Q-factor switching method and apparatus for detecting nuclear quadrupole and nuclear magnetic resonance signals

Abstract

A probe ( 80 ) for irradiating a sample with RF energy during transmitting periods and detecting an NQR or NMR signal from a substance contained within the sample during receiving periods. The probe ( 80 ) comprises a variable impedance unit ( 20 ) for changing the Q-factor of the probe and a probe coil. The probe ( 80 ) is responsive to powerful RF pulses applied thereto to excite an RF magnetic field in the probe coil during the transmitting periods. The variable impedance unit ( 20 ) is controllable to provide a Q-factor for the probe ( 80 ) at: (i) an optimal level during a prescribed transmitting period of an RF pulse for irradiating the sample with said RF energy; (ii) a minimal level during a prescribed recovery period immediately following said transmitting period to rapidly dampen transient signals from the probe; and (iii) a maximal level during a prescribed receiving period for detecting an NQR or NMR signal from the target substance if present, immediately following the recovery period. A method for detecting an NQR or NMR signal within a sample using the probe ( 80 ) is also described.

1 . An apparatus for changing the Q-factor of a probe, having an impedance that can be varied to achieve a Q-factor of minimal orders of magnitude and a Q-factor of high orders of magnitude, for irradiating a sample with RF energy during transmitting periods and detecting an NQR or NMR signal emitted from a substance contained within the sample from a signal received by the probe during receiving periods, the apparatus comprising: 
 Q-factor setting circuit for setting the Q-factor of a probe from minimal orders of magnitude to high orders of magnitude;    Q-factor changing circuit having low reactance for actively changing the Q-factor of the probe without injecting a parasitic charge therein; and    control circuit to control said Q-factor changing circuit so as to change the Q-factor of the probe in accordance with said Q-factor setting circuit to: 
 (i) an optimal level during a prescribed transmitting period of an RF pulse for irradiating the sample with said RF energy;  
 (ii) a minimal level during a prescribed recovery period immediately following said transmitting period to rapidly dampen transient signals from the probe; and  
 (iii) a maximal level during a prescribed receiving period for detecting an NQR or NMR signal from the target substance if present, immediately following the recovery period;  
   said optimal level being: 
 (a) sufficiently low to reduce said prescribed recovery period to a period in which said transient signals may be dampened and to develop the leading edge of the pulse envelope of said RF pulse during said transmitting period; and  
 (b) sufficiently high to reduce the bandwidth of said RF pulse during said transmitting period and so mitigate the power expended in amplifying the pulse envelope over the bandwidth; and  
   said maximal level being sufficiently high for the probe to receive a signal during the receiving period, after the recovery period, to enable an NQR or NMR signal emitted from a substance within the sample to be detected;    wherein said control circuit is adapted to control said Q-factor changing circuit in accordance with said Q-factor setting circuit to actively step the impedance of the probe down in a plurality of stages after said transmitting period for some period of time during the recovery period to provide the requisite minimal level of the Q-factor for the probe.    
   
   
       2 . An apparatus as claimed in  claim 1 , wherein the reactance of the Q-factor changing circuit that is low is the capacitive reactance thereof.  
   
   
       3 . An apparatus as claimed in  claim 1 , wherein the Q-factor changing circuit comprises a variable impedance unit that combines with the probe to form a tank resonant circuit that is capable of receiving powerful RF pulses applied to the probe to excite an RF magnetic field in the probe during transmitting periods, the value of the total Q-factor of the probe being determined by the impedance of the tank resonant circuit, whereby varying the impedance of the variable impedance unit changes the impedance of the tank resonant circuit.  
   
   
       4 . An apparatus as claimed in  claim 1 , wherein the Q-factor setting circuit is controlled to set the Q-factor of the probe during the recovery period to orders of magnitude of tenths to be considerably lower than the Q-factor of the probe before and after the recovery period so as to completely dampen the transient signals and provide for a rapid ring-down in the probe.  
   
