Patent Description:
Magnetic resonance imaging (MRI) is state of the art imaging technology which allows cross-sectional viewing of objects like the human body with unprecedented tissue contrast. MRI is based on the principles of nuclear magnetic resonance, a spectroscopic technique used by scientists to obtain microscopic chemical and physical information about molecules. The basis of both, nuclear magnetic resonance and MRI is the fact, that atomic nuclei with non-zero spin have a magnetic moment. In medical imaging, for example nuclei of hydrogen atoms are studied since they are present in the body in high concentrations like for example water. The nuclear spin of elementary particles can resonate at a resonant frequency, if a strong DC magnetic field is applied. This magnet resonance (MR) frequency is determined by the level of magnetic flux. In the MRI scanner, the magnetic field matches the selected resonance frequency only at a position in space. Only at this position the presence of these particles can be detected. By varying this position, an image can be measured.

The needed strong DC magnetic field (B0 field) is typically generated by superconducting magnets. In order to vary this field, such that it matches a given radio-frequency only at one position, a field gradient is generated using gradient coils. A field gradient can vary over time to achieve a scan.

To excite nuclear resonances, the RF coil generates a high frequency magnetic field at the nuclear resonance. The magnetic field must direct in a radial direction with respect to the axis of the MRI scanner. To achieve a radial magnetic field in all directions, a rotating field is used, which points in any radial direction at one point of time during one period. This is achieved using for example a so called 'birdcage' arrangement. Currents in opposing slabs of the birdcage flow in opposite direction and thus generate a radial field. Currents in neighbor leg or rung conductors have a phase shift, such that the field rotates.

The coil is generally a highly resonant antenna, designed for generating the well-defined magnetic field inside the human body. As a side effect, electric fields are causing losses which strongly change the input impedance of the coil. This mainly affects the real part of the impedance, the relative change being linked to the resonance equality factor change, also called the load factor. This is typically in the range of <NUM>-<NUM> for today's birdcage resonators which are the preferred implementation for MRI body coils.

The power fed into the body coil is produced by pulsed amplifiers, which demand a good or at least acceptable power matching at their output. Conventional birdcage resonators are directly fed at the coil ports using matching circuits. At one point <NUM> T in quadrature operation, this is typically realized by using a hybrid coupler to drive two quadrature channels of the coil simultaneously. This coupler is a <NUM>-port, which has the coil feeding ports connected to its outputs, the amplifier at one input and a load, typically <NUM> ohm, matching the transmission line impedance, connected to the fourth.

At 3T, the two separate individually transmit channels are connected via a transmit-receive box to the RF input ports of the body coil.

<CIT> relates to a nuclear magnetic resonance inspection apparatus comprising an eradiation coil, a detection coil and analogue-to-digital conversion means for sampling a nuclear magnetic resonance signal and converting the sample signal into a digital signal, and transmission means for transmitting wirelessly the digital signal to a signal processor. The international applicaiotn <CIT> dislsoes an RF coil with conductor loops on a conformal surface that substantially conforms with the subject to be imaged (for example the shoulder of a patient to be examined) The conductor loops form a resonant structure. This resonant structure is realised by coupling of the conductor loops which may be galvanically or inductively or both.

Embodiments of the invention provide for a magnetic resonance imaging system comprising a main magnet for generating a main magnetic field within an imaging zone, a radio-frequency, RF, antenna, comprising an RF input terminal and an RF output terminal, an RF system for supplying radio-frequency power to the RF input terminal to energize the antenna, the antenna being further adapted for picking up magnetic resonance signals from the imaging zone, the antenna e.g. comprising a plurality of coil elements; a data acquisition system for receiving the magnetic resonance signals from the RF output terminal; wherein the RF input terminal is in galvanic connection to the antenna and the RF output terminal is inductively coupled to the antenna.

