This invention relates to magnetic resonance (MR) imaging and spectroscopy, and particularly to radio-frequency (RF) coils.
U.S. Pat. No. 6,946,840 discusses an RF array coil comprising a plurality of transmitter and receiver coils. However, using their RF array coil to perform multi-nuclear (MN) imaging could result in a complicated and expensive system, as much of the hardware will need to be duplicated to enable the transmitter to excite different nuclear species, as well as to enable the receiver to receive data from each different nuclear species. It is therefore desirable to have a simpler and cheaper implementation of an RF coil system that is capable of MN imaging. It is also desirable to have a simpler and cheaper method of MN imaging using the said RF coil system, as well as to have an MR system capable of performing MN imaging in a simpler and cheaper fashion utilizing the said RF coil system.
Accordingly, an RF coil system that simplifies MN imaging is disclosed herein, the RF coil system comprising a transmitter coil for transmitting an RF signal to excite a target region, and a planar receiver coil assembly for receiving an MR signal from at least a portion of the target region, wherein the planar receiver coil assembly includes an on-board digital receiver circuit for processing the received magnetic resonance signal. A single transmitter coil reduces both the complexity and the cost involved in realizing MN imaging on the transmitter side, while including a digital receiver circuit on-board each receiver coil assembly does the same on the receiver side, as explained below. The term “on-board receiver circuit” indicates that the receiver circuit may be mounted on the circuit board holding the coil itself, or placed on a separate circuit board in close proximity to the receiver coil.
The operating frequency of a planar receiver coil assembly as disclosed herein is determined by its resonance frequency. The resonance frequency also determines the configuration of the on-board receiver circuit, for example the sampling or digitization frequency as well as the frequency of modulation and/or filtering. This operating frequency, which is thus defined almost entirely by the receiver coil assembly including the on-board digital receiver circuit, is therefore independent of the rest of the hardware on the MRI system. A planar receiver coil assembly tuned to a particular frequency or range of frequencies could connect directly to the same physical interface to the MR system, as another planar receiver coil assembly tuned to a different frequency or range of frequencies. Alternatively, a tunable planar receiver coil assembly initially tuned to a particular frequency or range of frequencies and connecting to the MR system via a particular physical interface could be retuned to a different frequency or range of frequencies and still connect to the MR system via the same physical interface. The remote MR system thus only has to support a digital coil data connection and is no longer frequency specific on the receive side. As a result, duplication of hardware is minimized, yielding a simpler and cheaper RF coil system capable of performing MN imaging.
Correspondingly, an MR system capable of performing MN imaging in a simpler and cheaper fashion utilizing the said RF coil system is also disclosed herein, the RF coil system comprising a transmitter coil for transmitting an RF signal to excite a target region, and a planar receiver coil assembly for receiving an MR signal from at least a portion of the target region, wherein the planar receiver coil assembly includes an on-board digital receiver circuit for processing the received magnetic resonance signal.
Furthermore, a simpler and cheaper method of MN imaging utilizing the said RF coil system is also disclosed herein, the method comprising transmitting an RF signal from a transmitter coil to excite a target region, receiving an MR signal from at least a portion of the target region using a planar receiver coil assembly, and processing the received magnetic resonance signal using a digital receiver circuit placed on-board the planar receiver coil assembly.
Furthermore, a computer program comprising instructions for switching such a radio-frequency coil system—comprising a transmitter coil and a planar receiver coil assembly—between transmit, receive and detune modes is also disclosed herein. The computer program comprises instructions to activate an active decoupling circuit to switch the planar receiver coil assembly from a receive mode to a detune mode when the transmitter coil is in operation, and to activate another active decoupling circuit to switch the transmitter coil from a transmit mode to a detune mode when the planar receiver coil assembly is in operation.
