Gradient coils for correcting higher order B0 field inhomogeneities in MR imaging

Abstract

A magnetic resonance apparatus corrects higher order B.sub.0 magnetic field inhomogeneities in the examination volume of an MR device. Currents through two or more coil sections (X.sub.1, X.sub.2) of at least one of a plurality of gradient coils (4) are independently controlled in such a manner that higher order field inhomogeneities of the main magnetic field B.sub.0 are compensated for by the magnetic field of the at least one gradient coil (4).

Claims

1. An MR device having functionality to correct magnetic field inhomogeneities of a nearly homogeneous main magnetic field B.sub.0 in the MR devices examination volume, which MR device comprises at least one main magnet coil for generating the nearly homogeneous steady magnetic field B.sub.0 within the examination volume, a number of gradient coils for generating switched magnetic field gradients in different spatial directions within the examination volume, each gradient coil comprising two or more coil sections (X.sub.1, X.sub.2), at least one body RF coil for generating RF pulses within the examination volume and/or for receiving MR signals from a body of a patient positioned in the examination volume, and a control unit for controlling the temporal succession of RF pulses and switched magnetic field gradients, wherein the currents flowing through the coil sections (X.sub.1, X.sub.2) of each gradient coil are controllable independently of each other, wherein each coil section (X.sub.1 or X.sub.2) is subdivided in an inner coil section and an outer coil section circuited in series and the MR device comprises a waveform generator and an amplifier, the waveform generator's output being connected to the amplifier's input and the amplifier's output coupled to the inner coil section to apply an electrical current to the inner coil section and individual current sources coupled to the respective outer coil sections to control an electrical current flowing through the outer coil sections of the respective coil sections.

2. The MR device of claim 1, wherein the individual current sources are configured for driving DC currents as constant offsets through the outer coil sections independently of the alternating currents generated by the amplifier.

3. The MR device of claim 1, wherein the coil sections are inner coil sections and outer coil sections of shielded gradient coils.

4. A magnetic resonance (MR) apparatus comprising: a main magnet configured to generate a nearly homogeneous steady-state magnetic field (B.sub.0) in an examination volume; at least first and second gradient coils configured to generate magnetic field gradients across the examination volume in first and second spatial directions, respectively, the first gradient coil including: at least two coil sections, each coil section being sub-divided into an inner coil section and an outer coil section, the inner and outer coil sections being connected in series; at least one RF coil configured to transmit RF pulses into the examination volume and/or receive magnetic resonance echo signals from the examination region; a waveform generator and a amplifier connected to an output of the first waveform generator, an output of the amplifier being coupled to the inner coil section; a current source coupled to the outer coil section configured to apply electric currents to the outer coil section; a host computer configured to control: the at least one RF coil, the waveform generator to apply an electric current to the inner coil section, and the current source to control electrical currents applied to the outer coil segment.

5. The magnetic resonance apparatus of claim 4, wherein the waveform generator is configured to apply gradient pulses, under control of the host processor, to the inner coil section.

6. The magnetic resonance apparatus of claim 5, wherein the current source is configured to apply a DC current through the outer coil section independently of the gradient pulses applied to the inner coil section.

7. The magnetic resonance apparatus of claim 4, wherein the outer coil section shields magnetic fields generated by the inner coil section.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a MR device for carrying out the method of the invention;

(2) FIG. 2 schematically shows a gradient coil arrangement comprising coil sections according to a first embodiment of the invention;

(3) FIG. 3 schematically shows a shielded gradient coil arrangement according to a second embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

(4) With reference to FIG. 1, a MR device 1 is shown. The device comprises superconducting or resistive main magnet coils 2 such that a nearly homogeneous, temporally constant main magnetic field B.sub.0 is created along a z-axis through an examination volume. The device further comprises a set of (1.sup.st, and—where applicable—2.sup.nd and 3.sup.rd order) shimming coils 2′, wherein the current flow through the individual shimming coils of the set 2′ is controllable for the purpose of minimizing B.sub.0 deviations within the examination volume.

(5) A magnetic resonance generation and manipulation system applies a series of RF pulses and switched magnetic field gradients to invert or excite nuclear magnetic spins, induce magnetic resonance, refocus magnetic resonance, manipulate magnetic resonance, spatially and otherwise encode the magnetic resonance, saturate spins, and the like to perform MR imaging.

(6) Most specifically, a gradient pulse amplifier 3 applies current pulses to selected ones of whole-body gradient coils 4, 5 and 6 along x, y and z-axes of the examination volume. A digital RF frequency transmitter 7 transmits RF pulses or pulse packets, via a send-/receive switch 8, to a body RF coil 9 to transmit RF pulses into the examination volume. A typical MR imaging sequence is composed of a packet of RF pulse segments of short duration which taken together with each other and any applied magnetic field gradients achieve a selected manipulation of nuclear magnetic resonance. The RF pulses are used to saturate, excite resonance, invert magnetization, refocus resonance, or manipulate resonance and select a portion of a body 10 positioned in the examination volume. The MR signals are also picked up by the body RF coil 9.

(7) For generation of MR images of limited regions of the body 10 by means of parallel imaging, a set of local array RF coils 11, 12, 13 are placed contiguous to the region selected for imaging. The array coils 11, 12, 13 can be used to receive MR signals induced by body-coil RF transmissions.

(8) The resultant MR signals are picked up by the body RF coil 9 and/or by the array RF coils 11, 12, 13 and demodulated by a receiver 14 preferably including a preamplifier (not shown). The receiver 14 is connected to the RF coils 9, 11, 12 and 13 via send-/receive switch 8.

