Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Reexamination Certificate
1999-07-26
2001-02-20
Oda, Christine K. (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
C324S322000, C324S321000
Reexamination Certificate
active
06191583
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an apparatus used in nuclear magnetic resonance (NMR) imaging, and more particularly, to a new and improved toroid cavity detector that reduces the effects of probe ringing when the toroid cavity detector is used in connection with NMR spectroscopy and tomographic imaging.
2. Background of the Invention
Large inventories of uncharacterized and heterogeneous nuclear material need to be characterized. Nuclear magnetic resonance (NMR) techniques have been used as a nondestructive assay (NDA) to characterize these materials before packaging (in particular to determine moisture content) and also to examine the materials after they have been packaged. One NMR technique that has been used involves a toroid cavity probe. This technique enables the determination of both the presence and quantity of individual elements as well as full chemical-shift resolution (i.e., chemical speciation). For example, determinations can be made as to the presence of hydrogen in such materials and as to whether hydrogen is bound with oxygen (water) or another element such as a salt. At the same time, protons in other forms, such as hydrides, hydrates and acids, also are detectable and quantifiable.
The ability to make such determinations and the sensitivity of toroid-based NMR imaging makes this type of technique ideal for special nuclear material (SNM) characterization before such material is packaged. In fact, such nondestructive methods of examination of such SNM provides reductions in work force radiation exposure. Moreover, such techniques possibly could be used to determine select fission and activation products for a wide variety of critical applications (such as waste management and SNM management, control and accountability) and could be used to meet international safeguards with respect to the management of such products.
In one type of toroid cavity detector, a sample to be analyzed is placed in the toroid cavity and the toroid cavity is placed in an externally applied static main homogeneous magnetic field (B
0
). The presence of the B
0
field causes the magnetic moments of a targeted class of nuclei in the sample to precess about the field's axis at a rate which is dependent on the magnetic field strength. Another magnetic field (B
1
) is produced that is perpendicular to the B
0
field and is alternately energized and de-energized within the cavity. In the case where the B
1
field is produced by a radio frequency (RF) transmitter pulse applied to an inner wire extending along the central axis of the cavity, the net sample magnetization is caused to rotate about the B
1
field axis. Following the RF pulse, the response of the sample to the applied field is detected and in most applications, received signals from the energized nuclei serve as input signals for spectroscopic analysis of the sample.
One of the unique features of the toroid cavity detector compared to some other types of detectors is the full containment of the RF magnetic field flux that is generated when the inner wire is pulsed with an RF current. As a result, the RF magnetic field flux that emanates from a sample contained within the toroid cavity and subjected to an excitation pulse of electromagnetic radiation is completely captured (i.e., detected). This feature makes the toroid cavity detector two to four times more sensitive to weak nuclear resonance signals than conventional coil resonators and allows a quantitative measurement of the total number of NMR-active nuclear spins in the sample, not possible with currently available NMR detectors.
For samples placed in a magnetic field of sufficiently high homogeneity, it is possible to record different chemical species containing a common nucleus. Not only can a proton be distinguished from a uranium nucleus, but also protons in water molecules can be distinguished from protons in hydrogen gas molecules. In fact, protons in water molecules that are in different physical environments can be distinguished. The resolving capability of chemical shift makes the toroid cavity detector a useful device for analyzing and monitoring various types of radioactive materials from spent nuclear fuel to plutonium ash.
Another technique that can be used with a toroid cavity detector is rotating frame imaging (RFI). An asymmetrically shaped transmitter coil is used to achieve a B
1
field gradient within the sample region. Different regions of the sample absorbs energy at different rates because the energy absorption process is controlled by the local strength of the B
1
field. Thus, RFI can be used to image a homogeneously filled toroid cavity such that the relative quantity of sample that is located at each accessible value of the B
1
field can be measured. As a result, this technique can be used to determine the diffusion coefficients of nuclei and the molecular species of the nuclei.
The toroid cavity detector also differs from other, more conventional electromagnetic detectors in that it produces a gradient in the generated RF magnetic field. This B
1
field gradient has a mathematically well-defined spatial distribution. This magnetic field gradient feature has two significant consequences. First, the sensitivity of the toroid cavity is radially distributed, with the greatest sensitivity near the central axis of the device. This enables measurements on samples of limited quantity. Second, concentric annular regions of a sample contained in the toroid cavity exchange energy with the resonator circuit at different rates. Thus, analysis of energy transfer rates in a toroid cavity can yield a radial spatial mapping of the different nuclear constituents in a cylindrical sample container. The toroid cavity also has been proven to function with asymmetric conductors.
A simple extension of the spatial imaging capability of the toroid cavity detector is the ability to spin-label the sample with nuclear magnetization alternating in opposite polarities in a series of concentric cylindrical shells. With this labeling procedure, mobility of nuclei within the cavity can be measured. The mobility can be stochastic or coherent and it can be measured on a variable length scale extending from a few micrometers to several millimeters. This technique is suitable for measuring transport velocities of fluids and gases within a sealed container or at the interior surface of a canister where there might be perfusion.
As is disclosed in U.S. Pat. No. 5,574,370, the toroid cavity of a toroid cavity detector can be a cylindrically shaped, hollow housing which is closed at both opposite ends and made for example of copper. A central conductor consisting of an inner wire in a Teflon outer jacket is fed through the base of the cylinder and is positioned coincident with the major axis of the cylinder. In alternate embodiments disclosed in the '370 patent, slots are machined in the side walls, top and bottom of the toroidal cylinder to provide access to the toroid cavity for a fluid or gas sample being analyzed by the toroid cavity detector or a series of openings and sample holders can be positioned in the top of the toroidal cylinder in order to provide access for tubes containing the samples to be analyzed by the toroidal cavity detector.
In the toroid cavity detector of the type disclosed in the '370 patent, the B
1
field is completely confined within the cavity formed in the housing of the toroid cavity detector and is generated when an RF signal is transmitted along the central conductor. The B
1
field is strongest near the central conductor and drops off as the inverse of the distance toward the outer wall of the toroid cavity. Both sensitivity and distance resolution increase with the gradient in the B
1
field and consequently, the toroid cavity detector provides a means to image simultaneously both the chemical shift of the targeted nuclei as well as their radial distance from the center and is well suited for NMR microscopy of films that uniformly surround the central conductor. In this regard, the
Gerald, II Rex E.
Nunez Luis H.
Rathke Jerome W.
Mason, Kolehmainen Rathburn & Wyss
Oda Christine K.
Shrivastav Brij B.
The University of Chicago
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