MRI gradient coil with variable field of view and apparatus and methods employing the same

• US 6,538,443 B2
• Filed: 03/20/2001
• Issued: 03/25/2003
• Est. Priority Date: 03/20/2001
• Status: Expired due to Term

• 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.
• Primary Examiner(s): Lefkowitz, Edward
• Assistant Examiner(s): Fetzner, Tiffany A.
• Application Number:
  US09/813,183
• Publication Number:
  US 20020171424A1
• Time in Patent Office: 735 Days
• Field of Search: 324/320, 324/319, 324/318, 324/322, 324/309, 324/307, 324/306, 324/300, 324/321, 600/410, 600/419
• US Class Current: 324/318
• CPC Class Codes: G01R 33/385 using gradient magnetic field coils
• CPC Class Codes: G01R 33/3875 using correction coil assemblies, e.g. active shimming

Abstract

A gradient coil assembly (22) generates magnetic field gradients across the main magnetic field of a magnetic resonance imaging apparatus and includes a base gradient coil set which generates magnetic field gradients which are substantially linear over a first useful imaging volume, and a correction gradient coil set which generates magnetic field gradients having substantially no first order moment. The correction gradient coil set produces third and higher order moments which combine with higher order terms of the base gradient coil set to produce magnetic field gradients which are substantially linear over a second useful imaging volume different from the first useful imaging volume. In a preferred embodiment, the second volume is continuously variable by adjusting the amounts of current applied to the base and correction coils.

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 base gradient coil set including a base primary coil and a base shield coil disposed about the examination region including an array of conductive coil loops arranged such that a current density flowing thereon generates a first order magnetic field moment and higher order magnetic field moments that define magnetic field gradients which are substantially linear over a first useful imaging volume; and

a correction gradient coil set including a correction primary coil and a correction shield coil disposed about the examination region including an array of conductive coil loops arranged such that a current density flowing thereon generates magnetic field gradients having substantially no first order moment, the correction gradient coil set producing third and higher order moments which combine with higher order terms of the base gradient coil set to produce magnetic field gradients which are substantially linear over a second useful imaging volume which is different from the first useful imaging volume when the correction gradient coil set is used in combination with the base gradient coil set, the change between the first and second useful imaging volume being caused by using a combination of the base and correction gradient coil sets.

2. The apparatus of claim 1, wherein the base gradient coil set and the correction gradient coil set are each separately shielded, the separate shielding substantially suppressing eddy currents produced by the gradient coil assembly over a range of second useful imaging volumes corresponding to a range of current densities flowing on the correction gradient coil set.

3. The apparatus of claim 1, further comprising an amplifier for supplying current to the base and correction coil sets.

4.The apparatus of claim 1, wherein the base coil set and the correction coil set occupy different radial positions about the examination region.

4. The appparatus of claim 1, further comprising one or both of:

an image distortion correction means for correcting a reconstructed image representation in accordance with preidentified spatially mapped nonuniformities in the magnetic field gradients, the spatially mapped nonuniformities varying in accordance with a selected imaging volume; and

a pre-emphasis correction means which compensates for 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.

5. The apparatus of claim 1, wherein the second useful imaging volume is larger than the first useful imaging volume.

6. A magnetic resonance imaging apparatus including:

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

a radio frequency coil 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; and

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

a base gradient coil set including base primary and shield coils disposed about the examination region including an array of conductive coil loops arranged such that a current density flowing thereon generates magnetic field gradients which are substantially linear over a first useful imaging volume, and

a correction gradient coil set including correction primary and shield coils disposed about the examination region including an array of conductive coil loops arranged such that a current density flowing thereon generates magnetic field gradients which combine with magnetic field gradients generated by the base gradient coil set to produce combined magnetic field gradients which are substantially linear over a second useful imaging volume; and

a means for continuously adjusting a current flow to at least the correction gradient coil, the second useful imaging volume being continuously variable in response to varying the amount of current supplied to one or both of the base coil set and the correction coil set.

