Gradient coil set with non-zero first gradient field vector derivative

• US 6,278,275 B1
• Filed: 10/18/1999
• Issued: 08/21/2001
• Est. Priority Date: 10/18/1999
• Status: Expired due to Fees

• Current Assignee: Picker International, Inc. (Marconi Holdings Company Limited)
• Original Assignee: Picker International, Inc. (Marconi Holdings Company Limited)
• Inventors: Petropoulos, Labros S., Schlitt, Heidi A.
• Primary Examiner(s): Oda, Christine
• Assistant Examiner(s): Shrivastav, Brij B
• Application Number:
  US09/419,597
• Time in Patent Office: 673 Days
• Field of Search: 324/318, 324/309, 324/307, 324/320, 324/300, 335/296
• US Class Current: 324/318
• CPC Class Codes: G01R 33/385 using gradient magnetic field coils

Abstract

A gradient coil assembly (22) generates substantially linear magnetic gradients across the central portion of an examination region (14). The gradient coil assembly (22) includes primary x, y, and z-gradient coils (62, 66, 68) which generate a gradient magnetic field (90) having a non-zero first derivative in and adjacent the examination region. Preferably, the gradient coil assembly (22) includes secondary, shielding x, y, and z coils which generate a magnetic field which substantially cancels, in an area outside a region defined by the shielding coils, a fringe magnetic field generated by the primary gradient coils. The existence of a non-zero first derivative in and adjacent the examination region eliminates aliasing effects attributable to the non-unique gradient field values on either side of a rollover point (82). The non-unique values of the gradient magnetic field adjacent the rollover point caused structure near the rollover point to overlay each other (FIGS. 7B, 8B). The unique non-linearity of the present gradient (90) adjacent the edges expands (magnifies) the image adjacent the edges (FIGS. 7A, 8A). Because the expansion is unique, distortions at the edges are readily and accurately mapped (52) back to linear.

Claims

1. A magnetic resonance imaging apparatus comprising:

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

a gradient coil assembly for generating gradient magnetic fields across the examination region, which gradient magnetic fields have a non-zero first derivative in and adjacent the examination region, the gradient coil assembly including;

a primary gradient coil set disposed adjacent the examination region, said primary gradient coil set including an array of conductive loops for generating the gradient magnetic fields along three orthogonal axes;

a secondary shielding coil set disposed between the primary coil assembly and the main magnet, said secondary shielding coil set including an array of conductive loops such that a current density flowing thereon causes a magnetic flux density which interacts with a magnetic flux density generated by the primary magnetic field to substantially zero a net magnetization flux density outside an area defined by the secondary shielding coil set;

an RF transmitter and coil assembly positioned adjacent the examination region such that it excites magnetic resonance dipoles in and adjacent the examination region;

an RF coil and receiver assembly which receives and demodulates magnetic resonance signals from the resonating dipoles; and

a reconstruction processor for reconstructing the demodulated magnetic resonance signals into an image representation.

2. The magnetic resonance imaging apparatus according to claim 1, wherein primary gradient coil set and secondary shielding coil set are arranged on formers.

3. The magnetic resonance imaging apparatus according to claim 2, wherein the formers are hollow cylindrical tubes arranged such that the examination region is defined inside the former of the primary gradient coil set with the former of the primary gradient coil set positioned inside the former of the secondary, shielding coil set.

4. A magnetic resonance imaging apparatus comprising:

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

a gradient coil assembly for generating gradient magnetic fields across the examination region, a gradient magnetic field generated along at least one axis having (i) a substantially constant slope along a central region of the examination region and (ii) an increasingly step slope adjacent edges of the examination region;

an RF transmitter and coil assembly positioned adjacent the examination region such that it excites magnetic resonance dipoles in and adjacent the examination region;

an RF coil and receiver assembly which receives and demodulates magnetic resonance signals from the resonating dipoles;

a reconstruction processor for reconstructing the demodulated magnetic resonance signals into an image representation; and

a linearity correction processor which adjusts at least one of the demodulated resonance signals and the image representation to correct for distortion attributable to the increasingly steep slope of the gradient magnetic field adjacent edges of the examination region.

5. A magnetic resonance imaging apparatus comprising:

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

a gradient coil assembly for generating gradient magnetic fields across the examination region, the gradient coil assembly including;

three primary gradient coil sets, one for generating a gradient magnetic field along each of three orthogonal axes, each of the primary gradient coil sets generating a corresponding gradient magnetic field which is linear adjacent a central region of the examination region and monotonically increasing adjacent edges of the examination region;

an RF transmitter and coil assembly positioned adjacent the examination region such that it excites magnetic resonance dipoles in and adjacent the examination region;

an RF coil and receiver assembly which receives and demodulates magnetic resonance signals from the resonating dipoles; and

a reconstruction processor for reconstructing the demodulated magnetic resonance signals into an image representation.

6. The magnetic resonance imaging apparatus according to claim 5, further including a secondary coil disposed around the primary gradient coil sets, the primary gradient coil sets and the secondary coil cooperating to generate the magnetic field gradients in the examination region and to minimize magnetic field gradients outside of the examination region.

