AU2008284170A1 - Method using composite preparation pulse for NQR testing of samples in inhomogenous RF magnetic fields - Google Patents

Description

WO 2009/020717 PCT/US2008/068081 METHOD USING COMPOSITE PREPARATION PULSE FOR NQR TESTING OF SAMPLES IN INHOMOGENOUS RF MAGNETIC FIELDS BACKGROUND Field of the Invention [0001] The field of the invention relates to nuclear quadrupole resonance ("NQR" or "QR") generally, and more particularly to a method using a composite preparation pulse that improves NQR sensitivity in inhomogeneous radio frequency ("RF") magnetic fields with little or no degradation in temperature bandwidth. Description of Related Art [0002] Nuclear quadrupole resonance ("NQR") is a chemically specific spectroscopic technique used to analyze a target sample (chemical, explosive, narcotic, and so forth) that contains quadrupole nuclei. Typically, one or more target samples will be hidden in and/or on a suspicious item (piece of luggage, person, parcel, etc.). Use of NQR sensors to rapidly and accurately detect such materials is particularly desirable in passenger and luggage screening operations, where many people and vast amounts of luggage must be thoroughly scanned in a non-invasive and efficient manner. [0003] By way of illustration, a piece of luggage (or other type of suspicious item) may be placed within an inductive coil that is part of a tuned resonance circuit. Radio frequency ("RF") excitation pulses may then be applied to the circuit to generate an oscillating RF magnetic field in the scanning volume occupied by the piece of luggage. The oscillating RF magnetic field temporarily aligns the nuclear magnetic moments of resonant quadrupole nuclei to create a macroscopic magnetization in the target sample. [0004] The macroscopic nuclear magnetization induced by a resonant RF magnetic field oscillates at the NQR frequency of the quadrupole nuclei and induces a voltage in the inductive element (coil in the NQR sensor) called the NQR signal. In order to observe the NQR signals, the high-voltage RF excitation is turned off and the nuclear signal is detected after a short time delay for recovery of the electronic components in the

