Patent Application
20170307704
The invention pertains to a radio frequency (RF) coil for use in an examination space of a magnetic resonance (MR) imaging system, an MR imaging system employing at least one such RF coil, a medical system employing such an MR imaging system and a medical device, and a method for applying a radio frequency field to an examination space of a magnetic resonance imaging system.
A state of the Art design of a magnetic resonance (MR) imaging system is for example an MR imaging system with a magnetic field strength of 3 Tesla. This state of the art MR imaging system employs e.g. a two-channel radio frequency (RF) body coil, which uses two geometrically decoupled feeding positions of a birdcage for RF-shimming. This technique provides a high field homogeneity and enables clinical imaging for additional applications at high field strengths. Although such MR imaging systems provide good imaging results, nowadays additional use cases for MR imaging systems emerge, which are the basis for additional requirements when designing an MR imaging system.
For example, the usage of MR imaging systems is becoming more and more common in the area of medical treatments, where the treatment is directed to a desired location of a subject of interest under guidance of an MR imaging system. E.g., in radiation therapy, an applicable dose can be directed to an exactly desired location, so that apart from the location, also the dose itself can be supervised during the treatment. Nevertheless, the applied radiation also affects the materials of the MR imaging system, so that for example increasing aging of material of the RF body coil may occur due to the applied radiation
Furthermore, also in diagnostic appliances, additional equipment can be required, which has to access the examination space. For example, bio sensors including e.g. a camera can be employed to supervise breathing or heartbeat of the subject of interest. These sensors preferably provide their sensor information from the subject of interest within the RF coil, where access to the subject of interest can be limited. Furthermore, connection of these sensor can require cabling, which may interfere with the fields generated by the MR imaging system, thereby reducing image quality of the MR imaging system.
It is an object of the invention to provide an RF coil, an MR imaging system having such an RF coil, and a medical system including such an MR imaging system, which enable efficient treatment and/or diagnosis when using MR imaging systems, and which are less susceptible to altering e.g. through applied radiation in medical treatments.
This object is achieved by 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 into two coil segments, and the two coil segments are spaced apart from each other in the longitudinal direction of the tubular body, whereby a gap is formed between the two coil segments. The RF coil could be a body coil, but could also be a local coil, e.g. a head coil. Preferably such head coil would comprise openings or a shape such that the shoulders of a patient can be fit in.
This object is also achieved by 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 as specified above.
This object is further achieved by a medical system comprising a magnetic resonance (MR) imaging system as specified above, and a medical device, which is arranged to access the examination space of the magnetic resonance (MR) imaging system through the gap of the RF coil.
This object is also achieved by 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 specified above, and commonly controlling the two RF coil segments to provide a homogenous B1 field within the examination space, in particular within the gap.
This object is still further achieved by 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 the above method.
Accordingly, with the gap provided between the two RF coil segments, the use of further devices used for e.g. medical treatment or analysis is facilitated, since treatments and/or analysis with medical devices can be performed through the gap. Hence, interferences with MR imaging system, in particular the RF coil, can be reduced. E.g. radiation applied to the examination space in conventional MR imaging systems employing conventional RF coils has to throughpass the material of the MR imaging systems and the conventional RF coil. Furthermore, when the radiation applied to the examination space in conventional MR imaging systems employing conventional RF coils throughpasses the conventional MR imaging system employing the conventional RF coil, the radiation alters the material of the MR imaging system and the conventional RF coil. Accordingly, accelerated aging of the materials occurs. These effects can be avoided, when the radiation is not directed towards the two RF coil segments, but passes through the gap between the two RF coil segments. Hence, the gap provides transparency for radiation therapy applications such as MR imaging guided linac or proton therapy. Furthermore, the localization of bio sensors such as camera detectors for the detection of motion (breathing, heart beat) can be facilitated by the gap. Another advantage of the proposed concept is a homogeneous attenuation of radiation. In case of using a state of the art RF coil, the attenuation is stronger in case of radiating through e.g. a coil conductor compared to radiating through air. This makes the treatment less efficient and less accurate. Having the RF coil separated into two segments with a gap in between, the radiation does not have to pass through e.g. a coil conductor, so that it is attenuated equally at different circumferential positions.
