Abstract:
The present invention provides nuclear magnetic resonance apparatus comprising means for applying a static magnetic field (H 0 ) to a sample under investigation, a radio frequency circuit arranged to be inductively coupled to the sample by means of an oscillating magnetic field disposed generally perpendicular to the static magnetic field for applying an excitation signal to the sample, and an electric potential sensor (E 1 , E 2 ) for detecting the excitation of the sample and for generating a detection output. The electric potential sensor comprises an electrode placed generally perpendicular to the axis of the oscillating magnetic field for capacitively coupling the electric potential sensor to the sample.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention concerns electric potential sensors for use in the detection of signals generated in nuclear magnetic resonance (NMR) apparatus, as well as to a method of NMR signal detection. 
         [0002]    The invention may find application in numerous areas, for example in NMR imaging and spectroscopy, and in quantum computers, as well as in medical applications such as Magnetic Resonance Imaging (MRI). 
       BACKGROUND TO THE INVENTION 
       [0003]    Nuclear magnetic resonance (NMR) is a phenomenon which occurs when the nuclei of certain atoms are subjected to a magnetic field in the presence of a second, perpendicular, oscillating magnetic field, and this phenomenon is commonly employed in NMR imaging and spectroscopy to study the physical, chemical or biological properties of samples of matter. This may best be understood by reference to  FIG. 1 . 
         [0004]    Many species of nuclei contain non zero spin and an associated magnetic dipole moment, and it is this which gives rise to nuclear paramagnetism and hence nuclear magnetic resonance (NMR). As shown in  FIG. 1 , when a static magnetic field H 0  is applied to a sample containing nuclear spins, the spins of the various nuclei become aligned in the direction of the Z axis. As a result of the underlying quantum mechanics of the system, the spins will precess around the Z axis at some angle θ, with a rate ω 0  given by the Larmor precession equation: 
         [0000]      ω 0 =γH 0    
         [0000]    where γ is the gyromagnetic ratio for the sample under consideration. If an additional radio frequency (RF) field H 1  is then applied in a direction perpendicular to the static magnetic field H 0  at a frequency 
         [0000]      ω RF =ω 0    
         [0000]    the angle θ will be increased due to resonant absorption. 
         [0005]    There are two conventional forms of NMR apparatus, known respectively as continuous wave NMR apparatus and pulse NMR apparatus, the signal detection in each case being by means of an inductive sensor as shown in  FIG. 2 . This inductive sensor comprises a resonant circuit  10  used to probe a sample  12  that is situated in a static magnetic field H 0 . In its simplest implementation, as illustrated, this circuit is used both to supply, and to read the output generated by, an excitation signal applied to the sample  12 . For this purpose, RF power is supplied by an RF source  14 , which is inductively coupled by means of a coil  16  to the sample  12 . 
       Continuous Wave NMR 
       [0006]    In the known continuous wave NMR apparatus, the static magnetic field H 0  is swept slowly in amplitude, and when 
         [0000]      ω 0 =ω RF    
         [0000]    absorption occurs and a dip in the signal voltage across the inductive coupling is observed. The same result may also be achieved by fixing the amplitude of the static magnetic field and sweeping the RF frequency. The signal voltage requires a large amount of amplification in order for the dip to be manifest, and is therefore applied to an amplifier  18  for output. 
         [0007]    In practice, the slow variation of the static magnetic field H 0  is generally achieved by adding a time varying field to the static one. This is often accomplished using additional coils. 
         [0008]      FIG. 3  shows a conventional magnetic continuous wave NMR absorption dip acquired using the apparatus of  FIG. 2 . The triangular ramp A represents a time varying signal (modulation) added to the static magnetic field H 0 , and the lower trace B shows the absorption dip for a sample of liquid glycerine. As expected, we see two dips per period of the modulation waveform, one for each time that the static field generates the result ω 0 =ω RF . 
       Pulse NMR 
       [0009]    In pulse NMR apparatus, the RF field H 1  consists of a short pulse of RF power, the RF pulse being applied to the sample  12  with an amplitude chosen such that the angle θ of the precessing spins becomes 90°, thereby tipping them from the Z plane into the X-Y plane. This is shown in  FIG. 4 . Clearly this corresponds to the maximum amplitude of the precession, and a pulse producing this deflection is called a 90° or π/2 pulse. A 180° or π pulse would simply flip the spins to the Z axis antiparallel to the applied static magnetic field. The pulse length is designated as:
       π/2 when θ is rotated by 90°   π when θ is rotated by 180°       
 
