Patent Application
20120169340
The invention relates to the field of magnetic resonance (MR) imaging. It concerns an MR imaging system comprising an RF waveguide for guiding traveling RF waves and at least one RF antenna configured to couple to at least one traveling mode of the RF waveguide. Further, the invention relates in general to an RF antenna for an MR system.
Image forming MR methods which utilize the interaction between magnetic field and nuclear spins in order to form two-dimensional or three-dimensional images are widely used nowadays, notably in the field of medical diagnostics, because for the imaging of soft tissue they are superior to other imaging methods in many respects, and do not require ionizing radiation and are usually not invasive.
According to the MR method in general, the body of the patient to be examined is arranged in a strong, uniform magnetic field whose direction at the same time defines an axis (normally the z-axis) of the coordinate system on which the measurement is based. The magnetic field produces different energy levels for the individual nuclear spins in dependence on the applied magnetic field strength which spins can be excited (spin resonance) by application of an alternating electromagnetic field (RF field) of defined frequency, the so called Larmor frequency or MR frequency. From a macroscopic point of view the distribution of the individual nuclear spins produces an overall magnetization which can be deflected out of the state of equilibrium by application of an electromagnetic pulse of appropriate frequency (RF pulse) while the magnetic field extends perpendicularly to the z-axis, so that the magnetization performs a precessional motion about the z-axis.
The variation of the magnetization can be detected by means of receiving RF antennas which are arranged and oriented within an examination volume of the MR device in such a manner that the variation of the magnetization is measured in the direction perpendicular to the z-axis.
In order to realize spatial resolution in the body, linear magnetic field gradients extending along the three main axes are superposed on the uniform magnetic field, leading to a linear spatial dependency of the spin resonance frequency. The signal picked up in the receiving antennas then contains components of different frequencies which can be associated with different locations in the body. The signal data obtained via the receiving antennas corresponds to the spatial frequency domain and is called k-space data. The k-space data usually includes multiple lines acquired with different phase encoding. Each line is digitized by collecting a number of samples. A set of k-space data is converted to an MR image, e.g. by means of Fourier transformation.
In recent years, two strong trends are observable in the design of MR imaging systems: on the one hand the clinical need and clinical acceptance of MR imaging systems operating at high magnetic field strength (three or more Tesla) became obvious. On the other hand, the dimensions of the examining volume (the inner bore diameter) of MR systems are steadily increasing.
Brunner et al (Nature, Volume 457, 2009, pages 994-998) have proposed a traveling wave approach for high field MR imaging. The examined patient is positioned within an RF waveguide that is used for guiding traveling RF waves along the longitudinal bore axis in at least one traveling mode of the RF waveguide. The traveling RF waves propagate through the examination volume of the MR imaging apparatus and are used for exciting and detecting magnetic resonance. The essential advantages of this concept are that it enables an excellent RF coverage as well as a high degree of RF homogeneity throughout the examination volume. For this reason, traveling wave MR imaging has the potential to facilitate the simultaneous exploration of the highest field strengths and larger bore diameters available for medical MR imaging.
However, MR imaging using such traveling waves requires a new type of RF antennas. Instead of coupling to the near field of the examined body, a traveling wave antenna must couple to the traveling modes of the RF waveguide. In the known approach, a circularly polarized patch antenna is used which is positioned at the open end of the cylindrical bore of the MR imaging apparatus. A problem of this setup is that it disables open access to the inner bore. This is critical for patient monitoring and patient accessibility which both are particularly important in MR imaging and MR guided medical interventions.
From the forgoing it is readily appreciated that there is a need for an improved MR imaging system. It is consequently an object of the invention to provide an MR imaging system having a large and easily accessible inner bore. Moreover, the MR imaging system shall enable high quality MR imaging at a high main magnetic field strength.