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WO 2009/020717 PCT/US2008/068081 sensitive receiver, called "dead time" of the receiver. The NQR signal can vanish quickly while the quadrupolar nuclei return to their original state of thermal equilibrium and loose their coherent oscillation. Under certain circumstances, the quantum states of the quadrupolar nuclei can be manipulated with additional RF pulses (called refocusing pulses) to recover partially the loss of coherence and create an echo signal. This refocusing process can be repeated with multiple RF pulses to create a train of multiple echo signals. [0005] The echo signals are detected by the NQR sensor during the periods of no irradiation between pulses and analyzed to determine whether a target sample is present or not within (and/or on) the interrogated piece of luggage. In this particular example, the suspicious item can be placed inside the coil and irradiated with fairly homogeneous RF magnetic fields. Thus, the target sample experiences a unifonn excitation and the NQR response (signal) can be maximized for that specific RF magnetic field strength. [0006] In certain applications of NQR technology, access to the interrogated item is restricted due to size or geometry constraints, and coils delivering non-uniform RF fields are used for interrogation of a limited volume. For instance, shoe-scanners or hand-held sensors for personnel screening single-sided sensors for landmine detection, etc., all use coils that produce inhomogeneous RF magnetic fields. The inhomogeneous RF magnetic fields greatly reduce and impair the ability of an NQR sensor to detect and identify a target sample because the target sample experiences variety of B] field intensities and the pulse sequence that produces NQR response signals can not be optimized for all B, values. Moreover, inhomogeneous RF magnetic fields cause the NQR sensitivity to vary depending on the configuration (shape/orientation) of the target sample and/or the position of the target sample with respect to the NQR sensor. Furthermore, the RF pulses used in standard NQR detection sequences may induce acoustic or magneto-acoustic ringing from certain materials that could be present in the scanning volume of a suspicious item. The acoustic or magneto-acoustic ringing further reduces the NQR sensitivity. Failure to improve the SNR, failure to minimize the NQR sensitivity's dependence on configuration and/or position of the target sample, and/or failure to eliminate the acoustic ringing can significantly degrade the sensitivity of the NQR 2 WO 2009/020717 PCT/US2008/068081 technique and may also cause explosives, narcotics, and/or other types of contraband to be wrongly identified or not identified at all. [0007] Before' examining several prior NQR detection approaches, it may be helpful to review a short description of what may be the most commonly used technique for effective detection of NQR echo signals from target samples - the spin-locked spin-echo ("SLSE") pulse sequence. [0008] Figure 1 is a time diagram that illustrates a standard SLSE pulse sequence 100. The standard SLSE pulse sequence 100 typically comprises several linear time periods, which for illustrative purposes, are preparatory pulse 101, time period (or gap) 102, refocusing pulse 103, recovery period or dead time 104, acquisition window 105, and a series of refocusing pulses 106. The preparatory pulse 101 has a pulse width pwl. The time period 102 has a length r. The refocusing pulse 103 has a pulse width pw2. The recovery period 104 has a length Dt. The acquisition window 105 has a length racq. [0009] The time diagram of the standard SLSE pulse sequence 100 is read from left to right. First, a single preparatory pulse 101 of length pwl occurs. This is followed by an interval 102 (of length U) of no irradiation. Then a series of identical refocusing pulses 106 occurs. Each refocusing pulse 103 generates an echo signal. Using processes known in the art, these echo signals may be amplified, conditioned, processed, and/or analyzed to identify a target sample as an explosive, an illegal narcotic, or other type of chemicals. Within the series of refocusing pulses 106, the recovery period 104 (of length Dt) and the acquisition window 105 (of length racq) are periods in which no irradiation occurs. Inclusion of these periods between refocusing pulses 103 permits the QR receiver to listen for and detect an echo signal. The series of refocusing pulses 106 may be repeated Ne times. [0010] One way to visualize a standard SLSE pulse sequence is to write it as Expression 1: f),-r- (- ~ Dt IN (1) 3 WO 2009/020717 PCT/US2008/068081 [0011] Expression 1, like other Expressions that follow, is not a mathematical equation (e.g., the horizontal lines (-) are dashes, not minus signs), but rather an exemplary textual representation of a pulse sequence (in this case, a standard SLSE pulse sequence). [0012] In Expression 1,/p = y/ppwis the flip angle for the preparatory pulse that occurs during pulse width pwl, and 0 = y,pw 2 is the flip angle for each of the refocusing pulses that occur during pulse width pw2. Note that these flip angles are valid for spin 1=1 nuclei, and are easily extended to I>1 by including appropriate factors, which would be known to a person of ordinary skill in the NQR detection field. The constant O is a known gyro-magnetic ratio of a predetermined type of quadrupolar nucleus. B, represents the strength of the RF magnetic field, which may vary in space depending on the type of apparatus used and/or the type of NQR detection being performed. Note that the "flip angle" pl for a quadrupole nucleus contained in a solid crystal of arbitrary orientation is scaled by an angular function that describes the relative orientation between the crystal and the direction of the external RF magnetic field. These "effective flip angles" depend on the angular coordinates and the NQR signal involves an average over all possible orientations for polycrystalline or powder samples. It should be pointed out that the variance in the flip angle with the orientation of the solid crystal is somehow equivalent to the effect of irradiating the sample with a spatially variable B, field. The intervals .Dt and 0 acq are free evolution periods with no RF irradiation. The time interval Dt comprises the dead time for recovery of the receiver and Eaq includes the interval for acquiring data from an echo NQR signal. This interval ] acq is sometimes referred to as an acquisition window. The subscript rp indicates the phase of the RF radiation during the pulse. There is a 90' or 7/2 phase shift (which can be positive or negative) between the preparatory pulse and the refocusing pulses. N is the number of refocusing pulses used for signal acquisition. [0013J Duration of each of the excitation pulses pwl and pw2 will vary, but typically each has a length that produces a maximum QR response at a given distance from the QR sensor at which the target sample experiences an average excitation field B, (usually 4 WO 2009/020717 PCT/US2008/068081 called the "nominal B, field"). In the presence of a uniform B 1 excitation field over the volume of the sample, the duration pwl of the preparation pulse is such that the flip angle pf is about 119' over a volume of the target sample (polycrystalline) and returns the maximum QR response for nuclei having spin 1=1. Note that as used herein, B, refers to the strength of an RF magnetic field, whereas #6, refers to a flip angle of a pulse of RF radiation. [0014] Several approaches have been suggested for improving NQR sensitivity over traditional pulse sequences with inhomogeneous RF magnetic fields. One approach involves replacing the standard preparatory pulse (of pulse width pwl) with a preparatory Adiabatic Half Passage ("AHP") pulse to enhance NQR sensitivity over the traditional SLSE pulse sequence. This technique is described in the article, "Applications of Adiabatic Half Passage to NQR," App. Magn. Reson. 