Abstract
A radiofrequency coil (1) for use in magnetic resonance to transmit or receive an oscillating magnetic field. The radiofrequency coil (1) comprises a plurality of conducting elements (2) connected together to form a continuous conducting path. Each conducting element (2) of the plurality of conducting elements are arranged in substantially parallel surfaces such that the area bounded by each conducting element (2) overlaps with at least 20% of the area bounded by another of the conducting elements (2) in another parallel surface.
Claims
1. A radiofrequency coil for magnetic resonance, wherein the radiofrequency coil is arranged to transmit and/or receive an oscillating magnetic field, the radiofrequency coil comprising: a continuous conducting path comprising a plurality of conducting elements, wherein each conducting element is connected to another of the conducting elements so as to form the continuous conducting path, and each conducting element defines an area bounded by the conducting element; wherein the plurality of conducting elements are arranged in substantially parallel surfaces such that the area bounded by each conducting element overlaps with at least 20% of the area bounded by another of the conducting elements.
2. The radiofrequency coil as claimed in claim 1, wherein the plurality of conducting elements are arranged about a common central region of the radiofrequency coil.
3. The radiofrequency coil as claimed in claim 2, wherein the plurality of conducting elements are confined to an annulus surrounding the common central region.
4. The radiofrequency coil as claimed in claim 1, wherein the bounded areas of the plurality of conducting elements overlap with each other at a central axis of the radiofrequency coil; and/or wherein the plurality of conducting elements are arranged such that the common area of overlap for the areas bounded by the plurality of conducting elements is at least 10% of a total area over which the area bounded by the plurality of conducing elements extend; and/or wherein the plurality of conducting elements are arranged such that less than 40% of the total area enclosed by the continuous conducting path does not have an overlap of at least some of the bounded areas defined by the plurality of conducting elements; and/or wherein at each of the overlaps of a pair of crossing conducting elements in the continuous conducting path, the conducting elements cross with an angle that is greater than 20°, e.g., greater than 30°, e.g. greater than 40°.
5-7. (canceled)
8. The radiofrequency coil as claimed in claim 1, wherein the plurality of conducting elements are arranged in a rotationally symmetric configuration; and/or wherein each conducting element is separated from the conducting element in the adjacent substantially parallel surface by a dielectric.
9. The radiofrequency coil as claimed in claim 8, wherein the order of rotational symmetry is greater than or equal to the number of conducting elements that form the continuous conducting path; and/or wherein the dielectric is arranged at least between the locations where pairs of conducting elements cross over each other; and/or wherein the dielectric does not extend over at least part of the common central region of the radiofrequency coil.
10-12. (canceled)
13. The radiofrequency coil as claimed in claim 1, wherein each conducting element is connected to another of the conducting elements at a periphery of the continuous conducting path; and/or wherein the radiofrequency coil comprises at least three conducting elements.
14. (canceled)
15. The radiofrequency coil as claimed in claim 1, wherein each conducting element is connected to another of the conducting elements by one or more discrete electrical components.
16. The radiofrequency coil as claimed in claim 15, where the one or more discrete electrical components comprise one or more capacitors and/or one or more LC traps.
17. The radiofrequency coil as claimed claim 1, wherein each conducting element comprises one or more discrete electrical components arranged part way along the length of the conducting element, and/or wherein the plurality of conducting elements are arranged such that each conducting element is positioned at a rotated position relative to another of the conducting elements; and/or wherein at least some of the plurality of conducting elements comprise an open shape.
18-19. (canceled)
20. The radiofrequency coil as claimed in claim 1, wherein the conducting elements are each arranged on a substrate; and/or wherein the continuous conducting path is mounted on a mounting substrate.
21. The radiofrequency coil as claimed claim 1, wherein the conducting elements comprise copper tracks on a substrate.
22. The radiofrequency coil as claimed in claim 21, wherein the copper tracks have a thickness between 10 μm and 150 μm.
23. The radiofrequency coil as claimed in claim 1, wherein the conducting elements comprise tubes; and/or wherein the conducting elements have a substantially constant width along their length, wherein the substantially constant width is between 1 mm and 20 mm; and/or wherein the conducting elements have a varying width along their length such that the conducting element is comprises of at least one thicker portion and at least one thinner portion.