The RF coil segments are preferably provided having essentially the same length in the longitudinal direction of the tubular body. Hence, the gap preferably results in a central area of the RF coil, which facilitates the provisioning of a homogenous B1 field. Furthermore, each of the RF coil segments itself may be separated into individual segments. The RF coil segments can be provided simply as a separation of a state of the Art RF coil. Preferably, the RF coil segments are provided with individual feeding ports. The RF coil segments in principal refer to an electrical separation of the RF coil into two RF coil segments, so that resonators of the RF coil segments are spaced apart from each other by the gap. Hence, the RF coil segments can be provided as single components, where the two RF coil segments are mechanically interconnected. Nevertheless, the two RF coil segments can also be mechanically split into two individual components.
The RF coil segments typically comprise rungs extending in a longitudinal direction of the RF coil. The rungs are typically provided at the outer circumferential surface of the RF coil. A set of typically 8 or 16 rungs is equally spaced apart in a circumferential direction of the RF coil. In general, the number of rungs is a multiple of four. The rungs are preferably arranged in parallel to the longitudinal direction of the RF coil. In an alternative embodiment, the rungs are arranged with an angular displacement out of the longitudinal direction of the RF coil, resulting in a “diagonal” arrangement of the rungs. The angular displacement can be up to 20° out of the longitudinal direction of the RF coil. The rungs are provided typically with a distance of few centimeters, preferably two to four centimeters, from the RF screen. The RF screen can be integral part of the RF coil, or a component of the MR imaging system. The distance of the rungs to the RF screen can be held variable for optimization purposes.
There are different ways to build up a z-segmented RF coil, e.g. a bodycoil. The individual RF coil segments can be made of TEM-resonators and/or birdcage resonators. Hence, the two RF coil segments can be made of TEM-resonators, birdcage resonators, or a combination thereof. This does not change the general behavior of the RF coil having two segments in its longitudinal direction. The longitudinal direction is usually referred to as z-direction. Furthermore, each RF coil segment itself may be provided having multiple RF coil segments. Hence, the RF coil can be provided e.g. with four RF coil segments, whereby the gap is provided in a central region between the RF coil segments, e.g. with two RF coil segments on each side of the gap.
The coil segments do not need to be spaced evenly around the tubular body. For example by providing a lower number of coil segments on a first part of the RF coil compared to other parts of the RF coil, extra space for treatment delivery can be provided in the first part.
The RF coil allows efficient parallel image reconstruction techniques such as the SENSE algorithm in the longitudinal direction, i.e. in the z-direction, with a reduction factor of two. The SENSE algorithm is known in the art. Since each RF coil segment only covers a 50% of an examination space, an increase in the signal-to-noise ratio (SNR) is likely to occur, assuming that a patient loading is dominant. Nevertheless, also in cases where coil noise is dominant, an increase in the signal-to-noise ratio (SNR) is likely to occur. This might happen e.g. in case of using a very small distance to the RF-screen. The SNR is typically proportional to the sqrt(Q), i.e. the square root of the quality factor Q of the coil resonance. Typical quality factors Q are in the range of 300-600 in case of empty coils. Due to patient loading, the quality factor Q may be decreased by a factor of about 2 to 6. For higher reduction factors in a left/right (L-R) and anterior/posterior (A-P) direction, the coils have to be configured in a degenerate design. Also RF shimming is feasible depending on the number of available independent RF channels of the RF coil. For an RF coil having four independent RF channels, RF shimming can be achieved e.g. along the z-direction of the RF coil, i.e. the longitudinal direction of the RF coil, and the x-y direction of the RF coil.
In the MR imaging system, the RF screen, the magnetic gradient coil system, and the main magnet, are typically arranged concentrically to surround the examination space. Overall, a typical full setup of the MR imaging system comprises the subject of interest, when located in the examination space, a full body RF coil used as receive and transmit coil, e.g. a full body coil, the RF screen, the magnetic gradient coil system, and the main magnet, when starting at a center of the examination space and moving in a radial direction. In an alternative embodiment, the MR imaging system comprises additionally a local RF coil, which is typically used as receive coil only, and which is located within the RF coil provided as full body coil to surround the subject of interest at least partially. In this alternative embodiment, the RF coil provided as full body coil and is used as transmit coil only. Furthermore, the gradient coil system may be provided with shim coils, which are provided at an radially outer area of the gradient coil system.