   
       5 . (canceled)  
   
   
       6 . An apparatus as claimed in  claim 1 , wherein the impedance of the probe is stepped down to a first minimal level of magnitude during a first period of the recovery period immediately following said transmitting period and then to a second minimal level of magnitude lower than said first minimal level during a subsequent period of the recovery period, prior to said receiving period.  
   
   
       7 . An apparatus as claimed in  claim 1 , wherein said Q-factor setting circuit comprises a variable step impedance element and switching control elements that are able to be actively switched to step down the impedance of the probe to provide the requisite minimal level of the Q-factor of the probe.  
   
   
       8 . An apparatus as claimed in claim  29 , wherein the switching control elements are included only in the control circuits of the variable step impedance element.  
   
   
       9 . An apparatus as claimed in  claim 8 , wherein said Q-factor changing circuit comprises a variable impedance unit including said variable step impedance element that combines with the probe to form a tank resonant circuit that is capable of receiving powerful RF pulses applied to the probe to excite an RF magnetic field in the probe during transmitting periods, the value of the total Q-factor of the probe being determined by the impedance of the tank resonant circuit, whereby varying the impedance of the variable impedance unit changes the impedance of the tank resonant circuit, and the switching control elements are placed in electrical series with one another and in parallel with resistive elements to enable higher permissible voltages on the tank circuit and to achieve a plurality of possible Q-factor values during the transmitting, recovery and receiving periods.  
   
   
       10 . An apparatus as claimed in  claim 1 , wherein said Q-factor changing circuit comprises a variable impedance unit that combines with the probe to form a tank resonant circuit that is capable of receiving powerful RF pulses applied to the probe to excite an RF magnetic field in the probe during transmitting periods, the value of the total Q-factor of the probe being determined by the impedance of the tank resonant circuit, whereby varying the impedance of the variable impedance unit changes the impedance of the tank resonant circuit, and other elements comprising a capacitor network or zener diodes are included in the tank resonant circuit to improve the voltage sharing during switching.  
   
   
       11 . An apparatus as claimed in  claim 9 , including isolation circuit to electrically isolate low level drive signals for controlling the variable impedance unit.  
   
   
       12 . An apparatus as claimed in  claim 11 , wherein said isolation circuit includes optical isolation and pulse transformers.  
   
   
       13 . An apparatus as claimed in  claim 7 , wherein the probe is balanced with the switching control elements.  
   
   
       14 . An apparatus as claimed in  claim 7 , wherein said the impedance of the probe is stepped down to a first minimal level of magnitude during a first period of the recovery period immediately following said transmitting period and then to a second minimal level of magnitude lower than said first minimal level during a subsequent period of the recovery period, prior to said receiving period.  
   
   
       15 . An apparatus as claimed in  claim 7 , wherein said switching control elements have a low capacitive reactance.  
   
   
       16 . An apparatus as claimed in  claim 14 , wherein a said switching control element comprises a triac or thyristor.  
   
   
       17 . An apparatus as claimed in  claim 7 , wherein the impedance of the probe is stepped down to a first minimal level of magnitude during a first period of the recovery period immediately following said transmitting period and then to a second minimal level of magnitude lower than said first minimal level during a subsequent period of the recovery period, prior to said receiving period, and said switching control elements are coupled with an inductive element during the period that the impedance of the probe is stepped down to said first minimal level to protect said switching control elements from voltage applied thereto pursuant to the resultant change in impedance, and are decoupled from said inductive element during the period that the impedance of the probe is stepped down to said second minimal level to minimise the inductance of said switching control elements and thus the reactance of the variable impedance to minimise the impedance of the probe to change the Q-factor to said minimal level during the recovery period.  
   
   
       18 . An apparatus as claimed in  claim 1 , wherein said minimal level of the Q-factor is in orders of magnitude of ones or tenths.  
   
   
       19 . An apparatus as claimed in  claim 6 , wherein said first minimal level of the Q-factor is in orders of magnitude of ones, and said second minimal level of the Q-factor is in orders of magnitude of tenths.  
   
   
       20 . An apparatus as claimed in  claim 1 , wherein said maximal level of the Q-factor is in orders of magnitude of hundreds or thousand.  
   
   
       21 - 31 . (canceled)