In accordance with the present invention, an MR imaging system is disclosed which comprises a main magnet for generating a uniform, steady magnetic field within an examination volume. The MR system further comprises an RF waveguide for guiding traveling RF waves along an axis of the examination volume in at least one traveling mode of the RF waveguide. Further, the system comprises at least one RF antenna for transmitting RF pulses to and/or receiving MR signals from a body of a patient positioned in the examination volume, wherein the RF antenna is configured to couple to the at least one traveling mode of the RF waveguide, wherein the RF antenna is located on the imaging system such that the examination volume is freely accessible, i.e. the inner bore of the magnet comprising the examination volume is open accessible. Further, the system comprises a control unit for controlling the temporal succession of RF pulses and a reconstruction unit for reconstructing an MR image from the received MR signals.
This MR imaging system uses the above described traveling wave concept. The traveling wave concept enables high quality MR imaging at high magnetic field strength using wide bore magnet systems. In the conventional traveling wave approach, the RF antenna is placed outside the examination volume at the open end of the magnet bore in order to exploit the fact that the traveling waves can be generated and detected at a distance from the examined patient. However, open access to the inner bore of the magnet is obstructed in this way. According to the invention, in contrast, the RF antenna coupling to the traveling mode of the RF waveguide is either located within the examination volume in such a manner that the examination volume is freely accessible or the RF antenna is even located outside the examination volume, for example at some distance to the open end of the magnet bore such that the inner bore of the magnet is not obstructed in any way by the RF antenna.
As a consequence, the MR imaging system of the invention enables full access to the examination volume which is a substantial advantage over the conventional setup. Moreover, an improvement of the RF homogeneity can be achieved by placing the RF antenna within the examination volume according to the invention, for example by using a correspondingly optimized design of a (multi-element) RF antenna. In-bore antennas need not to be multi-element devices per se.
In comparison to the prior art, the MR imaging system of the invention provides thus more space within the magnet bore. This is advantageous for interventional applications and enables a patient-friendly design.
According to a preferred embodiment of the invention, the RF antenna is located beneath or integrated into a patient table of the MR imaging system. Optionally, the RF antenna may be located in a recess of the gradient coil of the system or an RF shield of the system or it may even be integrated into the RF shield or gradient coil of the system itself. As a consequence, in the MR imaging system according to the invention the conventional head or body RF antennas coupling to the near field of the examined body can be completely dismissed with. These conventional RF antennas, typically birdcage or TEM (transversal electromagnetic) resonators closely surround the body of the patient and thereby limit the free space within the magnet bore. An increase of the free bore diameter is achieved by placing the RF antenna for transmitting RF pulses to and/or receiving RF signals from the body of the patient in or beneath the patient table, in recesses of the RF shield or gradient coil or integrate the RF antenna even into the RF shield and/or the gradient coils. It has to be noted that the free bore diameter may even be significantly increased using a traveling wave approach and omitting a conventional volume transmitter like a quadrature body coil for magnetic field strengths beyond 3 T. The reason is that conventional (near-field) transmitters do not yield the desired homogeneous excitation at field strengths beyond 3 T.
Preferably, the at least one RF antenna of the MR imaging system according to the invention is formed by an electrically conductive plate in which at least one recess is left open. The recess may be, for example, slot-shaped. In general, a slot-line antenna may be used which may be for example realized as a slit in a metallic plate, a slitted metallic box (cavity backed slit), a slitted waveguide structure, an antenna array using a number of slits in one of the above mentioned possible designs or even a curved or arbitrarily shaped slit for generation of a desired field shape in a desired region of interest.
The electromagnetic field distribution at the edges of recesses (or slits) causes the emission of electromagnetic radiation which is coupled into the RF waveguide. A traveling wave transmit and/or receive RF antenna can be realized by an array of elongate slot-shaped recesses within the conductive plate. Such an RF antenna does not necessarily require discreet tuning capacitors. The tuning to the MR resonance frequency can be achieved by means of appropriate capacitive or low-loss dielectric loading and/or by geometrical design. The arrangement of the slot-shaped recesses within the conductive plate enables the optimization of the RF coverage and homogeneity within the examination volume. The conductive plate may for example be curved matching the curvature of the inner bore of the MR magnet system in order to optimally adapt to the MR system regarding a maximum space available in the examination volume.
