WO2008078239A1

WO2008078239A1 - Multi-element coil array for mr systems

Info

Publication number: WO2008078239A1
Application number: PCT/IB2007/055119
Authority: WO
Inventors: Henk Guido Jan Tilleman
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2006-12-21
Filing date: 2007-12-14
Publication date: 2008-07-03

Abstract

A multi-element coil array comprising at least two coil elements (A, C) is disclosed, which are inductively and capacitively decoupled from each other by providing an overlapping area which is delimited by conductors (5,7) of the adjacent coil elements (A, C) which do not run parallel over each other, seen in a plane spread out by the coil elements, and which cross each other such that they include an angle between approximately 45- and approximately 90-. The coil array is provided for use as a RF antenna in a magnetic resonance (MR) imaging system.

Description

Multi-element coil array for MR systems

FIELD OF THE INVENTION

The invention relates to a multi-element coil array comprising at least two coil elements which coil array is especially provided for use as a RF antenna in a magnetic resonance (MR) imaging system. Furthermore, the invention relates to a RF receiver/transmitter system and to a MR imaging system comprising such a multi-element RF coil array.

BACKGROUND OF THE INVENTION

US 6,543,983 discloses a multi-channel phased array coil for use in a magnetic resonance system. The phased array coil includes a number N of coils (at least four coils) configured in an array, each of the N coils having a geometric shape and overlapping with (N-I) other coils to form an overlap area within the array. The geometric shape of each of the coils and the overlap area are configured to cause a mutual inductance between every pair of the coils to be less than 10 percent of the self- inductance of each of the N coils. The geometric shape of each of the coils is for example a beveled square with a square protrusion at one of its corners, or a faceted diamond with a trapezoid protrusion.

SUMMARY OF THE INVENTION

It has revealed that a disadvantage of the coil array as disclosed in the above US 6,543,983 is that an electromagnetic coupling can be observed between the N individual coils and especially between such coils which are positioned adjacent in the array, wherein this electromagnetic coupling is mainly a capacitive coupling.

Consequently, it would be desirable to provide a multi-element coil array comprising at least two coil elements as mentioned in the introductory part above in which the electromagnetic coupling, i.e. a capacitive coupling and/or an inductive coupling, especially between adjacent coil elements of the coil array is further reduced.

According to claim 1 , a multi-element coil array comprising two or more coil elements, each in the form of a conductor loop is provided, wherein at least two adjacent coil elements are arranged to provide an overlapping area, which overlapping area is delimited by conductors of the adjacent coil elements which do not run parallel over each other, seen in a plane spread out by the coil elements, and wherein crossings of the conductors are provided such that the conductors include an angle between approximately 45° and approximately 90° between each other. By the overlapping of the coil elements, the mutual inductive coupling between adjacent coil elements is minimized, whereas by the said arrangement and crossing of the conductors, the mutual capacitive coupling is minimized.

The subclaims disclose advantageous embodiments of the invention.

The embodiment according to claim 2 has the advantage that by the protrusions an overlapping also of such coil elements can be obtained which e.g. have a rectangular form and are adjacent in a crosswise or diagonal direction.

The embodiment according to claim 3 is advantageous when composing a multi-element coil array with a desired extension.

The embodiments according to claims 4 to 6 have the advantage that an effective overlapping area can be obtained especially between such coil elements which are adjacent in a crosswise or diagonal direction.

It will be appreciated that features of the invention are susceptible to being combined in any combination without departing from the scope of the invention as defined by the accompanying claims. These and further aspects, details, features and advantages of the invention will become apparent from the following description of preferred and exemplary embodiments of the invention which are given with reference to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows a diagrammatic side elevation of an MR imaging system;

Fig. 2 schematically shows three coil elements;

Fig. 3 schematically shows a first embodiment of a multi-element coil array;

Fig. 4 schematically shows a second embodiment of a multi-element coil array; Fig. 5 schematically shows a third embodiment of a multi-element coil array

Fig. 6 shows in an enlarged scale a central part of Figure 5; and

Fig. 7 schematically shows a fourth embodiment of a multi-element coil array.

DETAILED DESCRIPTION OF EMBODIMENTS Figure 1 shows substantial components of a known MR imaging system, which are of essential importance for the generation of magnetic fields and RF excitation pulses and for receiving MR relaxation signals in an examination zone 10. The MR imaging system is shown in the form of a vertical (open) system, however, the multi-element coil array and a related RF antenna comprising such a multi-element coil array according to the invention can be used in a cylindrical or horizontal MR imaging system as well.

Above and underneath the examination zone 10 there are provided respective magnet systems 20, 30 which serve to generate an essentially uniform main magnetic field (Bo field for aligning the nuclear spins in the object to be examined) whose magnetic flux density (magnetic induction) may be in the order of magnitude of from some tenths of Tesla to some Tesla. The main magnetic field essentially extends through a patient P in a direction perpendicular to the longitudinal axis of the patient (that is, in the x direction).

