Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Reexamination Certificate
2003-03-06
2004-08-17
Arana, Louis (Department: 2859)
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
C324S322000
Reexamination Certificate
active
06777937
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a nuclear quadrupole resonance (NQR) method and device for detecting a target material using a rotating radio frequency (RF) magnetic field. More particularly, the present invention is directed to an improved NQR surface coil design and method for generating a rotating RF field in a detecting region outside the surface coil.
BACKGROUND ART
There are many situations where it is desirable to detect the presence of a target material, that is, a specific substance. For example, detection systems are often used to detect sub-kilogram quantities of narcotics and explosives against a background of more benign materials. Such detection systems are used in airports and other locales to detect these materials, e.g. when hidden in luggage.
NQR is a known technique for detecting a target material. It is an effective means of detecting nitrogenous and chlorine-containing materials, such as explosives and narcotics, owing to the presence of quadrupolar nuclei, e.g.
14
N and
35,37
Cl, in these substances. This general NQR approach is referred to as ‘pure’ NQR to indicate that no externally applied static magnetic field is required. NQR is generally excited with an RF pulse that induces an NQR signal known as a free induction decay (FID). Trains of pulses induce NQR signals known as spin echoes, related to FIDs. An RF pulse at a specific frequency transmitted to a coil in proximity to a sample of interest induces an NQR signal from the nuclear spins of quadrupole nuclei in these specific materials but not in others. The NQR signal is generally proportional to the quantity of material excited, but the exact magnitude of the NQR signal depends on the relative orientation of the RF field and the molecules of the sample. The direction of the RF field with respect to the sample is, in turn, determined by the geometry of the RF field and the position of the sample relative to the coil.
FIG. 1
is a diagram illustrating a conventional NQR apparatus. A transmitter
20
and a receiver
22
are connected to a probe
24
through a transmit/receive (T/R) switch
26
. Probe
24
includes an inductor, such as a solenoid coil
28
, forming part of a resonance circuit with other inductors L and capacitors C. To detect the presence of a target material, T/R switch
26
connects transmitter
20
to probe
24
while disconnecting receiver
22
from probe
24
. Transmitter
20
then generates and transmits a pulse to probe
24
. The pulse is typically formed by an RF signal having a frequency corresponding to the resultant resonance signal of the nuclei of the target material to be detected. Probe
24
receives the pulse, causing coil
28
to store (RF) energy, which in turn produces a corresponding RF magnetic field. When a sample (not illustrated) is positioned near, or inside, coil
28
, the RF magnetic field irradiates the sample inducing an NQR signal in a target material.
After subjecting the sample to the RF magnetic field, T/R switch
26
connects receiver
22
to probe
24
while disconnecting transmitter
20
from probe
24
. Coil
28
then detects NQR induced in a target material and probe
24
produces a corresponding output signal. The output signal is received and analyzed by receiver
22
, thereby establishing the presence of a target material in the sample.
In real world use of NQR for detecting narcotics and explosives, a sample may or may not contain the target material of interest. Due to the selection of a narrow bandwidth of the RF magnetic field for irradiating a sample and because of the large range of NQR frequencies in benign, non-targeted materials, it is unlikely that an NQR signal is induced in such non-targeted materials by the RF magnetic field. Accordingly, an NQR device provides accurate identification of target materials without producing false alarms (false positives).
Unfortunately, a major obstacle to detecting sub-kilogram quantities of these types of materials is the low sensitivity of NQR detection devices. One approach to this problem employs spatially rotating RF magnetic fields generated by two spatially orthogonal RF magnetic fields 90° out of phase. In the related fields of nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI), the use of rotating RF magnetic fields can increase the detected maximum signal by a factor of 2, with a concomitant increase in the signal-to-noise ratio (SNR) of 1.4.
A similar increase in SNR can be obtained for NQR, but for very different physical reasons. For the typical case of a powdered or polycrystalline sample the NQR signal is independent of the direction of the RF field, however only a fraction of the nuclei are excited by the pulse. As described in U.S. Ser. No. 08/904,937, filed Aug. 1, 1997, and incorporated herein by reference, a rotating RF magnetic field can increase the number of nuclei excited by the RF pulse. The detected maximum signal from single pulse excitation increases by a factor of almost 2, with a concomitant increase in the SNR of approximately 1.2. The SNR gain is about 1.4 for spin echo sequences. Also, smaller pulse flip angles may be used to excite the spins, with a corresponding savings in RF transmitter power. Also as set forth in U.S. Ser. No. 08/904,937, the use of rotating RF magnetic fields in NQR can make the detector less sensitive to acoustic and piezoelectric ringing artifacts.
FIG. 2
is a diagram illustrating another NQR apparatus for applying and detecting rotating RF magnetic fields. It operates in a manner similar to the conventional NQR apparatus shown in
FIG. 1. A
transmitter
30
and a receiver
32
are connected to a probe
34
through T/R switch
36
. Probe
34
includes a coil
38
, such as a birdcage coil for generating and detecting rotating RF magnetic fields. Transmitter
30
differs from its conventional counterpart
20
in that it provides two RF signals 90° out of phase to create the rotating RF magnetic field. Likewise, receiver
32
detects both components of the rotating NQR signal and T/R switch
36
can connect the two transmitter outputs and two receiver inputs to probe
34
.
One implementation of a device capable of generating the rotating RF field inside the enclosed coil volume employs what is commonly referred to as a “birdcage” coil geometry. However, some scenarios require a device for excitation and detection outside the coil volume. There are two criteria for such a device: i) the device must generate the two RF magnetic field components substantially orthogonal to each other; and ii) if multiple coils are used, they must have negligible mutual inductance. U.S. Pat. No. 5,682,098 describes such an MRI device that employs overlapping surface coils for this purpose. “Planar quadrature coil design using shielded-loop resonators”,
J. Magnetic Resonance
, Vol. 125, pp. 84-91, A. Stensgaard (1997), describes another MRI device using a ‘dual mode’ single surface coil for rotating field detection. Such designs work well in MRI where the nuclei are insensitive to RF fields in the direction of the polarizing static magnetic field, i.e. where the RF magnetic fields need only be orthogonal in two dimensions. However in NQR, parallel RF magnetic field components introduce a non-rotating field component that reduces the SNR improvement.
FIG. 3
illustrates a conventional surface coil array
40
for applying and detecting rotating RF magnetic fields that can be used in an NQR device, e.g. such as the one shown in FIG.
2
. Coil array
40
includes a first coil
48
that is coplanar and co-centered (i.e. fully overlapping) with a second coil
49
. More particularly, coils
48
and
49
as shown are what are commonly referred to as figure-8 coils, based on the geometrical shape and appearance. A “figure-8” coil is understood to include any coil consisting of two coplanar sections enclosing an area. The sections are configured such that an external uniform RF magnetic field induces equal but opposite currents in the sections, the figure-8 coil thus performing like a receiving gradiometer. (By reciprocity, a coil may b
Garroway Allen
Miller Joel
Arana Louis
Karasek John J.
Legg L. George
The United States of America as represented by the Secretary of
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