These and other aspects will be described in detail hereinafter by way of example on the basis of the following embodiments, with reference to the accompanying drawings, wherein:
a-1e show various embodiments of the disclosed RF coil system;
Corresponding reference numerals used in the various figures represent corresponding elements in the figures.
a-1e show various embodiments of an RF coil system comprising a transmitter coil 102 and one or more planar receiver coil assemblies 104, 110. Each planar receiver coil assembly 104, 110 includes an on-board digital receiver circuit 106, 112 capable of processing MR signals received by the respective planar receiver coil assembly 104, 110. In some embodiments the planar receiver coil assembly 104, 110 is configured to overlap the transmitter coil 102 (
The transmitter coil 102 and the planar receiver coil assembly 104, 110 are electrically independent of each other, and serve only one function each. In other words, the transmitter coil 102 is exclusively used for transmitting RF signals, while the planar receiver coil assembly 104, 110 is used exclusively to receive MR signals. During use, the coils are intended to be placed either concentric (i.e., fully overlapping), partially overlapping or orthogonal to each other. Various possible configurations of the receiver coil system comprising the transmitter coil 102 and the planar receiver coil assembly 104, 110 are shown in
Separating transmit and receive functionality in this way also allows for greater flexibility with respect to realizing multi-channel, MN imaging. For example, by combining the use of a multiply-tuned transmitter coil with multiple planar receiver coil assemblies tuned to corresponding multiple frequencies, it is possible to realize multi-channel imaging of multiple nuclear species of interest. It is also possible to use a tunable planar receiver coil assembly wherein the planar receiver coil assembly initially tuned to a particular frequency is subsequently retuned to another frequency, thereby facilitating MN imaging in a sequential fashion. The tunable planar receiver coil assembly may be sequentially retuned to more than one additional frequency. Thus MN imaging may be either done separately for each of the nuclear species of interest or simultaneously for different nuclei.
As shown in
Since both the transmitter coil 102 and the planar receiver coil assembly 104, 110 can overlap in space, it would be advantageous to provide adequate electrical decoupling between the planar receiver coil assembly 104, 110 and the transmitter coil 102, both during the transmit stage as well as the receive stage. In other words, the planar receiver coil assembly 104, 110 needs to be RF-invisible during RF-transmit, while the transmitter coil 102 should not affect the planar receiver coil assembly 104, 110 during RF-receive.
Adequate decoupling of the transmitter coil 102 and the planar receiver coil assembly 104, 110 may be achieved by combining active decoupling with passive decoupling elements, as shown in
The active decoupling elements 204, 304 contain a capacitor and inductor in parallel with a diode in series with the inductor. Together these components form a local parallel resonant circuit to the resonant circuit formed by the coil loop 202, 302 and associated capacitances. The inductance is chosen so that this parallel resonant circuit matches the resonant frequency of the coil loop 202, 302. When the coil loop 202, 302 is in resonant mode, the diode is reverse-biased using a controlled DC voltage so that the parallel resonant circuit is not at resonance and the capacitor contributes to the series resonance of the coil loop 202, 302. When the diode is forward biased (by switching the DC voltage to the opposite polarity), the capacitor and inductor become resonant and create a high impedance at the place where they are located. As a consequence of the high impedance, the coil loop 202, 302 is no longer resonant. The passive decoupling element is similar to the active element except that it contains two parallel reversed diodes. The diodes remain “open circuited”, i.e., non-conducting, until the transmitter coil, during transmit, induces sufficient voltage in the receive coil to forward-bias both diodes and complete the local parallel resonant circuit. Once again, when the diodes conduct, the local resonant circuit creates a high impedance at that point which breaks the resonance of the larger coil loop.
The additional passive decoupling elements 303 ensure adequate detuning of the planar receiver coil assembly 104, 110 and prevent a hazard in case of failure or mistiming of the active decoupling circuitry 204, 304. The electronics may include additional components to provide tuning and matching as well as active decoupling capability.
Active decoupling of the transmitter coil 102 is also required during receive mode of the planar receiver coil assembly 104, 110 in order to prevent coupling of noise into the planar receiver coil assembly 104, 110. During operation, the transmitter coil 102 is either transmitting RF signals or is in decoupling mode. It is switched between the two modes under control of the transmit/detune switch which may be located remotely from the planar receiver coil assembly 104, 110, on the MR system.
From the perspective of patient safety, the use of optical or wireless data transfer reduces the possibility of RF burns by preventing coupling between transmitter and receiver components. This is true for both planar surface coils and volume body coils.