         [0012]    Once the RF pulse has ended, the spins will continue to precess but the amplitude of the precession, the angle θ, will decrease with time due to interaction of the spins of the nuclei with one another. This gives rise to what is known as the free induction decay (FID) signal from the precession of the spins, an exponentially decaying sinewave generally at the Larmor precession frequency. It is the FID signal which is detected in pulse NMR. 
         [0013]    In a variation of this arrangement, the applied RF field H 1  is selected to have a frequency close to but not quite satisfying the Larmor precession equation ω RF =ω 0 . Mixing then occurs, which gives rise to a lower frequency output signal, which is easier to amplify and filter. This process is known as nutation. 
         [0014]    Pulse NMR apparatus employs a sensor circuit generally the same as that shown on  FIG. 2 , with the exception that in this instance the RF source  14  supplies an oscillating pulse of short duration while the static magnetic field H 0  is of constant amplitude.  FIG. 5  shows a typical NMR π/2 RF pulse and the resulting FID signal occurring following the pulse. 
         [0015]    The known inductive arrangements have been in use for many years. However, they suffer from a number of disadvantages. 
         [0016]    In particular, in both the continuous wave NMR apparatus and the pulse NMR apparatus, the radio frequency excitation signal is applied to the sample inductively by means of a coil, and the nuclear precession signal is read out via inductive coupling. This necessarily introduces constraints, both physical and electronic on the performance of the system. Physically, the coil geometry may be restricted or determined by the nature and size of the sample. Further, an inherent electronic problem associated with the traditional approach is that the transmitter coil couples to the receiving coil inductively, leading to saturation of the receiver amplifier and a subsequent dead time during which the system recovers from overload. Numerous measures have been taken to alleviate this problem, including ensuring that the transmitter and receiver coils are perpendicular to each other, adding diode protection circuits, and the use of quarter wave transmission lines. However, despite these measures the inherent difficulty remains that a large amplitude RF transmitter pulse must be coupled to the sample, and that a small but significant proportion of this will couple inductively to the receiver coil and hence saturate the amplifier system. 
       SUMMARY OF THE INVENTION 
       [0017]    The present invention seeks to overcome the problems described above and to provide a novel means of NMR signal detection. 
         [0018]    The invention further seeks to replace the conventional inductive approach to signal detection in NMR apparatus with electric field detection of NMR signals. 
         [0019]    The invention also seeks to employ a specially designed electric potential sensor for the electric field detection of an NMR signal from a sample. 
         [0020]    According to one aspect of the present invention, there is provided a nuclear magnetic resonance apparatus comprising: 
         [0021]    means for applying a static magnetic field to a sample under investigation, a radio frequency circuit arranged to be inductively coupled to the sample by means of an oscillating magnetic field disposed generally perpendicular to the static magnetic field for applying an excitation signal to the sample, and an electric potential sensor for detecting the excitation of the sample and for generating a detection output, wherein the electric potential sensor comprises an electrode placed generally perpendicular to the axis of the oscillating magnetic field for capacitively coupling the electric potential sensor to the sample. 
         [0022]    According to another aspect of the present invention, there is provided method of nuclear magnetic resonance signal detection, comprising: 
         [0023]    applying a static magnetic field to a sample under investigation, employing an oscillating magnetic field disposed generally perpendicular to the static magnetic field for inductively applying an excitation signal to the sample, and capacitively coupling an electrode to the sample for detecting the excitation of the sample and generating a detection output, the electrode being placed generally perpendicular to the axis of the oscillating magnetic field. 