Planar or at least approximately planar RF conductor structures (RF surface resonators) in the form of RF transmission coils 40 serve to generate RF pulses (Bi field) of the MR frequency whereby the nuclear spins are excited in the tissue of the patient P to be examined, said RF transmission coils 40 being arranged on at least one of the magnet systems 20, 30. MR receiving coils 50 serve to receive subsequent MR relaxation signals from the tissue; these coils may also be formed by RF surface resonators provided on at least one of the magnet systems 20, 30. At least one common MR/RF surface resonator can also be used for RF transmission and MR reception if it is suitably switched over between transmitting and receiving, or the two RF surface resonators 40, 50 can both serve for the alternating transmission of RF pulses and reception of MR signals in common.

At least one of these RF conductor structures 40, 50 is provided in the form of a multi-element coil array according to the invention, comprising at least two coil elements which shall be explained below with reference to Figures 2 to 7.

Furthermore, regarding Figure 1, for the spatial discrimination and resolution of the relaxation (MR) signals emanating from the tissue of a patient P (localization of the excited states), there is also provided a plurality of gradient magnetic field coils 70, 80 by which three gradient magnetic fields are generated which extend in the direction of the x axis. A first gradient magnetic field then varies essentially linearly in the direction of the x axis, while a second gradient magnetic field varies essentially linearly in the direction of the y axis, and a third gradient magnetic field varies essentially linearly in the direction of the z axis. Finally, electrical accessory devices or auxiliary equipments are provided for given examinations. Such a device is, for example, a MR/RF surface coil 60 which is used in addition or as an alternative to the planar MR/RF receiving coils 50 (body coils) and which is arranged as a MR receiving coil directly on the patient P or the zone to be examined. Such a MR/RF surface coil 60 is preferably constructed as a flexible pad or a sleeve and can comprise a multi-element coil array according to the invention with at least two coil elements.

Figure 2 schematically shows examples for three individual coil element A, B, C. A multi-element coil array according to the invention is composed of two or more of such coil elements, wherein the coil elements must not have identical form (shape) and/or dimensions.

The coil elements A, B, C (and D to L according to Figures 4 to 7) each are at least substantially plane or level conductor loops, preferably having a rectangular shape, especially a square shape, with four sides of the rectangular shape in the form of first conductors 1 to 4. Furthermore, the coil elements each comprise at least one second conductor in the form of a protrusion 5 to 8 at their circumference which protrusion extends in a direction to the outside of the conductor loop, e.g. in a radial direction and which is provided by another conductor loop, which preferably as well has a rectangular form, especially a square form. Neither the coil elements of a multi-element coil array, nor the protrusions at one or more of the coil elements each must have the same form and dimensions as indicated in Figures 2 to 7, but both can have different forms and dimensions as well.

In case of a rectangular conductor loop as exemplarily indicated in Figures 2 to 7, a protrusion 5 to 8 is preferably provided at at least one corner of the loop, extending with an angel of preferably about 45 or about 135 degrees relative to the adjacent (first) conductors 1 to 4 of the loop.

Generally, the optimal forms and dimensions of the coil elements, especially the (possibly as well different) lengths of the (first) conductors 1 to 4 and the forms and dimensions of the protrusions 5 to 8 can be freely selected in dependence on the needs and especially on the type of application the coil array is provided for.

Figure 3 schematically shows a first embodiment of a multi-element coil array comprising two such coil elements A, B which are arranged side by side. More in detail, the two coil elements A, B according to Figure 2 are arranged in a direction side by side in an overlapping manner according to Figure 3 in such a way that neither the (first) conductors 3, 1 of the coil elements A, B, nor the (second) conductors of the protrusions 6, 5; 7, 8 of the adjacent coil elements A, B run parallel over each other, seen in a plane spread out by the coil elements (i.e. in a perpendicular projection onto the plane of Figure 3).

By this side by side overlap, in which the overlapping area is constituted or delimited by the adjacent first conductors 3, 1 of adjacent coil elements A, B and each two protrusions 6, 5; 7, 8 at the ends of both first conductors 3, 1, the mutual inductive coupling between the adjacent coil elements A, B is reduced to a minimum.

The extent or dimension of this overlapping area is selected according to the prior art (e.g. as disclosed in US 6,534,983) such that the mutual inductive coupling between the adjacent coil elements is minimized according to the needs of a proposed application of the multi-element coil array.

By the fact that neither the (first) conductors 3, 1 of the coil elements A, B, nor the (second) conductors of the protrusions 6, 5; 7, 8 of the adjacent coil elements A, B run parallel over each other, the mutual capacitive coupling between the adjacent coil elements A, B is reduced to a minimum as well.

Figure 4 schematically shows a second embodiment of a multi-element coil array comprising two coil elements A, C as indicated in Figure 2, which are arranged in a direction diagonal or crosswise to each other so that the two protrusions 7, 5 which are adjacent in a diagonal direction of the coil elements A, C overlap each other in such a way that the (second) conductors of the protrusions do not run parallel over each other, seen in a plane spread out by the coil elements (i.e. in a perpendicular projection onto the plane of Figure 4).