24. The radiofrequency coil as claimed in claim 23, wherein the cylindrical tubes have a wall thickness of between 50 μm and 2 mm; and/or wherein the thinner portion is arranged to be positioned at least where the pair of crossing conducting elements overlap.
25-28. (canceled)
29. The radiofrequency coil as claimed in claim 1, wherein the radiofrequency coil is curved, and/or wherein the radiofrequency coil is arranged to operate as a single coil or in combination with other coils.
30. (canceled)
31. A plurality of radiofrequency coils each as claimed in claim 1, wherein the plurality of radiofrequency coils are arranged to operate in combination with each other.
32. The plurality of radiofrequency coils as claimed in claim 31, wherein the radiofrequency coils are arranged to operate in a quadrature configuration or in an array configuration; and/or wherein the radiofrequency coils are spatially separated from each other; and/or wherein three or more radiofrequency coils are arranged in an array; and/or wherein one or more coils of the plurality of radiofrequency coils are geometrically overlapped with one or more other coils of the plurality of radiofrequency coils.
33-34. (canceled)
35. The plurality of radiofrequency coils as claimed in claim 32, wherein one or more pairs of overlapping radiofrequency coils have a centre-to-centre distance of between 33% to 100% of the diameter of the radiofrequency coils.
36. (canceled)
37. The plurality of radiofrequency coils as claimed in claim 35, wherein each of three radiofrequency coils overlaps with both of the other two radiofrequency coils; and/or wherein the centre-to-centre distance of the pair of radiofrequency coils is equal to the position at which the smallest mutual inductance between the two radiofrequency coils is found.
38. (canceled)
Description
[0076] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:
[0077] FIGS. 1a and 1b show a radiofrequency coil in accordance with an embodiment of the present invention;
[0078] FIGS. 2a and 2b show another radiofrequency coil in accordance with an embodiment of the present invention;
[0079] FIGS. 3a, 3b and 3c show another radiofrequency coil in accordance with an embodiment of the present invention;
[0080] FIG. 4 shows a circuit diagram for how a radiofrequency coil in accordance with an embodiment of the present invention may be used.
[0081] FIG. 5 shows another radiofrequency coil in accordance with an embodiment of the present invention;
[0082] FIG. 6 shows another radiofrequency coil in accordance with an embodiment of the present
[0083] FIGS. 7a and 7b show another radiofrequency coil in accordance with an embodiment of the present invention;
[0084] FIGS. 8a to 8d show simulated surface and cross-section oscillating radiofrequency magnetic (B.sub.1+) fields for two radiofrequency coils in accordance with embodiments of the present invention;
[0085] FIGS. 9a to 9d shows the on-axis simulated oscillating radiofrequency magnetic (B.sub.1+) fields and simulated signal-to-noise ratio results of FIG. 8 through the centre of the radiofrequency coil;
[0086] FIGS. 10a to 10d show simulated surface and cross-section oscillating radiofrequency magnetic (B.sub.1+) fields for two overlapping radiofrequency coils in accordance with an embodiment of the present invention; and
[0087] FIGS. 11a to 11f show simulated surface and cross-section oscillating radiofrequency magnetic (B.sub.1+) fields for three overlapping radiofrequency coils in accordance with an embodiment of the present invention; and
[0088] FIGS. 12a to 12c shows the simulated S.sub.21 parameter values as the overlap of a two radiofrequency surface coil system, in accordance with an embodiment of the present invention, is varied.
[0089] Magnetic resonance imaging (MRI) scanners are used to perform magnetic resonance measurements of a sample, for example magnetic resonance imaging or spectroscopy of a human or animal subject, typically to investigate the anatomy and physiology of the subject, e.g. to detect pathologies or abnormalities. MRI scanners include a large primary magnet that applies a static magnetic field over the scanning volume to polarise the nuclear spins of the sample being scanned, a gradient magnet that applies a magnetic field having a linear variation across the scanning volume to allow spatial localisation, and a radio frequency (RF) system to transmit an oscillating magnetic field (to excite nuclei in the sample) and receive RF radiation (from the subsequent relaxation of the excited nuclei).
