Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Reexamination Certificate
1996-11-15
2001-06-05
Arana, Louis M. (Department: 2857)
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
C324S318000
Reexamination Certificate
active
06242918
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an apparatus and method for reducing the recovery period of a probe in nuclear quadrupole resonance (NQR) and nuclear magnetic resonance (NMR) detection systems, and, more particularly, to an apparatus and method for reducing the recovery period by varying the impedance of a load to lower the total Q factor of the probe and load during the recovery period as compared to the total Q factor during transmission and reception.
2. Description of the Related Art
There are many situations where it is desirable to detect the presence of a specific substance. For example, with the unfortunate increase in drug trafficking and terrorist use of high explosives in aircraft and buildings, there is a strong interest for a reliable detection system that can detect sub-kilogram quantities of narcotics and explosives against a background of more benign materials in a rapid, accurate, and non-invasive fashion.
Nuclear quadrupole resonance (NQR) and nuclear magnetic resonance (NMR) are known techniques for detecting the presence of specific substances. More specifically, various substances produce a magnetic resonance signal when excited by radio frequency (RF) radiation at a particular frequency. Generally, RF radiation at a particular frequency will cause a precession of nuclei in a specific substance, but not in other substances. Nuclear quadrupole resonance (NQR) and nuclear magnetic resonance (NMR) take advantage of this phenomenon to detect the various substances (for NMR a magnet is included but not illustrated).
FIG. 1
illustrates a conventional NQR and NMR apparatus. As illustrated in
FIG. 1
, a transmitter
20
and a receiver
22
are connected to a probe
24
through a transmit/receive (T/R) switch
26
. Probe
24
includes a coil (not illustrated) forming part of a resonant circuit (not illustrated). To detect the presence of a specific substance, T/R switch
26
connects transmitter
20
to probe
24
while disconnecting receiver
22
from probe
24
. Then, transmitter
20
generates a pulse and supplies the pulse to probe
24
. Generally, the pulse is formed from a signal having a frequency corresponding to the resonance frequency of the nuclei of the specific substance that is intended to be detected. The pulse is transmitted to probe
24
, which causes the coil in probe
24
to store RF energy and generate an RF magnetic field at a target specimen (not illustrated). If the specific substance desired to be detected is present in the target specimen, the RF magnetic field generated by the coil will excite nuclear resonance in the quadrupolar nuclei of the specific substance and thereby cause the specific substance to produce a resonance signal.
After the RF magnetic field is generated by the coil of probe
24
, the T/R switch
26
connects receiver
22
to probe
24
while disconnecting transmitter
20
from probe
24
. The coil in probe
24
then detects the resonance signal produced by the specific substance, and probe
24
produces a corresponding output signal. The output signal of probe
24
is received and analyzed by receiver
22
, to confirm the presence of the specific substance in the target specimen.
Therefore, probe
24
generates an RF magnetic field at a specimen, and also receives resonance signals produced by the specific substance in the specimen. However, probe
24
cannot receive the resonance signal immediately after generating an RF magnetic field, because of the stored RF energy in the probe. Instead, immediately after the excitation pulse, probe
24
must “ring down”, i.e., dissipate the stored RF energy, before it can usefully receive the resonance signal.
FIG.
2
(A) illustrates a pulse
25
produced by transmitter
20
and provided to the coil of probe
24
from time t
1
to t
2
, and FIG.
2
(B) illustrates the corresponding RF energy
27
stored in the coil. As illustrated by FIGS.
2
(A) and
2
(B), some RF energy still remains in the coil after the end of pulse
25
at time t
2
until time t
3
. This remaining RF energy must be dissipated, that is, probe
24
must “ring down”, before probe
24
can effectively be used to receive a resonance signal. The time required for probe
24
to “ring down” to an appropriate level is referred to as a “recovery period”, and is illustrated in FIG.
2
(B) as recovery period R.
Unfortunately, a long recovery period will undesireably decrease the detection sensitivity of probe
24
, because of the loss of the resonance signal. For example, if the lifetime of a resonance signal is shorter than, or comparable to, the recovery period of the probe, then the resonance signal will have decayed substantially before it can be detected.
For a single coil employed in probe
24
, a rough rule of thumb is that a useful resonance signal cannot be received by probe
24
until approximately twenty (20) time constants have elapsed from the end of the RF pulse provided to the coil of probe
24
. The time constant of the coil is given by Q/&pgr;f, where f is the resonance frequency (in Hz), and Q is the quality factor of the coil. Therefore, the recovery period typically must be at least 20Q/&pgr;f. This recovery period becomes particularly long with NQR and NMR operating at relatively low frequencies, such as at frequencies less than 10 MHz and for high-Q coils.
Moreover, the signal-to-noise ratio (SNR) of a resonance signal received by the coil is increased when the coil has a high Q. Therefore, it is desirable to have a coil with a high Q. However, as can be seen from the time constant Q/&pgr;f, the recovery period can be relatively long for high Q coils. As a result, using the prior art, a system designer must compromise between the desire to have a high Q coil, and the desire to reduce the recovery period. For example, a coil with a Q of 1,000 at a frequency of 1 MHz has an estimated recovery period of 6 ms. This recovery period is too long to reliably detect many substances, and the Q of the coil may have to be reduced.
In addition, in NQR and NMR, a train of sequential pulses is typically generated by transmitter
20
and supplied to probe
24
for generating a series of RF magnetic fields at a specimen. After each pulse is provided to the coil in probe
24
and the corresponding RF magnetic field is generated, probe
24
must wait for a respective recovery period to elapse. After the recovery period elapses, probe
24
can receive resonance signals. Further, after a period of time elapses for receiving resonance signals, the next pulse is generated. Therefore, a recovery period must pass between each pulse. This recovery period dictates the time scale for which the train of RF pulses can be applied.
For example, in a conventional steady-state free-precession sequence of RF pulses, the pulses are spaced at a time interval &tgr;. Under certain conditions, it is observed that the amplitude of the resonance signal received in each time interval &tgr; desirably increases as the time interval &tgr; decreases. Moreover, a resonance signal produced by a specimen can be sampled more frequently by decreasing the time interval &tgr;, thereby increasing the overall signal-to-noise ratio (SNR). In addition, for methods employing stochastic excitation, the excitation bandwidth is limited by the time interval &tgr;. Therefore, a relatively short time interval &tgr; is generally preferred. Unfortunately, a long recovery period in the probe necessarily increases &tgr;.
Conventional methods have been used to “actively damp” the probe, and thereby reduce the recovery period. For example, the recovery period can be actively damped by switching the total Q factor back and forth from a high total Q factor to a low total Q factor, where the total Q factor is switched low during the recovery period. This low total Q factor during the recovery period reduces the recovery period. However, such methods modify the circuitry of the probe to switch the total Q factor. In other words, the electrical configuration of the probe is changed to switch the total Q factor. Such
Hepp Mark A.
McGrath Kenneth J.
Miller Joel B.
Arana Louis M.
Karasek John
McDonnell Thomas E.
The United States of America as represented by the Secretary of
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