Patent Application
20070096731
1. Field of the Invention
The present invention relates to sensors for the identification of substances and, more particularly, a system and method using nuclear quadrupole resonance under conditions of environmental interference for the simultaneous identification of one or more illicit substances, such as narcotics or explosives, which may be hidden on or inside a human body or personal belongings.
2. Description of the Related Art
Security technology for controlling the traffic of illicit substances is rapidly growing in demand. Nuclear Quadrupole Resonance (NQR)-based screening systems have been proven to provide reliable and noninvasive identification of materials containing the so-called quadrupolar nuclei, such as 14N or 35,37Cl, which are present in most explosives and in many of the narcotics. This methodology is not harmful to individuals or the scanned objects, and permits remote detection without the need for palpation or any mechanical contact. Additionally, automatic operation of the scanners is possible, making this technology much less dependent on a human error. The principles and the instrumentation used in NQR are, generally, similar to those employed in Nuclear Magnetic Resonance (NMR), which is a powerful and well developed technique for the investigations of solid and liquid materials, as well as for medical imaging (in the form commonly referred to as Magnetic Resonance Imaging or MRI). Both methods employ on-resonance radiofrequency (RF) magnetic field pulses (B1 field pulses) to excite transitions between the energy levels of the detected nuclei, by way of interacting with their intrinsic magnetic moments. This excitation is followed by a relaxation process, during which the nuclei emit a response RF signal that can be detected by the same or a different sensor that was utilized for the excitation. The frequency of this signal is, generally, specific to the local environment of the nucleus, and can be used to study molecular structure or to betray the presence of a certain type of a molecule in a sample.
There are some important differences between NQR and NMR, the most significant of which relates to the manner in which the energy levels are initially established. In NMR the nuclei possessing nonzero magnetic moments become polarized by an externally established static magnetic field, B0, whose magnitude mainly determines the resonance frequency at which the signals coming from the nuclei will oscillate. Stronger B0 fields lead to a greater extent of nuclear polarization and, therefore, to increased sensitivity of the measurements. In NQR, on the other and, an external magnet is not required because the nuclear levels are established due to coupling between the electric quadrupole moments of nuclei, eQ, and the electric field gradients, eq, internally generated by the charge distributions in the local molecular environments. Nuclei with nonzero electric quadrupole moment (non-spherically symmetrical electric charge distribution) are those with spin I>½, which includes such common nuclei as 14N and 35,37Cl. Although this interaction is purely electric in nature, since the nuclei also possess magnetic dipole moments, it is possible to induce transitions between the nuclear levels with B1 fields and detect the signals produced by the nuclei in response, much like in NMR. At the same time, no application of an external static magnetic field is required, which is why NQR spectroscopy is frequently referred to as “NMR at zero field”.
The Hamiltonian describing the quadrupole interaction in the principal axes frame of the electric field gradient is given in terms of the nuclear spin operators, I, Ix, Iy and Iz, as follows:
where the quantity e2Qq is defined as the quadrupole coupling constant of a nucleus in its environment, and η describes the asymmetry of the electric field gradient. The nuclear properties are represented by the quantity eQ and the influence of the electrostatic environment is described by η and eq. For the spin I=1 14N nucleus the three quadrupole eigenstates in terms of eigenstates of the Iz operator, |1>,|0> and |−1>, are |+>=(|1>+|−1>)/√{square root over (2)}|−>=(|1>−|−1>)/√{square root over (2)} and |0>. The transition frequencies are given by:
The NQR spectrum of a compound in which 14N nuclei experience non-axially symmetric electric field gradients (η≠0) will, therefore, consist of a doublet corresponding to the υ+ and υ− transitions and a line at a much lower frequency corresponding to υ0. The intensity of the transition at υ+ is at its maximum when the RF field is applied in the X direction of the principal axes frame for the electric field gradient tensor, and the intensity of the υ− transition is maximized when the B1 field lies in the Y direction. For a powder sample, a B1 field applied in the laboratory frame will be experienced by each crystallite in a different direction in its principal axes frame, with all directions being equally probable.