[0090] Embodiments of the present invention will now be described that provide the RF system in the form of a radiofrequency coil which both transmits and receives the (RF) oscillating magnetic field (it will be appreciated that these radiofrequency coils may be used only to transmit or only to receive an oscillating magnetic field).
[0091] FIGS. 1a and 1b show a radiofrequency coil 1 in accordance with an embodiment of the present invention. FIG. 1a shows the assembled radiofrequency coil 1 and FIG. 1b shows an exploded view of the different components (layers) of the radiofrequency coil 1.
[0092] As can be seen from FIGS. 1a and 1b, the radiofrequency coil 1 is formed from four conducting elements 2 that each have a triangular shape with rounded corners. The conducting elements 2 are each formed from a 10 mm wide track of copper which is deposited onto a printed circuit board. A bridging capacitor is positioned at the midpoint 3 of each of the conducting elements 2.
[0093] The four conducting elements 2 are connected together (using discrete electrical components at the gaps 5 between the conducting elements 2) to form a continuous conducting path that forms the radiofrequency coil 1. Each conducting element 2 is positioned at a rotation of the conducting elements 2 to which it is connected, such that the continuous conducting path is rotationally symmetric. The conducting elements 2 are each separated by an annular shaped dielectric layer 4 (in the form of a polyimide sheet). The formed radiofrequency coil 1 is mounted on a FR-4 substrate.
[0094] The dielectric layers 4 and conducting elements 2 are arranged on top of each other such that a dielectric layer 4 is provided between the conducting elements 2 at all of the crossing points 6 of the conducting elements 2. An aperture 8 is provided in the dielectric layers 4 in the centre of the radiofrequency coil 1. The ends of each conducting element 2 (and thus the connections between the conducting elements 2) extend radially outward of the dielectric layers 4, such that they can be connected together.
[0095] Two of the conducting elements 2 include contacts 10 at their ends to allow electrical contact to be made, in order to supply an oscillating current to the continuous conducting path of the radiofrequency coil 1 to transmit an oscillating magnetic field into the sample to be scanned and/or to receive an oscillating magnetic field that is emitted from the sample being scanned. The contacts 10 provide a gap 7 for one or more “matching” capacitors to impedance transform between the transmit and/or receive chain and the radiofrequency coil 1.
[0096] In operation, the radiofrequency coil 1 is placed over the part of the sample to be scanned. The sample and the radiofrequency coil 1 are inserted together into the bore of an MRI scanner. The primary and gradient magnets of the MRI scanner are operated to apply their respective magnetic fields over the volume of the scanner and the radiofrequency coil 1 is driven (through its contacts 10) by an oscillating current to generate and transmit an oscillating magnetic field into the sample to be scanned (i.e. the part of the sample over which the radiofrequency coil 1 has been placed).
[0097] The transmitted oscillating magnetic field acts to excite the (target species of) nuclei in the sample that have been polarised by the primary magnetic field. These excited nuclei then relax and emit an RF oscillating magnetic field which is then received and detected by the radiofrequency coil 1. Processing the detected oscillating magnetic field allows the position of the (target species of) nuclei in the sample to be scanned.
[0098] FIGS. 2a and 2b show another radiofrequency coil 21 in accordance with an embodiment of the present invention. The radiofrequency coil 21 shown in FIGS. 2a and 2b is similar to the radiofrequency coil 1 shown in FIGS. 1a and 1b in that it has four conducting elements 22 that have discrete electrical components (e.g. bridging capacitors) arranged at their midpoints 23 and are connected together (using discrete electrical components at the gaps 25 between the conducting elements 22 and a matching capacitor at the gap 27 where contacts to connect to an oscillating current supply are provided) to form a continuous conducting path, with the conducting elements 22 arranged in a rotationally symmetric configuration. Similarly, the conducting elements 22 are separated by respective dielectric layers 24.
[0099] The difference in the radiofrequency coil 21 shown in FIGS. 2a and 2b is in the shape of the conducting elements 22 and the dielectric layers 24. In FIGS. 2a and 2b the radiofrequency coil 21 is constructed from conducting elements 22 each having an “omega (Ω)” shape and the dielectric layers 24 are an octagonal annulus having a central aperture 28 (but again arranged such that the dielectric layers 24 are positioned between crossing points 26 of the conducting elements 22).
