Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
2002-11-05
2004-12-21
Fulton, Christopher W. (Department: 2859)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
C324S318000
Reexamination Certificate
active
06833701
ABSTRACT:
This application claims Paris Convention priority of DE 101 57 972.1 filed Nov. 27, 2001 the complete disclosure of which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
The invention concerns a nuclear magnetic resonance (NMR) spectrometer with a magnet arrangement for generating a homogeneous static magnetic field B
0
in the direction of a z axis and a radio frequency (RF) resonator containing one or more superconducting components for receiving NMR signals from a measuring volume.
An arrangement of this type is known from U.S. Pat. No. 5,619,140 (Reference [1]).
NMR is a highly distinctive and precise method for analyzing the structure of chemical compounds. However, it is not very sensitive. For this reason it is of primary importance in NMR to provide resonators which have maximum detection sensitivity, i.e. maximum S/N ratio.
The use of cooled and in particular superconducting RF resonators minimizes the losses in the resonator thereby considerably increasing the sensitivity. The currently most suitable superconductors are high-temperature superconductor (HTS) materials. They have a high transition temperature and are much less sensitive to static magnetic fields than are other superconductors.
The substance to be measured is usually a liquid and is contained in a measuring tube which is normally at room temperature, separated by an intermediate tube and a vacuum chamber from the cold NMR resonator which is at approximately 20K.
Arrangements with cooled superconducting RF coils are described e.g. in [1] or [7].
FIG. 2
schematically shows a cross-section through an NMR magnet with shim system and cryo probe head with cooled superconducting RF coil (resonator). An RF coil arrangement of this type is schematically shown in
FIGS. 3
a
and
b
.
FIG. 3
a
is a perspective view and
FIG. 3
b
a section in the xz plane.
The substantial problem in using superconductors in NMR receiving systems (RF receiver coil) is their static magnetization. If not controlled, it can produce field disturbances within the sample of such a magnitude that the line width becomes unacceptably large. A number of methods have been published which minimize this undesired magnetization [2], [3] or which at least try to minimize it [4].
The described methods have, however, the serious drawbacks which are described below. In particular, even with disturbance-free coils, subsequent undesired applied transverse magnetic fields lead to the recurrence of substantial disturbance fields.
It is therefore the underlying purpose of the present invention to prevent the occurrence of disturbing magnetization in RF coils of [1]. If the methods according to [2],[3] are used for these coils, their magnetization can be substantially eliminated. The major problem arises during subsequent operation, since undesired mechanical tilting of the coils with respect to the magnet could re-magnetize these coils. The present invention should also permit a coil that has been demagnetised once to always remain demagnetised during its entire subsequent operation.
In addition to the coil types [1] there is an additional type of known superconducting RF coil (described in [5] and [6]) which are already highly insensitive to possible disturbing transverse fields.
The positive effects of the inventive device are also compatible with these additional coil types ([5],[6]) and are, in fact, cumulative, i.e. the present invention further reduces the remaining, very small disturbances of these coil types by an additional substantial factor.
The range of applications of certain configurations of these coils is also extended. The combination results in extremely disturbance-free and long-term stable superconducting RF coil arrangements which meet the highest of standards, far beyond those of prior art, in terms of freedom from disturbance and stability, thereby offering new fields of application.
As shown in detail in [2] and [6], currents flowing in closed paths within a type II superconductor lead to an overall magnetization of the superconductor. They are determined by the previous history of the superconductor and remain for an essentially unlimited length of time due to the zero resistance of the superconductor, as long as the external conditions remain unchanged.
In the conventionally used thin layer coils (e.g. [1] or
FIGS. 3
a, b
), this magnetization has predominant importance mainly in the direction perpendicular to the substrate, since the surface in which the associated current flows is the largest. This transverse magnetization M
T
(
FIG. 6
a
) has substantially greater impact than does the longitudinal magnetization (in the z direction), since the extremely thin layer presents a very small surface available for longitudinal magnetization (which, together with the current loop, generates a dipole moment).
In any event, magnetization generates an additional magnetic field outside of the superconductor which can cause strong, undesired field disturbances in the sample volume.
FIG. 6
b
schematically shows the effects of transverse magnetization on the field disturbances outside of the RF coil and in particular on the B
z
component inside the NMR sample.
The task at hand is therefore to minimize the transverse magnetization M
T
which could otherwise greatly disturb operation.
Terminology
SC coils are discussed in detail below which are tilted with respect to the static field B
0
of the magnet. This requires precise definition of the terminology and, in particular, of the coordinate system used.
The term “transverse magnetic field change” is an additional magnetic field component dB
T
in the coordinate system of the superconducting coil, which is perpendicular to the direction of the static magnetic field B
0
existing prior to this change. This also corresponds, to first order in the coordinate system of the SC coil, to a rotation of the static magnetic field relative to the SC coil, which is equivalent to rotation of the coil with respect to the static magnetic field.
For the flat or sheet-like superconducting structures which are primarily considered herein and which are oriented substantially parallel to the magnetic field, this causes magnetization of the superconductor in the direction perpendicular to the superconductor surface. We call it “transverse magnetization”. This magnetization M
T
is oriented in first order perpendicularly to the static field B
0
.
The previous approach for minimizing disturbances in the spectra through inhomogeneities of the static magnetic field utilized the following strategies:
1. Minimize the maximum possible magnetization magnitude (through subdivision of the coil into sufficiently narrow strips [1], [5]).
2. Attempt to suppress generation of remaining possible magnetization through slow cooling of the superconductor in the field [4].
3. Post-treatment of the superconducting coil with a sequence of decreasing transverse magnetic fields for “demagnetisation” [2], [3] (thereby inducing a current structure with closely spaced, opposing current regions such that the sum of the individual magnetic field contributions cancels to a good approximation).
4. Design the superconducting structure of the coil such that disturbances in the magnetic field are allowed but the RF active field is limited through normally conducting elements to a field with only very slight disturbances [5].
5. Design the superconducting coil structure such that disturbances in the magnetic field are permitted while effecting a uniform distribution of magnetization in the z direction through an elongated construction of the coils having a macroscopically homogeneous distribution of the superconducting material, however with RF interruptions [6]. One can thereby show that these coils can strongly disturb the B
x
and B
y
components of the magnetic field but the B
z
c
Bruker Biospin AG
Fulton Christopher W.
Vargas Dixomara
Vincent Paul
LandOfFree
Stabilization of transverse magnetization in superconducting... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Stabilization of transverse magnetization in superconducting..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Stabilization of transverse magnetization in superconducting... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3326576