Efficiently shielded MRI gradient coil with discretely or continuously variable field of view

• US 6,479,999 B1
• Filed: 06/05/2001
• Issued: 11/12/2002
• Est. Priority Date: 06/05/2001
• Status: Expired due to Fees

• Current Assignee: Koninklijke Philips Electronics N.V. (Koninklijke Philips N.V.)
• Original Assignee: Koninklijke Philips Electronics N.V. (Koninklijke Philips N.V.)
• Inventors: Shvartsman, Shmaryu M., Morich, Michael A., DeMeester, Gordon D.
• Primary Examiner(s): Lefkowitz, Edward
• Assistant Examiner(s): Shrivastav, Brij B.
• Application Number:
  US09/874,773
• Time in Patent Office: 525 Days
• Field of Search: 324/318, 324/309, 324/308, 324/307, 324/319, 324/313, 335/216, 335/301
• US Class Current: 324/318
• CPC Class Codes: G01R 33/3875 using correction coil assemblies, e.g. active shimming
• CPC Class Codes: G01R 33/4215 of the gradient magnetic field, e.g. using passive or active shielding of the gradient magnetic field

Abstract

A gradient coil assembly generates magnetic field gradients across the main magnetic field of a magnetic resonance imaging apparatus and includes a primary gradient coil (22p) switchable between a first configuration which generates magnetic field gradients which are substantially linear over a first useful imaging volume, and a second configuration which generates magnetic field gradients which are substantially linear over a second useful imaging volume. A first shield coil set (22s₁) is complimentary to the primary gradient coil in one of the first and second configurations, and a second shield coil set (22s₂), when either used alone or in combination with the first shield coil, is complimentary to the other of the first and second configurations.

Claims

1. A magnetic resonance imaging apparatus comprising:

a main magnet system for generating a main magnetic field through an examination region;

a radio frequency coil disposed adjacent the examination region for transmitting radio frequency signals into the examination region and selectively exciting dipoles disposed therein;

a radio frequency transmitter for driving the radio frequency coil;

a receiver which receives magnetic resonance signals from resonating dipoles within the examination region;

an image processor which reconstructs an image representation from the received magnetic resonance signals for display on a human readable display; and

a gradient coil assembly for generating magnetic field gradients across the main magnetic field, the gradient coil assembly including;

a primary gradient coil set disposed about the examination region including an array of conductive coil loops which are switchable between a first configuration in which a current flowing thereon generates magnetic field gradients which are substantially linear over a first useful imaging volume and a second configuration in which a current flowing therein generates magnetic field gradients which are substantially linear over a second useful imaging volume which is smaller than the first;

a first shielding coil set disposed about the primary coil set including an array of conductive coil loops arranged such that a current flowing thereon substantially shields a fringe field from the primary coil set when the primary coil set is operating in the first configuration; and

a second shielding coil set disposed about the primary coil set including an array of conductive coil loops arranged such that a current flowing thereon substantially shields a fringe field from the primary coil set when the primary coil set is operating in the second configuration.

2. The apparatus of claim 1, wherein the second shielding coil set is disposed about the first shielding coil set.

3. The apparatus of claim 1, further including:

a switch for selectively switching the primary coil set electrically in series with i) the first shielding coil set and ii) one of the second shielding coil set and a combination of the first and second shielding coil sets.

4. The apparatus of claim 1, further including:

a power supply and switching circuitry for supplying current to the primary coil set and the first and second shielding coil sets.

5. The apparatus of claim 1, further including:

a first power supply for supplying current to a first group of coil turns of the primary coil set and the first shielding coil set; and

a second power supply for supplying current to a second group of coil turns of the primary coil set and the second shielding coil set.

6. The apparatus of claim 5, wherein the second useful imaging volume is continuously variable in response to varying the amount of current supplied to said second group of coil turns and the second shielding coil set.

7. The apparatus of claim 6, further comprising:

a user interface permitting selection of an imaging field of view; and

a sequence controller automatically varying the amounts of current supplied the second power supply in response to the selected field of view to produce magnetic field gradients which are substantially linear over a spatial extent substantially matching the selected field of view.

8. The apparatus of claim 1, further comprising one or both of:

an image distortion correction means for correcting a reconstructed image representation in accordance with identified spatial non-uniformities in the magnetic field gradients, the non-uniformities varying in accordance with a selected imaging volume; and

a pre-emphasis correction means which compensates for temporal distortions in a gradient energization profile attributable to eddy currents induced by the gradient energization profile, the pre-emphasis correction adaptive to a selected imaging volume.

9. A gradient coil assembly for inducing magnetic field gradients across an examination region in a magnetic resonance imaging apparatus of the type including a magnet for generating a temporally constant magnetic field through the examination region, the gradient coil assembly comprising:

a primary gradient coil set disposed about the examination region including an array of conductive coil loops which are switchable between a first configuration in which a current flowing thereon generates magnetic field gradients which are substantially linear over a first useful imaging volume and a second configuration in which a current flowing therein generates magnetic field gradients which are substantially linear over a second useful imaging volume which is smaller than the first;

a first shielding coil set disposed about the primary coil set including an array of conductive coil loops arranged such that a current flowing thereon substantially shields a fringe field from the primary coil set when the primary coil set is operating in the first configuration; and

a second shielding coil set disposed about the primary coil set including an array of conductive coil loops arranged such that a current flowing thereon substantially shields a fringe field from the primary coil set when the primary coil set is operating in the second configuration.

