GB2162641A

GB2162641A - Nuclear magnetic resonance

Info

Publication number: GB2162641A
Application number: GB08417947A
Authority: GB
Inventors: Peter John Feenan
Original assignee: Magnex Scientific Ltd
Current assignee: Magnex Scientific Ltd
Priority date: 1984-07-11
Filing date: 1984-07-11
Publication date: 1986-02-05
Also published as: GB8417947D0; GB2162641B

Abstract

Means for providing a magnetic field in NMR apparatus, e.g. for imaging, comprises first and second coils suitably wound on respective formers 1, the second coil being operable to reduce the fringe field produced by the first coil at a predetermined distance outside the sample volume to a predetermined level, e.g. less the 0.0005T at 3-5 metres thereby reducing the space required in a hospital or enabling the apparatus to be mobile by truck. The formers are preferably cylindrical, having circumferential channel compartments for corresponding coil windings connected via niches 6 which may also house superconductive switches. <IMAGE>

Description

SPECIFICATION Nuclear magnetic resonance Radio frequency electromagnetic signals which are nuclear magnetic resonance signals (i.e. NMR signals) can be utilised to derive information about nuclei (e.g. protons, SC, 23Na, 3ip, SF, or 14N) constituting excited sources of those signals. Various apparatus are known for enabling NMR signals for various applications. Some applications are imaging of locations of said sources (especially of protons in medical imaging of a patient's body), and/ or determining quality(s) of said sources (e.g. relaxation times of said sources). One example of said determination is spectroscopy for detecting characteristic nuclear magnetic resonance spectral date of nuclei, e.g. phosphorus nuclei in a patient's body.

Said apparatus for enabling NMR signals can comprise: reference magnetic means, for providing reference magnetic field(s) for aligning nuclei that will then be said excited sources. Evolution of NMR signals from said aligned nuclei will be in response to suitably changing magnetic field(s) andi or magnetic field gradient(s), superimposed on said reference magnetic field(s) by radio frequency (RF) currents (i.e. excitation currents) to be provided in radio frequency (RF) coil means suitably located relative to said sources, e.g. RF coils located on a patient's body. For imaging applications, said magnetic field gradient(s) can enable spatial information to be obtained from said excited sources. Said RF coil means can enable a preparation stage (in which said excitation currents are fed into said RF coil means); and a subsequent detection stage.Said excitation currents can be provided as pulses. Said detection stage is commenced such that NMR signals induce electrical signals of radio frequency into e.g. said RF coil means, those signals being used as output signals for subsequent processing to derive information about said sources. There can be gradient coil means, e.g. 3 orthogonally disposed relative to each other for super- imposing 3 corresponding magnetic field gradients on said reference magnetic field(s), at least one said gradient being provided at at least one time, or no said gradient being provided for at at least one time. Said enabling apparatus can comprise further component(s). One said further component is computer means for controlling said preparation and detection stages, e.g. control said excitation currents, sequence of said gradients, and compute images from said output signals.

Said reference magnetic means can comprise: a permanent magnet; a resistive electromagnet; or a superconductive electromagnet. Some examples of reference magnetic fields have strengths up to substantially 2T, e.g. substantially 0.15T or substantially 2.OT. It is desirable that said reference magnetic field(s) have a homogeneous flux pattern, e.g. with an inhomogeneity in field strength of less than 100 ppm in a volume (i.e. sampling volume) containing said sources while being subjected to said preparation and subsequent detection stages. The flux pattern and field strength of said reference magnetic field(s) should be uniform throughout said sampling volume, and those uniformities should be very stable relative to passage of time. Said sampling volume can be in a central bore contained in said superconductive electromagnet.That bore must be large enough to accommodate a variety of coil means (e.g. said RF coil means and said gradient coil means) and said sources (e.g. a patient comprising said sources).

