U.S. Philips Corporation

Koninklijke Philips Corporation N.v.

Gerardus Nerius Peeren

Johannes A Overweg

Gerardus B J Mulder

Paul Royston Harvey

Dennis K Everett

Gerardus Bernardus Jozef Mulder

Ronald Bryan Sharpless

Jouke Smink

Willem Marten Prins

Robert G Henderson

Cornelis L G Ham

William Amor

Paul Bernard Bunk

Gerardus B Mulder

Tatjana Claasen-Vujcic

Terrence M Doyle

Jacob Anne Den Boer

Jerome S Alden

Ronald B Sharpless

William H Amor

A magnetic resonance imaging scanner includes a generally cylindrical main magnet assembly () that defines a cylinder axis (). A first set of shims () are rigidly positioned inside the magnet assembly () at about a first distance (d) relative to the cylinder axis (). A second set of shims () are rigidly positioned inside the main magnet assembly () at about a second distance (d) relative to the cylinder axis (). The second distance (d) is different from the first distance (d). A generally cylindrical radio frequency coil () is arranged inside the main magnet assembly () at about a third distance (d) relative to the cylinder axis (). A plurality of gradient coils () are arranged inside the main magnet assembly () at about a fourth distance (d) relative to the cylinder axis ().

Magnetic resonance imaging scanner with molded fixed shims

A novel method is described for simulation of an electric stimulation of the nerve system subject to the rate of change of gradient fields. A gradient signal is filtered and a stimulation signal is derived, which is compared with a predetermined stimulation threshold value. An indicator signal is generated if the threshold value is exceeded. Therefore the time dependent and spatially dependent electric fields as defined by the scanning sequence and the gradient coil properties are calculated. A vector combination of said calculated electric field components from each gradient coil axis is performed, which results in a temporal diagram of the total electric field at various spatial locations within the gradient coil. The stimulation probability at each location from said temporal diagram and said stimulation signal is then calculated, and said stimulation probability is compared with the stimulation threshold value at each location within the gradient coil.

Method for simulation of an electric stimulation in an MR imaging system

Gradient fields G &equals;&part;B /&part;x etc. are generated in an MRI apparatus in order to indicate the position (x, y, z) of an object pixel to be imaged. In a known MRI system the x and y gradient fields are generated by a combination of four gradient coils, two of which are intended to receive the x and y gradient signals whereas the other two coils are intended to receive a mix of the x and y gradient signals in order to enable a simpler structure of the gradient coils and easier impedance matching. According to the invention, apart from the x, y and z gradient signals, other signals are applied to a conversion system ( ) in order to realize the desired gradient fields but also to achieve degrees of freedom that can be used for optimizing system parameters such as heat dissipation or the uniformity of the distribution of power among the gradient amplifiers.

MRI apparatus with gradient control conversion unit

A magnetic resonance imaging facilitates a pre-compensating phase encoding of spuriously generated magnetizations. The method includes generating a preparatory excitation of spins represented upon an RF-excitation line, applying subsequently a temporary magnetic compensation gradient as represented along a phase-encoding axis, generating subsequently an RF excitation pulse as represented upon the RF-excitation line, applying subsequent to the RF excitation pulse, and before an acquisition interval for receiving magnetic resonance signals, a phase-encoding gradient as represented along the phase-encoding axis. The phase-encoding gradient is substantially the same magnitude as, and opposite in direction relative to, the direction of the temporary magnetic compensation gradient. The last step includes receiving magnetic resonance signals during the acquisition interval as represented along the RF-excitation line.

Method to reduce artefacts in the magnetic resonance image due to spurious magnetic signals

In an MRI apparatus for medical purposes it is desirable to minimize the length of the magnet system for generating the main field, inter alia in order to keep the accessibility of the region to be examined in the imaging volume as high as possible for the attending staff. In order to save costs it is also desirable to make the diameter of the magnet system as small as possible. According to the invention a short, actively shielded magnet system is obtained in that the turns of the magnet system are situated in an imaginary U-shaped space,with the opening of the U facing the axis,. The current component in the bottom,of the U has a value which is smaller than that of the current component in the limbs,of the U. The inner space,of the U does not contain a current for generating a contribution to the main field in the measuring space,of the apparatus. Consequently, a free inner space is formed in the magnet system; this free space can be used, for example to accommodate gradient coils and/or RF coils.

