NUCLEAR MAGNETIC RESONANCE APPARATUS

Abstract: About an electromagnetic magnet system of an NMR tomography apparatus there is arranged a second electromag­ netic magnet system which is capable of generating a magnet­ ic field such that the external stray field of the first magnet sys­ tem is compensated for. When use is made of superconduct­ ing magnet coils, the reduction of the measurement field which is caused by the compensation magnet can be readily can­ celled. Using a construction where the surface O₂ of the cross-­ section of the second magnet system amounts to twice the surface O₁ of the cross-section of the first system, the field at the area of the windings of the first system is also compen­ sated for in the case of energizing where O₁ × H₁ = O₂ × H₂, with the result that substantially larger persistent currents are permissible.

Description: Nuclear magnetic resonance apparatus. The invention relates to a nuclear magnetic resonance (NMR) apparatus which comprises a magnet with a first electromagnetic coil system for generating a homogeneous magnetic field in an examination space within the magnet, and also relates to a magnet for such an apparatus. Such a nuclear magnetic resonance apparatus in the form of an NMR tomography apparatus is known from Computertomographie 1, 1981, pages 2-10. In apparatus of this kind compratively strong stray fields occur outside the magnet. These stray fields are extensively dealt with in an article in Diagnostic Imaging, April 1983, pp 32-35; it is noted therein that such stray fields do not occur in a magnet system consisting of permanent magnets. The drawbacks of such a magnet system are also mentioned in this article. The drawback of external stray fields is not only that they are so strong, particularly in NMR tomography apparatus, that they are also liable to have a disturbing effect on equipment arranged outside the magnet, but also that the measurement field in the magnet is liable to be disturbed by external variations of the stray field, for e xample due to the displacement of comparatively large pieces of magnetic material therein. For NMR apparatus comprising superconducting magnet such as used notably for NMR tomography apparatus, it is to be noted that the magnet field, and hence also the stray field, is always present therein. For a more detailed description of superconducting magnets for notably diagnostic NMR tomography apparatus, reference is made to the chapter "Superconductivity" on pp 116-127 of the book "Nuclear Magnetic Resonance Imagine by C.L. Partain et al, WB Saunders Company, 1983; this process will be referred to hereinafter as NMR imaging. It is the object of the invention to provide an NMR apparatus which comprises an electromagnetic coil system for generating a homogeneous magnetic field and in which the external magnetic stray field is reduced. To this end, an NMR apparatus of the kind set forth in accordance with the invention is characterized in that a second electromagnetic coil system is arranged concentrically about the first electromagnetic coil system, said second coil system generating a second magnetic field which at least partly compensates for an external magnetic stray field of the first magnet. Because the external magnetic stray field is compensated for in an NMR apparatus in accordance with the invention, the disturbing effects thereof are cancelled. This substantially simplifies the steps and facilities required for the use and installation of the apparatus. Moreover, the possibly detrimental effects on the homogeneity of the magnetic field in the measurement space of known apparatus by external influencing of the stray field is avoided. In comparison with the shielding by means of a shield of magnetic material, a substantial saving is achieved as regards space and weight. In a preferred embodiment comprising a first coil system having a cross-section 071and a field strength H1, the second coil system has a cross-section #2 and is adapted to generate a field strength H2 so that H1 x #1 = -H2 x O2S Therein, 2 may be equal for example, to approximately from 1.4 to 4 times 01; so in the case of circular coil systems a diameter ratio of the two coils of between approximately 1.2 and 2 is obtained. This construction combines a suitably usable diameter of the entire magnet system with an acceptable spacing of the two coil systems and an acceptable current intensity for notably the inner coil. A particularly attractive construction is obtained when the ratio of the cross-sections of the coils equals 2, because the Lorentz force on the current conductors of the first coil system is then reduced to zero. In a further preferred embodiment, ends of current conductors of the second coil system are electrically conductively interconnected. Consequently, a magnetic flux generated in the first coil system will induce such a magnetic flux in the second coil system that the previously described condition is satisfied. Similarly, the first coil system may also be short-circuited and, moreover, both coil systems may be interconnected in order to carry a corresponding winding current, thus enhancing the stability of the magnet system. Notably for NMR tomography apparatus as used for medical diagnosis it is attractive to compose the magnet system from superconducting coil systems. In the case of short-circuits, the electrical connections will also consist of a superconducting material. The desired magnetic flux for the coils can then also be generated by means of a magnetic flux pump. The drawback that the measurement field is reduced by the shield in superconducting magnets can be easily mitigated by increasing the superconducting current through the coils. When the magnet system is suitably constructed, this increase will not be pro% & natic, because the external magnetic field at the area of the current windings in existing apparatus acts as a restrictive factor for the permissible current (see nmr imaging, pp. 116-120) and this magnetic field is now completely eliminated or substantially reduced. In a further preferred embodiment involving superconducting coil systems, the second coil system is formed by a single cylinder or a plurality of cylinders of superconducting material which are concentrically or consecutively arranged and which have comparatively thin walls. The desired magnetic field for the second coil system is then generated by induction from the first coil system in this second coil system. By constructing the second coil system so that, viewed in the axial direction, it is longer than the first coil system, the compensation for the stray