For example, a known superconductive electromagnet has a clear said central bore of 1.05 metres diameter, and will provide a central reference magnetic field of substantially 0.5T, the field uniformity being better than 25 ppm over a 0.5 metre diameter spherical volume at the centre of said bore. A problem with such an electromagnet is that said reference magnetic field provides magnetic field circumambient or circumjacent said sampling volume. Such a field (i.e. fringe field) can have a strength greater than 0.0005T throughout a roughly ellipsoidal volume whose major axis is substantially 17.0 metres, and whose minor axis is substantially 13.0 metres. That volume is large and the fringe field is a problem. Thus, a large region of expensive hospital space contains the fringe field.

Magnetic fields greater in strength that 0.0005T can adversely affect heart pacemakers and other sensitive equipment found in hospitals. The volume in which said fringe field is greater than 0.0005T is therefore 'restricted volume', given public access to that volume is preferably restricted; great care must be taken about access and type of equipment located in the restricted volume. Some proposals to alleviate said problems of fringe field use ferromagnetic screening material placed close to said superconductive electromagnet or at several metres from it. Size, shape, and positioning of that material cancels or partly cancels the reference magnetic field provided from said super- conductive electromagnet. Such cancellation is outside some region in places smaller than said restricted volume corresponding to the original reference magnetic field.Said calculation requires use of a large computor and relatively long time of calculation. In one example, 10 to 20 tonnes of feromagnetic screening material might be required to reduce the fringe field strength to below 0.0005T outside a said restricted volume whose maximum dimension is substantially 8 metres. A said superconductive electromagnet provided with said ferromagnetic screening material can comprise distinct coils (sometimes wound onto the same former) whose magnetic senses are the same, i.e.

the magnetic fields produced by each of those coils and at the centre of said central bore are in the same direction.

A first aspect of the present invention provides reference magnetic means (preferably superconductive) suitable for use as a component of apparatus for enabling NMR signal(s), said reference magnetic means comprising: at least one first coil means (preferably superconductive), for surrounding at least a portion of a sampling volume for containing source(s) of NMR signal(s), said first coil means being for providing at least one first magnetic field component having a first sense; and at least one second coil means (preferably superconductive) disposed relative to said at least one first coil means, said second coil means being for providing at least one second magnetic field component having a second sense, said at least one first coil means and said at least one second coil means being adapted such that said first and second senses will enable said first and second magnetic field components at least partly to cancel fringe field outside said sampling volume, said cancellation preferably being such that said fringe field has a strength of 0.0005T at a predetermined distance (preferably in the range of 3 to 5 metres) from the centre of said sampling volume.

A second aspect of the present invention provides apparatus for enabling NMR signal(s), said apparatus comprising: at least one reference magnetic means of said first aspect of the invention.

A third aspect of the present invention provides a method of providing a reference magnetic field for exciting source(s) of NMR signal(s), said method comprising: utilising reference magnetic means of said first aspect, or utilising apparatus of said second aspect.

The flux pattern of the resultant composite magnetic field provided by interaction of said first and second magnetic field components can have any suitable location andSor suitable shape inside or outside said sampling volume. Preferably, said interaction enables the reference magnetic field in said sampling volume to be substantially homogeneous in flux density. The at least part cancellation of fringe field can result in stray magnetic field produced outside said sampling volume to be low in all directions (e.g. not isotropically); or relative to the centre of said sampling volume, the location at which stray magnetic field has dropped to 0.0005T is less than would be in the absence of said at least one second coil means.Sufficient cancellation of fringe field can enable some embodiments of said enabling apparatus to be mobile because any fringe field will not extend beyond the physical limits of transport means (e.g. a truck) for such an embodiment. Another advantage is that, from the centre of the sampling volume, the distance at which attractive force acting on small ferromagnetic objects is equal to their gravitational attraction will be less than in the absence of said at least one second coil means. A further advantage is that the existence of ambient ferromagnetic material (e.g. structural beams or rods in a building) will have less disturbing effect on the homogeneity of the composite magnetic field inside the sampling volume.