MRI apparatus having a short uniform field magnet with an internal space

The design of a gradient system may be aimed at a high degree of linearity of the gradient field or a high speed during the generating of the gradient pulses, depending on the wishes of the user. These two wishes imply contradictory design criteria. In order to comply with both user wishes, the gradient system,according to the invention is constructed so as to include a gradient coil,having a comparatively poor linearity, and a correction coil,which is intended to correct the linearity of the gradient coil; the linearity of the correction coil,itself thus is not important. The linearity of the system is enhanced, relative to that of the gradient coil alone, by addition of the fields of the two coils,. If a high speed is desired at the expense of the linearity, the gradient coil,alone may be activated; if a high linearity is desired at the expense of the speed, both coils,can be switched on. The apparatus is provided with switching means,for driving the gradient coil and the correction coil,mutually independently or not, as desired, and with a first and a second drive amplifier,for driving the first coil,and the second coil,, respectively.

Magnetic resonance imaging apparatus including a graident coil system with a correction coil

Magnetic resonance apparatus includes a magnet system for generating a steady magnetic field in a measuring space, which steady magnetic field is oriented mainly parallel to one of the axes of an orthogonal coordinate system, a gradient system with gradient coil systems and with a controlled source for supplying the gradient coil systems with excitation currents with a predetermined variation in time. The gradient coil systems include a number of linear gradient coil systems, each of which is arranged to generate a main gradient field which is dependent on the location of the field in the measuring space in such a manner that a magnetic field formed by superposition of one of the main gradient fields on the steady magnetic field can be described, as a function of the coordinates of the coordinate system, as a series containing first-order terms and higher-order terms, the first-order terms having predetermined coefficients which are equal to zero for two of the coordinates. The gradient system is arranged so that at least a part of the second-order terms of the series describing the magnetic field formed by superposition of the magnetic fields generated by the gradient system on the steady magnetic field have predetermined coefficients. The undesirable effects of concomitant gradient fields can thus be counteracted.

Magnetic resonance apparatus

Magnetic resonance apparatus includes an essentially cylindrical magnet system (2) for generating a steady magnetic field (H) in a measurement space (28), a coil system (4) for generating gradient fields in the measurement space, and at least one RF coil (10) which is arranged in the immediate vicinity of the measurement space. In order to increase the homogeneity of the main magnetic field (H), approximately annular first elements (44) of a magnetically conductive material are provided within the magnet system (2) so as to extend coaxially with respect to the axis (9) of the magnet system. In order to minimize the amount of magnetically conductive material required, the first elements (44) are situated in the immediate vicinity of the RF coil (10).

Magnetic resonance apparatus including elements for homogenizing the

An extremely efficient magnet system having a comparatively large angle of aperture can be obtained for magnetic resonance imaging by a reduction of fields of all orders in a common approach affecting active and passive as well as positive and negative oriented coil elements. Passive soft-magnetic ring segments are arranged in two pairs and located within a plurality of larger diameter active magnetic coils. The active coils include a central coil and outer coils wherein the outer coils are smaller than the central coil. The coils are in a helium Dewar vessel and arranged such that the system has an aperture of about 90.degree.. The central active coil may have a larger radius than the smaller outer coils.

Magnet system for magnetic resonance imaging

In a magnetic resonance apparatus comprising a superconducting magnet system (1) for generating a steady magnetic field there is also included a superconducting shielding coil system (22) in order to compensate for magnetic field variations induced in the magnet by external magnetic flux variations. The shielding coil system (22) comprises coil sections (24) of different diameters.

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Eindhoven, Netherlands

Gerardus Nerius Peeren