The term 'galvanic connection' refers to an electrical connection in which the current path of the RF input terminal and the antenna are coupled via a common impedance, such that the respective currents flowing in the antenna and the RF input terminal are flowing over the common impedance. The common impedance may be realized by means of a direct conduction path, i.e. a hardwire electrical connector. However, the impedance may also be realized by means of a coupling of the antenna and the RF input terminal via a capacitance.

In contrast thereto, the term 'inductive coupling' refers to an electrical isolation between the RF output terminal and the antenna such that no direct conduction path is permitted. Currents in the antenna are only inductively generating respective currents in the RF output terminal through electromagnetic induction.

Further, the term 'terminal' relates to an electrical connection point to which electrical energy can be supplied or from which electrical energy can be derived by a direct electrical connection using for example a hardwired electrical connection from e.g. an RF amplifier in case of the RF input terminal and e.g. an RF receiver in case of the RF output terminal.

Embodiments may have the advantage that due to the inductive feeding only for the receive path and galvanic feeding for the transmit path, a separate send-receive box is eliminated which typically has to be able to operate the coil in a respective manner during the transmit phase and the receive phase: it is the goal on one hand to ensure to direct a high powered transmit signal from the RF amplifier to the antenna, while also allowing a low loss connection between the antenna and the RF receiver. The conventional transmit-receive box, also called T/R switch, causes a signal path to be created between the RF amplifier and the antenna whenever a system is in transmit mode, wherein during a receive mode, the T/R switch generates a signal path between the antenna and the RF receiver, for example via a pre-amplifier.

In contrast thereto, by using the inductive feeding only for the receive path and galvanic feeding for the transmit path, the T/R switch (or send-receive box) is eliminated and the overall image quality may be improved. Since the RF output terminal is only inductively coupled to the coil, noise contributions from cable currents and ground loops are omitted, since the transmit path is separate and galvanically coupled to the coil structure such that any B1 excitation is not disturbed. In addition, Tx and Rx paths can be optimized in this way more independently since they do not have to share a common cable anymore (e.g. if the cable length is applied to optimize the matching).

In accordance with an embodiment of the invention, the magnetic resonance imaging system further comprises a pre-amplifier, wherein the inductive coupling of the RF output terminal is comprising an inductive coupling of the pre-amplifier to the antenna and the galvanic connection of the pre-amplifier in the output terminal. This may have the advantage that the RF signal acquired by the antenna can be picked up and amplified by the pre-amplifier at an extremely short distance from the coil since there is no need for respecting any cable length between the antenna and the pre-amplifier that would need to satisfy a certain impedance matching due to a common transmission and receive path shared between a transmit-receive box and the antenna. Further, since the pre-amplifier is inductively coupled to the antenna, the pre-amplifier is not connected to the antenna via ground such that the formation of sheath waves in coaxial cables between the antenna and the RF pre-amplifier are avoided. This may further increase the signal-to-noise ratio at which the pre-amplifier can provide the RF signals to the output terminal.

In accordance with an embodiment of the invention, the RF input terminal and/or the RF output terminal are each comprising an independently adjustable impedance matching circuit for matching the impedance of the respective terminal to a desired impedance. For example the impedance matching circuit of the RF input terminal is providing a matching impedance to the radio-frequency power supplied from the RF system to the RF input terminal and a permanent high impedance to the current of the magnetic resonance signals picked up by the antenna. Typically, the matching impedance will be <NUM> Ohm such that RF power supplied from the RF system to the RF input terminal can be fed to the RF antenna with minimal losses. Any magnetic resonance signals picked up as RF signals by the antenna are blocked from being transmitted towards the RF system due to the high impedance. In contrast thereto, the RF output terminal is matched regarding its impedance to the electrical circuits directly electrically coupled to the RF output terminal and used for transmitting the pre-amplified RF signals (MR signals) to the respective receiving unit. These circuits may include A/D converters and for example an optical or wireless transmitter to optically or wirelessly (using RF transmission) transmit the acquired MR signals to the receiver.