As a result the effect of the B1 field applied to an isotropic powder sample in every laboratory frame direction will appear the same, in the sense that the generated signal will have similar properties, although it will be originating from different crystallites in the sample. Since explosives or narcotics are isotropic substances, the direction of the B1 field used for their identification is unimportant, the only relevant measure being its amplitude.
The frequencies of the NQR measurements are, generally, on the order of several MHz, much lower then those of NMR or MRI, which are on the order of several tens or hundreds of MHz. The sensitivity of the measurements is also much lower. There is, however, an important advantage of not having to place objects in strong external magnetic fields, which led to a tremendous interest in this technology in the field of illicit substance detection, where accurate, noninvasive and remote identification of materials is necessary, but the use of the external magnetic fields is undesirable, as it can damage the magnetic parts of the studied objects and endanger the people in the vicinity. Additionally, the NQR signals exhibit very high specificity to the molecules being observed, thereby providing very reliable material identification, unlike NMR, which is more suitable for structure investigations.
Various sensor designs are currently used in conjunction with the NQR scanners. Cylindrical or rectangular close-shaped RF coils may be used (solenoid, single-turn, multiple loop, etc.) for the screening of such objects as luggage or mail, which can be put through the internal volume of the sensors. These coils offer uniform B1 fields and can be easily shielded from the RF environmental interference by placing an RF shield around the entire sensor (the coil with the screened items contained inside). There are, however, many situations when it is impossible or undesirable to place the studied objects inside a restricted volume, such as during the scanning of a minefield or of a human subject. In this case, surface devices may be used (single turn, spiral, planar solenoid, etc.). While these devices offer greater accessibility, they suffer from the environmental radiofrequency interference, coming from far away sources, such as commercial radio stations, or from the presence of other equipment in the vicinity, such as computers, switching power supplies, etc.
One design aimed at introducing environmental interference rejection properties into the surface sensors uses gradiometer coils that are immune to the environmental noise by being sensitive only to a spatial derivative of the electromagnetic field. Noise coming from a distant source can be assumed linear in space (wavelengths are much larger than the size of the coil) and, therefore, is not detected. These coils can be made, for example, by forming two electrically connected loops, one above the other, that are wound in the opposite direction. The noise from a distant source induces equal and opposite currents in the loops, canceling itself out. The sample is placed closer to one loop than to the other, and produces a stronger current in one of them than in the other. It is, therefore, detected by the coil assembly.
Another system uses two separate planar solenoid coils wound in an opposite sense and connected in series or in parallel or driven by a common circuit that couples them together and to a transmitter or receiver. The coils are positioned one above the other or side by side. Alternatively, the coils are wound in the same sense, but a phase inversion is performed in one of them before the signals from both are combined at the receiver. Noise coming from a distant source is picked up by the two coils and arrives at the receiver as two signals with opposite phases, leading to its self-cancellation. This coil assembly, therefore, possesses the property of common mode rejection. The sample is always placed closer to one coil than to the other, and its signal is, therefore, not self-cancelled. The approach of having a dedicated interference detector to be half of the sensor assembly has a general disadvantage of reducing the coil filling factor, η, by half, which leads to a reduction in the SNR, since it is proportional to √{square root over (η)}.
It has been proposed that the simultaneous detection of two samples may be realized if each of them is placed within the active volumes of each of the two coils comprising a sensor assembly similar those described above. For example, a two-coil detector may be used for the control of forbidden substances hidden in shoes. The coils are constructed such that the distant source noise signals are attenuated due to their being detected equally by each coil, followed by a phase inversion in one of the coils, leading to self-cancellation upon summation at the receiver. Both coils are involved in sample excitation performed with opposite phases in the two coils. The sample signals are, therefore, also detected with opposite phases, after which one of them undergoes a phase inversion, leading to their constructive interference at the receiver. This approach, however, assumes some prior knowledge of the possible illicit substance location, and provides no detection capability outside of this region (in the region between the coils, for example).