field can be further improved. It can be improved even further by providing axial ends of the second coil system with radially directed elements which also consist of a superconducting material when superconducting magnets are used, said elements being shaped, for example as plates, rings or spirals. These elements preferably extend beyond the turns of the first coil system and force the field lines even further towards the space between the two coil systems due to the superconducting currents to be generated. When the second coil system acts as a heat shield for the first coil system, it may be advantageous to use a material for the second coil system which has a comparatively high transition point for the superconductivity. This offers an additional protection for the first coil system and a substantial saving as regards the consumption of helium can thus be obtained, for example in the case of external cooling of the second system (see nmr imaging, p 126). In order to obtain minimum dimensions of the complete magnet system, both coil systems are preferably accommodated in a single Dewar vessel which is to be filled with liquid helium. Because the second coil system is in principle arranged as closely as possible about the first coil system, a comparatively strong reduction of the measurement field will occur. Therefore, the first coi#l system is preferably made of materials which allow for comparatively high persistent superconducting currents, In a preferred embodiment, gradient coils which are to be included in a magnet system in accordance with the invention in order to generate position-determining gradient fields in the measurement space are constructed so that optimum use is made of the compensation of magnetic fields near the gradient coil windings. Some preferred embodiments in accordance with the invention will be described in detail hereinafter with reference to the drawing. Therein: Figure 1 diagrammatically shows an NMR apparatus in accordance with the invention, and Figure 2 shows various embodiments of a coil system for such an apparatus. An NMR apparatus as shown in figure 1 comprises a magnet with an electromagnetic coil system 1 which in this case comprises four wire windings 2 for generating a strong, steady, homogeneous magnetic field in a measurement space 3 which is situated inside the coil system. For various feasible coil systems, reference is made to nmr imaging, pp 121-123. For the use as an NMR tomography apparatus to be described hereinafter, that is to say for the slice-wise imaging of an object, gradient coils 4 are used. NMR spectrometry apparatus, that is to say apparatus for the detection of different types of spin nuclei of elements or molecules in an object, do not require gradient coils. Furthermore, RF coils 5 which act as transmission coils for an RF electromagnetic field and as detection coil for nuclear spin resonance signals which are generated thereby are arranged comparatively closely about the measurement space. Alternatively, separate coils can be used for these two functions. Nuclear spin resonance signals stimulated by the RF electromagnetic field in an object to be measured are picked up via a signal amplifier 6 in order to be applied, via a phase-sensitive amplifier 8, to a central arithmetic and control unit 10. Also shown are a modulator 12 for an RF power supply source 14, a power supply source 16 for the main coil system, a power supply source 17 for the gradient coils, a high-frequency oscillator 18 and a monitor 20 for image display. The oscillator 18 controls the modulator 20 for the RF fields as well as the rectifier 10 which processes the measurement signals. In accordance with the invention, a second magnet coil system 22 is arranged about the first magnet coil system which comprises the windings 2. When resistance coils are used for the first coil system, the second coil system preferably also comprises resistance coils; however, when superconducting magnet coils are used for the first coil system, the second coil system preferably also comprises superconducting coils. The second magnet coil system can be powered by a power supply source 24 which may be a current source as well as a flux pump in the case of superconducting magnets. It is an important aspect that the power supply source is continuously activated in the case of resistance magnets and forms a closed circuit in conjunction with the windings, whilst the source is only active to generate the persisten current in the case of superconducting magnets. By a suitable choice of the geometry and the energizing of the second coil system, which may again be a single solenoid or the like, outside the two magnet coil systems a magnetic field can be generated which compensates for the magnetic stray field of the first coil system which prevails at that area. The strongly disturbing magnetic stray field otherwise occurring at this area is thus cancelled to a high degree, so that it is achieved not only that all very strict measures for the protection of humans and materials outside the magnet can be dispensed with, but also that the measurement field is shielded against disturbances by fluctuations in the stray field. The latter shielding results in a more homogeneous magnetic field in the measurement space and hence in an increased resolution, i.e. spatially in the case of tomography as well as spectrally in the case of spectrome try. For example, if 0 1 x H1 = -O x H2 in a construction as shown in figure 2a with a radial crosssectional area 1 of the first coil system for generating an axial magnetic field H3 and a radial cross-sectional area 0 of the second coil system in which an opposed axial magnetic field H2 is generated, the resultant external magnetic field will be cancelled to a very high degree. If, for example 02 = 2 x H1, so in the case of circular coil windings R2 = Vt x R2, H2 = -2 x H2. A magnetic field H3 thus produced in the measurement space 3 will then be equal to -H2 or or1. It now holds good that a magnetic flux enclosed by the first coil system is equal to but oppositely directed with respect to a flux enclosed by the space between the two coil systems. Consequently, there is no flux which is enclosed by the coil systems together, so that no magnetic flux remains in the form of a stray field about the coil system. The field strength in the measurement space is reduced to one half of the original value in this construction. This may be considered to be objectionable, but this reduction can be compensated for by increased energizing of the first coil system. For a comparatively small magnet, for