GB2170957A - Coil assembly for NMR imaging - Google Patents

Abstract

In a gradient coil assembly for use in NMR imaging, the assembly receives a pulsed electrical current and usually emits an oscillation in the audible range, which can be objectionable. The invention includes a former on which the coils are supported which is designed for reduced sound emission. The former comprises four axially spaced rings 55-58 joined by axially extending members. The rings are made of a rigid non-magnetic material and the rings and members may be integrally formed. <IMAGE>

Description

SPECIFICATION improvements in imaging using the resonance of nuclei This invention relates to imaging systems using the resonance of atomic nuclei, for use in diagnostic medicine. The principles involved in causing nuclei to resonate using magnetic fields, are already well established. Atomic nuclei, when placed in a high magnetic field and stimulated by a particular radio frequency, emit measurable radio signals. This phenomenon characterises all stable atomic nuclei which contain an odd number of protons or neutrons, or both. These particles possess two vital properties, a spin moment and a magnetic moment and can be likened to a spinning magnetic gyroscope.

Approximately two thirds of all stable atomic nuclei have spin and magnetic moments. The hydrogen nucleus, or proton, is the simplest of this type. It is also the most abundant element in the human body, approximately two thirds of which is water.

During imaging using magnetically induced resonance of nuclei, the patient is subjected simultaneously to three different energy sources: 1) A static magnetic field of high strength 2) A time varying or gradient magnetic field of significantly lower strength, and, 3) The radio frequency magnetic field.

1) The high strength magnetic field affects the protons in the patient's body. Without this field the protons would be randomly oriented; in this magnetic field the protons are aligned with or against the field (there is no intermediate state).

More protons are aligned with the field than against it because their energy state favours this.

Since there are more protons aligned with the field than against it, a magnetisation exists in the general direction of the magnetic field, and in a state of equilibrium, the magnetisation aligns with the magnetic field in a manner similar to a compass needle in the earth's field. If the protons are deflected away from the direction of the magnetic field they will not return directly to the equilibrium state, but instead will rotate in ever decreasing circles around the direction of the magnetic field like gyroscopes precessing under the influence of an impulsive force. The behaviour of the protons when the precession is decaying provides information about the type of material in the slice under examination and it is this information which is reproduced in the form of an image for medical examination.

The high strength magnetic field is provided by a superconducting magnet. This consists of a hollow cylindrical vessel which contains the magnet, is hermetically sealed and inside which the superconducting magnet operates in liquid helium at about 4 degrees Kelvin.

2) The time varying or gradient magnetic field is provided by windings which enable the gradient field to be superimposed on the main field and thus enable planar images or slices to be taken through parts of the patient's body. These windings usually comprise X,Y and Z gradient coils and the requisite windings can generate a field gradient perpendicular to the plane of the slice required.

When the radio frequency signal is applied, the necessary resonance conditions for the stimulation of the nuclei are only fulfilled in the one plane which it is desired to examine whilst in the rest of the patient's body the magnetic field and the frequency required for resonance are either too large or too small.

3) The radio frequency magnetic field is provided by further windings. The frequency is selected to resonate the nuclei and, as described above, this field deflects the nuclei away from the main magnetic field to cause precession.

The radio frequency coils are arranged on a cylindrical former and this assembly fits inside the cylindrical shape of the X, Y and Z coils.

The gradient coils receive a pulsed direct current of approximately square waveform in which the time the current flow differs from the time during which the current is zero. This current is pulsed at a frequency in the range 1 Hz to 20 Hz, the actual frequency being set by the overall imaging control system to match the operating requirements of the system.

When a pulsed current is passed along a conductor which lies across a magnetic field, a pulsating force is exerted on the conductor; the conductor may have to be mechanically supported against this force. In the case of the gradient coils the forces generated by the pulsating current are sufficient to cause the coils to oscillate and deflect the former into oscillation.

This action tends to produce a noise which has the frequency of the pulsating current and is therefore in the audible range, albeit at the lower end, and is often obtrusive and objectionable to patients and operators. The present invention enables a design of gradient coil assembly to be produced which reduces significantly the noise from this source.

According to the present invention there is provided a former for a gradient coil assembly intended for use in an imaging system which uses the resonance of nuclei, comprising a plurality of annular rings each capable of supporting one of the main effective arms of a gradient coil, and a plurality of members extending generally in the axial direction of the magnet, securing said hoops together.

A feature of this invention is the provision of flexible supports for the gradient coil former which reduce the transmission of noise.

A further feature of this invention is the fab rication of the gradient coil former in a noise absorbing material.

An embodiment of the invention will now be described with reference to the accompanying drawings in which, Figure 1 is an end view of a magnet assembly for use in diagnostic medicine, Figures 2, 3 and 4 are respectively end elevation, front elevation and plan of a gradient coil assembly for the magnet assembly of Figure 1, and, Figure 5 is a diagrammatic perspective view of the windings of the gradient coils of the assembly of Figures 2 to 4.

