Multi-channel balun for magnetic resonance apparatus

• US 6,320,385 B1
• Filed: 09/17/1999
• Issued: 11/20/2001
• Est. Priority Date: 09/17/1999
• Status: Expired due to Fees

• Current Assignee: Howmet Research Corporation (Howmet Aerospace, Inc.), Picker International, Inc. (Koninklijke Philips N.V.)
• Original Assignee: Picker International, Inc. (Koninklijke Philips N.V.)
• Inventors: Chmielewski, Thomas, Braum, William O., Burl, Michael
• Primary Examiner(s): Williams, Hezron
• Assistant Examiner(s): Fetzner, Tiffany A.
• Application Number:
  US09/399,292
• Time in Patent Office: 795 Days
• Field of Search: 324/307, 324/309, 324/311, 324/312, 324/313, 324/314, 324/318, 324/320, 324/322
• US Class Current: 324/322
• CPC Class Codes: G01R 33/3657 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

Abstract

A multi-channel balun (70, 72) blocks stray RF current from flowing on shield conductors of coaxial RF cables of a magnetic resonance apparatus. The balun comprises a parallel combination of an even number of helical coils of shielded transmission cable (L10, L12; L14, L16, L18, L20) wound in alternate directions such that voltages iduced by their external RF fields cancel. One capacitor (C10; C16, C18) is connected in parallel symmetrically with each pair of helical coils, with trim capacitors (C12; C22) fixed in the coil plug (86, 90) to retune the balun as required. The multi-channel balun (70, 72) accommodates magnetic resonance systems with an odd or even number of channels without requiring shielding. Preferably, the balun (70, 72) is constructed on a single circuit board in a close-packed relationship that is compact and space efficient, yet provides better decoupling from the transmit field.

Claims

1. A magnetic resonance apparatus having a main magnet which generates a main magnetic field through an examination region, at least one RF coil assembly positioned adjacent the examination region, a radio frequency transmitter which drives the RF coil assembly to excite magnetic resonance dipoles in a RF transmit field within the examination region, a multi-channel receiver connected by shielded transmission cables to the at least one RF coil assembly which receives and demodulates the magnetic resonance signals, a multi-channel balun connected with the shielded transmission cables between the RF coil assembly and the multi-channel receiver to block stray induced RF currents and noise from entering the receiver, the multi-channel balun comprising:

a parallel combination of at least two helical coils wound of shielded transmission cable such that the helical coils are shielded against picking up RF fields, said helical coils being wound in opposite directions; and

at least one capacitor connected in parallel with said helical coils.

2. The magnetic resonance apparatus according to claim 1, wherein the shielded transmission cable is one of coaxial cable, triaxial cable, and twinaxial cable.

3. The magnetic resonance apparatus according to claim 1, wherein said balun is located adjacent an edge of the RF transmit field.

4. The magnetic resonance apparatus according to claim 1, said multi-channel balun further comprising:

a trim capacitor for retuning the multi-channel balun, said trim capacitor being mounted on a coil plug.

6. The magnetic resonance apparatus according to claim 1, wherein the helical coils are of similar geometry.

7. The magnetic resonance apparatus according to claim 1, wherein the RF coil assembly receives in quadrature and is connected with two coaxial cables, one for carrying each quadrature channel;

the two helical coils are connected in parallel, each helical coil connected to one of the coaxial cables.

8. The magnetic resonance apparatus according to claim 1, wherein there are an even number of helical coils, which helical coils positioned next to one another having alternate winding directions, such that voltages induced by their respective RF fields cancel.

5. A magnetic resonance apparatus having a main magnet which generates a main magnetic field through an examination region, at least one RF coil assembly positioned adjacent the examination region, a radio frequency transmitter which drives at least one RF coil of the at least one RF coil assembly to excite magnetic resonance dipoles in a RF transmit field within the examination region, a multichannel receiver connected by shielded transmission cables to a plurality of RF coils of the at least one RF coil assembly which receives and demodulates the magnetic resonance signals, a multi-channel balun connected with the shielded transmission cables between the RF coil assembly and the multi-channel receiver to block stray induced radio frequency currents and noise from entering the channels of the multichannel receiver, the multi-channel balun comprising:

a parallel combination of a plurality of helical coils wound of shielded cable, such that the helical coils are shielded against picking up RF fields, said helical coils being wound in opposite directions, each helical coil being connected with a corresponding shielded transmission cable; and

at least one capacitor connected in parallel with and symmetrically between said helical coils.

9. A multi-channel balun for blocking stray RF current from flowing in RF lines, said multi-channel balun comprising:

a parallel combination of at least a first helical coil wound of shielded transmission cable and a second helical coil wound of shielded transmission cable, said helical coils being wound in opposite directions such that the helical coils are shielded against picking up RF fields; and

at least one capacitor connected in parallel with said helical coils.

10. The multi-channel balun according to claim 9, wherein the shielded transmission cable is one of coaxial cable, triaxial cable, and twinaxial cable.

11. The multi-channel balun according to claim 9, wherein said capacitor is located symmetrically between said first helical coil and said second helical coil.

12. A multi-channel balun for blocking stray RF currents from flowing in RF lines, said multi-channel balun comprising:

a parallel combination of an even number of helical coils wound of shielded transmission cable, which coils are positioned next to one another having alternate winding directions, such that voltages induced by their respective RF fields cancel with no additional RF fields being received; and

,

at least one capacitive element connected in parallel with said helical coils.

13. A method for blocking stray RF currents from flowing in signal cables carrying RF data of a selected frequency spectrum in a magnetic resonance apparatus, said method comprising:

winding at least two helical coils of shielded transmission cable in opposite directions, such that the helical coils are shielded against picking up RF fields; and

connecting said helical coils in parallel;

tuning the parallel connected helical coils to the selected RF data frequency spectrum; and

passing RF data along inner conductors of the two helical coils such that stray RF currents on outer shield conductors of the signal cables are blocked.

14. The method according to claim 13, wherein the shielded transmission cable is one of coaxial cable, triaxial cable, and twinaxial cable.

15. The method according to claim 13, wherein the tuning step includes:

adjusting a capacitance in parallel with the helical coils, said capacitance connected through a coil plug.