By providing the overlapping area by means of the protrusions 7, 5, the mutual inductive coupling between the diagonal or crosswise coil elements A, D is as well reduced to a minimum.

The capacitive coupling between the diagonal or crosswise coil elements A, D according to Figure 4 is again reduced to a minimum by the overlapping area in such a way that the conductors of the protrusions 7, 5 do not run parallel over each other.

Figure 5 schematically shows a third embodiment of a multi-element coil array comprising four coil elements A to D, each formed as indicated in Figure 2.

The coil elements A to D are arranged in two lines and two rows so that an at least substantially square multi-element coil array is provided. The overlap between the coil elements A and B, between C and D, between A and D and between B and C, which are positioned side by side or above and below each other is provided as described above in connection with Figure 3, whereas the crosswise or diagonal coil elements A, C and B, D overlap each other as explained above in connection with Figure 4.

The central area in which all four coil elements A to D overlap is indicated in an enlarged view in Figure 6. In this Figure, the conductors are denoted with the letters A to D in accordance with the related coil elements A to D to which each conductor belongs. From this Figure it becomes clear that the (second) conductors cross each other with an angle of about 90° or about 45° (or greater) in order to avoid or minimize any capacitive coupling between the coil elements A to D.

Finally, Figure 7 schematically shows a fourth embodiment of a multi-element coil array comprising 12 coil elements A to L, each formed as indicated in Figure 2, wherein the coil elements A to L are arranged such that a rectangular multi-element coil array is obtained.

Generally, the number of coil elements and the optimal forms and dimensions of each coil element, especially the length of the first conductors 1 to 4 and the forms and dimensions of the protrusions 5 to 8 (second conductors), can be evaluated for a desired application and resonance frequency by means of a known simulation software programs.

Furthermore, the protrusions at the outer corners of the multi-element coil array which do not overlap with an adjacent coil element according to Figures 3, 4, 5 and 7, can be omitted, and the corners of the coil elements can instead be provided by each directly connecting the adjacent first conductors of the related coil element.

Furthermore, especially in order to compensate influences of cables, cable trucks or electronic devices on the electromagnetic coupling of the coil elements, the length and/or width of one or more of the coil elements A, ....L and/or of one or more of the protrusions 5 to 8 of one or more of the coil elements A, ....L can be varied, in order to make the multi-element coil array at least substantially insensitive against such cables or devices which are possibly present in certain areas of the multi-element coil array.

A multi-element RF coil array according to the invention, comprising two or a plurality of the above described coil elements, can advantageously be used as a RF antenna 40, 50, 60 (Figure 1) in a magnetic resonance (MR) imaging system. Furthermore, such a multi-element RF coil array can be a part of a RF receiver/transmitter system for use in a MR imaging system.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive, and the invention is not limited to the disclosed embodiments. Variations to embodiments of the invention described above, especially with respect to the number and the arrangement of the coil elements, are possible without departing from the scope of the invention as defined by the accompanying claims.

Variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope but only to assist understanding of the claims.

Claims

CLAIMS:

1. Multi-element coil array comprising two or more coil elements (A,...L) each in the form of a conductor loop, wherein at least two adjacent coil elements are arranged to provide an overlapping area which is delimited by conductors (1, 2, 3, 4; 5, 6, 7, 8) of the adjacent coil elements (A,...L) which do not run parallel over each other, seen in a plane spread out by the coil elements, and wherein crossings of the conductors (5, 6, 7, 8) are provided such that the conductors (5, 6, 7, 8) include an angle between approximately 45° and approximately 90° between each other.

2. Multi-element coil array according to claim 1, wherein at least two adjacent coil elements (A,...L) are each provided with at least one conductor in the form of a protrusion (5, 6, 7, 8) in order to provide at least a part of or the whole overlapping area between the two coil elements.

3. Multi-element coil array according to claim 1, wherein the coil elements (A, ... L) have a rectangular form.

4. Multi-element coil array according to claim 3, comprising at least one conductor in the form of a protrusion (5, 6, 7, 8) which is provided at at least one corner of the coil elements (A,...L).

5. Multi-element coil array according to claim 4, wherein the protrusion (5, 6, 7, 8) has a rectangular form.

6. Multi-element coil array according to claim 4, wherein the overlapping area of coil elements (A, C; B, D) which are adjacent in a diagonal or crosswise direction is provided by means of the protrusions (5, 7; 6; 8).

7. RF receiver and/or transmitter system comprising a RF antenna in the form of a multi-element coil array according to at least one of claims 1 to 6.

8. MR imaging system in the form of a cylindrical (horizontal) system or in the form of a vertical (open) system, comprising a RF transmission and/or receiving body coil (40, 50) and/or a surface coil (60), which comprises a multi-element coil array according to at least one of claims 1 to 6.