Said first coil means can be embodied in any suitable manner, e.g. as to any of: material and configuration. Said at least one first coil means can comprise any suitable number (fractional or whole) of ampere turns. A plurality of said first coil means can be an odd or even number. At least two first coil means can be integral or connected together in any suitable manner, e.g. by at least one join andlor at least one coupling means. Respective first coil means can be located in respective first compartments constituted in any suitable manner (e.g. by a first former -see below). There can be a spatial interval between adjacent two said first compartments, that interval preferably being constituted by at least one partition means. But, said adjacent two compartments can be contiguous with each other if such contiguity is required.Preferably, at least one said first coil means is superconductive, e.g. comprises superconductive cable or wire.

Said second coil means can be embodied in any suitable manner, e.g. as to any of: material and configuration. Said at least one second coil means can comprise any suitable number (fractional or whole) of ampere turns. Preferably, the number of ampere turns of exterior second coil means is less than the number of ampere turns of interior first coil means. A plurality of said second second coil means can be an odd or even number. At least two second coil means can be integral or connected together in any suitable manner, e.g. by at least one join and/or at least one coupling means. Respective second coil means can be located in respective second compartments constituted in any suitable manner (e.g. by a second former -see below).

There can be a spatial interval between adjacent two said second compartments, that interval preferably being constituted by at least one partition means. But, said adjacent two second compartments can be contiguous with each other if such contiguity is required.

Said at least one first coil means can be supported by at least one first support means constituting at least two said first compartments and at least one first said partition means. Preferably, said first support means is elongate (e.g. cylindrical).

Preferably, the periphery of said elongate first support means comprises channels (e.g. circumferential channels) respectively constituting said first compartments. Preferably, the periphery of said elongate first support means comprises at least one recess constituting a first niche for receiving said at least one join and!or said at least one coupling means for adjacent first compartments. Preferably, a said first partition means comprises a said first niche, which preferably comprises first and second niche portions, said first niche portion preferably extending from a said first compartment, said second niche portion extending from and angularly offset from said first niche portion.

Preferably, said second niche portion is circumferentially disposed (e.g. a circumferential channel), when said first niche portion is axially directed of said elongate first support means. Said first support means can comprise at least one first guide means, for leading in a predetermined path from a first compartment into said first niche portion an inlet lead and/or an outlet lead of a said first guide means comprises at least one post as one example of suitable guide means. Said first support means can comprise at least one second guide means, for leading in a predetermined path from said first niche portion into said second niche portion an in let lead and/or an outlet lead of a said first coil means. Preferably, said at least one second guide means comprises: at least one post; and at least one contoured portion of surface of said first support means.First and second guide means can enable various convenient changes of direction of a path to contain said inlet lead and/or an outlet lead of a said at least one coupling means. In general, said at least one first support means can be constituted in any suitable manner, e.g. as a frame, framework, or former.

Said at least one second coil means can be supported by at least one second support means constituting at least two said second compartments and at least one second said partition means. Preferably, said second support means is elongate (e.g.

cylindrical). Preferably, the periphery of said elongate second support means comprises channels (e.g. circumferential channels) respectively constituting said second compartments. Preferably, the periphery of said elongate second support means comprises at least one recess constituting a second niche for receiving said at least one join and/or said at least one coupling means for adjacent second compartments. Preferably, a said second partition means comprises a said second niche, which preferably comprises respective first and second niche portions, said first niche portion preferably extending from a said second compartment, said second niche portion extending from and angularly offset from said first niche portion.Preferably, that second niche portion is circumferentially disposed (e.g. a circumferential channel), when said respective first niche portion is axially directed of said elongate second support means. Said second support means can comprise at least one respective first guide means, for leading in a predetermined path from a second compartment into the respective said first niche portion an inlet lead andior outlet lead of a said second coil means. Preferably, said at least one first guide means for said second support means comprises at least one post as one example of suitable guide means. Said second support means can comprise at least one respective second guide means, for leading in a predetermined path from a said first niche portion of said second support means into the respective said second niche portion an inlet lead and/or an outlet lead of said second coil means.Preferably, such at least one second guide means comprises: at least one post; and at least one contoured portion of surface of said second support means. First and second second guide means can enable various convenient changes or direction of a path to contain said inlet lead and/or an outlet lead of said second coil means. The second niche portion of said second support means can at least partly house said at least one join and/or said at least one coupling means of said second coil means. In general, said at least one second support means can be constituted in any suitable manner, e.g. as a frame, framework, or former.

