Open MRI Magnetic Field Generator

• US 20090085700A1
• Filed: 12/06/2006
• Published: 04/02/2009
• Est. Priority Date: 12/19/2005
• Status: Active Grant

• Current Assignee: Grum Teklemariam, Jianyu Lian, Xiaoping Hu
• Original Assignee: Grum Teklemariam, Jianyu Lian, Xiaoping Hu
• Inventors: Hu, Xiaoping, Lian, Jianyu, Teklemariam, Grum
• Granted Patent: US 8,077,002 B2
• Time in Patent Office: Days
• US Class Current: 335/302
• CPC Class Codes: G01R 33/3806 Open magnet assemblies for improved access to the sample, e.g. C-type or U-type magnets
• CPC Class Codes: G01R 33/383 using permanent magnets

Abstract

A magnet primarily for use in MRI applications comprises a pair of poles oriented about a plane of symmetry parallel to each therebetween defining an air gap region, magnetic field sources secured on the surfaces of the poles opposite the air gap that have yokes disposed on them, the yokes connected to each other by returns so that the entire magnet assembly can form a closed magnetic flux circuit to substantially confine the magnetic fields generated by the apparatus in the air gap where an imaging region is formed to place subjects for the purposes of examination. The main assembly being cylindrical in geometry has permanent magnets for magnetic field sources that are composed of two regions, a central disk-like portion magnetized substantially along the axial direction and an outer ring-like region magnetized substantially along the radial direction extending axially to form part of the pole together producing a very efficient and even flux distribution throughout the entire magnet assembly with minimal flux leakage. A further means of reducing flux leakage is incorporated in the yokes which have two sections, a disk-like region and an ring-like section to enclose the permanent magnets. The poles are made of multiple sections with a central disk-like region and an outer ring-like region that is a combination of permanent magnets and high permeability materials. This magnet assembly can achieve 1.0 Tesla or greater magnetic fields for whole-body scanning without saturating the magnet pole and other structures.

Claims

1. A magnet for use in MRI applications, said magnet generally comprising:

i. a pair of L-poles oriented about a plane of symmetry parallel to each therebetween defining an air gap region;

ii. a pair of L-magnets as magnetic field sources secured on the surfaces of the L-poles opposite the air gap, one pair of the L-magnets on one L-pole and the other pair on the other L-pole;

iii. said pair of L-magnets having disposed on them a pair of T-yokes on their surfaces opposite the L-poles, one pair of the T-yokes on one L-magnet and the other pair of the T-yokes on the other L-magnet;

iv. said pair of T-yokes connected to each other by returns so that the entire magnet assembly can form a closed magnetic flux circuit to substantially confine the magnetic fields in said air gap, L-poles, L-magnets, T-yokes and returns whereby said air gap, being cylindrical in geometry, forms an imaging region to place subjects for the purposes of examination.

2. The L-poles of claim 1 wherein,

i. each L-pole is comprised of a base section that is disk-like and is diametrically wide enough to generate a homogeneous magnetic field in the air gap;

ii. said base material usually made of a magnetically soft, high permeability material;

iii. the ends of each said base section having a ring-like section protruding into said air gap forming an L-segment;

iv. said ring-like section being made of a magnetically soft, high permeability material;

v. further said ring-like section having a height that can vary in length from zero to a finite value as required by the specific magnet design.

3. The L-magnets of claim 1 wherein,

i. each L-magnet is comprised of a solid disk-like central portion and an annular outer ring portion forming the L-section;

ii. said disk-like central section being magnetized substantially in the axial direction;

iii. and said annular outer ring section being magnetized substantially in the radial direction;

iv. said first L-magnet of the pair having its disk-like central section main magnetization orientation pointed axially with the North Pole pointing towards the T-yoke so that said adjacent annular outer ring portion relative to this orientation has its main magnetization orientation pointed radially out;

v. then, said second L-magnet of the pair will have its disk-like central section main magnetization orientation pointed axially as well with the North Pole pointing away from the T-yoke so that said adjacent annular outer ring portion of this second L-magnet of the pair relative to this orientation has its main magnetization orientation pointed radially in;

vi. and finally, an opposite orientation for the pair of L-magnets being possible if in the first L-magnet of the pair the North Pole for the magnetization in the disk portion points away from the T-yokes and said adjacent radial section magnetization points radially in so that in the second L-magnet of the pair the disk portion magnetization has the North Pole pointing towards the T-yokes and the radial portion pointing radially out.

4. The T-yokes of claim 1 wherein,

i. each T-yoke provides a support structure for the L-magnets;

ii. and said each T-yoke being comprised of a base yoke section and an annular ring section;

iii. said base section being sufficiently massive to carry magnetic field flux through it without substantial leakage;

iv. and said ring section of the T-yokes being sufficiently massive to carry magnetic field flux from the ring sections of the L-magnet;

v. said ring section of the T-yokes being located adjacent to the ring portions of the L-magnets for an efficient and even flux distribution throughout the magnetic circuit;

vi. and said ring section of the T-yokes containing all the magnetic flux within it without substantial leakage into surrounding magnetic circuit structures and air regions.

5. The L-poles of claim 2 wherein,

i. said base section can be polygonal in shape;

ii. so to can said ring section be polygonal in shape;

6. The L-poles of claim 5 wherein,

i. said base section can be made of a magnetically hard material as well as a magnetically soft material as claimed in claim 2 and also combinations thereof;

ii. and still further the outer circumferential edge of said base section can also be comprised of a permanent magnet material;

7. The L-poles of claim 5 wherein said ring section can also be comprised of a permanent magnet material.

8. The L-magnets of claim 3 wherein,

i. said central disk-like sections can be polygonal in shape;

ii. so to can said annular ring sections be polygonal in shape;

9. The L-magnets of claim 8 wherein,

i. each said central disk-like or polygonal section having subsections that are substantially axially magnetized but exhibiting slightly different axial magnetization orientations relative to each other;

ii. and within each said annular or polygonal outer ring section having subsections that are substantially radially magnetized but exhibiting slightly different radial magnetization orientations relative to each other.

10. The T-yokes of claim 4 wherein,

i. said ring portion improves the high reluctance path in the surrounding regions exclusive of the magnetic circuit, in particular, the regions between the said top portion of the T-yoke and the permanent magnet sections thereby enabling the return posts to be placed closer into the air gap even making contact with said ring portion of the T-yoke;

ii. and said ring portion being either tapered from the yoke end towards the air-gap end or completely eliminated to save weight.

11. The T-yokes of claim 10 wherein the ring portion can be a permanent magnet with magnetization orientation substantially in the axial direction to extend the homogeneous region in air gap and aid in further shimming the central imaging region.

12. The L-magnets of claim 3 having a ferromagnetic section to reduce mechanical forces during assembly, increase central field value and reduce saturation of the L-poles.

13. The separate ring portions of the L-magnets in claim 9 able to be mechanically moved so that the central field and homogeneity can be adjusted.