         [0024]    The invention thus replaces the conventional magnetic (inductive) readout system with an electric (capacitive) one. 
         [0025]    It is believed that by comparison with the prior art, a number of possible advantages accrue from the invention as described, namely:
       The problem of cross coupling between transmitter and receiver coils in the conventional apparatus is avoided, thereby reducing recovery and dead times in the receiver system.   A simpler implementation is possible, requiring no front-end tuned trap, quarter wave line or crossed diode networks on the receiver side. A simple gating of a preamplifier is found to be sufficient.   The use of an untuned (broadband) receiver system allows several nuclear species to be measured simultaneously.   Smaller probe sizes reduce filling factor problems, which exist in conventional systems, allowing the imaging of very small sample volumes.   High spatial resolution may be achieved using the intrinsic resolution of electric field probes. This will enable raster scanning, rather than field gradient techniques, to be used for imaging applications.       
 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0031]    The invention will be described further, by way of example, with reference to the accompanying drawings, in which: 
           [0032]      FIG. 1  is a diagram representing the phenomenon of nuclear magnetic resonance (NMR); 
           [0033]      FIG. 2  is a circuit diagram of a conventional NMR apparatus; 
           [0034]      FIG. 3  is a signal diagram illustrating an output signal obtained from the apparatus of  FIG. 2  when set up to operate as continuous wave NMR apparatus; 
           [0035]      FIG. 4  is a diagram representing the application of a short pulse of RF power in pulse NMR apparatus; 
           [0036]      FIG. 5  is a signal diagram representing an output signal obtained from the apparatus of  FIG. 2  when set up to operate as pulse NMR apparatus; 
           [0037]      FIG. 6  is a circuit diagram of NMR apparatus according to the present invention; 
           [0038]      FIG. 7  is a diagram of an electrode of the apparatus of  FIG. 6 ; 
           [0039]      FIG. 8  is a signal diagram comparing an output signal of the apparatus of  FIG. 6  with the output signal of the apparatus of  FIG. 2  when set up for continuous wave operation; 
           [0040]      FIG. 9  is a circuit diagram of an electric potential sensor of  FIG. 6 ; 
           [0041]      FIG. 10  is a block diagram of a continuous wave NMR system according to the invention; 
           [0042]      FIG. 11  is a block diagram of a pulse NMR system according to the invention; 
           [0043]      FIGS. 12 to 14  are circuit diagrams of certain of the blocks shown in  FIG. 11 ; 
           [0044]      FIG. 15  is a signal diagram representing the signals at various points of the system of  FIG. 11 ; 
           [0045]      FIGS. 16 and 17  are graphs showing a comparison of pulse NMR data obtained respectively using conventional pulse NMR apparatus and the pulse NMR system of  FIG. 11 , in which  FIG. 16  shows a respective free induction decay signal in both cases and  FIG. 17  is an identical plot for the situation where there is no static magnetic field and therefore no free induction decay signal in either case; and 
           [0046]      FIG. 18  is a graph representing a Fourier transform of the data obtained from the apparatus of  FIG. 11  as shown in  FIG. 16 . 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Theory of the Use of Electric Potential Sensors 
       [0047]    The present invention proposes to employ an electric field sensor in place of the known inductive sensor arrangement of  FIG. 2  for signal detection in NMR apparatus. 
         [0048]    The principle of operation for electric field detection of NMR signals is to utilise the electric field (E) associated with the precessing magnetic field (B) of the nuclear spins. From the theory of electromagnetism for the far field approximation of a dipole, it is known that: 
         [0000]    
       