In accordance with an embodiment of the invention, the system further comprises an analogue-to-digital, AD, converter, the analogue port of the AD converter being coupled with the output terminal.

In accordance with an embodiment the invention, the system further comprises a switch for selectively performing the coupling and a decoupling of the antenna with the RF output terminal. In the above example of a pre-amplifier, the switch is selectively performing the coupling and decoupling of the pre-amplifier with the antenna. This may have the advantage that during RF transmission, which is typically performed at rather high power, the RF output terminal or in the above example the pre-amplifier, can be blanked such that an overloading of the pre-amplifier or even the RF receiver behind the RF output terminal is avoided. Thus, this may serve as a protection of electrical components.

In accordance with an embodiment of the invention, the RF antenna comprises a plurality of coil elements, a plurality of the RF input terminals and the RF output terminals, wherein each one of the RF input terminals is galvanically coupled to at least one of the coil elements and each one of the RF output terminals is inductively coupled to at least one of the coil elements. By for example means of an optional selector, a specific one of the RF output terminals can be selected for providing the magnetic resonance signals from said selected RF output terminal to the data acquisition system or receiver. Even though it was mentioned above that the goal is to achieve a radio magnetic field in all directions using a rotating field, in reality the currents in the slabs of the birdcage are not everywhere the same in the birdcage. The currents strongly depend on the load of the coil, for example the patient that is currently imaged using the coil. By means of the selector it is possible to select the one or multiple ones of the RF output terminals that provide the magnetic resonance signals at highest quality.

For example, the system comprises a memory for storing machine-executable instructions and a processor for controlling the magnetic resonance imaging system, wherein execution of the machine-executable instructions causes the processor to control the system to determine which specific one of the RF output terminals provides the magnetic resonance signals with at least a predefined signal-to-noise ratio and to control the selector to select the determined RF output terminal(s).

All this can be performed completely independent of the transmit path since the transmit path is decoupled and independent from the receive path. Especially there are not multiple individual transmit-receive boxes necessary for performing the selection such that the total costs for implementing such a selector for an antenna with multiple coil elements is minimized.

In accordance with an embodiment of the invention, the coil elements of the RF antenna have a birdcage or TEM configuration or a combination of both.

For example, the galvanic connection and/or inductive coupling is across two of the coil elements. Said two coil elements may be coupled to each other via a capacitance.

In accordance with an embodiment of the invention, execution of the machine-executable instructions further causes the processor to control the magnetic resonance imaging system to acquire imaging magnetic resonance data using imaging pulse sequence commands, wherein the imaging pulse sequence commands are configured to control the magnetic resonance imaging system to acquire the imaging magnetic resonance signals according to a magnetic resonance imaging protocol; and reconstruct the magnetic resonance image using the imaging magnetic resonance data. The imaging magnetic resonance signals are received from the RF output terminal.

The term 'imaging protocol' may include any one of: one or more imaging scans, one or more pre-scans, loading of scan protocols, performing a predetermined processing of the received MRI signals and storing the processed MRI signals. The imaging scan protocol may also comprise instructions regarding the reconstruction of the MR image data acquired using the imaging.

The term 'imaging scan' includes both scans including only a single 2D image frame acquisition pass as well as 3D scanning techniques wherein each individual scan is performed as a time series of individual acquisition passes which are equal in terms of parameters and contrast. The term 'scan' may refer to a data acquisition sequence including applying a static magnetic field, a gradient magnetic field, transmitting an RF pulse, receiving an MRI signal, storing the received MRI signal.

In another aspect, the invention relates to an RF antenna for use in a magnetic resonance imaging system, the antenna e.g. comprising a plurality of coil elements, an RF input terminal and an RF output terminal, the RF input terminal being in galvanic connection to the antenna and adapted for receiving an RF input signal to energize the antenna; the RF output terminal being inductively coupled to the antenna and adapted for providing a magnetic resonance signal picked up by the antenna.