7. The apparatus of claim 7, 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 base coil set and the correction coil set 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. A gradient coil assembly for inducing a magnetic field gradient in an examination region of a magnetic resonance imaging apparatus, the gradient coil assembly comprising:

a shielded base gradient coil set including primary and shield coils disposed about the examination region including an array of conductive coil loops arranged such that a current density flowing thereon generates a base magnetic field gradient which includes a substantially linear region over a first imaging volume and nonlinear portions outside of the first imaging volume; and

a shielded correction gradient coil set including primary and shield coils disposed about the examination region, the shielded correction gradient coil set being separately shielded from the shielded base gradient coil set, the correction gradient coil set including an array of conductive coil loops arranged such that a current density flowing thereon generates a correction magnetic field gradient that combines with the base magnetic field gradient to produce a combined magnetic field gradient which is substantially linear over a second imaging volume which encompasses the first imaging volume and at least some of the non-linear portions of the first magnetic field gradient.

9. 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 base gradient coil set including base primary and shield coils disposed about the examination region including an array of conductive coil loops arranged such that a current density flowing thereon generates the substantially linear magnetic field gradients defining a first useful imaging volume; and

a correction gradient coil set including correction primary and shield coils disposed about the examination region including an array of conductive coil loops arranged such that a current density flowing thereon generates substantially no linear magnetic gradient, the correction gradient coil set producing third and higher order and substantially no first order gradient, the third and higher order gradients combining with higher-order terms of the base gradient coil set to produce the substantially linear magnetic field gradients defining a second useful imaging volume which is different from the first useful imaging volume caused by using the correction gradient coil set in combination with the base gradient coil set.

10. The method of claim 10, further comprising:

continuously adjusting a current applied to the correction coil set to adjust the imaging volume.

11. The method of claim 10, wherein the step of inducing gradient magnetic fields includes:

applying a first current to the base coil set and a second current to the correction coil set to generate a combined gradient field having a substantially linear region; and

the first and second currents selectively variable to selectively adjust the gradient strength and spatial extent of said substantially linear region.

12. The method of claim 12, wherein the first and second currents are selected in response to user input selecting an imaging field of view.

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

selecting a first current in a base gradient coil to produce a first magnetic field gradient, the first magnetic field gradient being generally linear over a first useful imaging volume that is smaller than the selected imaging volume, the first magnetic field gradient increasing proportionally with the supplied current and substantially defining the target magnetic field gradient within the first useful imaging volume; and

selecting a second current in a correction coil to produce a second magnetic field gradient, the second magnetic field gradient having substantially no first order moment, the second magnetic field gradient combining with the first magnetic field gradient to extend the linear region from the first useful imaging volume to the selected imaging volume.

14. The method of claim 14, further comprising:

simultaneously supplying the first current to the base gradient coil and the second current to the correction gradient coil, the quantity of current supplied to each coil being continuously adjustable to produce a magnetic field gradient which is generally linear over a continuous range of imaging volumes.

15. The method of claim 14, further comprising:

applying current pulses to the base and correction gradient coils in variable amounts to generate a plurality of magnetic field gradients which are substantially linear over a plurality of useful imaging volumes and which span a range of useful imaging volumes obtainable by combining the base and correction gradient coils; and

calculating resultant gradient field nonuniformities in each of the plurality of imaging volumes.

16. The method of claim 16, further comprising:

computing eddy current corrections for the range of useful imaging volumes based on each of the calculated gradient field nonuniformities;

reconstructing an image representation of a selected imaging volume;

selecting eddy current corrections which correspond to the selected imaging volume; and

correcting the reconstructed image representation with the selected eddy current corrections.

17. Method of claim 16, further comprising:

based on the applying of currents, measuring eddy current effects for the range of useful imaging volumes to calculate a plurality of pre-emphasis corrections; and

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

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

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

designing a correction coil that produces a second magnetic field gradient having non-zero moments of orders higher than first order and substantially zero first order moment, the first and second magnetic field gradients combining to produce a third magnetic field gradient that is generally linear over a second imaging volume that is different from the first imaging volume.

19. The method of claim 19, wherein the step of designing a correction coil includes:

calculating a first current distribution that produces the first magnetic field gradient from the base coil or a combination of the base and correction coils;

calculating a second current distribution that produces the third magnetic field gradient from the combination of the base and correction coils; and

subtracting the first current distribution from the second current distribution to calculate a third current distribution defining the correction coil current distribution.