Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Reexamination Certificate
2002-01-22
2003-09-09
Gutierrez, Diego (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
C324S309000, C324S318000
Reexamination Certificate
active
06617853
ABSTRACT:
The application claims Paris Convention priority of DE 101 04 054.7 filed Jan. 31, 2001 the complete disclosure of which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
The invention concerns a magnet arrangement comprising a superconducting magnet coil system for generating a magnetic field in the direction of a z axis in a working volume disposed on the z axis about z=0, wherein the field of the magnet coil system in the working volume has at least one inhomogeneous contribution H
n
·z
n
with n≧2 whose contribution to the total field strength on the z axis about z=0 varies with the nth power of z and with a field shaping device of magnetic material, which is substantially cylindrically symmetrical with respect to the z axis. The invention also concerns a method for determining the production tolerances of the magnetic field shaping device.
Such an arrangement comprising a superconducting magnet coil system and a magnetic field shaping device is known from U.S. Pat. No. 5,396,208.
Superconducting magnets are used in many fields of application, including, in particular, magnetic resonance methods, wherein one must differentiate between imaging methods (Magnetic Resonance Imaging, MRI) and spectroscopic methods. To obtain good spatial or spectral resolution in such methods, the magnetic field in the sample volume must have good homogeneity. The geometric arrangement of the field-generating magnet coils can optimize the basic homogeneity of the superconducting magnet. Typically, recesses must be provided (so-called notch structures) wherein no wire is wound. This reduces the space for magnet windings which renders the magnet more expensive and increases the stray field.
In an arrangement according to U.S. Pat. No. 5,396,208, an MRI system is designed to be more compact by providing several soft-magnetic rings which replace certain notch structures. These soft-magnetic rings can reduce the size of the superconducting coil system in the direction of the magnet axis such that the system has an opening angle of approximately 90 degrees. A large opening angle for MRI magnet systems is advantageous for claustrophobic patients when the magnet system is used in human medical diagnosis.
In accordance with U.S. Pat. No. 5,396,208, a magnet coil system can be more effectively homogenized using a magnetic field shaping device than with notch structures. Application of this idea to a magnet arrangement for high-resolution resonance spectroscopy instead of an MRI magnet system, produces new problems. To obtain a sufficiently effective field shaping device from magnetic material, at least parts of this field shaping device must typically be mounted close to the working volume of the magnet arrangement. These parts of the field shaping device can also produce, in addition to the desired magnetic field, undesired local distortions of the magnetic field in the working volume of the magnet arrangement due to manufacturing tolerances. This problem is much greater for high-resolution resonance spectroscopic applications than for MRI applications, mainly, for the following two reasons. First of all, the homogeneity requirements on a magnet arrangement for high-resolution resonance spectroscopy are considerably higher than those for an MRI system (typically 2*10
−4
ppm in a working volume of 0.2 cm
3
compared to 5 ppm in a working volume of 34 liters for MRI systems). Secondly, the desired field shaping effect of the magnetic field shaping device of a magnet arrangement for high-resolution resonance spectroscopy is typically obtained with much less magnetic material due to the more compact dimensions as compared to an MRI magnet arrangement. This will produce greatly increased, undesired field distortions if the actual location of some magnetic material of the field shaping device differs from the nominal position by a certain amount. In other words: The efficiency of the magnetic field shaping device is larger with respect to its desired as well as to its undesired effects.
It is the object of the present invention to realize a field shaping device of magnetic material with a suitable geometrical shape in a magnet arrangement for high-resolution resonance spectroscopy such that at least part of the notch structures for field homogenization in the magnet coil system can be omitted and such that the influence of unavoidable manufacturing tolerances of the magnetic field shaping device on the magnetic field shape in the working volume can be sufficiently compensated.
SUMMARY OF THE INVENTION
This object is achieved in the invention by using the magnet coil system in an apparatus for high-resolution magnetic resonance spectroscopy, wherein the radial separation of the field shaping device from the z axis is at least partly less than 80 millimeters and compensates for at least 50% of at least one of the inhomogeneous field contributions H
n
·z
n
of the magnet coil system, and at least one additional coil system is provided which acts as a shim device in the magnet arrangement.
At least one of the inhomogeneous field contributions which occur in the magnet coil system due to omission of the notch structures is compensated by at least 50% by the magnetic field shaping device. The inhomogeneous field contributions have the dependence H
n
·z
n
with n≧2, i.e., their contributions to the overall field strength of the magnet coil system vary along the magnet axis (z axis), about z=0, with the nth power of z.
A particular advantage of a magnet arrangement comprising such a positioned magnetic field shaping device is that field inhomogeneities H
n
·z
n
, with n≧4, can also be compensated for with small amounts of magnetic material. The efficiency of the field shaping device for compensating such field inhomogeneities would be drastically reduced if the field shaping device were completely positioned at a separation larger than 80 millimeters from the magnet axis.
One embodiment of the inventive magnet arrangement is particularly preferred, wherein the actual surface positions of the field shaping device at all locations differ at the most by &Dgr;
P
from the calculated surface positions, wherein &Dgr;
P
is given by
Δ
P
=
0.2
·
∫
∫
C
∫
&LeftBracketingBar;
Δ
C
(
r
,
z
,
ϕ
)
&RightBracketingBar;
·
&LeftBracketingBar;
J
C
(
r
,
z
,
ϕ
)
&RightBracketingBar;
(
r
2
+
z
2
)
k
/
2
ⅆ
V
∫
∫
P
∫
&LeftBracketingBar;
∇
→
×
M
→
(
r
,
z
,
ϕ
)
&RightBracketingBar;
(
r
2
+
z
2
)
k
/
2
ⅆ
V
with
&Dgr;
c
Production tolerance for the maximum (radial or axial) displacement of a volume element dV of the magnet winding in the magnet coil system,
k lowest degree, except for zero, of all those coefficients of the field of the magnet coil system, when expanded in spherical harmonic functions, for whose compensation no shim coil system is provided, wherein the degree characterizes the lower index of the Legendre function P
k
m
which occurs in the associated spherical harmonic,
J
c
current density in the magnet coil system,
{right arrow over (M)} magnetization of the field shaping device,
r radial separation of the volume element dV from the z axis,
&phgr; azimuthal angle of the volume element dV.
∫
∫
C
∫
…
ⅆ
V
and
∫
∫
P
∫
…
ⅆ
V
:
volume integral over the volume of the magnet coil system and the field shaping device.
The above integral over the volume of the magnet coil system is a measure of the field distortions in the working volume of the magnet arrangement which are produced by deviations of the position of the wire windings in the magnet coil system from their nominal positions due to manufacturing tolerances. The corresponding integral over the volume of the magnetic field shaping device is a measure of the field dist
Amann Andreas
Bovier Pierre-Alain
Eckert Daniel M.
Schauwecker Robert
Bruker Biospin AG
Gutierrez Diego
Shrivastav Brij B.
Vincent Dr. Paul
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