Abstract
The radio frequency (RF) antenna assembly has sets of antenna conductors that leave an opening between the sets. A radiotherapy beam path may pass through the opening so that the antenna conductors are at most minimally exposed to the radiation. Each set of antenna conductors has a surface conductor loop and a transverse conductor loop. The surface conductor loop is arranged on cylindrical surface and generates an RF field mostly in its axial range. The transverse conductor loop extends radially and generates an RF field in the axial range of the opening. In this way a homogeneous RF field within the RF antenna assembly.
Claims
1. A radio frequency antenna assembly comprising a plurality of sets of antenna conductors arranged in groups on a cylindrical surface and said groups of sets being mutually axially offset leaving an axially and angularly extending opening between the sets of antenna conductors each of the sets of antenna conductors including a surface conductor loop having its area in the cylindrical angular and axial directions and at least one transverse conductor loop having its area extending radially with respect to the cylindrical surface.
2. A radio frequency antenna assembly as claimed in claim 1, wherein in individual sets of antenna conductors a cylindrical area section forming the surface conductor loop and connected to a transverse protrusion section forming the transverse conductor loop are provided, where the cylindrical area section is arranged on the cylinder surface and the transverse protrusion section extends radially.
3. A radio frequency antenna assembly as claimed in claim 2, where in the cylindrical area section and the transverse protrusion section form a single electrically conducting loop.
4. A radio frequency antenna assembly as claimed in claim 1, wherein in individual sets of antenna conductors two transverse coil loops are associated with the surface conductor loop.
5. A radio frequency antenna assembly as claimed in claim 1, wherein the transverse coil loop extends axially up to the opening.
6. A radio frequency antenna assembly as claimed in claim 1, wherein the surface conductor extends axially beyond the transverse coil loop towards the RF antenna assembly's axial ends.
7. A radio frequency antenna assembly as claimed in claim 1, wherein the transverse coil loops from a TEM resonator.
8. A radio frequency antenna assembly as claimed in claim 1, wherein the surface conductor loop is a surface coil loop.
9. A radio frequency antenna assembly as claimed in claim 1, further comprising a RF screen and wherein the transverse coil loop is an electrically conducting strip that is electrically connected to the RF screen.
10. A radio frequency antenna arrangement comprising an anterior radio frequency antenna assembly as claimed in claim 1, and a posterior radio frequency antenna, wherein the radius of curvature of the cylinder surface of the posterior radio frequency assembly is different form the radius of curvature of the cylinder surface of the anterior radio frequency assembly.
11. A magnetic resonance examination system comprising a patient carrier with a support face, wherein a radio frequency antenna assembly of claim 1 is mounted at the patient carrier's side opposite its support face, or is integrated in the patient carrier.
12. A magnetic resonance examination system as claimed in claim 11, in which the radio frequency assembly is mounted to or integrated moveably with respect to the patient carrier.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] FIG. 1 shows a three-dimensional schematic view of an embodiment of the RF antenna assembly of the invention,
[0028] FIGS. 2 and 3 show a schematic views of examples of a set of antenna conductors for the RF antenna assembly of the invention FIGS. 4 and 5 show a schematic views of an another example of several sets of antenna conductors for the RF antenna assembly of the invention,
[0029] FIGS. 6 and 7 show diagrammatic representation if the RF field distribution of examples of sets of antenna elements of the RF antenna assemblies of the invention and
[0030] FIG. 8 shows a schematic front view of a magnetic resonance examination system in which the RF antenna assembly of the invention is incorporated
[0031] FIG. 9 shows a diagrammatic representation of an magnetic resonance examination system in which the invention is incorporated.
DETAILED DESCRIPTION OF THE EMBODIMENTS
[0032] FIG. 1 shows a three-dimensional schematic view of an embodiment of the RF antenna assembly of the invention. The RF antenna assembly of the invention has a number of sets 10 of antenna conductors. The antenna conductors form the antenna elements that constitute the resonant structure. The sets of antenna conductors 10 leave the angularly and axially extending opening 40. The opening is left between groups of the sets of antenna elements. In the example shown there are twelve of these sets of antenna conductors 11,13; six sets are arranged at either sides of the opening 40. Each of these six sets forms a group of sets of antenna conductors. The sets are axially displaced so as to leave the opening between them. These sets of antenna elements are arranged on a cylindrical surface 12 that is schematically partially indicated in FIG. 1. The cylindrical surface may be a cylindrical coil former that carries the antenna conductors. The cylinder axis 21 is orientated axially. When in use in an magnetic resonance examination system the cylinder axis is orientated along the direction of the magnetic resonance examination system's main magnetic field. the sets of antenna conductors are radially 22 apart so as to leave an examination zone 23 within the cylindrical surface. When in use, the RF antenna assembly can generate the dynamic RF (B.sub.1) field in the examination zone. When in use in an MR image guide therapy system, a therapeutic radiation beam, such as a beam of -rays, high-energy x-rays, protons or high-intensity focused ultrasound passes through the opening 40 towards a target region of a patient to be treated in the examination zone 23. To that end the RF antenna assembly of the invention is mounted in the MR image guided system such that the beam path of the therapeutic radiation beam passes through the opening.