NQR active materials normally exhibit multiple resonance lines at a range of frequencies. Simultaneous detection at more than one frequency can be utilized to make the detection very specific, drastically decreasing the possibility of false-positive alarms. Additionally, a sensor with multi-frequency capability could be used for simultaneous detection of various target substances, which is an important practical necessity. The measurements performed with different frequency channels of such sensor need to be independent, and, therefore, the channels have to possess a high degree of isolation (−20 dB is usually sufficient). Common multi-tuned coils, such as surface of solenoid coils, generally rely on the difference in frequency between the channels as a source of this isolation, and, consequentially suffer from the inability to have close frequency positioning, that may be required. Geometric decoupling is proposed as a means to alleviate this issue, utilizing surface coils with mutually perpendicular B1 fields. This approach, however, requires complex shaping of the sensors, restricting their applicability. Additionally, only three such universally decoupled cannels are possible, while any additional resonance frequencies are attained by multi-tuning the individual coils, which makes these frequencies susceptible to the abovementioned limitation.
It is well known that the transmission efficiency and sensitivity of the radiofrequency sensors is inversely proportional to the square root of their active volumes and directly proportional to their filling factors, η. When a sensor is used for scanning of electrically conducting objects, such as a human body, restricting the active volume leads to an increase in the the quality factor (Q), providing a further increase in the SNR, which is proportional to Q1/2. The active volume of a coil can be controlled by adjusting the penetration depth of the B1 field that it generates, and, therefore, that it is able to detect, according to the principle of reciprocity. The coil's η can be adjusted by choosing a shape most suitable for the object being scanned.
It is also becoming increasingly important to be able to rapidly and accurately determine the presence of illicit substances, such as explosives or drugs, which may be concealed and transported not only in the personal belongings of travelers, but also in the garments or even inside their bodies. Increasing security threats start to demand such measures as installation of checkpoints at the entrances to public transportation systems, buildings, stadiums, public events, etc. Inspection of a human body, however, is a very challenging task, since many of the bulk detection methods commonly utilized in baggage screening, for example, X-ray absorption-based systems, are inapplicable due to their harmful side effects on the health of those being screened. Body imaging methods, for example, X-ray diffraction-based, involve much lower amounts of harmful radiation, but require extensive image interpretation efforts by specially trained personnel and cannot check for the objects hidden inside a body. Additionally, since these imaging methods reveal the body's surface along with the hidden objects, they have raised privacy-related concerns.
It is therefore a principal object and advantage of the present invention to provide a system and method for detecting illicit substances in the presence of environmental noise.
It is an additional object and advantage of the present invention to provide a system and method for detecting illicit substances that does not require prior knowledge of the possible locations of target substances.
It is a further object and advantage of the present invention to provide a system and method for detecting illicit substances that has multiple, well isolated (orthogonal) channels, useful at different frequencies simultaneously and independently without requiring complicated sensor shapes.
It is another object and advantage of the present invention to provide a system and method for detecting illicit substances that can select the penetration depth of the B1 field so that the active volume, filling factor, and quality factor may be optimized for maximal efficiency and sensitivity.
It is yet a further object and advantage of the present invention to provide a system and method for detecting illicit substances that is capable of being adapted to closely match the shape of the object to be scanned.
It is yet an additional object and advantage of the present invention to provide a walk-through checkpoint system suitable for the reliable and rapid human body scanning.
In accordance with the foregoing objects and advantages, the present invention provides a system and method using the noise-resilient resonant modes of open-shape, multi-element sensors for nuclear quadrupole resonance detection of target materials. The embodiments of the present invention comprise designs and techniques for designing sensors for the NQR detection of a wide range of illicit substances, such as explosives or narcotics, or to any other NQR application, such as industrial process monitoring, that is carried out in the presence of environmental interference and/or in the situations where open-shape devices are preferred. The embodiments of the present invention further comprise a methodology and design criteria for the construction of the surface or open-volume sensors possessing properties such as noise-rejection, horizontally uniform B1 field magnitude (no blind spots along the surface), capacity for simultaneous multi-frequency operation, penetration depth control and shape adaptability, which are the characteristics identified as necessary in the previous section. The embodiments of the present invention can be utilized with any NQR spectrometer system capable of producing RF pulses of appropriate power and frequency, and of receiving the NQR signals. The embodiments of the present invention are, however, preferably used with a multi-channel system capable of delivering RF pulses and acquiring signals at different frequencies simultaneously and independently through its different channels.