      
       | E |=c| B | 
      
     
         [0000]    where c is the velocity of light. According to the present invention, an electric field sensor is provided for coupling, via a capacitive mechanism, to a sample in order to acquire the NMR signal. 
         [0049]    The electric field vector (E) is perpendicular to the magnetic field component (B) and rotating at the Larmor precession frequency. Formally, the two components are related by a Maxwell equation: 
         [0000]      Curl  E=−δB/δt    
         [0050]    This provides the information needed to position an electrode arrangement for the sensor in order to pick up the electric field component. As described below, the electrode arrangement is placed perpendicular to the axis of the coil providing the RF input. The electrode arrangement may comprise a single electrode arranged to measure the potential with respect to the signal ground, or it may comprise a pair of electrodes. In the following description, it will be assumed that each sensor only includes a single electrode. It is advantageous to use two electrometers or sensors in a differential arrangement to enhance the signal to noise ratio by noise subtraction. For this to operate effectively, the to electrodes of the two sensors must be at two unequal radial distances from the centre of the coil axis in order to provide a differential signal for amplification. 
       NMR Apparatus Employing an Electric Potential Sensor 
       [0051]      FIG. 6  illustrates the implementation of this principle in an electric field sensor arrangement  20 , which is applicable to both continuous wave and pulse NMR applications. The apparatus of  FIG. 6  is similar to that of  FIG. 2 , except in that two electric field sensors E 1  and E 2  have been added. Similar reference numerals designate the same elements and therefore need not be further described. The amplifier  18  is not essential but is retained to allow a magnetic signal to be acquired at the same time as the electric signal, for comparison purposes. 
         [0052]    In order to acquire the electric field NMR signal, each such sensor E is coupled to the sample  12  through a glass sample tube  24  via a capacitive mechanism by means of a respective electrode  22 , such as the one shown in  FIG. 7 . For most applications, the capacitive coupling will be very weak. For example, the coupling capacitance between the tip of a 0.2 mm diameter electrode  22  and the sample  12  through a 0.5 mm thick glass sample tube  24  is estimated to be of the order of a few × 10   −14  F. This may be increased to ˜0.5 pF by the addition of a small (˜1 mm 2 ) foil electrode plate  26  on the tip of the sensor electrode  22  as shown. 
         [0053]    In order to acquire the NMR signal without significant attenuation, it is necessary for each sensor E to have an input capacitance comparable to or lower than this coupling capacitance. It is also a requirement that the sensor has sufficient bandwidth and slew rate to allow efficient amplification of the RF NMR signal. In the present example, the operating frequency was chosen to be between 1.5 MHz and 2 MHz. Typically, NMR systems operate over the whole of the RF spectrum up to many hundreds of MHz. 
         [0054]    It should be noted that the intrinsic spatial resolution of each electric field sensor E is limited by the physical size of the tip of the electrode  22 . This is scalable over a wide range and has been shown to operate from &lt;1 μm to ˜10 cm. In principle, the resolution can range from the atomic scale (˜nm) to many metres depending on the application. The spatial resolution can easily be improved if a sample tube  24  with a thinner wall is used, or if the electrode  22  is insulated by a thin dielectric layer, so as to maintain the coupling capacitance with a much smaller cross sectional area. 
         [0055]    Thus, the design of each electric field sensor E for this application must meet certain stringent requirements such as: low input capacitance, and high operating bandwidth. 
         [0056]      FIG. 8  shows comparison data obtained from the NMR apparatus of  FIG. 6 , when operated as continuous wave NMR apparatus, including particularly the output signal from the amplifier  18  representing the conventional magnetic NMR signal and that obtained from the sensors E representing the new electric field NMR signal. It is evident that the outputs are comparable in each case. 
       A Specific Design of Electric Field Sensor 
       [0057]      FIG. 9  shows a schematic circuit diagram for a respective one of the sensors E of  FIG. 6 . 
         [0058]    As shown, an input  30  includes the electrode  22  and supplies the signal from this electrode to an operational amplifier  32 . The input impedance of the amplifier  32  is enhanced by the use of guarding, which is a positive feedback technique and which involves physically surrounding all the high impedance parts of the circuit, including the input circuitry, wiring and electrode  22 , as completely as possible with a guard  34  driven by the output of the amplifier  32 . The surfaces of the guard  34  enclose and are maintained at the same potential as the input  30 . Charging effects on stray capacitance are alleviated by maintaining the same potential (signal potential) on the guard  34  as on the input electrode  22 . 
         [0059]    The amplifier  32  is configured as a non-inverting amplifier with a voltage gain determined by a series connection of two resistors, R 2  and R 3 , having typical values of 1-100 kΩ. A capacitor C 2  (typically ˜1 μF) in series with the resistors R 2  and R 3  acts as a high pass filter by reducing the gain to unity at zero frequency (DC). This reduces the low frequency noise pick up considerably and thus aids the signal to noise ratio and stability of the sensor E. 
         [0060]    Another positive feedback technique in the form of bootstrapping is also employed in order to increase the effective input impedance of the amplifier  32  at signal frequency. More especially, a resistor R 1  (typically in the range 1MΩ to 100 GΩ) connected to the input  30  provides a DC input bias current, via resistors R 4  and R 5  (typically 1-100 kΩ), that is required by the amplifier  32  for stable operation. A capacitor C 1  is utilised to apply the output voltage of the amplifier  32  to the junction of the resistors R 1 , R 4  so that the input voltage appears at both terminals of the upper resistor R 1 , resulting in zero current flow and an infinite impedance. Thus, the resistor R 1  is bootstrapped by a positive feedback signal provided by the capacitor C 1  (typically 10 nF to 100 μF). This ensures that the potential across the resistor R 1  at the signal frequency is kept as small as possible, thus effectively increasing or bootstrapping the value of the resistor R 1  many times. Adjustment of the resistor R 5  permits the DC operating point of the amplifier  32  to be zeroed. The capacitor C 1  together with the resistors R 4  and R 5  also act as a high pass filter by restricting the range of frequencies over which the bootstrapping will operate effectively. 
         [0061]    The sensor E may also have the facility for gating the amplifier  32  rapidly, which in application of the sensor in pulse NMR apparatus enables the apparatus  20  to be protected from the large amplitude RF pulses that are applied to the sample  12  and thus allows the complex protection circuitry conventionally associated with the receiver side of an NMR spectrometer to be dispensed with. Gating of the amplifier  32  is achieved using an appropriate voltage signal applied to the enable pin  36  of the amplifier  32 . The symbols + and − in  FIG. 9  refer to the power supply connections for the amplifier  32 . 
         [0062]    For the purposes of evaluation, a test electric field sensor for use in the NMR apparatus shown in  FIG. 6  was produced according to the design of  FIG. 9  and based on a commercial operational amplifier with the following specifications: 
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Gain Bandwidth Product 
                 200 MHz 
               
