RF coil assembly for magnetic resonance imaging and spectroscopy systems

• US 20070279061A1
• Filed: 03/22/2007
• Published: 12/06/2007
• Est. Priority Date: 04/19/2006
• Status: Active Grant

• Current Assignee: Wisconsin Alumni Research Foundation (University of Wisconsin System)
• Original Assignee: Wisconsin Alumni Research Foundation (University of Wisconsin System)
• Inventors: Holmes, James, Kurpad, Krishna, Fain, Sean, Erickson, Matthew, Grist, Thomas
• Granted Patent: US 7,714,581 B2
• Time in Patent Office: Days
• US Class Current: 324/322
• CPC Class Codes: G01R 33/345 of waveguide type G01R33/343 takes precedence
• CPC Class Codes: G01R 33/3453 Transverse electromagnetic [TEM] coils
• CPC Class Codes: G01R 33/3635 Multi-frequency operation
• CPC Class Codes: G01R 33/365 Decoupling of multiple RF coils wherein the multiple RF coils have the same function in MR, e.g. decoupling of a receive coil from another receive coil in a receive coil array, decoupling of a transmission coil from another transmission coil in a transmission coil array
• CPC Class Codes: G01R 33/3678 involving quadrature drive or detection, e.g. a circularly polarized RF magnetic field
• CPC Class Codes: G01R 33/3685 Means for reducing sheath currents, e.g. RF traps, baluns
• CPC Class Codes: G01R 33/422 of the radio frequency field
• CPC Class Codes: G01R 33/5659 caused by a distortion of the RF magnetic field, e.g. spatial inhomogeneities of the RF magnetic field G01R33/56509, G01R33/56518, G01R33/56536 take precedence

Abstract

An RF coil assembly for an MRI system includes a resonator formed by a cylindrical shield and pairs of opposing conductive legs disposed symmetrically around a central axis and extending the axial length of the shield. Drive circuitry for each pair of opposing conductive legs includes a current balun that maintains substantially equal and opposite currents in the two conductive legs. Terminal susceptance elements are used to maintain maximum currents and minimum voltage at the midpoints of the conductive legs. Multinuclear measurements can be made simultaneously at different Larmor frequencies.

Claims

1. A coil assembly for a magnetic resonance imaging (MRI) system, the coil assembly comprising:

a resonator having a cylindrical shield formed around a central axis and having a plurality of pairs of opposing conductive legs arranged symmetrically around the central axis, each conductive leg extending from one end of the cylindrical shield to another end of the cylindrical shield;

means for tuning each conductive leg to a Larmor frequency; and

drive circuitry connected to each pair of opposing conductive legs and being operable to establish substantially equal and opposite current flow in opposing conductive legs.

2. The coil assembly as recited in claim 1, wherein the drive circuitry for each pair of opposing conductive legs includes a current balun that maintains the substantially equal and opposite current flow in opposing conductive legs.

3. The coil assembly as recited in claim 2, wherein each current balun is driven by a respective quadrature signal.

4. The coil assembly as recited in claim 2, wherein a direct digital synthesizer chip controls the phase and the amplitude of current applied to an input of each current balun.

5. The coil assembly as recited in claim 2, wherein the current baluns are formed as one of a sleeve balun, a coil balun, and a balun having high impedance beads on an outer surface of a coaxial cable.

6. The coil assembly as recited in claim 2, further including a tune and match circuit located between each conductive leg and a respective current balun for nulling the reactive component of the impedance thereat.

7. The coil assembly as recited in claim 6, wherein a tune and match circuit includes a predetermined length of feedline.

8. The coil assembly as recited in claim 7, further including means for changing a length of the feedline.

9. The coil assembly as recited in claim 6, wherein a tune and match circuit includes a shunt stub matching circuit.

10. The coil assembly as recited in claim 6, wherein a tune and match circuit includes a capacitive L network.

11. The coil assembly as recited in claim 6, wherein a tune and match circuit includes a variable butterfly capacitor.

12. The coil assembly as recited in claim 1, wherein the means for tuning includes terminal susceptance elements selected from the group including capacitors and termination stubs, each terminal susceptance element connected to one end of a respective conductive leg, with the drive circuitry connected to the other end of the respective leg.

13. The coil assembly as recited in claim 12, wherein the terminal susceptance elements include coaxial cable termination stubs each having a center conductor connected to its respective conductive leg and having an outer sheath connected to the shield at a first end thereof, and wherein the drive circuitry includes coaxial cables each having a center conductor connected to its respective conductive leg and having an outer sheath connected to the shield at a second end thereof.

14. The coil assembly as recited in claim 13, wherein one or more termination stubs are open circuited at the end opposite its shield connected end, and each open circuited stub has a length that combined with a half length of its respective conductive leg is approximately equal to nλ

/4, where λ

is a wavelength corresponding to a desired Larmor frequency, and n is an odd integer.

15. The coil assembly as recited in claim 13, wherein one or more termination stubs are short circuited at the end opposite its shield connected end, and each short circuited stub has a length that combined with a half length of its respective conductive leg is approximately equal to nλ

/4, where λ

is a wavelength corresponding to the Larmor frequency, and n is an even integer.

16. The coil assembly as recited in claim 1, wherein the means for tuning includes terminal susceptance elements selected from the group including capacitors and termination stubs, each conductive leg has a terminal susceptance element at each end thereof, and the drive circuitry is connected to the midpoint of each conductive leg.

