Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
  • G01R33/00—Arrangements or instruments for measuring magnetic variables
  • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
  • G01R33/28—Details of apparatus provided for in groups G01R33/44 - G01R33/64
  • G01R33/32—Excitation or detection systems, e.g. using radio frequency signals
  • G01R33/34—Constructional details, e.g. resonators, specially adapted to MR
  • G01R33/341—Constructional details, e.g. resonators, specially adapted to MR comprising surface coils
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
  • G01R33/00—Arrangements or instruments for measuring magnetic variables
  • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
  • G01R33/28—Details of apparatus provided for in groups G01R33/44 - G01R33/64
  • G01R33/288—Provisions within MR facilities for enhancing safety during MR, e.g. reduction of the specific absorption rate [SAR], detection of ferromagnetic objects in the scanner room
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
  • G01R33/00—Arrangements or instruments for measuring magnetic variables
  • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
  • G01R33/28—Details of apparatus provided for in groups G01R33/44 - G01R33/64
  • G01R33/32—Excitation or detection systems, e.g. using radio frequency signals
  • G01R33/36—Electrical details, e.g. matching or coupling of the coil to the receiver
  • G01R33/3642—Mutual coupling or decoupling of multiple coils, e.g. decoupling of a receive coil from a transmission coil, or intentional coupling of RF coils, e.g. for RF magnetic field amplification
  • G01R33/3657—Decoupling of multiple RF coils wherein the multiple RF coils do not have the same function in MR, e.g. decoupling of a transmission coil from a receive coil
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
  • G01R33/00—Arrangements or instruments for measuring magnetic variables
  • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
  • G01R33/28—Details of apparatus provided for in groups G01R33/44 - G01R33/64
  • G01R33/32—Excitation or detection systems, e.g. using radio frequency signals
  • G01R33/36—Electrical details, e.g. matching or coupling of the coil to the receiver
  • G01R33/3692—Electrical details, e.g. matching or coupling of the coil to the receiver involving signal transmission without using electrically conductive connections, e.g. wireless communication or optical communication of the MR signal or an auxiliary signal other than the MR signal
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
  • G01R33/00—Arrangements or instruments for measuring magnetic variables
  • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
  • G01R33/28—Details of apparatus provided for in groups G01R33/44 - G01R33/64
  • G01R33/32—Excitation or detection systems, e.g. using radio frequency signals
  • G01R33/34—Constructional details, e.g. resonators, specially adapted to MR
  • G01R33/34084—Constructional details, e.g. resonators, specially adapted to MR implantable coils or coils being geometrically adaptable to the sample, e.g. flexible coils or coils comprising mutually movable parts