Preferably, said at least one second support means is substantially concentric around said at least one first support means. Preferably, there is a cylindrical said second support means (e.g. a former made of suitable aluminium alloy) concentric around a cylindrical said first support means (e.g. a former made of suitable aluminium alloy).

Temporary retaining means (e.g. a sledge) can be mounted to any said former during winding of a coil thereon, such retaining means being used for contacting any of: a said lead, said join, or said coupling means, utilised in that winding.

To prevent or reduce the creation of unwanted current loops that will create unwanted magnetic field, it is preferred that all leads between adjacent first coil means, and/or adjacent second coil means, should be pairs of leads carrying equal but opposite currents, i.e. 'cancelled pairs'. For that purpose, preferably the start and end leads from any said coil use the same respective said niche, so as to enter and exit at as nearly the same location as possible. Some leads may run axially across compartments.Those leads preferably run across the top of finished windings to a suitable point for reentering a respective said second niche portion, e.g. to form a joint therein, or for jointing or coupling to a superconducting switch, Support means (e.g. a bar or rod) can be provided for those axial leads; preferably, facility is provided to guide leads from said support means into any respective said second niche portion. Said at least one first support means and said at least one second support means can be provided with their respective said coil means, and then disposed suitably relative to each other.

Said enabling apparatus of the second aspect of the invention can be embodied in any suitable manner, e.g. comprise: at least one said RF coil means; and optionally at least one said gradient coil means. Said enabling apparatus can comprise said computer means. Said enabling apparatus can comprise any suitable cryostat means for housing at least one superconductive reference magnetic means of the first aspect of the invention.

In the accompanying drawings, which are given by way of schematic example of the present invention: Fig. 1 shows a general former shape for winding thereon a superconductive reference magnet. Fig. 2 is a detail showing a lead passing into or out of a compartment constituted as a circumferential channel in the exterior of the former shape of Fig. 1. Fig. 3 shows an NMR apparatus (e.g. suitable for imaging a patient) containing a composite superconductive reference magnet comprising two concentric formers and their magnets according to Fig. 1.

A concentric pair of formers each according to Figs. 1 and 2 are suitably wound with superconductive material to provide corresponding concentric superconductive reference magnets. The general former shape 1 to permit the windings is shown in Fig. 1 as being cylindrical (e.g. a suitably grooved aluminium alloy cylinder), and having circumferential channels 2 each constituting a respective compartment for receiving windings of a corresponding superconductive coil (not shown) for providing magnetic field. Adjacent channels 2 are separated by a respective partition 3 compris ing two circumferential walls 4,5 spaced apart.

Each of walls 4, 5 has a respective axial first niche portion 6 communicating with the adjacent channel 2 but not as deep as that channel. Between walls 4,5 is a circumferential second niche portion 7 e.g.