example for spectrometry on small objects, this need not be objectionable. For resistance magnets for tomograpny or spectrometry on patients, in which case the apparatus are comparatively large, this drawback could render the use of the invention problematic. In the case of superconducting coil systems, this drawback is irrelevant from a point of view of energy, consumption or heat dissipation. The persistent currents can be increased to high values. An extremely attractive additional advantage of the invention is that the known limitations as regards the intensity of the persistent currents (see nmr imaging, pages 116-121) are completely eliminated in the last preferred construction described, because no external magnetic field strength occurs therein at the area of the windings of the first coil system, so that no interference is experienced from Lorentz forces or the quenching etc. described in nmr imaging p.120. In this preferred embodiment the current intensity of the persistent current for the first coil system, consequently, is limited only by its critical value with regard to the materials used without an additional magnetic field; this value is many times higher than that for known superconducting magnet systems. In order to avoid the adverse effects of Lorentz forces also when the magnet systems are energized, both coil systems are preferably simultaneously energized. This can be achieved in the present coil system by electrically short-circuiting ends of turns of the second coil system via a diagrammatically shown connection 26. In the case of superconducting coils, this short-circuit will also consist of superconducting material. When the first coil system is energized via a current source or a flux pump, a current will be induced in the second coil system which generates a magnetic field of an intensity such that the second coil system does not enclose a resultant flux; thus, this is the field strength required for optimum compensation of the stray field. The energizing of the first coil system will be such that the resultant field strength of both systems in the measurement space has the value desired for the measurements. A similar superconducting electric shortcircuit 28 can also be used for the first coil system. Using a suitably adapted geometry of both coil systems, the simultaneous energizing can also be achieved, maintaining the favourable 2 : 1 ratio of the coil surfaces, by composing both coil systems from continuous current conductors. The current intensity in both coil systems is then always equal, so that an extremely stable system is obtained. A superconducting version of such a system can also be energized by a current source or a flux pump. In the embodiment shown in figure 2b, the second coil system comprises one or more superconducting cylinders 30 which are concentrically or axially arranged with respect to one another. The risk of disturbing the magnetic field by a short-circuit connection of the second coil system is thus eliminated. In this construction, the second coil system is preferably energized by induction from the first coil system. The first coil system may also consist of such eylinders. When a superconductor having a comparatively high transition temperature is chosen as the material for the cylinder or cylinders of the second coil system, the second coil system can act as a heat shield for the first coil system. The cylinders of the second coil system can then be cooled, for example by an external refrigerator; this may have a favourable effect on the consumption of helium. Figure 2c shows an embodiment in which the second coil system 22 is longer, viewed in the axial direction, than the first coil system 1; at its ends radially directed annular plates 32 are provided. In these plates, which may also be shaped as rings or spirals, circular currents are introduced which will force the magnetic field lines of the first system even further between the two coil systems, so that the external flux will even be better compensated for. In each of the described coil systems auxiliary coils may be included for further homogenizing of the resultant magnetic field. These auxiliary coils are preferably also superconducting and form closed turns so that the desired persistent current can be produced therein by a flux pump or a current source. Although the 2 : 1 surface ratio of the two coil systems is extremely favourable, it need not be restricted to this value. Actually, any structurally feasible ratio may be chosen. In a system where one coil system is arranged as closely as possible about the other coil system, a substantial saving of space will be thus achieved, notably also for the helium dewar vessel in the case of superconducting magnets. The resultant magnetic field in the measurement space, however, will then be comparatively small. For a diameter of 0.5 m of the first coil system and 0.6 m of the second coil system, the surface ratio will be approximately 3:4 and the field strength in the measurement space will amount to one quarter of the field strength without compensating coil system. Therefore, the persistent current in the first coil system will have to be increased by a factor 4 in the same circumstances. Because the magnetic field near the turns of the first coil system will not be completely compensated for when this geometry is used, it may be objectionable for some applications. This objection need not exist, for example in the case of comparatively amall superconducting systems for spectrome try where a suitable shielding of the coil screen may be extremely desirable, notably for the surrounding equipment. On the other hand, a second coil system having a much larger diameter will only slightly reduce the measurement field. A second coil system having a surface which is 10 times larger than the surface of the first system will reduce the measurement field by only 1056. It is a draw back that because of this large dimension, which itself is already a drawback, the shielding will be less complete. For a superconducting construction of the second coil system this configuration is comparatively unattractive because use must certainly be made of two different cooling systems. In the case of a resistance magnet, this construction may be advantageous because the energy consumption of the first coil system will not be excessively increased. An extreme consequence of the invention is formed by a Helmholtz coil system which is so much larger than the first coil system that the measurement field is hardly reduced at all. These Helmholtz coils may also be made of a superconducting material.

