Z-SEGMENTED RF COIL FOR MRI WITH GAP AND RF SCREEN ELEMENT

Abstract

The present invention provides a radio frequency (RF) coil (140) for applying an RF field to an examination space (116) of a magnetic resonance (MR) imaging system (110) and/or for receiving MR signals from the examination space (116), whereby the RF coil (140) is provided having a tubular body (142), the RF coil (140) is segmented in a longitudinal direction (154) of the tubular body (142) into two coil segments (146), and the two coil segments (146) are spaced apart from each other in the longitudinal direction (144) of the tubular body (142), whereby a gap (148) is formed between the two coil segments (146). The present invention further provides a magnetic resonance (MR) imaging system (110) comprising at least one radio frequency (RF) coil (140) as specified above. The present invention still further provides a medical system (200) comprising the above magnetic resonance (MR) imaging system (110) and a medical device (202), which is arranged to access to the examination space (116) of the magnetic resonance (MR) imaging system (110) through the gap (148) of the RF coil (140). Even further, the present invention provides a method for applying a radio frequency (RF) field to an examination space (116) of a magnetic resonance (MR) imaging system (110), comprising the steps of providing at least one above radio frequency antenna device (140), and commonly controlling the two RF coil segments (146) to provide a homogenous B.sub.1 field within the examination space (116), in particular within the gap (148).

Claims

1. A radio frequency (RF) coil for applying an RF field to an examination space of a magnetic resonance (MR) imaging system and/or for receiving MR signals from the examination space, whereby the RF coil is provided having a tubular body, the RF coil is segmented in a longitudinal direction of the tubular body (142) into a first and a second coil segment, spaced apart from each other in the longitudinal direction of the tubular body whereby a gap is formed between the first and second coil segment, wherein the RF coil is provided as a hybrid RF coil having a hybrid design of a birdcage coil and a TEM coil, whereby the RF coil is TEM-like in its center region and birdcage-like at its end regions in the longitudinal direction by providing the first and second coil segment with a first and second conductive ring respectively in an area located apart from the gap and by providing the first and second coil segment with first and second conductive rungs extending from the first and second conductive ring respectively in a direction of the gap, wherein the first and second conductive rungs are configured to be coupled to an RF screen at their ends facing the gap.

2. The radio frequency (RF) coil according to preceding claim 1, wherein the first and second coil segment are arranged relative to each other with an rotational angle around the longitudinal axis of the tubular body.

3. The radio frequency (RF) coil according to claim 1, wherein the first and second coil segment are coupled together to generate a conventional birdcage field.

4. The radio frequency (RF) coil according to claim 1, wherein the first and second coil segment are decoupled from each other and driven independently.

5. The radio frequency (RF) coil according to claim 1, wherein the first and second coil segment can be driven with separate RF power amplifiers or using a hardware combiner or a splitter.

6. The radio frequency (RF) coil according to claim 1, wherein at least one segment of the RF coil is provided as a multi-element transmit-array.

7. A magnetic resonance (MR) imaging system, comprising: a tubular examination space provided to position a subject of interest therein, an RF screen for shielding the examination space, a magnetic gradient coil system for generating gradient magnetic fields superimposed to the static magnetic field, and a main magnet for generating a static magnetic field, whereby the RF screen, the magnetic gradient coil system and the main magnet are positioned in this order in a direction radially outward around the examination space, wherein the magnetic resonance (MR) imaging system comprises at least one radio frequency (RF) coil according to claim 1.

8. The magnetic resonance (MR) imaging system according to preceding claim 7, wherein at least one of the RF screen, the magnetic gradient coil system and the main magnet are segmented in the longitudinal direction of the examination space into two segments, which are spaced apart from each other in the longitudinal direction of the tubular body, whereby a gap is formed between the two segments.

9. The magnetic resonance (MR) imaging system according to claim 7, wherein the RF screen is segmented in the longitudinal direction of the examination space into two RF screen segments, the two RF screen segments are spaced apart from each other in the longitudinal direction of the tubular body, whereby a gap is formed between the two RF screen segments, and an alternative RF screen element is provided to connect the two RF screen segments through the gap.