of the same depth as the adjacent first niche portions 6. Niche portions 6,7 enable reception therein of a lead, a join, or coupling means (e.g. a superconductive switch), for interconnecting respective leads of coils in the adjacent channels 2. The inlet and outlet of each first niche portion have two guide posts 8,9 respectively at opposite sides of the inlet mouth, and two guide posts 10,11 at opposite sides of the outlet mouth. Guide posts 8,9,10,11 will enable suitable curving of the path of any lead L in guiding contact with such a post -see Fig. 2, which shows those posts bolted to the general former shape 1. A temporary retainer sledge 12 is mounted to the former shape 1 by a yoke 13, so that sledge 12 can abut e.g. lead L as it passes upwards and over the step constitutby the difference in depth of channel 2 and its adjacent first niche portion 6.There are six channels 2, three respectively at each of the opposite sides of a central circumferential partition 14 which is part of the former shape 1. Partition 14 can be used as an abutment to withstand axial thrusts during winding of coils. A service duct 15 is mounted to the walls 4,5 so as to extend axially of the former shape 1. Duct 15 guides and supports leads for communicating with the coils in compartments 2; duct 15 has a central connector 16 for connecting those leads to apparatus to be coupled to those leads (e.g. a source of electricity). Said mounting of yoke 13 and duct 11 can be provided in any suitable manners, e.g. by bolts or screws, for instance bolts 17 for yoke 13, and screws 18 for duct 15.

The windings on the concentric pairs of formers each according to Figs. 1 and 2 can be provided in accord- with the description given above before the first reference to the drawings. The coils on the outer former B are wound in opposite sense to those on the inner former A, so as at least partly to cancel fringe field that would otherwise be provided outside the composite magnet constituted by formers A,B and their windings. Outer former B can be disposed so that any of its windings is above and predetermined portion of any of the windings of former A. If desired, the relative dispositions of formers A and B can be adjustable (e.g.

axially andzor radially adjustable), but such a disposition can be a constant. At opposite ends of former shape 1 are end pieces 18 of annular shape that can be bolted to former shape 1.

In Fig. 3, an embodiment of NMR enabling apparatus is shown for imaging a patient. A sampling volume 20 for receiving the patient is surrounded by an annular channel 21 for receiving the concentric pair of former widings according to Figs. 1,2.

Liquefied helium can enter channel 21. Surround channel 21 is annular channel 22 for receiving a vacuum. Surrounding channel 22 is annular channel 23 for receiving liquefied nitrogen. Surrounding channel 23 is annular channel 24 for receiving a vacuum. Surround channel 24 is a cylindrical cryostat stainless steel vessel having opposite ends 25 (one is shown).

The present invention includes equivalents and modifications, e.g. the description of the drawings can be modified in any suitable manner in accordance with the description given above before the first reference to the drawings. It will be appreciated that former B provides exterior windings, whereas former A provides interior windings. Preferably, the ampere turns of the coils constituted by the exterior windings is less than the ampere turns of the coils constituted by the interior windings.

Claims

1. Reference magnetic means suitable for use as a component of apparatus for enabling NMR signal(s), said reference magnetic means comprising: at least one first coil means, for surrounding at least a portion of a sampling volume for containing source(s) of NMR signal(s), said first coil means being for providing at least one first magnetic field component having a first sense; and at least one second coil means disposed relative to said at least one first coil means, said second coil means being for providing at least one second magnetic field component having a second sense; wherein said at least one first coil means and said at least one second coil means are adapted such that said first and second senses will enable said first and second magnetic field components at least partly to cancel fringe field outside said sampling volume.

2. Reference magnetic means as claimed in claim 1, wherein said adaption of said at least one first coil means and of said at least one second coil means is such that said fringe field will have a strength of substantially 0.0005T at a predetermined distance from the centre of said sampling volume.

3. Reference magnetic means as claimed in claim 2, wherein said adaptation of said at least one first coil means and of said at least one second coil means is such that said fringe field will have a strength of substantiallv 0.0005T at a predetermined distance in the range substantially 3 to substantially 5 metres from the centre of said sampling volume.

4. Reference magnetic means as claimed in any one of claims 1 to 3, wherein said at least one first coil means and said at least one second coil means are adapted such that interaction of said first and second field components will sufficiently cancel said fringe field so as to enable said enabling apparatus to be mobile.

