Electricity: measuring and testing – Particle precession resonance
Reexamination Certificate
1997-08-22
2001-03-27
Arana, Louis (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
C324S314000
Reexamination Certificate
active
06208136
ABSTRACT:
The present invention relates to a method of and apparatus for nuclear quadrupole resonance testing a sample, as well as to a pulse sequence for exciting nuclear quadrupole resonance (NQR). The invention has particular application to the detection of the presence of a given substance in a sample. The sample may contain or be suspected of containing nuclei of integral or half-integral spin quantum number (I≧½).
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 nareotics. The detection may be of baggage at airports, or of explosives or drugs concealed on the person or buried underground or elsewhere.
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 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.
As described in U.S. Pat. No. 5,365,171 (Buess et al.), spurious interfering signals (also termed “ringing”) may sometimes arise from a sample during NQR tests which are not associated directly with or due to the nuclear resonance.
For example, one group of materials which can cause interference problems includes metallic conductors. Such materials may be commonly found in many types of objects in baggage. It has been discovered that the interference may be particularly pronounced when a sample includes metallic or ferromagnetic material as a layer of plating on another material, especially, it has been found, when the plating layer comprises Nickel. Objects which are particularly prone to such problems include screws or key-rings. The cause of this type of interference has not been proven, but it is believed to emanate from ferromagnetic or like resonance effects in the B
1
field of the sample coil, and be due to a form of magneto-acoustic ringing. It should be emphasised that this interference is not an artefact of the particular detection apparatus used, but a feature of the material itself. Also it will be understood that, in the context of the detection of the presence of a particular substance in a sample, it would usually not be the particular nuclear species to be detected but the remainder of the sample which would give rise to the interfering signals.
Included in the first group as a special case are magnets and magnetic materials. Such magnets can cause interference effects similar to those described in the preceding paragraph.
A further group of materials which can cause interference problems includes substances such as piezoelectric substances which may give rise to mechanical resonances. For instance, the silica in sand can produce interference at an r.f. frequency of 5 MHz.
Furthermore, it is to be noted that the above substances, as well as many others, may give rise to other interference problems than the spurious interfering signals referred to above. Particularly, they may alter the loading of the r.f. probe used to detect the nuclear response signals. Hence the probe may need to be re-tuned and/or have its quality factor adjusted according to the nature of the sample being tested.
The spurious interfering signals (or “artefacts”) commonly have decay characteristics very similar to those of true NQR-signals, and, furthermore, are often many times stronger; they can last for several milliseconds. The phase of those interfering signals and that of the resonance response signal following a single radio-frequency excitation pulse are entirely determined by the r.f. phase within the pulse. There is, however, one important distinction. When two or more pulses are used, the phase of the NQR response signal, whether it be a free induction decay (f.i.d.) or an echo, depends on the relative phases of the two preceding pulses, unlike that of the interfering signal, which is determined almost entirely by that of the immediately preceding pulse.
This distinction has been exploited in the afore-mentioned United States patent in an attempt to remove the interfering signal from an NQR response signal. The proposed solution involves the continuous use of phase alternating excitation pulses and the addition or subtraction of the response signals from the various pulses, which has the effect of reducing the spurious signals.
Whilst the afore-mentioned United States patent also describes a solution to the problem of spurious ringing, it has been found in practice that this solution may lack versatility, in that it is restricted to the use of one particular phase alternating pulse sequence. This sequence may not be the most favourable in all circumstances.
Indeed, and in particular, the technique proposed in this United States patent has been found to have the limitation that the separation between adjacent pulses in the phase-alternated sequence which is employed must be longer than the decay time of the interfering signal. If, contrary to the teachings of this patent, the interfering signal generated in response to a first pulse were to persist through to a subsequent pulse of different phase and be detected subsequent to that pulse, a portion of the interfering signal, rather than being subtracted out from the response signal, would actually be added to the signal. This imposes very severe restraints on the sensitivity of the technique.
Also, the technique proposed in the United States patent has been found not to be capable of completely cancelling all spurious instrumental artefacts, such as errors in the phases of the excitation pulses.
The present invention seeks to solve these and other problems.
According to the present invention, there is provided a method of nuclear quadrupole resonance testing a sample comprising a first type substance containing quadrupolar nuclei and a second type substance which may give rise to spurious signals which interfere with response signals from the quadrupolar nuclei, comprising:
applying a pulse sequence to the sample to excite nuclear quadrupole resonance, the pulse sequence comprising at least one pair of pulses;
detecting response signals; and
comparing, for the or each such pair, the respective response signals following the two member pulses of the pair;
the pulse sequence being such that the respective spurious signals following the two member pulses can be at least partially cancelled by the comparison without the corresponding true quadrupole resonance signals being completely cancelled; and
for the or each such pair, the two member pulses being of like phase.
By the feature of the two member pulses being of like phase (the principle of “phase equivalence”), for each such pair, the present invention can cancel the spurious signals significantly more effectively than has been possible hitherto.
By “like phase” is preferably meant that the phases differ by less than 90°, preferably by less than 45 or 30 or 15 or even 5°.
It will be understood that references in the present specification to phases differing or being equal equates to references to frequency dif
Peirson Neil Francis
Smith John Alec Sydney
Arana Louis
BTG International Limited
Pillsbury Madison & Sutro LLP
LandOfFree
Method of and apparatus for nuclear quadrupole resonance... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method of and apparatus for nuclear quadrupole resonance..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method of and apparatus for nuclear quadrupole resonance... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2507919