10. A method of magnetic resonance imaging, comprising:

generating a temporally constant magnetic field through an examination region of a magnetic resonance imaging apparatus;

exciting and manipulating magnetic resonance in selected dipoles in the examination region;

demodulating magnetic resonance signals received from the examination region;

reconstructing the demodulated resonance signals into an image; and

inducing gradient magnetic fields across the temporally constant magnetic field with a gradient coil assembly comprising;

a primary gradient coil set disposed about the examination region including an array of conductive coil loops which are switchable between a first configuration in which a current flowing thereon generates magnetic field gradients which are substantially linear over a first useful imaging volume and a second configuration in which a current flowing therein generates magnetic field gradients which are substantially linear over a second useful imaging volume which is smaller than the first;

a first shielding coil set disposed about the primary coil set including an array of conductive coil loops arranged such that a current flowing thereon substantially shields a fringe field from the primary coil set when the primary coil set is operating in the first configuration; and

a second shielding coil set disposed about the primary coil set including an array of conductive coil loops arranged such that a current flowing thereon substantially shields a fringe field from the primary coil set when the primary coil set is operating in the second configuration.

11. The method of claim 10, wherein the step of inducing gradient magnetic fields includes applying a current to one of:

a first group of coil turns of the primary coil set and the first shielding coil set;

a second group of coil turns of the primary coil set and the second shielding coil set; and

the second group of coil turns of the primary coil set, and the first and second shielding coil sets.

12. In a method of magnetic resonance imaging, a method of producing a magnetic field gradient which is generally linear over a selected imaging volume, comprising:

providing a primary gradient coil having a first configuration to produce a first magnetic field gradient in response to supplying current thereto, the first magnetic field gradient being generally linear over a first useful imaging volume;

identifying one or more turns of the primary gradient coil which, when electrically decoupled from the primary coil, reconfigures the primary gradient coil to a second configuration to produce a second magnetic field gradient in response to supplying the first current thereto, the second magnetic field gradient being generally linear over a second useful imaging volume;

configuring a first shield coil to substantially shield a fringe field from the primary coil when the primary coil set is operating in the first configuration; and

configuring a second shield coil to substantially shield a fringe field from the primary coil when the primary coil set is operating in the second configuration.

13. The method of claim 12, further comprising:

selecting one of the first and second imaging volumes;

in response to the first imaging volume being selected, providing current to the primary coil, said primary coil being in the first configuration, and the first shield coil; and

in response to the second imaging volume being selected, providing current to the primary coil, said primary coil being in the second configuration, and one of i) the second shield coil, and ii) the first and second shield coils.

14. The method of claim 12, further comprising:

selecting one of the first and second imaging volumes;

in response to the first imaging volume being selected, providing current to the primary coil, said primary coil being in the first configuration, and one of i) the first shield coil, and ii) the first and second shield coils; and

in response to the second imaging volume being selected, providing current to the primary coil, said primary coil being in the second configuration, and the second shield coil.

15. The method of claim 12, further comprising:

selecting an imaging volume between the first and second imaging volumes;

electrically decoupling said identified coils from the primary coil and providing a first current to the primary coil and the first shield coil;

selecting a second current which varies in accordance with the selected imaging volume;

providing the second current to said electrically decoupled coils and the second shield coil.

16. The method of claim 12, further comprising:

applying current pulses in variable amounts to generate a plurality of gradient magnetic fields which are substantially linear over a plurality of useful imaging volumes; and

mapping or calculating resultant gradient field non-uniformities in the plurality of imaging volumes to generate at least one of;

i) a plurality of correction maps, and ii) a plurality of correction coefficients.

17. The method of claim 16, further comprising:

reconstructing an image representation of a selected imaging volume;

selecting a one of the correction maps or correction coefficients which corresponds to the selected imaging volume; and

correcting the reconstructed image representation with the selected correction map or correction coefficients.

18. The method of claim 16, wherein the correction maps or correction coefficients are calculated from a combination of corresponding correction maps or correction coefficients for each gradient magnetic field.

19. The method of claim 12, further comprising:

applying current pulses in variable amounts to generate a plurality of magnetic field gradients which are substantially linear over a plurality of useful imaging volumes;

determining eddy current effects in the plurality of imaging volumes;

calculating a plurality of pre-emphasis corrections; and

applying a one of the pre-emphasis corrections which corresponds to the selected imaging volume to the current pulses.

20. The method of claim 19, wherein the eddy current effects are determined by measuring the eddy current effects, calculating the eddy current effects, or both.

21. The method of claim 19, wherein the pre-emphasis corrections are calculated from a combination of corresponding pre-emphasis corrections for each gradient magnetic field.

22. A method of designing a gradient coil system for a magnetic resonance imaging system having a variable useful imaging volume, comprising:

designing a primary gradient coil having a first configuration that produces a first magnetic field gradient that is generally linear over a first imaging volume;

identifying one or more turns of the primary gradient coil which, when electrically decoupled from the primary gradient coil, reconfigures the primary gradient coil to a second configuration which produces a second magnetic field gradient that is generally linear over a second imaging volume;

designing a first shield coil that produces a magnetic field which substantially cancels in an area outside a region defined by the shielding coil a first fringe magnetic field generated by the primary gradient coil in one of the first and second configurations;

designing a second shield coil that produces a magnetic field, either alone or in combination with the first shield coil, which substantially cancels in an area outside a region defined by the shielding coil a second fringe magnetic field generated by the primary gradient coil in the other of the first and second configurations.