5. Reference magnetic means as claimed in claim 4, wherein said at least one first coil means and said at least one second coil means are adapted such that interaction of said first and second field components will sufficiently cancel said fringe field so as to enable said enabling apparatus to be comprised by a truck.

6. Reference magnetic means as claimed in any one of claims 1 to 5, wherein respective first coil means are located in respective first compartments there being a spatial interval between adjacent two said first compartments, that interval being constituted by at least one first partition means, said at least one first coil means being supported by at least one first support means constituting at least two said first compartments and at least one said first partition means.

7. Reference magnetic means as claimed in claim 6, wherein said first support means is elongate; and the periphery of said elongate first support means comprises circumferential channels respectively constituting said first compartments.

8. Reference magnetic means as claimed in claim 7, wherein the periphery of said elongate first support means comprises at least one recess constituting a first niche for receiving at least one join and/or at least one coupling means for adjacent said first compartments, said at least one join and/or said at least one coupling means being for connecting together at least two first coil means.

9. Reference magnetic means as claimed in claim 8, wherein a said first partition means comprises a said first niche, that niche comprising first and second niche portions, said first niche portion extending from a said first compartment, said second niche portion extending from and angularly offset from said first niche portion.

10. Reference magnetic means as claimed in claim 9, wherein said first niche portion is axially directed of said elongate first support means; and said second niche portion is circumferentially disposed.

11. Reference magnetic means as claimed in any one of claims 8 to 10, wherein said elongate first support means comprises at least one first guide means, for leading in a predetermined path from a said first compartment into said first niche portion an inlet lead and/or an outlet lead of a said first coil means.

12. Reference magnetic means as claimed in claim 11, wherein said at least one first guide means comprises at least one guide post.

13. Reference magnetic means as claimed in any one of claims 8 to 12, wherein said elongate first support means comprises at least one second guide means. for leading in a predetermined path from said first niche portion into said second niche portion an inlet lead and/or an outlet lead of a said first coil means.

14. Reference magnetic means as claimed in claim 13, wherein said at least one second guide means comprises: at least one guide post; and at least one contoured portion of surface of said elongate first support means,

15. Reference magnetic means as claimed in any one of claims 11 to 14, comprising temporary retaining means mounted to said first support means and adapted to abut a lead of a said first coil means as that lead passes upwards and over an optional step constituted by a said first compartment and an adjacent said first niche portion.

16. Reference magnetic means as claimed in claim 15, wherein said at least one first guide means and said at least one second guide means are adapted to provide predetermined curving of said lead that undergoes said passage upwards and over said optional step.

17. Reference magnetic means as claimed in any one of claims 7 to 16, comprising a service duct mounted to said elongate first support means so as to extend in an axial direction of that support means, said duct being for guiding and supporting leads for communicating with a plurality of said first coil means, said duct having an optional central connector for connecting those leads to apparatus to be coupled to those leads.

18. Reference magnetic means as claimed in any one of claims 1 to 17, wherein respective second coil means are located in respective second compartments, there being a spatial interval between adjacent two said second compartments, that interval being constituted by at least one second partition means, said at least one second coil means being supported by at least one second support means constituting at least two said second compartments and at least one said second partition means.

19. Reference magnetic means as claimed in claim 18, wherein said second support means is elongate; and the periphery of said elongate second support means comprises circumferential channels respectively constituting said second compartments.

20. Reference magnetic means as claimed in claim 19, wherein the periphery of said elongate second support means comprises at least one recess constituting a first niche for receiving at least one join and/or at least one coupling means for adjacent said second compartments, said at least one join and/or said at least one coupling means for connecting together at least two second coil means.

21. Reference magnetic means as claimed in claim 20, wherein a said second partition means comprises a said second niche, that niche comprising first and second niche portions, said first niche portion extending from a said second compartment, said second niche portion extending from and angularly offset from said first niche portion.