10. The magnetic resonance (MR) imaging system according to claim 7, wherein the RF screen, the magnetic gradient coil system and the main magnet are segmented in the longitudinal direction of the examination space into two segments each, the two segments are spaced apart from each other in the longitudinal direction of the tubular body, whereby a gap is formed between each of the two segments, and the two RF screen segments extend along the gap in a ring-like manner in a direction radially outward of the examination space.

11. A medical system comprising: a magnetic resonance (MR) imaging system according to claim 7, and a medical device, which is arranged to access to the examination space of the magnetic resonance (MR) imaging system through the gap of the RF coils.

12. A method for applying a radio frequency (RF) field to an examination space of a magnetic resonance (MR) imaging system, comprising the steps of providing at least one radio frequency antenna device as claimed in claim 1, and commonly controlling the two RF coil segments to provide a homogenous B1 field within the examination space, in particular within the gap.

13. A software package for upgrading a magnetic resonance (MR) imaging system, whereby the software package contains instructions for controlling the MR imaging system according to method claim 12.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. Such an embodiment does not necessarily represent the full scope of the invention, however, and reference is made therefore to the claims and herein for interpreting the scope of the invention.

[0031] In the drawings:

[0032] FIG. 1 is a schematic illustration of a part of a generic embodiment of a magnetic resonance (MR) imaging system,

[0033] FIG. 2 is a schematic illustration of an RF coil according to a first embodiment,

[0034] FIG. 3 is a perspective view of an RF coil together with an RF screen according to a second embodiment,

[0035] FIG. 4 is a perspective view of the RF coil of FIG. 3 showing a simulated current distribution at a given point in time,

[0036] FIG. 5 is a perspective view of the RF coil of FIG. 3 showing a simulated current distribution at a given point of time for an RF coil with coupled and decoupled RF coil segments on the left and right side, respectively,

[0037] FIG. 6 is a diagrammatic illustration of scattering parameters in the top diagrams and smith charts in the bottom diagrams for the RF coil with coupled and decoupled RF coil segments on the left and right side, respectively,

[0038] FIG. 7 is a schematic illustration of an RF coil according to a third embodiment employed as multi-element transmit-array with capacitive decoupling,

[0039] FIG. 8 is a schematic illustration of an RF coil according to a fourth embodiment employed as multi-element transmit-array with inductive decoupling,

[0040] FIG. 9 is a perspective view of an RF coil together with an RF screen according to a fifth embodiment,

[0041] FIG. 10 is a diagrammatic illustration of simulated B1 fields using the RF coil of the fifth embodiment,

[0042] FIG. 11 is a diagrammatic illustration of input impedance over the frequency using the RF coil of the fifth embodiment,

[0043] FIG. 12 is a schematic illustration of a medical system comprising an MR imaging system with an RF coil and a medical device according to a sixth embodiment,

[0044] FIG. 13 is a schematic illustration of an MR imaging system with an RF coil and segmented RF screen with an alternative RF screen element located therebetween according to a seventh embodiment,

[0045] FIG. 14 is a schematic illustration of an RF coil with two RF coil segments and a decoupling circuit according to an eighths embodiment, and

[0046] FIG. 15 is a schematic illustration of an RF screen with two RF screen segments together with an alternative RF screen element according to a ninth embodiment.

DETAILED DESCRIPTION OF EMBODIMENTS

[0047] FIG. 1 shows a schematic illustration of a part of an embodiment of a magnetic resonance (MR) imaging system 110 comprising an MR scanner 112. The MR imaging system 110 is described here generically as a basis for all further embodiments.

[0048] The MR imaging system 110 includes a main magnet 114 provided for generating a static magnetic field. The main magnet 114 has a central bore that provides an examination space 116 around a center axis 118 for a subject of interest 120, usually a patient, to be positioned within. In this embodiment, the central bore and therefore the static magnetic field of the main magnet 114 have a horizontal orientation in accordance with the center axis 118. In an alternative embodiment, the orientation of the main magnet 114 can be different, e.g. to provide the static magnetic field with a vertical orientation. Further, the MR imaging system 110 comprises a magnetic gradient coil system 122 provided for generating gradient magnetic fields superimposed to the static magnetic field. The magnetic gradient coil system 122 is concentrically arranged within the bore of the main magnet 114, as known in the art.