[0033] The antenna conductors of the RF antenna assembly of the invention are configured to generate a spatially homogeneous B.sub.1-field distribution in the examination zone, while there are no antenna conductors in the axially and angularly extending opening. Each of the sets of antenna conductors includes a surface conductor loop 11 with one or more transverse conductor loops 13. In the example shown in FIG. 1, two transverse conductor loops are associated with each single surface conductor. The surface conductor loop is a flat conductor loop that is arranged generally in the cylindrical surface. The surface conductor loop generates the B.sub.1-field in the examination zone in the axial and angular range over which the surface conductor loop extends and along a radial range towards the cylinder axis. The transverse conductor loop extends radially and generates a B.sub.1-field component that extends axially from the transverse coil conductor. The B.sub.1-field contribution from the surface conductor loop and the transverse coil conductor(s) of the sets of antenna conductors combine to a spatially uniform B.sub.1-field distribution in the examination zone. The B.sub.1-field in the examination zone is controlled by controlling the (phase and amplitude, frequency, time, waveform) of AC electrical currents applied to the surface conductor loops and the transverse conductor loops. Most detailed control is achieved by independent control of electrical currents to each individual surface and each transverse conductor loop. Quite detailed control is achieved when electrical currents to each of the sets of antenna conductors is independently controlled.
[0034] As shown in FIG. 1, the surface conductor loop is formed as axially elongate conductor loop having its loop area in the cylinder surface. The transverse conductor loops are each transverse conductor loops that extend radially and axially. The transverse conductor loop have their loop areas extending radially and extend axially elongate.
[0035] FIGS. 2 and 3 show a schematic views of examples of a set of antenna conductors for the RF antenna assembly of the invention. FIG. 2 shows an example of a set 10 of antenna conductors with one surface conductor loop 11 and two transverse coil conductors 131, 133. The transverse conductor loop 131,133 extends axially all the way up to the axial boundary of the opening 40. That is, the boundary of the opening is formed by the angular conductor 111 and the radial conductors 135 at the side of the opening 40. At the axial end of the transverse coil conductor facing away from the opening 40, the surface conductor loop 11 extends axially beyond the axial extension of the transverse coil conductor 131 133. That is, the transverse coil conductors do not extend all the way to the axial end of the RF antenna assembly. Thus, the B.sub.1-field component of the transverse coil conductor extends well into the opening to contribute the B1-field in axial range of the opening 40, while the transverse coil conductor does not generate an appreciable B.sub.1-field component extending axially outside of the RF antenna assembly. Thus, no undesired stray B.sub.1-field is produced and the power efficiency of the RF antenna assembly is improved.
[0036] In FIG. 3, an example of a set of antenna conductors for the RF antenna assembly of the invention is shown. Here the transverse conductor loop 13 is a transverse conductor loop that is elongate along the angular direction of the cylinder surface of the RF antenna assembly and extends radially. The transverse conductor loop is located at the edge of the opening 40, i.e. near the angular conductor 111 of the surface coil loop 11. This transverse conductor loop predominantly generates its B.sub.1-field into the axial direction into the opening 40. Decoupling of the transverse conductor loops is performed by capacitive and/or inductive decoupling. Capacitive decoupling is realized by connecting the protrusion sectors in parallel and using a common capacitor. Inductive decoupling. can be realized in a similar approach.
