CROSS-REFERENCE TO RELATED APPLICATIONS

This is the U.S. national stage of International application PCT/EP2019/065913, filed Jun. 17, 2019 designating the United States and claiming priority to European patent application no. EP 18178152.7, filed Jun. 17, 2018 and European patent application no. EP 18192134.7, filed Aug. 31, 2018.

FIELD OF THE INVENTION

The present invention generally relates to a sheath wave barrier for suppressing electromagnetic RF coupling phenomena of an electrical cable in an MR apparatus.

BACKGROUND OF THE INVENTION

The operation of an MR apparatus, including but not limited to MR imaging (MRI), constitutes a harsh electro-magnetic environment for any devices used in the vicinity of the imaging volume. Such devices must withstand strong magnetic fields and high-power radio-frequency (RF) pulses.

Any cable running within the scanner bore of an MRI apparatus is prone to interact with the RF field leading to surface currents on the cable shield. These currents can have a number of adverse effects:in close vicinity of patients, they can produce excessive power absorption in the human tissue, potentially resulting in tissue burns;RF-power can flow along the cable out of the bore;the currents on the cables alter the RF field distribution within the bore; andsurface currents can enter the signal paths if shielding is not perfect.

To avoid such problems, so-called cable traps intended to act as surface wave suppressors, also called “sheath wave barriers”, are typically placed onto cables exposed to strong RF fields. However, broadband suppression by means of components with ferro- or ferrimagnetic properties is typically not applicable in MRI due to highly undesirable interactions with the strong magnetic background fields present within the scanner bore. Rather than that, conductive and dielectric structures introducing a high impedance are typically used in MR systems. While unwanted RF signals on DC and low-frequency signal lines are typically blocked by introducing inductors, analogously constructed devices for coaxial cables and cables with multiple lines would become too large for most applications. However, by tuning the trap circuit to resonance by lumped or distributed capacitances, it produces a localized high impedance (enhanced by the quality factor of the resonance) for surface waves at one or several suppression frequencies despite the comparably low reactance of the inductive geometrical structure. By appropriate tuning of the resonance properties, such traps can be designed to have at least one predetermined suppression frequency. The downside of such trap structures is that the electromagnetic resonance field within and around the trap can interact directly with the RF field. In such a case, the trap can convert into an RF pickup-coil. Furthermore, due to the resonance conditions, the sensitivity of the trap to external electromagnetic fields can become very high. In the worst case, the surface currents on the cable can then even be increased by the trap. Equally, if several traps need to be arranged in a setup, mutual coupling between two traps can either detune the individual trapping circuits and/or introduce unwanted coupling between the MR antennas and signal lines. Finally, the traps can also couple to the MRI antenna structure itself resulting in a loss of efficiency, SNR and other performance parameters as well as compromising the safety and stability of the entire setup.

In principle, these undesirable effects can be avoided byi) placing the trap away from any strong external RF fields, other traps and antennas, e.g. by placing the trap in a region where the cable leaves the sensitive volume of any RF transmit coils of the MR apparatus and distant from each other;ii) configuring the trap in such manner that the coupling between the transmit field generated by the RF transmit coils and the trap internal field is minimized.

The first approach has strong practical limitations due to restricted available space in the magnet bore. The second approach is partially accomplished with common concepts of “figure-8” or toroid winding patterns (U.S. Pat. No. 5,294,886 A), (seeFIGS. 1aand 1bof the present application), where induced currents (partially) superimpose destructively, thus strongly suppressing the effective coupling with the external field or by shielded structures (U.S. Pat. No. 4,682,125 A). However, given that in MR applications mostly low-loss coaxial cables are employed, the structures of such traps can become quite large due to the finite bending radius such cables. The previously presented toroidal structure offers imperfect decoupling to external fields due to the limited density of windings and also due to the missing compensation of the net winding around the main axis of the toroid. Adding an appropriate compensating turn would require a narrow U-turn of the cable which is usually prohibited by the mechanical properties of the coaxial cable. The resulting toroid has therefore at least one effective inductive cross-section equal to that of a circle with a radius corresponding to the mean radius of the toroid. Also, structures like the figure-8 disclosed in U.S. Pat. No. 5,294,886 A are, due to their incomplete symmetry, not decoupled from nonuniform RF fields. On the other hand, shielded traps and traps employing large conductive surfaces often exhibit unwanted interaction with the MRI scanner's gradient coils due to eddy current induction. Local magnetic fields generated by such eddy currents can impair image quality and generate vibrations and large forces on the electronics in the MRI system. Finally, the size of floating trapping structures (see U.S. Pat. No. 6,605,775 B1) as well the amount of conductive material required for their construction often prohibit application of such structures to the decoupling individual signal lines and signal lines in close vicinity to the imaging region.

Further traps according to the state of the art are disclosed in U.S. Pat. No. 4,922,204 A and in U.S. Pat. No. 5,543,713 A.

WO 2018/077679 A1 discloses a balun for use in MR imaging. The balun comprises two concentrically arranged solenoids.