               
                   
                 Slew Rate 
                 200 V/μs 
               
               
                   
                 Input Capacitance 
                 0.5 pF 
               
               
                   
                 Gating facility 
                 25 ns off, 200 ns on 
               
               
                   
                 Voltage Noise 
                 10 nV/{square root over (Hz)} 
               
               
                   
                   
               
             
          
         
       
     
         [0063]    Using the illustrated circuit, the operational bandwidth is typically 5 kHz to 10 MHz with an open circuit voltage noise of 30-70 nV/√{square root over (Hz)}. 
         [0064]    A particular benefit of the design of sensor E used here is the ability to gate the amplifier  32 . This allows the receiver chain to be isolated during the transmitter pulse without the need for either diode networks or quarter wave lines, as would usually be the case. In principle, the off and on times for this procedure are 25 ns and 200 ns respectively, extremely fast by the standards of NMR recovery time in conventional systems. 
       Continuous Wave NMR System According to the Invention 
       [0065]      FIG. 10  shows a block diagram for a continuous wave NMR system according to the invention, where the continuous wave excitation signal is provided by a conventional marginal oscillator and the static magnetic field H o  is created by a simple electromagnet. More particularly, the functions of RF source and signal amplifier are combined into a marginal oscillator  40  connected to a resonant circuit  42  surrounding the sample  12 . The static magnetic field section of the system includes a feedback stabilised power supply (PSU)  44  designed to maintain a constant current, and hence a constant magnetic field, through the coils  46  of an electromagnet  48 . For this purpose, the feedback loop  50  for the PSU  44  includes a PSU control circuit  52 , and the output of the PSU  44  is supplied by way of a low pass filter  54  to the coils  46 . Added to this static field, is a slowly time varying field created by a function generator  48  and fed to a secondary pair of coils  56  fitted to the pole pieces  58  of the electromagnet  48 . Two electric field detection sensors  20  as already described and as shown in  FIGS. 6 and 9  are connected to a differential operational amplifier  60 , and the output of the operational amplifier  60  is supplied to a band pass filter  62  and thence to a precision rectifier  64  and via a low pass filter  66  and a high pass filter  68  to an output  70 . 
       Pulse NMR System According to the Invention 
       [0066]    Another NMR system according to the invention is shown in  FIG. 11  comprising a pulse NMR system. This pulse NMR system includes some of the same parts as the continuous wave NMR system illustrated in  FIG. 10 , and the same parts are designated by the same reference numerals and need not be described further. In particular, the pulse NMR system of  FIG. 11  includes a static field power supply section that is identical to that used for the continuous wave system. However, to implement a pulse system, the RF electronics must be modified to include a high power (˜10 W) coherent source, a pulse modulator, appropriate diode isolation circuits and an active Q damping mechanism for the resonant transmitter probe. In addition, it is necessary to optimise parameters such as the pulse amplitude and duration as well as the mark to space ratio of the pulse sequence. 
         [0067]    In the present embodiment, a single electric field sensor  20  as described with reference to and as shown in  FIGS. 6 and 9  is provided, the NMR signal being coupled to the electric field sensor  20  via capacitive coupling. The sensor  20  is electronically gated by means of gating pulses supplied by a pulse sequence generator  70  through an adjustable transistor-transistor logic (TTL) pulse delay circuit  72 , which also supplies blanking pulses for controlling a sensor isolation switch or circuit  74  as well as Q damping pulses for controlling a Q damping circuit within the resonant circuit  42  as described below. It is to be noted that a conventional magnetic signal receiver (inductively coupled system) would require elaborate measures in order to prevent the large RF pulse from destroying or at least saturating the receiver electronics. By comparison, the electric field sensor requires no such protection, since the electronic gating is sufficient and much simpler to implement. 
