, the amplitude is constant. The tailored rf pulses may be applied in the presence of pulsed magnetic gradient fields of the conventional type to produce the phase variation.

Abstract: In the disclosed pulse and gradient switching sequences, refocusing pulses are timed to provide spectral encoding so that a resulting series of spin echoes each include both spatial and spectral information for chemical shift imaging. Atoms within an object are excited and may then be spatially encoded, as by a phase encoding gradient. A series of refocusing pulses is then applied, each followed by an observation gradient field. Each of the refocusing pulses after the first one may be displaced in time relative to the observation gradients in order to provide spectral encoding. In an alternative embodiment for limited spectral resolution of fat and water, the observation gradients may also be timed so that echoes may be added to compensate for the decreasing amplitudes of the successive spin echoes.

Abstract: A circuit and method are disclosed for providing high, constant amplitude sinusoidally modulated NMR gradient fields. A capacitor is connected in parallel with the gradient coil, but separated by a switch. A pre-determined amount of energy is stored in the capacitor and then the switch is closed to permit a sinusoidal oscillation of energy between the capacitor and gradient coil. By precharging appropriately, sine wave or cosine wave oscillation can be obtained for the NMR system. The oscillation is terminated by opening the switch at a time when all of the oscillating energy is in the capacitor. In a preferred embodiment of the invention, the switch is a bridge circuit, each of whose sides is a back-to-back thyristor pair, so that the gradient field may be modulated either as a full wave, half-wave rectified or non-rectified sinusoidal oscillation. In one embodiment, a full-wave rectified sine wave is used, and all the oscillating energy is caught in the capacitor during intervals in which 180.degree.

Abstract: A radio frequency (rf) coil for an NMR device is disclosed in which at least two active sections each have a ground end connected to a common potential and a drive end opposite the ground end. The rf coil also includes at least one transmission line section for coupling the drive ends of two of the active sections. The coil is tuned to a resonant frequency by a variable capacitor connected to the drive end of one of the active sections. Two active sections may be arranged opposite each other about an axis and connected by a half wavelength transmission line to provide a linearly polarized magnetic field. Four active sections may be arranged 90.degree. apart from each other in a quadrature coil to provide a circularly polarized magnetic field. Each active section may include two wires, each connected at one end to a lower ground ring and having an opposite end adjacent an upper ground ring which provides a ground plane.