IMPROVEMENTS IN MAGNETIC AND QUADRUPOLE RESONANCE ANALYSIS

Abstract: Apparatus for transmitting an RF magnetic field pulse for use in an NMR or NQR experiment. The apparatus comprising a transmit antenna (10) for transmitting the RF magnetic field pulse and a switching element (12) such as a TRIAC which can be selectively switched on to couple a damping circuit to the transmit antenna. The damping circuit damps the transmit antenna by carrying a damping current. The switching element (12) is responsive to the damping current so as to switch off when the transmit antenna (10) has been damped to an acceptable level.

Description: IMPROVEMENTS IN MAGNETIC AND OUADRUPOLE RESONANCE ANALYSIS The present invention relates to improvements in Nuclear Magnetic Resonance (NMR) analysis and Nuclear Quadrupole Resonance (NQR) analysis. In the majority of pulsed NMR or NQR spectrometers a resonant probe (including a transmit antenna) is used. A strong radio frequency (RF) electrical pulse, typically with a power of hundreds of watts, is applied to the probe. The RF magnetic field pulse which the probe transmits in response to the RF electrical signal excites the nuclear spins in a sample under investigation which resonate at the RF frequency used. After this strong RF pulse the resonant probe needs to lose its stored energy quickly (in other words, the probe needs to "ring down" quickly) . After the probe has rung down the NMR or NQR signal from the nuclear spins can be picked up by the same probe (in some NMR or NQR spectrometers separate antennae for transmitting and receiving are used, although these still suffer from the ring-down problem) . The probe ring-down prevents the detection of the NMR/NQR signal straight after the RF pulse. This is in many cases very unfortunate as vital information may be contained in the NMR/NQR signal just after the RF excitation pulse. Various techniques have been used to shorten the ring- down time, including permanent resistive damping and switched resistive damping. Permanent resistive damping uses a resistive damping element which is permanently electrically coupled to the probe, thereby permanently damping the probe. A problem with this technique is that due to losses caused by the resistive damping element, increased pulse power or longer pulse times are needed. Furthermore, the signal-to-noise ratio (SNR) of the NMR/NQR signal is reduced due to Johnson noise in the resistive damping element. Switched resistive damping uses actively switched switching elements (such as field-effect transistors or actively switched diodes) to connect a damping circuit with the probe only during the ring-down period. A problem with this technique is that when the switching elements are actively switched-off a parasitic charge is injected into the NMR/NQR probe via the parasitic capacitances of the diodes or transistor. This parasitic charge causes the probe to ring anew. In accordance with a first aspect of the present invention there is provided apparatus for transmitting an RF magnetic field pulse for use in an NMR or NQR experiment, the apparatus comprising a transmit antenna for transmitting the RF magnetic field pulse and a switching element which can be selectively switched on to couple a damping circuit to the transmit antenna whereby the damping circuit damps the transmit antenna by carrying a damping current, characterised in that the switching element is responsive to the damping current so as to switch off when the transmit antenna has been damped to an acceptable level. The first aspect of the invention is a variant of switched resistive damping, which does not suffer the problems caused by a switch-off signal. The switching element switches itself off after a certain recovery time after the damping current (and hence the load current through the transmit antenna) has fallen below a certain small limit. As the switching element does not need a switch-off control signal no charge is injected when the damping is removed, and therefore no new ringing is started. Any suitable switching element may be used, but typically the element is a thyristor or most preferably a triac. The switching element itself may also comprise the damping circuit (i.e. the switching element may provide the damping resistance) . Alternatively a separate damping circuit (such as a resistor) may be provided to give further damping. The switching element and/or the damping circuit may be resistively coupled to the transmit antenna (i.e. connected by a wire) . Alternatively the switching element and/or the damping circuit may be capacitively or inductively coupled to the transmit antenna. In accordance with a second aspect of the present invention there is provided a method of transmitting an RF magnetic field pulse for use in an NMR or NQR experiment using apparatus according to the first aspect of the present invention, the method comprising applying an RF electrical signal to the transmit antenna; and selectively switching on the switching element to damp the transmit antenna. We have also considered the possibility of improving the performance of a conventional switched damping circuit which needs to be actively switched off. In accordance with a third aspect of the present invention there is provided apparatus for