         [0068]    The output of the sensor  20  is connected to the isolation circuit  74  situated between the output of the sensor  20  and the input to subsequent amplification stages  76 . This isolation circuit  74  is controlled by the blanking pulses supplied by the TTL pulse delay circuit  72  and serves to isolate the output side of the system during and immediately following the application of the RF pulses to the resonant circuit  42 . Such isolation aids the recovery of the later stage amplifiers after the RF pulse has ended. In this instance, two further stages of amplification are used, including a signal preamplifier  78  and an amplifier  80 , followed by a bandpass filter  82  designed to reject unwanted noise. The output of the bandpass filter  82  constitutes the output  84  of the system. 
         [0069]      FIG. 12  shows the isolation circuit  74  from  FIG. 11 , which is used after the electric potential sensor  20  to protect the following amplifiers  80 ,  82  from overload and saturation and aid their recovery. As shown, the circuit  74  comprises a parallel combination of crossed diodes  110 ,  112  and an FET  114  presenting a low impedance termination to large pulses (since the diodes will conduct under these conditions and the FET  114  will be turned on by the blanking signal during the RF pulse). This, and gating the electric potential sensor  20 , are the main protective measures necessary in order to acquire an electric field NMR signal and are significantly less complex than would be required for the conventional magnetic signal approach. Note also that no additional circuitry is required before the electric field sensor  20 , whereas conventional NMR systems need circuits to protect the preamplifier stage from overload and to aid signal recovery. 
         [0070]    The RF pulse in the system of  FIG. 11  is generated using two phase locked RF synthesisers  86 ,  88 . One such synthesiser  86  acts as a generator providing the RF pulse, while the other such synthesiser  88  provides a reference frequency for the pulse generator  86 . The continuous RF output from the first synthesiser  86  is fed to a pulse modulator  90 , which is controlled using a TTL pulse produced by the pulse sequence generator  70 . The resulting RF pulse is then amplified to produce the requisite power. This is achieved using a commercial power amplifier  92 , a voltage amplifier  94  based on an operational amplifier, and a second power amplifier  96  with a 50Ω matching transformer  98 . The RF pulse is then passed through a diode network  100  to the resonant circuit  42 . This network  100  is arranged to isolate the resonant circuit  42  from the power amplifier chain when the pulse is off. 
         [0071]    The RF pulse diode isolation network  100  is further illustrated in  FIG. 13  and comprises a crossed diode brigade designed to prevent noise from the RF power amplifier  96  coupling to the resonant circuit  42  during the period when the pulse is off and the NMR signal is being acquired. As shown, the network  100  comprises pairs  120  of crossed diodes  122 ,  124  presenting low impedance to large signals (larger than the forward turn on voltage of ˜0.6V) and a relatively high impedance to small signals (e.g. the amplifier noise). This high impedance forms an effective potential divider with a parallel connection of a pair of resistors  126 ,  128  typically having a resistance of ˜470Ω each. Each pair of diodes  122 ,  124  provides additional attenuation such that the cascaded total effect is of the order of −60 dB. 
         [0072]    As already mentioned, the pulse sequence generator  70  and the associated TTL pulse delay unit  72  generate a series of gating pulses for gating the sensor  20  and a series of blanking pulses for controlling the isolation circuit  74 . They also generate a series of Q damping pulses, which are used to control the Q damping circuit within the resonant circuit  42 . Q damping is the process of dumping, as rapidly as possible, the energy stored in the resonant circuit  42  at the end of each RF pulse. Thus, each Q damping pulse is arranged to commence at the end of an RF pulse and to continue for a period of time optimised as the shortest possible time consistent with the energy stored in the resonant circuit being dumped. This enables the very small free induction decay signal to be seen more easily and is achieved using an FET switch to terminate excitation by the resonant circuit with a resistor which meets the criterion for critical damping. 
