Patent Publication Number: US-7710116-B2

Title: Method for reducing the coupling during reception between excitation and receive coils of a nuclear quadrupole resonance detection system

Description:
This application is a continuation of U.S. patent application Ser. No. 11/292,742, filed Dec. 2, 2005, now pending. 

   TECHNICAL FIELD 
   This invention relates to a method for improving the performance of a nuclear quadrupole resonance detection system by reducing the coupling during reception between the excitation coils and receive coils thereof, wherein the receive coils are preferably high temperature superconductor receive coils. 
   BACKGROUND 
   The use of nuclear quadrupole resonance (NQR) as a means of detecting explosives and other contraband has been recognized for some time. See, e.g., T. Hirshfield et al,  J. Molec. Struct.  58, 63 (1980); A. N. Garroway et al,  Proc. SPIE  2092, 318 (1993); and A. N. Garroway et al,  IEEE Trans. on Geoscience and Remote Sensing  39, 1108 (2001). NQR provides some distinct advantages over other detection methods. NQR requires no external magnet such as required by nuclear magnetic resonance, and NQR is sensitive to the compounds of interest, i.e. there is a specificity of the NQR frequencies. 
   One technique for measuring NQR in a sample is to place the sample within a solenoid coil that surrounds the sample. The coil provides a radio frequency (RF) magnetic field that excites the quadrupole nuclei in magnetic field that excites the quadrupole nuclei in the sample and results in their producing their characteristic resonance signals. This is the typical apparatus configuration that might be used for scanning mail, baggage or luggage. 
   There is also a need, however, for a NQR detector that permits detection of NQR signals from a source outside the detector, e.g. a wand detector, that could be passed over persons or containers as is done with existing metal detectors, or a panel detector that persons could stand on or near. Problems associated with such detectors using conventional systems are the decrease in detectability with distance from the detector coil and the associated equipment needed to operate the system. 
   A detection system can have one or more coils that serve as both excitation and receive coils, or it can have separate coils that only excite and only receive. An excitation, i.e. transmit, coil of an NQR detection system provides a radio frequency (RF) magnetic field that excites the quadrupole nuclei in the sample, and results in the quadrupole nuclei producing their characteristic resonance signals that the receive coil detects. 
   It can be especially advantageous to use a receive coil, i.e. a sensor, made of a high temperature superconductor (“HTS”) rather than copper since the HTS self-resonant coil has a quality factor Q of the order of 10 3 -10 6 . The NQR signals have low intensity and short duration. In view of the low intensity NQR signal, it is important to have a signal-to-noise ratio (S/N) as large as possible. The signal-to-noise ratio is proportional to the square root of Q so that the use of a HTS self-resonant coil as a sensor results in an increase in S/N by a factor of 10-100 over that of a copper coil. Therefore, the use of a high temperature superconductor receive coil with a large Q provides a distinct advantage over the use of an ordinary conductor coil. 
   Separate excitation and receive coils having the same resonance frequencies result in a coupling between the coils. This coupling can result in interference with the performance of the coils as well as damage to the receive coils. 
   An object of this invention is thus to provide a method for reducing the coupling between the excitation and receive coils in a NQR resonance detection system. 
   SUMMARY 
   This invention is directed to a method for shifting, during reception, the resonance frequency of the excitation coil(s) to thereby reduce the coupling between the excitation and receive coil(s). 
   During reception the resonance frequency of the transmit coil(s) is shifted. The shift in frequency, which is an increase or decrease in frequency, is performed in an amount that is sufficient to reduce the coupling between the coils to an acceptable level. After such a shift in resonance frequency, the resonance frequencies of the excitation and receive coils differ by at least about 10%. 
   Shifting resonance frequency in this manner to avoid coupling of coils improves the performance of a nuclear quadrupole resonance detection system, and this improvement in performance has particular value when the nuclear quadrupole resonance detection system is used for detecting the nuclear quadrupole resonance of an analyte material that constitutes a harmful or potentially harmful substance such as explosives, drugs (controlled substances) and/or other contraband. When screening samples, the presence of such a harmful or potentially harmful substance may be difficult to detect in the absence of the ability to verify its presence by detecting therein a certain nuclear quadrupole resonance that is characteristic of an analyte material of interest. 