In another aspect the invention relates to a method of operating a magnetic resonance imaging system, the system comprising a main magnet for generating a main magnetic field within an imaging zone, an RF antenna comprising an RF input terminal and an RF output terminal, the method comprising supplying by an RF system radio-frequency power to the RF input terminal to energize the antenna; picking up by the antenna magnetic resonance signals from the imaging zone; receiving by a data acquisition system the MR signals from the RF output terminal; wherein the RF input terminal is in galvanic connection to the antenna and the RF output terminal is inductively coupled to the antenna.

In another aspect, the invention relates to a computer program product comprising machine-executable instructions to perform the method as described above.

As will be appreciated by one skilled in the art, aspects of the present invention may be embodied as an apparatus, method or computer program product. Accordingly, aspects of the present invention may take the form of an entirely hardware embodiment, an entirely software embodiment (including firmware, resident software, microcode, etc.) or an embodiment combining software and hardware aspects that may all generally be referred to herein as a "circuit," "module" or "system. " Furthermore, aspects of the present invention may take the form of a computer program product embodied in one or more computer readable medium(s) having computer executable code embodied thereon.

A 'computer-readable storage medium' as used herein encompasses any tangible storage medium which may store instructions which are executable by a processor of a computing device. The computer-readable storage medium may be referred to as a computer-readable non-transitory storage medium. The computer-readable storage medium may also be referred to as a tangible computer readable medium. In some embodiments, a computer-readable storage medium may also be able to store data which is able to be accessed by the processor of the computing device. Examples of computer-readable storage media include, but are not limited to: a floppy disk, a magnetic hard disk drive, a solid state hard disk, flash memory, a USB thumb drive, Random Access Memory (RAM), Read Only Memory (ROM), an optical disk, a magneto-optical disk, and the register file of the processor. Examples of optical disks include Compact Disks (CD) and Digital Versatile Disks (DVD), for example CD-ROM, CD-RW, CD-R, DVD-ROM, DVD-RW, or DVD-R disks. The term computer readable-storage medium also refers to various types of recording media capable of being accessed by the computer device via a network or communication link. For example a data may be retrieved over a modem, over the internet, or over a local area network. Computer executable code embodied on a computer readable medium may be transmitted using any appropriate medium, including but not limited to wireless, wire line, optical fiber cable, RF, etc., or any suitable combination of the foregoing.

'Computer memory' or 'memory' is an example of a computer-readable storage medium. Computer memory is any memory which is directly accessible to a processor. 'Computer storage' or 'storage' is a further example of a computer-readable storage medium. Computer storage may be any volatile or non-volatile computer-readable storage medium.

A 'processor' as used herein encompasses an electronic component which is able to execute a program or machine executable instruction or computer executable code. References to the computing device comprising "a processor" should be interpreted as possibly containing more than one processor or processing core. The processor may for instance be a multi-core processor. A processor may also refer to a collection of processors within a single computer system or distributed amongst multiple computer systems. The term computing device should also be interpreted to possibly refer to a collection or network of computing devices each comprising a processor or processors. The computer executable code may be executed by multiple processors that may be within the same computing device or which may even be distributed across multiple computing devices.

Computer executable code may comprise machine executable instructions or a program which causes a processor to perform an aspect of the present invention. Computer executable code for carrying out operations for aspects of the present invention may be written in any combination of one or more programming languages, including an object oriented programming language such as Java, Smalltalk, C++ or the like and conventional procedural programming languages, such as the C programming language or similar programming languages and compiled into machine executable instructions. In some instances the computer executable code may be in the form of a high level language or in a pre-compiled form and be used in conjunction with an interpreter which generates the machine executable instructions on the fly.

Aspects of the present invention are described with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems) and computer program products according to embodiments of the invention. It is understood that each block or a portion of the blocks of the flowchart, illustrations, and/or block diagrams, can be implemented by computer program instructions in form of computer executable code when applicable. It is further understood that, when not mutually exclusive, combinations of blocks in different flowcharts, illustrations, and/or block diagrams may be combined.