[0049] Further, the MR imaging system 110 includes a radio frequency (RF) coil 140 designed as a whole-body coil having a tubular body. In an alternative embodiment, the RF coil 140 is designed as a head coil or any other suitable coil type for use in MR imaging systems 110. The RF coil 140 is provided for applying an RF magnetic field to the examination space 116 during RF transmit phases to excite nuclei of the subject of interest 120, which shall be covered by MR images. The RF coil 140 is also provided to receive MR signals from the excited nuclei during RF receive phases. In a state of operation of the MR imaging system 110, RF transmit phases and RF receive phases are taking place in a consecutive manner. The RF coil 140 is arranged concentrically within the bore of the main magnet 114. As is known in the art, a cylindrical metal RF screen 124 is arranged concentrically between the magnetic gradient coil system 122 and the RF coil 140.

[0050] In this context, it is to be noted that the RF coil 140 has been described as transmit and receive coil. Nevertheless, the RF coil 140 can also be provided as transmit or receive coil only.

[0051] Moreover, the MR imaging system 110 comprises an MR image reconstruction unit 130 provided for reconstructing MR images from the acquired MR signals and an MR imaging system control unit 126 with a monitor unit 128 provided to control functions of the MR scanner 112, as is commonly known in the art. Control lines 132 are installed between the MR imaging system control unit 126 and an RF transmitter unit 134 that is provided to feed RF power of an MR radio frequency to the RF antenna device 140 via an RF switching unit 136 during the RF transmit phases. The RF switching unit 136 in turn is also controlled by the MR imaging system control unit 126, and another control line 138 is installed between the MR imaging system control unit 126 and the RF switching unit 136 to serve that purpose. During RF receive phase, the RF switching unit 136 directs the MR signals from the RF coil 140 to the MR image reconstruction unit 130 after pre-amplification.

[0052] FIG. 2 shows an RF coil 140 for applying an RF field to the examination space 116 of the MR imaging system 110 and for receiving MR signals from the examination space 116 according to a first embodiment. The subject of interest 120 is located within the RF coil 140. The RF coil is provided having a tubular body 142 and is segmented in a longitudinal direction 144 of the tubular body 142 into two RF coil segments 146. The longitudinal direction 144 is usually referred to as z-direction. The two RF coil segments 146 are spaced apart from each other in the longitudinal direction 144 of the tubular body 142, whereby a gap 148 is formed between the two RF coil segments 146. Accordingly, the two RF coil segments 146 are spaced apart a distance 150, as shown in FIG. 2.

[0053] FIG. 3 shows an RF coil 140 for applying an RF field to the examination space 116 of the MR imaging system 110 and for receiving MR signals from the examination space 116 according to a second embodiment. Principles of the RF coil 140 according to the first embodiment also apply to the RF coil 140 of the second embodiment, unless otherwise stated.

[0054] The RF coil of the second embodiment is provided as a hybrid RF coil 140 having a hybrid design of a birdcage coil and a TEM coil. As can be seen in FIG. 3, the RF coil 140 is TEM-like in its center region 152 and birdcage-like at its end regions 154 in the longitudinal direction 144. Accordingly, the two RF coil segments 146 are provided with a conductive ring 156 in the end regions 154, which are located apart from the gap 148, and conductive rungs 158 extending from the conductive ring 156 in the direction of the gap 148. Each RF coil segment 146 in this embodiment is provided with a set of 16 conductive rungs 158, which are equally spaced apart in a circumferential direction of the RF coil 140. In an alternative embodiment, the RF coil 140 is provided with two sets of eight conductive rungs 158, i.e. one set of eight conductive rungs 158 is provided in each RF coil segment 146. The conductive rungs 158 are provided with a distance of few centimeters, preferably two to four centimeters, from the RF screen 124.