SUMMARY OF THE INVENTION

In view of the above, there is a need for an improved field sheath wave barrier for use in a magnetic resonance (MR) imaging or spectroscopy apparatus that overcomes or at least reduces the above-mentioned disadvantages and limitations.

Therefore, according to one aspect of the invention, there is provided a sheath wave barrier for suppressing electromagnetic RF coupling phenomena of an electrical cable at a predetermined suppression frequency (ω0) in a magnetic resonance (MR) imaging or spectroscopy apparatus, the cable being configured as a shielded cable with at least one inner conductor and a peripherally surrounding electrically conducting cable sheath, the sheath wave barrier comprising:a segment of said shielded cable;a primary inductor formed from said shielded cable segment between a first cable location and a second cable location;a secondary inductor formed by a conductor, said secondary inductor being concentrically arranged within or around the primary inductor between said first and second cable locations;said secondary inductor being electrically connected to the cable sheath at said first and second cable connections over respective first and second RLC network members;said primary and secondary inductors being configured in compensating manner such that magnetic field generated by said primary and secondary inductors is substantially cancelled in any region surrounding the sheath wave barrier.

In the present context, “compensating” shall mean that the magnetic field produced by the secondary inductor substantially cancels the field produced by the primary inductor in a region outside the two inductors such that the magnetic field produced by the current flowing through both inductors substantially produces no field in said region.

By virtue of the fact that the primary and secondary inductors are electrically connected over respective first and second RLC network members, compensating configurations can be achieved in a geometrically simple manner, without the need to apply sharp bends to the primary inductor, which consists of a shielded cable with generally poor bendability.

Advantageous embodiments are defined in the dependent claims and described further below.

A sheath wave barrier according to the present invention is illustrated inFIG. 2a. It is based on the principle of a field compensated inductor. The current of a simple solenoid inductor is returned through a concentric solenoid with fewer windings and larger diameter. If dimensioned properly, the magnetic field lines passing through the center are forced to return within the annular space between the inner and outer solenoids. This leads to an almost complete cancelling of the magnetic field in the spatial region surrounding the sheath wave barrier.

Depending on the specific situation, it may be preferable to arrange the secondary inductor concentrically either within or around the primary inductor. An arrangement with the secondary inductor being arranged within the primary inductor may be useful when seeking size minimization. This is because it will generally be possible to form smaller windings from the secondary inductor than from the primary inductor, as the latter is formed by a shielded cable which will generally be stiffer that the conductor used to form the secondary inductor. In other situations, it may be preferable to arrange the primary inductor internally, i.e. surrounded by the secondary conductor, for example if the primary conductor is wound around a suitable central support and kept there.

The secondary compensating inductor can analogously also be added to traps whose primary inductor is formed by strand of cable equipped with an additional surrounding shielding material in analogy to U.S. Pat. No. 5,543,713 A.

In certain embodiments, the sheath wave barrier is permanently disposed in a shielded cable of a component used for certain MR applications. Alternatively, the sheath wave barrier comprises a segment of shielded cable which is provided at its ends with coaxial cable connectors.

As generally known in the technical field and illustrated inFIG. 3a, a sheath wave impedance can be defined for a given shielded cable4. In particular, a sheath wave impedance magnitude can be defined as

Zs=VSIS
wherein VSdenotes a potential of the cable sheath and wherein ISdenotes a current flowing along the cable sheath. In general, the sheath wave impedance magnitude of a shielded cable is frequency-dependent. In the following, the sheath wave impedance magnitude ZS(ω0) at the predetermined suppression frequency wo will be of particular interest.

Analogously, one can define an impedance of the sheath wave barrier, which for brevity will be called “trap impedance”. As illustrated inFIG. 3b, a trap impedance magnitude can be defined as

ZT=VTIT
wherein VTdenotes a potential difference across the sheath wave barrier2and wherein ITdenotes a current flowing through the sheath wave barrier. The trap impedance magnitude at the predetermined suppression frequency will be denoted as ZT(ω0). It is understood that the all components of the trap, in particular the mentioned first and second inductor as well as mentioned first and second RLC network will contribute to the trap impedance. In particular, resonance conditions found in the entire circuit or sub-circuits can be employed to obtain a high impedance at one or several frequencies.

While any non-zero trap impedance will contribute in reducing unwanted sheath currents, it is generally advantageous if ZT(ω0) is at least equal to or preferably (claim2) substantially larger than ZS(ω0). A ratio ZT(ω0)/ZS(ω0) of at least 5, particularly at least 10 is preferred.

According to one embodiment (claim3), the primary inductor and the secondary inductor are each configured as a solenoid comprising a plurality of windings. In order to be configured in compensating manner minimizing the extent of the magnetic field produced by the inductor, the two solenoids are disposed in mutually counter circulating manner. Due to the concentric arrangement of the primary and secondary inductor, their respective solenoid axes are substantially collinear. In one embodiment, the two solenoids are cylindrically symmetric.