(9) A host computer 15 controls the shimming coils 2′ as well as the gradient pulse amplifier 3 and the transmitter 7 to generate any of a plurality of MR imaging sequences, such as echo planar imaging (EPI), echo volume imaging, gradient and spin echo imaging, fast spin echo imaging, and the like. For the selected sequence, the receiver 14 receives a single or a plurality of MR data lines in rapid succession following each RF excitation pulse. A data acquisition system 16 performs analog-to-digital conversion of the received signals and converts each MR data line to a digital format suitable for further processing. In modern MR devices the data acquisition system 16 is a separate computer which is specialized in acquisition of raw image data.

(10) Ultimately, the digital raw image data is reconstructed into an image representation by a reconstruction processor 17 which applies a Fourier transform or other appropriate reconstruction algorithms, such like SENSE or SMASH. The MR image may represent a planar slice through the patient, an array of parallel planar slices, a three-dimensional volume, or the like. The image is then stored in an image memory where it may be accessed for converting slices, projections, or other portions of the image representation into appropriate format for visualization, for example via a video monitor 18 which provides a man-readable display of the resultant MR image.

(11) A first practical embodiment of the invention is described as follows with reference to FIG. 2.

(12) FIG. 2 shows (a part of) the gradient pulse amplifier 3 and the gradient coil 4 of the MR device 1 in more detail. The gradient coil 4 is sectioned, which means that two coil sections X.sub.1 and X.sub.2 are present for generating a magnetic field gradient in the X-direction. Corresponding gradient coil halves Y.sub.1, Y.sub.2, Z.sub.1 and Z.sub.2 are present in the gradient coils 5 and 6, respectively. The currents through the coil sections X.sub.1 and X.sub.2 are applied by means of amplifiers (current sources) 20 and 21. Each amplifier 20, 21 is connected to one coil half X.sub.1, X.sub.2. The design of the gradient coil 4 as shown in FIG. 2 is realized in many MR devices in clinical use today. However, in known MR devices the amplifiers 20, 21 driving the sections X.sub.1 and X.sub.2 are driven simultaneously based on a single wave form generator per gradient axis X, Y, Z. In FIG. 2, in contrast, each amplifier 20, 21 is driven by an individual wave form generator 22, 23, respectively. In case only a static higher order field distribution needs to generated, the second wave form generator 23 can be simplified to a controllable DC current offset. This enables to control the currents flowing through the coil sections X.sub.1 and X.sub.2 of the gradient coil 4 independently of each other in accordance with the invention. Higher order spatial magnetic field distributions can be generated by means of the gradient coil 4 as shown in FIG. 2. The magnetic field of the gradient coil 4 is superimposed onto the main magnetic field B.sub.0 within the examination volume of the MR device 1. By appropriate control of the currents flowing through the coil sections X.sub.1 and X.sub.2 via the wave form generators 22, 23, higher order shimming of the main magnetic field B.sub.0 is achieved. When currents are flowing through the coil sections X.sub.1 and X.sub.2 such that a magnetic field gradient is generated, for example during an imaging sequence, a linear magnetic field gradient plus a certain amount of higher order field components (mainly 3.sup.rd order) will be generated. By inverting the current in one of the coil sections X.sub.1, X.sub.2, a B.sub.0 field (i.e. without linear gradient field) plus a higher order field distribution (mainly 2.sup.nd order) will be generated by the gradient coil. This higher order spatial field distribution generated by the gradient coil can be used, as explained above, in a targeted manner for compensating for corresponding higher order field inhomogeneities of the main magnetic field B.sub.0.

(13) The afore-described technique can be applied on the other two gradient coils 5 and 6 as well. By using gradient coils 4, 5, 6 that are sectioned in an appropriate fashion (wherein the coil sections will generally be asymmetrically arranged with respect to the symmetry planes of the examination volume), the gradient system of the MR device 1 can produce z.sup.2, x.sup.2 and y.sup.2 terms of the magnetic field distribution within the examination volume. This corresponds to Legendre coefficients C.sub.20, C.sub.21 and S.sub.21 of a conventional shim system.

(14) An alternative practical embodiment of the invention is shown in FIG. 3. In FIG. 3, the coil halves X.sub.1 and X.sub.2 are subdivided into inner coil sections 31, 32, and outer coil sections 33 and 34 respectively. The outer coil sections 33, 34 are used for shielding the magnetic field generated by the inner coil sections 31, 32. The coil sections 31 and 33 are connected in series as well as the coil sections 32 and 34. The outer coil sections 33, 34 are connected with individual current sources 35, 36 controlling the current flowing through the outer coil sections 33, 34. The currents of the current sources 35, 36 are superimposed onto the currents generated by amplifier 37. Appropriate control of the current sources 35, 36 results in a higher order field pattern generated by the gradient coil 4 which can be used in accordance with the invention for shimming of the main magnetic field B.sub.0. In the embodiment shown in FIG. 3, the amplifiers 35, 36 can be used for driving DC currents as constant offsets through the outer coil sections 33, 34 which are independent of the alternating currents generated by amplifier 37 (during gradient switching). In the embodiment of FIG. 3 only a single wave form generator 38 is required.

(15) As illustrated in FIG. 3, it is generally possible in accordance with the invention that one or more of the coil sections X.sub.1, X.sub.2, 31, 32, 33, 34 is connected with a (DC driven) current source controlling the current flowing through the respective coil section independently of the current flowing through another coil section of the gradient coil 4. The currents generated by the corresponding current sources can be controlled individually, for example for the purpose of dynamic shimming.