Claims: 1. A nuclear magnetic resonance (NMR) apparatus which comprises a magnet with an electromagnetic coil system (1) for generating a homogeneous magnetic field (H) in an examination space (3) within the magnet, characterized in that a second electromagnetic coil system (22) is arranged concentrically about the first electromagnetic coil system, said second electromagnetic coil system generating a second magnetic field which at least partly compensates for the external magnetic field which at least partly compensates for the external magnetic stray field of the first magnetic coil system. 2. An NMR apparatus as claimed in Claim 1, characterized in that the product of the surface ( 2) of a radial cross-section of the second coil system and a field strength (H2) which is generated thereby is at least substantially equal to the negative value of the corresponding product (01 x H1) of the first coil system. 3. An NMR apparatus as claimed in Claim 2, characterized in that the ratio of the coil surfaces is approximately 1:2. 4. An NMR apparatus as claimed in any one of the preceding Claims, characterized in that ends of current conductors of the second coil system are mutually electrically short-circuited. 5. An NMR apparatus as claimed in Claim 4, characterized in that the second coil and a short-circuit (26) between the ends thereof are made of a superconducting material. 6. An NMR apparatus as claimed in any one of the preceding Claims, characterized in that the first coil system is made of a superconducting material. 7. An NMR apparatus as claimed in Claim 6, characterized in that ends of current conductors of the first coil system are interconnected by way of a superconducting short-circuit (28). 8. An NMR apparatus as claimed in any one of the preceding Claims, characterized in that at least one of the coil systems is adapted to be energized by a magnetic flux generator. 9. An NMR apparatus as claimed in Claim 6, 7 or 8, characterized in that the second coil system is composed from a number of mutually radially and/or axially arranged cylinders of a superconducting material. 10. An NMR apparatus claimed in any one of the Claims 1, 2, 3, 6, 7, 8 or 9, characterized in that the length of the second coil system exceeds that of the first coil system. 11. An NMR apparatus as claimed in Claim 10, characterized in that the second coil system is made of a superconducting material and is provided, at least at one of its ends, with elements of a superconducting material which extend radially at least as far as the vicinity of the first coil system. 12. An NMR apparatus as claimed in any one of the preceding Claims, characterized in that the second coil system is made of a material having a superconducting temperature transition point which is comparatively high with respect to the material of the first coil system. 13. An NMR apparatus as claimed in Claim 12, characterized in that an external refrigerating machine is included for the cooling of the second coil system. 14. An NMR apparatus as claimed in Claim 12 or 13, tharacterized in that the second coil system is constructed so that it forms a heat shield for the first coil system. 15. An NMR apparatus as claimed in any one of the preceding Claims, characterized in that current conductors of the first coil system which are made of a superconducting material are suitable for carrying persistent currents having an intensity in excess of 100 A. 16. An NMR apparatus as claimed in Claim 15, characterized in that the geometry of the current conductors of the first coil system and the geometry of the current conductors of the gradient coils included in the magnet are suitable to achieve minimum Lorentz forces from the local magnetic field resulting from the magnetic fields of the two coil systems. 17. An NMR apparatus as claimed in any one of the preceding Claims, characterized in that, using superconducting material for both coil systems, the second coil system is arranged as closely as possible about the first coil system. 18. An NMR apparatus as claimed in any one of the preceding Claims, characterized in thatt using a superconducting material for both coil systems, the coils are accommodated together in a helium Dewar vessel. 19. An NMR apparatus as claimed in Claim 18, characterized in that, using an adapted cross-section and number of wire turns for the second soil system, both coil systems are superconductively connected in series in order to carry the same persistent current. 20. An NMR apparatus as claimed in any one of the preceding Claims, characterized in that auxiliary coils which homogenize the magnetic field are included in the magnet system. 21. A magnet system, evidently intended for use in an NMR apparatus as claimed in any one of the preceding Claims.

Inventors: GEUNS JOHANNES RUDOLPHUS VAN, VAN GEUNS, JOHANNES RUDOLPHUS
Applicants: KONINKL PHILIPS ELECTRONICS NV, N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1984
Main classification: G01N24
IPC codes: G01R33, A61B10, A61B5