   Yet another embodiment of this invention is consequently a method as described above wherein the nuclear quadrupole resonance detection system detects nuclear quadrupole resonance that is characteristic of an analyte material that constitutes a harmful or potentially harmful substance. This is accomplished, for example, by applying the excitation of a transmit coil to a sample to be screened for the detection of the presence of explosives, drugs or other contraband. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1A and 1B  each show circuit configurations with a reactance that can be altered to shift the resonance frequency of a shielded loop resonator excitation coil. 
       FIG. 2  shows a resonance frequency-shifting circuit comprised of a single loop inductively coupled to the coil whose frequency is to be shifted, a reactance in series with the single loop, and means for connecting the reactance to, and disconnecting the reactance from, the single loop. 
       FIG. 3  shows a resonance frequency-shifting circuit comprised of a single loop inductively coupled to a receive coil whose frequency is to be shifted, and a capacitor and cross-diode pair switch in series with the single loop. 
       FIG. 4  illustrates the use of the circuit that decouples two receive coils to shift the resonance frequencies of the coils during excitation. 
   

   DETAILED DESCRIPTION 
   This invention addresses the problem of coupling that may occur between an excitation coil and a high temperature superconductor receive coil in a nuclear quadrupole detection system. The resonance frequency of the excitation coil during excitation, and the resonance frequency of the receive coil during reception, must be set to be equal or essentially equal to each other, and must also be set to be equal or essentially equal to the nuclear quadrupole resonance frequency of an analyte material of interest, which will be or be contained in a sample to be analyzed or screened for the detection of nuclear quadrupole resonance. While it is preferred that such frequencies be exactly equal, it is sufficient if they are essentially equal in the sense that the frequency of the transmit coil(s) is in a range that will excite nuclear quadrupole resonance in the analyte material, and the frequency of the receive coil(s) is in a range that will detect nuclear quadrupole resonance in the analyte material. 
   Coupling between a transmit coil and a receive coil is greatest when they have the same or essentially the same frequency, and this coupling can result in serious performance problems. During sample excitation, the excitation magnetic field will induce a voltage in the receive coil. When a high Q high temperature superconductor receive coil is used, the induced or “ring-up” voltage could be large enough to damage the receive coil. A receive coil Q-spoiling circuit would prevent this from happening, but would also, as a result of the coupling, spoil the Q of the excitation coil. During reception, i.e. detection, of the NQR signal, the coupling would result in a degradation of the receive coil Q and a reduction in the sensitivity of the detector. 
   This invention provides a method for improving the performance of a NQR detection system by shifting the resonance frequency of the coil(s) not performing their function at that time, i.e. the frequency of the excitation coil(s) is shifted during reception. If the resonance frequencies of the excitation coil(s) and the high temperature superconductor receive coil(s) are adequately separated, their coupling is minimal. This allows each coil to perform its respective function, i.e. excitation or detection, as if the other coil(s) were not present. When using high-Q, high-temperature superconductor receive coils, it is preferred that the resonance frequencies of the receive coil(s) and the excitation coil(s) be separated by an amount that may, for example, be at least about 10% to reduce the coupling between coils to an acceptable level. Coupling is reduced to an acceptable level when, as stated above, each coil is able to perform its respective function, i.e. excitation or detection, as if the other coil(s) were not present. 
   The excitation coils used in this invention can be made of copper, silver, aluminum or a high temperature superconductor. A copper, silver or aluminum coil is preferably in the form of a shielded-loop resonator (SLR) coil. SLR&#39;s have been developed to eliminate the detuning effect of the electrical interaction between the coil and the surrounding material. 
   Preferably, one or more SLR copper excitation coils are used to apply the RF signal to the sample. The receive coils are high temperature superconductor coils. A high temperature superconductor receive coil is preferably in the form of a self-resonant planar coil, i.e. a surface coil, with a coil configuration of HTS on one or both sides of a substrate. High temperature superconductors are those that superconduct above 77K. The high temperature superconductors used to form the HTS self-resonant coil are preferably selected from the group consisting of YBa 2 Cu 3 O 7 , Tl 2 Ba 2 CaCu 2 O 8 , TlBa 2 Ca 2 Cu 3 O 9 , (TlPb)Sr 2 CaCu 2 O 7  and (TlPb)Sr 2 Ca 2 Cu 3 O 9 . Most preferably, the high temperature superconductor is YBa 2 Cu 3 O 7  or Tl 2 Ba 2 CaCu 2 O 8 . 
