Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
2001-09-25
2003-09-09
Smith, Ruth S. (Department: 3737)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
C324S318000
Reexamination Certificate
active
06618606
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a vertical field-type MRI apparatus for forming magnetic resonance images, including at least one field generating superconducting coil system for producing a substantially homogeneous magnetic field in an imaging volume of the apparatus, which coil system includes an outer coil that is situated in a flat outer coil plane and a supplementary coil that is situated within the outer coil, and at least one gradient coil system for producing a magnetic gradient field in the imaging volume of the apparatus. The gradient coil system includes a flat main gradient coil and a shielding coil.
BACKGROUND OF THE INVENTION
An apparatus of this kind is known from U.S. Pat. No. 5,939,962. The homogeneous magnetic field required for magnetic resonance imaging in such a vertical field type apparatus is usually generated by two oppositely situated magnetic poles wherebetween the patient to be examined can be arranged. Generally speaking, said magnetic field then has a vertical direction. Apparatus of this kind offer the advantage that the patient keeps a comparatively broad view of the surroundings when arranged in such an apparatus, so that sensations of claustrophobia occur less frequently.
An iron circuit that is capable of transporting the complete flux through the system becomes very heavy in the case of magnet systems having a field strength beyond approximately 0.5 T. A sensible alternative in that case is to omit the iron circuit completely and to construct the magnet system as an actively shielded air coil system. In that case there are no poles in the sense of iron structures that bound the space of the magnet system that is accessible to the patient, but the surfaces of the magnet system that bound the patient space will also be referred to hereinafter as “poles” for the sake of simplicity. For field strengths beyond 0.5 T the coils must be constructed so as to be superconducting. They are kept at the operating temperature in a cryostat. The “poles” are then formed by the outer wall of the vacuum envelope of the cryostat.
The cited United States patent discloses a superconducting coil system which consists of a round outer coil (a so-called “side coil” that is denoted by the reference
12
a
therein), a round supplementary coil (referred to therein as “the fourth coil” which bears the reference
12
d
), and a number of further coils (referred to as “the second and the third coils” bearing the references
12
b
and
12
c
therein). The homogeneous field in the imaging volume is generated mainly by the first two coils
12
a
and
12
d
and the other coils mentioned superpose a further homogenizing field thereon.
As is generally known, and also described in the cited United States patent, for this type of apparatus the aim is to arrange the field generating coil in the upper magnetic pole at an as short as possible distance from the field generating coil in the lower magnetic pole. This aim stems from the fact that the production costs of such a system increase by approximately a power of five of said pole distance, so that it is advantageous to keep this distance as small as possible. Because of this aim, the outer coils in the known apparatus are arranged practically directly against the boundary of the freely accessible space between the magnetic poles.
When the outer coils are mounted in that manner, the gradient coils in the known apparatus must extend to practically the diameter of the associated outer coils because of the necessary linearity of the gradient field in the imaging volume. Consequently, room for the supplementary coil (also having a voluminous and heavy construction so as to achieve the required homogeneous field) can be found only above the upper gradient coil and below the lower gradient coil. Consequently, the construction of this already bulky and heavy coil must be even larger; however, the outer coil must then also become larger. Moreover, in the case of actively shielded magnetic coils the shielding coils must then also become larger. The ultimate effect of the foregoing is that the costs of the apparatus are significantly increased again.
OBJECTS AND SUMMARY OF THE INVENTION
It is an object of the present invention to provide a vertical field-type MRI apparatus in which the distance between the outer coils and between the supplementary coils is as small as possible. To achieve this object and others, in the apparatus in accordance with the invention, the supplementary coil is situated in the outer coil plane, the energizing of the outer coil and the supplementary coil is such that these coils generate magnetic fields of opposite direction, the ratio D
a
/D
o
of the diameter D
a
of the supplementary coil to the diameter D
o
of the outer coil is between 0.7 and 0.9, the apex of the shielding coil of the gradient coil system is situated at a distance from the main gradient coil which is larger than the distance between the edge of the shielding coil and the main gradient coil, and the gradient coil system is situated at least partly in a space within the supplementary coil.
The invention is based on the recognition of the fact that the minimum distance between the field-generating poles can be realized by imparting a special shape to the shielding coil of the gradient coil system, that is, a shape such that the center or apex of the shielding coil is situated at a distance from the main gradient coil which is larger than the distance from the edge of the shielding coil to the main gradient coil. The shape of the shielding coil is thus substantially conical. As a result of the shape of the gradient coil system, the supplementary coil which is energized in an opposite sense or direction relative to the outer coil can be situated in the same plane (the outer coil plane) as the outer coil. This yields a suitably homogeneous field in the imaging volume if the ratio D
a
/D
o
of the diameter D
a
of the supplementary coil to the diameter D
o
of the outer coil is between 0.7 and 0.9. A space still remains in the supplementary coil so as to accommodate the gradient coil system thus formed.
Said special shape of the gradient coil system also yields an additional advantage is obtained. This is because it has been found that said system can be proportioned in such a manner that a slit-like space remains between the shielding coil of the system and a corresponding recess in the cryo container of the coil system that generates the homogeneous field. This space can be used to accommodate the so-called shim iron which is used in known manner so as to compensate changes of the magnetic environment, that is, to compensate circumstances degrading the homogeneity. Periodic adaptations of the shim state are necessary and to this end the shim iron is then readily accessible and can be displaced without it being necessary to remove the entire system of gradient coils. This advantage is due to a generally non-angular profile of said special shape of the shielding coil.
The shielding coil in a preferred embodiment of the invention extends across a substantially conical surface. This conical shape is then taken up in said free space within the supplementary coil and the flat main gradient coil is then situated substantially in the outer coil plane.
An advantageous embodiment of the apparatus in accordance with the invention is provided with three further coils, each of which is situated in a respective further coil plane, the outer coil plane being situated between the imaging volume and each of said further coil planes
It has been found that a suitable compromise can thus be achieved between production costs (condition: few and small further coils) and field strength and homogeneity (condition: many and large further coils).
In a further advantageous embodiment in accordance with the invention the three further coils are situated on a conical surface, the apex of the conical surface being directed away from the imaging volume.
Apart from the fact that this configuration very well satisfies the requirements as regards field str
Aldefeld Bernd
Overweg Johannes Adrianus
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