Advantageously (claim4), the primary inductor is configured as a cylindrical solenoid with a first number of windings n1and a first coil diameter d1, the secondary inductor is configured as a cylindrical solenoid with a second number of winding n2and a second coil diameter d2, and the ratio d1/d2of said first and second coil diameters is substantially equal to the square root √(n2/n1) of the ratio of said second and first number of windings. Such an embodiment leads to a high degree of coil compensation.

In principle, the secondary inductor can be formed from virtually any type of electrical conductor. In an advantageous embodiment (claim5), the secondary inductor is a solid wire, a hollow conductor, a braided conductor or a litz wire. The primary and secondary inductors do not need to be formed of the same material or be of the same conductor type. In particular, the secondary inductor can be formed with a comparatively thin and flexible wire, thereby allowing for a compact design of the sheath wave barrier.

The first and second RLC network members used to form an electric connection between the ends of the secondary inductor and the first and second cable locations of the shielded cable segment can be configured in several manners. According to an advantageous embodiment (claim6), the first RLC network member is a first capacitor with a first capacitance (C1) and the second RLC network member is either a galvanic connection or a second capacitor with a second capacitance (C2). As will be understood, a galvanic connection can be understood as an RLC network just comprising a resistor with very low resistance.

According to a particularly advantageous embodiment (claim7), the primary and secondary inductors together with the first and second RLC network members form a parallel resonance circuit at the predetermined suppression frequency (ω0). Optionally, a resonant behavior and hence a suppression effect is achieved at one further suppression frequency or at even more suppression frequencies.

According to another aspect, a coil assembly for MR imaging applications comprisesa carrier structure made of non-conducting material; andat least one electrically conducting MR coil for receiving and/or emitting signal at an MR coil operating frequency, each coil being attached to and extending along a face of said carrier structure;at least one magnetic field probe (P) mounted to said carrier structure, the magnetic field probe comprisinga nuclear magnetic resonance (NMR) active substance located in a resonance region within an elongated probe compartment mutually oriented along a longitudinal probe axis and having a proximal end and a distal end;means for pulsed NMR excitation of said substance at a probe RF and for receiving an NMR signal generated by said substance;said excitation and receiving means comprising an electrically conductive structure comprising a coil member disposed around said resonance region and further comprising a pair of connecting leads, said connecting leads forming an electrical connection between respective ends of the coil member and an RF coupling unit arranged near said proximal end, said connecting leads being oriented substantially along said longitudinal probe axis, said RF coupling unit being configured for connecting to an RF line which is connectable to external RF driver and receiver means;
the coil assembly further comprising at least one sheath wave barrier according to one of the preceding claims.

The term “RF coupling unit” shall be understood to include some kind of connecting device for attaching an external RF line, which is typically a shielded cable. Furthermore, the RF coupling unit will generally include some appropriate tuning and matching electronics. The RF coupling unit is located at one end of the elongated probe compartment, which is conveniently constituted by a capillary tube. According to this design, the connecting leads are hence disposed along the capillary. This allows the entire field probe to be encased within a substantially cylindrical shield of ca. 20 mm length and 8 mm diameter.

According to one embodiment (claim9), the sheath wave barrier is disposed in a shielded electrical cable forming an RF line of a magnetic field probe. In one particular embodiment (claim10), the sheath wave barrier has a predetermined suppression frequency which is substantially equal to the operating frequency of an MR coil in the system. With such an arrangement, the NMR signal acquired from an NMR probe is protected from interference caused by the high power being sent to the MR coils.

According to another embodiment (claim11), the sheath wave barrier is disposed in a shielded electrical cable of an RF line of one of said electrically conducting MR coils. In one particular embodiment (claim12), the sheath wave barrier has a predetermined suppression frequency which is substantially equal to said probe RF frequency. With such an arrangement, the MR signal obtained with said acquisition means is protected from interference caused by the high RF power being sent to the NMR probes.

According to a further aspect, there is provided an MR apparatus for carrying out MR imaging or spectroscopy of a subject, the MR apparatus comprising:a) magnet means for generating a main magnetic field (B) along a field direction in a sample region;b) encoding means for generating encoding magnetic fields superimposed to the main magnetic field,c) RF transmitter means for generating radiofrequency fields at said MR coil operating frequency;d) driver means for operating said encoding means and RF transmitter means to generate superimposed time dependent encoding fields and radiofrequency fields according to an MR sequence for forming images or spectra; ande) acquisition means for acquiring an MR signal from said subject,
the MR apparatus further comprising at least one sheath wave barrier as defined above disposed in a shielded electrical cable of the MR apparatus.

According to one embodiment (claim14), at least two sheath wave barriers are serially disposed in respective segments of a common shielded cable of an MR apparatus. In particular, two serially arranged sheath wave barriers with different suppression frequencies may be used in certain multi-frequency MR applications.

According to a further embodiment (claim15), the MR apparatus is operatively connected to a coil assembly as defined further above. The term “operatively connected” means, in particular, that each one of said electrically conducting MR coils is connected to an associated RF transmitter means c) and/or acquisition means d).

