Electricity: measuring and testing – Particle precession resonance
Reexamination Certificate
1997-11-07
2001-03-13
Arana, Louis (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
C324S318000
Reexamination Certificate
active
06201392
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the field of nuclear magnetic resonance apparatus and particularly to radio frequency (rf) probe structures utilizing high temperature superconducting coils.
BACKGROUND OF THE INVENTION
Nuclear magnetic resonance (NMR) spectrometers first became available in 1946. In 1950 observations of “shifted” resonances in nitrogen spectra by W. G. Proctor & F. C. Yu,
Phys. Rev
. 77, 717, (1950) stimulated efforts to improve the homogeneity and stability of magnets used in the experiments and led to the observation of chemically shifted resonances in proton spectra by J. T. Arnold, S. S. Dharmatti, and M. E. Packard,
Jour. Chem. Phys
. 19, 1608, (1951). This marked the beginning of high resolution NMR and its application as an analytical tool for chemistry, and sparked rapid growth in the development of NMR spectrometers. This development continues today at a pace limited only by the availability of relevant technology. Recent work is predicated upon improvements in rf probe performance incorporating receiver coils made from recently available high temperature superconducting (HTS) materials.
Nuclei of most isotopes of the elements have non-zero spin and exhibit gyromagnetic properties. They behave like microscopic spinning bar magnets and possess a coupled nuclear magnetic moment. In the absence of an externally applied magnetic field the nuclear moments of an ensemble of non-zero spin nuclei are randomly oriented in their atomic or molecular environment. When a static homogeneous magnetic field B is applied, the magnetic moments interact with the field and become oriented with respect to it. The spins are then said to be “polarized” by the field. Only certain orientations are allowed in accordance with well known quantum mechanical principles as described in “
Nuclear Magnetic Resonance—Principles and Theory
”, R Kitamaru, eds. Elsevier Science Publishers, 1990, Chap 2, pp. 25-36. As a consequence of the interaction of the field with the nuclear spin system, the nuclear energy level splits into multiple discrete levels corresponding to the different allowed orientations. Nuclei with spin equal ½ are the most suitable and most frequently used for high resolution NMR experiments. For simplicity and without loss of generality with respect to this present work, spin equal ½ nuclei will be assumed hereinafter. The split into two energy levels, corresponds to magnetic quantum numbers +½ and −½ for a spin ½ nucleus. The separation of the energy levels is proportional to the intensity of the magnetic field at the nucleus and to a proportionality constant &ggr; called the magnetogyric ratio. In thermal equilibrium in the static field, a Boltzmann distribution of energies is maintained. There are more spins in the lower energy state than in the higher energy state. This difference is called the Boltzmann excess.
When an ensemble of nuclei are simultaneously subjected to both a static magnetic field and an appropriate rf magnetic field of frequency &ngr; such that the energy of a quantum of radiation h&ngr;, where h is Planck's constant, is equal to the energy difference between the two spin energy levels, transitions can occur with equal probability from one state to the other. Due to the Boltzmann excess there is a net absorption of energy by the nucleus from the rf field. This transfer of energy is a necessary condition for obtaining a NMR signal.
In the aggregate, large ensembles of nuclei such as would be present in practical size samples obey the laws of classical dynamics. For convenience of visualization and ease of understanding, a classical vector model of the NMR phenomenon is hereinafter described.
With the static magnetic field applied, individual nuclei align themselves to the field, some with their microscopic magnetization vector substantially in the direction of the field, which is the low energy state corresponding to spin equal +½. Also, some nuclei align with their magnetization vector substantially in the direction opposite to the field, which is the high energy state corresponding to spin equal −½. In accordance with the Larmour Precession Theorem the individual nuclei precess about the direction of the field with an angular frequency &ohgr;
L
=&ggr;B, where &ggr; is the aforementioned magnetogyric ratio for each of the isotopic species, and B is the local field at the nuclei. More nuclei align in the direction of the field than in the direction opposite to the field, the difference being equal to the Boltzmann excess. Therefore, collectively the ensemble of nuclei exhibits a net macroscopic nuclear magnetization vector in the direction of the applied polarizing field B.
To generate a NMR signal, rf excitation is applied to the sample by a rotating magnetic field in the plane perpendicular to the direction of the polarizing field thereby enabling a transfer of energy to the spin system. The rotating field is provided by an alternating current in an excitation coil having its axis of symmetry perpendicular to the direction of the polarizing field. A linear oscillating magnetic field is generated along the x- axis of the excitation coil as shown in
FIG. 1
a
. The linear oscillating field can be decomposed into two counterrotating components, one of which, usually called the B
1
field, rotates in the direction of rotation of the aforementioned ensemble of nuclear spins, as shown in
FIG. 1
b
. When the angular frequency &ohgr; of the two rotating magnetic field components
20
and
22
is equal to &ohgr;
L
, the angular frequency of precession of the ensemble of nuclei
23
, a resonance condition exists and, as shown in
FIG. 1
c
, the net magnetization vector
8
tilts away from the z-axis
24
which is the direction of the static polarizing field
12
and precesses about it. As the magnetization vector
8
precesses about the polarizing field, it intersects the turns of a receiver coil, thereby generating a NMR signal. At resonance the angular frequency &ohgr; in the vector description equals 2&pgr;&ngr;, where &ngr; is the frequency of excitation which produces transitions between spin states in the quantum description of the phenomenon.
The broad general utility of NMR as a tool for determining the chemical structure of compounds is due to the influence of the molecular environment on the local magnetic field at the nuclei. The local magnetic field at the nucleus of a particular nuclear species at a particular site in a molecule is the vector addition of the externally applied field, altered slightly by the magnetic influence of its molecular environment. By way of example, circulation of electrons about the nucleus caused by the applied field results in an induced field at the nucleus which in some instances opposes the applied field (diamagnetism), and in some instances augments it (paramagnetism). By way of further example the local field at the nucleus can be additionally modified, taking on multiple values or “splitting” due to interactions with other non-zero spin nuclei in the molecule. As discussed hereinafter, these two effects, known as “chemical shift” and “spin-spin coupling” respectively are major sources of the fine structure seen in NMR spectra as described in “
Introduction To NMR Spectroscopy
”, R. Abrahms.; J Fisher, P. Loftus, pubs. J Wiley & Sons, 1993, chap. 2, pp. 13-33, chap. 3, pp. 34-53. NMR spectra which are characterized by resonance lines that are narrower than the shifts in resonance caused by chemical shift and spin-spin coupling are known as high resolution spectra and are primarily made possible by the application of an extremely homogeneous polarizing field.
An NMR spectrometer is comprised of: 1) a D.C. magnet which provides said stable homogeneous magnetic field for polarizing the spins, 2) an rf system which provides a suitable rf excitation signal, 3) a coil or a plurality of coils for coupling the rf excitation to the spins and for receiving the NMR response signal, 4) a detection system for detecting t
Anderson Weston
Delin Kevin A.
Fuks Luis Felipe
Withers Richard S.
Wong Wai Ha
Arana Louis
Berkowitz Edward H
Varian Inc.
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