   This invention provides a method for improving the performance of a NQR detection system by reducing the coupling between excitation and receive coils. The coupling is reduced by shifting the resonance frequency of the coils that are not performing their function at that time. The means for shifting frequency used has to provide the shift in frequency quickly, e.g. in less than about 1 μs. The means for shifting should not add significant losses to the receive coil, and should not interfere with the high power handling capabilities of the excitation coil. 
   There are many ways to shift the resonance frequencies of the coils. The resonance frequency may be shifted, for example, by providing means for tuning the resonance frequency with which the frequency is tuned, and then altering the means for tuning to shift the resonance frequency of the coil. Means for tuning the resonance frequency of a coil may include, for example, a reactance, and altering such a reactance has the effect of re-tuning, i.e. shifting, the resonance frequency of the coil. Shorting out all or a portion of a reactance used as means for tuning a resonance frequency, or adding additional reactance, can accomplish the desired re-tuning, or shifting, of the resonance frequency of a coil. 
   Where capacitors are used as the reactance,  FIGS. 1A and 1B  illustrate several ways of varying the tuning capacitance with a shielded loop resonator excitation coil  1 . In both figures, the input is fed to the SLR excitation coil  1  through the input capacitor  2  attached to one end  3  of the SLR excitation coil. The other end  4  of the SLR excitation coil is at ground. 
   In  FIG. 1A , capacitors  5  and  6  are shown in a series configuration. Switch  7  can switch capacitor  6  in and out of the circuit. In one embodiment, the resonance frequency of the SLR excitation coil  1  is equal or essentially equal to the nuclear quadrupole resonance frequency of the analyte material when capacitor  5  is in the circuit, and switch  7  is closed to switch capacitor  6  out of the circuit. This would be the configuration during excitation. During reception, switch  7  would be opened to switch capacitor  6  into the circuit and thereby shift the resonance frequency of the SLR excitation coil. In another embodiment, the resonance frequency of the SLR excitation coil  1  is equal or essentially equal to the nuclear quadrupole resonance frequency of the analyte material when capacitors  5  and  6  are in the circuit, i.e. when switch  7  is open. This would be the configuration during excitation for this embodiment. During reception, switch  7  would be closed to switch capacitor  6  out of the circuit and thereby shift the resonance frequency of the SLR excitation coil  1 . 
   In  FIG. 1B , capacitors  8  and  9  are shown in a parallel configuration. Switch  10  can switch capacitor  9  in and out of the circuit. In one embodiment, the resonance frequency of the SLR excitation coil  1  is equal or essentially equal to the nuclear quadrupole resonance frequency of the analyte material when capacitor  8  is in the circuit and switch  10  is open to switch capacitor  9  out of the circuit. This would be the configuration during excitation. During reception, switch  10  would be closed to switch capacitor  9  into the circuit and thereby shift the resonance frequency of the SLR excitation coil  1 . In another embodiment, the resonance frequency of the SLR excitation coil  1  is equal or essentially equal to the nuclear quadrupole resonance frequency of the analyte material when capacitors  8  and  9  are both in the circuit, i.e. when switch  10  is closed. This would be the configuration during excitation for this embodiment. During reception, switch  10  would be opened to switch capacitor  9  out of the circuit and thereby shift the resonance frequency of the SLR excitation coil. 
   A reactance that is a combination of capacitors, inductors and/or other circuit elements that effectively shift the resonance frequency of an excitation or receive coil can be used instead of the frequency shifting capacitors  6  and  9  shown, respectively, in  FIGS. 1A and 1B . In similar embodiments, a reactance as shown and described above can be used to shift the resonance frequency of an HTS receive coil. 
   Another way to shift the resonance frequency of a coil is to use a circuit comprised of a single loop or coil that is inductively coupled to the coil whose frequency is to be shifted. This is one of the preferred methods for shifting the frequency of a HTS receive coil. A reactance is in series with the single loop or coil, and means to connect the reactance to, and disconnect the reactance from, the single loop or coil is provided as well. The single loop or coil can be made of a regular conductor, such as copper, or a high temperature superconductor. The reactance can be an inductance, capacitance or combination of both. Means to connect the reactance to, and disconnect the reactance from, the single loop or coil may include at least one mechanical or electrical switch. A schematic diagram of such a circuit is shown in  FIG. 2 . A single loop  11  is inductively coupled to an excitation or receive coil  12 . Additional loops, such as a coil (not shown) can be used to provide the desired inductive coupling. Connected to the single loop  11  are a reactance  13  and a switch  14  that connects and disconnects the reactance  13  to the single loop  11 . The switch  14  can be a mechanical switch, or it can be an electrical switch such as a diode that conducts above a certain applied voltage. 