25, pp. 3-4, 2004, authored by researchers J. Miller and A.N. Garoway of the Navy Research Lab. Use of AHP as a preparatory pulse can provide near constant signal amplitude over variation (of up to a factor of two) in RF magnetic field strength, but reduces significantly the excitation frequency bandwidth over which NQR signals can be induced. In consequence, the range of temperatures over which NQR signals can be excited and detected (i.e., temperature bandwidth) is significantly reduced with this type of compensation schemes. Another problem with using AHP preparatory pulses is that AHP irradiation requires precise phase or frequency and amplitude control of the RF irradiation, and the long irradiation times needed for adiabatic excitation (typically, several milliseconds) can damage some electronic components, especially in the RF power transmitter. [00151 A second approach involves replacing each standard excitation pulse with a composite pulse that includes two or more substantially continuous sub-pulses, each having a different phase. This approach is described in U.S. Patent Application Publication No.: 2005/0030029AI to Sauer, Karen L., et al. A similar (third) approach that compensates for the effects of RF field inhomogeneity for a powder sample of spin-I nuclei is described in the article by Sauer, Karen L, et al., "Using Quatemions to Design Composite Pulses for Spin-1 NQR," Appl. Magn. Reson. 25, pp. 485-500 (2004). Unfortunately, these second and third approaches require utilizing a train of all composite 5 WO 2009/020717 PCT/US2008/068081 pulses. Additionally, as with using AHP, both the second and third approaches degrade temperature bandwidth. [0016] Other techniques for eliminating the effects of ringing are set forth in USPN 5,365,171 and USPN 6,392,408. For example, USPN 5,365,171 teaches irradiating a specimen with a modified steady state steady state free precession (SSFP) pulse sequence, which combines a phase-alternated pulse sequence (PAPS) with a non-phase alternated pulse sequence (NPAPS). The resulting signals from the PAPS and NPAS are then coadded to cancel out any free induction decay ("FID") contributions to the signals. By canceling out the FID contributions, the effects of probe ringing and other extraneous responses, as well as the effect of temperature variation, are minimized or removed. USPN 6,392,408 teaches irradiating a specimen with a specific sequence of electromagnetic pulses, referred to as a spin-locked inversion mid-echo (SLIME) pulse sequence with each pulse having a specified phase and duration. The pulses are separated by specified time intervals. The SLIME pulse sequence has a single excitation pulse. As a result of the characteristics of the SLIME pulse sequence, if the target substance to be detected is present in the specimen, sets of oppositely-phased NQR echo signals are generated from the target substance. NQR echo signals of one phase are subtracted from NQR echo signals having the opposite phase, rendering a cumulative echo signal, and simultaneously subtracting out the same-phase extraneous signal. These techniques, however, merely remove the adverse effects caused by ringing and other extraneous responses, and do not address the different problem of how to improve sensitivity of detection regardless of the geometry, orientation, and location of a target sample in the presence of an inhomogeneous RF field. [0017] In summary, several of the known techniques and approaches described above address the general problem of improving detection in inhomogeneous RF fields for different positions of a target sample or for different standoff distances of a NQR sensor. However, the specific problem of eliminating the decreased NQR sensitivity that is currently experienced with different geometries, orientation, and/or position of a target sample has not yet been addressed. 6 WO 2009/020717 PCT/US2008/068081 [0018] Thus, there is a need to develop a method that renders a NQR detection apparatus less sensitive to target sample configuration (shape and orientation) and/or distance from the NQR sensor when the target sample is in the presence of an inhomogeneous RF magnetic field. Additionally, there is a need for a method that improves (relative to a SNR obtained using previously developed techniques) a SNR of an NQR detection apparatus that produces an inhomogeneous RF magnetic field. Additionally, there is also a need for any such method to preserve the NQR sensor's temperature bandwidth for effective operation in the field. SUMMARY OF THE INVENTION [0019 Embodiments of the invention provide a method for improved excitation and detection of nuclear quadrupole resonance ("NQR") signals using an apparatus that delivers an inhomogeneous excitation radio frequency ("RF") magnetic field over the volume of the target sample. The method compensates for inhomogeneities in the RF magnetic field that result when surface coils are used to stimulate NQR responses from target samples of arbitrary geometry, orientation, and/or position. The method is particularly advantageous for NQR testing of explosive samples on single-sided systems; such as shoe scanners, mine detectors, or hand-held systems. Single-sided systems use surface coils or open coils as NQR sensors. [0020] Embodiments of the invention increase NQR sensitivity by compensating for RF field inhomogeneities without degrading the sensor's temperature bandwidth. One approach for rendering the NQR technique less sensitive to target configuration and location uses a uniquely-configured composite pulse ("CP") to prepare the spin system, followed by acquisition of a series of spin echoes resulting from a pulse sequence such as spin-lock, spin-echo ("SLSE"). The relative phase of the CP and refocusing pulses may be adjusted to optimize the temperature bandwidth. The method is advantageous in that overall QR sensitivity, or signal-to-noise ratio (SNR), is improved with respect to the standard SLSE excitation sequence that uses a single square preparation pulse. [0021] Embodiments of the invention can also known ringing cancellation techniques. 