[0055] The conductive rungs 158 are coupled to the RF screen 124 at their end facing the gap 148 with coupling capacitors 160. In an alternative embodiment, the conductive rungs 158 are galvanically connected or capacitively coupled to the RF screen 124, e.g. using pads close to the RF screen 124. In a further alternative embodiment, the RF screen 124 is part of the RF coil 140 itself. Hence, for the RF coil 140 results a hybrid design, which is TEM-like in its center region 152 and birdcage-like at the end regions 154. The RF coil 140 is provided with the RF screen 124 having radius of 370 mm, the RF coil 140 having radius of 355 mm and a coil length of 500 mm. The gap 148 has a length of approximately 20 cm. Accordingly, each RF coil segment 146 has a coil segment length of approximately 15 cm, e.g. RF coil length of 50 cm minus the length of the gap of 20 cm divided by 2.

[0056] As can be seen in detail in FIG. 3, the RF coil segments 146 are provided having essentially the same length in the longitudinal direction 144 of the tubular body 142. The RF coil segments 146 are provided with individual feeding ports, which are not shown in this figure. The RF coil segments 146 refer to an electrical separation of the RF coil 140 into two RF coil segments 146, so that resonators of the RF coil segments 146 are spaced apart from each other by the gap 148. The RF coil segments 146 in this embodiment are also mechanically split into two individual RF coil segments 146. In an alternative embodiment, the RF coil elements 146 are provided as single components, where the two RF coil segments 146 are mechanically interconnected.

[0057] FIG. 4 shows a simulated current distribution at a given point in time for the RF coil 140 of the second embodiment. As can be seen in FIG. 4, the currents through the two RF coil segments 146 are almost identical.

[0058] General techniques for decoupling of the RF coil segments 146 are known e.g. from US 2013/0063147 A1, which is incorporated herein by reference.

[0059] In FIG. 5 a simulated current distribution at a given point in time for the RF coil 140 of the second embodiment is shown. FIG. 6 shows the current distribution for the RF coil 140 with coupled and decoupled RF coil segments 146 on the left and right side, respectively.

[0060] In FIG. 6 an illustration of scattering parameters is given in the top diagrams for the RF coil 140 with coupled and decoupled RF coil segments 146 on the left and right side, respectively, in accordance with the drawing of FIG. 5.

[0061] Furthermore, in FIG. 6 an illustration of smith charts is given in the bottom diagrams for the RF coil 140 with coupled and decoupled RF coil segments 146 on the left and right side, respectively, in accordance with the drawing of FIG. 5.

[0062] FIG. 7 shows an RF coil 140 for applying an RF field to the examination space 116 of the MR imaging system 110 and for receiving MR signals from the examination space 116 according to a third embodiment. Principles of the RF coil 140 according to the first and second embodiments also apply to the RF coil 140 of the third embodiment, unless otherwise stated.

[0063] The RF coil 140 according to the third embodiment is employed as multi-element transmit-array with capacitive decoupling. Hence, multiple elements are provided as meshes 174, which can be fed via feeding ports 176. Coupling capacitors 178 are provided in the meshes 174, which are also denoted C.sub.ri and C.sub.ru, to easily distinguish the coupling capacitors 178. The RF coil 140 can be provided as degenerate RF coil 140 by choosing the correct ratio C.sub.ri/C.sub.ru, so that the individual meshes 174 are decoupled. Accordingly, each individual mesh 174 in the two RF coil segments 146 can be driven independently by a parallel Tx/Rx RF system.

[0064] FIG. 8 shows an RF coil 140 for applying an RF field to the examination space 116 of the MR imaging system 110 and for receiving MR signals from the examination space 116 according to a fourth embodiment. Principles of the RF coil 140 according to the third embodiment also apply to the RF coil 140 of the fourth embodiment, unless otherwise stated.

[0065] The RF coil 140 of the fourth embodiment differs from the RF coil 140 of the third embodiment in the decoupling. According to FIG. 8, inductive decoupling transformers 180 are provided between adjacent meshes 174. Apart from this difference, the RF coils 140 of the third and fourth embodiment are identical.