[0037] FIG. 4 shows a schematic view of an another example of several sets of antenna conductors for the RF antenna assembly of the invention. In this version each set is formed by a single conductor loop having a cylindrical area section 15 that is arranged in the cylinder surface and a transverse protrusion section 17 that extends at an angle to the area of the cylindrical area section 15. This single conductor loop may be combined with a transverse conductor loop 135 as shown with only one of the single conductor loops in FIG. 4. Preferably the transverse protrusion is along the radial direction, i.e. perpendicular to the area of the cylindrical area section. The transverse protrusion may be transverse to the cylindrical area section at an angle in the range of 60-120, while very good results are achieved when =90. This set of antenna conductors is formed from a single conductor loop and is easy to manufacture. Because only the electrical current to each set of conductors, viz. the single conductor loop is applied, the electrical currents to each of the sets of antenna elements is simple to control. The transverse protrusion section generates its B.sub.1-field component that extents predominantly into the axial range of the opening 40. The cylindrical area section generates its B.sub.1-field component predominantly along the axial range of the elongate cylindrical area section. In order to achieve proper resonant properties, tuning capacitors 31 are provided in the cylindrical area sections 15. Decoupling capacitors 32 link adjacent protrusion sections for capacitive decoupling of the protrusion sections 17.
[0038] In the embodiment of FIG. 5, the decoupling inductances 42 inductively decouple the protrusions 17. Decoupling inductances 32 link adjacent protrusions for inductively decoupling the protrusions.
[0039] FIGS. 6 and 7 show diagrammatic representations of the RF field distribution of examples of sets of antenna elements of the RF antenna assemblies of the invention. FIG. 6 shows diagrammatically the B.sub.1-field distribution in the angular and radially directions of one of the sets of antenna conductors 15,17 of FIG. 4. FIG. 6 shows a diagrammatic representation of the B1-field distribution in a lateral plane through the cylinder axis 21 in a plane 10 cm below the coil loop. FIG. 6 shows that the B.sub.1-field extends axially into the opening 40 at negative z-values). At the opposite axial end of the cylindrical area section 15 (positive z-values) the B.sub.1-field extends hardly or not at all beyond the axial extension of the RF antenna assembly. FIG. 7 shows diagrammatically the B.sub.1-field distribution in the angular and radially directions of one of the sets of antenna conductors 15,17 of FIG. 3. The phase of the electrical current applied to the surface conductor loop 11 and the transverse conductor loop 13 can be adjusted. In this particular simulation the electrical current in the vertical, smaller transverse loop 13 was 3.75 times that of the larger surface coil loop 11. The lateral deviation of the field generation inside the gap appears to be controlled by the current ratio and the phase of the current intended. Allowing one more degree of freedom by changing also the phase the current ratio (more difficult to realize), this can be improved as shown in FIG. 7. This field plot was generated by choosing amplitude ratio of 3.75, together with the phase ratio of 45.
[0040] The B.sub.1-field is shown to extend into the opening 40. The angular distribution of the B.sub.1-field, notably, the predominant axis 120 along which the B.sub.1-field extends from the set of conductors 10 is orientated with respect to the axial direction is determined by the phase difference between the electrical current applied to the surface conductor loop and the transverse conductor loop.
[0041] FIG. 8 shows a schematic front view of a magnetic resonance examination system in which the RF antenna assembly of the invention is incorporated. The magnetic resonance examination system includes a gantry 50 in which the magnet system with a main magnet and gradient system are mounted. In the gantry 50 the patient carrier 31, such as a patient table is mounted. The patient carrier has a support surface 32 on which a patient to be examined (and/or treated) is positioned. The patient carrier is axially moveable along the axis 21. The axis 21 is along the direction of the main magnetic field. The RF antenna element has an anterior 241 and posterior 242 antenna assembly located at opposite sides of the patient carrier. The anterior RF antenna assembly is mounted at the side of the support surface 32 onto which the patient to be examined is placed. The posterior RF antenna assembly is located at the opposite side of the patient carrier, generally that is underneath the patient carrier 31. The cylinder surface 12 of the anterior RF antenna assembly and of the posterior RF antenna assembly have different radius of curvature. This allows to make efficient use of the available bore space in the main magnet. Further, the RF assembly is configured in this way to correspond to the cross-sectional shape of the patient. This correspondence can be further improved by disposing the sets of antenna elements on a flexible carrier.
[0042] In the example of FIG. 8, the surface conductors 11 of the sets 10 of antenna conductors are formed as electrically conducting strips that are axially orientated. The RF antenna assembly further comprises a radio frequency (RF) screen located radially from the sets of antenna elements. The electrically conducting strips are coupled to the RF screen which provides for a return current path. This coupling may be galvanic, inductive or capacitive. The RF antenna elements with eh RF screen are electrically circuited as a TEM resonator.
