Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Reexamination Certificate
2002-04-10
2004-04-27
Bennett, G. Bradley (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
C324S322000
Reexamination Certificate
active
06727700
ABSTRACT:
This application claims Paris Convention priority of DE 101 18 835.8 filed on Apr. 17, 2001 the complete disclosure of which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
The invention concerns an RF (=radio frequency) receiver coil arrangement for receiving measuring signals from a measuring sample in the measuring volume of an NMR (=nuclear magnetic resonance) spectrometer, comprising an RF resonator having superconductive, inductively and capacitively acting conductive structures, which form resonant circuits, which are disposed on planar substrate elements, and which are disposed externally around the measuring sample.
An arrangement of this type is known from U.S. Pat. No. 5,585,723.
NMR is a very powerful and exact method for analyzing the structure of chemical compounds. Unfortunately however, it is not very sensitive. For this reason, it is of central interest in NMR to provide resonators which have as high a detection sensitivity as possible, i.e. as high a signal to noise (S/N) ratio as possible. The use of cooled and in particular superconducting radio frequency resonators permits the losses in the resonator to be kept very small thereby considerably increasing the sensitivity.
HTS material is currently the most suitable superconductor. This material has a high transition temperature and, compared to other superconductors, is very insensitive to static magnetic fields. These advantageous properties are obtained only when the superconductor is made from very thin epitaxial layers which are formed on oriented monocrystalline substrates. Since such substrates are normally only available as flat plates, the geometric shape of the resonators is constrained to be in the form of flat plates, thereby considerably limiting the possible geometric arrangements.
The substance to be measured is normally a liquid enclosed in a measuring tube usually at room temperature, which is separated from the cold NMR resonator (at approximately 20K) by an intermediate tube and a vacuum chamber.
Such arrangements are known e.g. from U.S. Pat. No. 5,585,723. Superconducting receiving resonators have the following principal problems. The static magnetization produced by the superconductor can cause field inhomogeneities which substantially prevent the generation of narrow lines in high-resolution NMR. Moreover, the finite critical current in the superconductor limits the maximum coil current for the transmitter pulse, thereby complicating or impeding short pulse widths for a predetermined NMR flip angle.
As mentioned above, the sensitivity of the resonator, i.e. its S/N ratio, plays a primary role in NMR. It depends on the volume integral of the term B
1
(x,y,z)/sqrt(P), wherein B
1
designates the high frequency field which is produced by the resonator at the location x, y, z when provided with power P. If the contributions of the currents in the conductors all have identical values, the conventional equation P=R·I
2
can be inserted for the power P, wherein the term in the volume integral becomes B
1
(x,y,z)/I·sqrt(R). This means that the larger the B
1
field produced by the resonator per unit current I and the smaller the overall loss resistance R of all conducting paths, the higher the sensitivity.
The NMR resonator must not only have high sensitivity but its conductors must also carry large currents to permit production of as high B
1
fields as possible during the transmitting phase. High B
1
fields permit short transmitter pulses for obtaining a given desired NMR flip angle. Such short transmitter pulses are highly desirable in NMR for various reasons which will not be discussed herein. Although resonator sensitivity allows optimum conversion of the current into a B
1
field, this is not sufficient in and of itself. The current should also be as high as possible. This, in turn, depends i.a. on the critical current of the superconductor and on the width of the conductor. The geometry of the resonator should therefore be such that those conductors which are mainly responsible for producing the B
1
field are as broad as possible and can carry the full critical current.
A further parameter which is of major importance to the NMR spectrum is the resolution. The resolution substantially depends on the relative line width which can be obtained for the NMR signal which, in turn, depends on the homogeneity of the stationary magnetic field H
0
in the active region of the measuring sample. Since the static magnetism of the superconducting material in the resonator can have a strong negative effect on the homogeneity of the H
0
field due to the short separation of the superconductor from the measuring sample, the surfaces of the individual superconducting conducting elements must be specially designed. One solution of prior art provides for the subdivision of all conductors into narrow strips to minimize the influence of magnetism to the greatest possible extent.
The Q of the resonator is also an important value which can influence the NMR spectrum. It should not be too high in order to keep transient and decay processes, mainly caused by the transmitter pulses, short for minimizing undesired artifacts in the NMR spectrum. Since the Q value is equal to the ratio &ohgr;L/R, i.e. the impedance of the overall inductance of the resonator, divided by the overall resistance of all conducting paths, the inductance L of the resonator should be as small as possible for a given resistance R. This can be obtained by keeping only those inductances which are mainly responsible for generating the B1 field in the measuring sample, and by reducing or compensating for all others to the extent possible.
Moreover, the resonator should permit a second resonator to be mounted as close to the measuring sample as possible such that its magnetic field is orthogonal to that of the first for minimizing magnetic coupling between the two resonators. The resonance frequencies of the two resonators can either be the same or different. Of these two options, the latter is usually chosen and the second resonator serves to excite and/or receive signals from a second type of nucleus. Orthogonal resonator arrangements of this type are often used in NMR for special experiments.
In particular, two types of resonators are known, i.e. the coplanar Helmholtz resonator (see U.S. Pat. No. 5,585,723) and the hybrid resonator (see U.S. Pat. No. 6,121,776).
Coplanar Helmholtz Resonator
This resonator consists of two planar resonant circuits in a Helmholtz arrangement (see
FIGS. 7 and 8
) which are tuned to the same resonance frequency f
0
and which produce two resonant modes due to their strong inductive coupling, one below and the other above f
0
. The mode with the lower resonance frequency is the only one which can produce a homogeneous B
1
field in the measuring volume. The other mode produces a highly inhomogeneous field and can therefore not be used as an NMR detector.
The opening angle &agr; of the resonant circuit arrangement is usually selected to be 120° (see
FIG. 7
) to obtain the best homogeneity of the B
1
field.
Since the coil consists exclusively of superconducting material, the losses are very small and the sensitivity could be very high were it not for other impairing factors. In particular, the planar geometry prevents the conducting paths from being placed in the direct vicinity of the measuring sample, therefore producing a correspondingly smaller B
1
field at the location of the measuring sample. The sensitivity is consequently reduced.
The structure of each of the two resonant circuits is shown in FIG.
9
. All conductors perform a dual function, namely generation of both the resonance inductance and the resonance capacitance of the resonant circuit. It can be shown that the largest possible average current through such conductors is only half of the critical current for the superconductor should the individual conductors have the same widths. The maximum B
1
field which can therefore be obtained is likewise only half the size of the highest theoretically possible field
Bennett G. Bradley
Bruker Biospin AG
Fetzner Tiffany A.
Vincent Paul
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