The present invention comprises various sensor types, such as planar, half-cylindrical open-volume birdcage, or transverse electromagnetic (TEM) coils, that are designed specifically for use in NQR-based applications in order to provide the necessary parameters for the detection of illicit substances in environmental noise and permit their use in low-frequency NQR application. The designs of the sensors of the present invention are based on the general principles of conventional open birdcage and the open TEM coil designs. More specifically, the embodiments of the current invention are based on an 8-window open-shape birdcage coil design and on a 9-element open-shape TEM coil design. Both designs have 9 legs carrying the current, responsible for the generation and the reception of the B1 fields in the sensor's working area. An open birdcage or TEM coil can be viewed as a half-wave resonator where a standing wave is formed in the direction perpendicular to the coil's legs. The current amplitudes in the legs are modulated sinusoidally going from one leg to the next, such that an integer number of half-periods fit between the first and the last leg. Modes are formed at different frequencies according to the number of the half-periods. In the current document, we will refer to the modes by the number of the formed half-periods. The correspondence between the frequencies and the mode's number depends on the type of the coil and is, for example, not the same in a high-pass or a low-pass birdcage coil. It is, however, important to point out that any of the modes may be excited independently from the others, and that it is possible to separately adjust the frequencies of the B1 fields generated and detected by these modes.
In another embodiment, the present invention comprises a NQR checkpoint inspection system that permits identification of substances hidden on or inside a human body as well as other objects, such as carry-on items. The spectrometer part of the system comprises a single or a plurality of scanning channels, depending on a single or a plurality of prohibited substances to be screened and/or a single or a plurality of localizations of the contraband substances on or in the human body to be scanned. Each individual detection channel includes a transmitter for generating and amplifying a resonant frequency to be delivered to the scanned objects, a transmit/receive switch, a preamplifier and a receiver for the NQR signal detection. A sensor with one or multiple channels is utilized in conjunction with the spectrometer and is connected to it through a matching network. Instead of one such sensor, a decoupled array of multiple sensors may be used, providing some important advantages, as mentioned below. The side of the structure opposite to the entrance is capable of separating into two door-like parts, permitting a convenient exit for the persons upon opening. The sensor that is incorporated into the structure is based on the TEM-type half-cylindrical coil, which has a multi-channel capability, a uniform radiofrequency field amplitude distribution along its surface and is composed of elements that are coupled to each other only by virtue of their radiofrequency magnetic fields, without any electrical connections being necessary. Therefore, opening and closing of the sensor structure does not require interrupting and reforming any such connections, which, otherwise, would lead to their oxidation or other type of degradation, and would decrease the sensor's performance.
The present invention will be more fully understood and appreciated by reading the following Detailed Description in conjunction with the accompanying drawings, in which:
a and 5b are schematics showing the preferred shapes for planar and half-cylindrical open-shape birdcage sensors according to the present invention.
a and 6b are schematics showing the preferred shapes for planar and half-cylindrical open-shape TEM sensors according to the present invention.
a through 8h is a schematic of the current distributions in the legs of the preferred embodiments of the open-shape sensors according to the present invention corresponding to the surface mode up to the fourth mode, as well as the B1 field patterns.
a through 9c is a schematic of the B1 field patterns for the butterfly, third, and fourth modes on both sides of a planar sensor according to the present invention when no shield is utilized.
a and 10b is a shows an inductive (a) and a capacitive (b) method of simultaneously driving the third and the fourth modes of a planar embodiment of an open-shape sensor accordingly to present invention useful for the simultaneous dual-frequency operation.
Referring now to the drawings, wherein like reference numerals refer to like parts throughout, there is seen in
More specifically, there is seen in
There is seen in
Since most NQR measurements are performed at low frequencies (below a few MHz), the standard birdcage and TEM designs may present some serious disadvantages due to their associated low inductances that require the use of the unreasonably large capacitance values to achieve low-frequency resonance conditions. Some of the embodiments of the current invention are, therefore, preferably constructed with high-inductance windows or elements, introduced as a part of the current invention for the birdcage coils (shown in
The preferred embodiments of the current invention are based on an 8-window open-shape birdcage coil design and on a 9-element open-shape TEM coil design. Both designs have 9 legs carrying the current, responsible for the generation and the reception of the B1 fields in the sensor's working area. An open birdcage or TEM coil can be viewed as a half-wave resonator where a standing wave is formed in the direction perpendicular to the coil's legs. The current amplitudes in the legs are modulated sinusoidally going from one leg to the next, such that an integer number of half-periods fit between the first and the last leg. Modes are formed at different frequencies according to the number of the half-periods. In the current document, we will refer to the modes by the number of the formed half-periods. The correspondence between the frequencies and the mode's number depends on the type of the coil and is, for example, not the same in a high-pass or a low-pass birdcage coil. It is, however, important to point out that any of the modes may be excited independently from the others, and that it is possible to separately adjust the frequencies of the B1 fields generated and detected by these modes.