[0043] FIG. 9 shows diagrammatically more details of a magnetic resonance imaging system in which the invention is used. The magnetic resonance imaging system includes a main magnet with a set of main coils 10 whereby the steady, uniform magnetic field is generated. The main coils are constructed, for example in such a manner that they from a bore to enclose a tunnel-shaped examination space. The patient to be examined is placed on a patient carrier which is slid into this tunnel-shaped examination space. The magnetic resonance imaging system also includes a number of gradient coils 111, 112 whereby magnetic fields exhibiting spatial variations, notably in the form of temporary gradients in individual directions, are generated so as to be superposed on the uniform magnetic field. The gradient coils 111, 112 are connected to a gradient control 121 which includes one or more gradient amplifier and a controllable power supply unit. The gradient coils 111, 112 are energised by application of an electric current by means of the power supply unit 121; to this end the power supply unit is fitted with electronic gradient amplification circuit that applies the electric current to the gradient coils so as to generate gradient pulses (also termed gradient waveforms) of appropriate temporal shape. The strength, direction and duration of the gradients are controlled by control of the power supply unit. The magnetic resonance imaging system also includes transmission and receiving antennae (coils or coil arrays) 113, 116 for generating the RF excitation pulses and for picking up the magnetic resonance signals, respectively. The transmission coil 113 is preferably constructed as a body coil 13 whereby (a part of) the object to be examined can be enclosed. The body coil is usually arranged in the magnetic resonance imaging system in such a manner that the patient 30 to be examined is enclosed by the body coil 113 when he or she is arranged in the magnetic resonance imaging system. The body coil 13 acts as a transmission antenna for the transmission of the RF excitation pulses and RF refocusing pulses. Preferably, the body coil 113 involves a spatially uniform intensity distribution of the transmitted RF pulses (RFS). The same coil or antenna is generally used alternately as the transmission coil and the receiving coil. Typically, a receiving coil includes a multiplicity of elements, each typically forming a single loop. Various geometries of the shape of the loop and the arrangement of various elements are possible The transmission and receiving coil 113 is connected to an electronic transmission and receiving circuit 115.
[0044] It is to be noted that is that there is one (or a few) RF antenna elements that can act as transmit and receive; additionally, typically, the user may choose to employ an application-specific receive antenna that typically is formed as an array of receive-elements. For example, surface coil arrays 116 can be used as receiving and/or transmission coils. Such surface coil arrays have a high sensitivity in a comparatively small volume. The receiving coil is connected to a preamplifier 123. The preamplifier 123 amplifies the RF resonance signal (MS) received by the receiving coil 116 and the amplified RF resonance signal is applied to a demodulator 124. The receiving antennae, such as the surface coil arrays, are connected to a demodulator 124 and the received pre-amplified magnetic resonance signals (MS) are demodulated by means of the demodulator 124. The pre-amplifier 123 and demodulator 124 may be digitally implemented and integrated in the surface coil array The demodulated magnetic resonance signals (DMS) are applied to a reconstruction unit. The demodulator 124 demodulates the amplified RF resonance signal. The demodulated resonance signal contains the actual information concerning the local spin densities in the part of the object to be imaged. Furthermore, the transmission and receiving circuit 115 is connected to a modulator 122. The modulator 122 and the transmission and receiving circuit 115 activate the transmission coil 113 so as to transmit the RF excitation and refocusing pulses. In particular the surface receive coil arrays 116 are coupled to the transmission and receive circuit by way of a wireless link. Magnetic resonance signal data received by the surface coil arrays 116 are transmitted to the transmission and receiving circuit 115 and control signals (e.g. to tune and detune the surface coils) are sent to the surface coils over the wireless link.
[0045] The reconstruction unit derives one or more image signals from the demodulated magnetic resonance signals (DMS), which image signals represent the image information of the imaged part of the object to be examined. The reconstruction unit 125 in practice is constructed preferably as a digital image processing unit 125 which is programmed so as to derive from the demodulated magnetic resonance signals the image signals which represent the image information of the part of the object to be imaged. The signal on the output of the reconstruction is applied to a monitor 126, so that the reconstructed magnetic resonance image can be displayed on the monitor. It is alternatively possible to store the signal from the reconstruction unit 125 in a buffer unit 127 while awaiting further processing or display.
[0046] The magnetic resonance imaging system according to the invention is also provided with a control unit 120, for example in the form of a computer which includes a (micro)processor. The control unit 120 controls the execution of the RF excitations and the application of the temporary gradient fields. To this end, the computer program according to the invention is loaded, for example, into the control unit 120 and the reconstruction unit 125.