Definitions

• the present invention relates to magnetic resonance (MR) systems, and in particular to the decoupling of receiving coils in MR systems.
• MR systems such as MR imaging (MRI) and MR scanning (MRS) systems generally comprise a main magnetic field generator arranged to generate a constant magnetic field, and gradient magnetic field coils arranged to produce local gradients in the main magnetic field.
• a pulsed radio frequency (RF) signal is generated by an RF coil which excites the nuclei of the object.
• the nuclei emit RF radiation at a frequency that depends on the strength of the magnetic field at their location, and the nature of the material of which they are part.
• This magnetic resonance radiation is detected by receiver coils which are generally located close to the area to be imaged. Variation of the magnetic field produced by the gradient coils and monitoring of the radiation produced over a number of cycles enables information to be obtained about the nature of the material throughout the imaged object, and this can be used to generate a three dimensional image of the object.
• the receiver coil can be connected by electrical wires to the processing unit that analyses the signals they produce.
• inductive coupling to transmit the receiver coil signals to the processing unit.
• the receiver coil is part of a receiver unit and is arranged to resonate in response to the magnetic resonance signals, which produces a further signal which is transmitted to a further receiver which is connected to the processing unit.
• the receiver coil in the receiver unit can resonate in response to the RF excitation pulses and can significantly interfere with the RF excitation signal. In order to avoid this it is known to decouple the receiver coils while the RF excitation signal is being generated.
• EP 1 130 413 discloses a system which includes a DC link from the system controller to the receiver coil which can be used to de-couple the receiver coil while the excitation signal is being produced.
• the system described in this document also includes inductors which are switched into the receiver coil by means of PIN diodes and provide a de-tuning of the receiver coil when the excitation signal is being generated.
• this de-tuning still allows the receiving coil to generate a significant level of interference as the excitation signal builds up to the point where any decoupling will occur.
• the receiver coil can have a very high Q factor, which may be of the order of 50. In such cases significant distortion can occur while the excitation signal is only at a small fraction of its full power.
• the present invention therefore aims to provide a fully wireless receiver coil for use in such, and other, applications.
• the present invention therefore provides a magnetic resonance system comprising magnetic field generating means arranged to generate a magnetic field, excitation means arranged to produce an excitation signal to produce magnetic resonance in an object, and a resonance signal receiving means arranged to receive magnetic resonance signals generated in the object, wherein the system further comprises decoupling means arranged to receive the excitation signal and, in response thereto, to decouple the resonance signal receiving means.
• the decoupling means may comprise a decoupling receiver arranged to receive the excitation signal and switching means arranged to decouple the receiving means.
• the receiving means may comprise a closed conductive loop and the switching means is arranged to open the conductive loop thereby to prevent current from flowing around the loop. This can allow complete decoupling of the receiver coil so that it provides no significant interference.
• Such a switching arrangement optionally with some amplification of the received signal, can allow the decoupling to be done very quickly as the excitation signal starts.
• the present invention further provides a receiving unit for a magnetic resonance signal comprising resonance signal receiving means arranged to receive resonance signals from an object, and a decoupling receiver arranged to receive a wireless signal, and decoupling means arranged to decouple the receiving means on receipt of the wireless signal.
• the wireless signal may be an excitation signal arranged to excite magnetic resonance in the object.
• it may be a control signal generated in response to the excitation signal, in which case it may be frequency shifted to a frequency remote from that of the excitation signal.
• the present invention further provides a receiving system for a magnetic resonance system, the receiving system comprising a receiving unit according to the invention, detection means arranged to detect transmission of an excitation signal arranged to excite magnetic resonance in the object, and decoupling transmitter arranged to transmit a wireless decoupling signal to the decoupling receiver at a different frequency from the excitation signal.
• Figure 1 is a schematic diagram of a magnetic resonance imaging system according to an embodiment of the invention
• Figure 2 is a diagram of a receiving unit of the system of Figure 1 ;
• Figure 3 is a diagram of part of a magnetic resonance imaging system according to a second embodiment of the invention.
• Figure 4 is a diagram of a transmitter/receiver unit of the system of Figure 3.
• a magnetic resonance imaging system comprises a main magnet 10 arranged to produce a fixed magnetic field in an imaging zone 12, and a number of gradient coils 14 arranged to produce gradients in the magnetic field.
• the system further comprises a control system 16 arranged to control the field generated by the gradient coils 14.
• An excitation coil 18 is also controlled by the control system 16 and arranged to produce pulses of radio frequency (RF) excitation field.
• RF radio frequency
• This field is arranged to excite magnetic resonance in the nuclei of an object 20, typically part of a patient.
• the system further comprises a receiver unit 22 arranged to detect signals in the form of radiation generated by the magnetic resonance, and to transmit these signals wirelessly to a further receiver 24, which is connected to the control system 16.
• the receiver unit 22 is in the form of a probe having a first end arranged to be inserted into an orifice in the body of a patient, and a second end which is arranged to be supported by a robot arm so that it can be manipulated as required.
• the receiver unit 22 comprises a receiver coil 30 which is located at the first end of the probe and which includes a loop of wire 32.
• the receiver unit 22 further comprises a control circuit 34 which includes a switch 36 which can be closed to close the conductive loop of the receiver coil 30, so that current can flow round the receiver coil, and opened to prevent current flowing round the receiver coil and prevent resonance in the coil.
