Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
1997-01-15
2001-06-12
Oda, Christine (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
C324S307000, C324S309000
Reexamination Certificate
active
06246237
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method of testing a sample, and to apparatus for testing the sample. It relates especially to Nuclear Quadrupole Resonance (NQR) testing a sample containing or suspected of containing nuclei of integral or half-integral spin quantum number (I>½).
BACKGROUND OF THE INVENTION
NQR testing is used for detecting the presence or disposition of specific substances. It depends on the energy levels of quadrupolar nuclei, which have a spin quantum number I greater than ½, 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. One particular use of the technique of the present invention is in the detection of the presence of substances such as explosives or drugs. The detection may be of baggage at airports, or of explosives or drugs concealed on the person or buried underground.
In the molecular environment of compounds in crystals, the nature and disposition of the electrons and other atomic nuclei close to the nucleus of interest produce an electric field gradient at the latter which interacts with its electric quadrupole moment to generate a set of energy levels, the transition frequencies between which are characteristic for a given substance. The presence of this frequency or frequencies not only indicates which nuclei are present, but also their chemical environment, thus indicating specific substances or types of substances in any tested sample.
In conventional NQR 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 a precessing 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.
In conventional NQR testing, either a substantial part of the f.i.d. is measured after each pulse or the responses are measured as echoes in the relatively short sampling periods between or following a succession of two or more pulses. Usually the results from a number of test pulses or test sequences are accumulated to improve the signal-to-noise ratio. Various schemes of pulse sequences have been used.
The present invention arises, in one aspect, from the surprising discovery that, in the detection of the presence of a particular substance in a given sample using nuclear resonance techniques, interfering signals may arise from the sample which may swamp the genuine nuclear resonance signals. This is particularly so in circumstances where the amount of the particular substance is much less than the amount of the remainder of the sample, and the interfering signals arise from the remainder of the sample. Such circumstances, it has now been discovered, occur commonly. For example, small amounts (maybe only a few tens of grams) of narcotics are frequently concealed within bulky pieces of airport baggage. Many common household items carried in baggage, it has been found, are likely to give rise to interfering signals.
The problem of one kind of interference when testing a particular substance is known from a book entitled “Experimental pulse NMR”, by Fukushima, E. et al, Addison-Wesley Publishing Company, Inc., pp. 466ff. The interference takes the form of spurious ringing caused by piezo-electric resonance of the substance. However this problem is not disclosed in the context of detecting the presence of the substance within a given sample.
It is known from this book to solve this particular interference problem by using an electrostatic shield (Faraday shield) between the sample and the NMR probe (usually a coil) to reduce the interference. However, such a shield can be bulky and unwieldy. Furthermore, it has been discovered pursuant to the present invention that, particularly with NQR testing, there may be different types of interference other than that caused by piezo-electric resonance, and that the Faraday shield may not successfully reduce all these types of interference.
Also, it is known from a book by Cady, E. B. entitled “Clinical Magnetic Resonance Spectroscopy”, Plenum Press (1990) (see pages 160ff.) in the field of Nuclear Magnetic Resonance Imaging to adjust the NMR probe according to the nature (for instance, size) of the body being imaged. This adjustment is not to take account of interfering signals from the body; rather, the body is actually the “substance” under test, and the adjustment is to ensure appropriate “matching” of the probe and the body. A similar technique is known from European Patent No. 0 180 121.
Such adjustment has not hitherto been contemplated for nuclear resonance techniques involving the detection of the presence of a particular substance in a given sample, partly, perhaps, because the amount of the substance under test is generally so small in relation to the total amount of the sample and the volume probed by the probe (that is, the filling factor is so small), that the possibility of requiring matching between the probe and the substance/sample has ever been contemplated. Typically, the filling factor might be only fraction of one percent.
According to a first aspect of the present invention, there is provided a method of detecting the presence of a particular substance in a given sample comprising exciting nuclear resonance in the substance and detecting the response signals from the substance, using a probe, and adjusting the probe in dependence on the character of the sample.
It has been found pursuant to the present invention that adjustment of the probe in dependence on the character of the sample can have the advantage of reducing interference from many types of substances (other than the substance of interest) commonly found in a typical sample such as airport baggage. Hence the invention can provide more accurate and sensitive tests than if such adjustment does not take place, and can also reduce the false alarm rate.
In the present invention, there is no need to adjust the probe in dependence on the character of the particular substance at all. The filling factor of the substance would be assumed to be so small (a fraction of one percent, say, corresponding to a few tens or hundreds of grams of explosive in a suitcase) that adjustment of the probe in dependence on the character of the substance is simply not necessary.
Preferably, the probe is adjusted (or adjustable) whilst it is in operation. This feature is particularly advantageous if baggage is being checked for the presence of explosives or drugs, for instance on a conveyor at an airport. The nature of the interference from each individual item of baggage may be different, and hence the sensitivity of the tests can be improved by adjusting the probe individually for each item of baggage.
The principles underlying this aspect of the invention are as follows. In the case of NQR testing, it has been determined pursuant to the present invention that there are two groups of materials which produce interference problems that may require adjustment of the r.f. probe usually used in NQR testing.
The first group includes metallic conductors which conduct electronically, such as brass, copper and aluminium. Such materials may be commonly found in many types of objects in baggage, for instance, in electronic circuitry. For this group, the interference effects are caused largely by eddy currents induced by the NQR r.f. excitation. These effects may be particu
Blanz Martin
Peirson Neil Francis
Smith John Alec Sidney
BTG International Ltd.
Oda Christine
Pillsbury & Winthrop LLP
Shrivastav Brij B.
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