7 WO 2009/020717 PCT/US2008/068081 [0022] Embodiments of the method and RF excitation pulse sequence herein described improve overall performance of NQR-based shoe scanners (and/or other detection systems (such as mine detectors and hand-held detectors). When such NQR-based detection systems utilize the invented method and RF excitation pulse sequence, the systems will deliver better detection performance than standard RF excitation pulse sequences currently used to scan target samples placed within an inhomogeneous RF magnetic field, where each target sample has an arbitrary shape, orientation, and position. More importantly, a NQR-based detection system utilizing embodiments of the method and RF excitation pulse sequence herein described will maintain at least its present temperature bandwidth and levels of ringing cancellation. Additionally, implementation of the embodiments of the invention is relatively simple and cost effective, requiring only changes in existing software code, but no hardware modifications. Finally, currently deployed NQR-based detection systems can be easily retrofitted with embodiments of the invention at minimum cost. [0023] Other features and advantages of the disclosure will become apparent by reference to the following description taken in connection with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS [0024] For a more complete understanding of embodiments of the invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: [0025] Figure 1 (Prior Art) is a time diagram illustrating a standard SLSE pulse sequence; [0026] Figure 2 is a time diagram illustrating a composite preparatory pulse sequence incorporated with a standard refocusing pulse train in accordance with an embodiment of the invention to compensate for an inhomogeneous RF magnetic field; [0027] Figure 3 is a graph of signal-to-noise ratio plotted versus distance for a target sample in a first configuration and demonstrates improvements and advantages of an embodiment of the invention; 8 WO 2009/020717 PCT/US2008/068081 [0028] Figure 4 is a graph of signal-to-noise ratio plotted versus distance for the target sample of Figure 3 in a second configuration and demonstrates improvements and advantages of an embodiment of the invention. [0029] Figure 5 is a graph of signal-to-noise ratio plotted versus temperature for a target sample in a first configuration and demonstrates improvements and advantages of an embodiment of the invention; [0030] Figure 6 is a graph of signal-to-noise ratio plotted versus temperature for a target sample in a second configuration and demonstrates improvements and advantages of an embodiment of the invention; and [0031] Figure 7 is a flowchart illustrating a method of performing NQR detection using a composite pulse sequence. [0032] Like reference characters designate identical or corresponding components and units throughout the several views. DETAILED DESCRIPTION [0033] The inventors of the present invention have discovered that detection of NQR signals with spin-lock, spin echo ("SLSE") techniques in inhomogeneous RF fields can be improved using a uniquely configured composite preparatory pulse followed by a series of refocusing pulses. In particular, it has been discovered that NQR sensitivity can be greatly improved by configuring the composite preparatory pulse to compensate for variations in the RF magnetic field, and following the composite preparatory pulse with a series of square refocusing pulses. [0034] A composite pulse includes a series of square ("hard") pulses, each of which may have a different phase and duration. The composite preparatory pulse developed by the inventors of the present invention is designed to induce uniform excitation of the spin magnetization in the presence of inhomogeneities in the RF magnetic field and resonance offset (or shifts in the NQR resonance frequency of the target). 9 WO 2009/020717 PCT/US2008/068081 [0035] The inventors perforned theoretical analysis of the spin dynamics of a quadrupole system during the preparatory pulse, produced the design of a new excitation scheme, and showed that, to improve sensitivity of detection regardless of orientation, location, and/or geometry of a target sample being interrogated in the presence of an inhomogeneous RF magnetic field, the first (preparatory) pulse of the CPMG sequence (#), should be replaced by a composite pulse of the form: [(#A), (# 2 ), (f,, C3).-(NV ] (2) [0036] In other words, Equation (2) demonstrates that a composite preparatory pulse with N segments can be denoted as a series of concatenated flip angles (#i), (82), (#3), and (pN). Figure 2 provides an example of the composite preparatory pulse that Equation (2) represents. p may range from 0 to 2u. In one aspect of the invention, the composite preparatory pulse may take the following form: (# #12), (163), (#04), (3) [0037] In Equations (2) and (3), the flip angles, = B,pw,,, with k = 1, 2, 3, 4 (or alternatively, "N", where N is an integer), are the different segments in the composite preparatory pulse with durations PW,A. The modified SLSE sequence now reads: [(#),__ (# 2), to3t (84), - '- 0 I - Dt -r ] (4) 2 2 _ , +I - c Ne [0038] In Equation (4), the delay x' may be different than the value used in the standard SLSE sequence. The inventors determined experimentally that the pulse sequence of Equation (4) yields good compensation of RF inhomogeneities for on-resonance spins, but it is not very effective in exciting NQR responses when the resonance frequency of 10 WO 2009/020717 PCT/US2008/068081 the target sample shifts relative to the irradiation frequency. It is not effective because the shifted resonance frequency of the target sample reduces the effective temperature bandwidth of the NQR detector. [0039] Figure 2 is a time diagram illustrating a composite pulse sequence 200 configured in accordance to an embodiment of the invention to compensate for an inhomogeneous RF magnetic field. The composite pulse sequence 200 may be written as Equation (4) above, and may comprise several linear time periods: a composite preparatory pulse 201, time period (or gap) 202, refocusing pulse 203, recovery period 204, acquisition window 205, and a series of refocusing pulses 206. The series of refocusing pulses 206 may be repeated N, times. The preparatory pulse 201 has a pulse width pwl. The time period 202 has a length r. The refocusing pulse 203 has a pulse width pw2. The recovery time period 204 has a length D1. The acquisition window 205 has a length raq. [00404 The composite preparatory pulse 201 may include two or more sub-pulses 210, 211, 212, 213, and each of the two or more sub-pulses 210, 211, 212, 213 may have a phase and/or duration that is the same as or different from the phase and/or duration of the other sub-pulses that comprise the composite preparatory pulse 201. For example, a first sub-pulse 210 may have a phase (py and a width pw 11 . The second sub-pulse 211 may have a phase PI2 and a width pw 1 2 . The third sub-pulse 212 may have a phase (pj,3 and a width pw 1