A 'user interface' as used herein is an interface which allows a user or operator to interact with a computer or computer system. A 'user interface' may also be referred to as a 'human interface device. ' A user interface may provide information or data to the operator and/or receive information or data from the operator. A user interface may enable input from an operator to be received by the computer and may provide output to the user from the computer. In other words, the user interface may allow an operator to control or manipulate a computer and the interface may allow the computer indicate the effects of the operator's control or manipulation. The display of data or information on a display or a graphical user interface is an example of providing information to an operator. The receiving of data through a keyboard, mouse, trackball, touchpad, pointing stick, graphics tablet, joystick, gamepad, webcam, headset, pedals, wired glove, remote control, and accelerometer are all examples of user interface components which enable the receiving of information or data from an operator.

A 'hardware interface' as used herein encompasses an interface which enables the processor of a computer system to interact with and/or control an external computing device and/or apparatus. A hardware interface may allow a processor to send control signals or instructions to an external computing device and/or apparatus. A hardware interface may also enable a processor to exchange data with an external computing device and/or apparatus. Examples of a hardware interface include, but are not limited to: a universal serial bus, IEEE <NUM> port, parallel port, IEEE <NUM> port, serial port, RS-<NUM> port, IEEE-<NUM> port, Bluetooth connection, wireless local area network connection, TCP/IP connection, Ethernet connection, control voltage interface, MIDI interface, analog input interface, and digital input interface.

A 'display' or 'display device' as used herein encompasses an output device or a user interface adapted for displaying images or data. A display may output visual, audio, and or tactile data. Examples of a display include, but are not limited to: a computer monitor, a television screen, a touch screen, tactile electronic display, Braille screen, Cathode ray tube (CRT), Storage tube, Bi-stable display, Electronic paper, Vector display, Flat panel display, Vacuum fluorescent display (VF), Light-emitting diode (LED) display, Electroluminescent display (ELD), Plasma display panel (PDP), Liquid crystal display (LCD), Organic light-emitting diode display (OLED), a projector, and Head-mounted display.

Magnetic Resonance (MR) data is defined herein as being the recorded measurements of radio frequency signals emitted by atomic spins using the antenna of a magnetic resonance apparatus during a magnetic resonance imaging scan. Magnetic resonance data is an example of medical imaging data. A Magnetic Resonance (MR) image is defined herein as being the reconstructed two or three dimensional visualization of anatomic data contained within the magnetic resonance imaging data.

In the following, like numbered elements in the figures are either similar elements or perform an equivalent function.

<FIG> shows an example of a magnetic resonance imaging system <NUM> with a magnet <NUM>. The magnet <NUM> is a superconducting cylindrical type magnet with a bore <NUM> through it. The use of different types of magnets is also possible; for instance it is also possible to use both a split cylindrical magnet and a so called open magnet. A split cylindrical magnet is similar to a standard cylindrical magnet, except that the cryostat has been split into two sections to allow access to the iso-plane of the magnet, such magnets may for instance be used in conjunction with charged particle beam therapy. An open magnet has two magnet sections, one above the other with a space in-between that is large enough to receive a subject: the arrangement of the two sections area similar to that of a Helmholtz coil. Open magnets are popular, because the subject is less confined. Inside the cryostat of the cylindrical magnet there is a collection of superconducting coils. Within the bore <NUM> of the cylindrical magnet <NUM> there is an imaging zone <NUM> where the magnetic field is strong and uniform enough to perform magnetic resonance imaging. A region of interest <NUM> is shown within the imaging zone <NUM>. A subject <NUM>, for example a patient, is shown as being supported by a subject support <NUM>, for example a moveable table, such that at least a portion of the subject <NUM> is within the imaging zone <NUM> and the region of interest <NUM>.