transmitting an RF magnetic field pulse for use in an NMR or NQR experiment, the apparatus comprising a transmit antenna for transmitting the RF magnetic field pulse; and a switching element coupled to the transmit antenna and which can be selectively switched on to damp the transmit antenna, and switched off to terminate the damping of the transmit antenna, wherein the transmit antenna has a damped ring- down time when the switching element is switched on characterised in that the switching element exhibits a response time between being switched off and termination of the damping which is of the order of the damped ring-down time of the transmit antenna. The third aspect of the present invention utilises the properties of a "slow" switching element to reduce ring- down problems. Any additional ringing caused by switching- off the switching element is damped before the switching element is disabled. The switching element may be switched on and off by separate switch-on and switch-off signals (e.g. separate voltage pulses) . Alternatively the switching element may be switched on by applying a switch-on signal, and switched off when the switch-on signal is removed. In a preferred example the switching element comprises one or more transistors of some kind. Typically the response time is greater than 50% of the damped ring-down time. In a preferred embodiment the response time is 80%-120% of the damped ring-down time. In accordance with a fourth aspect of the present invention there is provided a method of transmitting an RF magnetic field pulse for use in an NMR or NQR experiment using apparatus according to the third aspect of the present invention, the method comprising transmitting an RF magnetic field pulse with the transmit antenna; switching on the switching element to damp the transmit antenna; and switching off the switching element to terminate damping of the transmit antenna. Embodiments of the present invention will now be described with reference to the accompanying figures in which: Figure 1 is a schematic block diagram of a pulsed NMR system for determining the oil concentration in oil seeds; Figure 2 illustrates two RF pulses; Figure 3 illustrates the radiation signals transmitted and received by the probe; Figure 4 illustrates the probe with a TRIAC damper; Figure 5 illustrates an alternative damping system which employs a slow transistor; and, Figures 6A-6C illustrate the operation of the system of Figure 5. Figure 1 illustrates an NMR system for the determination of oil concentration in oil seeds in large samples (1 litre or more) at proton resonance frequencies of 1MHz and below. The system comprises a probe 1 which transmits and receives RF signals to/from a sensitive region. This probe is located in a static magnetic field, usually generated by a magnet (not shown in Figure 1) . An RF pulse generator generates an RF signal at a frequency of approximately lMHz which is amplified by TX amplifier 3 and applied to the probe 1 via TX/RX switch 4. Event generator 5 generates a pulsed electric signal of the form illustrated in Figure 2, comprising a series of pulses 13,14 etc. The pulses 13,14 cause the RF pulse generator 2 to apply corresponding RF pulses to the probe 1. The resulting probe radiation signal is illustrated in Figure 3. The probe 4 transmits a sequence of RF radiation pulses 15,16 etc in response to the pulsed electric signal. The TX/RX switch 4 is a passive switch which connects the probe 1 to the TX amplifier 3 during the transmit pulses 15,16, and connects the probe 1 to receiver electronics (RX amplifier 6 and data acquisition and processing module 7) between the transmit pulses 15,16. During RX periods, the module 7 monitors the signal on the probe 1 to detect NMR/NQR signals 19,27. In the absence of a damping element, each pulse 15,16 etc would have a relatively long ring down period (typically many hundred microseconds) after the pulse 13,14 has finished when the probe is emitting a ring-down signal. These undamped ring-down signals are illustrated in dashed lines at 20,21. However in the present case a damping element 8 damps the probe 1 to produce more rapid ring-down as indicated in solid lines 23,24. Figure 4 illustrates the probe 1 and damping element 8. Switched resistive damping is achieved by means of a T0812NJ triac 12 (manufactured by Tag Semiconductors Ltd) in parallel with the probe 1, which comprises an inductor 10 and a variable capacitor 11. The inductor 10 transmits the RF signal into a sensitive region in the bore of the coil in which a sample 25 to be analysed is placed. The event generator 5 triggers the triac 12 into an on-state at the end of each pulse 13,14. A damping current then passes through the triac 12 and damps the probe 1, resulting in ring-down signals 23,24 with lower ring-down times 17,18 (Figure 3) . The triac 12 is an electronic switching element which, by a short pulse of a few volts and a few ten mA, can be triggered into the on-state. It stays in the on-state as long as the damping current is above a certain limit. Once the damping current falls below this limit the triac slowly recovers and switches itself off gradually. The device is bipolar and can switch bipolar load currents. The soft recovery is very useful. This is unlike a transistor