[0100] Operation of the radiofrequency coil 21 shown in FIGS. 2a and 2b is very similar to the operation of the radiofrequency coil 1 shown in FIGS. 1a and 1b. The different shape of the continuous conducting path of the radiofrequency coil 21 of FIGS. 2a and 2b will generate an oscillating magnetic field having a different shape to that generated by the radiofrequency coil 1 shown in FIGS. 1a and 1b, but will otherwise operate in a very similar manner.
[0101] FIGS. 3a, 3b and 3c show another radiofrequency coil 31 in accordance with another embodiment of the present invention. The radiofrequency coil 31 shown in FIGS. 3a, 3b and 3c is similar to the radiofrequency coil 21 shown in FIGS. 2a and 2b in that it has “omega (Ω)” shaped conducting elements 32 that have discrete electrical components (e.g. bridging capacitors) arranged at their midpoints 33 and are connected together (using discrete electrical components at the gaps 35 between the conducting elements 32 and a matching capacitor at the gap 37 where contacts to connect to an oscillating current supply are provided) to form a continuous conducting path, with the conducting elements 32 arranged in a rotationally symmetric configuration. Similarly, the conducting elements 32 are separated by respective dielectric layers 34.
[0102] The difference in the radiofrequency coil 31 shown in FIGS. 3a, 3b and 3c is in the number and rotational symmetry of the conducting elements 32. In FIGS. 3a, 3b and 3c the radiofrequency coil 31 is constructed from six conducting elements 32, such that the continuous conducting path has six-fold rotational symmetry (compared to the four-fold symmetry of the radiofrequency coil 21 in FIGS. 2a and 2b). The dielectric layers 34 of the radiofrequency coil 31 shown in FIGS. 3a, 3b and 3c are a hexagonal annulus having a central aperture 38 (but again arranged such that the dielectric layers 34 are positioned between crossing points 36 of the conducting elements 32).
[0103] As can be seen from FIG. 3c, the radiofrequency coil 31 is curved around the surface of a cylinder 39 (e.g. the radiofrequency coil 31 may be mounted on a portion of a cylindrical sheet of glass reinforced plastic). This helps to conform the radiofrequency coil 31 to the part of the sample to be scanned, which moves the effective centre of the radiofrequency coil 31 closer to the internal part of the sample to be scanned.
[0104] Operation of the radiofrequency coil 31 shown in FIGS. 3a, 3b and 3c is very similar to the operation of the radiofrequency coils 1, 21 shown in FIGS. 1a and 1b and FIGS. 2a and 2b. The different number of conducting elements 32 and thus the different shape of the continuous conducting path of the radiofrequency coil 31 of FIGS. 3a, 3b and 3c will generate an oscillating magnetic field having a different shape to that generated by the radiofrequency coils 1, 21 shown in FIGS. 1a and 1b and FIGS. 2a and 2b, but will otherwise operate in a very similar manner.
[0105] One of the main operational differences of the radiofrequency coil 31 shown in FIGS. 3a, 3b and 3c is owing to its curved shape. As indicated above, the curved shape of the radiofrequency coil moves the effective centre of the radiofrequency coil 31 closer to the internal part of the sample to be scanned. This helps the generated and transmitted oscillating magnetic field to penetrate deeper into the part of the sample to be scanned, and helps the radiofrequency coil 31 to receive and detect the emitted oscillating magnetic field from a deeper penetration.
[0106] FIG. 4 shows a circuit diagram according to an embodiment of the present invention that may be used with and to operate the radiofrequency coil 31 shown in FIGS. 3a, 3b and 3c (a similar circuit may be used with the radiofrequency coils 1, 21 shown in FIGS. 1a and 1b, and FIGS. 2a and 2b, e.g. with the appropriate number of sets of discrete electrical components).