[0066] FIG. 9 shows an RF coil 140 for applying an RF field to the examination space 116 of the MR imaging system 110 and for receiving MR signals from the examination space 116 according to a fifth embodiment. Principles of the RF coil 140 according to the above described embodiments also apply to the RF coil 140 of the fifth embodiment, unless otherwise stated.

[0067] The RF coil 140 of the fifth embodiment is almost identical to the RF coil 140 of the second embodiment. The RF coils 140 of the fifth and second embodiments differ in that the two coil segments 146 of the fifth embodiment are arranged relative to each other with a rotational angle 182 around the longitudinal axis of the tubular body 142. Accordingly, the conductive rungs 158 from the one RF coil segment 146 point in a direction between the conductive rungs 158 of the other RF coil segment 146.

[0068] In FIG. 10 can be seen a diagrammatic illustration of simulated B1 fields using the RF coil of the fifth embodiment. Coronal and transversal B1 field homogeneity of simulated coil design is shown in the right and left diagram of FIG. 10, respectively. Contour lines are plotted in 10% steps compared to isocenter field. As can be seen, in the provided gap 148 of the RF coil 140, a homogeneous radial field is provided. On the central (z) axis, the field is very similar compared to a standard birdcage coil. Accordingly, the two RF coil segments 146 are commonly controlled to provide a homogenous B.sub.1 field within the examination space 116, in particular within the gap 148.

[0069] In FIG. 11 can be seen input impedance over the frequency using the RF coil 140 of the fifth embodiment. The Input impedance shows two very close resonances. The homogeneous mode is tuned to 63.86 MHz, the second mode appears at 63.53 MHz. Accordingly, mode separation is generated by separating the RF coil 140 into two RF coil segments 146. The two modes are split by just approximately 300 kHz. Hence, for the RF coil 140 of the fifth embodiment, four-port feeding for a quadrature coil is proposed. Alternatively, additional decoupling can be performed for using the coil like a 2×2=4 channel z-segmented bodycoil.

[0070] FIG. 12 schematically shows a medical system 200 according to a sixth embodiment. The medical system 200 comprises the above MR imaging system 110 with the RF coil 140 and a medical device 202.

[0071] As can be seen in FIG. 12, the MR imaging system 110 comprises an RF coil 140 as described above in respect to the first to fifth embodiment, RF screen 124, a magnetic gradient coil system 122 and a main magnet 114, as described above in respect to FIG. 1. The RF coil 140, the RF screen 124, the magnetic gradient coil system 122, and the main magnet 114 are arranged concentrically to surround the examination space 116. The RF coil 140, the RF screen 124, the magnetic gradient coil system 122, and the main magnet 114 are segmented in the longitudinal direction 144 of the examination space 116 into two segments each, i.e. two RF coil segments 146, two RF screen segments 204, two gradient coil segments 206, and two magnet segments 208, which are all spaced apart from each other in the longitudinal direction 144 of the tubular body 142, so that a gap 148 is formed between the respective segments 146, 204, 206, 208. The gap 148 is provided as single gap 148 for the RF coil segments 146, the RF screen segments 204, the gradient coil segments 206 and the main magnet segments 208 by aligning the respective segments 146, 204, 206, 208.

[0072] As can be further seen in FIG. 12, the two RF screen segments 204 are provided each with a ring-like extension 210. The ring-like extensions 210 extend from the respective RF screen segments along the gap 148 in a direction radially outward of the examination space 116.

[0073] The medical device 202 is arranged to access the examination space 16 of the MR imaging system 110 through the gap 148 of the RF coil 140, the RF screen 124, the gradient coil system 122, and the main magnet 116. Accordingly, with the provided gap 148, application of the medical device to the subject of interest 116 can be performed through the gap 148, e.g. when using a medical treatment/therapeutic device as medical device 202 to apply medical treatment through the gap 148.

