Electricity: measuring and testing – Particle precession resonance
Reexamination Certificate
1999-02-26
2003-05-20
Arana, Louis (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
C324S314000
Reexamination Certificate
active
06566873
ABSTRACT:
The present invention relates to a method of and apparatus for nuclear quadrupole resonance testing a sample, and is applicable in one aspect to imaging a sample based on its nuclear quadrupole resonance (NQR) response. The invention has particular application to the detection of the presence of a given substance in a sample, and more particularly to the determination of the position and/or amount of material.
Nuclear Magnetic Resonance (NMR) techniques are now widely used for imaging, particularly medical imaging, e.g. using proton resonance. However, NMR investigations require a strong and highly homogeneous static magnetic field to operate, and this requires bulky and expensive equipment. In addition, due to the strong magnetic field, the method cannot be used in the presence of ferrous objects.
Nuclear quadrupole resonance (NQR) responses can be detected without requiring the presence of a strong static magnetic field, and so interest in using the NQR response of a body to probe its structure has recently developed. However, because NQR is a different phenomenon to NMR, existing NMR techniques cannot be directly applied to NQR investigations.
NQR testing has been increasingly widely used for detecting the presence or disposition of specific substances. The phenomenon depends on transitions between energy levels of quadrupolar nuclei, which have a spin quantum number I greater than or equal to 1, of which
14
N is an example (I=1).
14
N nuclei are present in a wide range of substances, including animal tissue, bone, food stuffs, explosives and drugs. The basic techniques of NQR testing are well-known and are discussed in numerous references and journals, so will only be mentioned briefly herein.
In conventional Nuclear Quadrupole Resonance testing a sample is placed within or near to a radio-frequency (r.f.) coil and is irradiated with pulses or sequences of pulses of electro-magnetic radiation having a frequency which is at or very close to a resonance frequency of the quadrupolar nuclei in a substance which is to be detected. If the substance is present, the irradiant energy will generate an oscillating magnetization which can induce voltage signals in a coil surrounding the sample at the resonance frequency or frequencies and which can hence be detected as a free induction decay (f.i.d.) during a decay period after each pulse or as an echo after two or more pulses. These signals decay at a rate which depends on the time constants T
2
* for the f.i.d., T
2
and T
2e
for the echo amplitude as a function of pulse separation, and T
1
for the recovery of the original signal after the conclusion of the pulse or pulse sequence.
The present invention, in one aspect, is particularly concerned with probing a sample to obtain information dependent on the position or distribution of resonant nuclei within a sample. This may be used to produce an image of the sample.
It is known that the NQR response of nuclei in a crystal is dependent on the environment of the nuclei, and also on factors such as the strength of the exciting field. If the exciting radio-frequency (r.f.) field strength varies throughout the sample, then the resonance response will also be dependent on position within the sample, and this can in principle be used to give an indication of the position of resonant nuclei within a sample.
A method for obtaining positional information using NQR, employing an r.f. field gradient, and not requiring a static magnetic field, has been proposed by Rommel, Kimmich et al. (Journal of Magnetic Resonance 91, 630-636 (1991) and also U.S. Pat. No. 5,229,722). Those disclosures (see page 631, line 25 of the paper and column 6, lines 46-50 of the patent) teach that NMR techniques such as phase-encoding (in which both the phase and the amplitude of the r.f. signal received from the sample are used to obtain information about the sample) cannot be applied to NQR imaging, and that only amplitude encoding is possible with NQR imaging. In other words, it is stated that it is only possible to extract a single parameter (signal amplitude) from an NQR imaging experiment which uses an r.f. field gradient in the absence of a static magnetic field. This is stated to be consistent with the theory that the transverse magnetisation associated with an NQR response oscillates, in contrast to the precession about the applied magnetic field observed in an NMR experiment.
Our earlier United Kingdom Patent Number GB-2,257,525 discloses a method of imaging using NQR in which a field gradient is imposed upon a sample. Reference should be made to that disclosure for useful background information and further discussion of the art of imaging using NQR which is not repeated here. In that patent, surprisingly advantageous results were obtained by subjecting a sample to a field having a particular positional dependence. Although that method can enable a satisfactory image to be obtained, there is still some room for improvement.
The present invention seeks to provide a method and apparatus for probing a sample by detecting its NQR response which alleviates some or all of the drawbacks of previous methods. Preferred arrangements disclose a probing technique in which positional information may be obtained even in the absence of a controlled static magnetic field.
The invention is applicable to detection of quadrupolar nuclei (I≧1) and is particularly applicable to nuclei such as
14
N (I=1) in which advantageous results can readily be obtained in the absence of a static magnetic field, but may be used for detecting other quadrupolar nuclei, for example I=3/2, I=5/2 etc. The invention is particularly applicable to polycrystalline samples, or samples containing one or more polycrystalline clusters of quadrupolar nuclei.
In developing the invention, it has been appreciated that there are many NQR applications in addition to imaging in which it would be desirable to obtain more information than signal amplitude, but this has hitherto not been possible from a single measurement.
Surprisingly, the inventors have found that two independently varying components (e.g. phase and amplitude dependent components) can be extracted from a received NQR response signal if the excitation is selected appropriately. A preferred method of achieving this is to use two excitation pulses of selected phase. This can lead to a more reliable classification of the object under test.
The prior art has not reported detection of two independently resolvable components resulting from NQR interactions. Indeed, theory predicts only a single component is to be found, and Rommel et al. states that phase encoding is not possible in NQR experiments.
The phase and amplitude dependent components may actually be phase and amplitude, but it is to be understood that references herein to phase and amplitude dependent components are intended to include components derived from or related to the phase and amplitude of the response signal without necessarily being directly representative thereof. In particular, the signal may be resolved into two components, both of which vary as functions of both phase and amplitude. For example, in a preferred arrangement, the received signal is (initially) resolved into two components having a quadrature relationship. Phase-related information may be obtained by combining the two components in a first manner (e.g. comprising determining a ratio of the components) and amplitude-related information may be obtained by combining the components in a second manner (e.g. comprising summing a function of the components).
The extra information obtainable by the provision of both phase and amplitude information in an NQR experiment may be useful in a number of ways, as will be understood by one skilled in the art based on the discussion below.
In an imaging experiment, the provision of both phase and amplitude information can provide better classification of the sample than the amplitude encoding alone technique of Rommel et al. where the received signal amplitude is dependent on both the position (as
Peirson Neil Francis
Smith John Alec Sydney
Arana Louis
BTG International Limited
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