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3 . The fourth sub-pulse 213 may have a phase 1,4 and a width pw1,4. [0041] Further research led to the finding that the temperature bandwidth (off-resonance response) can be improved by adjusting the relative phase between the last segment of the preparatory pulse (# 4 ), and the refocusing pulses 0. This means that a 900 or c/2phase shift between the preparatory pulse and the refocusing pulses is no longer the optimum as in the standard SLSE pulse sequence. The newly invented pulse sequence, which has been named CP-SLSE, can be written as: [(# ( #2), (3) (#4) -r[, -Dt - l, (5) 11 WO 2009/020717 PCT/US2008/068081 [0042] In Equation (5), a phase value 9 = g4 yields good compensation of RF field inhomogeneities and temperature offsets, but other phase values may be used. The new pulse sequence of Equation (5) provides enhanced detection performance for NQR sensors that uses non-uniforn B 1 fields over the volume of a target sample. The CP SLSE pulse sequence of Equation (5) can also be used in combination with known methods for ringing cancellation. One such method of ringing cancellation is disclosed in USPN 6,392,408, discussed above, which teaches reversing the phase of the NQR echo signals while maintaining ringing signals substantially constant; but other methods of ringing cancellation may also be used. An example of a CP-SLSE pulse sequence that also achieves ringing rejection is: \tint2I 2-0,-Dt r,] - 2 where 0 is an inverting pulse (or set of inverted pulses) to reverse the phase of the NQR signal as described in USPN 6,392,408. [0043] In Equation (6), ringing cancellation may be achieved by adding all the responses excited during the train of M N, refocusing pulses. The parameter M indicates the number of repetitions of the sets of refocusing pulses 206 and inverted refocusing pulse(s) 0 shown between braces {}. Typically M = an even integer (2, 4, 6 ... ). The parameter Ne indicates the number of repetitions of the set of refocusing pulses 206 and the number of repetitions of a set of inverted refocusing pulses 0. Typically Ne and M are determined by the spin-lock relaxation time constant (commonly referred to as "T2E") that varies with the pulse-to-pulse period of the pulse sequence, material, and temperature. The parameter Ne may be adjusted (by selecting values from 1 and upwards) for maximum signal-to-noise ratio or sensitivity. For example, some target samples exhibit a fast echo-decay time. For such target samples, the CP-SLSE pulse sequence may use a Ne of not more than 20 or 40. On the other hand, some target samples that exhibit a slower echo-decay time constant may use a Ne of several hundreds. It will be appreciated that the benefits of the new composite preparatory pulse SLSE technique disclosed herein does not depend on the parameter Ne. 12 WO 2009/020717 PCT/US2008/068081 [0044] A CP-SLSE sequence was experimentally tested with a sheet test sample of compound #1, which is not identified here for security reasons. Results were obtained for two different configurations of the test sample and as a function of temperature of the sample and distance from the NQR coil to the bottom of the test sample. The pulse lengths pw ,k were each optimized for NQR detection with the target sample placed in front of the sensor at a fixed distance [0045] Figures 3 and 4 are graphs 300,400, respectively, that show the experimental SNR values measured with CP-SLSE and the SNR values measured with standard SLSE as a function of distance for the two configurations of the test sample. For configuration 1, the CP-SLSE sequence of Equation (5) performs better than the standard SLSE pulse sequence of Equation (1) for most of the measured distances, while for configuration II, the new CP-SLSE pulse sequence improves sensitivity for all positions of the test sample. The improvements with the new pulse sequence of Equations (4) and (5) are more noticeable with the test sample further away from the NQR coil, which is typically the most challenging scenario for NQR signal detection. [0046] Figures 5 and 6 show experimental SNR values measured with CP-SLSE and SNR values measured with standard SLSE as function of temperature with the target sample in one of two configurations at a pre-determined fixed distance from the NQR coil. Clearly, for both configurations of the test sample, the temperature bandwidth (off resonance response) of CP-SLSE pulse sequence of Equations (4) or (5) are similar or better than that with the standard SLSE pulse sequence of Equation (1). [0047] Figure 7 is a flowchart illustrating a method 700 of performing NQR detection using a composite pulse sequence. One or more of the steps 701, 702, 703, and 704 may be performed in any suitable order (and/or simultaneously). The steps of embodiments of the method 700 may be performed using any suitable NQR detection device having at least a RF radiation source, an echo detector, and a computer processor configured to operate both the RF radiation source and the echo detector. In particular, the computer processor may be configured to produce a train of spin-lock, spin-echo excitation pulses, wherein the preparatory pulse is a composite pulse having the form of Equation (2), 13 WO 2009/020717 PCT/US2008/068081 above. The computer processor may also be configured to identify a target sample by processing and analyzing detected spin echoes. [0048] Referring to Figure 7, an embodiment of performing a method of producing a composite preparatory pulse in a spin-lock, spin echo NQR sequence may proceed as follows. First, a suspicious item (as defined above) is placed in the scanning area of a NQR detection device. Next, the NQR detection device is operated to generate (701) a radio frequency ("RF") magnetic field about a target sample. The target sample may be hidden within the suspicious item or may be present on an exterior surface of the suspicious item. When the RF magnetic field is generated, one or more of the RF pulses used to create the RF magnetic field are compensated (702) for one or