In the medical system, the medical device can be any suitable kind of device, e.g. a diagnostic/analytic or therapeutic device. The diagnostic devices may comprise any suitable kind of diagnostic/analytic devices including devices for detection of breathing/breath-hold, heartbeat detection devices, positron emission tomography (PET) devices, in particular PET receivers, bio sensors, camera detectors, or others. The therapeutic devices may comprise any suitable kind of therapeutic devices including radiotherapy systems, linear accelerator (LINAC) devices, proton treatment devices, MR hyperthermia devices or others. In an alternative embodiment, the gap can also be used for positioning RF amplifiers of the MR imaging system.
The medical device can be located depending on size, form and particular needs for accessing the examination space and/or a subject of interest located in the examination space. Accordingly, the medical device can be located in the gap, or the medical device can access the examination space and/or a subject of interest through the gap. For example, a typical LINAC device is provided rotatable around the examination space and the accelerated particles can be directed to the subject of interest through the gap without the risk of interfering with components of the RF coil.
In other cases, the medical device can be positioned e.g. within the examination space, like an MR hyperthermia device. The MR hyperthermia device can be accessed and/or connected through the gap, thereby reducing coupling with the MR imaging device, in particular with the RF coil. Since individual coil elements of the two segments of the RF coil are not directly under the applicator, i.e. the MR hyperthermia device in this case, a good decoupling can be achieved.
According to a preferred embodiment, the two coil segments are arranged relative to each other with a rotational angle around the longitudinal axis of the tubular body. Accordingly, rungs of the two RF coil segments, which extend in the longitudinal direction of the RF coil, can be aligned between the two RF coil segments, or they can be arranged such that rungs from the one RF coil segment point in a direction between the rungs of the other RF coil segment.
According to a preferred embodiment, the two coil segments are coupled together to generate a conventional birdcage field. Preferably, the two coil segments are coupled by a (n times) lambda/2 transmission line, which provides one possibility to couple the two RF coil segments to generate a conventional birdcage field. With the lambda/2 coupling, the two coil segments can be driven like a conventional coil without the gap, e.g. like a conventional birdcage coil. Hence, the RF coil can be used to substitute conventional RF coils in existing MR imaging systems. The replacement can be performed even though the MR imaging system is a stand-alone device, which is not used as part of a medical system, i.e. even though the MR imaging system is not used together with an additional therapeutic or diagnostic device, which requires access to the examination space.
According to a preferred embodiment, the two coil segments are decoupled from each other and driven independently. Decoupling of the two RF coil segments enables that the RF coil as a whole can be driven as a four channel coil array. Hence, excitation of RF fields can be realized in a very accurate and efficient way.
According to a preferred embodiment, the two coil segments can be driven with separate RF power amplifiers or using a hardware combiner or a splitter. Hence, the two coil segments can be driven independently with the two RF power amplifiers. Alternatively, the two coil segments are driven in a combined way with just a single driver.
According to a preferred embodiment, 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. Accordingly, the two RF coil segments are provided with a conductive ring in the area located apart from the gap, and conductive rungs extend from the conductive ring in the direction of the gap. The conductive rungs are coupled to the RF shield, which can be part of the RF coil itself, or which can be part of the MR imaging system. The RF coil comprises an RF screen, to which the conductive rungs are coupled at their ends facing the gap. Alternatively, the screen can be part of the MR imaging system, and the conductive rungs are coupled at their end facing the to the RF screen. Hence, for the overall RF coil results a hybrid design, which is TEM-like in its center region and birdcage-like at the ends in the longitudinal direction. Typical QBC-dimensions of a conventional RF coil comprise a shield radius of 370 mm, a coil radius of 355 mm, and a coil length of 500 mm. For such a typical, conventional RF coil, a gap of approximately 20 cm can be achieved without affecting the operation and the imaging quality of the MR imaging system. Preferably, the gap has a width in the longitudinal direction of the RF coil of at least 5 cm, further preferred gap has a width of at least 10 cm, and still further preferred the gap has a width of 15 cm to 20 cm. The above coil dimensions are given by way of example only. For other coil dimensions, the width of the gap may be different.