Referring now to Figure 1, the magnet assembly comprises a hollow cylindrical body 10 which contains a superconducting magnet operating in liquid helium at about 4 degrees Kelvin. When the superconducting magnet has been run up and current is circulating in it, a magnetic field of between 0.2T and 2.5T is generated, depending on the design and operating conditions of the assembly. A gradient coil assembly 11 is positioned concentrically within the vessel 10 and a radio frequency coil assembly 12 is position concentrically within assembly 11. - Referring now to Figures 2 to 4, the assembly 11 comprises a cylindrical former 15 on which are wound a set of X-coils 16, a set of Y-coils 17 and a set of Z-coils 18.The X-coils and the Y-coils each comprise two diametrically opposed sets of windings; the X and Y coils are substantially identical in shape and magnetic magnitude and they differ only in that they are disposed 90 degrees circumferentially one from the other.

Figure 3 is an elevation which shows the Xcoils with the Y-coils removed, for the sake of clarity. One winding 20 of the two diametrically opposed sets of windings of the X-coils can be seen. The winding 20 consists of a single length of rectangular cross-section conductor wound into two substantially rectangular sets of turns 21 and 22 spaced axially along the former. The turns of the conductor conform to the cylindrical shape of the former 15.

The rectangular turns 21 comprise opposed straight side portions 23 and 24, where the turns of the conductor lie adjacent one to the other. The turns 22 also have similar side portions 25 and 26. These conductor side portions 23 to 26 form the main effective arms of the coil and provide the gradient field of the X-coil when combined with the main effective arms of the other, diametrically opposed set of X-windings.

At the other opposed portions 27 and 28, and 29 and 30, the turns extend parallel to the axis of the magnet and are spaced one from the other. They only contribute a magnetic effect parallel to the axis of the magnet and thus do not contribute to the transverse gradient field.

The ends of the conductor are brought out at 31 and the ends 32 of the other, diametrically opposed X-windings can be seen. Also shown are the ends 33, 34 of the two sets of windings of the Y-coil and the ends 35, 36 of the two sets of windings of the Z-coil (which will be described later). Only one of the X-coils is shown in Figure 3; the other diametrically opposed coil is the mirror image of the one shown and the two Y-coils are identical to the X-coils except that they are transposed by 90 degrees.

Referring now to Figure 5, the electrical current path through one of the windings of the X-coils is shown diagrammatically. The successive portions of the conductor are numbered 40-51. The entry lead is portion 40 and portions 41-44 represent six turns of the conductor on the actual coil: portion 45 is used to carry the current to the second part of the coil where portions 46-50 represent a further six turns of the conductor on the actual coil: portion 51 is the return connection.

The former 15 is basically cylindrical. However, the former comprises four axially spaced annular rings 55 to 58 and each of these rings provides the necessary mechanical support needed by two diametrically opposed main effective arms of the X-coil and two diametrically opposed main effective arms of the Y-coils. The rings are joined by integral longitudinal spacers, as at 59, so that the former 15 can conveniently be made from a single cylindrical tube with approximately rectangular holes cut out to leave rings 55 to 58 and the spacers 59. The former is made from a rigid non-magnetic material, such as glass fibre with resin impregnation, spirally wound paper with resin impregnation or a cloth resin matrix.

The material of the former needs to be strong in bending and have a high Young's Modulus.

Mechanical support for the windings is provided by nonmagnetic mountings which secure the windings to the former.

With the design of former of the present invention the oscillating magnetic forces generated in the X and Y coil windings do not propagate any significant amount of noise. In addition, the former is made of a noise absorbing material and this helps to attenuate any noise generated in operation. Also the whole gradient coil assembly is mounted within the main magnet on flexible mountings and this again helps to attenuate any noise produced.

The Z-coils each comprise a series of circumferentially wound turns positioned centrally of one of the X and Y coil centres. Each Zcoil is wound from a single conductor and the entry and return connections are brought as shown at 60, 61. These coils produce a magnetic field in the axial direction; they do not exert any diametric mechanical force on the former nor do they contribute to any mechanically produced noise.

Claims (7)

1. A gradient coil assembly intended for use in an imaging system which uses the resonance of nuclei, comprising a plurality of annular rings each capable of supporting at least one of the main effective arms of a gradient coil, and a plurality of members extending generally in the axial direction of the magnet, securing said rings together.

2. A gradient coil former as claimed in claim 1, wherein said rings form flexible supports for said arms.

3. A gradient coil assembly as claimed in claim 1 or 2, wherein said rings and members are formed integrally from a single cylindrical element.

4. A gradient coil assembly as claimed in claim 1, 2 or 3 having four said rings.

5. A gradient coil assembly as claimed in claim 1, 2, 3 or 4, wherein said rings and members form substantially rectangular apertures there between.

6. A gradient coil assembly as claimed in any one of claims 1 to 5, wherein said rings are formed of noise absorbing material.

7. A gradient coil assembly as claimed in any one of claims 1 to 6, wherein said arms are flexibly supported on said rings.