17. The coil assembly as recited in claim 16, wherein the drive circuitry includes a twin lead cable corresponding to each one of the conductive legs, each conductive leg includes two leg segments to form a conjugate dipole, with one lead of the twin lead cable driving one of the leg segments and the other lead driving the other leg segment, further wherein, for each pair of opposing conductive legs, the leads of the twin lead cable driving one of the conductive legs of the pair are interchanged with respect to the leads of the twin lead cable driving the other of the conductive legs of the pair.

18. The coil assembly as recited in claim 1 wherein at least one pair of opposing conductive legs is tuned to one Larmor frequency and at least one other pair of opposing conductive legs is tuned to another Larmor frequency.

19. The coil assembly as recited in claim 18 wherein the drive circuitry simultaneously operates with the pairs of opposing conductive legs at their respective Larmor frequencies.

20. A coil assembly for a multi-nuclear magnetic resonance imaging (MRI) system, the coil assembly comprising:

a resonator having a cylindrical shield formed around a central axis and having a plurality of pairs of opposing conductive legs arranged symmetrically around the central axis, each conductive leg extending from one end of the cylindrical shield to another end of the cylindrical shield;

first means for tuning a first pair of opposing conductive legs to a first Larmor frequency corresponding to a first nucleus;

second means for tuning a second pair of opposing conductive legs to a second Larmor frequency corresponding to a second nucleus; and

at least two current baluns, each current balun connected to a respective pair of opposing conductive legs and being operable to maintain substantially equal and opposite current flow in those opposing conductive legs.

21. The coil assembly as recited in claim 20 in which the drive circuitry simultaneously operates with the pairs of opposing conductive legs at their respective Larmor frequencies.

22. The coil assembly as recited in claim 20, wherein the first and second means for tuning include termination stubs, each termination stub is connected to one end of a respective conductive leg, and a cable from a respective current balun is connected to the other end of the respective leg.

23. The coil assembly as recited in claim 22, wherein each of the termination stubs for the first means for tuning has a length that combined with a half length of a respective conductive leg is approximately equal to n₁λ

₁/4, where λ

₁ is a wavelength corresponding to the first Larmor frequency, and n₁ is an odd integer if the termination stub is open circuited, and n₁ is an even integer if the termination stub is short circuited, and further wherein each of the termination stubs for the second means for tuning has a length that combined with a half length of a respective conductive leg is approximately equal to n₂λ

₂/4, where λ

₂ is a wavelength corresponding to the second Larmor frequency, and n₂ is an odd integer if the termination stub is open circuited, and n₂ is an even integer if the termination stub is short circuited.

24. The coil assembly as recited in claim 22, wherein each of the termination stubs for the first means for tuning are open circuited at a distal end thereof and each has a length that combined with a half length of a respective conductive leg is approximately equal to n₁λ

₁/4, where λ

₁ is a wavelength corresponding to the first Larmor frequency and n₁ is an odd integer, and further wherein each of the termination stubs for the second means for tuning are short circuited at a distal end thereof and each has a length that combined with a half length of a respective conductive leg is approximately equal to λ

₂, where λ

₂ is a wavelength corresponding to the second Larmor frequency.

25. The coil assembly as recited in claim 20, wherein the first and second means for tuning include termination stubs, and for each conductive leg, a termination stub is connected at each end, and a cable of a respective current balun is connected to the midpoint of that conductive leg.

26. The coil assembly as recited in claim 25, wherein each of the termination stubs for the first means for tuning has a length that combined with a half length of a respective conductive leg is approximately equal to n₁λ

₁/4, where λ

₁ is a wavelength corresponding to the first Larmor frequency, and n₁, is an odd integer if the termination stub is open circuited and n₁ is an even integer if the termination stub is short circuited, and further wherein each of the termination stubs for the second means for tuning has a length that combined with a half length of its respective conductive leg is approximately equal to n₂λ

₂/4, where λ

₂ is a wavelength corresponding to the second Larmor frequency, and n₂ is an odd integer if the termination stub is open circuited and n₂ is an even integer if the termination stub is short circuited.

27. The coil assembly as recited in claim 25, wherein each of the termination stubs for the first means for tuning are open circuited at a distal end thereof and each has a length that combined with a half length of a respective conductive leg is approximately equal to n₁λ

₁/4, where λ

₁ is a wavelength corresponding to the first Larmor frequency and n₁ is an odd integer, and further wherein each of the termination stubs for the second means for tuning are short circuited at a distal end thereof and each has a length that combined with a half length of a respective conductive leg is approximately equal to λ

₂, where λ

₂ is a wavelength corresponding to the second Larmor frequency.

28. A coil assembly for a magnetic resonance imaging (MRI) system, the coil assembly comprising:

a resonator having a cylindrical shield formed around a central axis and having a plurality of pairs of opposing conductive legs arranged symmetrically around the central axis, each conductive leg extending from one end of the cylindrical shield to another end of the cylindrical shield;

means for tuning each conductive leg to a Larmor frequency;

a current balun connected to each pair of opposing conductive legs and being operable to establish substantially equal and opposite current flow in opposing conductive legs, wherein each current balun is driven by a respective quadrature signal; and

wherein a direct digital synthesizer chip controls the phase and the amplitude of current applied to an input of each current balun to allow for arbitrary adjustment of the three dimensional shape of the resulting B₁ magnetic field on transmit to provide a transmitter-based parallel imaging scheme.

29. The coil assembly of claim 28, wherein the direct digital synthesizer chip is operable to provide a transmit SENSE parallel imaging scheme.