According to a preferred embodiment, at least one segment of the RF coil is provided as a multi-element transmit-array. Hence, in combination with a hardware combiner, a decoupling of the two RF coil segments is presumably obsolete, since the coupling between the individual RF coil segments is low.
According to a preferred embodiment 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. Preferably, the gap provided between the RF screen, the magnetic gradient coil system and/or the main magnet are aligned with the gap between the two RF coil segments. Accordingly, the advantages achieved by the gap between the separation of the RF coil into two RF coil segments apply also to the RF screen, the magnetic gradient coil system, or the main magnet. In the case of the RF screen, RF screen segments can be provided as single components, where the two RF screen segments are mechanically interconnected. Nevertheless, the two RF screen segments can also be mechanically split into two individual components. The longitudinal direction of the examination space and of the tubular body are aligned, i.e. the directions are identical.
According to a preferred embodiment, 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. To achieve an efficient RF screening, the RF screen is typically provided as a metal sheet or a metal web with a tight web structure, which is not transparent to RF fields. Furthermore, as already discussed above in respect to the rungs, also the RF screen is not transparent e.g. in respect to radiation when using a LINAC or other radiation devices together with the MR imaging system. To increase the transparency of the RF screen for radiation, the alternative RF screen element can be provided made from a non-conductive material, a mesh-like screen made of conductive material can be used, or a conductive layer with a higher transparency can be used. For example, a thin conductive layer made of copper with a thickness of about 15-40 μm, when used as alternative RF screen element, is almost transparent for radiation from a LINAC device. In an alternative embodiment, the alternative RF screen element can be provided as an overlap area of parts of the two RF screen segments, which overlap through the gap. In a further alternative embodiment, at least one conductive strip can be provided to galvanically connect the two RF screen segments through the gap. Preferably, multiple conductive strips are provided, which are spaced apart in a circumferential direction of the RF screen. Accordingly, an alternative RF screen element is formed as an element having at least one window in the gap. Furthermore, a capacitive coupling can be provided between the two RF screen segments. Hence, an electrical connection between the RF screen segments can be omitted, which enables the use of different kinds of alternative RF screen elements. The longitudinal direction of the examination space and of the tubular body are aligned, i.e. the directions are identical.
According to a preferred embodiment, 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. This design of the RF screen, i.e. of the two RF screen segments provides an extended RF screening in the direction of the gap to provide a shielding to the gradient coil. The slot formed in the gap is narrow compared to the typical dimensions of the RF coil and provides a suppression of radiation. Preferably, the RF screen segments or folded radially outwards. Preferably, the rungs of the RF coil segments are connected to the RF screen, so that an RF current can flow back via the RF screen, so that the gap can also be provided in the RF screen. The longitudinal direction of the examination space and of the tubular body are aligned, i.e. the directions are identical.
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.
In the drawings:
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.
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.
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.
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.
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
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.
As can be seen in detail in
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.
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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 Cri and Cru, to easily distinguish the coupling capacitors 178. The RF coil 140 can be provided as degenerate RF coil 140 by choosing the correct ratio Cri/Cru, 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.
The RF coil 140 of the fourth embodiment differs from the RF coil 140 of the third embodiment in the decoupling. According to
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.
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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.
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.
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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.
110 magnetic resonance (MR) imaging system
112 magnetic resonance (MR) scanner
114 main magnet
116 RF examination space
118 center axis
120 subject of interest
122 magnetic gradient coil system
124 RF screen
126 MR imaging system control unit
128 monitor unit
130 MR image reconstruction unit
132 control line
134 RF transmitter unit
136 RF switching unit
138 control line
140 radio frequency (RF) coil
142 tubular body
144 longitudinal direction
146 RF coil segment
148 gap
150 distance
152 center region
154 end region
156 conductive ring
158 conductive rung
160 coupling capacitor
174 mesh
176 feeding port
178 coupling capacitor
180 inductive decoupling transformers
182 rotational angle
200 medical system
202 medical device
204 RF screen segment
206 gradient coil segment
208 magnet segment
210 ring-like extension
212 alternative screen element
214 low loss cable
216 decoupling circuit
218 structure
220 opening
| Number | Date | Country | Kind |
|---|---|---|---|
| 14189300.8 | Oct 2014 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2015/074084 | 10/19/2015 | WO | 00 |