[0107] The circuit 41 shown in FIG. 4 includes a set of discrete electrical components for each “layer” (for or between each of the conducting elements) of the radiofrequency coil. In each layer this includes an LC trap 42 (a 51 pF capacitor in parallel with a 32.7 nH inductor, both of which are in series with a 87 pF capacitor in the adjacent layer) and a 87 pF bridging capacitor 43. The LC traps 42 are, for example, arranged at the gaps 35 between the conducting elements 32 of the radiofrequency coil 31 shown in FIGS. 3a, 3b and 3c. The bridging capacitors 43 are, for example, arranged at the midpoints 33 of the conducting elements 32.
[0108] The circuit 41 also includes a 155 pF matching capacitor 44 arranged between the ends of the continuous conducting path, between the external contacts of the radiofrequency coil. The matching capacitor 44 is, for example, arranged at the gap 37 in the radiofrequency coil 31 shown in FIGS. 3a, 3b and 3c, between the contacts for the external oscillating current supply to be provided. The circuit 41 includes a coaxial input line 45 and a coaxial ground line 46 which are connected into the continuous conducting path of the circuit via a carbon balun 47 and a proton balun 48.
[0109] FIG. 5 shows another radiofrequency coil 501 in an assembled configuration in accordance with an embodiment of the present invention. The shape of the radiofrequency coil conducting elements 502 are similar to the radiofrequency coil 1 shown in FIGS. 1a and 1b in that it has four conducting elements 502 that each have a triangular shape with rounded corners. The difference in the radiofrequency coil 501 shown in FIG. 5 is that the conducting elements have a varying width along their length, e.g. a tapered design, comprising multiple thicker portions 512 and multiple thinner portions 514. The thinner portions 512 of the conducting element 502 are positioned at crossing points 506 between conducting elements 502 arranged on top of each other and separated by a dielectric layer 504. The transition 516 between the thicker portions of the conducting elements and the thinner portions of the conducting elements is shown to be a continuous gradient, e.g. a slope design.
[0110] Operation of the radiofrequency coil 501 shown in FIG. 5 is very similar to the operation of the radiofrequency coil 1 shown in FIGS. 1a and 1b. The varying thickness of the continuous conducting path of the radiofrequency coil 501 of FIG. 5 will generate an oscillating magnetic field having a different shape and/or magnitude to that generated by the radiofrequency coil 1 shown in FIGS. 1a and 1b, but will otherwise operate in a very similar manner.
[0111] FIG. 6 shows another radiofrequency coil 601 in accordance with an embodiment of the present invention. The radiofrequency coil 601 is similar to the radiofrequency coil 2 shown in FIG. 2 in that each have four conducting elements with an “omega (Ω)” shape. The difference in the radiofrequency coil 601 is that the conducting elements vary in width along their length, e.g. they have a tapered design.
[0112] As with the radiofrequency coil 501 shown in FIG. 5, the conducting elements 602 have multiple thicker portions 612 and thinner portions 614 where the thinner portions are positioned at crossing points 606 between conducting elements 602 arranged on top of each other. Unlike the radiofrequency coil 501 shown in FIG. 5, radiofrequency coil 601 has thinner portions 614 which extend across multiple crossing points 606. The thicker portions have a width of 10 mm. The thinner portions have a width of 5 mm. The transition 616 between the thicker portions 612 of the conducting elements and the thinner portions 614 of the conducting elements 802 is shown to have a slope design.
[0113] Operation of the radiofrequency coil 601 shown in FIG. 6 is very similar to the operation of the radiofrequency coil 1 shown in FIGS. 1a and 1b. The varying thickness of the continuous conducting path of the radiofrequency coil 601 of FIG. 6 will generate an oscillating magnetic field having a different shape to that generated by the radiofrequency coil 1 shown in FIGS. 1a and 1b, but will otherwise operate in a very similar manner.
[0114] It will be appreciated that many other shapes and configurations may be provided for conducting elements than is shown in FIGS. 5 and 6. For example, a tapered design may be applied to any of the radiofrequency coils shown in FIGS. 1-3. It will also be appreciated that many other tapered designs are possible in addition to the designs shown in FIGS. 5 and 6, e.g. designs with curved or stepped transition.