• the receiver coil 30 is inductively coupled which means that it is arranged to resonate, when the switch is closed, in response to the magnetic resonance radiation emitted by the object and, when resonating, to generate a signal which is received by the receiver 24. Changes in the strength of the signal received by the receiver 24, and their dependence on the magnetic field gradient can be used in known manner to build up an image of the imaging zone 12.
• the receiver unit 22 further comprises a trigger coil, 38 which is located towards the second end of the probe and connected to the control circuit 34.
• the trigger coil 38 is arranged to resonate in response to the excitation field thereby generating an electrical signal in the control circuit. This signal is rectified and amplified to produce a trigger signal which is arranged to operate the switch 36 to open it.
• the transmitter unit 22 therefore includes an internal power source in the form of a battery 40 to provide power for the amplification. Opening of the switch 36 in response to the excitation signal results in the decoupling of the receiver coil 30 at any time when the excitation field is being produced. This in turn prevents the receiver coil 30 from resonating in response to the excitation field and therefore prevents the receiver coil from interfering with the excitation field.
• the receiver coil 30 is provided at a first end of the receiver unit 22 and the trigger coil 38 is provided at a second, opposite end.
• the receiver coil 30 is significantly spaced from the trigger coil 38 such that the extent or likelihood of the same current flowing in both coils is minimised or nullified altogether, i.e. the trigger coil 38 will not effectively detect the magnetic resonance radiation emitted by the object.
• the receiver coil 30 will be placed near to the object and the trigger coil 38 will therefore be spaced from the object.
• the first end of the receiver unit 22 is arranged to be inserted into an orifice in the body in preparation for imaging/scanning.
• the receiver coil 30 is at the first end so it can effectively detect magnetic resonance radiation emitted by the object.
• the trigger coil is further from the first end of the probe than the receiver coil, in this case being located at the second end of the receiver unit 22.
• the spacing between the first end and second end, and therefore between the coils, is such that, in use the trigger coil is prevented from interfering with the fixed magnetic field and the pulses of radio frequency (RF) excitation field during the imaging/scanning process. In this way the trigger coil detects the pulse but does not affect the pulse in the region of interest, i.e. the region being imaged.
• RF radio frequency
• the trigger coil is arranged, in use, to be located further away from the region being imaged than the receiver coil but one or both of the coils may not necessarily provided at the extreme ends of the unit.
• the trigger coil 38 is tuned to the frequency of the excitation signal it may be arranged to have a lower quality (Q) factor than the receiver coil 30 and therefore will resonate less, and generate less interference than the receiver coil would. This means that, as the strength of the excitation signal builds up, the trigger signal can be generated well before the excitation signal approaches its full strength.
• decoupling of the receiver coil 30 occurs when the excitation signal is at about 3% of its full strength.
• decoupling of the receiver coil occurs when the excitation signal is at about 1% of its full strength.
• the decoupling occurs at as low a level as is technically reliable.
• the receiver unit 22 is therefore completely wireless. Because it is inductively coupled it transmits signals wirelessly to the control system, and the trigger coil enables it to be decoupled automatically from the excitation signal without the need for any electrical input signal that requires a physical link to it. Any power that it needs for amplifying the trigger signal is provided by its own battery.
• the receiver unit 22 can therefore be used in conjunction with an MRI system without any need for a physical interface with that system. This allows receiver units to be tested, exchanged and disposed of easily.
• the system in a second embodiment of the invention, is the same as in the first embodiment except that it comprises an additional receiver/transmitter unit 150 which is located close to the excitation coil 118, and which is arranged to receive the excitation signal and re-transmit it at a different frequency.
• the receiver unit 122 is then arranged to receive this frequency shifted signal via a trigger coil similar to that of Figure 2.
• another control signal derived from the system such as a computer command line, is provided.
• the system is not dependent on the RF transmitter coils only.
• the receiver/transmitter unit 150 includes a search coil 152 which is arranged to resonate at the frequency of the excitation signal from the excitation coil 118, an amplifier 120 which has a local power source in the form of a battery in the unit 150, frequency shifting circuit 122 arranged to shift the frequency of the received signal to a frequency remote from that of the excitation signal, and a transmitter coil 156 arranged to transmit the frequency shifted signal.
• This frequency shifted signal forms a decoupling signal which is then picked up by the trigger coil of the receiver unit, which is tuned to that shifted frequency, and the receiver unit operates in the same way as in the first embodiment.
• the receiver unit may include a battery to provide amplification of the trigger signal, but if the frequency shifted signal is powerful enough this is not necessary.
• the trigger coil of the receiver unit will not resonate in response to the excitation signal as it is not tuned to the frequency of the excitation signal, and therefore the trigger coil will not cause disturbance of the excitation field for this reason.
• the search coil 152 of the transmitter/receiver unit 150 is remote from the imaging zone, or at least from the area of interest close to the receiver coil, and so provides little interference.
• the search coil 152 can also be arranged to have a lower Q factor than the trigger coil so as further to minimize the level of interference it produces.
• This embodiment also allows the receiver unit to be completely wireless which can bring the advantages outlined above for the first embodiment.

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Cited By (2)

Citations (8)

• 2006
  • 2006-09-14 GB GB0618087A patent/GB0618087D0/en not_active Ceased
• 2007
  • 2007-09-14 WO PCT/GB2007/003509 patent/WO2008032098A1/en active Application Filing

Patent Citations (8)

Non-Patent Citations (2)

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