Within the bore <NUM> of the magnet there is also a set of magnetic field gradient coils <NUM> which is used for acquisition of magnetic resonance data to spatially encode magnetic spins within the imaging zone <NUM> of the magnet <NUM>. The magnetic field gradient coils <NUM> connected to a magnetic field gradient coil power supply <NUM>. The magnetic field gradient coils <NUM> are intended to be representative. Typically magnetic field gradient coils <NUM> contain three separate sets of coils for spatially encoding in three orthogonal spatial directions. A magnetic field gradient power supply supplies current to the magnetic field gradient coils. The current supplied to the magnetic field gradient coils <NUM> is controlled as a function of time and may be ramped or pulsed.

Adjacent to the imaging zone <NUM> is a radio-frequency coil <NUM> "RF antenna" for manipulating the orientations of magnetic spins within the imaging zone <NUM> and for receiving radio transmissions from spins also within the imaging zone <NUM>. The radio frequency antenna may contain multiple coil elements. The radio frequency antenna may also be referred to as a channel or antenna. The radio-frequency coil <NUM> is connected to an RF amplifier <NUM>. The radio frequency amplifier <NUM> is providing RF power to the RF coil <NUM> for manipulating the orientations of magnetic spins within the imaging zone <NUM>. Further shown in <FIG> is an input terminal <NUM> of the RF coil <NUM>, the input terminal <NUM> being galvanically coupled to the RF amplifier <NUM>. An RF output terminal <NUM> of the RF coil <NUM> is inductively coupled to a receiver of the MR system <NUM>.

The amplifier <NUM>, the gradient controller <NUM> and the RF output terminal <NUM> are shown as being connected to a hardware interface <NUM> of a computer system <NUM>. Thus, the computer system <NUM> serves also as receiver for receiving and processing the MR signals acquired using the coil <NUM>.

The computer system further comprises a processor <NUM> that is in communication with the hardware system <NUM>, a memory <NUM>, and a user interface <NUM>. The memory <NUM> may be any combination of memory which is accessible to the processor <NUM>. This may include such things as main memory, cached memory, and also non-volatile memory such as flash RAM, hard drives, or other storage devices. In some examples the memory <NUM> may be considered to be a non-transitory computer-readable medium.

The computer memory <NUM> is shown as containing machine-executable instructions <NUM>. The machine-executable instructions contain commands or instructions which enable the processor <NUM> to control the operation and function of the magnetic resonance imaging system <NUM>. The computer memory <NUM> is shown as further containing imaging scan protocols <NUM>. Each imaging scan protocol may comprise pulse sequence commands for one or multiple pulse sequences which are either instructions or data which may be converted into instructions which enable the processor <NUM> to control the magnetic resonance imaging system <NUM> to acquire magnetic resonance data. The pulse sequence commands may therefore be part of an imaging scan protocol. The magnetic resonance data may for instance be used to cause the magnetic resonance imaging system to perform multiple pulse repetitions which cause magnetic resonance signals <NUM> to be acquired.

Magnetic resonance signals <NUM> are shown as being stored in the computer memory <NUM>. The magnetic resonance signals <NUM> for a particular pulse repetition may be collated into the magnetic resonance data <NUM>. The magnetic resonance data <NUM> may be used to generate a series of images <NUM>. The imaging scan protocols may further comprise instructions <NUM> regarding the reconstruction of the image data <NUM> acquired using the imaging.

For example, the antenna <NUM> has multiple coil elements in a birdcage configuration. Further, multiple ones of the output terminals <NUM> are provided which are inductively coupled to respective ones of the coil elements. Similarly, multiple ones of the RF input terminals <NUM> and optionally respective RF amplifiers <NUM> may be provided. Each one of the RF input terminals <NUM> may be galvanically coupled to at least one of the coil elements. A selector not shown in <FIG> may be controlled using instructions <NUM> to select specific ones of the RF input terminals and the RF output terminals to provide the RF power to the antenna and to receive the magnetic resonance signals from the antenna, respectively.

