MRI magnets

Details MRI magnets MRI magnets

1997-03-11

2001-04-03

Arana, Louis (Department: 2857)

Electricity: measuring and testing

Particle precession resonance

Spectrometer components

C324S320000

Reexamination Certificate

active

06211676

ABSTRACT:

This invention relates to electromagnets for use in magnetic resonance imaging (MRI) systems and in particular to magnets which are open, i.e., magnets in which an imaging volume wherein that portion of a patient being imaged is positioned is not surrounded by the magnet.
A special requirement for MRI is a strong uniform magnetic field, typically 0.2 to 2 Tesla, with a field homogeneity of a few parts per million in the imaging volume, typically a sphere of 30 cm to 50 cm in diameter. Most commonly such a field is produced by an electromagnet having a solenoid construction but this necessitates a patient being surrounded by the magnet and enclosed within a tube, which can cause a feeling of claustrophobia and limit access to a patient as may be required for some surgical or diagnostic procedures. However by using open magnets, to which this invention particularly relates, these problems are overcome.
Open electromagnets for use in MRI systems are well known. One known form of electromagnet comprises a pair of juxtaposed magnetic poles of opposite polarity between which the imaging volume is defined, which poles are linked and supported by a yoke which provides a magnetic flux return path and which principally comprises a generally C-shaped steel frame. Because large amounts of steel are required, these known C-shaped magnets are very heavy, especially for high field magnets which require many tons of steel to define the flux return path.
Very heavy magnets may cause problems with manufacture, transport, installation and maintenance as will be readily appreciated.
It is therefore an object of the present invention to provide an MRI system comprising an open electromagnet wherein the requirement for large amounts of steel, which adds undesirably to the weight of the system, is obviated.
According to the present invention an open electromagnet includes a pair of field coils which respectively comprise juxtaposed poles of the electromagnet between which an imaging volume of substantially homogeneous magnetic field is defined, toroidal shielding coil means arranged on a locus which forms a loop, so that when energized, the said shielding coil means produces a controlling magnetic field which configures a magnetic flux return path for the field coils so that the magnetic flux return path describes the said loop, and a support structure which serves rigidly to support the shielding coil means and the field coils.
The shielding coil means may in accordance with one aspect of the invention comprise a plurality of toroidal shielding coils arranged in spaced apart relationship on a common locus which describes the loop.
The toroidal shielding coils may be substantially identical to each other.
Alternatively, in accordance with another aspect of the invention, the shielding coil means may comprise a single, continuously wound, toroidal shielding coil which describes the loop.
By using the toroidal shielding coil means to configure the magnetic flux return path of magnetic flux produced by the field coils, the need for heavy steel yoke components which might otherwise be required to define the return path is obviated.
The electromagnet may be a superconducting electromagnet wherein the shielding coils and the field coils are contained in a cryostat maintained at a temperature close to absolute zero.
This low temperature may be achieved by immersing the coils within the cryostat in liquid helium and/or by using a two stage refrigerator and heat shields to which respective stages are thermally coupled.
The electromagnet may be a C-shaped electromagnet wherein the common locus defines a single loop which includes the magnetic flux return path and which is closed by a region between the juxtaposed poles which includes the imaging volume.
The support structure may be fabricated of non-magnetic material such as aluminium for example.
The magnetic field in the imaging volume may be finely adjusted to achieve high homogeneity by means of a plurality of pieces of magnetic material supported on a pair of non-magnetic plates positioned adjacent respective poles so that diametrical axes of the plates lie orthogonal to the direction of lines of flux passing between the said poles.
The open electromagnet may form a part of an MRI system which includes gradient coils and other components for magnetic resonance imaging as are well known to those skilled in the art.


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patent: 5347252 (1994-09-01), Ries
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patent: 2284061 (1995-05-01), None
patent: WO93/15514 (1993-08-01), None

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