It has to be noted here that the mentioned RF antenna which is formed by an electrically conductive plate in which at least one recess is left open, does not necessarily require the presence of an RF waveguide for guiding traveling RF waves along an axis of the examination volume in at least one traveling mode of the RF waveguide. By choosing appropriate spatial and electrical properties of the slot-line antenna, this antenna can be used in state of the art MR imaging systems without the presence of additional RF waveguides. Consequently, such slot-line antennas may be used to replace conventional RF antennas. However it has to be noted that also a combination with conventional RF antennas is possible.
For this reason, the invention also relates to an RF antenna for an MR imaging system wherein the RF antenna is formed by an electrically conductive plate comprising at least one recess. The invention also relates to an MR system imaging system which comprises a main magnet for generating a uniform, steady magnetic field within an examination volume, wherein the system further comprises at least one RF antenna for transmitting RF pulses to and/or receiving MR signals from a body of a patient positioned in the examination volume, wherein the RF antenna is formed by an electrically conductive plate comprising at least one recess. Further, the system comprises a control unit for controlling the temporal succession of RF pulses and a reconstruction unit for reconstructing an MR image from the received MR signals. In this case, a conventional MR imaging system being defined such that no traveling wave can propagate in its bore at the Larmor frequency may be employed. I.e. the traveling wave concept is optional in this case. Nevertheless, all concepts described throughout the description regarding an RF antenna formed by an electrically conductive plate comprising at least one recess in combination with the traveling wave approach may be used in a conventional MR imaging system being defined such that no traveling wave can propagate in its bore at the Larmor frequency.
Generally, an in bore transmit/receive slot-line resonator array may be used. Consequently, an antenna pattern may be provided which consists of an array of slot-line structures which require only very few or even no discreet tuning elements, like capacitors. The entire RF current flows over a broad distributed surface instead along discreet strips. The surface may be tuned via capacitive or low-loss dielectric loading, mechanical tuning or electrical tuning using e.g. (PIN)-diodes. A combination of these methods is also possible.
The slot-line concept can be combined with conventional near field coil elements and increases design freedom of the RF system and gradient coils. The slot-line antenna can be driven as a conventional mutual coupled volume resonator or as a multi-transmit coil array. Besides replacing the body coil in an MR system, slot-line antennas may also be used as surface (TxRx) (transmission/reception) array coils or insert volume coils, e.g. for head imaging.
This has the advantage that MR systems can be provided at lower costs. Since slot-line antennas require only a minor amount of space, this also results in more space in the bore of the MR system (i.e. in the examination volume) for example for interventional applications and patient-friendly designs.
According to a preferred embodiment of the invention, the geometry, i.e. the shape, size and/or position of the at least one recess is variable. This can be achieved for example mechanically. To this end, the conductive plate may comprise a number of plate sections that are movable relatively to each other. Alternatively, the at least one recess of the conductive plate may be bridged, as mentioned above, by one or more switchable PIN-diodes and/or one or more capacitors in order to modify the effective geometry of the recesses. This variability of the RF antenna can be used for tuning purposes as well as for the purpose of optimizing the RF field distribution within the examination volume, which is called RF shimming.
Using more than one slot forming a multi-element transmit system, the position, size and shape of the slots may be chosen such that an improved RF coverage, improved homogeneity in a given region of interest and/or improved, i.e. reduced specific absorption rate (SAR) is resulting.
In accordance with a further embodiment of the invention, the MR system further comprises a number of gradient coils for generating switched magnetic field gradients in different spatial directions within the examination volume, wherein the gradient coils comprise electrical conductors arranged on or in a curved body at least partially encompassing the examination volume, the conductive plate of the RF antenna being curved in a manner matching the curvature of the curved body, wherein the RF antenna is positioned contiguous to the curved body. In this embodiment, the gradient coils of the MR imaging system comprise electrical conductors arranged on or in a curve, for example cylindrical, body at least partially encompassing the examination volume, wherein the RF antenna is shaped corresponding to the shape of the gradient coil and is arranged contiguous to the gradient coil in order to obtain a maximum free space within the inner bore of the magnet. The RF antenna may be located, for example in a recess formed in the body of the gradient coil, as already mentioned above. An increased open access to the magnet bore is provided in this way.