more inhomogeneities in the RF magnetic field. Advantageously, this compensation may occur without degrading a temperature bandwidth of the NQR device. Additionally, the method 700 may include acquiring (703) a series of spin echoes having a spin-locked, spin-echo pulse sequence. Thereafter, the target sample may be identified (704) irrespective of one of the target sample's geometry, position, and orientation. The identification step (704) may be performed using known signal processing techniques. [0049] The step (702) of compensating for one or more inhomogeneities in the RF magnetic field may include applying a train of (SLSE) RF pulses to the target sample. The train of pulses may include a composite preparatory pulse followed by a series of single refocusing pulses. Each of the series of single refocusing pulses may be a square pulse. A relative phase of a last segment of the composite preparatory pulse and a first segment of a first of the series of single refocusing pulses may be other than ninety degrees (90"). Additionally, the composite preparatory pulse may be configured to produce a uniform rotation of spin magnetization in a presence of the one or more RF magnetic field inhomogeneities to compensate for the one or more inhomogeneities of the RF magnetic field strength. [0050] The following exemplary definitions are provided for terms used in the specification and claims. 14 WO 2009/020717 PCT/US2008/068081 [0051] Adiabatic Fast Passage - a technique in Nuclear Magnetic Resonance (NMR) that uses frequency, phase, and amplitude modulation of RF magnetic fields for excitation of a spin system. The AFP starts -with a radio-frequency field far from resonance. The resonance condition is approached by sweeping the amplitude and frequency of the excitation field. Adiabatic pulses can provide increased excitation bandwidths and accurate flip angles, with high tolerance to spatial variations in RF intensities. [0052] Adiabatic Half Passage - For an Adiabatic Half Passage (AHP) the frequency sweep during the adiabatic fast passage is terminated at resonance. For further explanation, see U.S. Patent No. 6,166,541. [0053] Composite Pulse - A radio frequency composite pulse consists essentially of two or more hard pulses with varying phases, durations and/or amplitudes that have no delay or negligible delays (<< T2*) between them. T2* (called T2 star) characterizes the lifetime of a free-induction decay (FID) NQR signal. A composite pulse with n segments can be denoted as a series of concatenated flip angles: 061p, (82)0 - -. - (AV, [0054] BI-Compensated Composite Pulse - a composite pulse developed by the inventors of the present invention and designed to correct for imperfections in the excitation amplitude of the RF magnetic field. These pulses are of importance in NQR where RF field inhomogeneity can severely undermine the sensitivity of detection. For example, if the sample is at an unknown distance from the RF coil used for exciting NQR signals it is difficult to excite the specimen with the optimal flip angle for maximum NQR response. As described above, composite preparatory pulses configured to compensate for variations in an RF magnetic field can enhance NQR sensitivity by providing close to optimal flip angle over wider range of RF field strengths. [0055] "Hard" Pulse - Pulsed electromagnetic radiation for excitation of nuclear signals from the specimen. In NQR and NMR, an ideal hard pulse is characterized by the strength of the radio frequency magnetic field B, the time duration pw, and the phase 15 WO 2009/020717 PCT/US2008/068081 rp of the radio frequency signal at the beginning of the pulse. Its effect on the nuclear spins is characterized by the flip angle (defined below). [0056] Flip Angle - Under the effect of a resonant radio frequency magnetic field Bt, the nuclear spin magnetization processes with an instantaneous angular velocity ,= y B,. Assuming all relaxation processes are negligible during application of the external radio frequency excitation, after a time pw of excitation with an electromagnetic field of constant amplitude the total rotation angle, or flip angle, of the magnetization is # 0 y pwB,. The parameter { is a function of the nuclear spin number; some values are (= - /2 for spin number I = 3/2, and = 1 for spin number I = 1. Therefore, the effect of a hard pulse is described by its flip angle and denoted (p),, where the subscript p denotes the phase of the radio frequency carrier signal at the beginning of the electromagnetic pulse. 100571 Gyromagnetic ratio, y - a scalar that defines the magnetic moment of a nucleus. It defines the magnitude of the magnetic moment of an atomic particle (nucleus, electron). [0058] Inhomogeneous - Non-uniform in space, spatially variable. [0059] Irradiation Frequency - a frequency (o of an electromagnetic field used for excitation of the nuclear spins. Typically, the irradiation frequency is at or close to the Quadrupole Resonance frequency co, of the target nuclei. [0060] Magneto-Acoustic Ringing - a phenomenon that arises from the interaction between the electromagnetic pulses and ferromagnetic metals with permanent magnetic moments that might be present in the interrogated specimen or nearby the sensor. [0061] Nominal B, (RF Magnetic Field) - Typically, all parameters of a pulse sequence in an NQR experiment are optimized to maximize the NQR sensitivity at a given RF magnetic field intensity. This optimal magnetic field value is sometime referred to as the nominal By. 16 WO 2009/020717 PCT/US2008/068081 [0062] Nuclear Quadrupole Moment - an intrinsic property of the nuclear charge distribution; it measures the departure of the nuclear electric charges from spherical symmetry. Nucleuses with a non-zero quadrupole moment in the presence of an electric field gradient experience the phenomena of Nuclear Quadrupole Resonance (NQR). [0063] Piezoelectric Ringing - Piezoelectricity is an electromechanical phenomenon of matter which involves the changes in electric polarization due to mechanical stress or the production of mechanical strain