Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Reexamination Certificate
2001-04-20
2003-08-12
Lefkowitz, Edward (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
C324S307000, C324S322000
Reexamination Certificate
active
06605944
ABSTRACT:
FIELD OF THE INVENTION
The present application is related to the field of magnetic resonance, in particular nuclear magnetic resonance (MMR).
More specifically, the invention is related to probeheads for NMR spectometers.
Still more specifically, the invention is related to a probehead for nuclear magnetic resonance measurements having a network comprising an NMR coil for receiving a sample, a line resonator as well as a probehead terminal.
BACKGROUND OF THE INVENTION
A probehead of the afore-mentioned type is disclosed in German patent specification DE 40 02 160 C2.
This prior art probehead is intended to be used for executing nuclear double resonance experiments. During such experiments a first kind of nuclei, commonly protons (
1
H) or fluorine (
19
F) is excited and/or observed, whereas as a second kind of nuclei e.g. an isotope of nitrogen (
15
N) or of phosphor (
31
P) or of carbon (
13
C) is excited and/or observed. In up to date high field nuclear resonance spectrometers the exciting frequency for protons (
1
H) may, for example, be set to be 400 MHz or even higher. The exciting frequency (always related to the same magnetic field strength) for the afore-mentioned isotopes of nitrogen (
15
N) would be approximately 40.5 MHz, for phosphor (
31
P) 162.0 MHz and for carbon (
13
C) 100.5 MHz. The entire frequency range within which such resonance experiments are conducted, may, therefore, exceed a range of between 40 and 400 MHz and, in extreme cases for certain kinds of nuclei may even begin below 40 MHz and, at higher fields, may extend above 400 MHz.
Probeheads of the type of interest in the present context have a frequency-determining element for optimizing the frequency. This element optimizes the probehead network for the respective center frequency or, in the case of double resonance experiments, for the two measuring frequencies. Elements of the most different type have been utilized for that purpose, the specific type of the component used resulting from the specific operating frequency on the one hand side and, from the very limited space available in a probehead for nuclear resonance measurements, on the other hand side. Within a probehead the available space is limited because the probehead, when in operation, is located within a magnet system, i.e. within a superconducting magnet system at high measuring frequencies, having an accessible bore of very narrow dimensions due to the required very high homogeneity of the magnetic field.
In the prior art probehead disclosed in German patent specification DE 40 02 160 C2 a line resonator of conventional coaxial design is utilized as the frequency-determining element. On one half of its length the line resonator comprises a switch by means of which the electrical length of the line resonator may be divided in two if necessity arises.
In another similar probehead of the type of interest in the present context, as disclosed in German patent specification 198 33 350 C1 a line resonator is likewise used as the frequency-determining element. A series capacitor is provided between this line resonator and the measuring coil, and a switch is provided for bridging the series capacitor.
Both afore-mentioned prior art probeheads are used, as mentioned before, for executing nuclear resonance measurements with at least two different kinds of nuclei, i.e. so-called double resonance experiments. The measuring frequency for exciting the protons (
1
H) may be fed to the NMR coil from the probehead terminal via the line capacitor, however, this is not absolutely necessary.
U.S. Pat. No. 5,861,748 discloses still another probehead for nuclear resonance measurements which is said to also allow measurements for a plurality of nuclear resonance measuring frequencies of various kinds of nuclei. This prior art probehead consists of a highly branched assembly of coaxial lines of various lengths and different tapping points with respective different matching elements for the various measuring frequencies. The coaxial line system of this prior art probehead is of particular disadvantage when during a change from one to another kind of nuclei one has to switch over to another piece of coaxial line for the respective lower operating frequency. Moreover, it cannot be seen how the disclosed coaxial line system shall be housed within an elongate probehead of small diameter, as is typically utilized for high field magnets.
Further probehends for nuclear resonance measurements are disclosed in U.S. Pat. No. 4,446,431 as well as in published European patent application 0 579 473. These prior art probeheads also utilize conventional coaxial lines as line resonators.
High frequency lines have inductances and capacitances distributed over the line length. A high frequency line of predetermined length may be used as an electromagnetic resonator (line resonator) allowing the generation of standing electromagnetic waves. The resonance frequency of a line resonator depends on the line length, the dielectric used in the line and on the complex impedances of the elements terminating the line at both its ends. A line resonator is characterized by its characteristic impedance, its loss resistance and its characteristic wavelength of the standing waves generated therein.
As already mentioned, line resonators have been used in NMR probeheads according to the prior art, wherein coaxial lines have been used in all such line resonators, i.e. lines having a straight cylindrical inner conductor within a cylindrical outer conductor. Electromagnetic waves may propagate or may be generated as standing waves on the line in such lines at all frequencies. The waves have only electrical and magnetic field components being directed perpendicular to the direction of propagation. Such waves are conventionally identified as transversal electromagnetic (TEM) modes. They have a characteristic wavelength &lgr;
0
, being equal to the wavelength of a planar wave propagating within a dielectric material of dielectric constant ∈
r
. Besides TEM-waves modes may appear, in particular at higher frequencies, having longitudinal components of the electric or the magnetic field strength.
In NMR probeheads lines having a straight inner conductor are, for example, configured as &lgr;
0
/4-resonators, i.e. a standing wave is generated on the line corresponding to a quarter of the wavelength. If the operating frequency of the probehead circuitry is set to a predetermined value, the required geometrical length of the line depends on the dielectric constant ∈
r
of the dielectric repleting the space between the inner and the outer conductor, and of the complex impedances of the elements terminating both ends of the line. These terminating impedances may be short-circuit impedances as well as capacitors, discrete coils, other lines or circuits made from such elements. Within NMR probeheads lines having a straight inner conductor may e.g. also be configurated as &lgr;
0
/2 resonators in which a standing half wave is generated on the line. The same principal relationships mentioned above with respect to the dielectric within the line and the complex terminating impedances apply for the geometrical length of such a &lgr;
0
/2 resonator. The configuration of the lines as &lgr;
0
/4 or &lgr;
0
/2 resonators shall only be understood as examples. Within high frequency circuits also other line lengths are used, depending on the particular requirement.
If, for example, a line is used being short-circuited at one terminal thereof and being open at its other terminal with air as a dielectric between the straight inner and outer conductors, an operating frequency of 300 MHz yields a line length of 25 cm for a &lgr;
0
/4 resonator because the free wave length for an operating frequency of 300 MHz is just 1 m. An equivalent line being, however, open at both terminal ends acts as a &lgr;
0
/2 resonator having a length of 50 cm at 300 MHz. If such lines are utilized in high frequency circuits within NMR probeheads, deviations of the line length to be selected occur depending on the complex terminating imp
Bruker Analytik GmbH
Lefkowitz Edward
Vargas Dixomara
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