7. A method of magnetic resonance imaging comprising:

inducing resonance in selected dipoles in an examination region such that the selected dipoles generate magnetic resonance signals;

applying a gradient magnetic field along three orthogonal axes across the examination region to encode the magnetic resonance signals, a first derivative of the gradient magnetic field being non-zero throughout the examination region, such that the gradient magnetic field along each axis is unique in and adjacent edges of the examination region;

receiving and demodulating the encoded resonance signals;

reconstructing the demodulated resonance signals into an image representation.

8. A magnetic resonance imaging method comprising:

inducing resonance in selected dipoles in an examination region such that the selected dipoles generate magnetic resonance signals;

applying a gradient magnetic field across the examination region to encode the magnetic resonance signals along at least one axis, the gradient magnetic field (i) having a non-zero first derivative through the examination region, (ii) being substantially linear across a central region of the examination region, and (iii) changing strength monotonically adjacent edges of the examination region;

receiving and demodulating the encoded resonance signals;

reconstructing the demodulated resonance signals into an image representation.

9. A magnetic resonance imaging method comprising:

inducing resonance in selected dipoles in an examination region such that the selected dipoles generate magnetic resonance signals;

applying a gradient magnetic field across the examination region to encode the magnetic resonance signals along at least one axis, the gradient magnetic field having (i) a substantially constant slope across the central region of the examination region and (ii) a continuously increasing slope adjacent an edge of the examination region;

receiving and demodulating the encoded resonance signals;

reconstructing the demodulated resonance signals into an image representation; and

adjusting one of (i) the demodulated resonance signals and (ii) the reconstructed image representation to correct for distortions attributable to the continuously increasing slope of the gradient magnetic field adjacent the edge of the examination region.

10. A method of designing a gradient coil assembly for magnetic resonance imaging systems, the method comprising:

(a) selecting radius and length for a primary gradient coil set and radius and length for a secondary, shielding coil set;

(b) generating a first continuous current distribution for the primary gradient coil set such that the first continuous current distribution is confined within predetermined finite geometric boundaries of a first surface defined in step (a), said first continuous current distribution generating a gradient magnetic field across an examination region whose first derivative in and adjacent the examination region is non-zero;

(c) generating a second continuous current distribution for the secondary, shielding coil set such that the second continuous current distribution is confined within the predetermined finite geometric boundaries defined in step (a), the first and second continuous current distributions generating a magnetic field which substantially cancels in an area outside a region defined by the secondary, shielding coil set;

(d) optimizing the primary gradient coil set with the secondary, shielding coil set utilizing an energy/inductance minimization algorithm; and

(e) discretizing the primary gradient coil set and the secondary, shielding coil set.

11. The method according to claim 10, wherein the method further comprises:

(f) applying the Biot-Savart law to the discrete current pattern to verify its validity; and

(g) measuring and mapping non-linearities present in one of (i) the gradient magnetic field near edges of the examination region and (ii) edges of magnetic resonance images of a subject extending substantially to the edges of the examination region in order to generate a correction map.

15. The gradient coil assembly according to claim 11, wherein the magnetic field gradients generated by at least one of the x, y, and z-gradient coils is substantially linear across a central portion of the examination region and monotonically increasing adjacent edges of the examination region.

12. A shielded gradient coil assembly designed by the method of claim 12.

13. A gradient coil assembly for generating magnetic gradients across a main magnetic field of a magnetic resonance apparatus, the gradient coil assembly comprising:

x and y-gradient coils configured to generate magnetic field gradients across an examination region along first and second orthogonal axes, a first derivative of the magnetic gradient field generated by the x and y-gradient coils in and adjacent the examination region being non-zero; and

a z-gradient coil for generating magnetic field gradients along a third axis orthogonal to the first and second axes, a first derivative of the magnetic field gradient generated by the z-gradient coil in and adjacent the examination region being non-zero.

14. The gradient coil assembly according to claim 13, wherein the x, y, and z-gradient coils include primary and secondary x, y, and z-gradient coils for generating the magnetic field gradients within the examination region and substantially zeroing magnetic field gradients outside of the gradient coils.

16. A magnetic resonance imaging apparatus comprising:

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

a gradient coil assembly for generating gradient magnetic fields along three orthogonal axes across the examination region, a first derivative of the gradient magnetic fields generated by the gradient coil assembly being non-zero in and adjacent the examination region;

an RF transmitter and coil assembly positioned adjacent the examination region such that it excites magnetic resonance dipoles in and adjacent the examination region;

an RF coil and receiver assembly which receives and demodulates magnetic resonance signals from the resonating dipoles; and

a reconstruction processor for reconstructing the demodulated magnetic resonance signals into an image representation.

17. The magnetic resonance imaging apparatus according to claim 16, wherein the gradient magnetic field generating along at least one axis has (i) a substantially constant slope along a central region of the examination region and (ii) an increasingly steep slope adjacent edges of the examination region.