         [0073]      FIG. 14  further illustrates the resonant circuit  42  used to probe the sample  12  and including the active Q damping circuit. The resonant frequency of the circuit is determined by a parallel connection of an inductance L and a capacitance C 2 . For this example, typical values for L and C 2  would be in the region respectively of 10 μH and 470 pF. A capacitor C 1  providing an input/output to the resonant circuit  42  is chosen to provide an optimum impedance match between the resonant circuit  42  and the transmission line cables (50Ω) used to couple to the RF power amplifier  92  and the signal preamplifier  78 . Typically, C 1  may be around 39 pF. An FET  130  is normally in the on state, but is switched off during the Q damping process. This action results in a resistor R 1  connected across the FET  130  being switched into a series connection with the resonant circuit  42 . The value for R 1  is chosen to satisfy the critical damping condition (R 2 =4 L/C) and may typically be ˜250Ω, which results in the rapid dissipation of the energy stored in the resonant circuit  42  at the end of the RF pulse. 
         [0074]    The RF pulse typically consists of a short (e.g. 20 μs) pulse followed by a long (1 ms to 10 s) gap, during which the sensor signal is acquired. The pulse synthesiser/generator  86  provides this short pulse as an output at point A of the system in  FIG. 11 , and the phase locked synthesiser  88  in conjunction with the pulse delay circuit  72  produces a delayed pulse as an output at points B 1  and B 2  of the system. The pulse at point B 1  comprises the gating pulse for controlling the sensor  20 , while the pulse at point B 2  is used to control the FET isolation switch  114  in the isolation circuit  74 . The Q damping pulse is generated as another output of the pulse delay circuit  72  at point C of the system and effectively consists of the overlap between the short pulse and the delayed blanking pulse. The Q damping pulse therefore exists for the time interval between the end of the RF pulse and the end of the blanking pulse. The duration of the Q damping pulse is optimised to give the shortest recovery time for the resonant circuit after the end of the RF pulse. The timing diagram in  FIG. 15  illustrates the relationship of these pulses and demonstrates that the delayed pulse and the Q damping pulse terminate together, at which point the sensor  20  will have been gated, the output amplifiers  78 ,  80  will have been isolated from the RF pulse and the energy stored in the resonant circuit  42  will have dissipated and the system is ready to acquire the free induction decay signal. 
         [0075]    The bandwidth limiting filter  82  is included in order to reject unwanted noise and so improve the signal to noise ratio. Signal averaging of the data is also used to improve the signal to noise ratio. This is only made possible because the pulse sequence generator  86  is phase locked to the second RF frequency synthesiser  88 . This means that the phase of the RF contained in each pulse is the same so that the data will build up to provide a true result rather than time average to zero. 
         [0076]    It should be noted that the described electric field sensor arrangement according to the invention offers a significantly simpler option for acquiring an NMR signal than does the conventional magnetic arrangement.  FIGS. 16 and 17  show comparable results for both arrangements using a similar circuit arrangement for generating the RF pulses, and a similar arrangement for the electromagnet  48  but the different sensor approaches. The plot in each case is the result of ˜100 averages. In  FIG. 16  comparative electric and magnetic signals are shown in the presence of the static magnetic bias field and for a glycerine sample as before, and in  FIG. 17  they are shown in the absence of a static magnetic bias field. Clearly in the latter case, only the recovery of the resonant circuit at the start of the trace is seen and no NMR decay signal (FID signal) thereafter. 
         [0077]    Since information is contained in the frequency of the precession, it is usual to present the data in the frequency domain, for example by means of a Fourier transform (FFT) to obtain the NMR spectrum of the sample. The result of Fourier transforming the electric field data of  FIG. 16  is shown in  FIG. 18 . One clear resonance peak corresponding to the NMR signal is displayed at 1.825 MHz for the glycerine sample, as expected. All other peaks are at least a factor of 1000 smaller and are the result of either noise or non-linear behaviour in the system. 
         [0078]    An estimate of the amplitude of the output electric field signal may be made as follows. First, in a conventional magnetic system as mentioned above, the output voltage (V) from the magnetic channel may be used to estimate the magnetic field (B) in the resonant probe coil due to the observed free induction decay signal using the equation: 
         [0000]    
       