The main coils 401 generate a steady and uniform static magnetic field, for example, of field strength 1 T, 1.5 T or 3 T. The disclosed RF coil system is applicable to any other field strength as well. The main coils 401 are arranged in such a way that they typically enclose a tunnel-shaped examination space, into which a subject 405 may be introduced. Another common configuration comprises opposing pole faces with an air gap in between them into which the subject 405 may be introduced by using the transport system 404. To enable MR imaging, temporally variable magnetic field gradients superimposed on the static magnetic field are generated by the multiple gradient coils 402 in response to currents supplied by the gradient driver unit 406. The power supply unit 412, fitted with electronic gradient amplification circuits, supplies currents to the multiple gradient coils 402, as a result of which gradient pulses (also called gradient pulse waveforms) are generated. The control unit 408 controls the characteristics of the currents, notably their strength, duration and direction, flowing through the gradient coils to create the appropriate gradient waveforms. The transmitter coil 403 generates RF excitation pulses in the subject 405, while the planar receiver coil assembly 416 receives MR signals generated by the subject 405 in response to the RF excitation pulses. The RF coil driver unit 407 supplies current to the transmitter coil 403 to transmit the RF excitation pulses. The characteristics of the transmitted RF excitation pulses, notably their strength and duration, are controlled by the control unit 408. The transmitter coil 403 is operated in one of two modes, namely transmit and detune modes, by the control unit 408 via the T/D switch 413. The T/D switch 413 is provided with electronic circuitry that switches the transmitter coil 403 between the two modes, and prevents the transmitter coil 403 from coupling noise during signal acquisition by the planar receiver coil assembly 416. The T/D switch 413 also switches the planar receiver coil assembly 416 between two modes during operation, namely receive mode and detune or decoupling mode. The planar receiver coil assembly 416 is switched to decoupling mode during the transmit mode of the independent transmitter coil 403 and to receive mode during the decoupling mode of the transmitter coil 403. The switching between the two modes of both the transmitter coil 403 and the planar receiver coil assembly 416 is coordinated under software control of the MR system.
The RF transmitter coil 403 may be integrated into the magnet in the form of a body coil or as separate surface coils. The transmitter coil 403 may have different geometries, for example, a birdcage configuration or a simple loop configuration, etc. The control unit 408 is preferably in the form of a computer that includes a processor, for example a microprocessor. The control unit 408 controls, via the T/D switch 413, the application of RF pulse excitations and the reception of MR signals. User input interface devices 411 like a keyboard, mouse, touch-sensitive screen, trackball, etc., enable an operator to interact with the MR imaging system.
The optical signal conversion unit 414 converts optical signals into electrical signals. The converted electrical signals contain the actual information concerning the local spin densities in a region of interest of the subject 405 being imaged. The received signals are reconstructed by the reconstruction unit 409, and displayed on the display unit 410 as an MR image. It is alternatively possible to store the signal from the reconstruction unit 409 in a storage unit 415, while awaiting further processing.
Physical connections to the transmitter coil 403 include an RF coaxial cable for transmitting RF power and a power supply cable for providing a switchable voltage source for reverse/forward biasing of the active decoupling circuits (204 in
The physical bulk of the transmitter coil 403 is minimized by locating the T/D switch 413 remotely from the transmitter coil 403 and connecting it via a flexible cable bunch (including the input cable for supplying the RF energy to the transmitter coil 403) and a suitable detachable plug/socket.
The computer program disclosed herein may reside on a computer readable medium, for example a CD-ROM, a DVD, a floppy disk, a memory stick, a magnetic tape, or any other tangible medium that is readable by a computer. The computer program may also be a downloadable program that is downloaded, or otherwise transferred to the computer, for example via the Internet. The computer program may be transferred to the computer via a transfer means such as an optical drive, a magnetic tape drive, a floppy drive, a USB or other computer port, an Ethernet port, etc.
It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. A person skilled in the art may change the order of steps or perform steps concurrently using threading models, multi-processor systems or multiple processes without departing from the disclosed concepts. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word “comprising” does not exclude the presence of elements or steps other than those listed in a claim. The word “a” or “an” preceding an element does not exclude the presence of a plurality of such elements. The disclosed method can be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In the system claims enumerating several means, several of these means can be embodied by one and the same item of computer readable software or hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.
Number | Date | Country | Kind |
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06116404.2 | Jun 2006 | EP | regional |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/IB2007/052529 | 6/29/2007 | WO | 00 | 12/18/2008 |