[0115] FIGS. 7a and 7b shows another radiofrequency coil 701 in accordance with an embodiment of the present invention. The shape of the radiofrequency coil 701 shown in FIGS. 7a and 7b is similar to the radiofrequency coil 2 shown in FIGS. 2a and 2b in that it has four conducting elements 702 each with an “omega (Ω)” shape. The radiofrequency coil 701 is set to have a diameter of 150 mm. The conducting elements 702 are cylindrical tubes comprising a circular cross-section with a uniform width of 10 mm along its length and a uniform wall thickness of 1 mm. FIG. 7a shows the dielectric layers 704 of the radiofrequency coil 701 arranged such that the dielectric layers 704 are positioned between the crossing points of the conducting elements 702. For the purposes of clarity, the dielectric layers 704 are not shown in FIG. 7b.
[0116] Frequency domain EM simulations were performed using the EM model radiofrequency coil 701. The transmit magnetic field, B.sub.1+ was calculated from 1 W of simulated power. The relative signal-to-noise ratio (SNR) was calculated from B.sub.1+. The results of this simulation are shown in FIGS. 8a to 8d and FIGS. 9a to 9d.
[0117] FIGS. 8a and 8b show the simulated surface (FIG. 8a) and cross-section (FIG. 8b) of the magnetic flux density (B.sub.1+) for the radiofrequency coil 701. FIGS. 8c and 8d show the simulated surface (FIG. 8c) and cross-section (FIG. 8d) of the magnetic flux density (B.sub.1+) for the radiofrequency coils having a comparative design to those shown in FIGS. 7a and 7b but comprising copper track conducting elements having a 10 mm uniform width.
[0118] The results shown in FIGS. 8a and 8b when compared to FIGS. 8c and 8d show that radiofrequency coils comprised of conducting elements with a cylindrical tube design have an improved B.sub.1+ flux density profile compared to an equivalent radiofrequency coils having copper tracks, in spite of the fact that the centre of the radiofrequency coil with tubular conducting elements will be further away from the sample.
[0119] FIG. 9a shows the on-axis simulated magnetic flux density (B.sub.1+) results shown in FIG. 8a as a cross-section through the centre of the radiofrequency coil. FIG. 9b shows an enlarged portion of FIG. 9a at penetration depths of interest corresponding to the depth of key organs within the human body. The results clearly show that the strength of the B.sub.1+ field for cylindrical tube conducting elements 921 is greater than that of the B.sub.1+ field for copper track conducting elements 922 of an equivalent design at all penetration depths.
[0120] FIG. 9c shows the on-axis simulated signal-to-noise ratio (SNR) corresponding to the results shown in FIGS. 8a and 8b. FIG. 9d shows an enlarged portion of FIG. 9c at penetration depths of interest corresponding to the depth of key organs within the human body. The results clearly show that the SNR for a radiofrequency coil comprising cylindrical tube conducting elements 923 is better than the SNR for a radiofrequency coil comprising copper track conducting elements 924 at all penetration depths.
[0121] The results shown in FIGS. 8a-8d and 9a-9d thus show that when compared to a copper track conductor design, cylindrical tube conductors help to improve both transmit and receive performance for radiofrequency surface coils due to the reduction in resistance due to proximity effects. However, radiofrequency coils comprising either cylindrical tube conductor elements or copper track conductor elements according to embodiments of the present invention help to provide an improved radiofrequency coil compared to those known in the art.
[0122] FIGS. 10a and 10b show a simulated surface (FIG. 10a) and cross-section (FIG. 10b) of the magnetic flux density (B.sub.1+) for two overlapping radiofrequency coils at an optimised position (calculated, in this embodiment, to be a centre-to-centre distance of 134.2 mm). Only one of the two radiofrequency coils within the pair is excited in the simulation. The radiofrequency coils depicted are of the design shown in FIGS. 2a and 2b, e.g. the four conducting elements of an “omega” shape. The overlap distance was determined using frequency domain EM simulations where the overlap distance was scanned incrementally to find a minimum S.sub.21 transmission parameter.
[0123] FIGS. 10c and 10d show a simulated surface (FIG. 10c) and cross-section (FIG. 10d) of the magnetic flux density (B.sub.1+) for two overlapping radiofrequency coils at a position other than optimised overlap (in this example, 139 mm). Only one of the two radiofrequency coils within the pair is excited in the simulation. The radiofrequency coils depicted are of the design shown in FIGS. 2a and 2b, e.g. the four conducting elements of an “omega” shape.