         
           
             B 
             = 
             
               
                 Φ 
                 . 
               
               QAf 
             
           
         
       
     
         [0000]    where 
         [0000]    
       
         
           
             
               V 
               = 
               
                 - 
                 
                   Φ 
                   . 
                 
               
             
             , 
             
               
                 Φ 
                 . 
               
               = 
               
                 
                    
                   Φ 
                 
                 
                    
                   t 
                 
               
             
             , 
           
         
       
     
         [0000]    Φ is the magnetic flux in the coil, Q is the quality factor of the resonant probe circuit, A is the cross sectional area of the coil and f is the precession frequency 
         [0000]    
       
         
           
             
               ( 
               
                 = 
                 
                   
                     ω 
                     0 
                   
                   
                     2 
                      
                     
                         
                     
                      
                     π 
                   
                 
               
               ) 
             
             . 
           
         
       
     
         [0000]    For typical data acquired using such a conventional system, this corresponds to B=1.8×10 −10  T. Next, such results may be converted to an electric field (E) using the plane wave approximation: 
         [0000]    
       
      
       | E |=c| B | 
      
     
         [0000]    where c is the velocity of light. This gives an electric field of 54 mV/m. The final step is to calculate the voltage at the electric sensors of the invention due to this electric field using the equation: 
         [0000]    
       
      
       V 
       out 
       =|E|d  
      
     
         [0079]    where d is the spacing of the electric sensors. For the example used here, this results in an estimated voltage at the input of the sensors of 6 μV. This corresponds to an estimated output voltage of ˜180 mV for the electric field channel, which is in good agreement with the observed output of ˜150 mV. 
         [0080]    Various modifications are possible to the described embodiments within the scope of the invention. 
         [0081]    For example, the embodiment of  FIG. 6  is described as having two electric field sensors, whereas that of  FIG. 11  is described as having one. In practice, any number of the sensors may be provided from one upwards. The use of two such sensors, however, employed differentially has the advantage of improving the signal to noise ratio by comparison with an arrangement employing only one such sensor. 
         [0082]    Further, the resistor R 1  included in the  FIG. 9  circuit for providing a DC input bias may be replaced with a number of other components serving the same purpose, such as for example a diode or a field effect transistor. 
         [0083]    The present invention as described is applicable both to continuous wave NMR apparatus and to pulse NMR apparatus, and offers a number of significant advantages by comparison with the conventional inductive approach to NMR signal detection. 
         [0084]    In particular, the problem of cross coupling between transmitter and receiver coils in the conventional apparatus is avoided, thereby reducing recovery and dead times in the receiver system. Furthermore, the implementation of an electric field sensor is greatly simplified by comparison with a conventional inductive sensor and requires no front-end tuned trap, quarter wave line or crossed diode networks on the receiver side. A simple gating of the preamplifier is found to be sufficient for protecting the sensor. 
         [0085]    It is also possible to employ smaller probe sizes than in the conventional systems and the sensor is only strongly coupled to a relatively small part of the sample. It is anticipated that this will reduce filling factor problems, which exist in conventional systems when using very small sample volumes. The present invention, therefore, should facilitate the imaging of very small sample volumes. 
         [0086]    In addition, high spatial resolution may be achieved using the intrinsic resolution of electric field probes. 
         [0087]    Further, the sensor electrodes are untuned and therefore intrinsically broadband. The use of an untuned (broadband) receiver system may allow several nuclear species to be measured simultaneously.