Further, instructions <NUM> may be provided which enable to blank the RF output terminal <NUM> during the excitation of the nuclear spins using high-power RF pulses by the RF amplifier <NUM>.

<FIG> is a flowchart of operating a magnetic resonance imaging, wherein the method starts with block <NUM> and the supplying of an RF pulse to the RF input terminal <NUM> in order to energize the antenna <NUM>. In block <NUM>, by the antenna <NUM> magnetic resonance signals are picked up from the imaging zone <NUM>. In block <NUM> the computer system <NUM> acting here as the data acquisition system is receiving the RF signals picked up by the antenna from the RF output terminal <NUM>.

<FIG> illustrates a circuit diagram of a birdcage resonator <NUM>. Typically a birdcage coil consists of two circular conductive loops referred to as end rings connected by a number of conductive straight elements called rungs. In <FIG>, the end rings are constituted by the horizontal conductor lines, whereas the rungs are constituted by the vertical conductor lines. Several capacitors <NUM> are arranged between the individual conducting elements based on the frequency characteristics of the coil <NUM> desired. In total, the antenna <NUM> therefore consists of multiple coil elements that can be used for exciting nuclear spins by supplying RF pulses to the coil elements and to receive magnetic resonance signals also using the individual coil elements.

In the configuration depicted in <FIG>, four RF input terminals <NUM> and two RF output terminals <NUM> are shown. The RF input terminals are electrically connecting respective RF amplifiers <NUM> to a respective match and detune circuit <NUM>, wherein each match and detune circuit <NUM> is galvanically coupled to two coil elements. In more detail, each match and detune circuit <NUM> is coupled to two conductive elements of each ring, wherein these two conductive elements are coupled to each other via a respective capacitor <NUM>. Thus, a coil element can be understood as a single electrical conductor slab of the coil or an arrangement of electrical conductors slabs and optional capacitors that form a coil as part of the antenna.

In contrast thereto, each RF output terminal <NUM> is galvanically coupled to a pre-amplifier <NUM>, whereas the pre-amplifier <NUM> is inductively coupled to one of the electrical conductors of the ring via a respective inductive coupler <NUM>. In the simplest case, the inductive coupler <NUM> may be a conductive loop placed in close vicinity to the respective conductor of the ring of the antenna <NUM>.

Not shown in <FIG> is an active or passive detuning circuit that may be placed between the inductive coupler <NUM> and the pre-amplifier <NUM> and that may be used to actively or passively blank the pre-amplifier <NUM> and therefore the output terminal <NUM> during providing the RF power by the amplifier <NUM> to the RF input terminal <NUM>. Further not shown in <FIG> is an analogue-to-digital converter which may be placed in between the pre-amplifier <NUM> and the output terminal <NUM> and which serves to digitize the pre-amplified MR signal.

As can be seen from <FIG>, the coil elements to which the RF input terminal is galvanically connected and the coil elements to which the RF output terminals is inductively coupled are different from each other. However, it will be understood by a skilled person that the coil elements may be identical since in reality the birdcage coil <NUM> has sufficient space to accommodate for the same coil element both, the galvanic connection to the RF input terminal and the inductive coupling to the RF output terminal.

Since the RF input terminal <NUM> is hardwired to the antenna <NUM> via the match and detune circuit <NUM>, the signal-to-noise ratio for coupling an MR excitation signal into the antenna can be easily optimized in such a manner that the impedance looking into the antenna <NUM> is matched or made equal to the transmission line impedance that connects the input terminal <NUM> to the RF amplifier <NUM>. The match and detune circuit <NUM> is further adapted in such a manner that a high impedance is provided for currents induced in the antenna <NUM> due to excited nuclear resonances. This even holds true in case the load impedance seen by the antenna <NUM> is varying due to the mass and composition of the material being located within the coil <NUM>, i.e. the imaging zone. The impedance adjustment to the RF coil can be easily managed since only the RF input terminal <NUM> has to be considered here.