[0074] The medical device 202 can be any suitable kind of device, e.g. a diagnostic or therapeutic device. The therapeutic devices may comprise radiotherapy systems, LINAC devices, proton treatment devices, MR hyperthermia devices or others.

[0075] FIG. 13 schematically shows a medical system 200 according to a seventh embodiment. The medical system 200 comprises the above MR imaging system 110 with the RF coil 140 and a medical device 202 and only differs from the medical system 200 of the first embodiment in the design of the RF screen 124, as detailed out below.

[0076] As can be seen in FIG. 13, the RF screen 124 is separated into two RF screen segments 204, as described above in respect to the sixth embodiment and spaced apart from each other. The two RF screen segments 204 according to the seventh embodiment are interconnected with an alternative RF screen element 212 located therebetween. Hence, the alternative RF screen element 212 is provided to connect the two RF screen segments 204 through the gap 148. To increase the transparency of the RF screen 124 for radiation, the alternative RF screen element 212 can be provided made from a non-conductive material, a mesh-like RF screen element 212 made of conductive material can be used, or a conductive layer with a higher transparency can be used as alternative RF screen element 212.

[0077] FIG. 14 schematically shows an RF coil 140 according to an eighth embodiment. The RF coil 140 is provided in accordance with the RF coils 140 of the above embodiments.

[0078] As can be seen in FIG. 14, the two RF coil segments 146 of the RF coil are decoupled using low loss cables 214, which are connected to a decoupling circuit 216. This prevents any cable or stripline connections between the two RF coil segments 146 via the gap 148. Each RF coil segment 146 is driven in quadrature mode or by separate independent transmitters. In an alternative embodiment, the RF coils segments 146 are decoupled via the gap 148 using thin stripline conductors or flexible thin PCB material, which provides low radiation and attenuation.

[0079] FIG. 15 shows an RF screen 124 of an RF coil 140 according to a ninth embodiment. The RF coil 140 and the RF screen 124 are provided in accordance with the above described embodiments. As can be seen in FIG. 15, the RF screen 124 is provided with two RF screen segments 204, which are spaced apart, thereby providing gap 148 therebetween. Each RF screen segment 204 is provided with structure extending in the longitudinal direction 144 to reduce gradient eddy currents and to allow RF current flow for mirror RF currents of the RF coil segments 146. In the gap, an opening 220 is provided for transparency of radiation. The opening 220 in this embodiment is provided without material in a window style. In alternative embodiments, the opening 220 is provided with a non-conductive material or conductive material like a thin mesh, or a thin conductive layer different from the RF screen segments 204.

[0080] 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; the invention is not limited to the disclosed embodiments. Other 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 or steps, 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.

REFERENCE SYMBOL LIST

[0081] 110 magnetic resonance (MR) imaging system

[0082] 112 magnetic resonance (MR) scanner

[0083] 114 main magnet

[0084] 116 RF examination space

[0085] 118 center axis

[0086] 120 subject of interest

[0087] 122 magnetic gradient coil system

[0088] 124 RF screen

[0089] 126 MR imaging system control unit

[0090] 128 monitor unit

[0091] 130 MR image reconstruction unit

[0092] 132 control line

[0093] 134 RF transmitter unit

[0094] 136 RF switching unit

[0095] 138 control line

[0096] 140 radio frequency (RF) coil

[0097] 142 tubular body

[0098] 144 longitudinal direction

[0099] 146 RF coil segment

[0100] 148 gap

[0101] 150 distance

[0102] 152 center region

[0103] 154 end region

[0104] 156 conductive ring

[0105] 158 conductive rung

[0106] 160 coupling capacitor

[0107] 174 mesh

[0108] 176 feeding port

[0109] 178 coupling capacitor

[0110] 180 inductive decoupling transformers

[0111] 182 rotational angle

[0112] 200 medical system

[0113] 202 medical device

[0114] 204 RF screen segment

[0115] 206 gradient coil segment

[0116] 208 magnet segment

[0117] 210 ring-like extension

[0118] 212 alternative screen element

[0119] 214 low loss cable

[0120] 216 decoupling circuit

[0121] 218 structure

[0122] 220 opening