[0124] Comparison of FIGS. 10a with 10c and 10b with 10d clearly shows that the system with optimised overlap (FIGS. 10a and 10b) has an improved inductive decoupling. The system shown in FIGS. 10c and 10d has a greater degree of coupling, with both radiofrequency coils becoming one resonant structure. This results in the optimised overlap system having an increased magnetic flux density peak strength and increased depth penetration.
[0125] FIGS. 11a to 11c show a simulated surface of the magnetic flux density (B.sub.1+) field for three overlapping radiofrequency coils at an optimised position of overlap where only one of the three radiofrequency coils within the system is excited. The radiofrequency coils depicted are of the design shown in FIGS. 2a and 2b, e.g. the four conducting elements of an “omega” shape. The optimised overlap distance between the first and second radiofrequency coil was approximated to be equal to the overlap distance determined via the simulations shown in FIGS. 10a and 10b (e.g. 134.2 mm). The distance between the first and third, and second and third radiofrequency coils was determined using frequency domain EM simulations where the overlap distance was scanned incrementally until a minimum for the S.sub.32 and S.sub.31 transmission parameters was found.
[0126] FIGS. 11d to 11f show a simulated surface of the magnetic flux density (B.sub.1+) for three overlapping radiofrequency coils at a position other than optimal overlap where only one of the two radiofrequency coils within the pair is excited. The radiofrequency coils depicted are of the design shown in FIGS. 2a and 2b, e.g. the four conducting elements of an “omega” shape.
[0127] Comparison of the two data sets shown in FIGS. 11a-11c and 11d-11f again shows that, like the two coil system shown in FIGS. 10a-10d, the three coil system has an improved inductive decoupling when the radiofrequency coils are arranged at a position of optimised overlap with a distance between coils 1 and 3 of 145 mm and a distance between coils 2 and 3 of 148 mm (FIG. 11a-11c). In contrast, the un-optimised overlap system has increased coupling, with the three elements becoming one resonant structure, resulting in a reduced B.sub.1+ depth penetration and peak strength.
[0128] FIG. 12a shows the simulated S.sub.21 transmission parameter value of the two radiofrequency coil array system shown in FIGS. 10a-10d as the overlap of the two radiofrequency coils is varied. FIG. 12b shows the simulated S.sub.21 parameter value of a system known in the prior art comprising two single “loop” type radiofrequency surface coils (e.g. radiofrequency surface coils of a single layered continuous circular conducting element). Comparison of FIG. 12a and FIG. 12b shows that the increased complexity of the radiofrequency coil surface (e.g. when compared to the simplicity of a single loop radiofrequency coil) does not result in a corresponding increase in difficulty in finding the optimum overlap for maximised decoupling.
[0129] FIG. 12c shows the simulated S.sub.21 transmission parameter values shown in FIG. 12a (for the two overlapping radiofrequency coil system in accordance with the embodiment of this invention shown in FIG. 10a-10d) and FIG. 12b (for the prior art system comprising two overlapped single “loop” type radiofrequency surface coils). This comparison further shows that the simulation results for the overlapped radiofrequency coils of the present invention (comprising “omega (Ω)” shaped conducting elements) 1225 provides an improved extent of decoupling, with a value of −17.63 dB at an optimised overlap position. In comparison, the simulation results for the overlapping single “loop” system known in the prior art 1226 shows a minimum decoupling value of −15.96 dB. This shows that arrays comprising a plurality of surface radiofrequency coils in accordance with embodiments of the present invention help to provide an improved receive array, compared to arrays comprising surface coils known in the art, with an improved sensitivity and improved signal-to-noise ratio of each individual element within the array.
[0130] It will be seen from the above that in at least preferred embodiments, the present invention provides a radiofrequency coil which has a significant overlap between the elements of the continuous conducting path. This helps to provide a radiofrequency coil that can generate an increased magnetic flux density for transmitting into a sample and/or receive an emitted magnetic field with an improved sensitivity.
[0131] Although the embodiments show the radiofrequency coil formed from conducting elements having particular shapes and arrangements relative to each other, it will be appreciated that many other different shapes and configurations may be provided.