Due to the inductive coupling of the RF output terminal <NUM> to the antenna <NUM> (or more specifically to the coil elements of the antenna <NUM>) no separate send-receive switch (hybrid box) unit is required which connects both, the RF amplifier and an RF receiver through a common match and detune circuit to the antenna <NUM>. Since the RF input terminal <NUM> and the RF output terminal <NUM> are 'isolated' from each other, RF system losses due to cables and connectors are reduced and the signal-to-noise ratio of the MR signals picked up by the antenna, transmitted via the inductive loop <NUM> to a pre-amplifier <NUM> and provided for example converted from the analogue to the digital domain to the output terminal <NUM> is optimized.

<FIG> depicts a further circuit diagram of an antenna <NUM> of a birdcage resonator. The general configuration regarding the arrangement of the rings and rungs, as well as the RF input terminals <NUM> is identical to the birdcage resonator that was discussed above with respect to <FIG>. The difference between <FIG> is that the RF output terminal <NUM> is coupled in <FIG> to a rung. More specifically, the induction loop <NUM> is inductively coupled simultaneously to two conductive elements of a rung, the two conductive elements being coupled to each other via a capacitor <NUM>. Thus, <FIG> depicts an example of a separate rung and ring feeding for the transmit and receive chain. This can also be inverted, thus receive in ring and transmit in rung.

The idea of having the RF input terminal in galvanic connection to the antenna and the RF output terminal in inductive coupling to the antenna can be used in a specific manner for performing an efficient load-dependent matching of antennas of any kind. However, preferred applications are for example an improved matching of large MRI transmit coils since these coils show a high load variation regarding the impedances.

<FIG> shows a block diagram of an MR coil, for example one of the antennas <NUM> previously discussed with respect to <FIG>. Schematically shown are the input terminals <NUM> and the output terminals <NUM>. Again, the input terminals <NUM> are connected galvanically for example via a respective match and detune circuit to the hardwire slabs of which the antenna is made up, whereas the RF output terminals are only inductively coupled to the antenna <NUM>, i.e. the hardwires of the antenna <NUM>.

Besides the computer system <NUM> which was discussed in <FIG> above, additionally a selector <NUM> is shown. The selector <NUM> is controlled by the computer system <NUM> and permits to selectively perform the feeding of RF power to one or more specific ones of the RF input terminals <NUM> and to selectively receive MR signals from one or more specific ones of the RF output terminals <NUM>. , the selector <NUM> as controlled by the computer system <NUM> specifically selects the RF output terminals <NUM> and the RF input terminals <NUM> such that the signal-to-noise ratio of the acquired MR signals is maximized.

In one example, the load of the subject to be imaged in the imaging zone is considered and by means of an electromagnetic simulation the RF input terminals <NUM> are selected which permit for a most efficient coupling of RF excitation signals into the antenna <NUM>. While this configuration is maintained, the selector <NUM> under control of computer system <NUM> is used to acquire MR signals from either individual ones of the RF output terminals <NUM> or from a variety of different combinations of RF output terminals <NUM>. Such acquired MR signals are then evaluated regarding their signal-to-noise ratio. Based on this evaluation, the RF output terminals <NUM> yielding the highest signal-to-noise ratio of the acquired MR signals are selected for performing the subsequent imaging scan according to the desired imaging protocol.

Claim 1:
A radio frequency (RF) antenna (<NUM>) for use in a magnetic resonance imaging system (<NUM>), with an RF input terminal (<NUM>) and an RF output terminal (<NUM>),
- the RF input terminal (<NUM>) being in galvanic connection to the antenna (<NUM>) and adapted for receiving an RF input signal to energize the antenna (<NUM>),
- the RF output terminal (<NUM>) being inductively coupled to the antenna (<NUM>) and adapted for providing a magnetic resonance signal picked-up by the RF antenna (<NUM>).