under electric fields. Piezoelectric materials in a target sample will interact with the radio frequency electric field of the electromagnetic pulses and the extraneous signal will be received by the NQR detection system. [0064] Preparatory Pulse -the first pulse or group of pulses at the beginning of a multiple pulse sequence that precedes signal acquisition. Typically, no NQR signals are measured during or right after a preparatory pulse. Preferably, the flip angle of the preparatory pulse is such as to generate the maximum NQR response. [0065] Quadrupolar Nucleus - an atomic nucleus possessing a non-zero nuclear quadrupole moment. Non-limiting examples are Chlorine, Nitrogen, Bromine, Iodine, Copper, Antimony, Aluminum, and others. [0066] Quadrupole Resonance Frequency - a characteristic transition frequency between energy levels resulting from the interaction between the electric field gradient with the nuclear quadrupole moment of a nucleus. It is determined by the nuclear quadrupole moment and the averaged strength of the electric field gradient at the position of the quadrupolar nucleus. [0067] Radio Frequency (RF) Field -Electromagnetic radiation with a domain of frequencies from thousands of MHz down to kHz. [0068] Refocusing Pulse - Pulsed magnetic field used to evoke spin echo signals after a preparatory pulse or group of preparatory pulses. [0069] RF Magnetic Field - Magnetic component of a radio frequency field. Using standard notation in physics, the oscillatory magnetic field vector can be described as 17 WO 2009/020717 PCT/US2008/068081 B5 =Ble*''% where B, is the amplitude of the field, t is time, c is the irradiation frequency in rad/sec, (o is the phase of the oscillation, and b is a unit vector that defines the direction of the magnetic field. [0070] Ringing - an undesirable response in magnetized materials (magneto-acoustic or magnetostrictive effect) or piezo-electric materials that is induced by the RF pulses transmitted by a NQR sensor. Other sources of ringing intrinsic to a NQR system include, but are not limited to, electronic circuit ringdown and baseline offsets. For further explanation, see U.S. Patent No. 5,365,171 and U.S. Patent No. 6,392,408. [0071] Single-Sided System - a NQR system intended to operate with the target sample outside, or partially outside, the NQR coil. The sensors are usually surface coils; such as spiral ("pancake") and meander-line. A non-limiting example of a single-sided NQR system is described in U.S. Patent No. 5,365,171. Other examples of single-sided systems include, but are not limited to, NQR-based shoe scanners and landmine detectors. A single-sided NQR sensor accesses the target sample from one side of the interrogated volume. [0072] SNR - an abbreviation for Signal-to-Noise Ratio, which is the ratio between the spectral peak intensity and thermal noise level. [0073] SLSE - abbreviation for spin-locked spin-echo. This multiple pulse sequence developed by Marino and Klainer (Marino R.A., Klainer S.M., "Multiple Spin Echoes in Pure Quadrupole Resonance," J. Chem. Phys. Vol. 67 (7), 1977) for efficient detection of echo signals in NQR by exploiting the phenomena of spin locking. It allows sensitivity enhancement by coherently adding the individual echoes from a train of spin echoes. A SLSE sequence consists of a first excitation pulse (called preparatory pulse) followed by a number of identical pulses called refocusing pulses (identical pulses means they all have the same amplitude, phase, and duration) having a phase that is ninety degrees from the phase of the preparatory pulse. This pulse sequence evokes a response that can persist 18 WO 2009/020717 PCT/US2008/068081 for a time much longer than the spin-spin relaxation time. The pulse sequence has specific time intervals between pulses to maximize sensitivity (SNR per unit time). [0074] Spin System - an assembly of atomic particles possessing a non-zero spin angular momentum. Nuclear Magnetic Resonance (NMR) and Nuclear Quadrupole Resonance (NQR) deal with assemblies of atomic nuclei. [0075] Spin Dynamics - Refers to the collective behavior of an assembly of atomic nuclei (nuclear spins) as the atomic nuclei interact with magnetic and or electric fields. [0076] Surface Coils - See single-sided system. [0077] Temperature Bandwidth - NQR resonance frequencies of the quadrupole nuclei contained in a target sample are sensitive to the temperature of the material that forms the target sample. In most materials that exhibit NQR responses a functional relationship can be established between frequency and temperature o = F(T). Any excitation pulse sequence has a limited irradiation bandwidth over which NQR responses can be excited and detected. This frequency or irradiation bandwidth can be translated into temperature of the material using the functional relationship T = F-'(we) to describe the temperature bandwidth of the excitation pulse sequence. For example, for many materials the relationship between temperature and NQR frequency can be described, over a narrow temperature range, by a linear equation (first-order polynomial) as o', = aT + b Therefore, the excitation frequency bandwidth can be described as a temperature bandwidth using the inverse relationship T = (we - b)/a. The parameter a is known as the temperature coefficient of the NQR resonance. [0078] Embodiments of the methods herein described for testing of samples in inhomogeneous RF magnetic fields, shown and described herein are illustrative only. Although only a few embodiments of the invention have been described in detail, those skilled in the art who review this disclosure will readily appreciate that substitutions, modifications, changes and omissions may be made in the design, operating conditions and arrangement of the preferred and other exemplary embodiments without departing 19 WO 2009/020717 PCT/US2008/068081 from the spirit of the embodiments as expressed in the appended claims. Accordingly, the scopes of the appended claims are intended to include all such substitutions, modifications, changes, and omissions. 20

