Actively shielded planar gradient coil for pole plate magnets of a magnetic resonance imaging apparatus

• US 5,581,187 A
• Filed: 06/15/1995
• Issued: 12/03/1996
• Est. Priority Date: 06/29/1994
• Status: Expired due to Term

• Current Assignee: Siemens AG
• Original Assignee: Siemens AG
• Inventors: Pausch, Guenther
• Primary Examiner(s): O'Shea, Sandra L.
• Assistant Examiner(s): MAH, RAYMOND
• Application Number:
  US08/490,673
• Time in Patent Office: 537 Days
• Field of Search: 324/318, 324/322, 324/300, 324/314, 324/320, 335/299, 335/301
• US Class Current: 324/318
• 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 33/385 using gradient magnetic field coils

Abstract

Each pole plate of a pole plate magnet of a magnetic resonance imaging apparatus has a sub-coil of the gradient coil allocated to it. Each sub-coil is composed of a primary and a secondary coil, respectively lying on one of two overlapping surfaces of a frustrum. The winding curve of each sub-coil changes multiply over a surface of the frustrum between the surfaces. The gradient coil can thus be optimized such that the required, maximum current densities become significantly lower compared to conventional, actively shielded arrangements.

Claims

1. A magnetic resonance imaging apparatus comprising:

two pole plates disposed opposite each other and defining an examination volume therebetween;

first and second planar gradient coils;

each gradient coil being composed of first and second sub-coils respectively associated with said pole plates, and each sub-coil being composed of a primary coil and a secondary coil;

the primary coil and the secondary coil of each sub-coil being respectively disposed on opposed surfaces of a frustrum and said primary coil and said secondary coil being connected by a plurality of windings extending in a winding path over a surface of said frustrum between said opposing surfaces with said winding path changing multiply over said surface between said opposing surfaces; and

means for supplying said primary coil and said secondary coil of each sub-coil with respective currents for causing each primary coil and secondary coil of a sub-coil to generate respective magnetic fields toward said pole plates which are substantially cancelled for actively shielding the respective gradient coils.

2. A magnetic resonance imaging apparatus as claimed in claim 1 wherein said primary coil and said secondary coil and said windings are disposed on surfaces of a frustrum of a cylinder.

3. A magnetic resonance imaging apparatus as claimed in claim 2 wherein said primary coil and said secondary coil and said windings are disposed on surfaces of a frustrum of an elliptical cylinder.

4. A magnetic resonance imaging apparatus as claimed in claim 2 wherein said primary coil and said secondary coil and said windings are disposed on surfaces of a frustrum of a circular cylinder.

5. A method for constructing a gradient coil formed by a conductor for producing a target field distribution with a prescribed current in a magnetic resonance system, said method comprising the steps of:

composing a gradient coil of first and second sub-coils;

composing each sub-coil of a primary coil and a secondary coil;

orienting the primary coil and the secondary coil composing a gradient coil on opposing surfaces of a frustrum and connecting said primary coil and said secondary coil with multiple windings in a winding path extending over a surface of said frustrum between said opposing surfaces with said winding path changing multiply over said surface between said opposing surfaces;

subdividing all surfaces of said frustrum by means of a grid mesh network placed over the frustrum, said network having grid openings and mesh branches;

occupying each grid opening in said network with a modeled elementary coil in the form of a closed turn, each elementary coil generating a respective magnetic field;

calculating the magnetic field generated by each of said elementary coils;

defining a number of ampere-turns for each elementary coil using a fit algorithm based on said target field distribution;

calculating a number of ampere-turns for each mesh branch by superimposing the ampere-turns for all of the elementary coils adjacent each mesh branch and thereby obtaining an ampere-turn density distribution over said network;

successively integrating said ampere-turn density distribution over whole-numbers of turns along an integration path based on said prescribed current to obtain a plurality of points on said frustrum; and

positioning said conductor on said frustrum in a configuration conforming to said points.

6. A method as claimed in claim 5 comprising the additional step of minimizing an overall ohmic loss of said gradient coil.

7. A method as claimed in claim 6 comprising the additional step of minimizing an overall coil energy of said gradient coil in combination with minimizing said ohmic loss.