In the prior art magnetic resonance studies, the use of the B1 fields with uniform magnitudes and phases is preferred. This requirement provides restrictions on the use of the modes available in the multi-modal sensors (only the mode 1 and the mode 2 in the region restricted to the central area of the sensor are used). NQR measurements of randomly oriented substances, such as explosives or narcotics, on the other hand, are insensitive to the direction of the B1 fields, as shown above. Consequentially, any or all of the available modes may be utilized. As described below, the use of the higher modes provides a number of important advantages.
The current distributions in the legs 50 of these devices correspond to the naturally formed resonant modes, as seen in
Homogeneous interference signals coming from distant sources will be better attenuated than those arriving from the more near sources. This is due to the fact that the noise rejection properties rely on the fact that the phase of the B1 field is rotated by one full cycle along the sensor's surface, and if the noise source can be considered to be closer to one side of the sensor than the other, cancellation will not be complete. Noise rejection properties of this mode are expected to be improved in the double-sided embodiment of the sensor, as shown in
Accordingly, sensors possessing the described modes with numbers higher then one are noise-resilient, do not have any blind spots along their surfaces, capable of multi-frequency operation via independent channels, have selectivity over the penetration depths of the associated fields (by mode selection) and have adaptable shapes (planar or curved sensors may be used). These sensors, thereby, satisfy all of the requirements identified above.
The first preferred embodiment of the current invention is a planar shielded 8-window birdcage-type sensor, as seen in
The second preferred embodiment of the current invention is a planar unshielded 8-window birdcage-type sensor, similar to that seen in
The third preferred embodiment of the current invention is an open half-cylindrical shielded 8-window birdcage-type sensor, shown in
The fourth preferred embodiment of the current invention is a planar shielded 9-element TEM-type sensor seen in
The fifth preferred embodiment of the current invention is a planar unshielded 8-window TEM-type sensor, similar to that seen in
The sixth preferred embodiment of the current invention is an open half-cylindrical shielded 9-element TEM-type sensor, seen in
As an example of another embodiment of the present invention, there is seen in
In other preferred embodiments, multi-sensor arrays 70, such as that seen in
According to the present invention, the method of inspecting for concealed substances is as follows. First, a person enters the active area inspection system 72, which has its barrier 78 closed and is ready for a scan. The presence of person 76 is either automatically detected or is registered by an operator. Any tuning and matching adjustments are automatically made, if needed. Next, single, or multiple-frequency scan is initiated, depending on the chosen settings. The results of the scan are provided to the operator in the form that does not require significant interpretation (e.g., a green/yellow/red light). In case of inconclusive scan (e.g., a yellow light), the exhaustive scanning mode is initiated. In case of positive illicit substance detection (e.g., a red light), the doors remain closed, and the appropriate action may be conducted. In case of negative illicit substance detection (e.g., a green light), barrier 78 opens, allowing person 76 to exit. Finally, barrier 78 is closed and the system is prepared to receive next person 76.
In addition to illicit substance detection, the present invention may be used for biomedical applications of NQR, such as muscle scanning. It is to be understood that various modifications in form and detail of the specific preferred embodiments referenced here may be made by those skilled in the art without departing from the scope of the present inventions.
This application claims priority to U.S. Provisional Patent Application Ser. No. 60/733,286, filed on Nov. 3, 2005, and U.S. Provisional Patent Application Ser. No. 60/766,749, filed on Feb. 9, 2005.
| Number | Date | Country | |
|---|---|---|---|
| 60733286 | Nov 2005 | US | |
| 60766749 | Feb 2006 | US |