Claims (10)

1. A method for improving accuracy of NQR target identification, the method compnsing: generating a radio frequency ("RF") magnetic field about a target sample; compensating for one or more inhomogeneities in the RF magnetic field; and identifying the target sample irrespective of one of a geometry of the target sample, position, and an orientation of the target sample.

2. The method of claim 1, wherein the compensating for one or more inhomogeneities in the RF magnetic field does not degrade a temperature bandwidth of the NQR sensor.

3. The method of claim 1, further comprising: acquiring a series of spin echoes having a spin-locked, spin-echo pulse sequence.

4. The method of claim 1, wherein the target sample is one of an explosive and a narcotic.

5. The method of claim 1, wherein the step of compensating for one or more inhomogeneities in the RF magnetic field further comprises: applying a train of pulses to the target sample, wherein the train of pulses comprises a composite preparatory pulse followed by a series of single refocusing pulses.

6. The method of claim 3, wherein each of the series of single refocusing pulses is a square pulse. 21 WO 2009/020717 PCT/US2008/068081

7. The method of claim 5, wherein a relative phase of a last segment of the composite preparatory pulse and a first segment of a first of the series of single refocusing pulses is other than ninety degrees (90').

8. The method of claim 5, wherein composite preparatory pulse is configured to produce a uniform rotation of spin magnetization in a presence of the one or more RF magnetic field inhomogeneities to compensate for the one or more inhomogeneities of the RF magnetic field strength.

9. The method of claim 5, wherein the composite preparatory pulse has a form: [(# )9, (#62)e (# G3) 9, --- (18N I wherein, (3 ;),(p2), (Th), aiid ()N) represent flip angles of individual subpulses, and wherein 9P1,, (P,2, (P,3 and p[,N respectively represent phases of the individual subpulses.

10. The method of claim 5, wherein the composite preparatory pulse has a form: [(#8 ) 8 (#2 83),, (#4),1 2 2 wherein (p1), (82), (83), and (84) represent concatenated flip angles of individual subpulses, and wherein T represents a relative phase of each of the individual subpulses. 22