22. Reference magnetic means as claimed in claim 21, wherein for a said second niche, said first niche portion is axially directed of said elongate second support means; and said second niche portion is circumferentially disposed.

23. Reference magnetic means as claimed in any one of claims 19 to 22, wherein said elongate second support means comprises at least one first guide means. for leading in a predetermined path from a said second compartment into said first niche portion an inlet lead and/or an outlet lead of a said second coil means.

24. Reference magnetic means as claimed in claim 23, wherein for said elongate second support means, said at least one first guide means comprises at least one guide post.

25. Reference magnetic means as claimed in any one of claims 19 to 26, wherein said elongate second support means comprises at least one second guide means, for leading in a predetermined path from said first niche portion into said second niche portion an inlet lead and'our an outlet lead of a said second coil means.

26. Reference magnetic means as claimed in claim 25, wherein for said elongate second support means, said at least one second guide means comprises: at least one guide post; and at least one contoured portion of surface of said elongate second support means.

27. Reference magnetic means as claimed in any one of claims 21 to 26, comprising temporary retaining means mounted to said second support means and adapted to abut a lead of a said second coil means as that lead passes upwards and over an optional step constituted by a said second compartment and an adjacent said first niche portion.

28. Reference magnetic means as claimed in claim 27, wherein for said elongate second support means, said at least one first guide means and said at least one second guide means are adapted to provide predetermined curving of said lead that undergoes said passage upwards and over said optional step.

29. Reference magnetic means as claimed in any one of claims 19 to 28, comprising a service duct mounted to said elongate second support means so as to extend in an axial direction of that support means, said duct being for guiding and supporting leads for communicating with a plurality of said second coil means, said duct having an optional central connector for connecting those leads to apparatus to be coupled to those leads.

30. Reference magnetic means as claimed in any one of claims 18 to 29, wherein said at least one second support means is substantially concentric around said at least one first support means.

31. Reference magnetic means as claimed in any one of claims 18 to 30, wherein said at least one first support means comprises a first cylindrical former of aluminium alloy; and said at least one second support means comprises a second cylindrical former of aluminium alloy.

32. Reference magnetic means as claimed in any one of claims 1 to 31, wherein all leads between adjacent said first coil means, andior between adjacent said second coil means, are adapted to be pairs of leads for carrying equal but opposite currents.

33. Reference magnetic means as claimed in any one of claims 1 to 32, wherein said at least one first coil means and said at least one second coil means are adapted to enable said sampling volume to be provided with a reference magnetic field substantially homogeneous in flux density.

34. Reference magnetic means as claimed in claim 1, substantially as herein before described with reference to and as shown in the accompanying drawings.

35. Apparatus for enabling NMR signal(s), said apparatus comprising at least one reference magnetic means as claimed in any one of claims 1 to 34.

36. Apparatus as claimed in claim 35, when mobile.

37. Apparatus as claimed in claim 36, when comprised by a truck or other transport means.

38. Apparatus as claimed in claim 35, substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

39. A method of providing a reference magnetic field for exciting source(s) of NMR signal(s), said method comprising: utilising reference magnetic means as claimed in any one of claims 1 to 34; or utilising apparatus as claimed in any one of claims 35 to 38.

40. A method as claimed in claim 39, wherein said utilising enables fringe field having a strength of substantially 0.0005T to be produced at a predetermined distance from the centre of said sampling volume.

41. A method as claimed in claim 40, wherein said strength of substantially 0,0005T is produced at a predetermined distance in the range substantially 3 to substantially 5 metres from the centre of said sample volume.

42. A method as claimed in any one of claims 39 to 41, wherein said at least one second coil means have a number of ampere turns less than a number of ampere turns comprised by said at least one first coil means.

43. A method as claimed in any one of claims 39 to 42, wherein said at least one first coil means and said at least one second coil means are superconductive.

44. A method as claimed in claim 39, substantially as hereinbefore described with reference to and as shown in the accompanying drawings.