Preferably, the curved gradient coil body is split (or partially split) along the axis of the examination volume. The recess in the conductive plate may be formed in this embodiment as a circumferential slot running along the gap between the split parts of the gradient coil body. An RF antenna with a dipole-like characteristic is obtained in this way, wherein the dipole axis parallels the longitudinal axis perpendicularly to the longitudinal axis of the magnet bore.
In accordance with a further embodiment of the invention, the RF antenna is tuned to an RF frequency using only non-discreet elements. This tuning may be even a static or dynamically achievable tuning, for example as mentioned above by means of mechanically movable elements which vary the size of the recess. However, in general in case the slot-line structure for a given frequency is realized without using any discreet elements like capacitors, the production costs of such a slot-line antenna are kept rather low. Further, the risk of failure of electrical elements is minimized. Changing of the antenna properties of the slot-line structure may even be achieved by varying the spatial dimensions of the structure (as mentioned above) or by providing dielectric materials to the structure. For example, the antennas may be individually loaded inside or on a dielectric material. The antenna may also be made from other materials than metal, for example from artificial magnetic conductors (AMCs) or even from carbon nanotubes.
In accordance with a further embodiment of the invention, a slot-line array structure of the antenna may be combined with local surface receive coils. Also a combination with transmit or receive coils tuned at lower frequencies is possible, wherein one coil may be for example used for fluorine MR imaging and the other coil may be used for proton MR imaging. Further, slot-line antennas, dipole antennas, TEM antennas, patch antennas and loop elements may be mixed in a suitable manner in order to obtain optimized RF transmission or reception capabilities of the MR system. This may be combined with multi-resonance excitation patterns, active slit length tuning using PIN-diodes for active shimming and further, as mentioned above, RF shimming by variable dielectric loading of the cylindrical bore.
In accordance with a further embodiment of the invention, the RF antenna is a directional antenna, wherein the directional antenna comprises directional antenna characteristics directed towards the examination volume.
While a conventional (body-) coil couples to the reactive near field of the sample and is thus loaded by sample losses, a propagation wave excitation antenna couples to a mode of the cylindrical waveguide of the MR system. While conventional MR coils are operated in the near field regime inside a cylindrical conducting bore at ultra-high magnetic fields, the cylinder itself acts as a waveguide as soon as the MR frequency is below the cut-off frequency of the waveguide. The electromagnetic energy is then transported through the cylinder by a traveling wave. The cut-off frequency of a given cylindrical bore can be considerably lowered by a dielectrical loading.
In order to realize traveling wave excitation in a bore, which initially does not allow wave propagation, dielectric filling with material of high permittivity on the surface of the cylinder or partly below the patient support may be applied. The presence of the patient body further reduces the cut-off frequency due to the additional dielectric loading effect. By using a directional RF antenna which is located on the imaging system such that the examination volume is freely accessible, for example outside the MR bore comprising the examination volume, a traveling wave is excited inside the examination volume of the MR system.
Having an antenna gain substantially larger than one in the direction of the main beam, also an array of directional antennas may be used to lower the RF amplifier power needed for a given B1 field in the bore. Moreover, the directional antenna characteristics allows for tailoring the excitation conditions of such MRI systems.
Furthermore, such an antenna system may also be integrated into the bore or placed at the edges of the magnet. Antenna elements may for example be located at the service end of the scanner still allowing free access to the bore. This may provide a significant gain in the bore diameter while keeping the system compact.
In accordance with a further embodiment of the invention, the RF antenna comprises a periodic antenna structure providing said antenna characteristics directed towards the examination volume.
In accordance with an embodiment of the invention, the MR system may comprise a phased array of RF antennas which has the advantage of providing optimized transmission and reception capabilities by such antennas. Consequently, an excitation field in the examination volume may be formed by external antenna design.
In accordance with a further embodiment of the invention, the RF antenna is a Yagi type antenna or a helically structured antenna. Further, the antenna may be a dipole and/or quarter wave line structured antenna.