   The switches used herein can be mechanical or electrical switches. One useful type of electrical switch is a cross-diode pair switch. The use of a cross-diode pair switch in a resonance frequency-shifting circuit for shifting the resonance frequency of a HTS receive coil is illustrated in  FIG. 3 . A single loop  21  is inductively coupled to the receive coil  22 . Additional loops, such as a coil (not shown), can be used to provide the desired inductive coupling. Connected to the single loop  21  are a capacitor  23  and two diodes  24  arranged as a cross-diode pair switch. The capacitance of capacitor  23  and the inductance of the loop  21  result in a resonance frequency for the resonance frequency shifting circuit. The value of the capacitance is chosen so that when the cross-diode pair switch acts essentially as a short circuit, i.e. has low resistance, the resonance frequency of the resonance frequency shifting circuit is essentially the same as that of the receive coil, which is set to the NQR signal frequency to be detected. 
   When a low power RF signal impinges on loop  21 , the induced voltage is not sufficient to turn on the diodes  24 . Under these conditions, the cross-diode pair switch is effectively a high resistance, and the resonance frequency shifting circuit has minimal effect on the receive coil, i.e. the Q of the receive coil is not significantly decreased. As the power of the RF signal impinging on the coil increases, e.g. during excitation, the induced voltage increases and reaches a level at which the diodes are turned on and thus conduct. Under these conditions, the effective resistance of the cross-diode pair switch is low, and the resonance frequency of the resonance frequency shifting circuit is the same as that of the receive coil. As a result of the coupling between the receive coil and the loop, the degenerate frequencies split into two modes, one of higher frequency and one of lower frequency. Therefore, the deleterious effects of having the resonance frequencies of the receive coil and the excitation coil the same during sample excitation are avoided. 
   In another embodiment, a switch is connected between the two ends of a coil so that the coil can be shorted out when not carrying out the function for which it was designed, i.e. excitation or reception. 
   When there is an array of n HTS receive coils in a nuclear quadrupole detection system, where n is 2 or more, the receive coils may couple with one another as well as with the excitation coil. A capacitive circuit can be used in this situation to decouple the n receive coils. The capacitive circuit is comprised of n single loops or coils, each of which is inductively coupled to one of the n receive coils, and a capacitor that is connected to each pair of the single loops or coils. The capacitors are chosen so that each receive coil has a resonance frequency that is equal or essentially equal to the resonance frequency of the analyte material. 
   Two receive coils with a capacitive decoupling circuit as described in this embodiment are shown schematically in  FIG. 4 . The capacitive decoupling circuit is comprised of two single loops, each of which is inductively coupled to one of the receive coils, and a capacitor connected to the single loops. However, the capacitive decoupling circuit has been altered to include a switch. Two high temperature superconductor receive coils  31  and  32  are in the form of HTS planar coils. The HTS receive coils  31  and  32  are coupled as a result of their mutual inductance. As the distance between the centers of the HTS coils  31 / 32  decreases the coupling increases. The receive coils  31  and  32  are shown with single loops  33  and  34  that are inductively coupled to the receive coils  31  and  32 , respectively. The single loops can be made from high temperature superconductors or from conducting metals such as copper. Electrical conductor  35  connects the one end of loop  33  to one end of loop  34 . Electrical conductor  36  connects the other end of loop  33  to the switch  37  that is connected to capacitor  38 . Electrical conductor  39  connects capacitor  38  to the other end of loop  34 . The capacitive decoupling circuit can contain a single capacitor as shown in  FIG. 4  where the capacitor has a fixed or variable value. Alternatively, however, the capacitive decoupling circuit can be comprised of three or more capacitors arranged in various configurations. When the switch  37  is closed, the receive coils  31  and  32  have resonance frequencies equal or essentially equal to the nuclear quadrupole resonance frequency of the analyte material. This would be the configuration during reception. When the switch is open, the resonance frequencies of the receive coils is shifted, and this would be the configuration during excitation.