where inevitably charge is injected when the transistor is switched off. This charge would cause new ringing. The triac 12 switches off by itself without a charge injection and therefore no ringing is caused.. For T0812NJ we found the recovery time to be less than 200 μs. An important specification of the triac 12 is the Critical Rate of Voltage Rise dV/dt which is 500 V/μs for the T0812NJ triac. In an alternative embodiment (not shown) the triac 12 may only be turned on when the voltage across the probe 1 (as monitored by a comparator, not shown) passes through zero. This results in a slight delay after trigger before the triac 12 carries a large damping current which may be kinder to the triac. In an alternative embodiment shown in Figure 5 the triac 12 is replaced by a slow transistor 26 in series with a damping resistor 27. The slow transistor 26 exhibits a response time (between receiving a switch-off signal and actually switching off) which is of the order of the damped ring-down time of the probe 1. This property is exploited to reduce the new ring-down signal caused by switching off the transistor, as illustrated in Figures 6A-6C. Figure 6A illustrates the radiation signal transmitted and received by the probe 1 of Figure 5. Figure 6A corresponds with Figure 3. The resonant probe 1 has a characteristic ring-down time (in the absence of the damping circuit) shown by dashed line 20. This is reduced to the damped ring-down time 17 when damping is applied. Figure 6B illustrates the timing of a switch-on pulse 80 which is applied to the transistor 26 by the event generator 5. Figure 6C illustrates the delayed ON/OFF state of the transistor 26. The transistor switches on at 82 in response to the switch-on signal 80, after a switch- on response time 83. The transistor switches off at 84, after a switch-off response time 85. Since the switch-off response time 85 of the transistor is of the order of the damped ring-down time 17, it is possible to turn off the switch-on signal 80 well before the transistor actually switches off at 84. Therefore any additional ringing caused by turning off the switch-on signal 81 is damped by the transistor/damping circuit. It should be noted that with a greater switch-off response time 85, the switch on pulse 81 may be turned off before the end of the pulse 13, shown in Figure 2. An NQR system may be constructed along similar lines to the NMR system of Figure 1, except with the omission of the static magnetic field at the location of the probe.

Claims: CLAIMS 1. Apparatus for transmitting an RF magnetic field pulse for use in an NMR or NQR experiment, the apparatus comprising a transmit antenna for transmitting the RF magnetic field pulse and a switching element which can be selectively switched on to couple a damping circuit to the transmit antenna whereby the damping circuit damps the transmit antenna by carrying a damping current, characterised in that the switching element is responsive to the damping current so as to switch off when the transmit antenna has been damped to an acceptable level. 2. Apparatus according to claim 1 wherein the switching element comprises a triac or thyristor. 3. Apparatus according to claim 1 or 2 wherein the switching element is coupled across the antenna. 4. Apparatus according to claim 1 or 2 further comprising a damping circuit for damping the transmit antenna, wherein the switching element can be selectively switched on to couple the damping circuit to the transmit antenna. 5. A method of transmitting an RF magnetic field pulse for use in an NMR or NQR experiment using apparatus according to any of the preceding claims, the method comprising applying an RF electrical signal to the transmit antenna; and selectively switching on the switching element to damp the transmit antenna. 6. Apparatus for transmitting an RF magnetic field pulse for use in an NMR or NQR experiment, the apparatus comprising a transmit antenna for transmitting the RF magnetic field pulse; and a switching element coupled to the transmit antenna and which can be selectively switched on to damp the transmit antenna, and switched off to terminate the damping of the transmit antenna, wherein the transmit antenna has a damped ring-down time when the switching element is switched on characterised in that the switching element exhibits a response time between being switched off and termination of the damping which is of the order of the damped ring-down time of the transmit antenna. 7. Apparatus according to claim 6 wherein the switching element comprises one or more transistors. 8. Apparatus according to claim 6 or 7 , wherein the response time is greater than 50% of the damped ring-down time. 9. A method of transmitting an RF magnetic field pulse for use in an NMR or NQR experiment using apparatus according to any of claims 6 to 8 , the method comprising transmitting an RF magnetic field pulse with the transmit antenna; switching on the switching element to damp the transmit antenna; and switching off the switching element to terminate damping of the transmit antenna.

Inventors: BLANZ MARTIN, BLANZ, MARTIN
Applicants: BLANZ MARTIN, OXFORD ANALYTICAL INSTR LTD, BLANZ, MARTIN, OXFORD ANALYTICAL INSTRUMENTS LIMITED
Application year: 1997
Main classification: G01R33
IPC codes: G01R33