Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Reexamination Certificate
2002-03-29
2003-03-18
Lefkowitz, Edward (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
C324S309000
Reexamination Certificate
active
06534984
ABSTRACT:
This application claims Paris Convention priority of DE 101 17 595.7 filed Apr. 7, 2001 the complete disclosure of which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
The invention concerns a main field magnet arrangement in one-sided magnetic resonance devices for generating a homogeneous magnetic field B in a working volume, wherein the working volume and the main field magnet are disposed on different sides of a plane E and the main field magnet has a geometric shape which permits arrangement of a radio frequency (RF) receiver coil system and optionally a gradient coil system on the same side of the plane E as the main field magnet wherein field-generating elements of permanent-magnetic material are provided which are magnetized in the same sense and in the direction of the magnetic field B in the working volume and which have a radially outer part and a radially inner part relative to an axis A extending through the center of the working volume, wherein the surface, facing the working volume, of the radially inner part is axially spaced further apart from the center of the working volume than the surface, facing the working volume, of the radially outer part, with a depression being formed in the axial direction in the radially inner part.
An arrangement of this type is e.g. known from U.S. Pat. No. 5,959,454.
An appropriate mathematical description of the spatial dependence of the magnetic induction B(&rgr;,&thgr;,&phgr;) as a function of the spherical coordinates &rgr;,&thgr;,&phgr; (&rgr;: radius, &thgr;: polar angle relative to the axis, &phgr;: azimuthal angle) about a point in the center of the working volume is given by the expansion according to spherical harmonic functions:
B
(
ρ
,
θ
,
φ
)
=
∑
l
=
0
∞
∑
m
=
-
1
+
1
C
l
,
m
ρ
1
P
l
,
m
(
cos
θ
)
(
cos
m
φ
+
i
sin
m
φ
)
P
l,m
(cos &thgr;): associated Legendre polynomial of degree l and order m,
C
l,m
: Amplitude, i
2
=−1
The amplitudes C
l,m
are complex numbers. Since the magnetic induction B is real, the amplitudes C
l,m
with identical l and oppositely equal m are complex conjugates of each other. For magnets which should generate as homogeneous a magnetic field in a working volume as possible, the term with l=m=0 is the only desired term since it describes an ideally homogeneous magnetic field. All other terms are disturbance terms which impair homogeneity. In the geometrical design of main field magnets for generating homogeneous magnetic fields, one therefore tries to eliminate as many disturbance terms of a degree l>0 as possible up to as large a value l
0
as possible through optimization of the geometric arrangement of the main field magnet. In these cases, the dependence of magnetic induction B along any straight line through the center of the working volume is given by superposition of the desired constant term with parabolas of order >l
0
+1. The latter provide only negligible contributions to the magnetic field proximate the origin which is therefore homogeneous through a limited working volume, and is suitable for magnetic resonance method examinations. For rotationally symmetric magnet arrangements, the magnetic field is also rotationally symmetric such that for arrangements of this type all terms with m unequal 0 theoretically disappear. The second summation completely vanishes and the number of possible disturbance terms of amplitudes C
l,m
to be compensated when designing the arrangement, is considerably reduced. For magnet arrangements which are additionally mirror-symmetrical in the axial direction relative to the center of the working volume, all disturbance terms disappear with even-numbered degree l. In these cases, the geometric design of the main field magnet must eliminate only very few, e.g. five, disturbance terms to obtain a theoretically very large working volume. For these reasons, commercial main field magnets for magnetic resonance devices according to current prior art usually have a mirror symmetry in the axial direction relative to the center of the working volume and are always rotationally symmetric. Deviations from pure rotational symmetry exist, if at all, due to magnetic components of the magnet arrangement which are far away from the respective working volume, such as magnetic yokes of iron in magnet arrangements having iron shielding. The components of magnet arrangements which are close to the working volume are in any case rotationally symmetric. In practice, none of the magnet arrangements mentioned herein is suited for generating sufficiently homogeneous magnetic fields on its own since unavoidable tolerances during production generate relatively small disturbance terms of a low degree l and low order l which are, however, still inadmissibly large for the application and which therefore must be compensated for with a suitable additional magnet arrangement, the shim system, which can be precisely set.
U.S. Pat. No. 4,689,591 describes arrangements of magnetic field generators, mainly magnet coils which are generally rotationally symmetric relative to an axis, however, not symmetrical in the direction of the axis. In this reference, permanent magnets are also used as magnetic field generators which are preferably rod-shaped and are magnetized in the direction of their longitudinal axes. A concrete design of such magnet arrangements is not disclosed.
U.S. Pat. No. 5,959,454 discusses a class of main field magnets which consists exclusively or principally of permanent-magnetic components which are exclusively rotationally-symmetrical about a common axis. The magnet arrangements according to U.S. Pat. No. 5,959,454 comprise a radially outer ring of permanent magnetic material which is magnetized in the direction of the axis. A rotationally-symmetrical insert of permanent-magnetic material, which is also magnetized in the same direction as the outer ring, is disposed inside the outer ring. In the most simple form, the insert also comprises one or more nested rings and possibly an full inner cylinder. The particularly advantageous property of these arrangements consists in that the insert represents a depression on the side of the magnet arrangement facing the working volume which can be sufficiently large that a relatively large working volume with sufficiently homogeneous and relatively strong magnetic field becomes possible.
Moreover, there is sufficient space in the depression for receiving an actively shielded one-sided gradient coil system and an RF coil. The magnetic field in the working volume of these arrangements has an axial orientation (z direction). The magnetic induction in the working volume in the embodiment of U.S. Pat. No. 5,959,454 is approximately 14% of the magnetic remanence polarization of the magnetic material. With the magnetic material NdFeB having a remanence polarization of 1.3 T, one obtains an induction of 0.18 T which is within the usual range of commercial magnetic resonance devices.
The magnet arrangements according to U.S. Pat. No. 5,959,454 are rotationally symmetrical but are designed for applications where mirror-symmetry relative to the center of the working volume is impossible. In the embodiments disclosed therein, all disturbance terms of the order m=0 disappear due to the rotational symmetry and all disturbance terms of the degrees 1, 2, 3, 4 disappear due to the further geometrical design. However, disadvantageously, the exclusively permanent-magnetic components must be rotationally symmetric. NdFeB, which is preferably used for such magnets, is not suited for rotationally-symmetric shaping since it cannot be processed by turning on the lathe. Grinding into shape is the preferred processing method for this material. Although grinding into a cylindrical shape is possible, flat surfaces are nevertheless preferable. A further disadvantage consi
Bruker BioSpin GmbH
Lefkowitz Edward
Shrivastav Brij B.
Vincent Paul
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