A further advantage of such directional antennas is that these antennas comprise a simpler and less expensive antenna structure even without any capacitors. The antenna size may be adapted in a desired manner by individually loading the antennas for example inside or on a dielectric material. It has to be noted that a combination of a traveling wave antenna setup with conventional RF antennas is possible in order to achieve a combination of traveling/propagating mode excitation and conventional near field excitation. For example, in a phase locked mode the traveling mode antennas may be used for ‘base’ polarization purposes and local antennas like for example TEM or strip-line antennas may be used for additional purposes like for example RF shimming. Furthermore, RF shimming is possible by variable dielectric loading of the cylindrical bore.
In accordance with a further embodiment of the invention, resonant passive antenna structures may be used close to the examination volume in order to facilitate traveling wave propagation. For example slots or dipoles may be used in the patient table as resonant structures in order to provide a locally enhanced B1 which permits reducing the power required for driving the traveling wave antenna setup.
In accordance with a further embodiment of the invention, the traveling wave antennas may be hidden under the cover of the MR magnet or even may be integrated into the gradient coils of the MR system.
In accordance with a further embodiment of the invention, the RF room enclosing the MR magnet, as well as the MR magnet itself has RF absorbing properties in order to avoid unwanted reflections of RF waves in case the RF antenna is located outside the examination volume and even outside the MR magnet itself.
External traveling wave antennas may be driven with amplifiers located on or near the antennas themselves, allowing for the construction of compact antenna setups.
According to yet another preferred embodiment of the invention, the RF waveguide is formed by an open-ended tube surrounding the examination volume. The tube defines the magnet bore of the MR imaging system. The tube may have a circular or elliptical shape. The tube acts as a waveguide provided that the MR frequency is beyond a cut-off frequency determined by the dimensions of the tube. This may be the case at high magnetic field strength and large inner bore diameters. The electromagnetic energy of the RF fields generated within the bore is then transported through the tube by traveling waves. For example, an electrically conductive screen or mesh lining the inner bore of the magnet may be used as a waveguide according to the invention.
The tube may comprise an electrically conductive pattern structure so as to enable guiding of the traveling RF waves in a selected traveling mode. The electrically conductive pattern determines the current path within the waveguide. The propagation of undesirable higher order modes can be suppressed in this way.
According to still a further preferred embodiment of the invention, the MR imaging system comprises at least one surface antenna located within the examination volume for receiving MR signals from a limited region of the body. In this way, traveling wave RF excitation can be combined with local detection of MR signals, for example by means of an array of conventional (tunable) RF surface coils coupling to the near field of the examined body. This hybrid approach provides additional degrees of freedom in the design of the RF system of the MR imaging apparatus and advantageously combines the improved RF coverage and homogeneity of traveling wave excitation with the high sensitivity of close range detection via RF surface antennas.
It has to be mentioned, that both, the slot-line structured antenna as well as the traveling wave antenna may be used for either RF excitation purposes, receiving MR signals after excitation or a combination thereof.
The enclosed drawings disclose preferred embodiments of the present invention. It should be understood however, that the drawings are designed for the purpose of illustration only and not as a definition of the limits of the invention.
In the drawings:
With reference to
A magnetic resonance generation and manipulation system applies a series of RF pulses and switched magnetic field gradients to invert or excite nuclear magnetic spins, induce magnetic resonance, refocus magnetic resonance, manipulate magnetic resonance, spatially or otherwise encode the magnetic resonance, saturate spins and the like to perform MR imaging.
More specifically, a gradient pulse amplifier 3 applies current pulses to select ones of whole body gradient coils 4, 5 and 6 along x, y and z-axis of the examination volume. An RF transmitter 7 transmits RF pulses or pulse packets, via a send/receive switch 8, to an RF antenna 9 to transmit RF pulses into the examination volume. A typical MR imaging sequence is composed of a packet of RF pulse sequences of short duration which taken together with each other and any applied magnetic field gradients achieve a selected manipulation of nuclear magnetic resonance. The RF pulses are used to saturate, excite resonance, invert magnetization, refocus resonance, or manipulate resonance and select a portion of a body 10 positioned in the examination volume. The MR signals may also be picked up by the RF antenna 9.
