Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Reexamination Certificate
1999-10-08
2001-07-03
Patidar, Jay (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
C324S319000, C324S322000
Reexamination Certificate
active
06255822
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a magnetic resonance imaging apparatus which includes a coil system which is substantially rotationally symmetrically arranged about a symmetry axis, and a power supply device for feeding the coil system with current in order to generate a steady, homogeneous magnetic field in a measuring space of the apparatus, which coil system consists of a first and a second substantially rotationally symmetrical coil sub-system, said two coil sub-systems being situated in a substantially rotationally symmetrical space which has a U-shaped cross-section in a plane containing the symmetry axis, the open side of said U-shaped cross-section facing the symmetry axis and the rotationally symmetrical space of U-shaped cross-section containing a first sub-space parallel to the symmetry axis and a second sub-space transverse the symmetry axis, the first coil sub-system being situated exclusively in the first sub-space whereas the second coil sub-system is situated exclusively in the second sub-space, the first coil sub-system conducting a first current component whereas the second coil sub-system conducts a second current component, the two current components having mutually opposed directions.
2. Description of Related Art
A magnetic resonance imaging apparatus for medical purposes, also referred to as an MRI apparatus, is arranged to form images of cross-sections of a body. To this end, in such an apparatus a strong, steady, homogeneous magnetic field is generated in a volume intended for imaging (the imaging volume). On this homogeneous field a gradient field is superposed in order to indicate the location of the cross-section to be imaged. The atoms in the tissue present in the imaging volume are then excited by means of an RF field; the radiation released upon relaxation of the excited atoms is used to form an image of the cross-section indicated by the gradient field. The steady, homogeneous field (also referred to as the main field) is generated by means of a coil system (superconducting or not). Together with the associated envelope, the coil system is shaped as a short tube which contains the imaging volume. The diameter of this tube is determined by the dimensions of the patients to be examined so that it has a given minimum value, for example of the order of magnitude of 90 cm. Generally speaking, the aim is to minimize the length of this tube relative to the diameter in order to mitigate feelings of claustrophobia for the patients to be examined and to keep the area to be examined in the imaging volume as accessible as possible for the attending staff.
The cited European patent application describes an MRI apparatus having a comparatively short coil system, so a short tubular patient space. Notably
FIG. 5
of the cited patent application shows a rotationally symmetrical coil system which is composed of a number of coils (denoted by the references
19
,
20
and
21
) which form part of two imaginary coil sub-systems. Thus, the coils
19
and
21
shown form part of a first imaginary coil sub-system and the coils
20
shown form part of a second imaginary coil sub-system.
These two coil sub-systems are arranged in a rotationally symmetrical space about a symmetry axis (denoted by the reference
7
). This space can be considered to be the envelope of the coils
19
-
21
such that it has a U-shaped cross-section in a plane through the symmetry axis, the open side of the U facing the symmetry axis. The first sub-space is then formed by the part of the U which extends parallel to the symmetry axis whereas the second sub-space is formed by the two limbs of the U which, therefore, extend perpendicularly to the symmetry axis. The first coil sub-system (i.e. the sub-system consisting of the coils
19
and
21
) is then situated in the first sub-space and the second coil sub-system (i.e. the sub-system consisting of the coils
20
) is situated in the second sub-space. A power supply device applies a current through the coils; this current may be considered to consist of two components, i.e. a first current component which flows through the first coil sub-system and a second current component which flows through the second coil sub-system. The two current components thus defined have mutually opposed directions in the known MRI apparatus as appears from the “Table 3” included in the cited document and from the associated description. In the known coil system a further coil (denoted by the reference numeral
18
) is situated in the space between the two limbs of the U and also contributes to the generating of the main field.
Even though this known coil assembly realizes a short tubular patient space, it does not provide shielding of the main field. If such shielding were required, the known system would have to be provided with additional means for shielding, for example active shielding coils which counteract the magnetic stray field outside the imaging volume as well as possible. Furthermore, in the tubular patient space of the known system a space has to be reserved for gradient coils and for coils for forming the RF field, so that the accessibility of the region to be examined is restricted to an undesirable extent.
Citation of a reference herein, or throughout this specification, is not to construed as an admission that such reference is prior art to the Applicant's invention of the invention subsequently claimed.
SUMMARY OF THE INVENTION
It is an object of the invention to provide an MRI apparatus of the kind set forth in which the main field of the apparatus is provided with active shielding and in which suitable accessibility of the region to be examined is achieved while preserving a sufficiently short tubular patient space nevertheless.
To achieve this, the MRI apparatus according to the invention is characterized in that in an absolute sense the value of the first current component is smaller than that of the second current component, and that the space which is enclosed by the first sub-space and the symmetry axis and does not belong to the second sub-space does not contain a current for generating a contribution to the steady, homogeneous magnetic field in the measuring space of the apparatus.
Even though both current components contribute to the formation of the main field, the principal function of the first current component is to provide the active shielding whereas the principal function of the second current component is the formation of the main field. The invention is also based on the recognition of the fact that a configuration of coils can be realized for which it is not necessary to provide current conductors in the space which is enclosed by the first sub-space and the symmetry axis and does not belong to the second sub-space (in the coil configuration which is known from the cited patent specification, this is the space between the limbs of the U in which the coil
18
is situated). The intermediate space thus vacated can be used to accommodate other components such as gradient coils and/or RF coils.
The second sub-space in an embodiment of the invention consists of two mutually substantially parallel sub-sub-spaces, said sub-sub-spaces being situated to both sides of the first sub-space. An attractive degree of symmetry of the construction of the coil system is thus achieved, so that the number of different coils is minimized; this is attractive from a manufacturing point of view.
The second current component in a preferred embodiment of the invention consists of a first sub-component whose direction is the same as that of the second current component, and a second sub-component whose direction opposes that of the second current component, the conductors of the second sub-component being situated nearer to the symmetry axis than those of the first sub-component. A computer simulation of this configuration reveals that the field thus obtained has a very high degree of homogeneity and that the previously mentioned intermediate space remains sufficiently large.
REFERENCES:
patent: 45875
Mulder Gerardus B. J.
Peeren Gerardus N.
Patidar Jay
Shrivastav Brij B.
U.S. Philips Corporation
Vodopia John F.
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