For generation of MR images of limited regions of the body 10, for example by means of parallel imaging, a set of local array RF coils 11, 12 and 13 are placed contiguous to the region selected for imaging. The array coils 11, 12 and 13 can be used to receive MR signals induced by RF transmissions effected via the RF antenna. However, it is also possible to use the array coils 11, 12 and 13 to transmit RF signals to the examination volume.
The resultant MR signals are picked up by the RF antenna 9 and/or by the array of RF coils 11, 12 and 13 and are demodulated by a receiver 14 preferably including a preamplifier (not shown). The receiver 14 is connected to the RF coils 9, 11, 12 and 13 via a send/receive switch 8.
A host computer 15 controls the gradient pulse amplifier 3 and the transmitter 7 to generate any of a plurality of imaging sequences, such as echo planar imaging (EPI), echo volume imaging, gradient and spin echo imaging, fast spin echo imaging and the like. For the selected sequence, the receiver 14 receives a single or a plurality of MR data lines in rapid succession following each RF excitation pulse. A data acquisition system 16 performs analogue to digital conversion of the received signals and converts each MR data line to a digital format suitable for further processing. In modern MR devices the data acquisition system 16 is a separate computer which is specialized in acquisition of raw image data.
Ultimately, the digital raw image data is reconstructed into an image representation by a reconstruction processor 17 which applies a Fourier transform or other appropriate reconstruction algorithms. The MR image may represent a planar slice through the patient, an array of parallel planar slices, a three-dimensional volume, or the like. The image is then stored in an image memory where it may be accessed for converting slices, protections or other portions of the image representation into appropriate formats for visualization, for example via a video monitor 18, which provides a man-readable display of the resultant MR image.
Also shown in
With further reference to
Consequently, the RF antenna 9 is located on the imaging system such that the examination volume 21 is freely accessible.
This has the advantage that the examination volume and thus the patient 10 is freely accessible from both, the left side 208 and the right side 210 of the MR system 1, wherein the left and right side 208 and 210 are defined in
Again, it has to be mentioned that the slot-line antenna 200 may also be applied and used without the presence of the RF waveguide 19. The waveguide is thus optional in this embodiment.
An embodiment of the slot structure 200 (see also
The antenna element depicted in
It has to be noted again, that the geometry of the RF antenna 9 is adapted in such a manner that the examination volume 21 is freely accessible and not blocked by the RF antenna 9 in order to gain free access to the examination volume and thus to the patient 10.
As can be further seen in
In
This principle can also be applied to the recess 23 which may be extended in length, as indicated by the dashed lines 704. By bridging this extended part 704 of the recess 23 by further appropriate capacitors 706, this part 704 can be made non-resonant thus also acting as Eddy current barrier. Consequently, only the recess 23 is resonant in a desired manner.
It has to be mentioned that in case multiple slot-line antennas are used for providing multi-element transmit/receive capabilities, a decoupling network between the individual antennas may be inserted realizing a suitable impedance between recesses of individual slot-line antennas. Alternatively, inductive decoupling may also be used for this purpose.
An alternative embodiment is shown with respect to
It has to be noted that any of the above mentioned slot-line array structures and incorporations in the MR system may be combined with local surface receive coils as known in the art.
As can be seen from
In accordance with a further embodiment of the invention, the open ends of the magnet are inclined, wherein the antenna 1400 may be comprised on the surface 1406 of the inclined parts of the magnet 2. Again, the antenna 1400 is not blocking the open ends 1402 or 1404 of the magnet 2 thus permitting a free access to the examination volume 21.
The antennas 1400 depicted in
In
| Number | Date | Country | Kind |
|---|---|---|---|
| 09171488.1 | Sep 2009 | EP | regional |
| 10151987.4 | Jan 2010 | EP | regional |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/IB2010/054136 | 9/14/2010 | WO | 00 | 3/8/2012 |
